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SDLRC - Scientific Articles all years by Author - St+
The Sheahan Diamond Literature Reference Compilation
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcementscalled the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Resource Center
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
The SDLRC provides 3 types of references identified in the reference code. DS for scientific article, DM for a media article, and DC for a corporate announcement. Consider DS0512-0001. The DS stands for "diamond scientific". 05 stands for 2005, the year the reference was posted. 12 represents the month the reference was posted. For all years prior to 2015 the default month is 12. -0001 is the reference's identifier and it does not mean anything. The number below the refence code, ie 2015, is the year the article was published. Note that the posted year may sometimes be later than the published year.
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The Trans Hudson Orogen of North America and the Himalayan Karakoram Tibetan Orogen of Asia: structural and thermal evolution of the lower and upper plates.
GAC Annual Meeting Halifax May 15-19, Abstract 1p.
Trans Hudson Orogen of North America and Himalaya Karakoram Tibetan Orogen of Asia: structural and thermal characteristics of the lower and upper plates.
St.Onge, M.R., Van Gool, A.M., Garde, A.A., Scott, D.J.
Correlation of Archean and paleoproterozoic units between northeastern Canada and western Greenland: constraining the pre-collisional upper plate accretionary history
Geological Society of London, Special Publication Earth Accretionary systems in Space and Time, No. 318, pp. 193-235.
Perceptions of the impact of board members' individual perspectives on the social and environmental performance of companies. ( Based on SA and not junior companies).
Journal of the South African Institute of Mining and Metallurgy, Vol. 114, Nov. pp. 957-969.
Diamond & Related Materials, Vol. 109, 108049, 6p. Pdf
Mantle
nitrogen
Abstract: Single crystal diamond (<5?ppm nitrogen) containing native NV centers with coherence time of 150??s was irradiated with 2?MeV alpha particles, with doses ranging from 1012 ion/cm2 to 1015 ion/cm2. The effect of ion damage on the coherence time of NV centers was studied using optically detected magnetic resonance and supplemented by fluorescence and Raman microscopy. A cross-sectional geometry was employed so that the NV coherence time could be measured as a function of increasing defect concentration along the ion track. Surprisingly, although the ODMR contrast was found to decrease with increasing ion induced vacancy concentration, the measured decoherence time remained undiminished at 150us despite the estimated vacancy concentration reaching a value of 40?ppm at the end of range. These results suggest that ion induced damage in the form of an increase in vacancy concentration does not necessarily result in a significant increase in the density of the background spin bath.
Cartigny, P., Farquar, J., Thomassot, E., Harris, J.W., Wing, B., Masterson, A., McKeegan, K., Stachel, T.
A mantle origin for Paleoarchean peridotite diamonds from the PAnd a kimberlite, Slave Province: evidence from 13C, 15N and 34,34S stable isotope systematics.
Aulbach, S., Stachel, T., Craeser, R.A., Heaman, L.M., Shirey, S.B., MUehlenbachs, K., Eichenberg, D., Harris
Sulphide survival and diamond genesis during formation and evolution of Archean subcontinental lithosphere: a comparison between the Slave and Kaapvaal cratons.
Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A delta 13 C-N study of eclogite hosted diamonds from the Jericho kimberlite.
Geochimica et Cosmochimica Acta, Vol. 75, pp. 6027-6047.
Hunt, L., Stachel, T., Grutter, H., Armstrong, J., McCandless, T.E., Simonetti, A., Tappe, S.
Small mantle fragments from the Renard kimberlites, Quebec: powerful recorders of mantle lithosphere formation and modification beneath the eastern Superior Craton.
Howell, D., Stern, R.A., Griffin, W.L., Southworth, R., Mikhail, S., Stachel, T., Verchovsky, A.B., O'Reilly, S.Y., Pearson, N.J.
New thermodynamic models and calculated phase equilibration temperatures in NCFMAS for basic and ultrabasic compositions through the transition zone into the uppermost lower mantle.
Hua, C., Zhili, Q., Taijin, L., Stern, R., Stachel, T., Yuan, S., Jian, Z., Jie, K., Shyu, P., Shecai, Q.
Variations in carbon isotopic composition in the subcontinental lithospheric mantle beneath the Yangtze and North Chin a cratons; evidence from in-situ analysis of diamonds using SIMS.
Multiple growth episodes or prolonged formation of diamonds? Inferences from infrared absorption data.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 281-296.
Palot, M., Pearson, D.G., Stern, R.A., Stachel, T., Harris, J.W.
Multiple growth events, processes and fluid sources involved in diamond genesis: a micro-analytical study of sulphide bearing diamonds from Finsch mine, RSA.
Geochimica et Cosmochimica Acta, Vol. 106, pp. 51-70.
Palot, M., Pearson, D.G., Stern, R.A., Stachel, T., Harris, J.W.
Isotopic constraints on the nature and circulation of deep mantle C-H-O-N fluids: Carbon and nitrogen systematics within ultra-deep diamonds from Kankan ( Guinea).
Geochimica et Cosmochimica Acta, Vol. 139, pp. 26-46.
Abstract: Size frequency distributions are the principal tool for predicting the macro-diamond grade of new kimberlite discoveries, based on micro-diamonds (i.e., diamond ? 0.5 mm) recovered from small exploration samples. Lognormal size frequency distributions – as observed for the Artemisia kimberlite (Slave Craton, Canada) – suggest a common source for micro- and macro-diamonds recovered from single samples, an implication that has never been conclusively tested. We analyzed 209 diamonds between 0.2 and 2 mm in size from the Artemisia kimberlite for their carbon isotopic compositions and nitrogen characteristics to determine the nature of the micro-/macro-diamond relationship.-Despite overall similarity in the ?13C distributions of micro- and macro-diamonds – both are bimodal with peaks in classes ? 5.0 to ? 4.5‰ and ? 3.5 to ? 3.0‰ – rare diamonds with ?13C between ? 14.2 and ? 24.5‰ of presumed eclogitic origin are restricted to macro-diamonds, whereas positive values are only observed for micro-diamonds. In addition, a shift in main mode and median value in ?13C of about +1‰ is observed for micro- relative to macro-diamonds. Fundamental differences between micro- and macro-diamonds at Artemisia were revealed through the analysis of nitrogen concentrations: 68% of micro-diamonds are Type II (“nitrogen free”) versus 21% of macro-diamonds, and only 19% of micro-diamonds have nitrogen contents > 100 atomic ppm versus 43% of macro-diamonds. Similarly, the presence of a detectable hydrogen related peak (at 3107 cm? 1) increases from 40% for micro-diamonds to 94% for macro-diamonds.-Previous studies on diamond populations from individual deposits have documented that single batches of ascending kimberlite or lamproite magma sample multiple diamond subpopulations formed during distinct growth events in compositionally variable sources and at various depth levels. The Artemisia data clearly show that even over a fairly narrow size interval, spanning the micro- to macro-diamond transition, the specific diamond subpopulations present and their relative proportions may vary significantly with diamond size. At Artemisia, we conclude that the observed lognormal size distribution is not a reflection of an entirely common origin of micro- and macro-diamonds.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 47.
Canada, Northwest Territories
Garnet chemistry
Abstract: In diamond exploration, the use of compositional data to identify diamond-related peridotitic xenocrysts has long been a widely used and powerful tool. In contrast, the application of similar methods to eclogitic garnet chemistry remains a challenge. The inability to unequivocally classify certain “eclogitic” garnet compositions as either mantle- or crust-derived implies that a high abundance of lower-crustal garnets will increase diamond-exploration expenditures by introducing a number of “false positives.” Revising existing classification schemes (e.g., Schulze, 2003) to reduce the abundance of “false positives” may, however, increase the number of “false negatives” through the misclassification of mantle-derived garnets as crustal. This study presents new geochemical and petrographical data for garnet and clinopyroxene from 724 kimberlite-hosted, crust- and mantle-derived xenoliths from localities worldwide, with a focus on samples whose lithology is constrained petrographically, rather than single mineral grains from concentrate. Mantle samples are primarily eclogitic and pyroxenitic, as constrained by mineral assemblage and garnet and clinopyroxene mineral chemistry, while crustal samples are dominantly plagioclase-bearing garnet-granulites. For those localities where an established geothermal gradient is available from literature resources, garnet-clinopyroxene pairs are employed in the estimation of pressure-temperature conditions of equilibration through the iterative coupling of the Krogh (1988) geothermometer and the relevant geothermal gradient. Our preliminary results suggest that closure temperatures for Fe-Mg exchange exceed the temperatures of residence of many lower-crustal samples, as geotherm-based calculated pressures of equilibration exceed the apparent stability of plagioclase (see Green and Ringwood, 1972). Comparison of equilibration pressures with sodium contents in garnet for mantle-derived samples (the diamond-facies criterion of Gurney, 1984) shows a positive correlation at localities for which an adequate range of pressures is observed (e.g., the Diavik mine). Other populations, such as mantle eclogitic garnets from Roberts Victor, plot at a much more restricted range of pressures and hence fail to demonstrate this correlation; instead, these samples may reflect the influence of a broader range of bulk-compositions, providing varying amounts of sodium to their constituent garnets. The results presented here demonstrate clearly that garnets from mantle- and crust-derived samples show significant overlap in geochemical character, for example in garnet Ca# vs. Mg# space (discrimination diagram of Schulze, 2003), where approximately 66% of our crust-derived garnet analyses plot in the “mantle” field. This percentage varies among locations. A selection of particularly high-Mg#, low-Ca# garnets derived from crustal, plagioclase-bearing lithologies in this study highlights the potential for crust-mantle confusion, as these garnets have Mg# in-excess of many mantle-derived eclogitic/pyroxenitic garnets. As a consequence, Fe-Mg-Ca-based classifications alone cannot reliably discriminate mantle and crustal garnets. The next step in this project will be to obtain trace element data for the entire sample suite. This will allow us to test the Li-geobarometer of Hanrahan et al. (2009) for eclogites and to search for trace element signatures that can be used as robust indicators of a diamond-facies origin of eclogitic garnets. Trace element data will also be employed in the refinement of the crust/mantle division discussed above.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 98.
Mantle
Diamond genesis
Abstract: Studies of mineral inclusions in diamond have conclusively established that the principal diamond substrates in Earth's mantle are peridotitic (about 2/3) and eclogitic (about 1/3) domains located at 140-200 km depth in the subcratonic lithosphere. There, the formation of the dominant harzburgitic diamond association generally occurred under subsolidus (melt-absent) conditions. In eclogitic and lherzolitic substrates, however, diamond grew in the presence of a melt, with relatively rare exceptions relating to formation from strongly reducing fluids or at relatively low pressure (<50 kbar) and temperature (<1050°C). Complex internal growth structures indicate that in many instances, diamond formation did not occur in a single short lived event. The observed close agreement of radiometric ages involving different isotope systems and inclusion minerals for diamonds from individual occurrences, however, cannot be coincidental and implies that the temporal extent of individual diamond growth events is contained within the uncertainty of the age dates. Diamond formed through most of Earth's history, from the Paleoarchean to at least the Mesozoic. Diamond forming episodes occur on regional to global scales in response to tectonothermal events such as suturing, subduction and plume impact. Individual diamond forming episodes may be associated with particular substrates, with harzburgitic paragenesis diamonds generally yielding Paleoarchean (3.6-3.2 Ga) ages and lherzolitic paragenesis diamonds forming mostly in the Paleoproterozoic at ~2 Ga. Peridotitic diamond growth, however, continued through Earth's history, with the youngest age date being ~90 Ma. Formation of diamonds hosted by eclogite is documented from the Mesoarchean to the Neoproterozoic (2.9 and 0.6 Ga) and may well continue up to the present. Multiple lines of evidence suggest that formation of fibrous diamonds and diamond coats often is penecontemporaneous to kimberlite magmatism and hence, for the Central Slave, may even extent into the Tertiary. When it comes to the actual process(es) driving the precipitation of diamond, our knowledge is much less complete. Diamond grows during the infiltration of carbon-bearing fluids or melts into a suitable substrate. But what exactly is the diamond forming reaction that occurs there? The conventional view that redox reactions between percolating fluids/melts and wall rocks are nature's diamond recipe is inconsistent with both the low redox capacity of lithospheric mantle and the occurrence of large diamonds. Based on thermodynamic modeling, we instead propose that isochemical cooling or ascent of carbon-bearing fluids is a key mechanism of diamond formation. It operates particularly efficiently in chemically depleted mantle rocks (harzburgite), where a high melting temperature precludes dilution of the infiltrating fluid (see above), thereby explaining the long observed close association between diamond and harzburgitic garnet.
Abstract: Canada is host to at least six separate cratons that comprise a significant proportion of its crustal extent. Of these cratons, we possess knowledge of the cratonic lithospheric roots beneath only the Slave craton and, to a lesser extent, the Superior craton, despite the discovery of many new diamond-bearing kimberlites in Canada's North. Here we present the first age, composition and geothermal information for kimberlite-borne peridotite xenoliths from two localities within the central Rae craton: Pelly Bay and Repulse Bay. Our aim is to investigate the nature and evolution of the deep lithosphere in these regions and to examine how events recorded in the mantle may or may not correlate with the complex history of crustal evolution across the craton. Peridotite xenoliths are commonly altered by secondary processes including serpentinization, silicification and carbonation, which have variably affected the major element compositions. These secondary processes, as well as mantle metasomatism recorded in pristine silicate minerals, however, did not significantly modify the relative compositions of platinum-group elements (PGE) and Os isotope ratios in the majority of our samples from Pelly Bay and Repulse Bay, as indicated by the generally high absolute PGE concentrations and mantle-like melt-depleted PGE patterns. The observed PGE signatures are consistent with the low bulk Al2O3 contents (mostly lower than 2.5%) of the peridotites, as well as the compositions of the silicate and oxide minerals. Based on PGE patterns and Os model ages, the peridotites from both localities can be categorized into three age groups: Archean (3.0-2.6 Ga overall; 2.8-2.6 Ga for Pelly Bay and 3.0-2.7 Ga for Repulse Bay), Paleoproterozoic (2.1-1.7 Ga), and "Recent" (<1 Ga, with model ages similar to the ca. 546 Ma kimberlite eruption age). The Archean group provides the first direct evidence of depleted Archean lithospheric mantle forming coevally with the overlying Archean crustal basement, indicating cratonization of the Rae during the Archean. The subtle difference in Os model ages between Pelly Bay and Repulse Bay coincides with the age difference between crustal basement rocks beneath these two areas, supporting the suggestion that the Rae craton was assembled by collision of separate two Archean blocks at 2.7-2.6 Ga. The Paleoproterozoic peridotites are interpreted to represent newly formed lithospheric mantle, most likely associated with regional-scale underplating during the 1.77-1.70 Ga Kivalliq-Nueltin event via removal of the lower portion of Archean lithospheric mantle followed by replacement with juvenile Paleoproterozoic lithospheric mantle. The existence of multiple age clusters in the lithosphere at each locality is consistent with the observation of present-day seismic lithospheric discontinuities (0540 and 0545) that indicate two or more layers of fossil lithospheric mantle fabric beneath this region. Our data define a shallow mantle lithosphere layer dominated by Archean depletion ages underlain by a layer of mixed Archean and Paleoproterozoic ages. This lithospheric mantle structure is probably a response to complex tectonic displacement of portions of the lithospheric mantle during Paleoproterozoic orogeny/underplating. The best equilibrated Archean and Paleoproterozoic peridotites at both Pelly Bay and Repulse Bay define a typical cratonic geotherm at the time of kimberlite eruption, with a ?200 km thick lithospheric root extending well into the diamond stability field, in keeping with the diamondiferous nature of the kimberlites. Such thick lithosphere remains in place to the present day as suggested by seismic and magnetotelluric studies (0540, 0545 and 0550). The metasomatically disturbed peridotites in the Rae lithospheric mantle, yielding model ages indistinguishable from kimberlite eruption, may represent parts of the Rae craton mantle root that show anomalous magnetotelluric signatures.
Abstract: Canada is host to at least six separate cratons that comprise a significant proportion of its crustal extent. Of these cratons, we possess knowledge of the cratonic lithospheric roots beneath only the Slave craton and, to a lesser extent, the Superior craton, despite the discovery of many new diamond-bearing kimberlites in Canada's North. Here we present the first age, composition and geothermal information for kimberlite-borne peridotite xenoliths from two localities within the central Rae craton: Pelly Bay and Repulse Bay. Our aim is to investigate the nature and evolution of the deep lithosphere in these regions and to examine how events recorded in the mantle may or may not correlate with the complex history of crustal evolution across the craton. Peridotite xenoliths are commonly altered by secondary processes including serpentinization, silicification and carbonation, which have variably affected the major element compositions. These secondary processes, as well as mantle metasomatism recorded in pristine silicate minerals, however, did not significantly modify the relative compositions of platinum-group elements (PGE) and Os isotope ratios in the majority of our samples from Pelly Bay and Repulse Bay, as indicated by the generally high absolute PGE concentrations and mantle-like melt-depleted PGE patterns. The observed PGE signatures are consistent with the low bulk Al2O3 contents (mostly lower than 2.5%) of the peridotites, as well as the compositions of the silicate and oxide minerals. Based on PGE patterns and Os model ages, the peridotites from both localities can be categorized into three age groups: Archean (3.0-2.6 Ga overall; 2.8-2.6 Ga for Pelly Bay and 3.0-2.7 Ga for Repulse Bay), Paleoproterozoic (2.1-1.7 Ga), and “Recent” (<1 Ga, with model ages similar to the ca. 546 Ma kimberlite eruption age). The Archean group provides the first direct evidence of depleted Archean lithospheric mantle forming coevally with the overlying Archean crustal basement, indicating cratonization of the Rae during the Archean. The subtle difference in Os model ages between Pelly Bay and Repulse Bay coincides with the age difference between crustal basement rocks beneath these two areas, supporting the suggestion that the Rae craton was assembled by collision of separate two Archean blocks at 2.7-2.6 Ga. The Paleoproterozoic peridotites are interpreted to represent newly formed lithospheric mantle, most likely associated with regional-scale underplating during the 1.77-1.70 Ga Kivalliq-Nueltin event via removal of the lower portion of Archean lithospheric mantle followed by replacement with juvenile Paleoproterozoic lithospheric mantle. The existence of multiple age clusters in the lithosphere at each locality is consistent with the observation of present-day seismic lithospheric discontinuities (0540 and 0545) that indicate two or more layers of fossil lithospheric mantle fabric beneath this region. Our data define a shallow mantle lithosphere layer dominated by Archean depletion ages underlain by a layer of mixed Archean and Paleoproterozoic ages. This lithospheric mantle structure is probably a response to complex tectonic displacement of portions of the lithospheric mantle during Paleoproterozoic orogeny/underplating. The best equilibrated Archean and Paleoproterozoic peridotites at both Pelly Bay and Repulse Bay define a typical cratonic geotherm at the time of kimberlite eruption, with a ?200 km thick lithospheric root extending well into the diamond stability field, in keeping with the diamondiferous nature of the kimberlites. Such thick lithosphere remains in place to the present day as suggested by seismic and magnetotelluric studies (0540, 0545 and 0550). The metasomatically disturbed peridotites in the Rae lithospheric mantle, yielding model ages indistinguishable from kimberlite eruption, may represent parts of the Rae craton mantle root that show anomalous magnetotelluric signatures.
Contributions to Mineralogy and Petrology, Vol. 171, 15p.
Canada, Northwest Territories
Deposit - Diavik
Abstract: Fibrous diamonds are often interpreted as direct precipitates of primary carbonate-bearing fluids in the lithospheric mantle, sourced directly from common reservoirs of “mantle” carbon and nitrogen. Here we have examined fibrous growth layers in five diamonds (as three rims or “coats” and two whole-crystal cuboids) from the Diavik Diamond Mine, Canada, using in situ C- and N-isotope and N-abundance measurements to investigate the origin and evolution of their parental fluids, and in particular, to test for isotopic variability within a suite of fibrous diamonds. High-resolution growth structure information was gleaned from cathodoluminescence (CL) imaging and, in combination with the isotopic data, was used to assess the nature of the transition from gem to fibrous growth in the coated diamonds. The two cuboids are characterized by fine concentric bands of fibrous and/or milky opaque diamond, with one sample (S1719) having intermittent gem-like growth layers that are transparent and colourless. The three coated diamonds comprise octahedral gem cores mantled by massive or weakly zoned fibrous rims, with sharp and well-defined gem-fibrous boundaries. For the two cuboid samples, ? 13C and ? 15N values were ?7.7 to ?3.2 ‰ (mean ?6.3 ± 1.3 ‰; 1 SD; n = 84) and ?5.6 to ?2.1 ‰ (mean ?4.0 ± 0.8 ‰; 1 SD; n = 48), respectively. The three fibrous rims have combined ? 13C values of ?8.3 to ?4.8 ‰ (mean ?6.9 ± 0.7 ‰; 1 SD; n = 113) and ? 15N values of ?3.8 to ?1.9 ‰ (mean ?2.7 ± 0.4 ‰; 1 SD; n = 43). N-abundances of the combined cuboid-fibrous rim dataset range from 339 to 1714 at. ppm. The gem cores have ? 13C and ? 15N values of ?5.4 to ?3.5 ‰ and ?17.7 to +4.5 ‰, respectively, and N-abundances of 480 to 1699 at. ppm. Broadly uniform C- and N-isotope compositions were observed in each of the gem cores (variations of ~<1 ‰ for carbon and ~<3 ‰ for nitrogen). This limited C- and N- isotope variability implies that the gem cores formed from separate pulses of fluid that remained isotopically uniform throughout the duration of growth. Significant isotopic and abundance differences were observed between the gem and fibrous growth zones, including in one detailed isotopic profile ? 13C and ? 15N offsets of ~?2.4 and ~?3.7 ‰, respectively, and a ~230 at. ppm increase in N-abundance. Combined with the well-defined gem-fibrous boundaries in plane light and CL, these sharp isotopic differences indicate separate parental fluid histories. Notably, in the combined fibrous diamond dataset prominent C- and N-isotope differences between the whole-crystal cuboid and fibrous rim data were observed, including a consistent ~1.3 ‰ offset in ? 15N values between the two growth types. This bimodal N-isotope distribution is interpreted as formation from separate parental fluids, associated with distinct nitrogen sources. The bimodal N-isotope distribution could also be explained by differences in N-speciation between the respective parental fluids, which would largely be controlled by the oxidation state of the fibrous rim and cuboid growth environments (i.e., N2 vs. NH4 + or NH3). We also note that this C- and N-isotope variability could indicate temporal changes to the source(s) of the respective parental fluids, such that each stage of fibrous diamond growth reflects the emplacement of separate pulses of proto-kimberlitic fluid—from distinct carbon and nitrogen sources, and/or with varying N-species—into the lithospheric mantle.
Bussweiler, Y., Pearson, D.G., Luth, R.W., Kjarsgaard, B.A., Stachel, T.
The evolution of calcite-bearing kimberlite by rock-melt reaction during ascent - evidence from polymineralic inclusions within Cr- diopside and Cr-pyrope megacrysts from Lac de Gras kimberlites, Northwest Territories, Canada.
GAC MAC Meeting Special Session SS11: Cratons, kimberlites and diamonds., abstract 1/4p.
Abstract: First predictions of the macrodiamond grade of newly discovered kimberlites are commonly obtained using size frequency distributions of microdiamonds. The success of this approach suggests a common origin of microdiamonds and macrodiamonds, an implication not yet conclusively established or disproved. In contrast to previous comparative studies on microdiamonds and macrodiamonds from single deposits, here all diamonds analyzed originate from the same microdiamond samples (558 diamonds, ranging from 0.212 to 3.35 mm). The diamonds were analyzed for their carbon isotope compositions and nitrogen characteristics, and, based on this dataset, statistical comparisons were conducted across the size range to assess cogenesis. As a whole, the Misery diamond suite shows high nitrogen contents (median = 850 at. ppm), a bimodal distribution in time-averaged mantle residence temperatures (two distinct subpopulations in mantle residence temperatures: ?1,125° and ?1,175°C), a high degree of platelet degradation, and ?13C compositions that are isotopically slightly heavier (median = ?4.4‰) than the global median. Statistical comparisons of the various size classes indicate the presence of subtly different subpopulations at Misery; however, the nature and magnitude of these geochemical differences are very small in the context of the global diamond database and are viewed as petrogenetically insignificant. The general geochemical similarity of diamonds from different size fractions at Misery reinforces the use of size-frequency analysis to predict diamond grade in kimberlite diamond deposits.
GAC MAC Meeting Special Session SS11: Cratons, kimberlites and diamonds., abstract 1/4p.
Canada, Northwest Territories
Diamond indicators
Abstract: The Central Mackenzie Valley (CMV) area of the Northwest Territories (NWT) comprises a Phanerozoic sedimentary basin that lies between the western margin of the Slave craton and the Cordillera. Although the region is considerably outside the bounds of the exposed Slave craton, both LITHOPROBE and more recent regional-scale surface wave studies (e.g., Priestley and McKenzie, 2006) indicate the likely presence of lithospheric mantle extending into the diamond stability field. Recent work conducted by Olivut Resources Ltd. led to the discovery of 29 kimberlites in the CMV. However, the indicator mineral chemistry of discovered kimberlites does not appear to be a good match (www.olivut.ca) with those during regional till and stream sediment sampling by the Geologic Survey of Canada (GSC) and Northwest Territories Geologic Survey (NTGS) in August 2003 and July 2005. We present new geochemical data on the regional indicator minerals with the aim of obtaining geotherm and depth of mantle sampling constraints on those indicator minerals discovered to date. A statistical evaluation of the data will compare the similarities to indicator mineral chemistry with parts of the Slave craton to evaluate whether the CMV indicators may ultimately be derived from that region. In total 3600 kimberlite indicator mineral grains were picked from the 0.25-2.0 mm size fractions. Peridotitic garnet grains dominate (46%), followed by magnesium ilmenite (26%), with decreasing individual proportions >15% of chromite, low-chrome diopside, olivine, chrome-diopside and eclogitic garnet. A sub-sample of these grains (3143) were analysed by EPMA. Garnet grains classify (after Grütter et al., 2004) as 1015 (62.1%) G9, 270 (16.5%) G11, 113 (6.9%) G10, 103 (6.3%) G12, 57 (3.5%) G1, 46 (2.8%) G10D, and the remaining 31 (1.9%) as G0, G3, G3D, G4, and G5. A sub-set of garnet grains (~700) were selected for LA-ICP-MS trace element analysis. Of the grains selected 74% G9, 14% G10 (and G10D), and 8% G11, with only 4% G12 and G0 (Grütter et al., 2004). Nickel concentrations from these grains range from 2.6-168.2 ppm, with the majority (>80%) between 20-100 ppm, yielding TNi (Canil, 1999) values ranging from 643-1348°C, with the majority between ~1000-1200°C. Using a central Slave craton geothermal gradient (Hasterok and Chapman, 2011), equilibration pressures for these garnet grains range from 20-80 kbars with the majority between 40-60 kbars (120-185 km). Preliminary analysis has 581 (81%) of the erupted peridotitic mantle garnet grains plotting within the diamond stability field (Kennedy and Kennedy, 1976). Of the 128 clinopyroxene grains analysed, only a few represent garnet peridotite (lherzolite) facies KIM clinopyroxene grains following compositional screening. Thermobarometry of these grains (Nimis and Taylor, 2000), assuming they were all derived from the same lithospheric section, yields P-T arrays identical to the central Slave geotherm that was 220 km thick at the time of eruption. These results are encouraging for diamond exploration. We thank Overburden Drilling Management Ltd. for grain picking and recovery of the small diamond, SGS Lakefield Research for mounting grains, and the GSC for probing of the grains.
Geochimica et Cosmochimica Acta, Vol. 157, pp. 1-12.
Technology
Diamond morphology
Abstract: Nitrogen isotope values from mantle diamonds are a commonly used tracer in the quest to track volatiles within the Earth’s mantle through deep time. Interpretations of this isotope data are valid so long as stable isotope fractionation processes in the mantle are understood. The fractionation of nitrogen isotopes between {1 1 1} and {1 0 0} growth sectors is well documented for high-pressure high-temperature (HPHT) synthetic diamonds, but there is little data on whether it also occurs in natural mixed-habit diamonds. We present 91 in-situ nitrogen isotope (?15N) measurements, along with carbon isotope (?13C) values and nitrogen abundances [N], obtained from three mixed-habit diamonds by secondary ion mass spectrometry (SIMS). While the well-documented enrichment of nitrogen concentrations in octahedral sectors compared to contemporaneous cuboid sectors is observed, a similarly clear disparity is not obvious in the ?15N data. Whereas HPHT synthetic diamonds exhibit 15N enrichment in the {1 0 0} sectors by ?+30‰, the mixed-habit diamonds studied here show enrichment of the octahedral sectors in 15N by only 0.4-1‰. This major difference between HPHT synthetic and natural mixed-habit diamonds is proposed to be the result of different physical properties of the growth interfaces. The smooth interfaces of the octahedral sectors are the same in both types of crystal, but the outermost atoms on the smooth cube interfaces of an HPHT synthetic diamond behave differently to those on the rough cuboid interfaces of the natural mixed-habit diamonds, resulting in different ?15N values. Both the ?13C (average of ??8.7‰) and ?15N (average of ?0‰) data show only minor offsets from the typical mantle values (?13C = ?5 ± 3‰, ?15N = ?5 ± 4‰). This may indicate diamond formation from a mantle derived fluid/melt containing a minor subducted component (lowering ?13C values and elevating ?15N) or relate to moderate degrees of isotopic fractionation of a pure mantle fluid/melt by prior diamond precipitation. The homogeneous nature of both the carbon and nitrogen isotopic compositions of all three diamonds, however, documents continuous and unlimited supply of diamond forming fluid/melt, with a constant composition. Such homogenous isotopic compositions exclude fluid mixing or isotopic fractionation close to the site of diamond formation and preclude distinguishing between these two processes based on diamond analyses alone.
Contributions to Mineralogy and Petrology, Vol. 171, 7, 25p.
Canada, Northwest Territories
Deposit - Lac de Gras arena
Abstract: Megacrystic (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) xenocrysts within kimberlites from Lac de Gras (Northwest Territories, Canada) contain fully crystallized melt inclusions. These ‘polymineralic inclusions’ have previously been interpreted to form by necking down of melts at mantle depths. We present a detailed petrographical and geochemical investigation of polymineralic inclusions and their host crystals to better understand how they form and what they reveal about the evolution of kimberlite melt. Genetically, the megacrysts are mantle xenocrysts with peridotitic chemical signatures indicating an origin within the lithospheric mantle (for the Cr-diopsides studied here ~4.6 GPa, 1015 °C). Textural evidence for disequilibrium between the host crystals and their polymineralic inclusions (spongy rims in Cr-diopside, kelyphite in Cr-pyrope) is consistent with measured Sr isotopic disequilibrium. The preservation of disequilibrium establishes a temporal link to kimberlite eruption. In Cr-diopsides, polymineralic inclusions contain phlogopite, olivine, chromite, serpentine, and calcite. Abundant fluid inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr2O3 and Al2O3. This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic melt + diopside ? forsterite + calcite + CO2, yielding the observed inclusion mineralogy and producing associated (CO2-rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical and chemical evidence for an origin of kimberlite that involves the reaction of high-pressure dolomitic melt with diopside-bearing mantle assemblages producing a lower-pressure melt that crystallizes a calcite-dominated assemblage in the crust.
Contributions to Mineralogy and Petrology, in press available 25p.
Canada, Northwest Territories
Deposit - Lac de Gras
Abstract: Megacrystic (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) xenocrysts within kimberlites from Lac de Gras (Northwest Territories, Canada) contain fully crystallized melt inclusions. These ‘polymineralic inclusions’ have previously been interpreted to form by necking down of melts at mantle depths. We present a detailed petrographical and geochemical investigation of polymineralic inclusions and their host crystals to better understand how they form and what they reveal about the evolution of kimberlite melt. Genetically, the megacrysts are mantle xenocrysts with peridotitic chemical signatures indicating an origin within the lithospheric mantle (for the Cr-diopsides studied here ~4.6 GPa, 1015 °C). Textural evidence for disequilibrium between the host crystals and their polymineralic inclusions (spongy rims in Cr-diopside, kelyphite in Cr-pyrope) is consistent with measured Sr isotopic disequilibrium. The preservation of disequilibrium establishes a temporal link to kimberlite eruption. In Cr-diopsides, polymineralic inclusions contain phlogopite, olivine, chromite, serpentine, and calcite. Abundant fluid inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr2O3 and Al2O3. This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic melt + diopside ? forsterite + calcite + CO2, yielding the observed inclusion mineralogy and producing associated (CO2-rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical and chemical evidence for an origin of kimberlite that involves the reaction of high-pressure dolomitic melt with diopside-bearing mantle assemblages producing a lower-pressure melt that crystallizes a calcite-dominated assemblage in the crust.
Abstract: In this study, we report the first direct evidence for water-bearing fluids in the uppermost lower mantle from natural ferropericlase crystal contained within a diamond from São Luíz, Brazil. The ferropericlase exhibits exsolution of magnesioferrite, which places the origin of this assemblage in the uppermost part of the lower mantle. The presence of brucite-Mg(OH)2 precipitates in the ferropericlase crystal reflects the later-stage quenching of H2O-bearing fluid likely in the transition zone, which has been trapped during the inclusion process in the lower mantle. Dehydration melting may be one of the key processes involved in transporting water across the boundary between the upper and lower mantle.
Abstract: Using stable isotope data obtained on multiple aliquots of diamonds from worldwide sources, it has been argued that carbon and nitrogen in diamond are decoupled. Here we re-investigate the carbon-nitrogen relationship based on the most comprehensive microbeam data set to date of stable isotopes and nitrogen concentrations in diamonds (n = 94) from a single locality. Our diamond samples, derived from two kimberlites in the Chidliak Field (NE Canada), show large variability in ?13C (? 28.4 ‰ to ? 1.1‰, mode at ? 5.8‰), ?15N (? 5.8 to + 18.8‰, mode at ? 3.0‰) and nitrogen contents ([N]; 3800 to less than 1 at.ppm). In combination, cathodoluminescence imaging and microbeam analyses reveal that the diamonds grew from multiple fluid pulses, with at least one major hiatus documented in some samples that was associated with a resorption event and an abrupt change from low ?13C and [N] to mantle-like ?13C and high [N]. Overall, ?13C appears to be uncorrelated to ?15N and [N] on both the inter- and intra-diamond levels. Co-variations of ?15N-log[N], however, result in at least two parallel, negatively correlated linear arrays, which are also present on the level of the individual diamonds falling on these two trends. These arrays emerge from the two principal data clusters, are characterized by slightly negative and slightly positive ?15N (about ? 3 and + 2‰, respectively) and variable but overall high [N]. Using published values for the diamond-fluid nitrogen isotope fractionation factor and nitrogen partition coefficient, these trends are perfectly reproduced by a Rayleigh fractionation model. Overall, three key elements are identified in the formation of the diamond suite studied: (1.) a low ?13C and low [N] component that possibly is directly associated with an eclogitic diamond substrate or introduced during an early stage fluid event. (2.) Repeated influx of a variably nitrogen-rich mantle fluid (mildly negative ?13C and ?15N). (3.) In waning stages of influx, availability of the mantle-type fluid at the site of diamond growth became limited, leading to Rayleigh fractionation. These fractionation trends are clearly depicted by ?15N-[N] but are not detected when examining co-variation diagrams involving ?13C. Also on the level of individual diamonds, large (? 5‰) variations in ?15N are associated with ?13C values that typically are constant within analytical uncertainty. The much smaller isotope fractionation factor for carbon (considering carbonate- or methane-rich fluids as possible carbon sources) compared to nitrogen leads to an approximately one order of magnitude lower sensitivity of ?13C values to Rayleigh fractionation processes (i.e. during fractionation, a 1‰ change in ?13C is associated with a 10‰ change in ?15N). As a consequence, even minor heterogeneity in the primary isotopic composition of diamond forming carbon (e.g., due to addition of minor subducted carbon) will completely blur any possible co-variations with ?15N or [N]. We suggest this strong difference in isotope effects for C and N to be the likely cause of observations of an apparently decoupled behaviour of carbon and nitrogen isotopes in diamond.
Abstract: Using stable isotope data obtained on multiple aliquots of diamonds from worldwide sources, it has been argued that carbon and nitrogen in diamond are decoupled. Here we re-investigate the carbon-nitrogen relationship based on the most comprehensive microbeam data set to date of stable isotopes and nitrogen concentrations in diamonds (n = 94) from a single locality. Our diamond samples, derived from two kimberlites in the Chidliak Field (NE Canada), show large variability in ?13C (? 28.4 ‰ to ? 1.1‰, mode at ? 5.8‰), ?15N (? 5.8 to + 18.8‰, mode at ? 3.0‰) and nitrogen contents ([N]; 3800 to less than 1 at.ppm). In combination, cathodoluminescence imaging and microbeam analyses reveal that the diamonds grew from multiple fluid pulses, with at least one major hiatus documented in some samples that was associated with a resorption event and an abrupt change from low ?13C and [N] to mantle-like ?13C and high [N]. Overall, ?13C appears to be uncorrelated to ?15N and [N] on both the inter- and intra-diamond levels. Co-variations of ?15N-log[N], however, result in at least two parallel, negatively correlated linear arrays, which are also present on the level of the individual diamonds falling on these two trends. These arrays emerge from the two principal data clusters, are characterized by slightly negative and slightly positive ?15N (about ? 3 and + 2‰, respectively) and variable but overall high [N]. Using published values for the diamond-fluid nitrogen isotope fractionation factor and nitrogen partition coefficient, these trends are perfectly reproduced by a Rayleigh fractionation model. Overall, three key elements are identified in the formation of the diamond suite studied: (1.) a low ?13C and low [N] component that possibly is directly associated with an eclogitic diamond substrate or introduced during an early stage fluid event. (2.) Repeated influx of a variably nitrogen-rich mantle fluid (mildly negative ?13C and ?15N). (3.) In waning stages of influx, availability of the mantle-type fluid at the site of diamond growth became limited, leading to Rayleigh fractionation. These fractionation trends are clearly depicted by ?15N-[N] but are not detected when examining co-variation diagrams involving ?13C. Also on the level of individual diamonds, large (? 5‰) variations in ?15N are associated with ?13C values that typically are constant within analytical uncertainty. The much smaller isotope fractionation factor for carbon (considering carbonate- or methane-rich fluids as possible carbon sources) compared to nitrogen leads to an approximately one order of magnitude lower sensitivity of ?13C values to Rayleigh fractionation processes (i.e. during fractionation, a 1‰ change in ?13C is associated with a 10‰ change in ?15N). As a consequence, even minor heterogeneity in the primary isotopic composition of diamond forming carbon (e.g., due to addition of minor subducted carbon) will completely blur any possible co-variations with ?15N or [N]. We suggest this strong difference in isotope effects for C and N to be the likely cause of observations of an apparently decoupled behaviour of carbon and nitrogen isotopes in diamond.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 9187 Abstract
Africa, Zimbabwe, Sierra Leone
Deposit - Marange, Zimmi
Abstract: Traditional models for diamond formation within the lithospheric mantle invoke either carbonate reduction or methane oxidation. Both these mechanisms require some oxygen exchange with the surrounding wall-rock at the site of diamond precipitation. However, peridotite does not have sufficient buffering capacity to allow for diamond formation via these traditional models and instead peridotitic diamonds may form through isochemical cooling of H 2 O-rich CHO fluids [1]. Marange mixed-habit diamonds from eastern Zimbabwe provide the first natural confirmation of this new diamond growth model [2]. Although Marange diamonds do not contain any silicate or sulphide inclusions, they contain Ni-N-vacancy complexes detected through photoluminescence (PL) spectroscopy that suggest the source fluids equilibrated in the Ni-rich depleted peridotitic lithosphere. Cuboid sectors also contain abundant micro-inclusions of CH 4 , the first direct observation of reduced CH 4-rich fluids that are thought to percolate through the lithospheric mantle [2]. In fluid inclusion-free diamonds, core-to-rim trends in ? 13 C and N content are used to infer the speciation of the diamond-forming fluid. Core to rim trends of increasing ? 13 C with decreasing N content are interpreted as diamond growth from oxidized CO 2-or carbonate-bearing fluids. Diamond growth from reduced species should show the opposite trends-decreasing ? 13 C from core to rim with decreasing N content. Within the CH 4-bearing growth sectors of Marange diamonds, however, such a 'reduced' trend is not observed. Rather, ? 13 C increases from core to rim within a homogeneously grown zone [2]. These contradictory observations can be explained through either mixing between CH 4-and CO 2-rich end-members of hydrous fluids [2] or through closed system precipitation from an already mixed CH 4-CO 2 H 2 O-maximum fluid with XCO 2 (CO 2 /[CO 2 +CH 4 ]) between 0.3 and 0.7 [3]. These results demonstrate that Marange diamonds precipitated from cooling CH 4-CO 2-bearing hydrous fluids rather than through redox buffering. As this growth mechanism applies to both the fluid-rich cuboid and gem-like octahedral sectors of Marange diamonds, a non-redox model for diamond formation from mixed CH 4-CO 2 fluids is indicated for a wider range of gem-quality peridotitic diamonds. Indeed, at the redox conditions of global diamond-bearing lithospheric mantle (FMQ-2 to-4; [4]), CHO fluids are strongly water-dominated and contain both CH 4 and CO 2 as dominant carbon species [5]. By contrast diamond formation in eclogitic assemblages, through either redox buffering or cooling of carbon-bearing fluids, is not as well constrained. Zimmi diamonds from the West African craton have eclogitic sulphide inclusions (with low Ni and high Re/Os) and formed at 650 Ma, overlapping with the timing of subduction [6]. In one Zimmi diamond, a core to rim trend of decreasing ? 13 C (-23.4 to-24.5 %¸) and N content is indicative of formation from reduced C 2 H 6 /CH 4-rich fluids, likely derived from oceanic crust recycled during Neoproterozoic subduction. Unlike mixed CH 4-CO 2 fluids near the water maximum, isochemical cooling or ascent of such reduced CHO fluids is not effficient at diamond precipitation. Furthermore, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the ?1 internal variation seen. Consequently, this Zimmi eclogitic diamond likely formed through redox buffering of reduced subduction-related fluids, infiltrating into sulphide-bearing eclogite.
Earth and Planetary Science Letters, Vol. 473, pp. 44-51.
Africa, Zimbabwe
deposit - Marange
Abstract: Because of the inability of depleted cratonic peridotites to effectively buffer oxygen fugacities when infiltrated by CHO or carbonatitic fluids, it has been proposed recently (Luth and Stachel, 2014) that diamond formation in peridotites typically does not occur by rock-buffered redox reactions as previously thought but by an oxygen-conserving reaction in which minor coexisting CH4 and CO2 components in a water-rich fluid react to form diamond (CO2 + CH4 = 2C + 2H2O). In such fluid-buffered systems, carbon isotope fractionation during diamond precipitation occurs in the presence of two dominant fluid carbon species. Carbon isotope modelling of diamond precipitation from mixed CH4CH4- and CO2-bearing fluids reveals unexpected fundamental differences relative to diamond crystallization from a single carbon fluid species: (1) irrespective of which carbon fluid species (CH4 or CO2) is dominant in the initial fluid, diamond formation is invariably associated with progressive minor (<1‰) enrichment of diamond in 13C as crystallization proceeds. This is in contrast to diamond precipitation by rock-buffered redox processes from a fluid containing only a single carbon species, which can result in either progressive 13C enrichment (CO2 or carbonate fluids) or View the MathML sourceC13 depletion (CH4 fluids) in the diamond. (2) Fluid speciation is the key factor controlling diamond ?13C?13C values; as XCO2 (XCO2 = CO2/[CO2 + CH4]) in the initial fluid increases from 0.1 to 0.9 (corresponding to an increase in fO2fO2 of 0.8 log units), the carbon isotope composition of the first-precipitated diamond decreases by 3.7‰. The tight mode in ?13C of ?5 ±1‰?5 ±1‰ for diamonds worldwide places strict constraints on the dominant range of XCO2 in water-rich fluids responsible for diamond formation. Specifically, precipitation of diamonds with ?13C values in the range ?4 to ?6‰ from mantle-derived fluids with an average ?13C value of ?5‰ (derived from evidence not related to diamonds) requires that diamond-forming fluids were relatively reduced and had methane as the dominant carbon species (XCO2 = 0.1–0.5). Application of our model to a recently published set of in-situ carbon isotope analyses for peridotitic diamonds from Marange, Zimbabwe (Smit et al., 2016), which contain CH4 fluid inclusions, allows us to perfectly match the observed co-variations in ?13C?13C, ?15N?15N and N content and at the same time explain the previously counter-intuitive observation of progressive View the MathML sourceC13 enrichment in diamonds that appear to have grown from a fluid with methane as the dominant carbon species. Similarly, the almost complete absence in the published record of progressive View the MathML sourceC13 depletion trends within diamonds likely reflects ubiquitous precipitation from CH4- and CO2-bearing water-rich fluids, rather than diamond formation exclusively by carbonate-bearing and CH4-free oxidized fluids or melts.
Abstract: Diamond inclusions are the only samples from the mantle transition zone (410-660 km) and the lower mantle. Majoritic garnet is a rare inclusion, limited to pressures of ~8-20 Gpa with Si content being indicative of depth of re-equilibration. These garnet inclusions can therefore provide information on properties of the transition zone such as oxidation state. In this study, we used Synchrotron Mössbauer Source (SMS) to determine the ferric-ferrous ratios of 13 small (30 to 100 micrometers diameter) majoritic inclusions in diamonds from Jagersfontein. The studied inclusions have pyroxenitic affinities [1], with compositions intermediate between typical peridotite and eclogite. They contain 4.62-11.2 wt% CaO, 0.03-0.34 wt% Cr2O3 and Mg# of 0.65-0.81. Minimum pressures for their equilibration using Beyer and Frost [2] barometer are between 8 and 18 GPa with at least 4 of these inclusions being formed in the transition zone. The Fe3+/Fetotal ratios in the garnets increase from 0.08±0.01 to 0.30±0.03 with increasing pressure. These values define a clear extension of the trend apparent in the data from peridotite xenoliths crystallised at lower pressures. Thermodynamic calculations suggest that these high ferric contents correspond to oxygen fugacities above the FeFeO (IW) buffer, which means that the high Fe3+ contents were not generated by disproportionation of Fe2+ to Fe3+ and Fe0 . It is more likely that carbonate was the oxidising agent responsible for generating the high Fe3+ of these garnets.
Abstract: Based on the mineral inclusion content, diamonds from the Argyle Mine, Western Australia, derive primarily (~90%) from eclogitic sources with a minor peridotitic contribution from both harzburgitic and lherzolitic lithologies. The eclogitic inclusions cover a large compositional range and show in part unusually high concentrations of mantle incompatible elements (P, Ti, Na and K). Coherent trends in major elements (e.g., of Ti or Na versus Mg-number) suggest that the eclogitic diamond source was created by a single process, namely igneous fractionation. Calculated bulk rock REEN patterns match a section of oceanic crust reaching from lavas and sheeted dykes to upper gabbros. Positive Eu anomalies for garnet and clinopyroxene, with calculated bulk rock REEN patterns similar to upper (non-layered) gabbros, are strong evidence for plagioclase accumulation, which is characteristic for the gabbroic portions of oceanic crust. Linking previously published oxygen isotope analyses of eclogitic garnet inclusions with their major element composition reveals a correlation between ?18O (mean of +7.2‰) and Na content, consistent with coupled 18O and Na enrichment during low temperature alteration of oceanic crust. The carbon isotopic composition of Argyle eclogitic diamonds forms a normal distribution around a ?13C value of -11‰, indicative of mixing and homogenization of mantle and crustal (organic matter) derived carbon prior to diamond precipitation. Previously published noble gas data on Argyle diamonds support this two component model. Inclusion and nitrogen-in-diamond based thermometry indicate an unusually hot origin of the eclogitic diamond suite, indicative of derivation from the lowermost 25 km (about 180-205 km depth) of the local lithospheric mantle. This is consistent with emplacement of an oceanic protolith during subduction along the Kimberley Craton margin, likely during the Halls Creek Orogeny (about 1.85 Ga). For Argyle eclogitic diamonds the relationship between the rate of platelet degradation and mantle residence temperature indicates that both temperature and strain play an important role in this process. Therefore, ubiquitous platelet degradation and plastic deformation of Argyle diamonds are consistent with derivation from a high temperature environment (softening the diamond lattice) close to the lithosphere-asthenosphere boundary (inducing strain). In combination, the Argyle data set represents a uniquely strong case for a subduction origin of an eclogitic diamond source followed by mixing of mantle and crustal components during diamond formation. Some lherzolitic inclusions show a similarity in incompatible element enrichments (elevated P, Na and K) to the eclogitic suite. The presence of a mildly majoritic lherzolitic garnet further supports a link to eclogitic diamond formation, as very similar majoritic components were found in two eclogitic garnet inclusions. The carbon isotopic composition of peridotitic diamonds shows a mode between -5 to -4 ‰ and a tail extending towards the eclogitic mode (-11 ‰). This suggests the presence of multiple generations of peridotitic diamonds, with indications for an origin linked to the eclogitic suite being restricted to diamonds of lherzolitic paragenesis.
Argyle diamonds – how subduction along the Kimberley Craton edge generated the world's biggest diamond deposit.
Journal of Geochemical Exploration, Vol. 186, pp. 24-35.
Mantle
garnet diamond exploration
Abstract: In diamond exploration, the accurate distinction between garnets from the crust or mantle, or from those having a cognate origin with kimberlite (low-Cr megacrysts), is important for the assessment of indicator mineral samples; misclassifications potentially result in costly misdirection of exploration efforts. Existing literature databases and graphical classification schemes for garnets suffer from a paucity of craton-derived, lower-crustal garnets that - as shown here - are among the most difficult to distinguish from garnets of mantle origin. To improve this situation, a large database of new and literature garnet major element analyses has been compiled. Using this dataset, it is shown that the conventionally used Mg# (Mg/(Mg + Fe)) vs. Ca# (Ca/(Mg + Ca)) plot (Schulze, 2003) for discrimination of crust and mantle garnets results in significant overlap (39.2% crustal failure rate using our dataset). We propose a new graphical classification scheme that uses the parameters ln(Ti/Si) and ln(Mg/Fe) to discriminate low-Cr garnets of crust origin from those of a mantle eclogite-pyroxenite origin with an error rate of 10.1 ± 2.1% at the 95% confidence level (assessed via K-fold cross-validation with ten random test datasets), significantly lower than existing methods. Multivariate statistical solutions, which incorporate a wide selection of geochemical variables, represent additional possibilities for discrimination. Using our new database, logistic regression (LR) and linear discriminant analysis (LDA) approaches are evaluated and new crust-mantle garnet discrimination schemes derived. The resulting solutions, using a wide variety of cations in garnet, provide lower misclassification rates than existing literature schemes. Both LR and LDA are successful discrimination techniques with error rates for the discrimination of crust from mantle eclogite-pyroxenite of 7.5 ± 1.9% and 8.2 ± 2.3%, respectively. LR, however, involves fewer stipulations about the distribution of training data (i.e., it is more "robust") and can return an estimate for probability of classification certainty for single garnets. New data from diamond exploration programs can be readily classified using these new graphical and statistical methods. As the discrimination of low-Cr megacrysts from mantle eclogite-pyroxenite is not the focus of this study, we recommend the method of Schulze (2003) or Grütter et al. (2004) for low-Cr megacryst discrimination to identify megacrysts in the "mantle" suite. Runstreams for our LDA and LR approaches using the freeware "R" are provided for quick implementation.
Earth Planetary Science Letters, Vol. 490, pp. 77-87.
Canada, Ontario
deposit - Victor
Abstract: The central Superior Craton hosts both the diamondiferous 1.1 Ga Kyle Lake and Jurassic Attawapiskat kimberlites. A major thermal event related to the Midcontinent Rift at ca. 1.1 Ga induced an elevated geothermal gradient that largely destroyed an older generation of diamonds, raising the question of when, and how, the diamond inventory beneath Attawapiskat was formed. We determined Re-Os isotope systematics of sulphides included in diamonds from Victor by isotope dilution negative thermal ionisation mass spectrometry in order to obtain insights into the age and nature of the diamond source in the context of regional tectonothermal evolution. Regression of the peridotitic inclusion data (n = 14 of 16) yields a 718 ± 49 Ma age, with an initial 187Os/188Os ratio of 0.1177 ± 0.0016, i.e. depleted at the time of formation (?Os -3.7 ± 1.3). Consequently, Re depletion model ages calculated for these samples are systematically overestimated. Given that reported 187Os/188Os in olivine from Attawapiskat xenoliths varies strongly (0.1012-0.1821), the low and nearly identical initial Os of sulphide inclusions combined with their high 187Re/188Os (median 0.34) suggest metasomatic formation from a mixed source. This was likely facilitated by percolation of amounts of melt sufficient to homogenise Os, (re)crystallise sulphide and (co)precipitate diamond; that is, the sulphide inclusions and their diamond host are synchronous if not syngenetic. The ?720 Ma age corresponds to rifting beyond the northern craton margin during Rodinia break-up. This suggests mobilisation of volatiles (C, N, S) and Os due to attendant mantle stretching and metasomatism by initially oxidising and S-undersaturated melts, which ultimately produced lherzolitic diamonds with high N contents compared to older Kyle Lake diamonds. Thus, some rift-influenced settings are prospective with respect to diamond formation. They are also important sites of hidden, intra-lithospheric volatile redistribution that can be revealed by diamond studies. Later emplacement of the Attawapiskat kimberlites, linking the carbon cycle to the surface, was associated with renewed disturbance during passage of the Great Meteor Hotspot. Lherzolitic diamond formation from oxidising small-volume melts may be the expression of an early and deep stage of the lithospheric conditioning required for the successful eruption of kimberlites, which complements the late and shallow emplacement of volatile-rich metasomes after upward displacement of a redox freezing front.
Mineralogy and Petrology, in press available, 12p.
Canada, Ontario, Attawapiskat
deposit - Victor
Abstract: The Jurassic Victor kimberlite (Attawapiskat Field) was emplaced into an area of the central Superior Craton that was affected by a lithosphere-scale thermal event at ~1.1 Ga. Victor diamonds formed ca. 400 million years after this event, in a lithospheric mantle characterized by an unusually cool model geotherm (37-38 mW/m2; Hasterok and Chapman 2011). The bulk of Victor diamonds derives from a thin (<10 km thick) layer that is located at about 180 km depth and represents lherzolitic substrates (for 85% of diamonds). Geothermobarometric calculations (average pressure and temperature at the 1 sigma level are 57?±?2 kbar and 1129?±?16 °C) coupled with typical fluid metasomatism-associated trace element patterns for garnet inclusions indicate diamond precipitation under sub-solidus (lherzolite + H2O) conditions. This conclusion links the presence of a diamond-rich lherzolitic layer in the lithospheric mantle, just above the depth where ascending melts would freeze, to the unusually low paleogeotherm beneath Attawapiskat, because along an average cratonic geotherm (40 mW/m2) lherzolite in the presence of hydrous fluid would melt at depths >140 km.
Mineralogy and Petrology, 10.1007/s00710 -018-0599-2, 14p.
Canada, Northwest Territories
deposit - Diavik, Ekati
Abstract: Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.
Mineralogy and Petrology, 10.1007/ s710-018-0576 -9, 10p.
South America, Brazil
deposit - Sao Luiz
Abstract: Three diamonds from Sao Luiz, Brazil carrying nano- and micro-inclusions of molecular ?-N2 that exsolved at the base of the transition zone were studied for their C and N isotopic composition and the concentration of N utilizing SIMS. The diamonds are individually uniform in their C isotopic composition and most spot analyses yield ?13C values of ?3.2?±?0.1‰ (ON-SLZ-390) and???4.7?±?0.1‰ (ON-SLZ-391 and 392). Only a few analyses deviate from these tight ranges and all fall within the main mantle range of ?5?±?3‰. Most of the N isotope analyses also have typical mantle ?15N values (?6.6?±?0.4‰, ?3.6?±?0.5‰ and???4.1?±?0.6‰ for ON-SLZ-390, 391 and 392, respectively) and are associated with high N concentrations of 800-1250 atomic ppm. However, some N isotopic ratios, associated with low N concentrations (<400 ppm) and narrow zones with bright luminescence are distinctly above the average, reaching positive ?15N values. These sharp fluctuations cannot be attributed to fractionation. They may reflect arrival of new small pulses of melt or fluid that evolved under different conditions. Alternatively, they may result from fractionation between different growth directions, so that distinct ?15N values and N concentrations may form during diamond growth from a single melt/fluid. Other more continuous variations, in the core of ON-SLZ-390 or the rim of ON-SLZ-392 may be the result of Rayleigh fractionation or mixing.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0604-9 12p.
Africa, Botswana
deposit - Karowe
Abstract: Mineral inclusions in diamonds play a critical role in constraining the relationship between diamonds and mantle lithologies. Here we report the first major and trace element study of mineral inclusions in diamonds from the Karowe Mine in north-east Botswana, along the western edge of the Zimbabwe Craton. From a total of 107 diamonds, 134 silicate, 15 oxide, and 22 sulphide inclusions were recovered. The results reveal that 53% of Karowe inclusion-bearing diamonds derived from eclogitic sources, 44% are peridotitic, 2% have a sublithospheric origin, and 1% are websteritic. The dominant eclogitic diamond substrates sampled at Karowe are compositionally heterogeneous, as reflected in wide ranges in the CaO contents (4-16 wt%) of garnets and the Mg# (69-92) and jadeite contents (14-48 mol%) of clinopyroxenes. Calculated bulk rock REEN patterns indicate that both shallow and deep levels of the subducted slab(s) were sampled, including cumulate-like protoliths. Peridotitic garnet compositions largely derive from harzburgite/dunite substrates (~90%), with almost half the garnets having CaO contents <1.8 wt%, consistent with pyroxene-free (dunitic) sources. The highly depleted character of the peridotitic diamond substrates is further documented by the high mean and median Mg# (93.1) of olivine inclusions. One low-Ca garnet records a very high Cr2O3 content (14.7 wt%), implying that highly depleted cratonic lithosphere at the time of diamond formation extended to at least 220 km depth. Inclusion geothermobarometry indicates that the formation of peridotitic diamonds occurred along a 39-40 mW/m2 model geotherm. A sublithospheric inclusion suite is established by three eclogitic garnets containing a majorite component, a feature so far unique within the Orapa cluster. These low- and high-Ca majoritic garnets follow pyroxenitic and eclogitic trends of majoritic substitution, respectively. The origin of the majorite-bearing diamonds is estimated to be between 330 to 420 km depth, straddling the asthenosphere-transition zone boundary. This new observation of superdeep mineral inclusions in Karowe diamonds is consistent with a sublithospheric origin for the exceptionally large diamonds from this mine.
deposit - Argyle, De Beers Pool, Jwaneng, Orapa, Udachnaya, Venetia, Wawa, Diavik
Abstract: Earth’s mantle is by far the largest silicate-hosted reservoir of carbon. Diamonds are unrivalled in their ability to record the cycle of mantle carbon and other volatiles over a vast portion of the Earth’s history. They are the product of ascending, cooling, carbon-saturated, metasomatic fluidsmelts and/or redox reactions, predominantly within peridotitic and eclogitic domains in the mantle lithosphere. This paper reports the results of a major secondary ion mass spectrometry (SIMS) carbon isotope study, carried out on 127 diamond samples, spanning a large range of geological time. Detailed transects across the incremental growth zones within each diamond were measured for C isotopes, N abundances and, for samples with N >~200 at.ppm, N isotopes. Given that all of the samples are fragments, recovered when the original crystals were broken to liberate their inclusions, 81 of the analytical traverses have confirmed growth direction context. 98 samples are from studies that have confirmed the dates of the individual diamonds through analysis of their silicate or sulphide inclusions, from source localities including Argyle, De Beers Pool, Jwaneng, Orapa, Udachnaya & Venetia. Additional samples come from Wawa (a minimum age) and Diavik where the samples are tied via inclusion paragenesis to published ages. The peridotitic dataset covers the age range of ~3.3 - 2.0 Ga, with the eclogitic data from 2.9 - 1.0 Ga. In total, 751 carbon isotope and nitrogen concentration measurements have been obtained (425 on peridotitic diamonds, and 326 on eclogitic diamonds) with 470 nitrogen isotope measurements (190 P, 280 E). We attempt to constrain the diamond carbon isotope record through time and its implications for (i) the mantle carbon reservoir, (ii) its oxygen fugacity, (iii) the fluid / melt growth environment of diamonds, (iv) fractionation trends recorded in individual diamonds, and (v) diamond population studies using bulk combustion carbon isotope analysis.
Abstract: Polycrystalline diamond aggregates (framesites, boart, diamondite) are an understudied variety of mantle diamond, but can make up 20% of the production in some Group I kimberlites. Their polycrystalline nature indicates rapid precipitation from carbon-oversaturated fluids and individual PDAs often contain a chemically heterogeneous suite of websteritic and pyroxenitic inclusions and minerals intimately intergrown with the diamond crystals. Geochemical and microstructural evidence suggests that fluid-driven redox reactions with lithospheric material occurring episodically over millions of years play a major role in freezing carbon in the subcratonic lithosphere (Jacob et al., 2000; 2016; Mikhail et al., 2014). A suite of 39 samples from the Venetia kimberlite pipe in South Africa allows a more detailed look at the diamondforming fluids. 13C values in the diamonds measured by secondary ion mass spectrometry range from +2 to -28 and cover the entire range for PDA from the literature. Nitrogen concentrations are mostly very low (less than 100 at ppm), but reach up to 2660 at ppm in individual samples. These high nitrogen concentrations in concert with mostly positive 15N values of up to +17 and some very negative 3C values suggest crustal material as the source of the nitrogen and the carbon. However, detailed analysis of the sample provides evidence for a more complex growth history followed by alteration. Individual diamond crystals show complex growth zonations by cathodoluminescence imaging that can be related with the carbon and nitrogen isotopic compositions and points to growth incorporating several pulses of carbon-nitrogen fluid with distinct isotopic compositions. Most of these growth events show decoupled carbon and nitrogen systematics. In addition, EBSD identifies deformation and recrystallization and nitrogen aggregation states range from pure IaA to pure IaB, supporting a heterogeneous and episodic growth history.
Africa, South Africa, Guinea, South America, Brazil
deposit - Kankan, Jagersfontein, Juina
Abstract: Inclusions in super-deep diamonds provide a unique window to the sublithospheric mantle (e.g. [1-4]). Here we present oxygen isotopes for Kankan majoritic garnet and former bridgmanite inclusions. The clustering of Kankan majorites around a ?18O of +9‰ is nearly identical to those reported from Jagersfontein [1]. This elevated and nearly constant ?18O signal indicates homogenization of partial melts from the uppermost part of altered basaltic slabs. Conversely, ?18O values in Juina majorites are highly variable [2] due to crystallization from small, discrete melt pockets in a heterogeneous eclogitic source. While all these majorites have eclogitic/pyroxenitic Cr2O3 and CaO contents, charge-balance for Si[VI] is achieved very differently, with Jagersfontein [3], Kankan [4], and Juina [2] majorites transitioning from eclogitic Na[VIII]Si[VI] to peridotitic-pyroxenitic [5] Mg[VI]Si[VI] substitutions. We interpret this shift as the result of homogenized eclogitic partial melts infiltrating and reacting with adjacent pyrolitic mantle at Kankan and Jagersfontein. Increases in Mg# and Cr2O3 with reductions in ?18O support this reaction. This model is in agreement with recent experiments in which majorites and diamonds form from a reaction of slab-derived carbonatite with reduced pyrolite at 300-700 km depth [6]. The Kankan diamonds also provide an opportunity to establish the chemical environment of the lower mantle. Four inclusions of MgSiO3, inferred to be former bridgmanite [4], provide the first-measured ?18O values for lower mantle samples. These values suggest derivation from primitive mantle, or unaltered subducted oceanic lithospheric mantle. The Kankan super-deep inclusions thus provide a cross-section of deep mantle that highlights slab-pyrolite reactions in the asthenosphere and primitive compositions in the lower mantle.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp.319-342.
Bulbuc, K.M., Galarneau, M., Stachel, T., Stern, R.A., Kong, J., Chinn, I.
Contrasting growth conditions for sulphide-and garnet-included diamonds from the Victor mine ( Ontario).
2018 Yellowknife Geoscience Forum , p. 97-98. abstract
Canada, Ontario, Attawapiskat
deposit - Victor
Abstract: The Victor Diamond Mine, located in the Attawapiskat kimberlite field (Superior Craton), is known for its exceptional diamond quality. Here we study the chemical environment of formation of Victor diamonds. We imaged eight sulphide-included diamond plates from Victor using cathodoluminescence (CL). Then, along core-rim transects, we measured nitrogen content and aggregation state utilizing Fourier Transform Infrared (FTIR) spectroscopy, and the stable isotope compositions of carbon (?13C) and nitrogen (?15N), using a multi-collector ion microprobe (MC-SIMS). We compare the internal growth features and chemical characteristics of these sulphide inclusion-bearing diamonds with similar data on garnet inclusion-bearing diamonds from Victor (BSc thesis Galarneau). Using this information, possible fractionation processes during diamond precipitation are considered and inferences on the speciation of the diamond forming fluid(s) are explored. Sulphide inclusion-bearing diamonds show much greater overall complexity in their internal growth features than garnet inclusion-bearing diamonds. Two of the sulphide-included samples have cores that represent an older generation of diamond growth. Compared to garnet inclusion-bearing diamonds, the sulphide-included diamonds show very little intra-sample variation in both carbon and nitrogen isotopic composition; the inter-sample variations in carbon isotopic composition, however, are higher than in garnet included diamonds. For sulphide-included diamonds, ?13C ranges from -3.4 to -17.5 and ?15N ranges from -0.2 to -9.2. Garnet inclusion-bearing diamonds showed ?13C values ranging from -4.6 to -6.0 and ?15N ranging from -2.8 to -10.8. The observation of some 13C depleted samples indicates that, unlike the lherzolitic garnet inclusion-bearing diamonds, the sulphide inclusion-bearing diamonds are likely both peridotitic and eclogitic in origin. The total range in N content across sulphide inclusion-bearing diamonds was 2 to 981 at ppm, similar to the garnet-included samples with a range of 5 to 944 at ppm. The very limited variations in carbon and nitrogen isotopic signatures across growth layers indicate that sulphide-included Victor diamonds grew at comparatively high fluid:rock ratios. This is contrasted by the garnet inclusion-bearing diamonds that commonly show the effects of Rayleigh fractionation and hence grew under fluid-limited conditions.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 343-358.
Canada, Ontario, Attawapiskat, Africa, South Africa
deposit - Victor, Finsch, Newlands
Abstract: Quantitative trace element data from high-purity gem diamonds from the Victor Mine, Ontario, Canada as well as near-gem diamonds from peridotite and eclogite xenoliths from the Finsch and Newlands mines, South Africa, acquired using an off-line laser ablation method show that we see the same spectrum of fluids in both high-purity gem and near-gem diamonds that was previously documented in fibrous diamonds. “Planed” and “ribbed” trace element patterns characterize not only the high-density fluid (HDF) inclusions in fibrous diamonds but also in gem diamonds. Two diamonds from two Finsch harzburgite xenoliths show trace element patterns similar to those of saline fluids, documenting the involvement of saline fluids in the precipitation of gem diamonds, further strengthening the link between the parental fluids of both gem and fibrous diamonds. Differences in trace element characteristics are evident between Victor diamonds containing silicate inclusions compared with Victor diamonds containing sulphide inclusions. The sulphide-bearing diamonds show lower levels of inter-element fractionation and more widely varying siderophile element concentrations - indicating that the silicate and sulphide-bearing diamonds likely formed by gradations of the same processes, via melt-rock reaction or from a subtly different fluid source. The shallow negative LREEN-HREEN slopes displayed by the Victor diamonds establish a signature indicative of original derivation of the diamond forming agent during major melting (~10% melt). Consequently, this signature must have been passed on to HDFs separating from such silicate melts.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0641-4 18p.
Canada, Northwest Territories
indicator minerals, geocthermobarometry
Abstract: The Central Mackenzie Valley (CMV) area of Northwest Territories is underlain by Precambrian basement belonging to the North American Craton. The potential of this area to host kimberlitic diamond deposits is relatively high judging from the seismologically-defined lithospheric thickness, age of basement rocks (2.2-1.7 Ga) and presence of kimberlite indicator minerals (KIMs) in Quaternary sediments. This study presents data for a large collection of KIMs recovered from stream sediments and till samples from two study areas in the CMV, the Horn Plateau and Trout Lake. In the processed samples, peridotitic garnets dominate the KIM grain count for both regions (> 25% each) while eclogitic garnet is almost absent in both regions (< 1% each). KIM chemistry for the Horn Plateau indicates significant diamond potential, with a strong similarity to KIM systematics from the Central and Western Slave Craton. The most significant issue to resolve in assessing the local diamond potential is the degree to which KIM chemistry reflects local and/or distal kimberlite bodies. Radiogenic isotope analysis of detrital kimberlite-related CMV ilmenite and rutile grains requires at least two broad age groups for eroded source kimberlites. Statistical analysis of the data suggests that it is probable that some of these KIMs were derived from primary and/or secondary sources within the CMV area, while others may have been transported to the area from the east-northeast by Pleistocene glacial and/or glaciofluvial systems. At this stage, KIM chemistry does not allow the exact location of the kimberlitic source(s) to be constrained.
Regier, M.E., Pearson, D.G., Stachel, T., Stern, R.A., Harris, J.
Tracing the formation and abundance of superdeep diamonds.
2018 Yellowknife Geoscience Forum , p. 63. abstract
Africa, Guinea
deposit - Kankan
Abstract: Super-deep diamonds from the transition zone and lower mantle are valuable targets for mining, as they are often large, gem-quality1 or ultra-valuable type IIb stones2. Hence, in mine prospects, it may become important to determine the various populations of sub-lithospheric diamonds. Unambiguously identifying a diamond’s depth of formation is difficult as some minerals can be indicative of various depth regimes (e.g., ferropericlase, Ca-walstromite, enstatite, clinopyroxene, coesite). Here, we use the oxygen isotope compositions of inclusions in Kankan diamonds from Guinea to distinguish between the various diamond-forming processes that happen at lithospheric, asthenospheric to transition zone, and lower mantle depths. In this way, we hope to establish a process by which isotope geochemistry can better constrain the populations of superdeep diamonds in kimberlites, and can assist in estimating a pipe’s propensity for large, valuable stones. Oxygen isotopic analysis by secondary ion mass spectrometry (SIMS) is a high-precision technique that can track hydrothermal alteration that occurred at or close below the ocean floor. Our analyses of inclusions from Kankan diamonds demonstrate that garnets with 3-3.03 Si cations (pfu) have ?18O that are well-constrained within the normal values expected for peridotitic and eclogitic inclusions, but that garnets with ?3.04 Si cations (pfu) have consistently high ?18O (median: 10‰) that slightly decreases with increasing Cr2O3. We interpret this signal as the reaction between a melted carbonate-rich oceanic slab and normal convecting asthenosphere3. In contrast, retrogressed, or former, bridgmanite has ?18O values similar to primitive mantle, suggesting little involvement of slab melts. In contrast to the worldwide suite of lithospheric inclusions of eclogitic paragenesis (median ?18O of 7.03‰)4,5, diamonds derived from ~250 to 500 km have inclusions with consistent, extremely high oxygen isotopes (median: 9.32‰)6,7, due to the melting of extremely enriched carbonated oceanic crust. Diamonds from the lower mantle, however, have inclusions with primitive mantle oxygen isotopes, suggesting a different formation process. The clear distinction in inclusion ?18O between lithospheric, asthenospheric to transition zone, and lower mantle diamond populations is useful in informing the depth regime of a suite of stones, especially those with inclusions of ambiguous depths (e.g., clinopyroxene, coesite, Ca-walstromite, enstatite, ferropericlase, etc.). For instance, we are currently searching for exotic oxygen isotopes in ferropericlase that indicate asthenospheric diamond growth, rather than the primitive mantle values expected for lower mantle ferropericlase. In conclusion, oxygen isotopic analyses of diamond inclusions can identify various sublithsopheric diamond populations, and may benefit the assessment of a mine’s potential for large gem-quality, or type IIb diamonds.
Siva-Jothy, W., Chinn, I., Stachel, T., Pearson, D.G.
Resorption features of macro and micro diamonds from Gahcho Kue.
2018 Yellowknife Geoscience Forum , p. 120. abstract
Canada, Northwest Territories
deposit - Gahcho Kue
Abstract: Studies into the relationship between oxygen fugacity of mantle fluids/melts and etch features on diamond surfaces have shown specific fluid/melt compositions correspond to associated etch features. A classification scheme has been proposed to determine the fluid composition within a kimberlite by examining etch features associated with diamond surfaces as a proxy for fluid composition in an ascending diamondiferous kimberlite. A suite of 388 microdiamonds (defined as diamonds which pass through a 0.5 mm square mesh screen) and 88 macrodiamonds taken from various drill hole depths in the Hearne kimberlite and 88 inclusion-bearing macrodiamonds from the Gahcho Kué mine (NWT) were viewed under a secondary electron microscope for their surface features in accordance with this scheme. Two hundred and thirty specimens show shallow-depth etch features that can be easily classified: the main features observed were trigons and truncated trigons on the {111} faces and/or tetragons on the {100} faces (indicating etching by fluids of variable CO2:H2O ratios). Thirty-four specimens show deeper etched features that represent either extreme degrees of regular etching (such as deeply-etched tetragons), or corrosion type etching, wherein the diamond lattice is etched in a fluid-free melt. Variability between crystal habits exists between the size fractions studied, with cubic habits only being observed in the microdiamond population. This implies variable formation conditions for the two different diamond size fractions studied from Gahcho Kué. Among microdiamonds, surface textures associated with fluid-related etching are markedly more variable, with truncated trigons, tetragons, and both positive and negative trigons being observed. However, these often occur in combination with features showing a large variability in their depth to size ratio between samples, which is typically caused by mantle-related etching. These observations suggest repeated interaction of fluids/melts with the Gahcho Kué diamond population, with at least some of the fluids affecting the microdiamonds being more CO2-rich than those that etched the macrodiamond fraction.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 145-168.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 271-2.
Global
diamond inclusions
Abstract: Through research on inclusions in diamonds over the past 50 years, a detailed picture has emerged of the mineralogical and chemical composition of diamond substrates in Earth’s mantle and of the pressure-temperature conditions during diamond formation. The exact diamond-forming processes, however, are still a subject of debate. One approach to constrain diamond-forming processes is through model calculations that aim to obtain the speciation and the carbon content of carbon-hydrogen-oxygen (CHO) fluids at particular O/(O+H) ratios and pressure-temperature conditions (using GFluid of Zhang and Duan, 2010, or other thermodynamic models of fluids). The predictions of such model calculations can then be tested against carbon and nitrogen stable isotopes and nitrogen content fractionation models, based on in situ analyses across homogenously grown diamond growth layers. Based on this approach, Luth and Stachel (2014) proposed that diamond precipitation occurs predominantly from cooling or ascending CHO fluids, composed of water with minor amounts of CO2 and CH4 (which in response to decreasing temperature may react to form diamond: CO2+ CH4 ? 2C + 2H2O). The second approach focuses on constraining the diamondforming medium by studying submicrometer fluid inclusions in fibrous-clouded and, more recently, gem diamonds. Such studies established the presence of four compositional end members of inclusions: hydrous-saline, hydrous-silicic, high-Mg carbonatitic, and low-Mg carbonatitic (e.g., Navon et al., 1988; Weiss et al., 2009). Although these fluid inclusions only depict the state of the diamond-forming medium after formation, they nevertheless provide unique insights into the major and trace-element composition of such fluids that otherwise could not be obtained. The apparent dichotomy between the two approaches—models for pure CHO fluids and actual observation of impure fluids (socalled high-density fluids) in clouded and fibrous diamonds—relates to the observation that in high-pressure and high-temperature experiments close to the melting temperature of mantle rocks, hydrous fluids contain 10–50% dissolved solid components (e.g., Kessel et al., 2015). Although at this stage the impurity content in natural CHO fluids cannot be included in numerical models, the findings for clouded and fibrous diamonds are not in conflict with the isochemical diamond precipitation model. Specifically, the fact that observed high-density inclusions are often carbonate bearing is not in conflict with the relatively reducing redox conditions associated with the O/(O+H) ratios of modeled diamond-forming CHO fluids. The model for the minimum redox stability of carbonate - bearing melts of Stagno and Frost (2010) permits fluid carbonate contents of up to about 30% at such redox conditions. Although additional data need to be obtained to build a thermodynamic model for CHO fluids with dissolved silicates and to better characterize the major and trace-element composition of high-density CHO fluids in equilibrium with typical diamond substrates (the rock types peridotite and eclogite), we already see sufficient evidence to suggest that the two approaches described above are converging to a unified model of isochemical diamond precipitation from cooling or ascending high-density CHO fluids.
Abstract: The oxidation state of iron in Earth’s mantle is well known to depths of approximately 200?km, but has not been characterized in samples from the lowermost upper mantle (200-410?km depth) or the transition zone (410-660?km depth). Natural samples from the deep (>200?km) mantle are extremely rare, and are usually only found as inclusions in diamonds. Here we use synchrotron Mössbauer source spectroscopy complemented by single-crystal X-ray diffraction to measure the oxidation state of Fe in inclusions of ultra-high pressure majoritic garnet in diamond. The garnets show a pronounced increase in oxidation state with depth, with Fe3+/(Fe3++ Fe2+) increasing from 0.08 at approximately 240?km depth to 0.30 at approximately 500?km depth. The latter majorites, which come from pyroxenitic bulk compositions, are twice as rich in Fe3+ as the most oxidized garnets from the shallow mantle. Corresponding oxygen fugacities are above the upper stability limit of Fe metal. This implies that the increase in oxidation state is unconnected to disproportionation of Fe2+ to Fe3+ plus Fe0. Instead, the Fe3+ increase with depth is consistent with the hypothesis that carbonated fluids or melts are the oxidizing agents responsible for the high Fe3+ contents of the inclusions.
Abstract: Quantitative trace element data from high-purity gem diamonds from the Victor Mine, Ontario, Canada as well as near-gem diamonds from peridotite and eclogite xenoliths from the Finsch and Newlands mines, South Africa, acquired using an off-line laser ablation method show that we see the same spectrum of fluids in both high-purity gem and near-gem diamonds that was previously documented in fibrous diamonds. "Planed" and "ribbed" trace element patterns characterize not only the high-density fluid (HDF) inclusions in fibrous diamonds but also in gem diamonds. Two diamonds from two Finsch harzburgite xenoliths show trace element patterns similar to those of saline fluids, documenting the involvement of saline fluids in the precipitation of gem diamonds, further strengthening the link between the parental fluids of both gem and fibrous diamonds. Differences in trace element characteristics are evident between Victor diamonds containing silicate inclusions compared with Victor diamonds containing sulphide inclusions. The sulphide-bearing diamonds show lower levels of inter-element fractionation and more widely varying siderophile element concentrations - indicating that the silicate and sulphide-bearing diamonds likely formed by gradations of the same processes, via melt-rock reaction or from a subtly different fluid source. The shallow negative LREEN-HREEN slopes displayed by the Victor diamonds establish a signature indicative of original derivation of the diamond forming agent during major melting (~10% melt). Consequently, this signature must have been passed on to HDFs separating from such silicate melts.
Geochemical Perspectives Letters, Vol. 9, pp. 6-10. 10.7185/geochemlet.1830
Mantle
peridotites
Abstract: The origin of the peridotites that form cratonic mantle roots is a central issue in understanding the history and survival of Earth’s oldest continents. A long-standing hypothesis holds that the unusual bulk compositions of some cratonic peridotites stem from their origin as subducted oceanic serpentinite, dehydrated during subduction to form rigid buoyant keels (Schulze, 1986; Canil and Lee, 2009). We present oxygen isotope data from 93 mantle peridotites from five different Archean cratons to evaluate their possible origin as serpentinites. Cratonic mantle peridotite shows remarkably uniform ?18O values, identical to modern MORB-source mantle, that do not vary with bulk rock Si-enrichment or Ca-depletion. These data clearly conflict with any model for cratonic lithosphere that invokes serpentinite as a protolith for cratonic peridotite, and place additional constraints on cratonic mantle origins. We posit that the uniform ?18O was produced by sub-arc and/or MOR depletion processes and that the Si-enriched nature of some samples is unlikely to be related to slab melt infiltration. Instead, we suggest a peridotitic source of Si-enrichment, derived from ascending mantle melts, or a water-fluxed depleted mantle. These variably Si-enriched, cratonic mantle protoliths were then collisionally compressed into the thick cratonic roots that have protected Earth’s oldest continental crust for over 2.5 Gyr.
Chemical Geology, doi.org/10,1016/j.chem geo.2019.04.014 37p.
Africa, Sierra Leone
deposit - Zimmi
Abstract: Here we present SIMS data for a suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the origin and redox-controlled speciation of diamond-forming fluids for these Neoproterozoic eclogitic diamonds. Low ?13C values below ?15‰ in three diamonds result from fluids that originated as carbon in the oceanic crust, and was recycled into the diamond-stable subcratonic lithospheric mantle beneath Zimmi during subduction. ?13C values between ?6.7 and ?8.3‰ in two diamonds are within the range for mantle-derived carbon and could reflect input from mantle fluids, serpentinised peridotite, or homogenised abiogenic and/or biogenic carbon (low ?13C values) and carbonates (high ?13C values) in the oceanic crust. Diamond formation processes in eclogitic assemblages are not well constrained and could occur through redox exchange reactions with the host rock, cooling/depressurisation of CHO fluids or during H2O-loss from CHO fluids. In one Zimmi diamond studied here, a core to rim trend of decreasing ?13C (?23.4 to ?24.5‰) and decreasing [N] is indicative of formation from reduced CH4-bearing fluids. Unlike mixed CH4-CO2 fluids near the water maximum, isochemical diamond precipitation from such reduced CHO fluids will only occur during depressurisation (ascent) and should not produce coherent fractionation trends in single diamonds that reside at constant depth (pressure). Furthermore, due to a low relative proportion of the total carbon in the fluid being precipitated, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the 1‰ internal core-to- rim variation. Consequently, this Zimmi eclogitic diamond showing a coherent trend in ?13C and [N] likely formed through oxidation of methane by the host eclogite, although the mineralogical evidence for this process is currently lacking.
Earth and Planetary Science Letters, Vol. 516, pp. 190-201.
Mantle
carbon
Abstract: A long-standing unresolved problem in understanding Earth's deep carbon cycle is whether crustal carbon is recycled beyond arc depths. While isotopic signatures of eclogitic diamonds and their inclusions suggest deep recycling of crustal material, the crustal carbon source remains controversial; seafloor sediment - the widely favored crustal carbon source - cannot explain the combined carbon and nitrogen isotopic characteristics of eclogitic diamonds. Here we examined the carbon and oxygen isotopic signatures of bulk-rock carbonate for 80 geographically diverse samples from altered mafic-ultramafic oceanic crust (AOC), which comprises 95 vol% of the crustal material in subducting slabs. The results show: (i) AOC contains carbonate with C values as low as ?24‰, indicating the presence of biogenic carbonate; (ii) carbonate in AOC was mainly formed during low-temperature (<100 °C) alteration processes. Modeling accounting for this newly recognized carbon source in the oceanic crust with formation temperatures <100 °C yields a global carbon influx of 1.5±0.3 × 1012 mol C/yr carried by subducting AOC into the trench, which is 50-90% of previous estimates, but still of the same order of the carbon influx carried by subducting sediments into the trench. The AOC can retain carbon better than sediment during subduction into the asthenosphere, transition zone and lower mantle. Mixing of asthenospheric and AOC fluids provides the first consistent explanation of the diverse record of carbon and nitrogen isotopes in diamonds, suggesting that AOC, instead of sediment, is the key carrier of crustal carbon into the deep mantle.
www.minsocam.org/ MSA/Centennial/ MSA_Centennial _Symposium.html The next 100 years of mineral science, June 20-21, p. 22. Abstract
Mantle
diamond inclusions
Abstract: Much of the temporal record of Earth’s evolution, including its trace of plate tectonics, is blurred due to the dynamic nature of the crust-mantle system. While zircon provides the highest fidelity crustal record, diamond takes over in the mantle as the go-to mineral, capable of retaining critical information for a variety of geochemical proxies, over billion year timescales. Here we use diamond and its inclusions to tell the story of the recycling of C, N, O, H and B from the crust to various depths in Earth’s mantle. In this story, altered oceanic crust (AOC) and lithospheric mantle will play a prominent role. The carbon isotope record of diamond has long been thought to reflect the mixing of primitive mantle carbon with carbon recycled from isotopically light organic material originating from the crust. A major difficulty has been reconciling this view with the highly varied nitrogen and carbon isotope signatures in diamonds of eclogitic paragenesis, which cannot be interpreted by the same mechanism. Recent work on AOC of igneous origin (Li et al., EPSL in press) shows how isotopically varied carbon and nitrogen can be subducted to great depth and retained in spatial juxtaposition with the mafic silicate component of AOC to form the complex C-N isotope systematics observed in diamonds and the varied O isotope compositions of their inclusions. In this model a large portion of the 13C depleted carbon originated from biogenic carbonate within the AOC rather than from overlying sediments. Metamorphosed and partially devolatilized AOC will have very variable C/N ratios and highly variable nitrogen isotopes, explaining why simple two component mixing between organic matter and convecting upper mantle cannot explain the complexity of C-N isotope systematics in diamonds. Igneous AOC and its underlying altered mantle are considerably more efficient than subducted sediment at retaining their volatile inventory when recycled to transition zone and even lower mantle depths. Hence, this combination of mixing between AOC-derived volatiles and those from the convecting mantle produces the isotopic fingerprints of superdeep diamonds and their inclusions. These amazing diamonds, some worth millions of dollars, can contain pristine ultra-high pressure mineral phases never before seen in terrestrial samples. The first hydrous ringwoodite found in Earth provides evidence in support of a locally water-saturated transition zone that may result from altered oceanic lithospheric mantle foundering at that depth in the mantle. The O isotope composition of deep asthenosphere and transition zone phases document clearly crustal precursors that have interacted with the hydrosphere before residing hundreds of km deep within the Earth. Finally, spectacular blue diamonds contain boron, an element of strong crustal affinities, transported into the deep Earth along with crustal carbon, by the plate tectonic conveyor system. Diamond - such a simple mineral - and its inclusions, will continue to provide a unique, brightly illuminating light into the darkest recesses of Earth’s mantle for many years to come.
Abstract: Primary diamond deposits are typically restricted to the stable Archean cores of continents, an association known as Clifford’s rule. Archean to Palaeoproterozoic crustal ages (3.3 - 2.1 Ga) have been reported for the Sask Craton, a small terrane in Western Canada, which hosts the diamondiferous Cretaceous Fort à la Corne (FALC) Kimberlite Field. Yet the craton is enclosed by the Palaeoproterozoic (1.9 - 1.8 Ga) Trans Hudson Orogen (THO). In this study we evaluate the age and geochemistry (major, trace, and platinum group elements data, as well as Re-Os isotope systematics) of the lithospheric mantle root beneath the Sask Craton to assess the timing of craton formation and the potential role played by the THO in its evolution. The lithospheric mantle root is dominated by lherzolite with average olivine Mg# of 91.5, which is more fertile than observed in other cratons. Garnets from concentrate further highlight the rarity of harzburgite in the lithospheric mantle. Single clinopyroxene thermobarometry provides temperaturepressure constraints for the garnet-bearing lithospheric mantle (840 to 1250 °C and 2.7 to 5.5 GPa), indicative of a cool geotherm (38 mW/m2) and a large diamond window of ~100 km thickness (from ~120-220 km depth). Most of the studied xenoliths show evidence for melt metasomatism in their trace and major element compositions, while retaining platinum group element patterns expected for melt residues. 187Os/188Os compositions span a broad range from 0.1109 to 0.1507, corresponding to Re-depletion (TRD) ages between 2.4 to 0.3 Ga, with a main mode in the Palaeoproterozoic (2.4 to 1.7 Ga). With the absence of Archean ages, the main depletion and stabilisation of the Sask Craton occurred in the Palaeoproterozoic, closely associated with the Wilson cycle of the THO. From a diamond exploration perspective this indicates that major diamond deposits can be found on cratons that were stabilised in the Palaeoproterozoic.
Abstract: The recently recognised Sask Craton, a small terrane with Archean (3.3-2.5 Ga) crustal ages, is enclosed in the Paleoproterozoic (1.9-1.8 Ga) Trans Hudson Orogen (THO). Only limited research has been conducted on this craton, yet it hosts major diamond deposits within the Cretaceous (~106 to ~95 Ma) Fort à la Corne (FALC) Kimberlite Field. This study describes major, trace and platinum group element data, as well as osmium isotopic data from peridotitic mantle xenoliths (n = 26) from the Star and Orion South kimberlites. The garnet-bearing lithospheric mantle is dominated by moderately depleted lherzolite. Equilibration pressures and temperatures (2.7 to 5.5 GPa and 840 to 1250 °C) for these garnet peridotites define a cool geotherm indicative of a 210 km thick lithosphere, similar to other cratons worldwide. Many of the peridotite xenoliths show the major and trace element signatures of carbonatitic and kimberlitic melt metasomatism. The Re-Os isotopic data yield TRD (time of Re-depletion) model ages, which provide minimum estimates for the timing of melt depletion, ranging from 2.4 to 0.3 Ga, with a main mode spanning from 2.4 to 1.7 Ga. No Archean ages were recorded. This finding and the complex nature of events affecting this terrane from the Archean through the Palaeoproterozoic provide evidence that the majority of the lithospheric mantle was depleted and stabilised in the Palaeoproterozoic, significantly later than the Archean crust. The timing of the dominant lithosphere formation is linked to rifting (~2.2 Ga - 2.0 Ga), and subsequent collision (1.9-1.8 Ga) of the Superior and Hearne craton during the Wilson cycle of the Trans Hudson Orogen.
Abstract: The recently recognised Sask Craton, a small terrane with Archean (3.3-2.5 Ga) crustal ages, is enclosed in the Paleoproterozoic (1.9-1.8 Ga) Trans Hudson Orogen (THO). Only limited research has been conducted on this craton, yet it hosts major diamond deposits within the Cretaceous (~106 to ~95 Ma) Fort à la Corne (FALC) Kimberlite Field. This study describes major, trace and platinum group element data, as well as osmium isotopic data from peridotitic mantle xenoliths (n = 26) from the Star and Orion South kimberlites. The garnet-bearing lithospheric mantle is dominated by moderately depleted lherzolite. Equilibration pressures and temperatures (2.7 to 5.5 GPa and 840 to 1250 °C) for these garnet peridotites define a cool geotherm indicative of a 210 km thick lithosphere, similar to other cratons worldwide. Many of the peridotite xenoliths show the major and trace element signatures of carbonatitic and kimberlitic melt metasomatism. The Re-Os isotopic data yield TRD (time of Re-depletion) model ages, which provide minimum estimates for the timing of melt depletion, ranging from 2.4 to 0.3 Ga, with a main mode spanning from 2.4 to 1.7 Ga. No Archean ages were recorded. This finding and the complex nature of events affecting this terrane from the Archean through the Palaeoproterozoic provide evidence that the majority of the lithospheric mantle was depleted and stabilised in the Palaeoproterozoic, significantly later than the Archean crust. The timing of the dominant lithosphere formation is linked to rifting (~2.2 Ga - 2.0 Ga), and subsequent collision (1.9-1.8 Ga) of the Superior and Hearne craton during the Wilson cycle of the Trans Hudson Orogen.
Contributions to Mineralogy and Petrology, Vol. 174, (12) doi: 10.1007/s00410-019-1634-y
Africa, Ghana
deposit - Akwatia
Abstract: Trace-element concentrations in olivine and coexisting garnets included in diamonds from the Akwatia Mine (Ghana, West African Craton) were measured to show that olivine can provide similar information about equilibration temperature, diamond paragenesis and mantle processes as garnet. Trace-element systematics can be used to distinguish harzburgitic olivines from lherzolite ones: if Ca/Al ratios of olivine are below the mantle lherzolite trend (Ca/Al??300 µg/g Ca or?>?60 µg/g Na are lherzolitic. Conventional geothermobarometry indicates that Akwatia diamonds formed and resided close to a 39 mW/m2 conductive geotherm. A similar value can be derived from Al in olivine geothermometry, with TAl-ol ranging from 1020 to 1325 °C. Ni in garnet temperatures is on average somewhat higher (TNi-grt?=?1115-1335 °C) and the correlation between the two thermometers is weak, which may be not only due to the large uncertainties in the calibrations, but also due to disequilibrium between inclusions from the same diamond. Calcium in olivine should not be used as a geothermobarometer for harzburgitic olivines, and often gives unrealistic P-T estimates for lherzolitic olivine as well. Diamond-hosted olivine inclusions indicate growth in an extremely depleted (low Ti, Ca, Na, high Cr#) environment with no residual clinopyroxene. They are distinct from olivines from mantle xenoliths which show higher, more variable Ti contents and lower Cr#. Hence, most olivine inclusions in Akwatia diamonds escaped the refertilisation processes that have affected most mantle xenoliths. Lherzolitic inclusions are probably the result of refertilisation after undergoing high-degree melting first. Trivalent cations appear to behave differently in harzburgitic diamond-hosted olivine inclusions than lherzolitic inclusions and olivine from mantle xenoliths. Some divalent chromium is predicted to be present in most olivine inclusions, which may explain high concentrations up to 0.16 wt% Cr2O3 observed in some diamond inclusions. Strong heterogeneity of Cr, V and Al in several inclusions may also result in apparent high Cr contents, and is probably due to late-stage processes during exhumation. However, in general, diamond-hosted olivine inclusions have lower Cr and V than expected compared to mantle xenoliths. Reduced Na activity in depleted harzburgites limits the uptake of Cr, V and Sc via Na-M3+ exchange. In contrast, Al partitioning in harzburgites is not significantly reduced compared to lherzolites, presumably due to uptake of Al in olivine by Al-Al exchange.
Diamonds & Related Materials, Vol. 101, 107642, 8p. Pdf
Canada, Northwest Territories
deposit - Ekati
Abstract: High pressure high temperature (HPHT) treatment has long been applied in the gem trade for changing the body colour of diamonds. The identification of HPHT-treated diamonds is a field of on-going research in gemological laboratories, as different parameters of treatment will result in either the creation or the destruction of a variety of lattice defects in diamonds. Some features that exist in treated diamonds can also be found in natural diamonds, and consequently must not be employed for the separation of treated and natural diamonds. In this research, we investigated the properties of 11 natural yellow diamonds (directly obtained from the Ekati Diamond Mine to ensure that they are untreated) before and after HPHT treatment, conducted at a temperature of 2100 °C and a pressure of 6 GPa for 10 min. We report spectroscopic data and fluorescence characteristics, collected using PL mapping, FTIR mapping and fluorescence imaging showing the distribution of lattice defects and internal growth structures. PL mapping indicates SiV defects exist in one of the nitrogen-rich natural diamonds prior to treatment. Silicon-related defects can also be created by HPHT treatment, and they seem to show a relationship with pre-existing NV? centres. SIMS analysis was conducted to confirm the presence of silicon in these diamonds. The increase in the hydrogen-related infrared absorption peak at 3107 cm?1 (VN3H) is very strong in some diamonds that do not form B-centres during treatment. NVH was observed in our HPHT-treated natural diamonds, so it is possible that this strong increase in VN3H suppresses the aggregation of A- to B-centres as the newly formed A-centres were captured by NVH lattice defects to form VN3H. HPHT-altered and HPHT-induced platelet peaks are different from their natural counterparts in peak width and shape. Strong green fluorescence over a large area of a diamond, which is linked to relatively high concentration of H3 centres, was produced after HPHT treatment. We are confident that the unusual platelet peaks and strong emission of H3 centres are reliable indicators for HPHT-treated diamonds as they are not observed in untreated natural diamonds.
Mineralogy and Petrology, in press available 13p. Pdf
Canada, Northwest Territories
deposit - Chidliak, Ekati
Abstract: Yellow diamonds from the CH-7 (Chidliak) and the Misery (Ekati Mine) kimberlites in northern Canada are characterised for their nitrogen characteristics, visible light absorption, internal growth textures, and carbon isotope compositions. The diamonds are generally nitrogen-rich, with median N contents of 1230 (CH-7) and 1030 at.ppm (Misery). Normally a rare feature in natural diamonds, single substitutional nitrogen (C centres) and related features are detected in infrared absorption spectra of 64% of the studied diamonds from CH-7 and 87% from Misery and are considered as the major factor responsible for their yellow colouration. Episodically grown diamonds, characterised by colourless cores containing some nitrogen in the fully aggregated form (B centres) and yellow outer layers containing C centres, occur at both localities. Carbon isotope compositions and N contents also are significantly different in such core and rim zones, documenting growth during at least two temporally distinct events and involving different diamond forming fluids. Based on their nitrogen characteristics, both the yellow diamonds and yellow rims must have crystallized in close temporal proximity (<<1 Ma) to kimberlite activity at CH-7 and Misery.
Geochimica et Cosmochimica Acta, Vol. 275, pp. 99-122.
Mantle
carbon
Abstract: Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earth’s history. Diamonds’ inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as ?3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from ?3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total ?¹³C data range from ?17.1 to ?1.9 ‰ (P = ?8.4 to ?1.9 ‰; E = ?17.1 to ?2.1‰) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in ?¹³C of peridotitic diamonds has been at ?5 (±2) ‰ since the earliest diamond growth ?3.5 Ga, and this mode is also observed in the eclogitic diamond record since ?3 Ga. The skewness of eclogitic diamonds’ ?¹³C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent. No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1‰, but also that the principal ?¹³C mode of ?5 ± 1‰ in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga.
Abstract: Because of their robust nature, diamonds survive mantle processes and protect occluded minerals since the time of diamond formation. For the Kaapvaal Craton - the archetype for craton formation and evolution - the geochemical signatures of inclusions in Koffiefontein diamonds tell a story from craton formation to evolution and from lithospheric (below about 160 km) to lower mantle (>660 km) environs. We analysed a suite of 94 lithospheric to lower mantle diamonds and their silicate and oxide inclusions. Geochemical results confirm that the diamond substrates are very depleted, with Mg#OL of 91.5-95.0 and a dominance of low-Ca (<1.8 wt% CaO), presumably dunite-derived garnet. The Si-rich nature and preserved high Mg# of the peridotitic diamond substrates beneath Koffiefontein and the formation of KNbO3 (goldschmidtite) from an extremely fractionated melt/fluid indicate that potentially both mantle- and subduction-related fluids are the cause of metasomatism in the Kaapvaal cratonic root. Mantle-like, restricted carbon isotopic compositions of both P- and E-type diamonds (avg. ?13C -5.7 ‰ and -6.6 ‰, respectively) indicate that an abundant, mantle-derived CHO fluid is responsible for diamond formation. Diamonds have a large range in nitrogen concentrations and isotopic compositions, suggesting decoupling from carbon and heterogeneous sources. ?18O of former bridgmanite and ?13C of its host diamond document a purely mantle-derived lower mantle component. Combined, these results reveal a complex and multistage evolution of the Kaapvaal Craton whereby multiple episodes of fluid and melt metasomatism re-enriched the craton already, prior to diamond formation, followed by diamond entrainment in a kimberlite possibly derived from the lower mantle.
Abstract: Among mineral inclusions in diamond, sulphides are the most abundant. Also, they are the keel tool for dating diamond formation given their high concentration of highlysiderophile elements. However, the mineralogical nature of these inclusions is not well understood, mainly due to the exsolution of the original, high temperature monosulphide solid solution (Mss) to Fe-, Ni- and Cu-rich endmembers during cooling, obscuring the original composition. This complex exsolution observed in sulphide inclusions in diamonds can also cause problems with Re-Os age determinations if the whole inclusion is not extracted. To overcome this issue, recently, sulphide inclusions have been homogenized at high temperature and controlled oxygen fugacity [1]. However, X-ray diffraction or Raman spectroscopy analyses, required to accurately identify the inclusion phases, and define their degree of crystallographic plus compositional homogeneity, have not been reported. Here we combine for the first time a thorough nondestructive multi-technique characterization of sulphide inclusions in diamonds from the Victor Mine (Canada) with homogenization experiments and isotopic analyses. In particular, we report X-ray diffraction data of the sulphides before and after homogenization, confirming a change from a polycrystalline assemblage of pyrrothite, pentlandite and chalcopyrite to single-crystal Mss. The data are used to reconstruct the Mss’ original bulk composition, define the true bulk isotopic ratios and document any difference in Re- Os isotope systematics.
Abstract: Majoritic garnet has been predicted to be a major component of peridotite and eclogite in Earth's deep upper mantle (>250 km) and transition zone. The investigation of mineral inclusions in diamond confirms this prediction, but there is reported evidence of other majorite-bearing lithologies, intermediate between peridotitic and eclogitic, present in the mantle transition zone. If these lithologies are derived from olivine-free pyroxenites, then at mantle transition zone pressures majorite may form monomineralic or almost monomineralic garnetite layers. Since majoritic garnet is presumably the seismically fastest major phase in the lowermost upper mantle, the existence of such majorite layers might produce a detectable seismic signature. However, a test of this hypothesis is hampered by the absence of sound wave velocity measurements of majoritic garnets with relevant chemical compositions, since previous measurements have been mostly limited to synthetic majorite samples with relatively simple compositions. In an attempt to evaluate the seismic signature of a pyroxenitic garnet layer, we measured the sound wave velocities of three natural majoritic garnet inclusions in diamond by Brillouin spectroscopy at ambient conditions. The chosen natural garnets derive from depths between 220 and 470 km and are plausible candidates to have formed at the interface between peridotite and carbonated eclogite. They contain elevated amounts (12-30%) of ferric iron, possibly produced during redox reactions that form diamond from carbonate. Based on our data, we model the velocity and seismic impedance contrasts between a possible pyroxenitic garnet layer and the surrounding peridotitic mantle. For a mineral assemblage that would be stable at a depth of 350 km, the median formation depth of our samples, we found velocities in pyroxenite at ambient conditions to be higher by 1.9(6)% for shear waves and 3.3(5)% for compressional waves compared to peridotite (numbers in parentheses refer to uncertainties in the last given digit), and by 1.3(13)% for shear waves and 2.4(10)% for compressional waves compared to eclogite. As a result of increased density in the pyroxenitic layer, expected seismic impedance contrasts across the interface between the monomineralic majorite layer and the adjacent rocks are about 5-6% at the majorite-eclogite-interface and 10-12% at the majoriteperidotite-boundary. Given a large enough thickness of the garnetite layer, velocity and impedance differences of this magnitude could become seismologically detectable.
Abstract: The transport of carbon into Earth’s mantle is a critical pathway in Earth’s carbon cycle, affecting both the climate and the redox conditions of the surface and mantle. The largest unconstrained variables in this cycle are the depths to which carbon in sediments and altered oceanic crust can be subducted and the relative contributions of these reservoirs to the sequestration of carbon in the deep mantle1. Mineral inclusions in sublithospheric, or ‘superdeep’, diamonds (derived from depths greater than 250 kilometres) can be used to constrain these variables. Here we present oxygen isotope measurements of mineral inclusions within diamonds from Kankan, Guinea that are derived from depths extending from the lithosphere to the lower mantle (greater than 660 kilometres). These data, combined with the carbon and nitrogen isotope contents of the diamonds, indicate that carbonated igneous oceanic crust, not sediment, is the primary carbon-bearing reservoir in slabs subducted to deep-lithospheric and transition-zone depths (less than 660 kilometres). Within this depth regime, sublithospheric inclusions are distinctly enriched in 18O relative to eclogitic lithospheric inclusions derived from crustal protoliths. The increased 18O content of these sublithospheric inclusions results from their crystallization from melts of carbonate-rich subducted oceanic crust. In contrast, lower-mantle mineral inclusions and their host diamonds (deeper than 660 kilometres) have a narrow range of isotopic values that are typical of mantle that has experienced little or no crustal interaction. Because carbon is hosted in metals, rather than in diamond, in the reduced, volatile-poor lower mantle2, carbon must be mobilized and concentrated to form lower-mantle diamonds. Our data support a model in which the hydration of the uppermost lower mantle by subducted oceanic lithosphere destabilizes carbon-bearing metals to form diamond, without disturbing the ambient-mantle stable-isotope signatures. This transition from carbonate slab melting in the transition zone to slab dehydration in the lower mantle supports a lower-mantle barrier for carbon subduction.
Abstract: Sulfides are the most abundant inclusions in diamonds and a key tool for dating diamond formation via Re-Os isotopic analyses. The manner in which fluids invade the continental lithospheric mantle and the time scale at which they equilibrate with preexisting (protogenetic) sulfides are poorly understood yet essential factors to understanding diamond formation and the validity of isotopic ages. We investigated a suite of sulfide-bearing diamonds from two Canadian cratons to test the robustness of Re-Os in sulfide for dating diamond formation. Single-crystal X-ray diffraction (XRD) allowed determination of the original monosulfide solid-solution (Mss) composition stable in the mantle, indicating subsolidus conditions of encapsulation, and providing crystallographic evidence supporting a protogenetic origin of the inclusions. The results, coupled with a diffusion model, indicate Re-Os isotope equilibration is sufficiently fast in sulfide inclusions with typical grain size, at mantle temperatures, for the system to be reset by the diamond-forming event. This confirms that even if protogenetic, the Re-Os isochrons defined by these minerals likely reflect the ages of diamond formation, and this result highlights the power of this system to date the timing of fluid migration in mantle lithosphere.
Journal of Petrology, doi.org/petrology/egab090 65p. Pdf
Africa, South Africa
deposit - Roberts Victor
Abstract: The origin of the eclogites that reside in cratonic mantle roots has long been debated. In the classic Roberts Victor kimberlite locality in South Africa, the strongly contrasting textural and geochemical features of two types of eclogites have led to different genetic models. We studied a new suite of 63 eclogite xenoliths from the former Roberts Victor Mine. In addition to major- and trace-element compositions for all new samples, we determined 18O/16O for garnet from 34 eclogites. Based on geochemical and textural characteristics we identify a large suite of Type I eclogites (n = 53) consistent with previous interpretations that these rocks originate from metamorphosed basaltic-picritic lavas or gabbroic cumulates from oceanic crust, crystallised from melts of depleted MORB mantle. We identify a smaller set of Type II eclogites (n = 10) based on geochemical and textural similarity to eclogites in published literature. We infer their range to very low ?18O values combined with their varied, often very low Zr/Hf ratios and LREE-depleted nature to indicate a protolith origin via low-pressure clinopyroxene-bearing oceanic cumulates formed from melts that were more depleted in incompatible elements than N-MORB. These compositions are indicative of derivation from a residual mantle source that experienced preferential extraction of incompatible elements and fractionation of Zr-Hf during previous melting.
Abstract: The Victor diamond mine in Ontario, Canada opened in 2008 and ceased operations in June 2019. Previous researchers documented that Victor diamonds are unusually young (~ 720 Ma, Aulbach et al., 2018) and grew predominantly in unusually fertile peridotite substrates, specifically garnet lherzolite and garnet wehrlite (Stachel et al., 2018). Our recent work on n=157 lherzolitic clinopyroxene (Cpx) xenocrysts from the Victor mine reveals profound major- and trace-element (re)fertilization of the deepest 1/3rd of the central Superior craton lithosphere. For example, Cpx Mg/(Mg+Fe) of 0.93 in shallow peridotite decreases across a steep gradient to Mg/(Mg+Fe) of 0.89 at depths of 4.2 to 5.6 GPa. We document marked compositional gradients over a similar depth range for certain minor (Ti, Mn, Ni) and trace elements (LREE and HREE) and attribute the gradients to chromatographic and/or crystal-chemical fractionation effects. We carefully categorized the Victor cpx xenocrysts in nine depth-composition classes and determined Pb-isotope ratios for representative grains from each class in a bold experiment aimed at capturing geochronological data from mantle Cpx. A resultant 207Pb/206Pb secondary isochron array at ~ 1075 Ma identifies craton-scale events related to the Mid-Continent Rift as the source of fluids and/or melts that (re)fertilized the central Superior craton at depth, some 355 Ma prior to diamond growth. Coordinated, systematic major- and trace-element relationships in clinopyroxene permit compositional discrimination of mantle (re)fertilization at ~1075 Ma from fluid-metasomatism attending diamond growth at ~ 720 Ma. Roughly 10% of the clinopyroxene xenocrysts analyzed in this work exhibit diamond-associated compositions.
Abstract: The term craton has a complex and confused etymology. Despite originally specifying only strength and stability - of the crust - the term craton, within the context of diamond exploration, has widely come to refer to a region characterised by crustal basement older than 2.5 Ga, despite the fact that some such “cratons” no longer possess their deep lithospheric root. This definition often precluded regions with deep lithospheric roots but basement younger than 2-2.5 Ga. Viscous, buoyant lithospheric mantle roots are key to the survival and stability of continental crust. Here we use a revised craton definition (Pearson et al., 2021, in press), that includes the requirement of a deep (~150 km or greater) and intact lithospheric root, to re-examine the link between cratons and diamonds. The revised definition has a nominal requirement for tectonic stability since ~ 1 Ga and recognises that some regions are “modified cratons” - having lost their deep roots, i.e., they may have behaved like cratons for an extended period but subsequently lost much of their stabilising mantle roots during major tectono-thermal events. In other words, despite being long-lived features, cratons are not all permanent. The 150 km lithospheric thickness cut-off provides an optimal match to crustal terranes with 1 Ga timescale stability. In terms of regional diamond exploration, for a given area, the crucial criterion is when a deep mantle root was extant, i.e., over what period was the lithospheric geotherm suitable for diamond formation, stability and sampling? A thick lithospheric root is key to the formation of deep-seated magmas such as olivine lamproites and to the evolution of sub-lithospheric sourced proto-kimberlites, all capable of carrying and preserving diamonds to Earth's surface. This criterion appears essential even for sub-lithospheric diamonds, that still require a diamond transport mechanism capable of preserving the high-pressure carbon polymorph via facilitating rapid transport of volatile-charged magma to the surface, without dilution from additional melting that takes place beneath thinner (<120 km) lithospheric "lids". Seismology can help to define the lateral extent of today's cratons, but a detailed understanding of the regional geological history, kimberlite eruption ages and geothermal conditions is required to evaluate periods of past diamond potential, no-longer evident today. This revised craton concept broadens the target terranes for diamond exploration away from only the Archean cores of cratons and an associated mentality that "the exception proves the rule". The revised definition is compatible with numerous occurrences of diamond in Proterozoic terranes or Archean terranes underpinned by Proterozoic mantle.
Abstract: The principal distinction between diamond substrates and the cratonic mantle roots as sampled by garnet peridotite xenoliths is the much higher proportion of harzburgite (-dunite) to lherzolite (-wehrlite) in the former (~85:15 %) compared to the latter (18:82 %). Dunitic mineralogies are common diamond substrates (~38%) but rarely documented in xenoliths (~2 %). Using mineral Mg# as an indicator of source depletion through melt extraction again documents the more depleted character of diamond substrates relative to the cratonic garnet-peridotite xenolith record. On a like-for-like paragenesis level, however, olivine inside and outside of diamond has statistically indistinguishable means in Mg#. This observation implies: (1) that the major element composition of inclusions is imposed largely by the substrate and not by the diamond forming medium and (2) that widespread Fe-rich metasomatism of the lithospheric mantle did not occur subsequent to diamond formation (Paleoarchean to Mesoproterozoic). The latter conclusion precludes neither localized metasomatic shifts in Mg#, nor metasomatism by small melt fractions/fluids subsequent to diamond formation, as such events have low fluid/rock ratios and hence limited impact on bulk rock Mg#. A distinctive feature of inclusions relative to xenolith minerals is the higher Cr/Al of garnet and chromite in diamond. Higher Cr/Al for inclusions is not limited to the harzburgitic-dunitic paragenesis, but also occurs among lherzolitic inclusions. This suggests that the almost exclusive restriction of Cr2O3 contents >13 wt% to inclusion garnets is not a consequence of higher degrees of primary melt depletion being restricted to, or preferentially preserved, in diamond substrates. Instead, the very high Cr contents in a subset of inclusions likely relate to the pressure and temperature dependence of the distribution of Cr between garnet and spinel. Experiments showed inclusion-like high Cr/Al for coexisting Cr-pyrope and Cr-spinel in harzburgite at high pressures and temperatures (>5 GPa and >1200 °C; Girnis and Brey 1999). High Cr/Al inclusion compositions thus likely reflect some diamond growth occurring over a wide range of temperatures, elevated above a cratonic geotherm during high-temperature thermal perturbations. Na and Ti are sensitive indicators of mantle metasomatism. Enrichment of Na and Ti in both inclusion and xenolith minerals is most prominent in the lherzolitic paragenesis and very intense Ti-rich metasomatism is almost entirely restricted to lherzolite xenoliths that resided at temperatures >1130 °C, i.e. above the hydrous solidus. Since equilibration temperatures of >1130 °C are common also for inclusions, the near absence of intense Ti-metasomatism in inclusions likely relates to either a diamond unfriendly character of such metasomatism or an increase in Ti-metasomatic intensity or frequency subsequent to principally Archean-Mesoproterozoic formation of peridotitic diamonds.
Geochimica et Cosmochimica Acta, Vol. 323, pp. 20-39.
Africa, Sierra Leone
deposit - Koidu
Abstract: Inclusion-bearing diamonds from the Koidu kimberlite complex, Sierra Leone (West African Craton) were analyzed in situ for carbon and nitrogen isotope compositions, nitrogen concentrations and nitrogen aggregation states. In a suite of 105 diamonds, 78% contain eclogitic mineral inclusions, 17% contain peridotitic mineral inclusions, and 5% - an unusually high proportion - contain co-occurring eclogitic and peridotitic mineral inclusions indicating a mixed paragenesis. Major and trace element compositions of mineral inclusions from two mixed paragenesis diamonds (one with omphacite + Mg-chromite, the other with eclogitic garnet + forsteritic olivine) were determined. The presence of positive Eu anomalies in centrally located omphacite and eclogitic garnet inclusions indicates derivation from subducted protoliths, formed as igneous cumulates in lower oceanic crust. Mg-chromite (Cr# 85.5; Mg# 65.2) and olivine (Mg# 94.5) inclusions, located in outer portions of the mixed paragenesis diamonds, have compositions indicative of derivation from strongly depleted cratonic peridotites. Given that the olivine Mg# of 94.5 is the highest reported to date for the West African Craton, the eclogitic and peridotitic inclusions in these mixed paragenesis diamonds cannot have precipitated during infiltration of peridotitic substrates by eclogite-derived fluids, as the consequent fluid-rock interaction should lead to Mg# lower than that for the original peridotitic diamond substrate. The different origins of eclogitic and peridotitic inclusions could be explained by physical transport of their host diamonds from eclogitic into peridotitic substrates, possibly along high-strain shear zones, before renewed diamond growth. Based on the ?¹³C-?¹?N systematics of the entire inclusion-bearing diamond suite from Koidu, three major compositional clusters are identified. Cluster 1 (eclogitic diamond cores; ?¹³C = -33.2 to -14.4 ‰ and ?¹?N = -5.3 to +10.1 ‰) bears the isotopic signature of recycled crustal material (± a mantle component). Cluster 2 (peridotitic diamonds and including the core of a diamond containing omphacite + Mg-chromite; ?¹³C = -6.0 to -1.1 ‰ and ?¹?N = -4.2 to +9.7 ‰) likely involves mixing of carbon and nitrogen from subducted and mantle sources. Cluster 3 (rims of eclogitic diamonds and including the eclogitic garnet + olivine included diamond and the rim of the omphacite + Mg-chromite included diamond; ?¹³C = -7.8 to -3.6 ‰ and ?¹?N = -7.9 to -2.1 ‰) matches convecting mantle-derived fluids/melts. The distinct isotopic signatures of the three diamond clusters, together with differences in nitrogen aggregation and cathodoluminescence response between diamond cores and rims, suggest episodic diamond growth during multiple fluid/melt pulses.
Abstract: The magnetotelluric component of the EarthScope USArray program has covered over 35% of the continental United States. Resistivity tomography models derived from these data image lithospheric structure and provide constraints on the distribution of fluids and melt within the lithosphere. We present a three-dimensional resistivity model of the northwestern United States which provides new insight into the tectonic assembly of western North America from the Archean to present. Comparison with seismic tomography models reveals regions of correlated and anti-correlated resistivity and velocity that help identify thermal and compositional variations within the lithosphere. Recent (Neogene) tectonic features reflected in the model include the subducting Juan de Fuca–Gorda plate which can be traced beneath the forearc to more than 100 km depth, high lithospheric conductivity along the Snake River Plain, and pronounced lower-crustal and upper-mantle conductivity beneath the Basin and Range. The latter is abruptly terminated to the northwest by the Klamath–Blue Mountains Lineament, which we interpret as an important structure during and since the Mesozoic assembly of the region. This boundary is interpreted to separate hot extended lithosphere from colder, less extended lithosphere. The western edge of Proterozoic North America, as indicated by the Cretaceous initial 87Sr/86Sr = 0.706 contour, is clearly reflected in the resistivity model. We further image an Archean crustal block (“Pend Oreille block”) straddling the Washington/Idaho border, which we speculate separated from the Archean Medicine Hat block in the Proterozoic. Finally, in the modern Cascades forearc, the geometry and internal structure of the Eocene Siletz terrane is reflected in the resistivity model. The apparent eastern edge of the Siletz terrane under the Cascades arc suggests that pre-Tertiary rocks fill the Washington and Oregon back-arc.
Perceptions of the impact of board members' individual perspectives on the social and environmental performance of companies. ( Based on SA and not junior companies).
Journal of the South African Institute of Mining and Metallurgy, Vol. 114, Nov. pp. 957-969.
Carbon speciation in the asthenosphere: experimental measurements of the redox conditions at which carbonate bearing melts coexist with graphite or diamond in peridotite assemblages.
Earth and Planetary Science Letters, Vol. 300, 1-2, Nov. 15, pp. 72-84.
Abstract: Chemical reduction-oxidation mechanisms within mantle rocks link to the terrestrial carbon cycle by influencing the depth at which magmas can form, their composition, and ultimately the chemistry of gases released into the atmosphere. The oxidation state of the uppermost mantle has been widely accepted to be unchanged over the past 3800 m.y., based on the abundance of redox-sensitive elements in greenstone belt-associated samples of different ages. However, the redox signal in those rocks may have been obscured by their complex origins and emplacement on continental margins. In contrast, the source and processes occurring during decompression melting at spreading ridges are relatively well constrained. We retrieve primary redox conditions from metamorphosed mid-oceanic ridge basalts (MORBs) and picrites of various ages (ca. 3000-550 Ma), using V/Sc as a broad redox proxy. Average V/Sc values for Proterozoic suites (7.0 ± 1.4, 2?, n = 6) are similar to those of modern MORB (6.8 ± 1.6), whereas Archean suites have lower V/Sc (5.2 ± 0.4, n = 5). The lower Archean V/Sc is interpreted to reflect both deeper melt extraction from the uppermost mantle, which becomes more reduced with depth, and an intrinsically lower redox state. The pressure-corrected oxygen fugacity (expressed relative to the fayalite-magnetite-quartz buffer, ?FMQ, at 1 GPa) of Archean sample suites (?FMQ -1.19 ± 0.33, 2?) is significantly lower than that of post-Archean sample suites, including MORB (?FMQ -0.26 ± 0.44). Our results imply that the reducing Archean atmosphere was in equilibrium with Earth's mantle, and further suggest that magmatic gases crossed the threshold that allowed a build-up in atmospheric O2 levels ca. 3000 Ma, accompanied by the first "whiffs" of oxygen in sediments of that age.
Abstract: Chemical reduction-oxidation mechanisms within mantle rocks link to the terrestrial carbon cycle by influencing the depth at which magmas can form, their composition, and ultimately the chemistry of gases released into the atmosphere. The oxidation state of the uppermost mantle has been widely accepted to be unchanged over the past 3800 m.y., based on the abundance of redox-sensitive elements in greenstone belt-associated samples of different ages. However, the redox signal in those rocks may have been obscured by their complex origins and emplacement on continental margins. In contrast, the source and processes occurring during decompression melting at spreading ridges are relatively well constrained. We retrieve primary redox conditions from metamorphosed mid-oceanic ridge basalts (MORBs) and picrites of various ages (ca. 3000-550 Ma), using V/Sc as a broad redox proxy. Average V/Sc values for Proterozoic suites (7.0 ± 1.4, 2?, n = 6) are similar to those of modern MORB (6.8 ± 1.6), whereas Archean suites have lower V/Sc (5.2 ± 0.4, n = 5). The lower Archean V/Sc is interpreted to reflect both deeper melt extraction from the uppermost mantle, which becomes more reduced with depth, and an intrinsically lower redox state. The pressure-corrected oxygen fugacity (expressed relative to the fayalite-magnetite-quartz buffer, ?FMQ, at 1 GPa) of Archean sample suites (?FMQ -1.19 ± 0.33, 2?) is significantly lower than that of post-Archean sample suites, including MORB (?FMQ -0.26 ± 0.44). Our results imply that the reducing Archean atmosphere was in equilibrium with Earth's mantle, and further suggest that magmatic gases crossed the threshold that allowed a build-up in atmospheric O2 levels ca. 3000 Ma, accompanied by the first "whiffs" of oxygen in sediments of that age.
Journal of Volcanology and Geothermal Research, in press available 15p.
Africa, Algeria
Lamproite
Abstract: The late Miocene (11-9 Ma) volcanic rocks of Kef Hahouner, ~ 40 km NE of Constantine (NE Algeria), are commonly classified as lamproites in literature. However, these rocks are characterized by an anhydrous paragenesis with plagioclase and Mg-rich olivine phenocrysts, set in a groundmass made up of feldspars, pyroxenes and opaque minerals. Thus, we classify the Kef Hahouner rocks as ultrapotassic shoshonites and latites, having K2O > 3 wt.%, K2O/Na2O > 2.5, MgO > 3-4 wt.%, SiO2 < 55-57 wt.% and SiO2/K2O < 15. All the investigated samples show primitive mantle-normalized multi-element patterns typical of orogenic (arc-type) magmas, i.e. enriched in LILE (e.g. Cs, Rb and Ba) and LREE (e.g. La/Yb = 37-59) with respect to the HFSE, peaks at Pb and troughs at Nb and Ta. Initial isotopic ratios are in the range of 87Sr/86Sr = 0.70874-0.70961, 143Nd/144Nd = 0.51222-0.51223, 206Pb/204Pb = 18.54-18.60, 207Pb/204Pb = 15.62-15.70 and 208Pb/204Pb = 38.88-39.16. The Kef Hahouner volcanic rocks show multi-element patterns similar to the other circum-Mediterranean lamproites and extreme Sr, Nd and Pb isotopic compositions. Nevertheless, the abundant plagioclase, the presence of Al-rich augite coupled with high Al2O3 whole rock compositions (9.6-21.4 wt.%), and the absence of phlogopite are all at inconsistent with the definition of lamproite. We reviewed the rocks classified as lamproites worldwide, and found that many of these rocks, as for the Kef Hahouner samples, should be actually defined as "normal" potassic to ultrapotassic volcanic rocks. Even the grouping of lamproites into "orogenic" and "anorogenic" types appears questionable.
Abstract: The deep carbon cycle and the origin of carbonatitic melts into the Earth’s mantle have been studied through the effect of CO2 on phase equilibria within carbonated eclogitic assemblage in the last decades. However the effect of temperature (T), pressure (P) and oxygen fugacity (fO2) on the melt composition remains unclear. This study aims to determine the melt composition of CO2-rich melts at fO2 buffered by the C/carbonate equilibrium as function of P and T. Experiments were performed using the Voggenreiter 840 t, Walker-type multi anvil press available at HP/HT Lab at National Institute of Geophysics and Volcanology (INGV) in Rome. The starting material employed for all the experiments is a mixture of synthetic omphacitic glass, quartz, dolomite and graphite representative of the Dolomite-CoesiteDiopside-Graphite buffering assemblage [DCDG; 1], doped with ilmenite and rutile and ~3 wt% iridium used as redox sensor to monitorate the oxygen fugacity during the experiment. The recovered quenched samples were polished for textural and chemical analysis of the mineral phases using Field emission scanning electron microscope and electron microprobe at the INGV. Preliminary results were combined with previous published data [2], and the determined fo2 compared with thermodynamic predictions. The obtained data show that at 800°C run product consists of a subsolidus mineral assemblage representative of the DCDG mineral assemblage. With increasing temperature, a carbonatitic melt forms with 1-5 wt% SiO2 at 900 °C, then evolves to a carbonate-silicate melt with 25 wt% SiO2 at 1100 °C, and to a silicate melt with ~32 wt% SiO2 at 1200 °C. Preliminary results demonstrate that magmas with compositions from carbonatitic to carbonate-silicate (hybrid) melts can form within less than 1 log unit of fO2 by redox melting of elemental carbon-bearing eclogite rocks.
Abstract: The composition of the early Earth’s atmosphere is believed to result from significant magma outgassing during the Archaean eon. It has been widely debated whether the oxygen fugacity (fo2) of the Earth’s mantle has remained constant over the last ~3.8 Ga to levels where volatiles were mostly in their mobile form [1,2], or whether the mantle has experienced a gradual increase of its redox state [3]. Both hypotheses raise fundamental questions on the effect of composition of the early Earth’s accreting material, the origin and availability of primordial carbon in Earth’s interior, and the migration rate of CO2-rich magmas. In addition, the occurrence in nature of carbonatites (or silicate-carbonatitic rocks), diamonds and carbides indicate a dominant control of the mantle redox state on the volatile speciation over time and, maybe, on mechanisms of their formation, reaction and migration through the silicate mantle. A recent model has been developed that combines both experimental results on the fo2 of preserved carbonaceous chondrites at high pressure and thermodynamic predictions of the the temporal variation of the mantle redox state, with the CO2-bearing magmas that could form in the early asthenospheric mantle. Since any variation in melt composition is expected to cause significant changes in the physical properties (e.g., viscosity and density), the migration rate of these magmas has been determined using recent in situ viscosity data on CO2-rich melts with the falling sphere technique. Our results allow determining the composition of CO2- bearing magmas as function of the increasing mantle redox state over time, and the mechanisms and rate for exchange of carbon between mantle reservoirs.
Abstract: Knowledge of the rheology of molten materials at high pressure and temperature is required to understand magma mobility and ascent rate at conditions of the Earth's interior. We determined the viscosity of nominally anhydrous sodium carbonate (Na2CO3), an analogue and ubiquitous component of natural carbonatitic magmas, by the in situ “falling sphere” technique at 1.7, 2.4 and 4.6?GPa, at 1200 to 1700?°C, using the Paris-Edinburgh press. We find that the viscosity of liquid Na2CO3 is between 0.0028?±?0.0001?Pa•s and 0.0073?±?0.0001?Pa•s in the investigated pressure-temperature range. Combination of our results with those from recent experimental studies indicate a negligible dependence on pressure from 1?atm to 4.6?GPa, and a small compositional dependence between molten alkali metal-bearing and alkaline earth metal-bearing carbonates. Based on our results, the viscosity of Na2CO3 is consistent with available viscosity data of both molten calcite (determined at high pressure and temperature) and Na2CO3 at ambient pressure. Molten Na2CO3 is a valid experimental analogue for study of the rheology of natural and/or synthetic near-solidus carbonatitic melts. Estimated values of the mobility and ascent velocity of carbonatitic melts at upper conditions are between 70 and 300?g?cm?3•Pa?1•s?1 and 330-1450?m•year?1, respectively, when using recently proposed densities for carbonatitic melts. The relatively slow migration rate allows magma-rock interaction over time causing seismic anomalies and chemical redox exchange.
Researchgate preprint, 10.31223/ofs.io/uh5c8 40p. Pdf
Mantle
carbonatite
Abstract: Over the last decades, many experimental studies have focused on the effect of CO2 on phase equilibria and melting behavior of synthetic eclogite and peridotite rocks as function of pressure and temperature. These studies have been of fundamental importance to understanding the origin of carbonated magmas varying in composition from carbonatitic to kimberlitic. The occurrence of diamonds in natural rocks is a further evidence of the presence of (reduced) carbon in the Earth’s interior. The oxygenation of the Earth’s interior (i.e. its redox state) through time has strongly influenced the speciation of carbon from the mantle to mantle-derived magmas and, in turn, to the released volcanic gases to the atmosphere. This paper explains how the knowledge of the oxygen fugacity recorded by mantle rocks and determined through the use of appropriate oxy-thermobarometers allows modeling the speciation of carbon in the mantle, its mobilization in the asthenospheric mantle by redox partial melting, and its sequestration and storage during subduction by redox freezing processes. The effect of a gradual increase of the mantle fo2 on the mobilization of C is here discussed along with the main variables affecting its transport by subduction down to the mantle.
Journal of the Geological Society of London, Vol. 176, pp. 375-387.
Global
carbonatite
Abstract: Over the last decades, many experimental studies have focused on the effect of CO2 on phase equilibria and melting behavior of synthetic eclogite and peridotite rocks as function of pressure and temperature. These studies have been of fundamental importance to understanding the origin of carbonated magmas varying in composition from carbonatitic to kimberlitic. The occurrence of diamonds in natural rocks is a further evidence of the presence of (reduced) carbon in the Earth’s interior. The oxygenation of the Earth’s interior (i.e. its redox state) through time has strongly influenced the speciation of carbon from the mantle to mantle-derived magmas and, in turn, to the released volcanic gases to the atmosphere. This paper explains how the knowledge of the oxygen fugacity recorded by mantle rocks and determined through the use of appropriate oxy-thermobarometers allows modeling the speciation of carbon in the mantle, its mobilization in the asthenospheric mantle by redox partial melting, and its sequestration and storage during subduction by redox freezing processes. The effect of a gradual increase of the mantle fo2 on the mobilization of C is here discussed along with the main variables affecting its transport by subduction down to the mantle.
Minerals MDPI, Vol. 10, 267 doi: 10.23390/min10030267 14p. Pdf
Mantle
Melililite, carbon
Abstract: Understanding the viscosity of mantle-derived magmas is needed to model their migration mechanisms and ascent rate from the source rock to the surface. High pressure-temperature experimental data are now available on the viscosity of synthetic melts, pure carbonatitic to carbonate-silicate compositions, anhydrous basalts, dacites and rhyolites. However, the viscosity of volatile-bearing melilititic melts, among the most plausible carriers of deep carbon, has not been investigated. In this study, we experimentally determined the viscosity of synthetic liquids with ~31 and ~39 wt% SiO2, 1.60 and 1.42 wt% CO2 and 5.7 and 1 wt% H2O, respectively, at pressures from 1 to 4.7 GPa and temperatures between 1265 and 1755 °C, using the falling-sphere technique combined with in situ X-ray radiography. Our results show viscosities between 0.1044 and 2.1221 Pa•s, with a clear dependence on temperature and SiO2 content. The atomic structure of both melt compositions was also determined at high pressure and temperature, using in situ multi-angle energy-dispersive X-ray diffraction supported by ex situ microFTIR and microRaman spectroscopic measurements. Our results yield evidence that the T-T and T-O (T = Si,Al) interatomic distances of ultrabasic melts are higher than those for basaltic melts known from similar recent studies. Based on our experimental data, melilititic melts are expected to migrate at a rate ~from 2 to 57 km•yr?1 in the present-day or the Archaean mantle, respectively.
Proceedings of the National Academy of Sciences, pnas.org/cgi/doi.10.1073 /pnas.2004269117 7p. Pdf
Mantle
diamond inclusions
Abstract: The recent discovery in high-pressure experiments of compounds stable to 24-26 GPa with Fe4O5, Fe5O6, Fe7O9, and Fe9O11 stoichiometry has raised questions about their existence within the Earth’s mantle. Incorporating both ferric and ferrous iron in their structures, these oxides if present within the Earth could also provide insight into diamond-forming processes at depth in the planet. Here we report the discovery of metallic particles, dominantly of FeNi (Fe0.71Ni0.24Cu0.05), in close spatial relation with nearly pure magnetite grains from a so-called superdeep diamond from the Earth’s mantle. The microstructural relation of magnetite within a ferropericlase (Mg0.60Fe0.40)O matrix suggests exsolution of the former. Taking into account the bulk chemistry reconstructed from the FeNi(Cu) alloy, we propose that it formed by decomposition of a complex metal M oxide (M4O5) with a stoichiometry of (Fe3+2.15Fe2+1.59Ni2+0.17Cu+0.04)? = 3.95O5. We further suggest a possible link between this phase and variably oxidized ferropericlase that is commonly trapped in superdeep diamond. The observation of FeNi(Cu) metal in relation to magnetite exsolved from ferropericlase is interpreted as arising from a multistage process that starts from diamond encapsulation of ferropericlase followed by decompression and cooling under oxidized conditions, leading to the formation of complex oxides such as Fe4O5 that subsequently decompose at shallower P-T conditions.
Abstract: he interior of the Earth is an important reservoir for elements that are chemically bound in minerals, melts, and gases. Analyses of the proportions of redox-sensitive elements in ancient and contemporary natural rocks provide information on the temporal redox evolution of our planet. Natural inclusions trapped in diamonds, xenoliths, and erupted magmas provide unique windows into the redox conditions of the deep Earth, and reveal evidence for heterogeneities in the mantle’s oxidation state. By examining the natural rock record, we assess how redox boundaries in the deep Earth have controlled elemental cycling and what effects these boundaries have had on the temporal and chemical evolution of oxygen fugacity in the Earth’s interior and atmosphere.
Contributions to Mineralogy and Petrology, Vol. 175, 107, 17p. Pdf
Russia
deposit - Udachnaya
Abstract: The formation of diamonds within eclogitic rocks has been widely linked to the fate of carbon during subduction and, therefore, referred to conditions of pressure, temperature, and oxygen fugacity (fo2). Mantle-derived eclogite xenoliths from Udachnaya kimberlite pipes represent a unique window to investigate the formation of carbon-free, graphite-diamond-bearing and diamond-bearing rocks from the Siberian craton. With this aim, we exploited oxy-thermobarometers to retrieve information on the P-T-fo2 at which mantle eclogites from the Siberian craton equilibrated along with elemental carbon. The chemical analyses of coupled garnet and omphacitic clinopyroxene were integrated with data on their iron oxidation state, determined both by conventional and synchrotron 57Fe Mössbauer spectroscopy. The calculated fo2s largely vary for each suite of eclogite samples from 0.10 to ? 2.43 log units (?FMQ) for C-free eclogites, from ? 0.01 to ? 2.91 (?FMQ) for graphite-diamond-bearing eclogites, and from ? 2.08 to ? 3.58 log units (?FMQ) for diamond-bearing eclogites. All eclogite samples mostly fall in the fo2 range typical of diamond coexisting with CO2-rich water-bearing melts and gaseous fluids, with diamondiferous eclogites being more reduced at fo2 conditions where circulating fluids can include some methane. When uncertainties on the calculated fo2 are taken into account, all samples essentially fall within the stability field of diamonds coexisting with CO2-bearing melts. Therefore, our results provide evidence of the potential role of CO2-bearing melts as growth medium on the formation of coexisting diamond and graphite in mantle eclogites during subduction of the oceanic crust.
Abstract: Over recent decades, many experimental studies have focused on the effect of CO2 on phase equilibria and melting behaviour of synthetic eclogites and peridotites as a function of pressure and temperature. These studies have been of fundamental importance to understanding the origin of carbonated magmas varying in composition from carbonatitic to kimberlitic. The occurrence of diamonds in natural rocks is further evidence of the presence of (reduced) carbon in the Earth's interior. The oxygenation of the Earth's interior (i.e. its redox state) through time has strongly influenced the speciation of carbon from the mantle to mantle-derived magmas and, in turn, to the volcanic gases released to the atmosphere. This paper explains how the knowledge of the oxygen fugacity recorded by mantle rocks and determined through the use of appropriate oxy-thermobarometers allows modelling of the speciation of carbon in the mantle, its mobilization in the asthenospheric mantle by redox partial melting, and its sequestration and storage during subduction by redox freezing processes. The effect of a gradual increase of the mantle fO2 on the mobilization of C is here discussed along with the main variables affecting its transport by subduction into the mantle.
Tappert, M.C., Rivard, B., Fulop, A., Rogge, D., Feng, J., Tappert, R., Stalder, R.
Characterizing kimberlite dilution by crustal rocks at the Snap Lake diamond mine ( Northwest Territories, Canada) using SWIR ( 1.90-2.36 um) and LWIR ( 8.1-11.1um) hypersprectal imagery collected from drill core.
Economic Geology, Vol. 110, 6, Sept-Oct. pp. 1375-1387.
Abstract: Garnets from kimberlite-hosted mantle and a few xenoliths from the lower crust were investigated for water, major, minor, and trace elements. Xenoliths from the mantle comprise pyroxenite, eclogite, alkremite, and peridotite, and crustal xenoliths are mafic high-pressure granulites. Samples from South Africa, Lesotho, and Namibia comprise two principal settings, Kaapvaal Craton (‘on craton’) and Rehoboth terrane (‘off craton’). The composition of garnet depends on rock type but is unrelated to the setting, except for Ti and Cr. In garnets from ‘off craton’ mantle xenoliths, Ti positively correlates with Cr whereas those from ‘on craton’ samples reveal a negative correlation between both elements. Rare earth element patterns indicative of a metasomatic overprint are observed in garnets from both settings, especially in eclogitic garnet. Water contents in garnet are low and range from <1 to about 40 ppm. No setting-related difference occurs, but a weak correlation between water and rock type exists. Water contents in garnets from eclogite and mafic granulite are lower than those in pyroxenite, alkremite, and peridotite. All garnets are water under-saturated, i.e. they do not contain the maximum amount of water that can be accommodated in the mineral structure. Cratonic and non-cratonic samples also show the same characteristics in the infrared (IR) absorption spectra. An absorption band at 3650 cm-1 is typical for most mantle garnets. Bands at 3520 and 3570 cm-1 are present only in TiO2-rich garnets from the Rehoboth terrane and are ascribed to a Ti-related hydrogen substitution. A number of garnets, especially from the Kaapvaal Craton, contain molecular water in addition to structural water. Molecular water is inhomogeneously distributed at grain scale pointing to local interaction with fluid and to disequilibrium at grain scale. These garnets consistently reveal either submicroscopic hydrous phases or additional IR bands at 3630 and 3610-3600 cm-1 caused by structural water. Both features do not occur in garnets in which molecular water is absent. The observations imply (i) relatively late introduction of fluid, at least in cases where hydrous phases formed, and (ii) a relatively dry environment because only water-deficient garnets are able to incorporate additional structural water. Most importantly, they imply (iii) that the low water contents are primary and not due to water loss during upward transport. This late water influx is not responsible for the metasomatic overprint indicated by garnet REE patterns. The results of this study suggest dry conditions in the lithosphere, including mantle and crustal sections of both the Kaapvaal Craton (‘on craton’) and the Rehoboth terrane (‘off craton’). If the low water contents contributed to the stabilization of the Kaapvaal cratonic root (Peslier et al., 2010) the same should apply to the Rehoboth lithosphere where the same variety of rock types occurs. The extremely low water contents in eclogite relative to pyroxenite may be explained by an oceanic crust origin of the eclogites. Subduction and partial melting would cause depletion of water and incompatible elements. The pyroxenites formed by crystal accumulation in the mantle and did not suffer melt depletion. Such a difference in origin can be reconciled with the low Ti contents in eclogitic garnet and the high Ti contents in pyroxenitic garnet.
Physics and Chemistry of Minerals, dor.org/10.1007/ d00269-018-0987-5 13p.
Russia, Kazakhstan, Alps
coesite, UHP
Abstract: The high-pressure silica polymorphs coesite and stishovite were synthesized under water-saturated conditions from a natural granitic composition doped with Li and B. Experiments were performed in a Multi-Anvil apparatus between 4 and 9.1 GPa and 900 and 950 °C, based on the conditions of a subducting continental crust as realistic for the ultrahigh-pressure metamorphic units Dora Maira and Kochetav massifs. Run products consisted of coesite/stishovite?+?kyanite?±?phengite?±?omphacite, and quench material. The synthesized silica polymorphs were successively analyzed by infrared spectroscopy, electron microprobe, and Secondary-Ion Mass Spectrometry (SIMS). No hydrous defects were observed in coesite synthesized at 4 GPa and 900 °C, whereas coesite grown at higher pressures revealed a triplet of infrared absorptions bands at 3575, 3523, and 3459 cm??1, two minor bands at 3535 and 3502 cm??1, and a small band at 3300 cm??1 that was only visible at 7.7 GPa. The total amount of Al was charge-balanced by H and the other monovalent cations. However, the band triplet could not be associated with AlOH defects, while the band doublet was inferred to BOH defects and the small band probably corresponded to interstitial H. Stishovite displayed one dominant band at 3116 cm??1 with a shoulder at 3170 cm??1, and a minor band at 2665 cm??1, probably all associated with AlOH defects. BOH defects were not observed in stishovite, and LiOH defects were neither observed in coesite nor stishovite, probably because of preferentially partition of Li in other phases such as omphacite. The total amount of defect protons increased with pressure and with metal impurity concentrations. The general increase in OH defects in silica polymorphs with increasing pressure (this study) contrasted the negative pressure trend of OH in quartz observed previously from the same starting material, and revealed an incorporation minimum of OH in silica polymorphs around the quartz/coesite phase transition.
Physics and Chemistry of Minerals, Vol. 46, pp. 77-89.
Russia, Europe, Alps
UHP
Abstract: The high-pressure silica polymorphs coesite and stishovite were synthesized under water-saturated conditions from a natural granitic composition doped with Li and B. Experiments were performed in a Multi-Anvil apparatus between 4 and 9.1 GPa and 900 and 950 °C, based on the conditions of a subducting continental crust as realistic for the ultrahigh-pressure metamorphic units Dora Maira and Kochetav massifs. Run products consisted of coesite/stishovite?+?kyanite?±?phengite?±?omphacite, and quench material. The synthesized silica polymorphs were successively analyzed by infrared spectroscopy, electron microprobe, and Secondary-Ion Mass Spectrometry (SIMS). No hydrous defects were observed in coesite synthesized at 4 GPa and 900 °C, whereas coesite grown at higher pressures revealed a triplet of infrared absorptions bands at 3575, 3523, and 3459 cm??1, two minor bands at 3535 and 3502 cm??1, and a small band at 3300 cm??1 that was only visible at 7.7 GPa. The total amount of Al was charge-balanced by H and the other monovalent cations. However, the band triplet could not be associated with AlOH defects, while the band doublet was inferred to BOH defects and the small band probably corresponded to interstitial H. Stishovite displayed one dominant band at 3116 cm??1 with a shoulder at 3170 cm??1, and a minor band at 2665 cm??1, probably all associated with AlOH defects. BOH defects were not observed in stishovite, and LiOH defects were neither observed in coesite nor stishovite, probably because of preferentially partition of Li in other phases such as omphacite. The total amount of defect protons increased with pressure and with metal impurity concentrations. The general increase in OH defects in silica polymorphs with increasing pressure (this study) contrasted the negative pressure trend of OH in quartz observed previously from the same starting material, and revealed an incorporation minimum of OH in silica polymorphs around the quartz/coesite phase transition.
Immiscible transition from carbonate rich to silicate rich melts in the 3 GPa melting interval of eclogite + CO2 and genesis of silica undersaturated Oceanic lavas.
Journal of Petrology, Vol. 47, 4, April pp. 647-671.
Earth and Planetary Science Letters, Vol. 474, pp. 309-321.
Canada, Nunavut
deposit - Jericho
Abstract: During ascent, kimberlites react with the lithospheric mantle, entrain and assimilate xenolithic material, loose volatiles and suffer from syn- and post-magmatic alteration. Consequently, kimberlite rocks deviate heavily from their primary melt. Experiments at 7 GPa, 1300–1480?°C, 10–30 wt% CO2 and 0.46 wt% H2O on a proposed primitive composition from the Jericho kimberlite show that saturation with a lherzolitic mineral assemblage occurs only at 1300–1350?°C for a carbonatitic melt with <8 wt% SiO2 and >35 wt% CO2. At asthenospheric temperatures of >1400?°C, where the Jericho melt stays kimberlitic, this composition saturates only in low-Ca pyroxene, garnet and partly olivine. We hence forced the primitive Jericho kimberlite into multiple saturation with a lherzolitic assemblage by adding a compound peridotite. Saturation in olivine, low- and high-Ca pyroxene and garnet was obtained at 1400–1650 °C (7 GPa), melts are kimberlitic with 18–29 wt% SiO2 + Al2O3, 22.1–24.6 wt% MgO, 15–27 wt% CO2 and 0.4–7.1 wt% H2O; with a trade-off of H2O vs. CO2 and temperature. Melts in equilibrium with high-Ca pyroxene with typical mantle compositions have ?2.5 wt% Na2O, much higher than the commonly proposed 0.1–0.2 wt%. The experiments allow for a model of kimberlite origin in the convective upper mantle, which only requires mantle upwelling that causes melting at the depth where elemental carbon (in metal, diamond or carbide) converts to CO2 (at ?250 km). If primary melts leading to kimberlites contain a few wt% H2O, then adiabatic temperatures of 1400–1500?°C would yield asthenospheric mantle melts that are kimberlitic (>18 wt% SiO2 + Al2O3) but not carbonatitic (<10 wt% SiO2 + Al2O3) in composition, carbonatites only forming 100–200?°C below the adiabat. These kimberlites represent small melt fractions concentrating CO2 and H2O and then acquire part of their chemical signature by assimilation/fractionation during ascent in the subcratonic lithosphere.
Contributions to Mineralogy and Petrology, Vol. 173, pp. 76- doi.org/10.1007/ s00410-018-1502-1
Africa, Lesotho
deposit - Letseng
Abstract: The Letšeng-la-Terae kimberlite (Lesotho), famous for its large high-value diamonds, has five distinct phases that are mined in a Main and a Satellite pipe. These diatreme phases are heavily altered but parts of a directly adjacent kimberlite blow are exceptionally fresh. The blow groundmass consists of preserved primary olivine with Fo86?88, chromite, magnesio-ulvöspinel and magnetite, perovskite, monticellite, occasional Sr-rich carbonate, phlogopite, apatite, calcite and serpentine. The bulk composition of the groundmass, extracted by micro-drilling, yields 24-26 wt% SiO2, 20-21 wt% MgO, 16-19 wt% CaO and 1.9-2.1 wt% K2O, the latter being retained in phlogopite. Without a proper mineral host, groundmass Na2O is only 0.09-0.16 wt%. However, Na-rich K-richterite observed in orthopyroxene coronae allows to reconstruct a parent melt Na2O content of 3.5-5 wt%, an amount similar to that of highly undersaturated primitive ocean island basanites. The groundmass contains 10-12 wt% CO2, H2O is estimated to 4-5 wt%, but volatiles and alkalis were considerably reduced by degassing. Mg# of 77.9 and 530 ppm Ni are in equilibrium with olivine phenocrysts, characterize the parent melt and are not due to olivine fractionation. 87Sr/86Sr(i)?=?0.703602-0.703656, 143Nd/144Nd(i)?=?0.512660 and 176Hf/177Hf(i)?=?0.282677-0.282679 indicate that the Letšeng kimberlite originates from the convective upper mantle. U-Pb dating of groundmass perovskite reveals an emplacement age of 85.5?±?0.3 (2?) Ma, which is significantly younger than previously proposed for the Letšeng kimberlite.
Abstract: Silicon carbide (SiC, moissanite) is a common industrial material that is rarely found in terrestrial rocks and meteorites. It has been found to adopt over 300 different crystal structures, most of which are polytypic: they consist of alternating layers of Si and C, with only small stacking faults or shears distinguishing them from one another. In nature, only a few polytypes of SiC have been found, primarily a cubic zincblende type (3C-SiC), several hexagonal wurtzite types (4H-SiC and 6H-SiC), and a rhombohedral type (15R-SiC). Our natural silicon carbide sample is from a Miocene tuff (Yizre’el Valley, Israel) related to interplate alkaline basalt volcanism. Three SiC grains with native silicon and metal silicide inclusions were analyzed using Raman spectroscopy and synchrotron Laue X-ray microdiffraction accompanied by mapping at a 5-8 um resolution. SiC is found to crystallize in only the 4H and 6H polytypes. Due to the crystal orientation of the grains, as well as the significant difference in the c-axis length (~10 vs. ~15 um in 4H and 6H respectively), we were able to unambiguously assign polytypes to each diffraction pattern. Each grain contains large areas where one polytype dominates as a single crystal. In some cases, multiple stacking faults and misoriented polycrystalline aggregates of SiC occur at the 4H/6H interface. In other cases we see intercalation of the 4H and 6H crystals throughout the diffracting volume without a significant change in their crystallographic axes orientation, pointing to a possibly incommensurate crystal structure. Stress and strain are also mapped for all three grains, showing a slight (< 2 ppt) compressive strain in the y direction of all three grains, and a tensile strain in the x and z directions. In the SiC-2 grain, a mostly single-crystalline Si inclusion was found, with an exposed surface diameter of ~30 um. We examine residual strain in Si by both Laue X-ray diffraction and Raman spectroscopy, and find results to generally agree between the two measurements.
Abstract: Moissanite, SiC, is an uncommon accessory mineral that forms under low oxygen fugacity. Here, we analyze natural SiC from a Miocene tuff-sandstone using synchrotron Laue microdiffraction and Raman spectroscopy, in order to better understand the SiC phases and formation physics. The studied crystals of SiC consist of 4H- and 6H-SiC domains, formed from either, continuous growth or, in one case, intergrown, together with native Si. The native Si is polycrystalline, with a large crystal size relative to the analytical beam dimensions (>1-2 ?m). We find that the intergrown region shows low distortion or dislocation density in SiC, but these features are comparatively high in Si. The distortion/deformation observed in Si may have been caused by a mismatch in the coefficients of thermal expansion of the two materials. Raman spectroscopic measurements are discussed in combination with our Laue microdiffraction results. Our results suggest that these SiC grains likely grew from an igneous melt.
A period of global uncertainty ( Blank spot) in the Precambrian history of the southern Siberian Craton and the problem of the transproterozoic supercontinent.
Flourine , yttrium and lanthaide rich cerianite (Ce) from carbonatitic rocks of the Kerimasi volcano and surrounding explosive craters Gregory Rift Tanzania.
Mineralogical Magazine, Vol. 75, 6, pp. 2813-2822.
Geochemistry: Exploration, Environment, Analysis, Vol. 17, 2, pp. 63-91.
Technology
classification
Abstract: Mineral deposit models strategically guide exploration. The lithologies from which these models are built have genetic connotations. Thus, rock classification must be accurate to ensure that mineral exploration is effective and successful. Rock classification is based on mineral proportions, and these are commonly determined by: (1) visual inspection, which is subject to large errors; (2) point counting, which is tedious and time-consuming; (3) image analysis of stained slabs or polished thin sections, which is expensive and constrained by the availability of appropriate stains; and (4) image analysis of spectrometric data, which is expensive. These features make rock classification difficult and undermine its quality, thereby negatively impacting geological conclusions and mineral exploration results. A novel alternative procedure for igneous rock classification involves using whole rock lithogeochemical data for classification on Streckeisen ternary diagrams. This approach employs several calculations that transform: (1) mass-based element concentrations (the original lithogeochemical data produced by the laboratory) sequentially into (2) unstandardized (do not sum to unity) molar element numbers; (3) unstandardized molar mineral numbers; (4) unstandardized volume mineral numbers; and finally (5) standardized (closed; sum to unity) volume mineral concentrations that estimate the mineral modes in rocks. These mineral mode estimates can then be plotted on (projected onto) Streckeisen ternary diagrams, to classify the rocks in the normal manner. This new approach has advantages over conventional classification strategies, in that it is relatively inexpensive, adaptable to all forms of igneous rocks, quantitative, accurate, and precise. Required petrographic information necessary to conduct such a classification includes only knowledge of chemical formulae of the ‘essential’ mineral assemblage. Essential minerals are, here, considered those minerals having concentrations exceeding 5% in 5% of the rocks under consideration. This criterion allows this lithogeochemical classification procedure to be applicable to a wide variety of igneous rocks. This lithogeochemical classification procedure has additional applications beyond the classification of plutonic igneous rocks. For example, if an essential mineral assemblage can be identified or hypothesized, classification of felsic or mafic volcanic rocks can also be achieved. Additionally, an essential mineral assemblage does not have to consist exclusively of igneous minerals. As a result, conversion from molar element numbers to molar mineral numbers can be undertaken using many mineral assemblages. This allows analogous lithogeochemical classification to be undertaken for almost any rock type (e.g. clastic sedimentary rocks, using the calculated proportions of quartz, feldspar, and clay minerals). Consequently, lithogeochemical calculation of the essential mineral modes in rocks can be used to establish mineral zoning maps in space or time, allowing exploration geoscientists to create down-hole logs depicting hydrothermal alteration mineral abundances, or surface maps of hydrothermal alteration zones on a mineral property. To demonstrate this new procedure, results from classifications of metaluminous, peraluminous, and alkaline felsic plutonic and volcanic rocks, and mafic and ultramafic plutonic and volcanic rocks are compared with mineral modes acquired by independent means (visual estimates, point counts, image analysis, spectrometry). These case studies demonstrate that the proposed lithogeochemical classification procedure is as or more accurate than conventional classification methods. Furthermore, because lithogeochemical samples are far larger, and thus more representative than the surfaces used to estimate mineral modes by conventional means, this lithogeochemical classification procedure is also far more precise. The resulting classification is thus especially effective when working with fine-grained rocks where mineral identification and volume estimation is difficult.
Documenting the chemical, physical and thermodynamic changes associated with all possible geochemical reactions in rocks using Gale vector space:Jericho
Geological Association of Canada, Gac-Mac Yellowknife 2007, May 23-25, Volume 32, 1 pg. abstract p.78-79.
Thompson-Howarth error analysis: unbiased alternatives to the large sample method for assessing non-normally distributed measurement error in geochemical samples.
Geochemistry, Exploration, Environment Analysis, Vol. 8, pp. 173-182.
Emerging computer techniques for the minerals industry
Society for Mining, Metallurgy and Exploration (SME), American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME) Publication, 400p. approx. $ 65.00
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 18924 Abstract
Africa
Geochronology
Abstract: Kimberlites provide rich information about the composition and evolution of cratonic lithosphere. Accurate geochronology of these eruptions is key for discerning spatiotemporal trends in lithospheric evolution, but kimberlites can sometimes be difficult to date with available methods. We explored whether (U-Th)/He dating of zircon and perovskite can serve as reliable techniques for determining kimberlite emplacement ages. We obtained zircon and/or perovskite (U-Th)/He (ZHe, PHe) dates from 16 southern African kimberlites. Most samples with abundant zircon yielded reproducible ZHe dates (?15% dispersion) that are in good agreement with published eruption ages. The majority of dated zircons were xenocrystic. Zircons with reproducible dates were fully reset during eruption or resided at temperatures above the ZHe closure temperature prior to entrainment in the kimberlite magma. Not dating hazy and radiation damaged grains can help avoid anomalous results for more shallowly sourced zircons that underwent incomplete damage annealing and/or partial He loss during the eruptive process. All seven kimberlites dated with PHe yielded reproducible (?15% dispersion) and reasonable results. We conducted two preliminary perovskite 4He diffusion experiments, which suggest a PHe closure temperature of >300°C. Perovskite in kimberlites is unlikely to be xenocrystic and its relatively high temperature sensitivity suggests that PHe dates will typically record emplacement rather than postemplacement processes. ZHe and PHe geochronology can effectively date kimberlite emplacement and provide useful complements to existing techniques.
Abstract: Topographic uplift of the southern African Plateau is commonly attributed to mantle causes, but the links between mantle processes, uplift, and erosion patterns are not necessarily straightforward. We acquired apatite (U-Th)/He (AHe) dates from eight kimberlite and basement samples from the lower reaches of the large westward-draining Orange River system with the goal of evaluating the roles of lithospheric modification and river incision on the erosion history here. Average AHe dates range from 79 to 118 Ma and thermal history models suggest that most samples are consistent with a main erosion phase at ca. 120-100 Ma, with some variability across the region indicating a complex erosion history. Major erosion overlaps with the timing of strong lithospheric thermochemical modification as recorded in xenoliths from the studied kimberlites, but the denudation pattern does not mimic the northward progression of lithospheric alteration across the study region. We attribute this area’s denudation history to a combination of mantle effects, rifting, establishment of the Orange River outlet at its current location, and later faulting. When considering these results with other kimberlite-derived surface histories from an ?1000-km-long E-W transect across the plateau, an eastward-younging trend in denudation is evident. The interplay of mantle processes and the shape of the large, west-draining Orange River basin likely control this first order-pattern.
Abstract: When one tectonic plate dives beneath another at a subduction zone, it recycles huge amounts of water and other chemicals into Earth’s mantle. The sinking plate carries seawater trapped in sediments and crust or chemically bound in minerals like serpentine. Later release of this water in the mantle contributes to key geological processes, such as earthquakes and the formation of volcano-feeding magma.
By volume, the largest portion of a subducting plate is its bottom layer, which comprises upper mantle material. Estimates of the amount of water in downgoing slabs of upper mantle vary widely: Some suggest that worldwide, subduction zones have swallowed more than two oceans’ worth of water in the past 540 million years. However, new research by Miller et al. suggests that water transport at the Middle America Trench subduction zone is an order of magnitude less than previously estimated. As a plate approaches a subduction zone, it bends downward, causing faults to form. Models and earlier observations have suggested that this bending and faulting allow seawater to infiltrate into the upper mantle, where it fills cracks in fault zones, reacts with olivine to produce serpentine, and is later carried deeper into the subduction zone. Previous estimates of how much water reaches the upper mantle along bending faults have relied on measurements of the speed of seismic waves as they pass through a subducting plate. However, those measurements and estimates could not discern whether the upper mantle layer is uniformly hydrated or whether water is confined to bending fault zones. To address that limitation, the new study accounted for seismic anisotropy characterizing how the speed of seismic waves depends on the direction they travel through a material. The researchers used data collected by seafloor seismometers to measure seismic anisotropy along the Middle America Trench near Nicaragua, which enabled a much more detailed picture of upper mantle hydration. The data revealed that in the region studied, water storage in the upper mantle is limited to serpentinized fault zones that thin rapidly with depth, suggesting that fault dynamics and serpentinization reaction kinetics prevent seawater from hydrating the mantle between bending faults. New estimates of water transport that incorporate this finding are an order of magnitude lower than previous estimates for the Middle America Trench. Because the same processes occur at other subduction zones, the researchers report that far less water may be transported worldwide than previously estimated. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/2020JB020982, 2021)
Abstract: A 5.6-km-long line of refraction and reflection seismic data spanning the Pliocene-Pleistocene fill of the Olduvai Basin, Tanzania is presented. The line is oriented along a northwest-southeast profile through the position of Olduvai Gorge Coring Project (OGCP) Borehole 2A. Our aims are to (1) delineate the geometry of the basin floor by tracing bedrock topography of the metaquartzitic and gneissic basement, (2) map synsedimentary normal faults and trace individual strata at depth, and (3) provide context for the sequence observed in OGCP cores. Results with refraction tomography and poststack migration show that the maximum basin depth is around 405?m (±25?m) in the deepest portion, which quadruples the thickness of the basin-fill previously known from outcrops. Variations in seismic velocities show the positions of lower density lake claystones and higher density well-cemented sedimentary sequences. The Bed I Basalt lava is a prominent marker in the refraction seismic results. Bottom-most sediments are dated to >2.2?Ma near where Borehole 2A bottoms out at the depth of 245?m. However, the seismic line shows that the basin-fill reaches a maximum stratigraphic thickness of around 380?m deep at Borehole 2A, in the western basin where the subsidence was greatest. This further suggests that potential hominin palaeoenvironments were available and preserved within the basin-fill possibly as far back as around 4?Ma, applying a temporal extrapolation using the average sediment accretion rate.
Abstract: We report new geological, mineralogical, geochemical and geochronological data about the Katugin Ta-Nb-Y-Zr (REE) deposit, which is located in the Kalar Ridge of Eastern Siberia (the southern part of the Siberian Craton). All these data support a magmatic origin of the Katugin rare-metal deposit rather than the previously proposed metasomatic fault-related origin. Our research has proved the genetic relation between ores of the Katugin deposit and granites of the Katugin complex. We have studied granites of the eastern segment of the Eastern Katugin massif, including arfvedsonite, aegirine-arfvedsonite and aegirine granites. These granites belong to the peralkaline type. They are characterized by high alkali content (up to 11.8?wt% Na2O?+?K2O), extremely high iron content (FeO?/(FeO??+?MgO)?=?0.96-1.00), very high content of most incompatible elements - Rb, Y, Zr, Hf, Ta, Nb, Th, U, REEs (except for Eu) and F, and low concentrations of CaO, MgO, P2O5, Ba, and Sr. They demonstrate negative and CHUR-close ?Nd(t) values of 0.0…?1.9. We suggest that basaltic magmas of OIB type (possibly with some the crustal contamination) represent a dominant part of the granitic source. Moreover, the fluorine-enriched fluid phases could provide an additional source of the fluorine. We conclude that most of the mineralization of the Katugin ore deposit occurred during the magmatic stage of the alkaline granitic source melt. The results of detailed mineralogical studies suggest three major types of ores in the Katugin deposit: Zr mineralization, Ta-Nb-REE mineralization and aluminum fluoride mineralization. Most of the ore minerals crystallized from the silicate melt during the magmatic stage. The accessory cryolites in granites crystallized from the magmatic silicate melt enriched in fluorine. However, cryolites in large veins and lens-like bodies crystallized in the latest stage from the fluorine enriched melt. The zircons from the ores in the aegirine-arfvedsonite granite have been dated at 2055?±?7?Ma. This age is close to the previously published 2066?±?6?Ma zircon age of the aegirine-arfvedsonite granites, suggesting that the formation of the Katugin rare-metal deposit is genetically related to the formation of peralkaline granites. We conclude that Katugin rare-metal granites are anorogenic. They can be related to a Paleoproterozoic (?2.05?Ga) mantle plume. As there is no evidence of the 2.05?Ga mantle plume in other areas of southern Siberia, we suggest that the Katugin mineralization occurred on the distant allochtonous terrane, which has been accreted to Siberian Craton later.
Abstract: The Arbarastakh Neoproterozoic ultramafic carbonatite complex is located in the southwestern part of the Siberian Craton (Aldan Shield) and contains ore-bearing Zr-Nb (REE) carbonatites and phoscorites. Carbonatites are mainly represented by calcite and silicocarbonatite varieties. The primary minerals composing the carbonatites are calcite and dolomite, as well as phlogopite, clinopyroxene, fluorapatite, amphibole, fluorite, K-feldspar and feldspathoids. Olivine (forsterite), Ti-magnetite, apatite, phlogopite, calcite, dolomite and the minor spinel group minerals form the primary phoscorites. The ore-bearing Zr-Nb mineral assemblages of the phoscorites and carbonatites include accessory zircon, zirconolite, perovskite, pyrochlore and baddeleyite. The Ba-Sr-REE hydrothermal mineralisation consists of ancylite-(Ce), bastnaesite-(Ce) and burbankite, as well as barite-celestite, strontianite, barytocalcite, and rare Cu-Fe sulphides. The silicocarbonatites and carbonatites formed in multiple stages from a single alkaline Ca-Na-K-silicocarbonatite melt, while the phoscorites are products of differentiation of the carbonatitic melt and were crystallised from an Fe-rich phosphate-carbonate melt at temperatures of more than 720 °C. The silicate-phosphate-carbonate melts were responsible for the Zr-Nb mineralisation of the carbonatites at temperatures of more than 540-575 °C; the hydrothermal REE-bearing mineral assemblages crystallised from saline (60-70 wt%) carbonatitic fluids of Na-Ca-Mg-F-carbonate composition at a minimum temperature range of 350-300 °C. The Ca-Sr-carbonate as well as the Na-hydro-carbonate fluids were responsible for the Ba-Sr-REE mineralisation of the phoscorites at ~500-480 and 450-430 °C.
Abstract: The Archean Yilgarn Craton in Western Australia is intruded by numerous mafic dykes of varying orientations, which are poorly exposed but discernible in aeromagnetic maps. Previous studies have identified two craton-wide dyke swarms, the 2408?Ma Widgiemooltha and the 1210?Ma Marnda Moorn Large Igneous Provinces (LIP), as well as limited occurrences of the 1075?Ma Warakurna LIP in the northern part of the craton. We report here a newly identified NW-trending mafic dyke swarm in southwestern Yilgarn Craton dated at 1888?±?9?Ma with ID-TIMS U-Pb method on baddeleyite from a single dyke and at 1858?±?54?Ma, 1881?±?37 and 1911?±?42?Ma with in situ SHRIMP U-Pb on baddeleyite from three dykes. Preliminary interpretation of aeromagnetic data indicates that the dykes form a linear swarm several hundred kilometers long, truncated by the Darling Fault in the west. This newly named Boonadgin dyke swarm is synchronous with post-orogenic extension and deposition of granular iron formations in the Earaheedy basin in the Capricorn Orogen and its emplacement may be associated with far field stresses. Emplacement of the dykes may also be related to initial stages of rifting and formation of the intracratonic Barren Basin in the Albany-Fraser Orogen, where the regional extensional setting prevailed for the following 300?million years. Recent studies and new paleomagnetic evidence raise the possibility that the dykes could be part of the coeval 1890?Ma Bastar-Cuddapah LIP in India. Globally, the Boonadgin dyke swarm is synchronous with a major orogenic episode and records of intracratonic mafic magmatism on many other Precambrian cratons.
Abstract: The Archean Yilgarn Craton in Western Australia hosts at least five generations of Proterozoic mafic dykes, the oldest previously identified dykes belonging to the ca. 2408-2401?Ma Widgiemooltha Supersuite. We report here the first known Archean mafic dyke dated at 2615?±?6?Ma by the ID-TIMS U-Pb method on baddeleyite and at 2610?±?25?Ma using in situ SHRIMP U-Pb dating of baddeleyite. Aeromagnetic data suggest that the dyke is part of a series of NE-trending intrusions that potentially extend hundreds of kilometres in the southwestern part of the craton, here named the Yandinilling dyke swarm. Mafic magmatism at 2615?Ma was possibly related to delamination of the lower crust during the final stages of assembly and cratonisation, and was coeval with the formation of late-stage gold deposit at Boddington. Paleogeographic reconstructions suggest that the Yilgarn and Zimbabwe cratons may have been neighbours from ca. 2690?Ma to 2401?Ma and if the Zimbabwe and Kaapvaal cratons amalgamated at 2660-2610?Ma, the 2615?Ma mafic magmatism in the southwestern Yilgarn Craton may be associated with the same tectonic event that produced the ca. 2607-2604?Ma Stockford dykes in the Central Zone of the Limpopo Belt. Paleomagnetic evidence and a similar tectonothermal evolution, including coeval low-pressure high-temperature metamorphism, voluminous magmatism, and emplacement of mafic dykes, support a configuration where the northern part of the Zimbabwe Craton was adjacent to the western margin of the Yilgarn Craton during the Neoarchean. Worldwide, reliably dated mafic dykes of this age have so far been reported from the Yilgarn Craton, the Limpopo Belt and the São Francisco Craton.
Osmium isotopes in Baffin Island and West Greenland picrites: implications for the 187 Os and 188 Os composition of the convection mantle and nature 3He/4he
Earth and Planetary Interiors, Vol. 278, 3-4, pp. 267-277.
Abstract: Olivine is distinguished from all other minerals in providing a remarkable chemical narrative about magmatic processes that occurred in Earth’s crust, mantle, and core over the entire age of Earth history. Olivines in mantle peridotite have Ni contents and Mg numbers that were largely produced by equilibrium crystallization in an early turbulently convecting magma ocean; subsequent stages of partial melting operated to slightly elevate Ni and Mg number in residual olivines. Olivines from Archean komatiites from the Abitibi greenstone belt have Ni contents and Mg numbers that are consistent with an extensively melted peridotite source at great depths in the mantle. Olivines from basaltic oceanic crust, the Icelandic mantle plume and other Phanerozoic occurrences have compositions that record magma chamber crystallization, recharge, mixing, and partial melting. Olivines from the present-day Icelandic mantle plume have compositions that are consistent the melting of a peridotite source; unlike Hawaii, the melting of recycled crust as a distinct pyroxenite lithology is not evident in the olivine chemistry of Iceland. Paleocene picrites from Baffin Island and West Greenland from the ancient Icelandic plume have olivines with Ni contents that are consistent with either Ni-rich peridotite that formed by core-mantle interaction or by low-pressure crystallization of hot and deep magmas. In general, hot magma oceans, mantle plumes, and ambient mantle magmatism form in ways that are captured by the compositions of the olivine crystals that they contain.
Journal of the Russell Society, Vol. 18, pp. 24-45.
Europe, United Kingdom
History
Abstract: The presence of quartz crystals in t he soils around Buxton has been known for centuries, and at one time theses so-called 'Buxton Diamonds' were, from published sources, apparently realtively abundant, and well-knwn to both visitors and to commentators. However, few specimens survive in museum collections, and there is considerable confusion in published accounts as to what exactly constitutes a 'Buxton Diamond'. No satisfactory description or explanation of their origin r occurrence has hitherto been published. Attractive specimens of quartz and amethyst are known from various occurrences in the Peak District, associated with igneous rocks, but these are not true 'Buxton Diamonds' . This paper presents the history of 'Buxton Diamonds', and confirms the occurrence of these, sometime highly attractive, crystals of quartz in the limestone of Diamond Hill and the surrounding area.
Organic petrology, organic geochemistry, palynology and petrophysics dat a from Lac de Gras kimberlites and associated sedimentary rocks and xenoliths.
Geological Survey of Canada Open File, No. 4272, 1 CD $ 32.50
Geostandards and Geoanalytical Research, in press available
Technology
REE mass fractions
Abstract: Olivine offers huge, largely untapped, potential for improving our understanding of magmatic and metasomatic processes. In particular, a wealth of information is contained in rare earth element (REE) mass fractions, which are well studied in other minerals. However, REE data for olivine are scarce, reflecting the difficulty associated with determining mass fractions in the low ng g?1 range and with controlling the effects of LREE contamination. We report an analytical procedure for measuring REEs in olivine using laser ablation quadrupole-ICP-MS that achieved limits of determination (LOD) at sub-ng g?1 levels and biases of ~ 5-10%. Empirical partition coefficients (D values) calculated using the new olivine compositions agree with experimental values, indicating that the measured REEs are structurally bound in the olivine crystal lattice, rather than residing in micro-inclusions. We conducted an initial survey of REE contents of olivine from mantle, metamorphic, magmatic and meteorite samples. REE mass fractions vary from 0.1 to double-digit ng g?1 levels. Heavy REEs vary from low mass fractions in meteoritic samples, through variably enriched peridotitic olivine to high mass fractions in magmatic olivines, with fayalitic olivines showing the highest levels. The variable enrichment in HREEs demonstrates that olivine REE patterns have petrological utility.
Abstract: It is now well established that the cratonic sub-continental lithospheric mantle (SCLM) represents a residue of extensively melted fertile peridotite. The widespread occurrence of garnet in the Archaean SCLM remains a paradox because many experiments agree that garnet is exhausted beyond c. 20% melting. It has been suggested that garnet may have formed by exsolution from Al-rich orthopyroxene [1,2,3]. However, the few examples of putative garnet exsolution in cratonic samples remain exotic and have not afforded a link to garnet that occurs as distinct grains in granular harzburgite. We present crystallographic (EBSD), petrographic and chemical (SEM-EDS and LA-ICP-MS) data for an exceptionally well-preserved orthopyroxene megacryst juxtaposed against granular harzburgite. Garnet lamellae within the megacryst show crystallographic continuity and have a strong fabric relative to the host orthopyroxene, strongly indicating that the megacryst formed by exsolution. Garnet lamellae are sub-calcic Cr-pyropes with sinusoidal rare earth element patterns, while the orthopyroxene host is high-Mg enstatite; the reconstructed precursor is clinoestatite. The megacryst shows evidence for disintegrating into granular peridotite, and garnet and orthopyroxene within the granular peridotite are texturally and chemically identical to equivalent phases in the megacryst. Collectively, this evidence supports a common origin for the granular and exsolved portions of the sample. The compositions of the exsolved Cr pyrope and enstatite are typical of harzburgites and depleted lherzolites from the SCLM. Furthermore, garnet inclusions within orthopyroxene in several granular peridotites exhibit the same fabric as those in the exsolved megacryst. We hypothesise that clinoenstatite was a common phase in cratonic SCLM and that exsolution is the likely origin of many sub-calcic garnets in depleted peridotites.
Abstract: It is well established that the cratonic subcontinental lithospheric mantle (C-SCLM) represents a residue of extensively melted peridotite. The widespread occurrence of garnet in C-SCLM remains a paradox because experiments show that it should be exhausted beyond ?20% melting. It has been suggested that garnet may have formed by exsolution from Al-rich orthopyroxene; however, the few documented examples of garnet exsolution in cratonic samples are exotic and do not afford a direct link to garnet in granular harzburgite. We report crystallographic, petrographic, and chemical data for an exceptionally well preserved orthopyroxene megacryst containing garnet lamellae, juxtaposed against granular harzburgite. Garnet lamellae are homogeneously distributed within the host orthopyroxene and occur at an orientation that is unrelated to orthopyroxene cleavage, strongly indicating that they formed by exsolution. Garnet lamellae are subcalcic Cr-pyrope, and the orthopyroxene host is high-Mg enstatite; these phases equilibrated at 4.4 GPa and 975 °C. The reconstructed precursor is a high-Al enstatite that formed at higher pressure and temperature conditions of ?6 GPa and 1750 °C. The megacryst shows evidence for disintegrating into granular peridotite, and garnet and orthopyroxene within the granular peridotite are texturally and chemically identical to equivalent phases in the megacryst. Collectively, this evidence supports a common origin for the granular and exsolved portions of the sample. We hypothesize that high-Al enstatite was a common phase in the C-SCLM and that exsolution during cooling and stabilization of the C-SCLM could be the origin of most subcalcic garnets in depleted peridotites.
Circumferential Faulting Around Wells Creek Basin, Houston And Stewart Counties, Tennessee- a Manuscript by Safford, J.m. and Lander, D.w.t. : Circa 1895.
Tennessee Academy of Science Journal, Vol. 41, No. 1, PP. 37-48.
Global
Kimberlite, Western Tennessee, Central States, Cryptoexplosion
Variable temperature 27Al and 29Si NMR studies of synthetic forsterite and Fe bearing Dora Maira pyrope garnet: temperature dependence and mechanisms of paramagnetically shifted peaks.
Smit, K.V., Shirey, S.B., Stern, R.A., Steele, A., Wang, W.
Diamond growth from C-H-N-O recycled fluids in the lithosphere: Evidence from CH4 micro-inclusions and dleta 13 C-Delta 15 N-N content in Marange mixed-habit diamonds.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 9187 Abstract
Africa, Zimbabwe, Sierra Leone
Deposit - Marange, Zimmi
Abstract: Traditional models for diamond formation within the lithospheric mantle invoke either carbonate reduction or methane oxidation. Both these mechanisms require some oxygen exchange with the surrounding wall-rock at the site of diamond precipitation. However, peridotite does not have sufficient buffering capacity to allow for diamond formation via these traditional models and instead peridotitic diamonds may form through isochemical cooling of H 2 O-rich CHO fluids [1]. Marange mixed-habit diamonds from eastern Zimbabwe provide the first natural confirmation of this new diamond growth model [2]. Although Marange diamonds do not contain any silicate or sulphide inclusions, they contain Ni-N-vacancy complexes detected through photoluminescence (PL) spectroscopy that suggest the source fluids equilibrated in the Ni-rich depleted peridotitic lithosphere. Cuboid sectors also contain abundant micro-inclusions of CH 4 , the first direct observation of reduced CH 4-rich fluids that are thought to percolate through the lithospheric mantle [2]. In fluid inclusion-free diamonds, core-to-rim trends in ? 13 C and N content are used to infer the speciation of the diamond-forming fluid. Core to rim trends of increasing ? 13 C with decreasing N content are interpreted as diamond growth from oxidized CO 2-or carbonate-bearing fluids. Diamond growth from reduced species should show the opposite trends-decreasing ? 13 C from core to rim with decreasing N content. Within the CH 4-bearing growth sectors of Marange diamonds, however, such a 'reduced' trend is not observed. Rather, ? 13 C increases from core to rim within a homogeneously grown zone [2]. These contradictory observations can be explained through either mixing between CH 4-and CO 2-rich end-members of hydrous fluids [2] or through closed system precipitation from an already mixed CH 4-CO 2 H 2 O-maximum fluid with XCO 2 (CO 2 /[CO 2 +CH 4 ]) between 0.3 and 0.7 [3]. These results demonstrate that Marange diamonds precipitated from cooling CH 4-CO 2-bearing hydrous fluids rather than through redox buffering. As this growth mechanism applies to both the fluid-rich cuboid and gem-like octahedral sectors of Marange diamonds, a non-redox model for diamond formation from mixed CH 4-CO 2 fluids is indicated for a wider range of gem-quality peridotitic diamonds. Indeed, at the redox conditions of global diamond-bearing lithospheric mantle (FMQ-2 to-4; [4]), CHO fluids are strongly water-dominated and contain both CH 4 and CO 2 as dominant carbon species [5]. By contrast diamond formation in eclogitic assemblages, through either redox buffering or cooling of carbon-bearing fluids, is not as well constrained. Zimmi diamonds from the West African craton have eclogitic sulphide inclusions (with low Ni and high Re/Os) and formed at 650 Ma, overlapping with the timing of subduction [6]. In one Zimmi diamond, a core to rim trend of decreasing ? 13 C (-23.4 to-24.5 %¸) and N content is indicative of formation from reduced C 2 H 6 /CH 4-rich fluids, likely derived from oceanic crust recycled during Neoproterozoic subduction. Unlike mixed CH 4-CO 2 fluids near the water maximum, isochemical cooling or ascent of such reduced CHO fluids is not effficient at diamond precipitation. Furthermore, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the ?1 internal variation seen. Consequently, this Zimmi eclogitic diamond likely formed through redox buffering of reduced subduction-related fluids, infiltrating into sulphide-bearing eclogite.
Abstract: The formation of Iron Oxide-Apatite (IOA) systems has long been enigmatic. The compositions of both magnetite and apatite and the other component elements suggest derivation from high temperature (T) magmatic systems, with genetic models including iron oxide magmas or igneous magnetite and apatite flotation. Ideas related to the role of H2O and associated oxidative mechanisms have resurfaced from models of the late 1960s. As such, salt melts forming in open, differentially degassing systems could represent an end-member to the formation of IOA deposits. Another end-member involves autometasomatic decarbonation reactions involving ferroan carbonatites with co-genetic melts or host rocks generating CO2 capable of oxidizing carbonatites to enhance magnetite-apatite saturation. The syntectic decarbonation end-member presented here examines the reactions of carbonate melts of mantle origin or from syntectic reactions with limestone, with cogenetic silicate magmas. Although carbonate and silicate melts can coexist at magmatic pressure (P) and T, their compositions must be peralkalic. However, as P decreases, immiscibility or reactivity between these melts is such that CO2 is exsolved (decarbonation) to the point that at near surface conditions, decarbonation is complete. The addition of CO2 to silicate melt will drive the conversion of FeO to Fe2O3 in order to make carbon monoxide (CO), thus shifting the redox equilibria. For most silicate magmas, the amount of dissolved carbonate and CO2 is quite limited, and differential CO2 degassing results. These carbonate: silicate melt reactions then may result in oxidation of the silicate magma, to enhance immiscibility of IOA (liquation) and elemental partitioning associated with liquid-liquid immiscibility. This could be an oxidative mechanism for Fe-Ti tholeiites (ferrobasalts) and diorites to reach a two-liquid field and form IOA melts via liquation. Carbonates would typically be consumed in these reactions, although CO2 is an important degassing product that would substantially increase ?V of the reaction, which has implications during high-level emplacement.
Abstract: Studies on fluid inclusions in carbonatitic rocks are essential to understand the physicochemical processes involved in carbonatite-related hydrothermal ore mineralization. Although little is known about the composition of carbonatite-derived fluids. We investigated fluid inclusions in the Kaiserstuhl carbonatites, SW Germany [1,2] and identified four different types typically known from carbonatitic systems worldwide [3]: (I): Vapor-poor H2O-NaCl fluids with <50 wt.% salinity. (II): Vapor-rich H2O-NaCl-CO2 fluids with <5 wt.% salinity. (III): Multi-component fluids with high salinity and CO2. (IV): Multi-component fluids with high salinity, no CO2. Homogenization temperatures (156 to 530°C) of all fluid types generally show a wide range [this study, 2]. Primary type I fluid inclusions occur in early magmatic olivine/monticellite, as well as paragenetically later apatites and calcites [2]. This indicates a ubiquitous existence of a saline brine, which does not reach saturation with respect to halite, during early to late crystallization stages. Liquidus surface modelling based quantifications for fluid type III suggest that carbonatite melts predomonantly exsolve Na-K-sulfate-carbonate/bicarbonate-chloride brines (type III or IV, respectively). Such fluid inclusions, with type III (CO2-free) on one side and type IV (and II, both CO2-rich) on the other side, may represent immiscible fluids that were trapped after segregation by boiling from a parental highly saline brine (type I). Fluid boiling, in turn, is probably triggered by a rapid pressure release during “pneumatic hammer-like,” discontinuous melt ascent.
Geochimica et Cosmochimica Acta, Vol. 277, pp. 224-242. pdf
Europe, Germany
carbonatite
Abstract: Studies of fluid inclusions in carbonatitic rocks are essential for understanding physicochemical processes involved in carbonatite-related hydrothermal ore mineralization and fenitization. However, the composition of many carbonatite-derived fluids is challenging to quantify, which hampers their detailed interpretation. Here, we present a systematic study of microthermometry of fluid inclusions found in carbonatites from the Kaiserstuhl (SW Germany), and a simple numerical model to estimate the compositions of such fluids, which are typical of numerous carbonatites worldwide. Four types of fluid inclusions have been identified in the Kaiserstuhl carbonatites: (I) vapor-poor H2O-NaCl fluids with <50?wt.% salinity; (II) vapor-rich H2O-NaCl-CO2 fluids with <5?wt.% salinity; (III) multi-component fluids with high salinity and high CO2 contents; and (IV) multi-component fluids with high salinity but little to no CO2. At present, it is only possible to quantify fluid compositions for types I and II. For the complex types III and IV, we conducted predictive modeling of the liquidus surface based on the Margules equations. The results suggest that carbonatite melts predominantly exsolve Na-K-sulfate-carbonate/bicarbonate-chloride brines (types III or IV). Such fluid inclusions may represent immiscible fluids that were trapped after segregation by boiling from a parental highly saline brine (type I). Fluid boiling, in turn, was probably triggered by a rapid pressure release during melt ascent. The present model enables quantification of fluid compositions associated with carbonatitic magmatism.
Geochimica et Cosmochimica Acta, Vol. 277, pp. 224-242. pdf
Europe, Germany
deposit - Kaiserstuhl
Abstract: Studies of fluid inclusions in carbonatitic rocks are essential for understanding physicochemical processes involved in carbonatite-related hydrothermal ore mineralization and fenitization. However, the composition of many carbonatite-derived fluids is challenging to quantify, which hampers their detailed interpretation. Here, we present a systematic study of microthermometry of fluid inclusions found in carbonatites from the Kaiserstuhl (SW Germany), and a simple numerical model to estimate the compositions of such fluids, which are typical of numerous carbonatites worldwide. Four types of fluid inclusions have been identified in the Kaiserstuhl carbonatites: (I) vapor-poor H2O-NaCl fluids with <50?wt.% salinity; (II) vapor-rich H2O-NaCl-CO2 fluids with <5?wt.% salinity; (III) multi-component fluids with high salinity and high CO2 contents; and (IV) multi-component fluids with high salinity but little to no CO2. At present, it is only possible to quantify fluid compositions for types I and II. For the complex types III and IV, we conducted predictive modeling of the liquidus surface based on the Margules equations. The results suggest that carbonatite melts predominantly exsolve Na-K-sulfate-carbonate/bicarbonate-chloride brines (types III or IV). Such fluid inclusions may represent immiscible fluids that were trapped after segregation by boiling from a parental highly saline brine (type I). Fluid boiling, in turn, was probably triggered by a rapid pressure release during melt ascent. The present model enables quantification of fluid compositions associated with carbonatitic magmatism.
Abstract: Aqueous fluids are critical agents in the geochemical evolution of Earth’s interior. Fluid circulation and fluid-rock reactions in the Earth take place at temperatures ranging from ambient to magmatic, at pressures from ambient to extreme, and involve fluids that range from nearly pure H2O through to complex, multicomponent solutions. Consequently, the physical and chemical properties of hydrothermal fluids vary widely as functions of geologic setting; this variation strongly impacts fluid-driven processes. This issue will focus on the nature of geologic fluids at hydrothermal conditions and how such fluids affect geologic processes in some major settings.
Earth Science Reviews , Vol. 215, 103509, 27p. Pdf
Mantle
magmatism
Abstract: Carbonatites are formed from volatile-rich melts, commonly associated with a characteristic hydrothermal footprint. However, studies of their fluid inclusions are relatively scarce and heterogeneous in terms of detail and completeness of the data presented. Here, we review and discuss comprehensively the results of previous studies and derive a general model for the formation and properties of fluids associated with carbonatitic magmatism. Worldwide, four types of fluid inclusion occur in carbonatites: (type I/HS) vapour-poor H2O-NaCl fluids with up to 50 wt% salinity; (type II/HC) vapour-rich H2O-NaCl-CO2 fluids with <5 wt% salinity; (type III/DS) multi-component fluids with high salinity and without CO2; and (type IV/CDS) multi-component fluids with high salinity and high CO2. This global dataset suggests continuous fluid release from deep to shallow-level intrusions. Modelling of the liquidus surface indicates that carbonatite magmas generally exsolve a saline brine (type I/HS). This brine separates/evolves into a Na-K-sulfate-carbonate/bicarbonate-chloride brine with or without CO2 (types III/DS and IV/CDS), trapped together with low salinity CO2-rich fluids produced by immiscibility. Fluid immiscibility is related to rapid pressure release during fast, forceful and discontinuous magma ascent, which we envisage as a "pneumatic jackhammer" model for carbonatite ascent and emplacement. In this model, cyclic and progressive fluid flux via pressure build-up and subsequent catastrophic pressure release results in a self-sustaining crustal ascent of the buoyant, low-viscosity magma. This process allows for rapid and efficient magma ascent, in particular above the brittle-ductile transition zone, where pressures that prevailed during apatite crystallization have been estimated in numerous complexes. Moreover, this model provides an explanation for the apparent absence of shallow carbonatite magma chambers (in a classical sense) and identifies fenitization as a phenomenon induced by both fluids released during magma ascent and residual fluids.
Sand, K.K., Nielsen, T.F.D., Secher, K., Steenfelt, A.
Kimberlite and carbonatite exploration in southern West Greenland: summary of previous activities and recent work by the kimberlite research group at the Geological Survey of Denmark and Greenland.
Steenfelt, A., Jensen, S.M., Nielsen, T.F.D., Sand, K.K.
Provinces of ultramafic lamprophyre dykes, kimberlite dykes and carbonatite in West Greenland characterised by minerals and chemical components in surface media.
Astheospheric source of Neoproterozoic and Mesozoic kimberlites from the North Atlantic craton, West Greenland: new high precision U-Pb and Sr-Nd isotope dat a on perovskite.
Earth and Planetary science Letters, Vol. 466, pp. 152-167.
Mantle
Metasomatism, magma, carbonatite
Abstract: Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr-Nd-Hf-Pb-Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ??Hf??Hf coupled with View the MathML source>+5‰?7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05-3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (?3.6 to View the MathML source?2.4‰?13C for carbonatites versus ?5.7 to View the MathML source?3.6‰?13C for kimberlites) require open-system fractionation at magmatic temperatures.
Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of ‘pure’ carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16-33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly differentiated ‘cratonic carbonatites’ have only little in common with those of metasomatic agents that act on the deeper lithosphere. Consequently, carbonatite trace element systematics should only be used with caution when constraining carbon mobility and metasomatism at mantle depths. Regardless of the exact nature of carbonate-bearing melts within the mantle lithosphere, they play an important role in enrichment processes, thereby decreasing the stability of buoyant cratons and promoting rift initiation - as exemplified by the Mesozoic-Cenozoic breakup of the North Atlantic craton.
Earth and Planetary Science Letters, Vol. 557, doi.org/10.1016/ j.epsl.2020.116730 9p. Pdf
Europe, Greenland
meteorite
Abstract: Large meteorite impacts have a profound effect on the Earth's geosphere, atmosphere, hydrosphere and biosphere. It is widely accepted that the early Earth was subject to intense bombardment from 4.5 to 3.8 Ga, yet evidence for subsequent bolide impacts during the Archean Eon (4.0 to 2.5 Ga) is sparse. However, understanding the timing and magnitude of these early events is important, as they may have triggered significant change points to global geochemical cycles. The Maniitsoq region of southern West Greenland has been proposed to record a ?3.0 Ga meteorite impact, which, if confirmed, would be the oldest and only known impact structure to have survived from the Archean. Such an ancient structure would provide the first insight into the style, setting, and possible environmental effects of impact bombardment continuing into the late Archean. Here, using field mapping, geochronology, isotope geochemistry, and electron backscatter diffraction mapping of 5,587 zircon grains from the Maniitsoq region (rock and fluvial sediment samples), we test the hypothesis that the Maniitsoq structure represents Earth's earliest known impact structure. Our comprehensive survey shows that previously proposed impact-related geological features, ranging from microscopic structures at the mineral scale to macroscopic structures at the terrane scale, as well as the age and geochemistry of the rocks in the Maniitsoq region, can be explained through endogenic (non-impact) processes. Despite the higher impact flux, intact craters from the Archean Eon remain elusive on Earth.
Abstract: The jigsaw fit of Earth’s continents, which long intrigued map readers and inspired many theories, was explained about 60 years ago when the foundational processes of plate tectonics came to light. Topographic and magnetic maps of the ocean floor revealed that the crust—the thin, rigid top layer of the solid Earth—is split into plates. These plates were found to shift gradually around the surface atop a ductile upper mantle layer called the asthenosphere. Where dense oceanic crust abuts thicker, buoyant continents, the denser crust plunges back into the mantle beneath. Above these subduction zones, upwelling mantle melt generates volcanoes, spewing lava and creating new continental crust.
Butler, J.P., Jamieson, R.A., Steenkamp, H.M., Robinson, P.
Discovery of coesite eclogite from the Nordyane UHP domain, Western Gneiss region, Norway: field relations, metamorphic history and tectonic significance.
Journal of Metamorphic Geology, in press available
Journal of the Southern African Institute of Mining and Metallurgy, 8p. Pdf
Africa, South Africa
legal
Abstract: Despite the importance that barrier identification has for policy-making and industry stakeholders alike; little guidance exists on consistent processes to systematically identify barriers that are hindering the different sectors of a value chain’s expansion and growth. This article describes the development of a framework that supports the identification of barriers to growth in mineral value chains. The resultant process was applied to the case of the manganese value chain in South Africa, and revealed 31 barriers within this industry. The results were validated by a panel of experts and the feedback was used to rework and improve the framework.
Preliminary report on the Texaco deep Precambrian drill hole in The midcontinent rift system
United States Geological Survey (USGS) Open file, United States Geological Survey (USGS)-Missouri G.S. Symp: Mineral resource potential of, p. 2. (abstract.)
Proterozoic mantle xenoliths in ultramafic dykes near Wawa, Ontario: implications for the lithospheic mantle underneath the central North American craton.
Geological Society of America Annual Meeting ABSTRACTS, Nov. 7-10, Paper 17-7, Vol. 36, 5, p. 47.
Contributions to Mineralogy and Petrology, Vol. 176, 10, 28p. Pdf
Canada, Quebec
deposit - Renard
Abstract: This study aims to constrain the nature of kimberlite-xenolith reactions and the fluid origin for Kimberley-type pyroclastic kimberlite (KPK). KPKs are characterized by an abundance of basement xenoliths (15-90%) and display distinct pipe morphology, textures, and mineralogy. To explain the KPK mineralogy deviating from the mineralogy of crystallized kimberlite melt, we study reactions between hypabyssal kimberlite transitional to KPK and felsic xenoliths. Here, we characterize the pectolite-diopside-phlogopite-serpentine-olivine common zonal patterns using petrography, bulk composition, thermodynamic modelling, and conserved element ratio analysis. To replicate the observed mineral assemblages, we extended the thermodynamic database to include pectolite, using calculated density functional theory methods. Our modelling reproduces the formation of the observed distinct mineralogy in reacted granitoid and gneiss. The assimilation of xenoliths is a process that starts from high temperatures (1200-600 °C) with the formation of clinopyroxene and wollastonite, continues at 600-200 °C with the growth of clinopyroxene, garnet, and phlogopite finishing at temperatures?
Geochimica et Cosmochimica Acta, Vol. 243, pp. 133-148.
Mantle
olivine
Abstract: Chemical exchange between seawater and the oceanic crust is thought to play a significant role in the regulation of the global magnesium (Mg) cycle, yet relatively little is known about the rates and mechanisms of Mg exchange in these crustal environments. In this study we experimentally characterize the extent, and nature, of Mg isotope fractionation during the carbonation and serpentinization of olivine (one of the principal minerals found in ultramafic rocks) under hydrothermal conditions. Olivine alteration was found to be incongruent, with the reactant fluid composition varying according to the extent of olivine dissolution and the precipitation of secondary minerals. In mildly acid water (pH???6.5), olivine dissolved to form Mg-Fe carbonate solid solutions and minor chrysotile. Upon carbonation and a decrease of CO2 in the water, the pH increased to >8, with chrysotile and brucite becoming the dominant alteration minerals. The Mg-rich carbonates preferentially incorporated lighter Mg isotopes, resulting in a ?0.5‰ increase of the ?26Mg composition of the fluid relative to olivine during the initial carbonation and serpentinization reactions. This was followed by a decrease in ?26Mg under higher pH conditions associated with the formation of brucite. Our experimental and modeling results therefore demonstrate that the ?26Mg composition of fluids involved in olivine alteration reflect the type and quantity of secondary Mg minerals formed, which in turn depend on the pH and CO2 concentration of the water. Comparison of these results with natural groundwaters and geothermal waters from basaltic terrains indicate that the ?26Mg composition of natural waters are likely to also be controlled by mafic rock dissolution and the preferential incorporation of isotopically light Mg into carbonates and isotopically heavy Mg into Mg-Si minerals. Together, these findings improve our understanding of Mg isotope systematics during water-rock interaction, and suggest that ?26Mg may be a useful tool for tracing reactions that are critical to geological CO2 sequestration.
Abstract: Nitrogen is the main constituent of the Earth’s atmosphere, but its provenance in the Earth’s mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth’s accretion versus that subducted from the Earth’s surface is unclear1,2,3,4,5,6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15N15N isotopologue of N2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle ?15N (the fractional difference in 15N/14N from air), N2/36Ar and N2/3He. Our results show that negative ?15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen7,8,9,10, in fact represent dominantly air-derived N2 that experienced 15N/14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15N15N data allow extrapolations that characterize mantle endmember ?15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly increased ?15N and N2/36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has ?15N values substantially greater than that of the convective mantle, resembling surface components12,13,14,15, its N2/36Ar and N2/3He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume ?15N values may both be dominantly primordial features.
Abstract: Botswana experienced a Mw 6.5 earthquake on 3rd April 2017, the second largest earthquake event in Botswana's recorded history. This earthquake occurred within the Limpopo-Shashe Belt, ~350 km southeast of the seismically active Okavango Rift Zone. The region has no historical record of large magnitude earthquakes or active faults. The occurrence of this earthquake was unexpected and underscores our limited understanding of the crustal configuration of Botswana and highlight that neotectonic activity is not only confined to the Okavango Rift Zone. To address this knowledge gap, we applied a regularized inversion algorithm to the Bouguer gravity data to construct a high-resolution crustal thickness map of Botswana. The produced crustal thickness map shows a thinner crust (35-40 km) underlying the Okavango Rift Zone and sedimentary basins, whereas thicker crust (41-46 km) underlies the cratonic regions and orogenic belts. Our results also show localized zone of relatively thinner crust (~40 km), one of which is located along the edge of the Kaapvaal Craton within the MW 6.5 Botswana earthquake region. Based on our result, we propose a mechanism of the Botswana Earthquake that integrates crustal thickness information with elevated heat flow as the result of the thermal fluid from East African Rift System, and extensional forces predicted by the local stress regime. The epicentral region is therefore suggested to be a possible area of tectonic reactivation, which is caused by multiple factors that could lead to future intraplate earthquakes in this region.
Abstract: Raman spectral characteristics of a range of diamond-based abrasives (powders and sprays) and drilling and cutting tools, originating from preparation laboratories worldwide, are presented. Some abrasives show strong broadening of the main diamond band [FWHM (full width at half band-maximum) > 5 cm? 1] accompanied by strong band-downshift (View the MathML source?˜ = 1316-1330 cm? 1). Others are characterised by moderate band broadening (FWHM = 1.8-5 cm? 1) at rather regular band position (View the MathML source?˜ = 1331-1333 cm? 1). In addition we found that a "fresh" abrasive and its used analogue may in some cases show vast differences in their Raman spectra. The Raman parameters of diamond-based abrasives overlap widely with Raman parameters of UHP (ultra-high pressure) microdiamond. It is hence impossible to assign diamond detected in a geological specimen to either an introduced artefact or a genuine UHP relict, from the Raman spectrum alone. Raman is an excellent technique for the detection of minute amounts of diamond; however it does not provide conclusive evidence for the identification of UHP microdiamond. The latter requires thorough verification, for instance by optical microscopy or, if doubts cannot be dispelled, transmission electron microscopy.
Global trench migration velocities and slab migration induced upper mantle volume fluxes: constraints to find an Earth reference frame based on minimizing viscous dissipation.
Earth Science Reviews, Vol. 88, 1-2, May pp. 118-144.
Earth and Planetary Science Letters, Vol. 500, pp. 156-167.
United States
geodynamics
Abstract: The origin of the complex pattern of SKS splitting over the western United States (U.S.) remains a long-lasting debate, where a model that simultaneously matches the various SKS features is still lacking. Here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy. These tests reveal a configuration of mantle deformation more complex than ever envisioned before. In particular, we find that both lithospheric thickness variations and toroidal flows around the Juan de Fuca slab modulate flow locally, but their co-existence enhances large-scale mantle deformation below the western U.S. The ancient Farallon slab below the east coast pulls the western U.S. upper mantle eastward, spanning the regionally extensive circular pattern of SKS splitting. The prominent E-W oriented anisotropy pattern within the Pacific Northwest reflects the existence of sustaining eastward intrusion of the hot Pacific oceanic mantle to beneath the continental interior, from within slab tears below Oregon to under the Snake River Plain and the Yellowstone caldera. This work provides an independent support to the formation of intra-plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume.
Ore and Energy Resource Geology, Vol. 7, 100013 12p. Pdf
Russia
mantle fluids
Abstract: P-T- Oxygen fugacity (fO2) conditions and fluid compositions were estimated for the formation conditions of pyrope garnet inclusions in diamonds and xenocrysts from diamond-bearing and diamond-free kimberlites using their total chemical analyses and single oxythermobarometry. Our data indicate that optimal conditions for diamond growth and preservation occur in the presumed water-rich mantle fluids containing the lowest abundance of free atomic carbon. The majority of the calculated C-H-O fluid compositions for diamond formation in peridotite xenoliths from high diamond grade kimberlites correspond to a high hydrogen and low carbon and oxygen atomic fluid percents, while those from the majority of peridotite xenoliths in the low grade diamond kimberlites corresponds to the low hydrogen, high carbon and oxygen atomic percent fluids. This new approach defines the conditions of diamond formation for kimberlitic deposits. It better characterizes diamond grades in kimberlites in comparison to the previous empirical mineralogical Ca-Cr methods and can be used as a more precise mineralogical-petrological method for prospecting for kimberlitic diamond deposits.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0627-2 9p.
Africa, Botswana
deposit - Orapa
Abstract: This paper presents the results of an investigation into the structure of eolian kimberlite indicator minerals (KIMs) haloes present within Quaternary Kalahari Group sediments (up to 20 m thick) overlying the Late Cretaceous kimberlites in the Orapa field in North-East Botswana. A database of more than 8000 samples shows that kimberlites create a general mineralogical blanket of KIMs of various distances of transportation from primary sources in the Orapa area. Models of the reflection and dispersion patterns of KIMs derived from kimberlite pipes including AK10/ AK22/AK23 have been revealed based on 200 selected heavy mineral samples collected during diamond prospecting activities in Botswana from 2014 to 2017. Short distance eolian haloes situated close to kimberlite bodies cover gentle slopes within plains up to 500 × 1000 m in size. They have regularly have oval or conical shapes and are characterized by the presence mainly of unabraded or only slightly abraded KIMs. A sharp reduction of their concentration from hundreds and thousands of grains / 20 l immediately above kimberlites toto 10 grains/20 l at a distance of only 100-200 m from the pipes is a standard feature of these haloes. The variation of concentration, morphology and abrasion of specific KIMs with increasing distance from the primary sources has been investigated and presented herein. Sample volumes recommended for pipes present within a similar setting as those studied, with different depth of sedimentary cover are as follows: up to 10-20 m cover at 20-50 l, 20-30 m cover at 50-100 l and 30-80 m cover at 250 l. It is important to appreciate that the discovery of even single grains of unabraded or slightly abraded KIMs in eolian haloes are of high prospecting significance in this area. The results of the research can be applied to in diamond prospecting programs in various regions with similar environments.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0628-1 14p.
Africa, Angola
kimberlites
Abstract: Based on a comprehensive analysis of kimberlite pipes of Angola, including the near surface structural setting, deep lithospheric structure, pipe morphology and emplacement, mineralogical and petrographic features, diamond characteristics and locations of secondary deposits four geographical regions have been outlined within Angola representing four types of diamond bearing potential. These areas include high diamond bearing potential pipes, possible potential, no potential, and unclear potential areas. It was found that the depth of magmatism and diamond potential of kimberlites increases from the Atlantic coast in southwestern Angola into the continent in the north-easterly direction. Areas prospective for the discovery of new primary diamond deposits have been identified.
Deep-seated magmatism, its sources and plumes, Proceedings of XIII International Workshop held 2014., Vol. 2014, pp. 178-202.
Africa, South Africa, Russia, Yakutia
Deposit - Dike Newlands, Nyurbinskaya
Abstract: Green garnets occur in concentrates of diamondiferous kimberlite bodies in Yakutia (Udachnaya, Mir, etc.), South Africa (Newlands, Bellsbank), Venezuela (Guaniamo sills), and Canada (Mud Lake field). Mantle xenoliths of rocks containing such garnets are very rare. We found peridotite xenoliths with green garnet in situ in kimberlites of the Newlands dike. Xenoliths are irregular in form, 4.5*1.9 cm, 1.5*0.8 cm, and 1.0*0.5 cm in size, and have similar modal compositions: gar(70)+ol(28)+sp(2), gar(9)+ol(90)+sp(1) and gar(50)+ol(30)+sp(20). Rock texture is medium-crystalline, while structure is massive. We also identified a garnet macrocryst of 0.5*0.4 cm in size with a pale green kelyphytic rim. Garnet composition in the studied samples is quite constant and is characterized by the high Cr2O3 content (10.94-11.99%) and CaO content (19.52-24.94%) at the reduced contents of TiO2 (0.24-0.52%). The chrome spinel is high Cr2O3 (55%) content and the low TiO2 (0.5-0.6%) content. Olivine is high-Mg (Fo95), but elevated CaO content (0.09%). Isotopic composition of oxygen in garnet (?18O = 4.05-4.25 pm) and olivine (?18O = 4.91 pm) differs drastically from the mantle values. Rb-Sr and Sm-Nd isotopic composition show the relatively "young" model age of the sample relative to the depleted mantle (1.78 billion years), the age of formation of this rocks is also relatively "young" - probable mezoproterozoic. In kimberlites and placers of the Nyurbinskaya pipe (Nakyn field, Yakutia) there are 4 green garnet grains of 0.5-2.0 mm in size, including one intergrowth gar+sp. Most garnets are characterized by the higher CaO (18.06-22.87%) and TiO2 (1.46, 1.65, 1.75%) contents not noted before for similar garnets. Studied green garnets have the similar "sine wave" type of REE distribution for low-Ti garnets and a "raised" type of REE distribution with enrichment in medium and light REE for high-Ti garnet.
All green garnets are characterized by an increased content of light REE and Sc. High-Ti garnets are characterized by an increased content of light and middle REE, as well as titanium, and a particularly sharply increased content of Zr (!). Paragenesis ol+sp is formed at 805oand 23.4 kbar, and paragenesis ol+gar is formed at 1080oand 23.8 kbar. The rocks are characterized by nonequilibrium paragenesis ol+sp+gar and formation at moderate depths (80-90 km) under conditions of high heat flow (52-55 mW/m2). Judging from modal composition of studied xenoliths (absence of clinopyroxene), variations in chemical compositions and trace element compositions, relatively "young" model age and non-mantle isotopy of oxygen in garnets, these rocks are not "wehrlites" and likely represent metasomatic rocks such as uvarovite-chromite veins or schlierens at the moderate depths of upper mantle - it is similar to uvarovite-chromite veins of the metasomatic or a hydrothermal origin in the crustal serpentinites.
Ashchepkov, I., Ntaflos, T., Logvinova, A., Vladykin, N., Ivanov, A., Spetsius, Z., Stegnitsky, Y., Kostrovitsky, S., Salikhov, R., Makovchuk, I., Shmarov, G., Karpenko, M., Downes, H., Madvedev, N.
Abstract: The PTX diagrams for the separate phases in Sytykanskaya (Ashchepkov et al., 2016) Dalnyaya (Ashchepkov et al., 2017), pipes shows that the PK show the relatively simple P-X trends and geotherms and shows more contrast and simple layering. The PK contain most abundant material from the root of the magma generation they are dunitic veins as the magma feeders represented by the megacrysts. New results for the Aykhal, Zarya and Komsomolskaya pipes in Alake field and Zarnitsa and Udachnaya pipes in Daldyn field show that evolution is accompanied by the developing of metasomatites and branching and veining of the wall rock peridotites . In Aykhal pipe in PK the Gar- dunites prevail, the xenoliths from the dark ABK "Rebus" contain Cr-Ti - rich garnets and ilmenites, more abundant compared with the grey carbonited breccia Nearly the same features were found for Yubileinaya pipe. The example of Komsomolskya pipes show that the ABK contain more eclogitic xenolith than PK. The developing of the magma channel shown in satellite Chukukskaya and Structurnaya pipe was followed by the separation of some parts of the magmatic feeders and crystallization of abundant Gar megacrysts near o the walls blocking the peridotites from the magma feeder. This drastically decrease diamond grade of pipes. Such blocking seems to be the common features for the latest breccias. In Zarnitsa pipe, the dark PK and ABK also contain fresh xenoliths but not only dunites but also sheared and metasomatic varieties and eclogites. Most of dark ABK in Yakutia contain the intergrowth of ilmenites with brown Ti- Cpx showing joint evolution trends. The late breccia contains completely altered peridotite xenoliths mainly of dunite- harzburgite type. The comparison of the trace elements of the coexisting minerals in megacryst show that they were derived from the protokimberlites but are not in complete equilibrium as well as other megacrystalline phases. Ilmenites show inflections of the trace element patterns of most Ilmenites but more regular for the Cpx and Garnets revealing the sub parallel patterns elevating LREE with the rising TRE. But commonly these are not continuous sequances because they developed in the pulsing moving systems like beneath Zarnitsa. The minerals from the feeders like dunites also show the inflected or S-type REE patterns. From the earlier to later phases the TRE compositions became more evolved reflecting the evolution of protokimberlites. The wall rocks also often show the interaction with the more evolved melts and sometimes "cut" spectrums due to the dissolution some phases and repeated melting events So we could suggest the joint evolution of the mantle column protokimberlites and megacrysts composition and type of kimberlites with the diamond grade. The mantle lithospheric base captured by the PK. The developing and rising protokimbelrites was followed by the crystallization of the diamonds in the gradient in FO2 zone in wall rocks due to reductions of C -bearing fluids and carbonatites (> 1 QMF) on peridotites ((< -2 -5 QMF). The most intensive reactions are near the graphite - diamond boundary where protokimberlites are breaking and where most framesites are forming.
Solid Earth Discussions, Vol. 5, pp. 1-75. pdf * note date
Russia, Yakutia
picroilmenites
Abstract: Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modeling, shows long compositional trends for the ilmenites. These are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite minerals from stage to stage and splitting of the magmatic system in the final stages.
High-pressure (5.5-7.0 GPa) Cr-bearing Mg-rich ilmenites (group 1) reflect the conditions of high-temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1-10/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb > 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) trace the high-temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10-100)/PM with La / Ybn ~ 10-25, similar to those derived from kimberlites, with high-field-strength elements (HFSE) peaks (typical megacrysts). A series of similar patterns results from polybaric Assimilation + fractional crystallization (AFC) crystallization of protokimberlite melts which also precipitated sulfides (Pb < 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high-Cr content (group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr-rich ilmenites are typical of veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. Ilmenites which have the highest trace element contents (1000/PM) have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet-spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn-Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.
Lithos, Vol. 406-407. doi: 10.1016/j.lithos.2021.106499 77p. Pdf
Russia
kimberlite genesis
Abstract: Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modeling, shows long compositional trends for the ilmenites. These are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite minerals from stage to stage and splitting of the magmatic system in the final stages. High-pressure (5.5–7.0 GPa) Cr-bearing Mg-rich ilmenites (group 1) reflect the conditions of high-temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1–10/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb > 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) trace the high-temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10–100)/PM with La / Ybn ~ 10–25, similar to those derived from kimberlites, with high-field-strength elements (HFSE) peaks (typical megacrysts). A series of similar patterns results from polybaric Assimilation + fractional crystallization (AFC) crystallization of protokimberlite melts which also precipitated sulfides (Pb < 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high-Cr content (group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr-rich ilmenites are typical of veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. Ilmenites which have the highest trace element contents (1000/PM) have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet–spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn–Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.
Volcanic rocks of the Nyurbinskaya pipe: a portrayal of regional upper mantle evolution from the Riphean to the Carboniferous time, and its geodynamic relationship.
Deep Seated Magmatism, its sources and plumes, Ed. Vladykin, N.V., 2008 pp. 71-103.
The diamond bearing territories of Africa and their importance for expansion of the raw material base of the Russian diamond mining industry. ***IN RUS
Mineral Resources of Russia: economics and Management *** IN RUS, No. 6, pp. 66-72. pdf
Abstract: Solanum adamantium is described from Serra de Grão Mogol, located in the Espinhaço range, in northern Minas Gerais State, Brazil. The new species is ecologically and morphologically similar to the prickly species S. buddleiifolium and S. thomasiifolium, from which it differs in a series of vegetative and reproductive characters. We discuss the morphological similarities and differences among these species, as well as certain aspects of the new taxon's ecology and geographic distribution. Images of diagnostic characters, a map of geographical distribution, a preliminary conservation assessment, and full specimen citations are provided.
Abstract: Continental rifts evolve along two possible paths. In one, a rift successfully evolves into seafloor spreading, leaving the rift structures buried beneath thick sedimentary and volcanic rocks at a passive continental margin. Alternatively, the rift fails and remains as a fossil feature within a continent. We consider insights into these processes from studies of North America's Midcontinent Rift (MCR). The MCR combines the linear geometry of a rift formed at a plate boundary and the huge igneous rock volume of a Large Igneous Province. The rift is a fault bounded basin filled with volcanics and sediments, which record a history of extension, volcanism, sedimentation, subsidence, and inversion. The MCR came close to evolving into an oceanic spreading center, but it instead failed and thus records a late stage of rifting. It thus preserves a snapshot of a stage of the process by which actively extending rifts, characterized by upwelling mantle and negative gravity anomalies, evolve either into failed and often inverted rifts without upwelling mantle and positive gravity anomalies or into passive continental margins. Many rifts can be viewed as following a generally similar evolutionary sequence, within which a complex combination of factors control the variability of structures within and among rifts. Study of the MCR also gives insight into passive continental margins. The MCR gives a snapshot of deposition of a thick, dense, and highly magnetized volcanic section during rifting. Surface exposures, seismic, and gravity data delineate a rift basin filled by inward dipping flood basalt layers, underlain by thinned and underplated crust. The fact that the MCR shows many features of a rifted volcanic margin suggests that it came close to continental breakup before it failed, and illustrates how many passive margin features form prior to breakup.
Abstract: North America's Midcontinent Rift (MCR) and Grenville Front (GF) jointly record aspects of the complex history of the assembly of Rodinia. The ~1100 Ma MCR, remaining from a failed major rifting event, is exposed along Lake Superior and well defined by gravity, magnetic, and seismic data. The GF, which results from collisions with Laurentia, is exposed in and identified by seismic and potential field data in Canada. In the eastern U.S., lineated gravity highs extending southward from Michigan to Alabama, along the trend of the front in Canada, have been interpreted either as a buried Grenville Front or as part of the MCR's east arm. We explore this issue by examining the gravity signatures of the MCR and GF. Both the MCR's arms have pronounced gravity highs, with the west arm's greater than the east arm's. Combining the gravity observations with seismic data suggests that the west arm contains 20-25 km thickness of volcanics, whereas the east arm contains 10-15 km of volcanics. Along the Grenville Front in Canada, thickened crust along the northern portion causes a broad gravity low, whereas the stacked thrusts along the southern portion cause essentially no gravity signature. Hence the lineated gravity highs in the eastern U.S. appear similar to those along the remainder of the MCR, and unlike those on either portion of the GF. These data favor the gravity anomalies traditionally interpreted as the Grenville Front in the eastern U.S. as instead being part of the MCR's east arm. A thrust sheet structure like that of the southern Canadian Grenville Front - which would have essentially no gravity effect - could also be present along the MCR's east arm, as implied by recent EarthScope seismic data.
Abstract: Continental rifts evolve along two possible paths. In one, a rift successfully evolves into seafloor spreading, leaving the rift structures buried beneath thick sedimentary and volcanic rocks at a passive continental margin. Alternatively, the rift fails and remains as a fossil feature within a continent. We consider insights into these processes from studies of North America's Midcontinent Rift (MCR). The MCR combines the linear geometry of a rift formed at a plate boundary and the huge igneous rock volume of a Large Igneous Province. The rift is a fault bounded basin filled with volcanics and sediments, which record a history of extension, volcanism, sedimentation, subsidence, and inversion. The MCR came close to evolving into an oceanic spreading center, but it instead failed and thus records a late stage of rifting. It thus preserves a snapshot of a stage of the process by which actively extending rifts, characterized by upwelling mantle and negative gravity anomalies, evolve either into failed and often inverted rifts without upwelling mantle and positive gravity anomalies or into passive continental margins. Many rifts can be viewed as following a generally similar evolutionary sequence, within which a complex combination of factors control the variability of structures within and among rifts. Study of the MCR also gives insight into passive continental margins. The MCR gives a snapshot of deposition of a thick, dense, and highly magnetized volcanic section during rifting. Surface exposures, seismic, and gravity data delineate a rift basin filled by inward dipping flood basalt layers, underlain by thinned and underplated crust. The fact that the MCR shows many features of a rifted volcanic margin suggests that it came close to continental breakup before it failed, and illustrates how many passive margin features form prior to breakup.
Abstract: North America's Midcontinent Rift (MCR) and Grenville Front (GF) jointly record aspects of the complex history of the assembly of Rodinia. The ~1100 Ma MCR, remaining from a failed major rifting event, is exposed along Lake Superior and well defined by gravity, magnetic, and seismic data. The GF, which results from collisions with Laurentia, is exposed in and identified by seismic and potential field data in Canada. In the eastern U.S., lineated gravity highs extending southward from Michigan to Alabama, along the trend of the front in Canada, have been interpreted either as a buried Grenville Front or as part of the MCR's east arm. We explore this issue by examining the gravity signatures of the MCR and GF. Both the MCR's arms have pronounced gravity highs, with the west arm's greater than the east arm's. Combining the gravity observations with seismic data suggests that the west arm contains 20-25 km thickness of volcanics, whereas the east arm contains 10-15 km of volcanics. Along the Grenville Front in Canada, thickened crust along the northern portion causes a broad gravity low, whereas the stacked thrusts along the southern portion cause essentially no gravity signature. Hence the lineated gravity highs in the eastern U.S. appear similar to those along the remainder of the MCR, and unlike those on either portion of the GF. These data favor the gravity anomalies traditionally interpreted as the Grenville Front in the eastern U.S. as instead being part of the MCR's east arm. A thrust sheet structure like that of the southern Canadian Grenville Front - which would have essentially no gravity effect - could also be present along the MCR's east arm, as implied by recent EarthScope seismic data.
Abstract: The outermost layer of the solid Earth consists of relatively rigid plates whose horizontal motions are well described by the rules of plate tectonics. Yet, the thickness of these plates is poorly constrained, with different methods giving widely discrepant results. Here a recently developed procedure to derive lithospheric thickness from seismic tomography with a simple thermal model is discussed. Thickness is calibrated such that the average as a function of seafloor age matches the theoretical curve for half-space cooling. Using several recent tomography models, predicted thickness agrees quite well with what is expected from half-space cooling in many oceanic areas younger than ? 110 Myr. Thickness increases less strongly with age for older oceanic lithosphere, and is quite variable on continents, with thick lithosphere up to ? 250 km inferred for many cratons. Results are highly correlated for recent shear-wave tomography models. Also, comparison to previous approaches based on tomography shows that results remain mostly similar in pattern, although somewhat more variable in the mean value and amount of variation. Global correlations with and between lithosphere thicknesses inferred from receiver functions or heat flow are much lower. However, results inferred from tomography and elastic thickness are correlated highly, giving additional confidence in these patterns of thickness variations, and implying that tomographically inferred thickness may correlate with depth-integrated strength. Thermal scaling from seismic velocities to temperatures yields radial profiles that agree with half-space cooling over large parts of their depth range, in particular for averaged profiles for given lithosphere thickness ranges. However, strong deviations from half-space cooling profiles are found in thick continental lithosphere above depth ? 150 km, most likely due to compositional differences.
Earth and Planteray Science Letters, Vol. 489, pp. 258-266.
Mantle
subduction
Abstract: The impact of remotely forced mantle flow on regional subduction evolution is largely unexplored. Here we investigate this by means of 3D thermo-mechanical numerical modeling using a regional modeling domain. We start with simplified models consisting of a 600 km (or 1400 km) wide subducting plate surrounded by other plates. Mantle inflow of ?3 cm/yr is prescribed during 25 Myr of slab evolution on a subset of the domain boundaries while the other side boundaries are open. Our experiments show that the influence of imposed mantle flow on subduction evolution is the least for trench-perpendicular mantle inflow from either the back or front of the slab leading to 10-50 km changes in slab morphology and trench position while no strong slab dip changes were observed, as compared to a reference model with no imposed mantle inflow. In experiments with trench-oblique mantle inflow we notice larger effects of slab bending and slab translation of the order of 100-200 km. Lastly, we investigate how subduction in the western Mediterranean region is influenced by remotely excited mantle flow that is computed by back-advection of a temperature and density model scaled from a global seismic tomography model. After 35 Myr of subduction evolution we find 10-50 km changes in slab position and slab morphology and a slight change in overall slab tilt. Our study shows that remotely forced mantle flow leads to secondary effects on slab evolution as compared to slab buoyancy and plate motion. Still these secondary effects occur on scales, 10-50 km, typical for the large-scale deformation of the overlying crust and thus may still be of large importance for understanding geological evolution.
Abstract: The outermost layer of the solid Earth consists of relatively rigid plates whose horizontal motions are well described by the rules of plate tectonics. Yet, the thickness of these plates is poorly constrained, with different methods giving widely discrepant results. Here a recently developed procedure to derive lithospheric thickness from seismic tomography with a simple thermal model is discussed. Thickness is calibrated such that the average as a function of seafloor age matches the theoretical curve for half-space cooling. Using several recent tomography models, predicted thickness agrees quite well with what is expected from half-space cooling in many oceanic areas younger than ? 110 Myr. Thickness increases less strongly with age for older oceanic lithosphere, and is quite variable on continents, with thick lithosphere up to ? 250 km inferred for many cratons. Results are highly correlated for recent shear-wave tomography models. Also, comparison to previous approaches based on tomography shows that results remain mostly similar in pattern, although somewhat more variable in the mean value and amount of variation. Global correlations with and between lithosphere thicknesses inferred from receiver functions or heat flow are much lower. However, results inferred from tomography and elastic thickness are correlated highly, giving additional confidence in these patterns of thickness variations, and implying that tomographically inferred thickness may correlate with depth-integrated strength. Thermal scaling from seismic velocities to temperatures yields radial profiles that agree with half-space cooling over large parts of their depth range, in particular for averaged profiles for given lithosphere thickness ranges. However, strong deviations from half-space cooling profiles are found in thick continental lithosphere above depth ? 150 km, most likely due to compositional differences.
Abstract: The instruction of Earth science courses often relies upon the observation of in-hand specimens which poses a significant barrier to delivering courses in an online format. While there are abundant resources for the digital delivery of 3-dimensional images of rock specimens, there are limited avenues to deliver microscopic materials to students in a manner that approximates the in-person experience. We have developed an accessible solution for creating and delivering microscopic educational materials to students. Our solution is an open-source device that combines a 3D-printed mechanism, to move a sample around the microscope, and an integrated camera that are both controlled by a central, inexpensive computer. The PiAutoStage system can be attached to almost any microscope and is capable of automatically imaging an entire microscopic sample by combining hundreds of collected images into a single panorama. We have found that the images permit an experience comparable to using a microscope and have the additional benefit of allowing students to examine, not only the field of view permitted in a microscope but an entire sample at once. The system is low-cost and utilizes widely available components making it universally accessible to any institution with an existing microscope.
Origin of the Archean Sask Craton and its extent within the Trans-Hudson orogen: evidence Pb Nd isotopic compositions basement rocks, post-orogenic intrusions.
Canadian Journal of Earth Sciences, Vol. 42, 4, April pp. 659-684.
Journal of Geophysical Research, Vol. 122, 10.1002/2017JB014501
Mantle
thermodynamics
Abstract: We present a newly developed software framework, MMA-EoS, that evaluates phase equilibria and thermodynamic properties of multicomponent systems by Gibbs energy minimization, with application to mantle petrology. The code is versatile in terms of the equation-of-state and mixing properties and allows for the computation of properties of single phases, solution phases, and multiphase aggregates. Currently, the open program distribution contains equation-of-state formulations widely used, that is, Caloric-Murnaghan, Caloric-Modified-Tait, and Birch-Murnaghan-Mie-Grüneisen-Debye models, with published databases included. Through its modular design and easily scripted database, MMA-EoS can readily be extended with new formulations of equations-of-state and changes or extensions to thermodynamic data sets. We demonstrate the application of the program by reproducing and comparing physical properties of mantle phases and assemblages with previously published work and experimental data, successively increasing complexity, up to computing phase equilibria of six-component compositions. Chemically complex systems allow us to trace the budget of minor chemical components in order to explore whether they lead to the formation of new phases or extend stability fields of existing ones. Self-consistently computed thermophysical properties for a homogeneous mantle and a mechanical mixture of slab lithologies show no discernible differences that require a heterogeneous mantle structure as has been suggested previously. Such examples illustrate how thermodynamics of mantle mineralogy can advance the study of Earth's interior.
A new method to simulate convection with strongly temperature and pressure dependent viscosity in a spherical shell: applications to the Earth's mantle.
Physics of the Earth and Planetary Interiors, in press available
Abstract: The Barchi-Kol terrain is a classic locality of ultrahigh-pressure (UHP) metamorphism within the Kokchetav metamorphic belt. We provide a detailed and systematic characterization of four metasedimentary samples using dominant mineral assemblages, mineral inclusions in zircon and monazite, garnet zonation with respect to major and trace elements, and Zr-in-rutile and Ti-in-zircon temperatures. A typical diamond-bearing gneiss records peak conditions of 49 ± 4 kbar and 950-1000 °C. Near isothermal decompression of this rock resulted in the breakdown of phengite associated with a pervasive recrystallization of the rock. The same terrain also contains mica schists that experienced peak conditions close to those of the diamond-bearing rocks, but they were exhumed along a cooler path where phengite remained stable. In these rocks, major and trace element zoning in garnet has been completely equilibrated. A layered gneiss was metamorphosed at UHP conditions in the coesite field, but did not reach diamond-facies conditions (peak conditions: 30 kbar and 800-900 °C). In this sample, garnet records retrograde zonation in major elements and also retains prograde zoning in trace elements. A garnet-kyanite-micaschist that reached significantly lower pressures (24 ± 2 kbar, 710 ± 20 °C) contains garnet with major and trace element zoning. The diverse garnet zoning in samples that experienced different metamorphic conditions allows to establish that diffusional equilibration of rare earth element in garnet likely occurs at ~900-950 °C. Different metamorphic conditions in the four investigated samples are also documented in zircon trace element zonation and mineral inclusions in zircon and monazite. -Pb geochronology of metamorphic zircon and monazite domains demonstrates that prograde (528-521 Ma), peak (528-522 Ma), and peak to retrograde metamorphism (503-532 Ma) occurred over a relatively short time interval that is indistinguishable from metamorphism of other UHP rocks within the Kokchetav metamorphic belt. Therefore, the assembly of rocks with contrasting P-T trajectories must have occurred in a single subduction-exhumation cycle, providing a snapshot of the thermal structure of a subducted continental margin prior to collision. The rocks were initially buried along a low geothermal gradient. At 20-25 kbar they underwent near isobaric heating of 200 °C, which was followed by continued burial along a low geothermal gradient. Such a step-wise geotherm is in good agreement with predictions from subduction zone thermal models.
Abstract: Abundant multiphase solid inclusions (MSI) were found in garnet in an ultrahigh-pressure (UHP) paragneiss from the Kokchetav complex, Kazakhstan. The MSI are composed of mineral associations that include rock-forming and accessory minerals, which crystallized during exhumation. We present experimental and analytical protocols for how such inclusions can be homogenized to glass and analysed for major and trace elements. After homogenization we identified two types of glass. One type is present in garnet porphyroblasts in the melanocratic part of the sample and represents a high-pressure melt formed close to peak conditions of >45 kbar, 1000°C. These inclusions are characterized by high concentrations of light rare earth elements (LREE), Th and U. Extraction of these melts resulted in a pronounced depletion of the Kokchetav gneisses in those elements. Measured partition coefficients of large ion lithophile elements (LILE) between phengite inclusions and melt inclusions are DRb?=?1•9-2•5, DBa?=?1•1-6•9 and DCs?=?0•6-0•8, resulting in limited depletion of these elements during partial melting in the presence of phengite. The Nb concentration in melts (27?ppm) is about double that in the restite (15?ppm), indicating slightly incompatible behaviour during UHP anatexis, despite the presence of residual accessory rutile and phengite. A second type of inclusion occurs in garnet from the leucocratic part of the rock and represents a late-stage melt formed during exhumation at 650-750°C and crustal pressures. These inclusions are characterized by low LREE and Nb and high U. Zircon domains formed during high-temperature melting are characterized by high Ti content (100-300?ppm) and unfractionated Th/U (0•4-0•8), whereas the low-temperature domains display low Ti (10?ppm) and Th/U (0•08). The composition of UHP melts with moderate enrichment in LILE, no depletion in Nb and extreme enrichment in LREE and Th is remarkably different from the trace element signature of arc basalts, arguing against involvement of this type of melting in the generation of arc crust. The composition of the UHP melt inclusions is similar to that of melt inclusions from HP crustal xenoliths from Pamir and also to some shoshonites from Tibet. UHP anatexis, as observed in the Kokchetav massif, might be related to the formation of shoshonitic alkaline igneous rocks, which are common in collisional settings.
Abstract: Kimberlite-borne mantle eclogites represent an important diamond source rock. Although the origin and stability of diamond, as opposed to its low-pressure polymorph graphite, have been studied for decades, their relationship in rare natural samples where both polymorphs coexist remains poorly constrained. To shed new light on this issue, seven graphite-diamond-bearing eclogites from the kimberlite pipe Udachnaya, Siberian craton were comprehensively investigated with respect to their petrography, mineral chemical composition and omphacite 87Sr/86Sr, acquired in situ by laser ablation multicollector inductively coupled plasma mass spectrometry. The calculated P-T conditions for basaltic group eclogites (Eu/Eu* < 1) correspond to a pressure range of 4•8-6•5?GPa and temperatures of 1060-1130?°C, whereas gabbroic eclogites with positive Eu- and Sr-anomalies have a smaller pressure variation (4•8-5•8?GPa), but a larger range in temperature (990-1260?°C). Reconstructed bulk compositions for gabbroic eclogites indicate an oceanic crustal origin for their protoliths, with accumulation of plagioclase and olivine ± clinopyroxene (gabbronorite or olivine gabbro). The protoliths of basaltic eclogites probably formed from the complementary residual melt. The presence of coesite and low Mg# in basaltic eclogites suggest that their light rare earth element depletion was the result of <10?% partial melting during subsequent subduction and emplacement into the cratonic lithosphere. Extremely unradiogenic 87Sr/86Sr (0•70091-0•70186 for six of seven samples) not only provides new evidence for the Archean age (2•5-2•9?Gyr) of Yakutian graphite-diamond-bearing eclogites and for formation of their protoliths in a depleted mantle source, but also suggests that they were not significantly metasomatically overprinted after their formation, despite their extended residence in the cratonic mantle lithosphere. The mineralogical and petrographic features indicate that the primary mineral association includes garnet, omphacite, ± coesite, ± kyanite, ± rutile, graphite, and diamond. Graphite occurs in the samples in the form of idiomorphic crystals (the longest dimensions being 0•4-1?mm) in garnet and kyanite and extends beyond their grain boundaries. Diamonds occur as octahedral cubic transparent, slightly colored or bright yellow crystals as large as 0•1-2?mm. Furthermore, idiomorphic and highly ordered graphite occurs as inclusions in diamond in four samples. The carbon isotope composition for diamond and graphite has a narrow range (?4 to ?6•6?‰) for both groups (gabbroic and basaltic), indicating a mantle source and limiting the role of subducted isotopically light biogenic carbon or reduction of isotopically heavy carbonate in diamond crystallization. Importantly, the presence of graphite and diamond inclusions in garnet, omphacite, and kyanite in three samples indicates a co-formation close in time to eclogitization. Combined, the petrographic and geochemical evidence suggests that both polymorphic carbon modifications can form in the diamond stability field, as also suggested by experiments and some natural examples, although the exact mechanism remains unresolved. Furthermore, this study provides natural evidence that graphite can be preserved (metastably) deep within the diamond stability field, without recrystallizing into diamond, for a long time, ?2•5?Gyr.
Bulletin of the Russian Academy of Sciences. Physics ** IN ENG, Vol. 80, 1, pp. 74-77.
South America, Brazil
Diamond formation
Abstract: Luminescence kinetics in the temperature range of 80 480 K and the red region of the spectrum is studied for Brazilian diamonds. Components with decay time constants of 23 and 83 ns are observed at room temperature after being excited by laser radiation with wavelengths of 375 and 532 nm, which differs considerably from the data published earlier for the luminescence kinetics of NV 0- and NV -centers.
Contributions to Mineralogy and Petrology, Vol. 176, 34 21p. Pdf
Mantle
carbonatites
Abstract: The reactions between pyroxenes and carbonates have been studied in the CaMgSi2O6 + MgCO3 (Di + 2Mgs), CaMgSi2O6 + NaAlSi2O6 + 2MgCO3 (Di + Jd + 2Mgs), CaMgSi2O6 + Na2Mg(CO3)2 (Di + Eit), and CaMgSi2O6 + K2Mg(CO3)2 (Di + K2Mg) systems at pressures of 3.0 and 4.5 GPa in the temperature range 850-1300 °C and compared with those established previously at 6.0 GPa. The Di + 2Mgs solidus locates at 1220 °C / 3 GPa and 1400 °C / 6 GPa. Near-solidus melt is carbonatitic with SiO2 < 4 wt% and Ca# 56. The Di + Jd + 2Mgs solidus locates near 1050 °C at 3 GPa, rises to 1200 °C at 4.5 GPa, and 1350 °C at 6 GPa. The solidus is controlled by the reaction: 4NaAlSi2O6.2CaMgSi2O6 (clinopyroxene) + 12MgCO3 (magnesite) = 2MgAl2SiO6.5Mg2Si2O6 (clinopyroxene) + 2[Na2CO3.CaCO3.MgCO3] (liquid) + 6CO2. As pressure increases, the composition of solidus melt evolves from 26Na2CO3?74Ca0.58Mg0.42CO3 at 3 GPa to 10Na2CO3?90Ca0.50Mg0.50CO3 at 6 GPa. Melting in the Di + Eit and Di + K2Mg systems is controlled by the reactions: CaMgSi2O6 (clinopyroxene) + 2(Na or K)2 Mg(CO3)2 (eitelite) = Mg2Si2O6 (orthopyroxene) + 2[(Na or K)2CO3?Ca0.5Mg0.5CO3] (liquid). The Di + Eit solidus locates at 925 °C / 3 GPa and 1100 °C / 6 GPa, whereas the Di + K2Mg solidus is located at 50 °C lower. The resulting melts have alkali-rich carbonate compositions, (Na or K)2CO3?Ca0.4Mg0.6CO3. The obtained results suggest that most carbonates belong to the ultramafic suite would survive during subduction into the deep mantle and experience partial melting involving alkaline carbonates, eitelite or K2Mg(CO3)2, under geothermal conditions of the subcontinental lithospheric mantle (35-40 mW/m2). On the other hand, the jadeite component in clinopyroxene would be an important fluxing agent responsible for the partial melting of carbonated rocks under the rift margin geotherm (60 mW/m2) at a depth of about 100 km, yielding the formation of Na-carbonatite melt.
Timing and duration of kimberlitic magmatism in the Zimnii Bereg Diamondiferous province: evidence from Rb Sr age dat a on kimberlitic sills along the Mela River.
Doklady Earth Sciences, Vol. 407, 2, Feb-Mar. pp. 304-307.
Abstract: Results from thermochronological studies have multiple applications to various problems in tectonics and landform evolution However, up to now a lack of thermochronological data from the northeastern Fennoscandian Shield has complicated the interpretation of tectonothermal evolution of the region Here, we use both new and previously published multimineral 40Ar/39Ar data (amphibole, mica, and feldspar) on the various Precambrian magmatic and metamorphic complexes to reconstruct the thermal history of NE Fennoscandia within the Kola Peninsula area in the interval 1900–360 Ma Using the apatite fission track method as well as a numerical model of the heating?cooling process of northeastern Fennoscandia's upper crust, we have reconstructed its thermal evolution for the interval 360–0 Ma According to our model, since Lapland?Kola orogenesis (1930–1905 Ma) northeastern Fennoscandia experienced a quasi?monotonous cooling with the average rate of ~0 15 °C/Myr, which is equal to an exhumation rate of ~1–2 m/Myr New apatite fission track data and time?temperature modeling reveal a “hidden” endogenous thermal event in the NE Fennoscandia that took place between 360 and 300 Ma This we attribute to an elevated geothermal gradient due to Baltica's drift over the African large low shear?wave velocity province in the lowest mantle and/or thermal blanketing by insulating Devonian?Carboniferous sedimentary/volcanic cover Our model is further supported by evidence of Late Devonian?Carboniferous rifting in the East and South?Western Barents Basin, as well as various 360–300 Ma magmatic events within SW Fennoscandia and the Baltic countries
Abstract: Results from thermochronological studies have multiple applications to various problems in tectonics and landform evolution. However, up to now a lack of thermochronological data from the northeastern Fennoscandian Shield has complicated the interpretation of tectonothermal evolution of the region. Here, we use both new and previously published multimineral 40Ar/39Ar data (amphibole, mica, and feldspar) on the various Precambrian magmatic and metamorphic complexes to reconstruct the thermal history of NE Fennoscandia within the Kola Peninsula area in the interval 1900-360 Ma. Using the apatite fission track method as well as a numerical model of the heating?cooling process of northeastern Fennoscandia's upper crust, we have reconstructed its thermal evolution for the interval 360-0 Ma. According to our model, since Lapland?Kola orogenesis (1930-1905 Ma) northeastern Fennoscandia experienced a quasi?monotonous cooling with the average rate of ~0.15 °C/Myr, which is equal to an exhumation rate of ~1-2 m/Myr. New apatite fission track data and time?temperature modeling reveal a “hidden” endogenous thermal event in the NE Fennoscandia that took place between 360 and 300 Ma. This we attribute to an elevated geothermal gradient due to Baltica's drift over the African large low shear?wave velocity province in the lowest mantle and/or thermal blanketing by insulating Devonian?Carboniferous sedimentary/volcanic cover. Our model is further supported by evidence of Late Devonian?Carboniferous rifting in the East and South?Western Barents Basin, as well as various 360-300 Ma magmatic events within SW Fennoscandia and the Baltic countries.
Abstract: New data on the age of dolerite dikes in the NE part of the Kola province of the Fennoscandinavian shield and the picrodolerite sills that cut the dikes are presented. The results of U-Pb ID-TIMS baddeleyite dating indicate that dolerites were formed between 2508 ± 6 and 2513 ± 16 Ma ago, simultaneously with the intrusions of the Monchegorsk group. A comparison of the composition of the dolerites studied with dykes of the same age found in other Archean cratons shows their significant similarity and suggests their formation at the same large magmatic province. The age of baddeleyite from the picrodolerites sills at 2403 ± 12 Ma ago indicates an event of basic magmatism that was not previously established in this part of the Fennoscandinavian shield. It is possible that, along with dolerite dykes with an age of 2405 Ma and komatiites of the Vetreny belt of the Karelian craton, sills of the Kola province are a component of a unified large magmatic event.
Stratigraphy and Geological Correlation, Vol. 28, 1, pp. 1-34.
Europe, Fennoscandia
magmatism
Abstract: The comprehensive geochemical and isotopic-geochronological study of Early Proterozoic volcanic rocks in structure of the Polmak-Pechenga-Imandra-Varzuga belt and dikes and sills of the Murmansk and Kola-Norwegian terranes is conducted. Abundant swarms of mafic dikes (2.06-1.86 Ga) are established in the northwestern frame of the belt, including swarms of metadolerites (2060 ± 6 Ma), ferropicrites and gabbronorites (1983 ± 5 Ma), and poikilophitic dolerites (1860 ± 4 Ma). The comparison of volcanic rocks of the Pechenga and Imandra-Varzuga structures shows asynchronous change in volcanism style, with a significant time lapse. The geochemical features of volcanic rocks of the Tominga Formation are typical of those of continental magmatism and can hardly be correlated with those of the Pilguyarvi Formation. According to isotopic-geochronological data, depleted mantle melts in the Pechenga and Imandra-Varzuga zones intruded at 2010-1970 and 1970-1980 Ma, respectively. The analysis of the conditions of formation of volcanic series shows that Neoarchean lithospheric mantle, which produced melts with low Zr/Nb ratios, was a source for primary melts of the Kuetsjarvi Formation of the Pechenga structure and their homologs of the Imandra-Varzuga structure. In contrast, the volcanic rocks of the Kolasjoki Formation, which were weakly contaminated with crustal material, and the related Ilmozero Formation, as well as the metadolerite dikes of the Kirkenes region, were sourced mostly from asthenosphere with separation of melt above the garnet stability depth. The formation of the volcanic rocks of the Pilguyarvi Formation is related, judging from the geochemical data, to two asthenospheric sources different in depth, which produced tholeiitic and ferropicritic melts.
Abstract: New petrographic, geochemical, and isotopic (Sr, Nd, and ?18?) data on olivine and pyroxene phenocrysts provide constraints on the composition and crustal evolution of primary melts of Paleoproterozoic (2.40 Ga) picrodoleritic sills in the northwest Kola province, Fennoscandian Shield. The picrodolerites form differentiated sills with S-shaped compositional profiles. Their chilled margins comprise porphyritic picrodolerite (upper margin) and olivine gabbronorite (bottom) with olivine and clinopyroxene phenocrysts. Analysis of the available data allows us to recognize three main stages in the crystallization of mineral assemblages. The central parts of large (up to 2 mm) olivine phenocrysts (Ol-1-C) crystallized at the early stage. This olivine (Mg# 85-92) is enriched in Ni (from 2845 to 3419 ppm), has stable Ni/Mg ratio, low Ti, Mn and Co concentrations, and contains tiny (up to 10 ?m) diopside-spinel dendritic lamella that probably originated due to the exsolution from high Ca- and Cr- primary magmatic olivine. All these features of Ol-1-C are typical of olivine from primitive picritic and komatiitic magmas (De Hoog et al., 2010; Asafov et al., 2018). Ol-1-C contains large (up to 0.25 mm) crystalline inclusions of high-Al enstatite (Mg# 80-88) and clinopyroxene (Mg# 82-90), occasionally in association with Ti-pargasite and chromian spinel (60.4 wt.% Al2O3). These inclusions are regarded as microxenoliths of wall rock that were captured by primary melt at depths more than 30 km and preserved due to the conservation in magmatic olivine. The second stage was responsible for the crystallization of Ol-1 rim (Ol-1-R), small (up to 0.3 mm) olivine (Ol-2, Mg# 76-85) grains, and central parts of large (up to 1.5 mm) clinopyroxene (Cpx-C) phenocrysts in the mid-crustal transitional magma chamber (at a depth of 15-20 km) at 1160-1350°C. At the third stage, Cpx-C phenocrysts were overgrown by low-Mg rims (Mg# 70-72) similar in composition to the groundmass clinopyroxene from chilled picrodolerite and gabbro-dolerite in the central parts of the sills. This stage likely completed the evolution of picrodoleritic magma and occurred in the upper crust at a depth of about 5 km. All stages of picrodoleritic magma crystallization were accompanied by contamination. Primary melts were contaminated by upper mantle and/or lower crust as recognized from xenocrystic inclusions in Ol-1-C. The second contamination stage is supported by the negative values of ?Nd(2.40) = -1.1 in clinopyroxene phenocrysts. At the third stage, contamination likely occurred in the upper crust when ascending melts filled gentle fractures. This caused vertical whole-rock Nd heterogeneity in the sills (Erofeeva et al., 2019), and difference in Nd isotopic composition of clinopyroxene phenocrysts and doleritic groundmass. It was also recognized that residual evolved melts are enriched in radiogenic strontium but have neodymium isotopic composition similar to other samples. It could be explained by the interaction of the melts with fluid formed via decomposition of biotite from surrounding gneisses under the effect of high-temperature melts.
Abstract: Available data suggest that the breakup of the Neoarchean Kenorland supercontinent at 2.5-2.4 Ga was likely triggered by a large mantle plume upwelling that caused significant magmatism. Here, we present 2D high-resolution magmatic-thermomechanical numerical models of extension of the continental crust underplated by a hot mantle plume material. Using this model, it is demonstrated that mantle plume underplating generates a large amount of mafic melt by decompression melting. This melt penetrates into the extending continental crust along normal faults thereby forming multiple generations of mafic dyke-like intrusions along normal faults. In case of extension velocity of 0.2-1 cm/yr, lower crustal heating and hot mafic melt emplacement may cause partial melting of the continental crust that can generate significant volume of felsic melts. This in turn triggers emplacement of felsic intrusions that temporarily and spatially associate with the mafic dyke-like intrusions. The modeling results agree well with geological data from the Karelian Craton and provide possible explanation for the observed association of Paleoproterozoic mafic dykes and felsic intrusions which formed in a relatively short time interval (up to 20 Myrs) in the early stages of the supercontinent breakup.
GPS constraints on continental deformation in the Africa Arabia Eurasia continental collision zone and implications for the dynamics of plate interactions.
Journal of Geophysical Research, Vol. 111,B5 B05411.
Abstract: We present a statistical approach to data mining and quantitatively evaluating detrital age spectra for sedimentary provenance analyses and palaeogeographic reconstructions. Multidimensional scaling coupled with density-based clustering allows the objective identification of provenance end-member populations and sedimentary mixing processes for a composite crust. We compiled 58 601 detrital zircon U-Pb ages from 770 Precambrian to Lower Palaeozoic shelf sedimentary rocks from 160 publications and applied statistical provenance analysis for the Peri-Gondwanan crust north of Africa and the adjacent areas. We have filtered the dataset to reduce the age spectra to the provenance signal, and compared the signal with age patterns of potential source regions. In terms of provenance, our results reveal three distinct areas, namely the Avalonian, West African and East African-Arabian zircon provinces. Except for the Rheic Ocean separating the Avalonian Zircon Province from Gondwana, the statistical analysis provides no evidence for the existence of additional oceanic lithosphere. This implies a vast and contiguous Peri-Gondwanan shelf south of the Rheic Ocean that is supplied by two contrasting super-fan systems, reflected in the zircon provinces of West Africa and East Africa-Arabia.
Journal of Geophysical Research, Vol. 121, 9, pp. 6966-6987.
Mantle
Geodynamics
Abstract: Crustal inheritance is often considered important in the tectonic evolution of the Wilson Cycle. However, the role of the mantle lithosphere is usually overlooked due to its difficulty to image and uncertainty in rheological makeup. Recently, increased resolution in lithosphere imaging has shown potential scarring in continental mantle lithosphere to be ubiquitous. In our study, we analyze intraplate deformation driven by mantle lithosphere heterogeneities from ancient Wilson Cycle processes and compare this to crustal inheritance deformation. We present 2-D numerical experiments of continental convergence to generate intraplate deformation, exploring the limits of continental rheology to understand the dominant lithosphere layer across a broad range of geological settings. By implementing a "jelly sandwich" rheology, common in stable continental lithosphere, we find that during compression the strength of the mantle lithosphere is integral in generating deformation from a structural anomaly. We posit that if the continental mantle is the strongest layer within the lithosphere, then such inheritance may have important implications for the Wilson Cycle. Furthermore, our models show that deformation driven by mantle lithosphere scarring can produce tectonic patterns related to intraplate orogenesis originating from crustal sources, highlighting the need for a more formal discussion of the role of the mantle lithosphere in plate tectonics.
Geological Society of London, Special Publication, Vol. 470, doi:10.1144 /SP470.7
Mantle
tectonics
Abstract: This review of the role of the mantle lithosphere in plate tectonic processes collates a wide range of recent studies from seismology and numerical modelling. A continually growing catalogue of deep geophysical imaging has illuminated the mantle lithosphere and generated new interpretations of how the lithosphere evolves. We review current ideas about the role of continental mantle lithosphere in plate tectonic processes. Evidence seems to be growing that scarring in the continental mantle lithosphere is ubiquitous, which implies a reassessment of the widely held view that it is the inheritance of crustal structure only (rather than the lithosphere as a whole) that is most important in the conventional theory of plate tectonics (e.g. the Wilson cycle). Recent studies have interpreted mantle lithosphere heterogeneities to be pre-existing structures and, as such, linked to the Wilson cycle and inheritance. We consider the current fundamental questions in the role of the mantle lithosphere in causing tectonic deformation, reviewing recent results and highlighting the potential of the deep lithosphere in infiltrating every aspect of plate tectonics processes.
Abstract: This review of the role of the mantle lithosphere in plate tectonic processes collates a wide range of recent studies from seismology and numerical modelling. A continually growing catalogue of deep geophysical imaging has illuminated the mantle lithosphere and generated new interpretations of how the lithosphere evolves. We review current ideas about the role of continental mantle lithosphere in plate tectonic processes. Evidence seems to be growing that scarring in the continental mantle lithosphere is ubiquitous, which implies a reassessment of the widely held view that it is the inheritance of crustal structure only (rather than the lithosphere as a whole) that is most important in the conventional theory of plate tectonics (e.g. the Wilson cycle). Recent studies have interpreted mantle lithosphere heterogeneities to be pre-existing structures and, as such, linked to the Wilson cycle and inheritance. We consider the current fundamental questions in the role of the mantle lithosphere in causing tectonic deformation, reviewing recent results and highlighting the potential of the deep lithosphere in infiltrating every aspect of plate tectonics processes.
N: Cycle Concepts in Plate Tectonics, editors Wilson and Houseman , Geological Society of London special publication 470, pp. 137-155.
Mantle
plate tectonics
Abstract: This review of the role of the mantle lithosphere in plate tectonic processes collates a wide range of recent studies from seismology and numerical modelling. A continually growing catalogue of deep geophysical imaging has illuminated the mantle lithosphere and generated new interpretations of how the lithosphere evolves. We review current ideas about the role of continental mantle lithosphere in plate tectonic processes. Evidence seems to be growing that scarring in the continental mantle lithosphere is ubiquitous, which implies a reassessment of the widely held view that it is the inheritance of crustal structure only (rather than the lithosphere as a whole) that is most important in the conventional theory of plate tectonics (e.g. the Wilson cycle). Recent studies have interpreted mantle lithosphere heterogeneities to be pre-existing structures and, as such, linked to the Wilson cycle and inheritance. We consider the current fundamental questions in the role of the mantle lithosphere in causing tectonic deformation, reviewing recent results and highlighting the potential of the deep lithosphere in infiltrating every aspect of plate tectonics processes.
Abstract: The Canadian High Arctic preserves a long and complex tectonic history, including craton formation, multiple periods of orogenesis, extension and basin formation, and the development of a passive continental margin. We investigate the possible preservation of deformational structures throughout the High Arctic subcontinental lithosphere using measurements of seismic anisotropy from shear wave splitting at 11 seismograph stations across the region, including a N-S transect along Ellesmere Island. The majority of measurements indicate a fast-polarisation orientation that parallels tectonic trends and boundaries, suggesting that lithospheric deformation is the dominant source of seismic anisotropy in the High Arctic; however, a sub-lithospheric contribution cannot be ruled out. Beneath Resolute in the central Canadian Arctic, distinct back-azimuthal variations in splitting parameters can be explained by two anisotropic layers. The upper layer is oriented E-W and correlates with tectonic trends and the inferred lithospheric deformation history of the region. The lower layer has a ?NNE-SSW orientation and may arise from present-day convective mantle flow beneath locally-thinned continental lithosphere. In addition to inferences of anisotropic structure beneath the Canadian High Arctic, measurements from the far north of our study region suggest the presence of an anisotropic zone in the lowermost mantle beneath northwest Alaska.
Abstract: Plate tectonic reconstructions are usually constrained by the correlation of lineaments of surface geology and crustal structures. This procedure is, however, largely dependent on and complicated by assumptions on crustal structure and thinning and the identification of the continent-ocean transition. We identify two geophysically and geometrically similar upper mantle structures in the North Atlantic and suggest that these represent remnants of the same Caledonian collision event. The identification of this structural lineament provides a sub-crustal piercing point and hence a novel opportunity to tie plate tectonic reconstructions. Further, this structure coincides with the location of some major tectonic events of the North Atlantic post-orogenic evolution such as the occurrence of the Iceland Melt Anomaly and the separation of the Jan Mayen microcontinent. We suggest that this inherited orogenic structure played a major role in the control of North Atlantic tectonic processes.
Ultramafic xenoliths from the Palo-Aike basalts: Implications for the nature and evolution of the subcontinental lithosphere below southern SouthAmerica
Proceedings of the Fourth International Kimberlite Conference, Held Perth, Australia, No. 16, pp. 343-345
Mantle xenoliths from the Quaternary Pali-Aike volcanic field of southernmost South America: implications for the accretion of Phanerozoic continentallithosphere
Proceedings of Fifth International Kimberlite Conference held Araxa June 1991, Servico Geologico do Brasil (CPRM) Special, pp. 395-397
Peru, South America
Petrochemical, mineral chemistry, geothermometry, Basalts
Hua, C., Zhili, Q., Taijin, L., Stern, R., Stachel, T., Yuan, S., Jian, Z., Jie, K., Shyu, P., Shecai, Q.
Variations in carbon isotopic composition in the subcontinental lithospheric mantle beneath the Yangtze and North Chin a cratons; evidence from in-situ analysis of diamonds using SIMS.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 306-7.
Mantle
diamond inclusions
Abstract: Nitrogen is one of the most common impurities in diamond, and its aggregation styles have been used as criteria for diamond classification. Pure type IaB diamonds (with 100% nitrogen in B aggregation) are rather rare among natural diamonds. The occurrence of the B center is generally associated with high temperature and a long residence time of the host diamond, which would potentially provide information on the earth’s deep interior. Seawater circulation is the unique process that shapes the surface of our planet and potentially has a profound effect on its interior due to slab subduction. In about 50 type IaB diamonds with detectable micro-inclusions submitted to GIA for screening, we found that more than 70% of them contained a typical mineral assemblage from the sublithosphere. Jeffbenite (TAPP), majorite garnet, enstatite, and ferropericlase have been observed, which could be retrograde products of former bridgmanite. CaSiO3-walstromite with larnite and titanite is the dominant phase present in approximately 40% of all diamond samples. Direct evidence from oxygen isotope ratios measured by secondary ion mass spectrometry, or SIMS, (?18OVSMOWin the range +10.7 to +12.5‰) of CaSiO3-walstromite with coexisting larnite and titanite that retrograde from CaSiO3-perovskite suggest that hydrothermally altered oceanic basalt can subduct to depths of >410 km in the transition zone. Incorporation of materials from subducted altered oceanic crust into the deep mantle produced diamond inclusions that have both lower mantle and subduction signatures. Ca(Si,Al)O3-perovskite was observed with a high concentration of rare earth elements (>5 wt.%) that could be enriched under P-Tconditions in the lower mantle. Evidence from ringwoodite with a hydroxide bond, coexisting tuite and apatite, precipitates of an NH3phase, and cohenite with trace amounts of Cl imply that the subducted brines can potentially introduce hydrous fluid to the bottom of the transition zone. In the diamonds with subducted materials, the increasing carbon isotope ratio from the core to the rim region detected by SIMS (?13C from -5.5‰ to -4‰) suggests that an oxidized carbonate-dominated fluid was associated with recycling of the subducted hydrous material. The deep subduction played an important role in balancing redox exchange with the reduced lower mantle indicated by precipitated iron nanoparticles and coexisting hydrocarbons and carbonate phases.
www.minsocam.org/ MSA/Centennial/ MSA_Centennial _Symposium.html The next 100 years of mineral science, June 20-21, p. 22. Abstract
Mantle
diamond inclusions
Abstract: Much of the temporal record of Earth’s evolution, including its trace of plate tectonics, is blurred due to the dynamic nature of the crust-mantle system. While zircon provides the highest fidelity crustal record, diamond takes over in the mantle as the go-to mineral, capable of retaining critical information for a variety of geochemical proxies, over billion year timescales. Here we use diamond and its inclusions to tell the story of the recycling of C, N, O, H and B from the crust to various depths in Earth’s mantle. In this story, altered oceanic crust (AOC) and lithospheric mantle will play a prominent role. The carbon isotope record of diamond has long been thought to reflect the mixing of primitive mantle carbon with carbon recycled from isotopically light organic material originating from the crust. A major difficulty has been reconciling this view with the highly varied nitrogen and carbon isotope signatures in diamonds of eclogitic paragenesis, which cannot be interpreted by the same mechanism. Recent work on AOC of igneous origin (Li et al., EPSL in press) shows how isotopically varied carbon and nitrogen can be subducted to great depth and retained in spatial juxtaposition with the mafic silicate component of AOC to form the complex C-N isotope systematics observed in diamonds and the varied O isotope compositions of their inclusions. In this model a large portion of the 13C depleted carbon originated from biogenic carbonate within the AOC rather than from overlying sediments. Metamorphosed and partially devolatilized AOC will have very variable C/N ratios and highly variable nitrogen isotopes, explaining why simple two component mixing between organic matter and convecting upper mantle cannot explain the complexity of C-N isotope systematics in diamonds. Igneous AOC and its underlying altered mantle are considerably more efficient than subducted sediment at retaining their volatile inventory when recycled to transition zone and even lower mantle depths. Hence, this combination of mixing between AOC-derived volatiles and those from the convecting mantle produces the isotopic fingerprints of superdeep diamonds and their inclusions. These amazing diamonds, some worth millions of dollars, can contain pristine ultra-high pressure mineral phases never before seen in terrestrial samples. The first hydrous ringwoodite found in Earth provides evidence in support of a locally water-saturated transition zone that may result from altered oceanic lithospheric mantle foundering at that depth in the mantle. The O isotope composition of deep asthenosphere and transition zone phases document clearly crustal precursors that have interacted with the hydrosphere before residing hundreds of km deep within the Earth. Finally, spectacular blue diamonds contain boron, an element of strong crustal affinities, transported into the deep Earth along with crustal carbon, by the plate tectonic conveyor system. Diamond - such a simple mineral - and its inclusions, will continue to provide a unique, brightly illuminating light into the darkest recesses of Earth’s mantle for many years to come.
Abstract: Impingement of a hot buoyant mantle plume head on the lithosphere is one of the few scenarios that can initiate a new subduction zone without requiring any pre-existing weak zones. This mechanism can start subduction and plate tectonics on a stagnant lid and can also operate during active plate tectonics where plume-lithosphere interactions is likely to be affected by plate motion. In this study, we explore the influence of plate motion on lithospheric response to plume head-lithosphere interaction including the effect of magmatic weakening of lithosphere. Using 3d thermo-mechanical models we show that the arrival of a new plume beneath the lithosphere can either (1) break the lithosphere and initiate subduction, (2) penetrate the lithosphere without subduction initiation, or (3) spread asymmetrically below the lithosphere. Outcomes indicate that lithospheric strength and plume buoyancy control plume penetration through the lithosphere whereas the plate speed has a subordinate influence on this process. However, plate motion may affect the geometry and dynamics of plume-lithosphere interaction by promoting asymmetry in the subduction zone shape. When a sufficiently buoyant plume hits a young but subductable moving lithosphere, a single-slab modern-style subduction zone can form instead of multiple subduction zones predicted by stagnant lid models. In the case of subduction initiation of older moving oceanic lithosphere, asymmetrical cylindrical subduction is promoted instead of more symmetrical stagnant lid subduction. We propose that the eastward motion of the Farallon plate in Late Cretaceous time could have played a key role in forming one-sided subduction along the southern and western margin of the Caribbean and NW South America.
Petrogenesis of mantle derived large-ion lithophile elements (LILE) enriched Archean monzodiorites and Trachy andesites (sanukitoids) in southwestern Superior Province
Canadian Journal of Earth Sciences, Vol. 26, No. 9, September pp. 1688-1712
The Geological Society of Canada (GSC) sensitive High Resolution Ion Microprobe (SHRIMP): analytical techniques of zircon uranium-thorium (U-Th) lead age..
Geological Society of Canada (GSC) Paper, No. 1997-F, p. 1-31.
Tectonic implications of new SHRIMP and TIMS U Pb geochronology of rocks from the Sask Craton, Peter Lake Domain and Hearne margin, Trans-Hudson Orogen.
Canadian Journal of Earth Sciences, Vol. 42, 4, April pp. 635-657.
Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A delta 13 C-N study of eclogite hosted diamonds from the Jericho kimberlite.
Geochimica et Cosmochimica Acta, Vol. 75, pp. 6027-6047.
Howell, D., Stern, R.A., Griffin, W.L., Southworth, R., Mikhail, S., Stachel, T., Verchovsky, A.B., O'Reilly, S.Y., Pearson, N.J.
New thermodynamic models and calculated phase equilibration temperatures in NCFMAS for basic and ultrabasic compositions through the transition zone into the uppermost lower mantle.
Palot, M., Pearson, D.G., Stern, R.A., Stachel, T., Harris, J.W.
Multiple growth events, processes and fluid sources involved in diamond genesis: a micro-analytical study of sulphide bearing diamonds from Finsch mine, RSA.
Geochimica et Cosmochimica Acta, Vol. 106, pp. 51-70.
Palot, M., Pearson, D.G., Stern, R.A., Stachel, T., Harris, J.W.
Isotopic constraints on the nature and circulation of deep mantle C-H-O-N fluids: Carbon and nitrogen systematics within ultra-deep diamonds from Kankan ( Guinea).
Geochimica et Cosmochimica Acta, Vol. 139, pp. 26-46.
Subduction related origin of eclogite xenoliths from the Wajrakarur kimberlite field, Eastern Dharwar craton, southern India: constraints from petrology and geochemistry.
Geochimica et Cosmochimica Acta, Vol. 166, pp. 165-188.
Abstract: Size frequency distributions are the principal tool for predicting the macro-diamond grade of new kimberlite discoveries, based on micro-diamonds (i.e., diamond ? 0.5 mm) recovered from small exploration samples. Lognormal size frequency distributions – as observed for the Artemisia kimberlite (Slave Craton, Canada) – suggest a common source for micro- and macro-diamonds recovered from single samples, an implication that has never been conclusively tested. We analyzed 209 diamonds between 0.2 and 2 mm in size from the Artemisia kimberlite for their carbon isotopic compositions and nitrogen characteristics to determine the nature of the micro-/macro-diamond relationship.-Despite overall similarity in the ?13C distributions of micro- and macro-diamonds – both are bimodal with peaks in classes ? 5.0 to ? 4.5‰ and ? 3.5 to ? 3.0‰ – rare diamonds with ?13C between ? 14.2 and ? 24.5‰ of presumed eclogitic origin are restricted to macro-diamonds, whereas positive values are only observed for micro-diamonds. In addition, a shift in main mode and median value in ?13C of about +1‰ is observed for micro- relative to macro-diamonds. Fundamental differences between micro- and macro-diamonds at Artemisia were revealed through the analysis of nitrogen concentrations: 68% of micro-diamonds are Type II (“nitrogen free”) versus 21% of macro-diamonds, and only 19% of micro-diamonds have nitrogen contents > 100 atomic ppm versus 43% of macro-diamonds. Similarly, the presence of a detectable hydrogen related peak (at 3107 cm? 1) increases from 40% for micro-diamonds to 94% for macro-diamonds.-Previous studies on diamond populations from individual deposits have documented that single batches of ascending kimberlite or lamproite magma sample multiple diamond subpopulations formed during distinct growth events in compositionally variable sources and at various depth levels. The Artemisia data clearly show that even over a fairly narrow size interval, spanning the micro- to macro-diamond transition, the specific diamond subpopulations present and their relative proportions may vary significantly with diamond size. At Artemisia, we conclude that the observed lognormal size distribution is not a reflection of an entirely common origin of micro- and macro-diamonds.
In situ oxygen isotope, major-, and trace element constraints on the metasomatic modification and crustal origin of a Diamondiferous eclogite from Roberts Victor, Kaapvaal Craton.
Geochimica et Cosmochimica Acta, in press available, 45p.
Contributions to Mineralogy and Petrology, Vol. 171, 15p.
Canada, Northwest Territories
Deposit - Diavik
Abstract: Fibrous diamonds are often interpreted as direct precipitates of primary carbonate-bearing fluids in the lithospheric mantle, sourced directly from common reservoirs of “mantle” carbon and nitrogen. Here we have examined fibrous growth layers in five diamonds (as three rims or “coats” and two whole-crystal cuboids) from the Diavik Diamond Mine, Canada, using in situ C- and N-isotope and N-abundance measurements to investigate the origin and evolution of their parental fluids, and in particular, to test for isotopic variability within a suite of fibrous diamonds. High-resolution growth structure information was gleaned from cathodoluminescence (CL) imaging and, in combination with the isotopic data, was used to assess the nature of the transition from gem to fibrous growth in the coated diamonds. The two cuboids are characterized by fine concentric bands of fibrous and/or milky opaque diamond, with one sample (S1719) having intermittent gem-like growth layers that are transparent and colourless. The three coated diamonds comprise octahedral gem cores mantled by massive or weakly zoned fibrous rims, with sharp and well-defined gem-fibrous boundaries. For the two cuboid samples, ? 13C and ? 15N values were ?7.7 to ?3.2 ‰ (mean ?6.3 ± 1.3 ‰; 1 SD; n = 84) and ?5.6 to ?2.1 ‰ (mean ?4.0 ± 0.8 ‰; 1 SD; n = 48), respectively. The three fibrous rims have combined ? 13C values of ?8.3 to ?4.8 ‰ (mean ?6.9 ± 0.7 ‰; 1 SD; n = 113) and ? 15N values of ?3.8 to ?1.9 ‰ (mean ?2.7 ± 0.4 ‰; 1 SD; n = 43). N-abundances of the combined cuboid-fibrous rim dataset range from 339 to 1714 at. ppm. The gem cores have ? 13C and ? 15N values of ?5.4 to ?3.5 ‰ and ?17.7 to +4.5 ‰, respectively, and N-abundances of 480 to 1699 at. ppm. Broadly uniform C- and N-isotope compositions were observed in each of the gem cores (variations of ~<1 ‰ for carbon and ~<3 ‰ for nitrogen). This limited C- and N- isotope variability implies that the gem cores formed from separate pulses of fluid that remained isotopically uniform throughout the duration of growth. Significant isotopic and abundance differences were observed between the gem and fibrous growth zones, including in one detailed isotopic profile ? 13C and ? 15N offsets of ~?2.4 and ~?3.7 ‰, respectively, and a ~230 at. ppm increase in N-abundance. Combined with the well-defined gem-fibrous boundaries in plane light and CL, these sharp isotopic differences indicate separate parental fluid histories. Notably, in the combined fibrous diamond dataset prominent C- and N-isotope differences between the whole-crystal cuboid and fibrous rim data were observed, including a consistent ~1.3 ‰ offset in ? 15N values between the two growth types. This bimodal N-isotope distribution is interpreted as formation from separate parental fluids, associated with distinct nitrogen sources. The bimodal N-isotope distribution could also be explained by differences in N-speciation between the respective parental fluids, which would largely be controlled by the oxidation state of the fibrous rim and cuboid growth environments (i.e., N2 vs. NH4 + or NH3). We also note that this C- and N-isotope variability could indicate temporal changes to the source(s) of the respective parental fluids, such that each stage of fibrous diamond growth reflects the emplacement of separate pulses of proto-kimberlitic fluid—from distinct carbon and nitrogen sources, and/or with varying N-species—into the lithospheric mantle.
Abstract: Plate tectonics plays a vital role in the evolution of our planet. Geochemical analysis of Earth’s oldest continental crust suggests that subduction may have begun episodically about 3.8 to 3.2 billion years ago, during the early Archaean or perhaps more than 3.8 billion years ago, during the Hadean. Yet, mantle rocks record evidence for modern-style plate tectonics beginning only in the late Archaean, about 3 billion years ago. Here we analyse the nitrogen abundance, as well as the nitrogen and carbon isotopic signatures of Archaean placer diamonds from the Kaapvaal craton, South Africa, which formed in the upper mantle 3.1 to 3.5 billion years ago. We find that the diamonds have enriched nitrogen contents and isotopic compositions compared with typical mantle values. This nitrogen geochemical fingerprint could have been caused by contamination of the mantle by nitrogen-rich Archaean sediments. Furthermore, the carbon isotopic signature suggests that the diamonds formed by reduction of an oxidized fluid or melt. Assuming that the Archaean mantle was more reduced than the modern mantle, we argue that the oxidized components were introduced to the mantle by crustal recycling at subduction zones. We conclude, on the basis of evidence from mantle-derived diamonds, that modern-style plate tectonics operated as early as 3.5 billion years ago.
In situ oxygen-isotope, major, and trace element constraints on the metasomatic modification and crust origin of a Diamondiferous eclogite from Roberts Victor, Kaapvaal craton.
Geochimica et Cosmochimica Acta, Vol. 174, pp. 345-359.
Abstract: First predictions of the macrodiamond grade of newly discovered kimberlites are commonly obtained using size frequency distributions of microdiamonds. The success of this approach suggests a common origin of microdiamonds and macrodiamonds, an implication not yet conclusively established or disproved. In contrast to previous comparative studies on microdiamonds and macrodiamonds from single deposits, here all diamonds analyzed originate from the same microdiamond samples (558 diamonds, ranging from 0.212 to 3.35 mm). The diamonds were analyzed for their carbon isotope compositions and nitrogen characteristics, and, based on this dataset, statistical comparisons were conducted across the size range to assess cogenesis. As a whole, the Misery diamond suite shows high nitrogen contents (median = 850 at. ppm), a bimodal distribution in time-averaged mantle residence temperatures (two distinct subpopulations in mantle residence temperatures: ?1,125° and ?1,175°C), a high degree of platelet degradation, and ?13C compositions that are isotopically slightly heavier (median = ?4.4‰) than the global median. Statistical comparisons of the various size classes indicate the presence of subtly different subpopulations at Misery; however, the nature and magnitude of these geochemical differences are very small in the context of the global diamond database and are viewed as petrogenetically insignificant. The general geochemical similarity of diamonds from different size fractions at Misery reinforces the use of size-frequency analysis to predict diamond grade in kimberlite diamond deposits.
Earth and Planetary Science Letters, Vol. 430, pp. 284-295.
Asia, Tibet
Luobusa Massif Type Iib
Abstract: For more than 20 years, the reported occurrence of diamonds in the chromites and peridotites of the Luobusa massif in Tibet (a complex described as an ophiolite) has been widely ignored by the diamond research community. This skepticism has persisted because the diamonds are similar in many respects to high-pressure high-temperature (HPHT) synthetic/industrial diamonds (grown from metal solvents), and the finding previously has not been independently replicated. We present a detailed examination of the Luobusa diamonds (recovered from both peridotites and chromitites), including morphology, size, color, impurity characteristics (by infrared spectroscopy), internal growth structures, trace-element patterns, and C and N isotopes. A detailed comparison with synthetic industrial diamonds shows many similarities. Cubo-octahedral morphology, yellow color due to unaggregated nitrogen (C centres only, Type Ib), metal-alloy inclusions and highly negative View the MathML source?C13 values are present in both sets of diamonds. The Tibetan diamonds (n=3n=3) show an exceptionally large range in View the MathML source?N15 (?5.6 to +28.7‰+28.7‰) within individual crystals, and inconsistent fractionation between {111} and {100} growth sectors. This in contrast to large synthetic HPHT diamonds grown by the temperature gradient method, which have with View the MathML source?N15=0‰ in {111} sectors and +30‰+30‰ in {100} sectors, as reported in the literature. This comparison is limited by the small sample set combined with the fact the diamonds probably grew by different processes. However, the Tibetan diamonds do have generally higher concentrations and different ratios of trace elements; most inclusions are a NiMnCo alloy, but there are also some small REE-rich phases never seen in HPHT synthetics. These characteristics indicate that the Tibetan diamonds grew in contact with a C-saturated Ni-Mn-Co-rich melt in a highly reduced environment. The stable isotopes indicate a major subduction-related contribution to the chemical environment. The unaggregated nitrogen, combined with the lack of evidence for resorption or plastic deformation, suggests a short (geologically speaking) residence in the mantle. Previously published models to explain the occurrence of the diamonds, and other phases indicative of highly reduced conditions and very high pressures, have failed to take into account the characteristics of the diamonds and the implications for their formation. For these diamonds to be seriously considered as the result of a natural growth environment requires a new understanding of mantle conditions that could produce them.
Geochimica et Cosmochimica Acta, Vol. 157, pp. 1-12.
Technology
Diamond morphology
Abstract: Nitrogen isotope values from mantle diamonds are a commonly used tracer in the quest to track volatiles within the Earth’s mantle through deep time. Interpretations of this isotope data are valid so long as stable isotope fractionation processes in the mantle are understood. The fractionation of nitrogen isotopes between {1 1 1} and {1 0 0} growth sectors is well documented for high-pressure high-temperature (HPHT) synthetic diamonds, but there is little data on whether it also occurs in natural mixed-habit diamonds. We present 91 in-situ nitrogen isotope (?15N) measurements, along with carbon isotope (?13C) values and nitrogen abundances [N], obtained from three mixed-habit diamonds by secondary ion mass spectrometry (SIMS). While the well-documented enrichment of nitrogen concentrations in octahedral sectors compared to contemporaneous cuboid sectors is observed, a similarly clear disparity is not obvious in the ?15N data. Whereas HPHT synthetic diamonds exhibit 15N enrichment in the {1 0 0} sectors by ?+30‰, the mixed-habit diamonds studied here show enrichment of the octahedral sectors in 15N by only 0.4-1‰. This major difference between HPHT synthetic and natural mixed-habit diamonds is proposed to be the result of different physical properties of the growth interfaces. The smooth interfaces of the octahedral sectors are the same in both types of crystal, but the outermost atoms on the smooth cube interfaces of an HPHT synthetic diamond behave differently to those on the rough cuboid interfaces of the natural mixed-habit diamonds, resulting in different ?15N values. Both the ?13C (average of ??8.7‰) and ?15N (average of ?0‰) data show only minor offsets from the typical mantle values (?13C = ?5 ± 3‰, ?15N = ?5 ± 4‰). This may indicate diamond formation from a mantle derived fluid/melt containing a minor subducted component (lowering ?13C values and elevating ?15N) or relate to moderate degrees of isotopic fractionation of a pure mantle fluid/melt by prior diamond precipitation. The homogeneous nature of both the carbon and nitrogen isotopic compositions of all three diamonds, however, documents continuous and unlimited supply of diamond forming fluid/melt, with a constant composition. Such homogenous isotopic compositions exclude fluid mixing or isotopic fractionation close to the site of diamond formation and preclude distinguishing between these two processes based on diamond analyses alone.
Contributions to Mineralogy and Petrology, in press available 19p.
Asia, Tibet
Melting
Abstract: Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F-Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800-850 °C, >15 kbar). Zircons show four main age groupings: 1.0-0.5 Ga, 50-45, 35-20, and 16-13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50-45 Ma zircons match the crystallization age and juvenile character (?Hfi +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35-20 Ma Miocene ages are derived from zircons with similar Hf-O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon ?18O (+8.73-8.97 ‰) values, and extremely low ?Hfi (?12.7 to ?9.4) values. Such evolved Hf-O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low ?18O (+6 to 7.5 ‰), intermediate (?18O +8.5 to 9.0 ‰), and high ?18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with ?18O 10-12 ‰. Both the low and intermediate ?18O groups show significant variation in Fe content, whereas the two high ?18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate ?18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high ?18O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35-20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust-mantle hybridization in the deep Tibetan crust.
Abstract: Using stable isotope data obtained on multiple aliquots of diamonds from worldwide sources, it has been argued that carbon and nitrogen in diamond are decoupled. Here we re-investigate the carbon-nitrogen relationship based on the most comprehensive microbeam data set to date of stable isotopes and nitrogen concentrations in diamonds (n = 94) from a single locality. Our diamond samples, derived from two kimberlites in the Chidliak Field (NE Canada), show large variability in ?13C (? 28.4 ‰ to ? 1.1‰, mode at ? 5.8‰), ?15N (? 5.8 to + 18.8‰, mode at ? 3.0‰) and nitrogen contents ([N]; 3800 to less than 1 at.ppm). In combination, cathodoluminescence imaging and microbeam analyses reveal that the diamonds grew from multiple fluid pulses, with at least one major hiatus documented in some samples that was associated with a resorption event and an abrupt change from low ?13C and [N] to mantle-like ?13C and high [N]. Overall, ?13C appears to be uncorrelated to ?15N and [N] on both the inter- and intra-diamond levels. Co-variations of ?15N-log[N], however, result in at least two parallel, negatively correlated linear arrays, which are also present on the level of the individual diamonds falling on these two trends. These arrays emerge from the two principal data clusters, are characterized by slightly negative and slightly positive ?15N (about ? 3 and + 2‰, respectively) and variable but overall high [N]. Using published values for the diamond-fluid nitrogen isotope fractionation factor and nitrogen partition coefficient, these trends are perfectly reproduced by a Rayleigh fractionation model. Overall, three key elements are identified in the formation of the diamond suite studied: (1.) a low ?13C and low [N] component that possibly is directly associated with an eclogitic diamond substrate or introduced during an early stage fluid event. (2.) Repeated influx of a variably nitrogen-rich mantle fluid (mildly negative ?13C and ?15N). (3.) In waning stages of influx, availability of the mantle-type fluid at the site of diamond growth became limited, leading to Rayleigh fractionation. These fractionation trends are clearly depicted by ?15N-[N] but are not detected when examining co-variation diagrams involving ?13C. Also on the level of individual diamonds, large (? 5‰) variations in ?15N are associated with ?13C values that typically are constant within analytical uncertainty. The much smaller isotope fractionation factor for carbon (considering carbonate- or methane-rich fluids as possible carbon sources) compared to nitrogen leads to an approximately one order of magnitude lower sensitivity of ?13C values to Rayleigh fractionation processes (i.e. during fractionation, a 1‰ change in ?13C is associated with a 10‰ change in ?15N). As a consequence, even minor heterogeneity in the primary isotopic composition of diamond forming carbon (e.g., due to addition of minor subducted carbon) will completely blur any possible co-variations with ?15N or [N]. We suggest this strong difference in isotope effects for C and N to be the likely cause of observations of an apparently decoupled behaviour of carbon and nitrogen isotopes in diamond.
Abstract: Using stable isotope data obtained on multiple aliquots of diamonds from worldwide sources, it has been argued that carbon and nitrogen in diamond are decoupled. Here we re-investigate the carbon-nitrogen relationship based on the most comprehensive microbeam data set to date of stable isotopes and nitrogen concentrations in diamonds (n = 94) from a single locality. Our diamond samples, derived from two kimberlites in the Chidliak Field (NE Canada), show large variability in ?13C (? 28.4 ‰ to ? 1.1‰, mode at ? 5.8‰), ?15N (? 5.8 to + 18.8‰, mode at ? 3.0‰) and nitrogen contents ([N]; 3800 to less than 1 at.ppm). In combination, cathodoluminescence imaging and microbeam analyses reveal that the diamonds grew from multiple fluid pulses, with at least one major hiatus documented in some samples that was associated with a resorption event and an abrupt change from low ?13C and [N] to mantle-like ?13C and high [N]. Overall, ?13C appears to be uncorrelated to ?15N and [N] on both the inter- and intra-diamond levels. Co-variations of ?15N-log[N], however, result in at least two parallel, negatively correlated linear arrays, which are also present on the level of the individual diamonds falling on these two trends. These arrays emerge from the two principal data clusters, are characterized by slightly negative and slightly positive ?15N (about ? 3 and + 2‰, respectively) and variable but overall high [N]. Using published values for the diamond-fluid nitrogen isotope fractionation factor and nitrogen partition coefficient, these trends are perfectly reproduced by a Rayleigh fractionation model. Overall, three key elements are identified in the formation of the diamond suite studied: (1.) a low ?13C and low [N] component that possibly is directly associated with an eclogitic diamond substrate or introduced during an early stage fluid event. (2.) Repeated influx of a variably nitrogen-rich mantle fluid (mildly negative ?13C and ?15N). (3.) In waning stages of influx, availability of the mantle-type fluid at the site of diamond growth became limited, leading to Rayleigh fractionation. These fractionation trends are clearly depicted by ?15N-[N] but are not detected when examining co-variation diagrams involving ?13C. Also on the level of individual diamonds, large (? 5‰) variations in ?15N are associated with ?13C values that typically are constant within analytical uncertainty. The much smaller isotope fractionation factor for carbon (considering carbonate- or methane-rich fluids as possible carbon sources) compared to nitrogen leads to an approximately one order of magnitude lower sensitivity of ?13C values to Rayleigh fractionation processes (i.e. during fractionation, a 1‰ change in ?13C is associated with a 10‰ change in ?15N). As a consequence, even minor heterogeneity in the primary isotopic composition of diamond forming carbon (e.g., due to addition of minor subducted carbon) will completely blur any possible co-variations with ?15N or [N]. We suggest this strong difference in isotope effects for C and N to be the likely cause of observations of an apparently decoupled behaviour of carbon and nitrogen isotopes in diamond.
Abstract: Alluvial diamonds from the Kasai River, Democratic Republic of the Congo (DRC) are sourced from Cretaceous kimberlites of the Lucapa graben in Angola. Analysis of 40 inclusion-bearing diamonds provides new insights into the characteristics and evolution of ancient lithospheric mantle of the Congo craton. Silicate inclusions permitted us to classify diamonds as peridotitic, containing Fo91-95 and En92-94, (23 diamonds, 70% of the suite), and eclogitic, containing Cr-poor pyrope and omphacite with 11-27% jadeite (6 diamonds, 18% of the suite). Fluid inclusion compositions of fibrous diamonds are moderately to highly silicic, matching compositions of diamond-forming fluids from other DRC diamonds. Regional homogeneity of Congo fibrous diamond fluid inclusion compositions suggests spatially extensive homogenization of Cretaceous diamond forming fluids within the Congo lithospheric mantle. In situ cathodoluminescence, secondary ion mass spectrometry and Fourier transform infrared spectroscopy reveal large heterogeneities in N, N aggregation into B-centers (NB), and ?13C, indicating that diamonds grew episodically from fluids of distinct sources. Peridotitic diamonds contain up to 2962 ppm N, show 0-88% NB, and have ?13C isotopic compositions from ? 12.5‰ to ? 1.9‰ with a mode near mantle-like values. Eclogitic diamonds contain 14-1432 ppm N, NB spanning 29%-68%, and wider and lighter ?13C isotopic compositions of ? 17.8‰ to ? 3.4‰. Fibrous diamonds on average contain more N (up to 2976 ppm) and are restricted in ?13C from ? 4.1‰ to ? 9.4‰. Clinopyroxene-garnet thermobarometry suggests diamond formation at 1350-1375 °C at 5.8 to 6.3 GPa, whereas N aggregation thermometry yields diamond residence temperatures between 1000 and 1280 °C, if the assumed mantle residence time is 0.9-3.3 Ga. Integrated geothermobaromtery indicates heat fluxes of 41-44 mW/m2 during diamond formation and a lithosphere-asthenosphere boundary (LAB) at 190-210 km. The hotter-than-average cratonic mantle may be attributable to contemporaneous rifting of the southern Atlantic, multiple post-Archean reactivations of the craton, and/or proximal Cretaceous plumes.
Abstract: Due to the acute scarcity of very ancient rocks, the composition of Earth’s embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today’s oceanic crust1, 2, 3, 4, or included significant amounts of continental crust5, 6, 7, 8 remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth—a 4,019.6 ± 1.8?Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.
Abstract: Due to the acute scarcity of very ancient rocks, the composition of Earth’s embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today’s oceanic crust1, 2, 3, 4, or included significant amounts of continental crust5, 6, 7, 8 remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth—a 4,019.6 ± 1.8?Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.
Smit, K.V., Shirey, S.B., Stern, R.A., Steele, A., Wang, W.
Diamond growth from C-H-N-O recycled fluids in the lithosphere: Evidence from CH4 micro-inclusions and dleta 13 C-Delta 15 N-N content in Marange mixed-habit diamonds.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 9187 Abstract
Africa, Zimbabwe, Sierra Leone
Deposit - Marange, Zimmi
Abstract: Traditional models for diamond formation within the lithospheric mantle invoke either carbonate reduction or methane oxidation. Both these mechanisms require some oxygen exchange with the surrounding wall-rock at the site of diamond precipitation. However, peridotite does not have sufficient buffering capacity to allow for diamond formation via these traditional models and instead peridotitic diamonds may form through isochemical cooling of H 2 O-rich CHO fluids [1]. Marange mixed-habit diamonds from eastern Zimbabwe provide the first natural confirmation of this new diamond growth model [2]. Although Marange diamonds do not contain any silicate or sulphide inclusions, they contain Ni-N-vacancy complexes detected through photoluminescence (PL) spectroscopy that suggest the source fluids equilibrated in the Ni-rich depleted peridotitic lithosphere. Cuboid sectors also contain abundant micro-inclusions of CH 4 , the first direct observation of reduced CH 4-rich fluids that are thought to percolate through the lithospheric mantle [2]. In fluid inclusion-free diamonds, core-to-rim trends in ? 13 C and N content are used to infer the speciation of the diamond-forming fluid. Core to rim trends of increasing ? 13 C with decreasing N content are interpreted as diamond growth from oxidized CO 2-or carbonate-bearing fluids. Diamond growth from reduced species should show the opposite trends-decreasing ? 13 C from core to rim with decreasing N content. Within the CH 4-bearing growth sectors of Marange diamonds, however, such a 'reduced' trend is not observed. Rather, ? 13 C increases from core to rim within a homogeneously grown zone [2]. These contradictory observations can be explained through either mixing between CH 4-and CO 2-rich end-members of hydrous fluids [2] or through closed system precipitation from an already mixed CH 4-CO 2 H 2 O-maximum fluid with XCO 2 (CO 2 /[CO 2 +CH 4 ]) between 0.3 and 0.7 [3]. These results demonstrate that Marange diamonds precipitated from cooling CH 4-CO 2-bearing hydrous fluids rather than through redox buffering. As this growth mechanism applies to both the fluid-rich cuboid and gem-like octahedral sectors of Marange diamonds, a non-redox model for diamond formation from mixed CH 4-CO 2 fluids is indicated for a wider range of gem-quality peridotitic diamonds. Indeed, at the redox conditions of global diamond-bearing lithospheric mantle (FMQ-2 to-4; [4]), CHO fluids are strongly water-dominated and contain both CH 4 and CO 2 as dominant carbon species [5]. By contrast diamond formation in eclogitic assemblages, through either redox buffering or cooling of carbon-bearing fluids, is not as well constrained. Zimmi diamonds from the West African craton have eclogitic sulphide inclusions (with low Ni and high Re/Os) and formed at 650 Ma, overlapping with the timing of subduction [6]. In one Zimmi diamond, a core to rim trend of decreasing ? 13 C (-23.4 to-24.5 %¸) and N content is indicative of formation from reduced C 2 H 6 /CH 4-rich fluids, likely derived from oceanic crust recycled during Neoproterozoic subduction. Unlike mixed CH 4-CO 2 fluids near the water maximum, isochemical cooling or ascent of such reduced CHO fluids is not effficient at diamond precipitation. Furthermore, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the ?1 internal variation seen. Consequently, this Zimmi eclogitic diamond likely formed through redox buffering of reduced subduction-related fluids, infiltrating into sulphide-bearing eclogite.
Geochimica et Cosmochimica Acta, in press available 55p.
Africa, South Africa
Deposit - Roberts Victor
Abstract: Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, collected at the Roberts Victor Diamond mine (South Africa), show a reaction texture in which kyanite is consumed. Geothermobarometric calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a steep, cool prograde metamorphic gradient of 2 °C/km to reach peak conditions of 5.8 GPa and 890 °C for the kyanite eclogite. The kyanite-out reaction formed bimineralic eclogite and is probably an integral part of the mineralogical evolution of most archetypal bimineralic eclogites at Roberts Victor and potentially elsewhere. The kyanite-out reaction occured at close to peak pressure (5.3 GPa) and was associated with a rise in temperature to 1380 °C. Mass balance calculations show that upon breakdown, the kyanite component is fully accommodated in garnet and omphacite via a reaction system with low water fugacity that required restricted fluid influx from metasomatic sources. The ?18O values of garnets are consistently higher than normal mantle values. Each sample has its characteristic trend of ?18O variance between garnets in the kyanite-bearing sections and those in the bimineralic parts covering a range between 5.1‰ and 6.8‰. No systematic change in O-isotope signature exists across the sample population. Differences in garnet trace element signatures between differing lithologies in the eclogites are significant. Grossular-rich garnets coexisting with kyanite have strong positive Eu-anomalies and low Gd/Yb ratios, while more pyrope-rich garnets in the bimineralic sections have lost their positive Eu-anomaly, have higher Gd/Yb ratios and generally higher heavy rare earth element contents. Garnets in the original kyanite-bearing portions thus reflect the provenance of the rocks as metamorphosed gabbros/troctolites. The kyanite-out reaction was most likely triggered by a heating event in the subcratonic lithosphere. As kyanite contains around 100 ppm of H2O it is suggested that the kyanite-out reaction, once initiated by heating and restricted metasomatic influx, was promoted by the release of water contained in the kyanite. The steep (high-P low-T) prograde P-T path defining rapid compression at low heating rates is atypical for subduction transport of eclogites into the lithospheric mantle. Such a trajectory is best explained in a model where strong lateral compression forces eclogites downward to higher pressures, supporting models of cratonic lithosphere formation by lateral collision and compression.
Geochimica et Cosmochimica Acta, Vol. 213, pp. 574-592.
Africa, Botswana
deposit - Orapa
Abstract: Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and ?18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy ?18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu?, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like ?18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ?3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
Abstract: Polycrystalline diamond aggregates (diamondites) are produced by rapid crystal nucleation caused by extreme carbon supersaturation in mantle fluids. They may form episodically and under variable chemical conditions, providing snapshots of diamond formation in the Earth’s mantle. Diamondites, thus, represent an extreme end member of diamond formation mechanisms, while forming via the same processes and ingredients as the gem-sized diamonds. We present results on a large suite of diamondites from the Venetia mine (South Africa), comprising a complete characterisation of the diamonds and their silicate inclusions and intergrowths. The highlighted characteristic of this sample suite is its heterogeneity in all aspects, from affiliated silicate to diamond composition and texture of the diamond aggregates. The diamond grains in the samples are intergrown with silicates (garnets, clinopyroxenes, phlogopites) comprising a websteritic-eclogitic and a peridotiticpyroxenitic suite of minerals. Diamonds, regardless of their affiliation based on their silicate phases, overlap in carbon and nitrogen composition and have ?13C values between -28 and -8 ‰, ?15N values of 0.8 to 16.3 ‰ and nitrogen contents of 4 to 2329 ppm. The entire range of carbon and nitrogen variability of the suite is also reflected in some individual samples. Cathodoluminescence imaging visualizes different zones in the samples that can be interpreted as different growth events with differing nitrogen contents and ?15N decoupled from ?13C values, in line with the variability off nitrogen aggregation states. Electron backscatter diffraction analyses identify an original texture of randomly intergrown diamond grains that is partly changed by deformation and newly grown smaller diamond grains. The large overall variability suggesting episodic formation of diamondite with nitrogen from crustal sources.
Abstract: Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, collected at the Roberts Victor Diamond mine (South Africa), show a reaction texture in which kyanite is consumed. Geothermobarometric calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a steep, cool prograde metamorphic gradient of 2 °C/km to reach peak conditions of 5.8 GPa and 890 °C for the kyanite eclogite. The kyanite-out reaction formed bimineralic eclogite and is probably an integral part of the mineralogical evolution of most archetypal bimineralic eclogites at Roberts Victor and potentially elsewhere. The kyanite-out reaction occured at close to peak pressure (5.3 GPa) and was associated with a rise in temperature to 1380 °C. Mass balance calculations show that upon breakdown, the kyanite component is fully accommodated in garnet and omphacite via a reaction system with low water fugacity that required restricted fluid influx from metasomatic sources. The ?18O values of garnets are consistently higher than normal mantle values. Each sample has its characteristic trend of ?18O variance between garnets in the kyanite-bearing sections and those in the bimineralic parts covering a range between 5.1‰ and 6.8‰. No systematic change in O-isotope signature exists across the sample population. Differences in garnet trace element signatures between differing lithologies in the eclogites are significant. Grossular-rich garnets coexisting with kyanite have strong positive Eu-anomalies and low Gd/Yb ratios, while more pyrope-rich garnets in the bimineralic sections have lost their positive Eu-anomaly, have higher Gd/Yb ratios and generally higher heavy rare earth element contents. Garnets in the original kyanite-bearing portions thus reflect the provenance of the rocks as metamorphosed gabbros/troctolites. The kyanite-out reaction was most likely triggered by a heating event in the subcratonic lithosphere. As kyanite contains around 100 ppm of H2O it is suggested that the kyanite-out reaction, once initiated by heating and restricted metasomatic influx, was promoted by the release of water contained in the kyanite. The steep (high-P low-T) prograde P-T path defining rapid compression at low heating rates is atypical for subduction transport of eclogites into the lithospheric mantle. Such a trajectory is best explained in a model where strong lateral compression forces eclogites downward to higher pressures, supporting models of cratonic lithosphere formation by lateral collision and compression.
Geochimica et Cosmochinica Acta, Vol. 213, pp. 574-592.
Africa, Botswana
deposit - Orapa
Abstract: Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and ?18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy ?18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu?, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like ?18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ?3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
Mineralogy and Petrology, in press available, 11p.
Africa, Botswana
deposit - Orapa
Abstract: Spatially resolved analyses reveal considerable isotopic heterogeneity within and among diamonds ranging in size from 0.15 to 4.75 mm from the Orapa Mine, Botswana. The isotopic data are interpreted in conjunction with nitrogen aggregation state data and growth zone relationships from cathodoluminescence images. The integrated information confirms that a distinct diamond growth event (with low IaAB nitrogen aggregation states, moderately high nitrogen contents and ?13C and ?15N values compatible with average mantle values) is younger than the dominant population(s) of Type IaAB diamonds and cores of composite diamonds with more negative and positive ?13C and ?15N values, respectively. A significant proportion of the older diamond generation has high nitrogen contents, well outside the limit sector relationship, and these diamonds are likely to reflect derivation from subducted organic matter. Diamonds with low ?13C values combined with high nitrogen contents and positive ?15N values have not been previously widely recognised, even in studies of diamonds from Orapa. This may have been caused by prior analytical bias towards inclusion-bearing diamonds that are not necessarily representative of the entire range of diamond populations, and because of average measurements from heterogeneous diamonds measured by bulk combustion methods. Two distinct low nitrogen/Type II microdiamond populations were recognised that do not appear to continue into the macrodiamond sizes in the samples studied. Other populations, e.g. those containing residual singly-substituted nitrogen defects, range in size from small microdiamonds to large macrodiamonds. The total diamond content of the Orapa kimberlite thus reflects a complex assortment of multiple diamond populations.
Mineralogy and Petrology, 10.1007/ s710-018-0576 -9, 10p.
South America, Brazil
deposit - Sao Luiz
Abstract: Three diamonds from Sao Luiz, Brazil carrying nano- and micro-inclusions of molecular ?-N2 that exsolved at the base of the transition zone were studied for their C and N isotopic composition and the concentration of N utilizing SIMS. The diamonds are individually uniform in their C isotopic composition and most spot analyses yield ?13C values of ?3.2?±?0.1‰ (ON-SLZ-390) and???4.7?±?0.1‰ (ON-SLZ-391 and 392). Only a few analyses deviate from these tight ranges and all fall within the main mantle range of ?5?±?3‰. Most of the N isotope analyses also have typical mantle ?15N values (?6.6?±?0.4‰, ?3.6?±?0.5‰ and???4.1?±?0.6‰ for ON-SLZ-390, 391 and 392, respectively) and are associated with high N concentrations of 800-1250 atomic ppm. However, some N isotopic ratios, associated with low N concentrations (<400 ppm) and narrow zones with bright luminescence are distinctly above the average, reaching positive ?15N values. These sharp fluctuations cannot be attributed to fractionation. They may reflect arrival of new small pulses of melt or fluid that evolved under different conditions. Alternatively, they may result from fractionation between different growth directions, so that distinct ?15N values and N concentrations may form during diamond growth from a single melt/fluid. Other more continuous variations, in the core of ON-SLZ-390 or the rim of ON-SLZ-392 may be the result of Rayleigh fractionation or mixing.
Geochimica et Cosmochimica Acta, Vol. 213, 1, pp. 574-592.
Africa, Botswana
deposit - Orapa
Abstract: Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and ?18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy ?18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu?, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like ?18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ?3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the - possibly oxidising - metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
deposit - Argyle, De Beers Pool, Jwaneng, Orapa, Udachnaya, Venetia, Wawa, Diavik
Abstract: Earth’s mantle is by far the largest silicate-hosted reservoir of carbon. Diamonds are unrivalled in their ability to record the cycle of mantle carbon and other volatiles over a vast portion of the Earth’s history. They are the product of ascending, cooling, carbon-saturated, metasomatic fluidsmelts and/or redox reactions, predominantly within peridotitic and eclogitic domains in the mantle lithosphere. This paper reports the results of a major secondary ion mass spectrometry (SIMS) carbon isotope study, carried out on 127 diamond samples, spanning a large range of geological time. Detailed transects across the incremental growth zones within each diamond were measured for C isotopes, N abundances and, for samples with N >~200 at.ppm, N isotopes. Given that all of the samples are fragments, recovered when the original crystals were broken to liberate their inclusions, 81 of the analytical traverses have confirmed growth direction context. 98 samples are from studies that have confirmed the dates of the individual diamonds through analysis of their silicate or sulphide inclusions, from source localities including Argyle, De Beers Pool, Jwaneng, Orapa, Udachnaya & Venetia. Additional samples come from Wawa (a minimum age) and Diavik where the samples are tied via inclusion paragenesis to published ages. The peridotitic dataset covers the age range of ~3.3 - 2.0 Ga, with the eclogitic data from 2.9 - 1.0 Ga. In total, 751 carbon isotope and nitrogen concentration measurements have been obtained (425 on peridotitic diamonds, and 326 on eclogitic diamonds) with 470 nitrogen isotope measurements (190 P, 280 E). We attempt to constrain the diamond carbon isotope record through time and its implications for (i) the mantle carbon reservoir, (ii) its oxygen fugacity, (iii) the fluid / melt growth environment of diamonds, (iv) fractionation trends recorded in individual diamonds, and (v) diamond population studies using bulk combustion carbon isotope analysis.
Abstract: Polycrystalline diamond aggregates (framesites, boart, diamondite) are an understudied variety of mantle diamond, but can make up 20% of the production in some Group I kimberlites. Their polycrystalline nature indicates rapid precipitation from carbon-oversaturated fluids and individual PDAs often contain a chemically heterogeneous suite of websteritic and pyroxenitic inclusions and minerals intimately intergrown with the diamond crystals. Geochemical and microstructural evidence suggests that fluid-driven redox reactions with lithospheric material occurring episodically over millions of years play a major role in freezing carbon in the subcratonic lithosphere (Jacob et al., 2000; 2016; Mikhail et al., 2014). A suite of 39 samples from the Venetia kimberlite pipe in South Africa allows a more detailed look at the diamondforming fluids. 13C values in the diamonds measured by secondary ion mass spectrometry range from +2 to -28 and cover the entire range for PDA from the literature. Nitrogen concentrations are mostly very low (less than 100 at ppm), but reach up to 2660 at ppm in individual samples. These high nitrogen concentrations in concert with mostly positive 15N values of up to +17 and some very negative 3C values suggest crustal material as the source of the nitrogen and the carbon. However, detailed analysis of the sample provides evidence for a more complex growth history followed by alteration. Individual diamond crystals show complex growth zonations by cathodoluminescence imaging that can be related with the carbon and nitrogen isotopic compositions and points to growth incorporating several pulses of carbon-nitrogen fluid with distinct isotopic compositions. Most of these growth events show decoupled carbon and nitrogen systematics. In addition, EBSD identifies deformation and recrystallization and nitrogen aggregation states range from pure IaA to pure IaB, supporting a heterogeneous and episodic growth history.
Africa, South Africa, Guinea, South America, Brazil
deposit - Kankan, Jagersfontein, Juina
Abstract: Inclusions in super-deep diamonds provide a unique window to the sublithospheric mantle (e.g. [1-4]). Here we present oxygen isotopes for Kankan majoritic garnet and former bridgmanite inclusions. The clustering of Kankan majorites around a ?18O of +9‰ is nearly identical to those reported from Jagersfontein [1]. This elevated and nearly constant ?18O signal indicates homogenization of partial melts from the uppermost part of altered basaltic slabs. Conversely, ?18O values in Juina majorites are highly variable [2] due to crystallization from small, discrete melt pockets in a heterogeneous eclogitic source. While all these majorites have eclogitic/pyroxenitic Cr2O3 and CaO contents, charge-balance for Si[VI] is achieved very differently, with Jagersfontein [3], Kankan [4], and Juina [2] majorites transitioning from eclogitic Na[VIII]Si[VI] to peridotitic-pyroxenitic [5] Mg[VI]Si[VI] substitutions. We interpret this shift as the result of homogenized eclogitic partial melts infiltrating and reacting with adjacent pyrolitic mantle at Kankan and Jagersfontein. Increases in Mg# and Cr2O3 with reductions in ?18O support this reaction. This model is in agreement with recent experiments in which majorites and diamonds form from a reaction of slab-derived carbonatite with reduced pyrolite at 300-700 km depth [6]. The Kankan diamonds also provide an opportunity to establish the chemical environment of the lower mantle. Four inclusions of MgSiO3, inferred to be former bridgmanite [4], provide the first-measured ?18O values for lower mantle samples. These values suggest derivation from primitive mantle, or unaltered subducted oceanic lithospheric mantle. The Kankan super-deep inclusions thus provide a cross-section of deep mantle that highlights slab-pyrolite reactions in the asthenosphere and primitive compositions in the lower mantle.
Bulbuc, K.M., Galarneau, M., Stachel, T., Stern, R.A., Kong, J., Chinn, I.
Contrasting growth conditions for sulphide-and garnet-included diamonds from the Victor mine ( Ontario).
2018 Yellowknife Geoscience Forum , p. 97-98. abstract
Canada, Ontario, Attawapiskat
deposit - Victor
Abstract: The Victor Diamond Mine, located in the Attawapiskat kimberlite field (Superior Craton), is known for its exceptional diamond quality. Here we study the chemical environment of formation of Victor diamonds. We imaged eight sulphide-included diamond plates from Victor using cathodoluminescence (CL). Then, along core-rim transects, we measured nitrogen content and aggregation state utilizing Fourier Transform Infrared (FTIR) spectroscopy, and the stable isotope compositions of carbon (?13C) and nitrogen (?15N), using a multi-collector ion microprobe (MC-SIMS). We compare the internal growth features and chemical characteristics of these sulphide inclusion-bearing diamonds with similar data on garnet inclusion-bearing diamonds from Victor (BSc thesis Galarneau). Using this information, possible fractionation processes during diamond precipitation are considered and inferences on the speciation of the diamond forming fluid(s) are explored. Sulphide inclusion-bearing diamonds show much greater overall complexity in their internal growth features than garnet inclusion-bearing diamonds. Two of the sulphide-included samples have cores that represent an older generation of diamond growth. Compared to garnet inclusion-bearing diamonds, the sulphide-included diamonds show very little intra-sample variation in both carbon and nitrogen isotopic composition; the inter-sample variations in carbon isotopic composition, however, are higher than in garnet included diamonds. For sulphide-included diamonds, ?13C ranges from -3.4 to -17.5 and ?15N ranges from -0.2 to -9.2. Garnet inclusion-bearing diamonds showed ?13C values ranging from -4.6 to -6.0 and ?15N ranging from -2.8 to -10.8. The observation of some 13C depleted samples indicates that, unlike the lherzolitic garnet inclusion-bearing diamonds, the sulphide inclusion-bearing diamonds are likely both peridotitic and eclogitic in origin. The total range in N content across sulphide inclusion-bearing diamonds was 2 to 981 at ppm, similar to the garnet-included samples with a range of 5 to 944 at ppm. The very limited variations in carbon and nitrogen isotopic signatures across growth layers indicate that sulphide-included Victor diamonds grew at comparatively high fluid:rock ratios. This is contrasted by the garnet inclusion-bearing diamonds that commonly show the effects of Rayleigh fractionation and hence grew under fluid-limited conditions.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 343-358.
Regier, M.E., Pearson, D.G., Stachel, T., Stern, R.A., Harris, J.
Tracing the formation and abundance of superdeep diamonds.
2018 Yellowknife Geoscience Forum , p. 63. abstract
Africa, Guinea
deposit - Kankan
Abstract: Super-deep diamonds from the transition zone and lower mantle are valuable targets for mining, as they are often large, gem-quality1 or ultra-valuable type IIb stones2. Hence, in mine prospects, it may become important to determine the various populations of sub-lithospheric diamonds. Unambiguously identifying a diamond’s depth of formation is difficult as some minerals can be indicative of various depth regimes (e.g., ferropericlase, Ca-walstromite, enstatite, clinopyroxene, coesite). Here, we use the oxygen isotope compositions of inclusions in Kankan diamonds from Guinea to distinguish between the various diamond-forming processes that happen at lithospheric, asthenospheric to transition zone, and lower mantle depths. In this way, we hope to establish a process by which isotope geochemistry can better constrain the populations of superdeep diamonds in kimberlites, and can assist in estimating a pipe’s propensity for large, valuable stones. Oxygen isotopic analysis by secondary ion mass spectrometry (SIMS) is a high-precision technique that can track hydrothermal alteration that occurred at or close below the ocean floor. Our analyses of inclusions from Kankan diamonds demonstrate that garnets with 3-3.03 Si cations (pfu) have ?18O that are well-constrained within the normal values expected for peridotitic and eclogitic inclusions, but that garnets with ?3.04 Si cations (pfu) have consistently high ?18O (median: 10‰) that slightly decreases with increasing Cr2O3. We interpret this signal as the reaction between a melted carbonate-rich oceanic slab and normal convecting asthenosphere3. In contrast, retrogressed, or former, bridgmanite has ?18O values similar to primitive mantle, suggesting little involvement of slab melts. In contrast to the worldwide suite of lithospheric inclusions of eclogitic paragenesis (median ?18O of 7.03‰)4,5, diamonds derived from ~250 to 500 km have inclusions with consistent, extremely high oxygen isotopes (median: 9.32‰)6,7, due to the melting of extremely enriched carbonated oceanic crust. Diamonds from the lower mantle, however, have inclusions with primitive mantle oxygen isotopes, suggesting a different formation process. The clear distinction in inclusion ?18O between lithospheric, asthenospheric to transition zone, and lower mantle diamond populations is useful in informing the depth regime of a suite of stones, especially those with inclusions of ambiguous depths (e.g., clinopyroxene, coesite, Ca-walstromite, enstatite, ferropericlase, etc.). For instance, we are currently searching for exotic oxygen isotopes in ferropericlase that indicate asthenospheric diamond growth, rather than the primitive mantle values expected for lower mantle ferropericlase. In conclusion, oxygen isotopic analyses of diamond inclusions can identify various sublithsopheric diamond populations, and may benefit the assessment of a mine’s potential for large gem-quality, or type IIb diamonds.
Geochemical Perspectives Letters, Vol. 9, pp. 6-10. 10.7185/geochemlet.1830
Mantle
peridotites
Abstract: The origin of the peridotites that form cratonic mantle roots is a central issue in understanding the history and survival of Earth’s oldest continents. A long-standing hypothesis holds that the unusual bulk compositions of some cratonic peridotites stem from their origin as subducted oceanic serpentinite, dehydrated during subduction to form rigid buoyant keels (Schulze, 1986; Canil and Lee, 2009). We present oxygen isotope data from 93 mantle peridotites from five different Archean cratons to evaluate their possible origin as serpentinites. Cratonic mantle peridotite shows remarkably uniform ?18O values, identical to modern MORB-source mantle, that do not vary with bulk rock Si-enrichment or Ca-depletion. These data clearly conflict with any model for cratonic lithosphere that invokes serpentinite as a protolith for cratonic peridotite, and place additional constraints on cratonic mantle origins. We posit that the uniform ?18O was produced by sub-arc and/or MOR depletion processes and that the Si-enriched nature of some samples is unlikely to be related to slab melt infiltration. Instead, we suggest a peridotitic source of Si-enrichment, derived from ascending mantle melts, or a water-fluxed depleted mantle. These variably Si-enriched, cratonic mantle protoliths were then collisionally compressed into the thick cratonic roots that have protected Earth’s oldest continental crust for over 2.5 Gyr.
Chemical Geology, doi.org/10,1016/j.chem geo.2019.04.014 37p.
Africa, Sierra Leone
deposit - Zimmi
Abstract: Here we present SIMS data for a suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the origin and redox-controlled speciation of diamond-forming fluids for these Neoproterozoic eclogitic diamonds. Low ?13C values below ?15‰ in three diamonds result from fluids that originated as carbon in the oceanic crust, and was recycled into the diamond-stable subcratonic lithospheric mantle beneath Zimmi during subduction. ?13C values between ?6.7 and ?8.3‰ in two diamonds are within the range for mantle-derived carbon and could reflect input from mantle fluids, serpentinised peridotite, or homogenised abiogenic and/or biogenic carbon (low ?13C values) and carbonates (high ?13C values) in the oceanic crust. Diamond formation processes in eclogitic assemblages are not well constrained and could occur through redox exchange reactions with the host rock, cooling/depressurisation of CHO fluids or during H2O-loss from CHO fluids. In one Zimmi diamond studied here, a core to rim trend of decreasing ?13C (?23.4 to ?24.5‰) and decreasing [N] is indicative of formation from reduced CH4-bearing fluids. Unlike mixed CH4-CO2 fluids near the water maximum, isochemical diamond precipitation from such reduced CHO fluids will only occur during depressurisation (ascent) and should not produce coherent fractionation trends in single diamonds that reside at constant depth (pressure). Furthermore, due to a low relative proportion of the total carbon in the fluid being precipitated, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the 1‰ internal core-to- rim variation. Consequently, this Zimmi eclogitic diamond showing a coherent trend in ?13C and [N] likely formed through oxidation of methane by the host eclogite, although the mineralogical evidence for this process is currently lacking.
Nature Research Scientific Reports, https://doi.org/10.1038/ s41598-019-55743-1 11p. Pdf
Mantle
melting, redox
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Oxygen fugacity (fO2) is an intensive variable implicated in a range of processes that have shaped the Earth system, but there is controversy on the timing and rate of oxidation of the uppermost convecting mantle to its present fO2 around the fayalite-magnetite-quartz oxygen buffer. Here, we report Fe3+/?Fe and ƒf2 for ancient eclogite xenoliths with oceanic crustal protoliths that sampled the coeval ambient convecting mantle. Using new and published data, we demonstrate that in these eclogites, two redox proxies, V/Sc and Fe3+/?Fe, behave sympathetically, despite different responses of their protoliths to differentiation and post-formation degassing, seawater alteration, devolatilisation and partial melting, testifying to an unexpected robustness of Fe3+/?Fe. Therefore, these processes, while causing significant scatter, did not completely obliterate the underlying convecting mantle signal. Considering only unmetasomatised samples with non-cumulate and little-differentiated protoliths, V/Sc and Fe3+/?Fe in two Archaean eclogite suites are significantly lower than those of modern mid-ocean ridge basalts (MORB), while a third suite has ratios similar to modern MORB, indicating redox heterogeneity. Another major finding is the predominantly low though variable estimated fO2 of eclogite at mantle depths, which does not permit stabilisation of CO2-dominated fluids or pure carbonatite melts. Conversely, low-fO2 eclogite may have caused efficient reduction of CO2 in fluids and melts generated in other portions of ancient subducting slabs, consistent with eclogitic diamond formation ages, the disproportionate frequency of eclogitic diamonds relative to the subordinate abundance of eclogite in the mantle lithosphere and the general absence of carbonate in mantle eclogite. This indicates carbon recycling at least to depths of diamond stability and may have represented a significant pathway for carbon ingassing through time.
Diamonds & Related Materials, Vol. 101, 107642, 8p. Pdf
Canada, Northwest Territories
deposit - Ekati
Abstract: High pressure high temperature (HPHT) treatment has long been applied in the gem trade for changing the body colour of diamonds. The identification of HPHT-treated diamonds is a field of on-going research in gemological laboratories, as different parameters of treatment will result in either the creation or the destruction of a variety of lattice defects in diamonds. Some features that exist in treated diamonds can also be found in natural diamonds, and consequently must not be employed for the separation of treated and natural diamonds. In this research, we investigated the properties of 11 natural yellow diamonds (directly obtained from the Ekati Diamond Mine to ensure that they are untreated) before and after HPHT treatment, conducted at a temperature of 2100 °C and a pressure of 6 GPa for 10 min. We report spectroscopic data and fluorescence characteristics, collected using PL mapping, FTIR mapping and fluorescence imaging showing the distribution of lattice defects and internal growth structures. PL mapping indicates SiV defects exist in one of the nitrogen-rich natural diamonds prior to treatment. Silicon-related defects can also be created by HPHT treatment, and they seem to show a relationship with pre-existing NV? centres. SIMS analysis was conducted to confirm the presence of silicon in these diamonds. The increase in the hydrogen-related infrared absorption peak at 3107 cm?1 (VN3H) is very strong in some diamonds that do not form B-centres during treatment. NVH was observed in our HPHT-treated natural diamonds, so it is possible that this strong increase in VN3H suppresses the aggregation of A- to B-centres as the newly formed A-centres were captured by NVH lattice defects to form VN3H. HPHT-altered and HPHT-induced platelet peaks are different from their natural counterparts in peak width and shape. Strong green fluorescence over a large area of a diamond, which is linked to relatively high concentration of H3 centres, was produced after HPHT treatment. We are confident that the unusual platelet peaks and strong emission of H3 centres are reliable indicators for HPHT-treated diamonds as they are not observed in untreated natural diamonds.
Mineralogy and Petrology, in press available 13p. Pdf
Canada, Northwest Territories
deposit - Chidliak, Ekati
Abstract: Yellow diamonds from the CH-7 (Chidliak) and the Misery (Ekati Mine) kimberlites in northern Canada are characterised for their nitrogen characteristics, visible light absorption, internal growth textures, and carbon isotope compositions. The diamonds are generally nitrogen-rich, with median N contents of 1230 (CH-7) and 1030 at.ppm (Misery). Normally a rare feature in natural diamonds, single substitutional nitrogen (C centres) and related features are detected in infrared absorption spectra of 64% of the studied diamonds from CH-7 and 87% from Misery and are considered as the major factor responsible for their yellow colouration. Episodically grown diamonds, characterised by colourless cores containing some nitrogen in the fully aggregated form (B centres) and yellow outer layers containing C centres, occur at both localities. Carbon isotope compositions and N contents also are significantly different in such core and rim zones, documenting growth during at least two temporally distinct events and involving different diamond forming fluids. Based on their nitrogen characteristics, both the yellow diamonds and yellow rims must have crystallized in close temporal proximity (<<1 Ma) to kimberlite activity at CH-7 and Misery.
Geochimica et Cosmochimica Acta, Vol. 275, pp. 99-122.
Mantle
carbon
Abstract: Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earth’s history. Diamonds’ inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as ?3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from ?3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total ?¹³C data range from ?17.1 to ?1.9 ‰ (P = ?8.4 to ?1.9 ‰; E = ?17.1 to ?2.1‰) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in ?¹³C of peridotitic diamonds has been at ?5 (±2) ‰ since the earliest diamond growth ?3.5 Ga, and this mode is also observed in the eclogitic diamond record since ?3 Ga. The skewness of eclogitic diamonds’ ?¹³C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent. No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1‰, but also that the principal ?¹³C mode of ?5 ± 1‰ in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga.
Nature Research Scientific Reports, Vol. 9:20190 doir.org/10.38 /s41598-019-55743-1, 11p. Pdf
Mantle
eclogite
Abstract: Oxygen fugacity (ƒO2) is an intensive variable implicated in a range of processes that have shaped the Earth system, but there is controversy on the timing and rate of oxidation of the uppermost convecting mantle to its present ƒO2 around the fayalite-magnetite-quartz oxygen buffer. Here, we report Fe3+/?Fe and ƒO2 for ancient eclogite xenoliths with oceanic crustal protoliths that sampled the coeval ambient convecting mantle. Using new and published data, we demonstrate that in these eclogites, two redox proxies, V/Sc and Fe3+/?Fe, behave sympathetically, despite different responses of their protoliths to differentiation and post-formation degassing, seawater alteration, devolatilisation and partial melting, testifying to an unexpected robustness of Fe3+/?Fe. Therefore, these processes, while causing significant scatter, did not completely obliterate the underlying convecting mantle signal. Considering only unmetasomatised samples with non-cumulate and little-differentiated protoliths, V/Sc and Fe3+/?Fe in two Archaean eclogite suites are significantly lower than those of modern mid-ocean ridge basalts (MORB), while a third suite has ratios similar to modern MORB, indicating redox heterogeneity. Another major finding is the predominantly low though variable estimated ƒO2 of eclogite at mantle depths, which does not permit stabilisation of CO2-dominated fluids or pure carbonatite melts. Conversely, low-ƒO2 eclogite may have caused efficient reduction of CO2 in fluids and melts generated in other portions of ancient subducting slabs, consistent with eclogitic diamond formation ages, the disproportionate frequency of eclogitic diamonds relative to the subordinate abundance of eclogite in the mantle lithosphere and the general absence of carbonate in mantle eclogite. This indicates carbon recycling at least to depths of diamond stability and may have represented a significant pathway for carbon ingassing through time.
Abstract: Because of their robust nature, diamonds survive mantle processes and protect occluded minerals since the time of diamond formation. For the Kaapvaal Craton - the archetype for craton formation and evolution - the geochemical signatures of inclusions in Koffiefontein diamonds tell a story from craton formation to evolution and from lithospheric (below about 160 km) to lower mantle (>660 km) environs. We analysed a suite of 94 lithospheric to lower mantle diamonds and their silicate and oxide inclusions. Geochemical results confirm that the diamond substrates are very depleted, with Mg#OL of 91.5-95.0 and a dominance of low-Ca (<1.8 wt% CaO), presumably dunite-derived garnet. The Si-rich nature and preserved high Mg# of the peridotitic diamond substrates beneath Koffiefontein and the formation of KNbO3 (goldschmidtite) from an extremely fractionated melt/fluid indicate that potentially both mantle- and subduction-related fluids are the cause of metasomatism in the Kaapvaal cratonic root. Mantle-like, restricted carbon isotopic compositions of both P- and E-type diamonds (avg. ?13C -5.7 ‰ and -6.6 ‰, respectively) indicate that an abundant, mantle-derived CHO fluid is responsible for diamond formation. Diamonds have a large range in nitrogen concentrations and isotopic compositions, suggesting decoupling from carbon and heterogeneous sources. ?18O of former bridgmanite and ?13C of its host diamond document a purely mantle-derived lower mantle component. Combined, these results reveal a complex and multistage evolution of the Kaapvaal Craton whereby multiple episodes of fluid and melt metasomatism re-enriched the craton already, prior to diamond formation, followed by diamond entrainment in a kimberlite possibly derived from the lower mantle.
Abstract: The transport of carbon into Earth’s mantle is a critical pathway in Earth’s carbon cycle, affecting both the climate and the redox conditions of the surface and mantle. The largest unconstrained variables in this cycle are the depths to which carbon in sediments and altered oceanic crust can be subducted and the relative contributions of these reservoirs to the sequestration of carbon in the deep mantle1. Mineral inclusions in sublithospheric, or ‘superdeep’, diamonds (derived from depths greater than 250 kilometres) can be used to constrain these variables. Here we present oxygen isotope measurements of mineral inclusions within diamonds from Kankan, Guinea that are derived from depths extending from the lithosphere to the lower mantle (greater than 660 kilometres). These data, combined with the carbon and nitrogen isotope contents of the diamonds, indicate that carbonated igneous oceanic crust, not sediment, is the primary carbon-bearing reservoir in slabs subducted to deep-lithospheric and transition-zone depths (less than 660 kilometres). Within this depth regime, sublithospheric inclusions are distinctly enriched in 18O relative to eclogitic lithospheric inclusions derived from crustal protoliths. The increased 18O content of these sublithospheric inclusions results from their crystallization from melts of carbonate-rich subducted oceanic crust. In contrast, lower-mantle mineral inclusions and their host diamonds (deeper than 660 kilometres) have a narrow range of isotopic values that are typical of mantle that has experienced little or no crustal interaction. Because carbon is hosted in metals, rather than in diamond, in the reduced, volatile-poor lower mantle2, carbon must be mobilized and concentrated to form lower-mantle diamonds. Our data support a model in which the hydration of the uppermost lower mantle by subducted oceanic lithosphere destabilizes carbon-bearing metals to form diamond, without disturbing the ambient-mantle stable-isotope signatures. This transition from carbonate slab melting in the transition zone to slab dehydration in the lower mantle supports a lower-mantle barrier for carbon subduction.
Journal of Petrology, doi.org/petrology/egab090 65p. Pdf
Africa, South Africa
deposit - Roberts Victor
Abstract: The origin of the eclogites that reside in cratonic mantle roots has long been debated. In the classic Roberts Victor kimberlite locality in South Africa, the strongly contrasting textural and geochemical features of two types of eclogites have led to different genetic models. We studied a new suite of 63 eclogite xenoliths from the former Roberts Victor Mine. In addition to major- and trace-element compositions for all new samples, we determined 18O/16O for garnet from 34 eclogites. Based on geochemical and textural characteristics we identify a large suite of Type I eclogites (n = 53) consistent with previous interpretations that these rocks originate from metamorphosed basaltic-picritic lavas or gabbroic cumulates from oceanic crust, crystallised from melts of depleted MORB mantle. We identify a smaller set of Type II eclogites (n = 10) based on geochemical and textural similarity to eclogites in published literature. We infer their range to very low ?18O values combined with their varied, often very low Zr/Hf ratios and LREE-depleted nature to indicate a protolith origin via low-pressure clinopyroxene-bearing oceanic cumulates formed from melts that were more depleted in incompatible elements than N-MORB. These compositions are indicative of derivation from a residual mantle source that experienced preferential extraction of incompatible elements and fractionation of Zr-Hf during previous melting.
Abstract: Metasomatic processes modify the composition of the subcratonic lithospheric mantle and can either form or destroy diamonds. The composition of these metasomatic agents is uncertain and has been mostly deduced from chemical zonation and overprints recorded by associated mantle silicates. Diamonds experience partial dissolution (resorption) during their residence in the mantle due to mantle metasomatism and later during their ascent in kimberlite magma. Diamonds, enclosed inside mantle xenoliths during the whole duration of ascent in kimberlite magma, can preserve their pre-kimberlite surface features, which record the last diamond-destructive metasomatic event to have occurred in the mantle. The geometry of diamond dissolution features acquired during mantle storage can provide information about diamond-destructive metasomatic events in the mantle. Diamond samples recovered from inside mantle xenoliths are extremely rare and mostly limited to eclogitic lithology, which suggests that variable resistance of different mantle lithologies to disintegration in kimberlite magma may affect representativity of these sample. Here we use whole diamond populations from exploration parcels and apply our earlier developed set of criteria to distinguish between kimberlitic and pre-kimberlitic surface features on diamonds. The study used diamonds (<1 to 4.5 mm size) from eight kimberlites in three localities: Orapa cluster, Botswana (BK1, AK15, and AK1 kimberlites), Ekati Mine, Northwest Territories, Canada (Grizzly, Leslie, Koala, and Misery kimberlites), and Snap Lake kimberlite dyke, Northwest Territories, Canada. The host kimberlites cover seven different volcaniclastic and coherent kimberlite lithologies, and our previous studies demonstrated a correlation between the style of kimberlitic resorption on diamonds and the host kimberlite lithology for these samples. From the total of 3256 studied diamonds, we identified 534 diamonds with pre-kimberlite surface textures. These pre-kimberlite surface textures display six distinct types, which are present in all the studied diamond parcels regardless of their geographic locality and host kimberlite lithology. The relative proportions of these types depend on the geographic locality showing linkage to a specific mantle source. We examined the relationship between the surface features on diamonds, their growth patterns revealed in cathodoluminescence (CL) images, the content and aggregation of nitrogen defects using Fourier transform infrared spectroscopy (FTIR), and nitrogen content in specific growth zones of diamonds obtained using secondary ion mass spectrometry (SIMS) for 82 Ekati diamonds. Our data show that growth step-faces develop on diamonds with complex multi-crystal cores, whereas flat-faced octahedra with simple oscillatory-zoned growth patterns derive from single growth events. Initial stages of dissolution affecting only outer growth zones develop simple serrate laminae on diamonds, while more extensive dissolution exposes more complex growth zones developing various shapes of laminae and etch features (trigons and irregular asperities). The effect of internal growth patterns on dissolution features is more profound during pre-kimberlitic than kimberlite-related resorption likely due to the greater role of defects in diamond dissolution at mantle conditions. Comparison with the results of diamond dissolution experiments shows that metasomatism by C-O-H fluid is not destructive to diamond, while carbonate-silicate melt-driven metasomatism causes diamond dissolution. Continuous change in the silicate content of silicate?carbonate melts and temperature variations within 200 °C can explain all pre-kimberlite dissolution features observed in this study. Similar pre-kimberlite dissolution features on diamonds from both the Zimbabwe and Slave cratons suggests that these metasomatic processes are widespread and affected the mantle below the eight studied kimberlites.
Geochimica et Cosmochimica Acta, Vol. 323, pp. 20-39.
Africa, Sierra Leone
deposit - Koidu
Abstract: Inclusion-bearing diamonds from the Koidu kimberlite complex, Sierra Leone (West African Craton) were analyzed in situ for carbon and nitrogen isotope compositions, nitrogen concentrations and nitrogen aggregation states. In a suite of 105 diamonds, 78% contain eclogitic mineral inclusions, 17% contain peridotitic mineral inclusions, and 5% - an unusually high proportion - contain co-occurring eclogitic and peridotitic mineral inclusions indicating a mixed paragenesis. Major and trace element compositions of mineral inclusions from two mixed paragenesis diamonds (one with omphacite + Mg-chromite, the other with eclogitic garnet + forsteritic olivine) were determined. The presence of positive Eu anomalies in centrally located omphacite and eclogitic garnet inclusions indicates derivation from subducted protoliths, formed as igneous cumulates in lower oceanic crust. Mg-chromite (Cr# 85.5; Mg# 65.2) and olivine (Mg# 94.5) inclusions, located in outer portions of the mixed paragenesis diamonds, have compositions indicative of derivation from strongly depleted cratonic peridotites. Given that the olivine Mg# of 94.5 is the highest reported to date for the West African Craton, the eclogitic and peridotitic inclusions in these mixed paragenesis diamonds cannot have precipitated during infiltration of peridotitic substrates by eclogite-derived fluids, as the consequent fluid-rock interaction should lead to Mg# lower than that for the original peridotitic diamond substrate. The different origins of eclogitic and peridotitic inclusions could be explained by physical transport of their host diamonds from eclogitic into peridotitic substrates, possibly along high-strain shear zones, before renewed diamond growth. Based on the ?¹³C-?¹?N systematics of the entire inclusion-bearing diamond suite from Koidu, three major compositional clusters are identified. Cluster 1 (eclogitic diamond cores; ?¹³C = -33.2 to -14.4 ‰ and ?¹?N = -5.3 to +10.1 ‰) bears the isotopic signature of recycled crustal material (± a mantle component). Cluster 2 (peridotitic diamonds and including the core of a diamond containing omphacite + Mg-chromite; ?¹³C = -6.0 to -1.1 ‰ and ?¹?N = -4.2 to +9.7 ‰) likely involves mixing of carbon and nitrogen from subducted and mantle sources. Cluster 3 (rims of eclogitic diamonds and including the eclogitic garnet + olivine included diamond and the rim of the omphacite + Mg-chromite included diamond; ?¹³C = -7.8 to -3.6 ‰ and ?¹?N = -7.9 to -2.1 ‰) matches convecting mantle-derived fluids/melts. The distinct isotopic signatures of the three diamond clusters, together with differences in nitrogen aggregation and cathodoluminescence response between diamond cores and rims, suggest episodic diamond growth during multiple fluid/melt pulses.
The Canadian Mineralogist, Vol. 60, pp. 67-90. pdf
Canada, Ontario
cathodluminenescence
Abstract: Forty-one samples of diamond from the Jurassic 95-2 kimberlite pipe in the Lake Timiskaming Kimberlite Cluster, Superior Craton, Canada, were imaged using cathodoluminescence and analyzed by secondary ion mass spectrometry and Fourier-transform infrared absorbance spectrometry to determine carbon stable isotope composition, total nitrogen abundance, and nitrogen aggregation state. The carbon isotope compositions results (?13CVPDB) range from -9.11 to -3.57‰, with a mean value of -5.8‰. Intra-stone variation is small (maximum ?2.2‰, and in most individual diamond samples <1‰). Nitrogen contents range from 0.5 to 2040 ppm (mean of 483 ppm). The greatest range of values in a single stone is 825 ppm. The samples are poorly aggregated in terms of nitrogen. The samples are mostly type IaA or IaAB, with a few bordering on type Ib. Diamond growth was episodic, with nitrogen behaving highly compatibly (i.e., D = [N]diamond/[N]fluid >> 1). Precipitation was likely from a carbonate-rich fluid in a peridotitic (lherzolitic) environment within the mantle of the central Superior Craton. This generation of diamond growth is very similar to those reported from the Jurassic age Victor and U2 pipes of the Attawapiskat Kimberlite Cluster, and distinct from a possibly much older (>1.1 Ga) generation of diamond reported in other older host rocks (T1, Wawa, Lynx, and Renard). This older generation of diamond at these other localities is also predominantly of the peridotitic (harzburgitic) paragenesis but contains far less nitrogen (although typically more aggregated as B centers) and has higher ?13CVPDB. The younger generation of diamond formed after mantle heating during formation of the Mid-Continental Rift (ca. 1.1 Ga) destroyed any proximal prior generation(s) of diamond. Igneous activity after 1.1 Ga subsequently refertilized the cratonic mantle to a lherzolitic paragenesis in which the younger generation precipitated.
Evidence from ophiolites, blueschists, and ultrahigh pressure metamorphic terranes that the modern episode of subduction tectonics began in Neoproterozoic time.
Abstract: We want to know when plate tectonics began and will consider any important Earth feature that shows significant temporal evolution. Kimberlites, the primary source of diamonds, are rare igneous features. We analyze their distribution throughout Earth history; most are young (?95% are younger than 0.75 Ga), but rare examples are found as far back as the Archean (older than 2.5 Ga). Although there are differing explanations for this age asymmetry (lack of preservation, lack of exposure, fewer mantle plumes, or lack of old thick lithosphere in the Archean and Proterozoic), we suggest that kimberlite eruptions are a consequence of modern-style plate tectonics, in particular subduction of hydrated oceanic crust and sediments deep into the mantle. This recycling since the onset of modern-style plate tectonics ca. 1 Ga has massively increased mantle CO2 and H2O contents, leading to the rapid and explosive ascent of diamond-bearing kimberlite magmas. The age distribution of kimberlites, combined with other large-scale tectonic indicators that are prevalent only in the past ?1 Ga (blueschists, glaucophane-bearing eclogites; coesite- or diamond-bearing ultrahigh-pressure metamorphic rocks; lawsonite-bearing metamorphic rocks; and jadeitites), indicates that plate tectonics, as observed today, has only operated for <25% of Earth history.
Abstract: How new subduction zones form is an emerging field of scientific research with important implications for our understanding of lithospheric strength, the driving force of plate tectonics, and Earth's tectonic history. We are making good progress towards understanding how new subduction zones form by combining field studies to identify candidates and reconstruct their timing and magmatic evolution and undertaking numerical modeling (informed by rheological constraints) to test hypotheses. Here, we review the state of the art by combining and comparing results coming from natural observations and numerical models of SI. Two modes of subduction initiation (SI) can be identified in both nature and models, spontaneous and induced. Induced SI occurs when pre-existing plate convergence causes a new subduction zone to form whereas spontaneous SI occurs without pre-existing plate motion when large lateral density contrasts occur across profound lithospheric weaknesses of various origin. We have good natural examples of 3 modes of subduction initiation, one type by induced nucleation of a subduction zone (polarity reversal) and two types of spontaneous nucleation of a subduction zone (transform collapse and plumehead margin collapse). In contrast, two proposed types of subduction initiation are not well supported by natural observations: (induced) transference and (spontaneous) passive margin collapse. Further work is therefore needed to expand on and understand the implications of these observations. Our future advancing understanding of SI will come from better geologic insights, laboratory experiments, and numerical modeling, and with improving communications between these communities.
Abstract: China covers approximately 10?million?km2 and its crust has a complicated evolution of amalgamation, igneous activity, and sedimentation. Many studies address various aspects of China's crust, but few provide a simple geological and geophysical overview that is accessible to students and non-specialists; Filling this void is the objective of this review. China is characterized by thick (40-75?km) crust in the west due to Cenozoic collision with India and thin (30-40?km thick) crust in the east due to E-W Mesozoic-Cenozoic backarc extension. In contrast, overall crustal fabric trends E-W, defined by ophiolite belts and ultra-high pressure metamorphic rocks. This crustal fabric indicates that China has grown like a sandwich, with crust progressively added through Phanerozoic time by closing various E-W oriented Tethys oceans and seaways. In map view, China consists of five E-W trending tiers. Tier 1 is defined by the Central Asian Orogenic Belt (CAOB) along the northern margin of China, which consists of the Xing'an-Mongolia orogenic belt in the NE and the Tianshan Orogen in the NW. The CAOB formed during ~1000?Ma to ~250?Ma and is an accretionary orogen of mostly Paleozoic age that formed through closure of the Paleo-Asian Ocean and collision between the Siberian Craton and Archean-Paleoproterozoic crust to the south, which constitutes Tier 2. The CAOB has a strong aeromagnetic signature. Sediments from the Amur River show detrital U-Pb zircon age peaks at 2.8-2.3?Ga, 1.8?Ga, 450-250?Ma, and 200-100?Ma, which is expected for erosion of the Xing'an-Mongolia belt. Tier 1 igneous rocks are mainly Paleozoic except in the NE (Xing'an-Mongolia orogenic belt) and reflect subduction of the Paleo-Asian Ocean and associated accretion events, whereas Paleozoic CAOB crust in the east is overprinted by Jurassic and Cretaceous igneous rocks related to subduction of ancient Pacific basin oceanic lithosphere. Tier 2 includes the North China Craton (NCC) to the east and Tarim Craton to the west. The NCC contains the oldest rocks in China and is dominated by Archean and Paleoproterozoic ages. The extent of Archean rocks in the NCC may have been overestimated, as suggested by detrital zircons from the Yellow River, which flows across the craton, showing age peaks at 2.5-2.2?Ga, ~1.9?Ga, 500-400?Ma, and 300-200?Ma. The Tarim Craton is dominated by Palaeoproterozoic- Mesoproterozoic metamorphic strata along with a significant proportion of Neoproterozoic (~0.8?Ga) rocks. U-Pb ages for detrital zircons from Tarim River sediments reflect this basement geology, with strong peaks of Early and Late Paleozoic age, less abundant Neoproterozoic ages, and scattered ages back to the Archean. The NCC also was affected by abundant Mesozoic igneous activity with voluminous Early Cretaceous rocks that are associated with lithospheric thinning and decratonization. Tier 3 - also known as the Central China Orogen - is composed of the Sulu-Dabie-Qinling-Kunlun Orogen and records closing of an arm of Prototethys during the Ordovician to Silurian and Paleotethys during the Triassic. Tier 3 contains one of Earth's three giant ultra-high pressure (UHP) terranes with well-documented peak metamorphism of 650-850?°C and 4?GPa, indicating that some of these rocks were deeply subducted and then exhumed from depths of over 120?km in Triassic time. Tier 3 magmatism occurred in two episodes, early-middle Paleozoic and Triassic. Tier 4 contains blocks rifted from Gondwana, which include the Songpan-Ganzi, Qiangtang, and Lhasa terranes of Tibet in the west and the South China Block in the east. These terranes are marked by broad magnetic anomalies with a NE-SW trend along the Pacific margin, and a broad N-S trending anomaly between Tibet and South China. The South China Block is made up of Proterozoic and minor Archean crust of the Yangtze and Cathaysia blocks, which collided at 1.0-0.8?Ga to form the Jiangnan Orogen and the South China Block. Age spectra for detrital zircons from the Yangtze and Pearl Rivers shows major peaks at ~1.8?Ga, 900-800?Ma, ~400?Ma, and 300-150?Ma, which is consistent with the age of South China Block crust. Early-Middle Paleozoic igneous rocks are also found in South China. Mesozoic igneous rocks are widespread in both South China and Tibet and are related to subduction of the Paleo-Pacific and Tethyan oceanic plates, respectively. The accretion of Tibetan terranes to southern Eurasia occurred in the Mesozoic before collision with India at ~55?Ma. Tier 5 is represented by the island of Taiwan on the SE margin of China and marks where China crust continues to grow. Taiwan lies on a complex convergent boundary between the South China Block to the NW, the Philippine Sea Plate to the SE, and the Sunda Plate to the SW.
Abstract: When Earth's tectonic style transitioned from stagnant lid (single plate) to the modern episode of plate tectonics is important but unresolved, and all lines of evidence should be considered, including the climate record. The transition should have disturbed the oceans and atmosphere by redistributing continents, increasing explosive arc volcanism, stimulating mantle plumes and disrupting climate equilibrium established by the previous balance of silicate?weathering greenhouse gas feedbacks. Formation of subduction zones would redistribute mass sufficiently to cause true polar wander if the subducted slabs were added in the upper mantle at intermediate to high latitudes. The Neoproterozoic Snowball Earth climate crisis may reflect this transition. The transition to plate tectonics is compatible with nearly all proposed geodynamic and oceanographic triggers for Neoproterozoic Snowball Earth events, and could also have contributed to biological triggers. Only extraterrestrial triggers cannot be reconciled with the hypothesis that the Neoproterozoic climate crisis was caused by a prolonged (200-250 m.y.) transition to plate tectonics.
Abstract: When Earth's tectonic style transitioned from stagnant lid (single plate) to the modern episode of plate tectonics is important but unresolved, and all lines of evidence should be considered, including the climate record. The transition should have disturbed the oceans and atmosphere by redistributing continents, increasing explosive arc volcanism, stimulating mantle plumes and disrupting climate equilibrium established by the previous balance of silicate?weathering greenhouse gas feedbacks. Formation of subduction zones would redistribute mass sufficiently to cause true polar wander if the subducted slabs were added in the upper mantle at intermediate to high latitudes. The Neoproterozoic Snowball Earth climate crisis may reflect this transition. The transition to plate tectonics is compatible with nearly all proposed geodynamic and oceanographic triggers for Neoproterozoic Snowball Earth events, and could also have contributed to biological triggers. Only extraterrestrial triggers cannot be reconciled with the hypothesis that the Neoproterozoic climate crisis was caused by a prolonged (200-250 m.y.) transition to plate tectonics.
Abstract: To understand how plate tectonics became Earth's dominant mode of convection, we need to address three related problems. (i) What was Earth's tectonic regime before the present episode of plate tectonics began? (ii) Given the preceding tectonic regime, how did plate tectonics become established? (iii) When did the present episode of plate tectonics begin? The tripartite nature of the problem complicates solving it, but, when we have all three answers, the requisite consilience will provide greater confidence than if we only focus on the long-standing question of when did plate tectonics begin? Earth probably experienced episodes of magma ocean, heat-pipe, and increasingly sluggish single lid magmatotectonism. In this effort we should consider all possible scenarios and lines of evidence. As we address these questions, we should acknowledge there were probably multiple episodes of plate tectonic and non-plate tectonic convective styles on Earth. Non-plate tectonic styles were probably dominated by ‘single lid tectonics’ and this evolved as Earth cooled and its lithosphere thickened. Evidence from the rock record indicates that the modern episode of plate tectonics began in Neoproterozoic time. A Neoproterozoic transition from single lid to plate tectonics also explains kimberlite ages, the Neoproterozoic climate crisis and the Neoproterozoic acceleration of evolution.
Abstract: The hypothesis that the Mesoproterozoic (1600-1000 Ma) tectonic regime was a protracted single-lid episode is explored. Single-lid tectonic regimes contrast with plate tectonics because the silicate planet or moon is encased in a single lithospheric shell, not a global plate mosaic. Single-lid tectonics dominate among the Solar System’s active silicate bodies, and these show a wide range of magmatic and tectonic styles, including heat pipe (Io), vigorous (Venus), and sluggish (Mars). Both positive and negative evidence is used to evaluate the viability of the Mesoproterozoic single-lid hypothesis. Four lines of positive evidence are: (1) elevated thermal regime; (2, 3) abundance of unusual dry magmas such as A-type granites and anorthosites; and (4) paucity of new passive continental margins. Negative evidence is the lack of rock and mineral assemblages formed by plate-tectonic processes such as ophiolites, blueschists, and ultra high-pressure terranes. Younger plate-tectonic-related and Mesoproterozoic mineralization styles contrast greatly. Paleomagnetic evidence is equivocal but is permissive that Mesoproterozoic apparent polar wander paths of continental blocks did not differ significantly. These tests compel the conclusion that the Mesoproterozoic single-lid hypothesis is viable.
Abstract: Oceanic crust is created by the extraction of molten rock from underlying mantle at the seafloor ‘spreading centres’ found between diverging tectonic plates. Modelling studies have suggested that mantle melting can occur through decompression as the mantle flows upwards beneath spreading centres, but direct observation of this process is difficult beneath the oceans. Continental rifts, however—which are also associated with mantle melt production—are amenable to detailed measurements of their short-term kinematics using geodetic techniques. Here we show that such data can provide evidence for an upwelling mantle flow, as well as information on the dimensions and timescale of mantle melting. For North Island, New Zealand, around ten years of campaign and continuous GPS measurements in the continental rift system known as the Taupo volcanic zone reveal that it is extending at a rate of 6-15?millimetres per year. However, a roughly 70-kilometre-long segment of the rift axis is associated with strong horizontal contraction and rapid subsidence, and is flanked by regions of extension and uplift. These features fit a simple model that involves flexure of an elastic upper crust, which is pulled downwards or pushed upwards along the rift axis by a driving force located at a depth greater than 15?kilometres. We propose that flexure is caused by melt-induced episodic changes in the vertical flow forces that are generated by upwelling mantle beneath the rift axis, triggering a transient lower-crustal flow. A drop in the melt fraction owing to melt extraction raises the mantle flow viscosity and drives subsidence, whereas melt accumulation reduces viscosity and allows uplift—processes that are also likely to occur in oceanic spreading centres.
Science Adavances, Vol. 6, eaba7118 May 27, 9p. Pdf
Asia, Java
geophysics -seismic
Abstract: New passive- and active-source seismic experiments reveal unusually high mantle P-wave speeds that extend beneath the remnants of the world’s largest known large igneous province, making up the 120-million-year-old Ontong-Java-Manihiki-Hikurangi Plateau. Sub-Moho Pn phases of ~8.8 ± 0.2 km/s are resolved with negligible azimuthal seismic anisotropy, but with strong radial anisotropy (~10%), characteristic of aggregates of olivine with an AG crystallographic fabric. These seismic results are the first in situ evidence for this fabric in the upper mantle. We show that its presence can be explained by isotropic horizontal dilation and vertical flattening due to late-stage gravitational collapse and spreading in the top 10 to 20 km of a depleted, mushroom-shaped, superplume head on a horizontal length scale of 1000 km or more. This way, it provides a seismic tool to track plumes long after the thermal effects have ceased.
Mathematics and the Buckyball.The elaborate symmetries of this soccer ball shaped molecule allow many of its properties to be calculated from firstprinciples
American Scientist, Vol. 81, January-February pp. 56-70
Investigated the connection between mineralogy and physical properties in the ultramafic magmatic Ni-Cu deposit at Kabanga Tanzania and Anuri Kimberlite NT
High pressure intermediate temperature metamorphism in the southern Barbarton granitoid greenstone terrain, South Africa: a consequence of subduction driven ...
Benn, K., Mareschal, J-C., Condie, K.C. Archean Geodynamics and Environments, AGU Geophysical Monograph, No. 164, pp. 239-254.
Abstract: Geodynamics of crustal growth and evolution consist in one of the thorniest questions of the early Earth. In order to solve it, Archean cratons are intensively studied through geophysical, geochemical and geochronological investigations. However, timing and mechanisms leading to accretion and stabilization of crustal blocks are still under question. In this study, new information on the evolution of Archean cratons is provided through complementary approaches applied to the northern margin of the Archean Kaapvaal craton (KC). The study area comprises the Pietersburg Block (PB) and the terrane immediately adjacent to the North: the Southern Marginal Zone of the Limpopo Complex (SMZ). We present a comprehensive petro-metamorphic study coupled with LA-ICP-MS U-Pb isotope examination of both Na- and K-rich granitoids from the two areas. This dataset points toward a new interpretation of the northern KC (PB?+?SMZ). Two significant magmatic events are newly recognized: (i) a ca. 3.2?Ga event, and (ii) a protracted magmatic event between ca. 2.95–2.75?Ga. These events affected in both investigated areas and are unrelated to the ca. 2.7?Ga-old event usually attributed to the SMZ. More importantly, phase equilibrium modelling of several lithologies from the SMZ basement points to middle-amphibolite facies conditions of equilibration instead of granulite-facies conditions historically assumed. This study has both important regional and global implications. Firstly, the presence of a continuous basement from the Thabazimbi-Murchison Lineament to the Palala Shear Zone, different than Central Zone of the Limpopo Complex basement, implies a complete reviewing of the whole Limpopo Complex concept. Secondly, the geometry observed in the northern Kaapvaal craton is assumed to testify for a complete accretionary orogenic sequence with formation of both mafic and TTG lithologies through arc-back arc geodynamic. This was followed by a long-lived lateral compression triggering partial melting of the lower continental crust and emplacement of Bt-granitoids bodies that stabilizes the continental crust. Lastly, partial melting of the underlying enriched mantle stabilized the entire lithosphere allowing long-term preservation of the crustal block.
Abstract: Eclogite mantle xenoliths from various kimberlite occurrences on the Kaapvaal craton show evidence for depth- and redox-dependent metasomatic events that led to variable base metal sulphide and incompatible element enrichments. Eclogite xenoliths from the Roberts Victor, Jagersfontein, Kimberley (Kamfersdam) and Premier kimberlites were investigated for their silicate and base metal sulphide geochemistry, stable oxygen isotope compositions and oxybarometry. The variably metasomatised eclogites had basaltic, picritic and gabbroic protolith compositions and have garnet ?18O values that range from +3.3 to +7.9‰, which, when coupled with the trace element characteristics, indicate oceanic lithosphere protoliths that had undergone variable degrees of seawater alteration. The deepest equilibrated eclogites (175220?km depth) from near the base of the Kaapvaal craton lithosphere are the most refractory and feature significant light rare earth element (LREE) depletions. They show the most oxidised redox compositions with ?logƒO2 values of FMQ-3.9 to FMQ-1.5. Subtle metasomatic overprinting of these eclogites resulted in base metal sulphide formation with relatively depleted and highly fractionated HSE compositions. These deepest eclogites and their included base metal sulphides suggest interaction with relatively oxidised melts or fluids, which, based on their HSE characteristics, could be related to precursor kimberlite metasomatism that was widespread within the Kaapvaal craton mantle lithosphere. In contrast, eclogites that reside at shallower, “mid-lithospheric” depths (140180?km) have been enriched in LREE and secondary diopside/phlogopite. Importantly, they host abundant metasomatic base metal sulphides, which have higher HSE contents than those in the deeper eclogites at the lithosphere base. The mid-lithospheric eclogites have more reducing redox compositions (?logfO2?=?FMQ-5.3 ? FMQ-3.3) than the eclogites from the lowermost Kaapvaal lithosphere. The compositional overprint of the shallower mantle eclogites resembles basaltic rather than kimberlitic/carbonatitic metasomatism, which is also supported by their relatively reducing redox state. Base metal sulphides from the mid-lithospheric eclogites have HSE abundances and distributions that are similar to Karoo flood basalts from southern Africa, suggesting a link between the identified shallow mantle metasomatism of the Kaapvaal cratonic lithosphere and the Karoo large igneous event during the Mesozoic. The sulphide-hosted platinum group element abundances of the mid-lithospheric eclogites are higher compared with their analogues from the deeper lithospheric eclogites, which in combination with their contrasting oxidation states, may imply redox-controlled HSE mobility during sulphur-rich metasomatism of continental mantle lithosphere.
deposit - Roberts Victor, Jagersfontein, Kimberley, Kamfersdam, Premier
Abstract: Eclogite mantle xenoliths from various kimberlite occurrences on the Kaapvaal craton show evidence for depth- and redox-dependent metasomatic events that led to variable base metal sulphide and incompatible element enrichments. Eclogite xenoliths from the Roberts Victor, Jagersfontein, Kimberley (Kamfersdam) and Premier kimberlites were investigated for their silicate and base metal sulphide geochemistry, stable oxygen isotope compositions and oxybarometry. The variably metasomatised eclogites had basaltic, picritic and gabbroic protolith compositions and have garnet ?18O values that range from +3.3 to +7.9‰, which, when coupled with the trace element characteristics, indicate oceanic lithosphere protoliths that had undergone variable degrees of seawater alteration. The deepest equilibrated eclogites (175-220 km depth) from near the base of the Kaapvaal craton lithosphere are the most refractory and feature significant light rare earth element (LREE) depletions. They show the most oxidised redox compositions with ?logƒO2 values of FMQ-3.9 to FMQ-1.5. Subtle metasomatic overprinting of these eclogites resulted in base metal sulphide formation with relatively depleted and highly fractionated HSE compositions. These deepest eclogites and their included base metal sulphides suggest interaction with relatively oxidised melts or fluids, which, based on their HSE characteristics, could be related to precursor kimberlite metasomatism that was widespread within the Kaapvaal craton mantle lithosphere. In contrast, eclogites that reside at shallower, “mid-lithospheric” depths (140-180 km) have been enriched in LREE and secondary diopside/phlogopite. Importantly, they host abundant metasomatic base metal sulphides, which have higher HSE contents than those in the deeper eclogites at the lithosphere base. The mid-lithospheric eclogites have more reducing redox compositions (?logfO2 = FMQ-5.3 ? FMQ-3.3) than the eclogites from the lowermost Kaapvaal lithosphere. The compositional overprint of the shallower mantle eclogites resembles basaltic rather than kimberlitic/carbonatitic metasomatism, which is also supported by their relatively reducing redox state. Base metal sulphides from the mid-lithospheric eclogites have HSE abundances and distributions that are similar to Karoo flood basalts from southern Africa, suggesting a link between the identified shallow mantle metasomatism of the Kaapvaal cratonic lithosphere and the Karoo large igneous event during the Mesozoic. The sulphide-hosted platinum group element abundances of the mid-lithospheric eclogites are higher compared with their analogues from the deeper lithospheric eclogites, which in combination with their contrasting oxidation states, may imply redox-controlled HSE mobility during sulphur-rich metasomatism of continental mantle lithosphere.
Contrasting lower crustal evolution across an Archean Proterozoic suture: physical, chemical and geochronologic studies of lower crustal xenoliths in southern Wyoming and northern Colorado.
American Geophysical Union, Geophysical Monograph, No. 154, pp. 139-162.
Abstract: Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry, and electromagnetic data can identify contemporary melt zones, magma reservoirs, and, or, crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs), and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community.
Abstract: Engineering coherent systems is a central goal of quantum science. Color centers in diamond are a promising approach, with the potential to combine the coherence of atoms with the scalability of a solid-state platform. We report a color center that shows insensitivity to environmental decoherence caused by phonons and electric field noise: the neutral charge state of silicon vacancy (SiV0). Through careful materials engineering, we achieved >80% conversion of implanted silicon to SiV0. SiV0 exhibits spin-lattice relaxation times approaching 1 minute and coherence times approaching 1 second. Its optical properties are very favorable, with ~90% of its emission into the zero-phonon line and near -transform-limited optical linewidths. These combined properties make SiV0 a promising defect for quantum network applications.
Abstract: Magmatic volatiles are critically important in the petrogenesis of igneous rocks but their inherent transience hampers the identification of their role in magmatic and mineralization processes. We present evidence that magmatic volatiles played a critical role in the formation of the 1894 Ma Paleoproterozoic Montviel alkaline-carbonatite complex, Canada, and the related carbonatite-hosted REE-Nb deposit. Field and drill core relationships indicate that lithological units of the complex were emplaced in the following order: clinopyroxenites, melteigites, ijolites, melanosyenites, leucosyenites, granites, lamprophyric silicocarbonatites, rare magnesiocarbonatites, calciocarbonatites, ferrocarbonatites, late mixed carbonatites, kimberlitic silicocarbonatites and polygenic breccias. Magmatic minerals within these units were systematically metasomatized. In undersaturated silicate rocks, augite recrystallized to aegirine–augite and aegirine, plagioclase recrystallized to albite, and nepheline recrystallized with analcime, cancrinite and albite. Primary biotite was replaced by secondary, REE-rich metasomatic biotite, particularly along fractures and alteration pockets. In carbonatites, liquidus phases consisted of calcite and dolomite and were recrystallized to ferroan dolomite, ankerite, siderite, barytocalcite, witherite and strontianite, which are intimately related to the REE-bearing carbonates and fluorocarbonates. Biotite is common to all lithologies, ranges in REE concentrations from 1.5 to 230 ppm and yielded subsolidus crystallization temperatures ranging from 770 °C to 370 °C. Sm-Nd isotope analyses from biotite and aegirine-augite yield a range of ?Nd values (+ 3.4 to ? 3.0) that suggests mixing of fluids from three sources during the crystallization of the Montviel magmas. The clinopyroxenites to melteigite, ijolites and melanosyenites crystallized augite and biotite with initial ?Nd value ? 3.4 and these minerals were metasomatized by a 1st fluid, lowering their ?Nd to values comprised between 0.8 and 3.4. Silicocarbonatites and carbonatites subsequently crystallized aegirine-augite and biotite with initial ?Nd value ? 2.6 and a 2nd fluid metasomatized the minerals to lower ? values. Both the 1st and the 2nd fluids eventually mixed with a 3rd recrystallizing aegirine-augite and biotite and lower their ?Nd values down to ? 3.0. The results presented herein suggest that the mantle magmas evolved through 4 distinct mantle pulses by fractional crystallization, mixing of depleted mantle fluids with crustal fluids, and metasomatism. Some of the silicate rocks also show evidence of assimilation of wall rock as part of their petrogenetic evolution. During the last stages of its evolution in carbonatites, the fluid source transited from the depleted mantle to the crust and we speculate that this resulted in a violent explosive eruption creating the diatreme-shaped, HREE-rich polygenic breccia.
Abstract: Alkaline and carbonatite rocks are relatively rare but offer the opportunity to study the contribution of fluids in the genesis of mantle and crustal rocks because they are commonly affected by metasomatism. Carbonate minerals represent versatile archives of mantle and crustal magmatic-hydrothermal processes because they can have magmatic, metasomatic or hydrothermal origins and because they host the trace elements, stable and radiogenic isotopes required to unravel their petrogenesis. Previous studies have shown that the 1894 Ma Montviel alkaline?carbonatite complex was emplaced through four injections of volatile-saturated, mantle magmas which evolved through fractional crystallization, mixing of mantle and crustal fluids and metasomatism. Trace element analyses and ?18O, ?13C, 87Sr/86Sr and 143Nd/144Nd isotope compositions of metasomatic and hydrothermal carbonates further support that each magma injection was accompanied by a volatile phase. Variations in trace element concentrations suggest that the carbonatite might have exsolved from a metasomatized mantle or hybrid silicate?carbonatite magma, and that the fluid composition evolved towards higher REE and lower HFSE with increasing degree of segregation of the carbonatite magma and the silicate source. A strong correlation between the C-O-Sr isotopic systems show that mantle fluids mixed with crustal fluids, increasing the 87Sr/86Sr from mantle to crustal values, and driving the C and O isotopic ratios towards respectively lighter and heavier values. The Sm/Nd isotopic system was weakly coupled with the other isotopic systems as depleted mantle fluids mixed with crustal fluids and metasomatized the crystallizing magmas, thereby redistributing the REE and affecting their Sm/Nd ratios. The Nd isotopes suggest that the mixed mantle/crustal fluids redistributed the rare earth elements, producing ultra-depleted (?Nd = + 10), normally depleted (?Nd = + 4) and slightly enriched (?Nd = ? 2) isotopic compositions.
Abstract: We propose to study the Kipawa peralkaline complex, a rare-earth deposit principally composed of eudialyte, mosandrite and britholite. The Kipawa complex is situated in the Parautochton zone of the Grenville Province in the Tesmiscamingue region of Quebec, 55 km south of contact with Superior Province. The complex consists of peralkaline syenites, amphibolites, gneisses that are intercalated with calc-silicate rocks and marble, and overlain by a peralkaline gneissic granite. The Kipawa complex differs geochemically and petrologically from other well-known peralkaline complexes such as the Illimausaq, Lovozero, Thor Lake or Strange Lake complexes. Classic peralkaline complexes are large, circular igneous complexes, with or without volcanism and have an isotopic signature reflecting mantle origin with different degrees of assimilation and crustal contamination (for example Illimausaq is reported with ?Nd values of 0.4 and -5.7). The Kipawa Complex is a thin, folded stack of sheet imbricates between Kikwissi Suite rocks, McKillop Lake sequence and Red Pine Chute gneiss, suggesting a regional tectonic control. Isotopic analyses carried out by other teamsindicate a strong crustal signature (?Nd = -8.7). Several hypotheses are possible: crustal contamination, hydrothermal activity, fluids alteration during formation, metamorphism or dominant crustal origin. Our objective is to characterize the geochemical and isotopic composition of the Kipawa complex in order to improve our understanding of the age and formation of the complex. Analyses of both whole rocks, eudialytes and zircons will be made to obtain isotopic signatures and determine formation ages and/or post-formation processes.
Roulleau, E., Pinti, D.L., Stevenson, R.K., Takahata, N., Sano, Y., Pitre, F.
N, Ar and Pb isotopic co-variation in magmatic minerals: discriminating fractionation processes from magmatic sources in Montregian Hills, Quebec, Canada.
Journal of Volcanology and Geothermal Research, Vol. 322, pp. 14-25.
Africa, Tanzania
deposit - Oldoinyo Lengai
Abstract: Volcanic ash leachate studies have been conducted on various volcanoes on Earth, but few have been done on African volcanoes until now. Tephra emissions may affect the environment and the health of people living in this area, and therefore we conducted a first tephra (ash and lapilli sized) leachate study on the Oldoinyo Lengai volcano, situated in northern Tanzania. The recent explosive eruption in 2007-2008 provided us with fresh samples from the first three weeks of the eruption which were used for this study. In addition, we also used a natrocarbonatitic sample from the activity prior to the explosive eruption, as the major activity at Oldoinyo Lengai is natrocarbonatitic. To compare the leaching process affecting the natrocarbonatitic lavas and the tephras from Oldoinyo Lengai, the 2006 natrocarbonatitic lava flow was resampled 5 years after the emplacement and compared to the initial, unaltered composition. Special interest was given to the element fluorine (F), since it is potentially toxic to both humans and animals. A daily intake of fluoride (F?) in drinking water of > 1.5 mg/l can lead to dental fluorosis, and higher concentrations lead to skeletal fluorosis. For this reason, a guideline value for fluoride in drinking water was set by the WHO (2011) to 1.5 mg/l. However, surface waters and groundwaters in the Gregory Rift have elevated fluoride levels of up to 9.12 mg/l, and as a consequence, an interim guideline value for Tanzania has been set at 8 mg/l. The total concentration of fluorine in the samples from the natrocarbonatitic lava flow is high (3.2 wt%), whereas we observed a significant decrease of the fluorine concentration (between 1.7 and 0.5 wt%) in the samples collected three days and three weeks after the onset of the explosive 2007-08 eruption. However, the total amount of water-extractable fluoride is lower in the natrocarbonatitic lavas (319 mg/l) than in the nephelinitic tephra (573-895 mg/l). This is due to the solubility of the different F-bearing minerals. In the natrocarbonatites, fluorine exists predominantly in fluorite (CaF2), and in the early tephra as Na-Mg bearing salts such as neighborite (NaMgF3) and sellaite (MgF2). All these three minerals have very low solubility in water (16-130 mg/l). The later nephelinitic tephras contain surface coating of villiaumite (NaF), which is highly soluble (42,200 mg/l) in water and can thus release the fluoride more readily upon contact with water. Although there is still the need for further data and a more precise study on this topic in Tanzania, we can already draw a first conclusion that the intake of water during or directly following the deposition of the tephra is not advisable and should be avoided, whereas the release of fluoride from the lava flow has less influence on the river waters.-
Geological Society of London Special Publication Supercontinent Cycles through Earth History., Vol. 424, pp. 47-81.
Australia
Supercontinents
Abstract: The Australian continent records c. 1860-1800 Ma orogenesis associated with rapid accretion of several ribbon micro-continents along the southern and eastern margins of the proto-North Australian Craton during Nuna assembly. The boundaries of these accreted micro-continents are imaged in crustal-scale seismic reflection data, and regional gravity and aeromagnetic datasets. Continental growth (c. 1860-1850 Ma) along the southern margin of the proto-North Australian Craton is recorded by the accretion of a micro-continent that included the Aileron Terrane (northern Arunta Inlier) and the Gawler Craton. Eastward growth of the North Australian Craton occurred during the accretion of the Numil Terrane and the Abingdon Seismic Province, which forms part of a broader zone of collision between the northwestern margins of Laurentia and the proto-North Australian Craton. The Tickalara Arc initially accreted with the Kimberley Craton at c. 1850 Ma and together these collided with the proto-North Australian Craton at c. 1820 Ma. Collision between the West Australian Craton and the proto-North Australian Craton at c. 1790-1760 Ma terminated the rapid growth of the Australian continent.
A review of diamond occurrences and potentials in Alberta
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) , Annual Meeting Abstracts approximately 10 lines, Vol. 86, No. 968, March POSTER ABSTRACT p. 67.
A review of diamond occurrences and potential in Alberta
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 98-99.
The Journal of the Southern African Insitute of Mining and Metallurgy, Vol. 119, Feb. 6p. Pdf
Africa, South Africa
DMC
Abstract: The petrology and bulk-rock sulphur isotopic compositions of kimberlite samples from four localities (Bultfontein, De Beers, Kimberley, Wesselton) of the archetypal Kimberley cluster, South Africa, were used to investigate the origin(s) of S in kimberlites and gain insights into the occurrence of recycled crustal material in the source of Mesozoic kimberlites. The samples, which show variable degrees of alteration, are all hypabyssal and were derived from coherent root-zones as well as dykes and sills. Typical sulphide minerals are Cu-Fe-Ni-sulphides with less common pyrite, galena, sphalerite, and djerfisherite. They occur in a variety of textural associations, for example as groundmass phases, secondary inclusions in olivine, inclusions in matrix phases (e.g., phlogopite), or in carbonate-serpentine segregations. Barite is the most commonly observed sulphate phase. Bulk-sample ?34SVCDT values of sulphides in fresh kimberlites, which mostly do not contain barite, vary from - 2.0 to -5.7 ‰. Slightly altered kimberlite samples, in which sulphides were generally associated with serpentine, returned somewhat higher bulk-sulphide ?34SVCDT (-3.8 to +1.1 ‰). One sample from the Wesselton Water Tunnel Sills complex contains abundant barite and pyrite in its groundmass, with the latter having ?34SVCDT (+0.2 to +1.9 ‰) similar to altered kimberlites. Two further altered samples returned ?34SVCDT values (-10.1 to -13.0 ‰) that suggest a contribution from the local country rocks (Dwyka shale: ?34SVCDT from -10.2 to -10.5 ‰). All samples have near-zero ?33S values, suggesting that material displaying mass-independent fractionation has not played an important role. The negative ?34SVCDT values of fresh kimberlites from Kimberley suggest the involvement of recycled crustal material in their source, which is consistent with radiogenic isotope compositions. Overall, it appears that most kimberlitic sulphide S isotopic compositions can be explained by the action of a few typical magmatic/hydrothermal processes. One of the most important performance indicators of a dense medium cyclone (DMC) circuit is the Tromp curve, and by extension the separation density and Ecart Probable (Ep) values. The densimetric profiles of DMC product streams have been traditionally acquired using heavy liquid sinkfloat analysis, which has certain disadvantages, such as the associated safety and health risks. More recently, non-toxic media such as lithium hetero-polytungstates (LST) have been used, with the desired densities being achieved by maintaining the solutions at specific temperatures. However, the high costs of these liquids can be prohibitive. The long turnaround time of the sink-float analysis is a further disadvantage for timeous interventions to the operating set-points of the DMC process. The RhoVol technology can generate the density distribution of a batch of particles in a rapid, accurate, repeatable, and safe manner. Additional data of interest, such as particle size and shape, are also measured and reported on a per-particle basis. Furthermore, samples can be sorted into discrete sorting bins based on any of the measured parameters of the particle, making further analyses of the material possible. This technology has applications across all commodities that use the DMC, particularly in the size fractions 25 +8 mm and 8 +3 mm. To date, laboratory results have proved very encouraging separation densities are within 5% of traditional sink-float results, and the technology is being introduced to diamond DMC plants.
The Immigrant's Guide: the Diamond Fields of South Africa, with a Map of the Country and Full Particulars As to Roads, Prices of Necessities, Orange and Vaal rivers
Canadian Institute of Mining and Metallurgy, Vol. 6, 3, 14p.
Africa, Botswana
Deposit - Orapa
Abstract: The degree of uncertainty associated with a natural diamond resource is important to quantify from the time of discovery through the production lifetime. Data collection occurs during the discovery, exploration, delineation, and production or recovery phases. Quantifying the relationship between data and uncertainty is an important component of project valuation. The value of data is measured as their potential to reduce uncertainty if they are collected. A method is developed using Monte Carlo simulation for predicting resource uncertainty and valuing data during critical phases of development, particularly discovery and exploration. The technique is applied to diamond pipe deposits.
De Wit, M., Bhebhe, Z., Davidson, J., Haggerty, S.E., Hundt, P., Jacob, J., Lynn, M., Marshall, T.R., Skinner, C., Smithson, K., Stiefenhofer, J., Robert, M., Revitt, A., Spaggiari, R., Ward, J.
Abstract: From the discovery of diamonds in South Africa in 1866 until the end of 2013, Africa is estimated to have produced almost 3.2 Bct out of a total global production of 5.03 Bct, or 63.6% of all diamonds that have ever been mined. In 2013 African countries ranked 2nd (Botswana), 3rd (DRC), 6th (Zimbabwe), 7th (Angola), 8th (South Africa), and 9th (Namibia), in terms of carat production and 1st (Botswana), 4th (Namibia), 5th (Angola), 6th (South Africa), 7th (Zimbabwe), and 9th (DRC), in terms of value of the diamonds produced. In 2013 Africa produced 70.6 Mct out of a global total of 130.5 Mct or 54.1%, which was valued at US$ 8.7 billion representing 61.5% of the global value of US$ 14.1 billion.
Canadian Institute of Mining and Metallurgy, Vol. 7, 4, pp. 216-239.
Global
microdiamonds
Abstract: Concerns around the use of micro-diamonds for resource estimation have been raised by some workers because: 1) multiple diamond populations are present in many parts of the mantle source region, 2) small diamonds in kimberlite could be exposed to proportionately greater levels of resorption and modification, and 3) euhedral micro-diamonds could crystallize immediately prior to kimberlite eruption. This paper addresses these concerns and discusses the geology of the mantle and the principal diamond host rocks, the impact of mantle processes, compares micro- and macro-diamond properties and features, and outlines several steps that can be undertaken to identify and mitigate the risk of resorption of diamond and its impact on the diamond grade size relationship.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0566-y 12p.
Global
microdiamonds
Abstract: Microdiamonds offer several advantages as a resource estimation tool, such as access to deeper parts of a deposit which may be beyond the reach of large diameter drilling (LDD) techniques, the recovery of the total diamond content in the kimberlite, and a cost benefit due to the cheaper treatment cost compared to large diameter samples. In this paper we take the first step towards local estimation by showing that micro-diamond samples can be treated as a regionalised variable suitable for use in geostatistical applications and we show examples of such output. Examples of microdiamond variograms are presented, the variance-support relationship for microdiamonds is demonstrated and consistency of the diamond size frequency distribution (SFD) is shown with the aid of real datasets. The focus therefore is on why local microdiamond estimation should be possible, not how to generate such estimates. Data from our case studies and examples demonstrate a positive correlation between micro- and macrodiamond sample grades as well as block estimates. This relationship can be demonstrated repeatedly across multiple mining operations. The smaller sample support size for microdiamond samples is a key difference between micro- and macrodiamond estimates and this aspect must be taken into account during the estimation process. We discuss three methods which can be used to validate or reconcile the estimates against macrodiamond data, either as estimates or in the form of production grades: (i) reconcilliation using production data, (ii) by comparing LDD-based grade estimates against microdiamond-based estimates and (iii) using simulation techniques.
Mineralogy and Petrology, in press available, 11p.
Africa, Botswana
deposit - Orapa
Abstract: Spatially resolved analyses reveal considerable isotopic heterogeneity within and among diamonds ranging in size from 0.15 to 4.75 mm from the Orapa Mine, Botswana. The isotopic data are interpreted in conjunction with nitrogen aggregation state data and growth zone relationships from cathodoluminescence images. The integrated information confirms that a distinct diamond growth event (with low IaAB nitrogen aggregation states, moderately high nitrogen contents and ?13C and ?15N values compatible with average mantle values) is younger than the dominant population(s) of Type IaAB diamonds and cores of composite diamonds with more negative and positive ?13C and ?15N values, respectively. A significant proportion of the older diamond generation has high nitrogen contents, well outside the limit sector relationship, and these diamonds are likely to reflect derivation from subducted organic matter. Diamonds with low ?13C values combined with high nitrogen contents and positive ?15N values have not been previously widely recognised, even in studies of diamonds from Orapa. This may have been caused by prior analytical bias towards inclusion-bearing diamonds that are not necessarily representative of the entire range of diamond populations, and because of average measurements from heterogeneous diamonds measured by bulk combustion methods. Two distinct low nitrogen/Type II microdiamond populations were recognised that do not appear to continue into the macrodiamond sizes in the samples studied. Other populations, e.g. those containing residual singly-substituted nitrogen defects, range in size from small microdiamonds to large macrodiamonds. The total diamond content of the Orapa kimberlite thus reflects a complex assortment of multiple diamond populations.
Mineralogy and Petrology, 10.1007/ s00710-018- 0589-4, 14p.
Africa, South Africa, Canada, Northwest Territories, Ontario
deposit - Venetia, Voorspoed, Gahcho Kue, Victor, Snap Lake
Abstract: De Beers kimberlite mine operations in South Africa (Venetia and Voorspoed) and Canada (Gahcho Kué, Victor, and Snap Lake) have the potential to sequester carbon dioxide (CO2) through weathering of kimberlite mine tailings, which can store carbon in secondary carbonate minerals (mineral carbonation). Carbonation of ca. 4.7 to 24.0 wt% (average?=?13.8 wt%) of annual processed kimberlite production could offset 100% of each mine site’s carbon dioxide equivalent (CO2e) emissions. Minerals of particular interest for reactivity with atmospheric or waste CO2 from energy production include serpentine minerals, olivine (forsterite), brucite, and smectite. The most abundant minerals, such as serpentine polymorphs, provide the bulk of the carbonation potential. However, the detection of minor amounts of highly reactive brucite in tailings from Victor, as well as the likely presence of brucite at Venetia, Gahcho Kué, and Snap Lake, is also important for the mineral carbonation potential of the mine sites.
The Southern African Institute of Mining and Metallurgy Diamonds - from source to use 2013, 9p. Pdf researchgate
Africa, Angola, South Africa
deposit - Mulepe, Voorspoed
Abstract: Construction of geological models for kimberlites has historically largely been based on visual volcanic textures and features, with chemical data only produced on an ad-hoc basis depending on the level of perceived complexity observed in the geological data. The focus of this manuscript is a high-level mineral resource management view of the impact which the initial absence of proper chemical data had on two kimberlite resource models as well as the increasing magnitude of the problem caused by the lack of chemical data once the resource estimation process commenced. The impact of the absence of chemical data on projects where micro-diamond estimation techniques are employed is also discussed. Metallurgical parameters have historically not been incorporated into kimberlite resource models, but recent work at De Beers Group Services has shown that geostatistical quantification of the DMS (Dense Media Separator) yield is useful for mine planning purposes, and will add value in the understanding of the kimberlite emplacement process. A case study is presented where a geostatistical analysis and conditional simulation was performed on the DMS yield in a kimberlite at a local block scale. These data were used to identify mining blocks where the DMS yield would exceed the capability of the treatment plant.
Abstract: Kimberlites are the main source of natural gem-quality diamonds. The intrepid diamond explorer faces three major problems. First, finding a small, usually less than 300 m diameter, kimberlite, which is often highly weathered. Second, evaluating the quantity of diamonds within a kimberlite that often consists of multiple phases of intrusive and extrusive kimberlite, each with potentially different diamond grades. Third, evaluating the rough diamonds, the value of which is dependent on carat-weight, shape, colour, and clarity. Modern advances in mantle petrology, geophysics, geochemistry, geomorphology, and geostatistics now complement historical exploration knowledge and aid in selecting prospective target areas, resource estimation, and evaluating kimberlite-hosted diamond deposits.
Abstract: Calcic garnets are an important – although somewhat neglected – member of the garnet group. Typically, these mineral are members of complex solid solutions involving largely substitutions in the Fe3+/Al and Si sites and at least eight different end-members. The absolute majority of garnets in this family are Ti-Mg-Fe2+(± Al ± Zr)-bearing andradite transitional to morimotoite and schorlomite. Importantly, these garnets occur as common accessory minerals in a wide range of igneous and rocks, including nepheline syenites, alkali feldspar syenites, melteigite-urtites, nephelinites, melilitolites, melilitites, calcite carbonatites, ultramafic lamprophyres, orangeites, contaminated kimberlites, skarns and rodingites. Calcic garnets have a great capacity for atomic substitutions involving high-field-strength elements and, even more importantly, rare earths (up to 4000 ppm, including Y), Th and U (both up to 100 ppm) at low levels of common Pb. Their (La/Yb)cn ratio varies over two orders of magnitude (from < 0.01 to ~1), making these minerals a sensitive indicator of crystal fractionation, degassing and other magma-evolution processes. Given these unique compositional characteristics and surprising lack of interest in these minerals in the previous literature, we explored the possibility of using calcic garnets as a U-Pb geochronometer. For this purpose, we selected samples of well-crystallized igneous garnet from four very different rock types of different age, including: carbonatite (Afrikanda) from the Devonian Kola Alkaline Province, carbonatite from the Neoproterozoic Belaya Zima complex (Central-Asian mobile belt), ijolite from the Chick Ordovician igneous complex (Central-Asian mobile belt), granitic pegmatite from the Eden Lake complex in the Paleoproterozoic Trans-Hudson orogen, and feldspathoid syenite from the Cinder Lake alkaline complex in the Archean Knee Lake greenstone belt. U-Pb TIMS ages of the studied garnets are mostly concordant and reveal perfect correspondence with reported U-Pb zircon or perovskite ages as well as Sm-Nd isochrone age for these complexes. Therefore we can advertise calcic garnets as a promising tool for U-Pb geochronological studies.
Abstract: Mantle xenoliths from the Middle-Late Jurassic Obnazhennaya kimberlite are often compared with those from the Udachnaya kimberlite (ca. 367 Ma) to inform the evolution of the Siberia craton. However, there are no direct constraints on the timing of the Obnazhennaya kimberlite eruption. Such uncertainty of the kimberlite age precludes a better understanding of the mantle xenoliths from the Obnazhennaya pipe, and thus also of the evolution of the Siberia craton. This paper reports U-Pb ages for both perovskite from the Obnazhennaya kimberlite and rutile in an Obnazhennaya eclogite xenolith. The fresh perovskite formed during the early stage of magmatic crystallization and yields a U-Pb age of 151.8 ± 2.5 Ma (2?). Rutile in the eclogite xenolith yields an overlapping U-Pb age of 154.2 ± 1.9 Ma (2?). Because rutile has a Pb closure temperature lower than the inferred residence temperature of the eclogite prior to eruption, the U-Pb isotope system in rutile was not closed until the host eclogite was entrained and delivered to the surface by the kimberlite and therefore records the timing of kimberlite eruption. These data provide the first direct constraints on the emplacement age of the Obnazhennaya kimberlite and add to the global ‘kimberlite bloom’ from ca. 250-50 Ma as well as to the largest pulse of kimberlite volcanism in Siberia from ca. 171-144 Ma. The timing of this Jurassic-Cretaceous pulse coincides with the closure of the Mongol-Okhotsk Ocean, but the depleted Sr-Nd isotopic characteristics of 171-144 Ma kimberlites are inconsistent with a subduction-driven model for their petrogenesis. Thus, the closure of the Mongol-Okhotsk Ocean may act as a trigger for the initiation of 171-144 Ma kimberlite emplacement of Siberia, but was not the source.
Harvey, S., Read, G., DesGagnes, B., Shimell, M., Danoczi, J., Van Breugel, B., Fourie, L., Stilling, A.
Utilization of olivine macrocryst grain size and abundance dat a as a proxy for diamond size and grade in pyroclastic deposits of the Orion South kimberlite Fort a la Corne, Sasakatchewan, Canada.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
Journal of the Geological Society, Vol. 174, pp. 609-626.
South America, Uruguay
magmatism
Abstract: The crystalline basement of Uruguay was assembled during the Brasiliano Orogeny in the Neoproterozoic Era and was later affected by discrete tectonic activity. A new multi-method low-temperature dataset including (U–Th)/He ages from both zircon and apatite, T–t modelling and K–Ar dating of fine sericite fractions and fault gouge reveal a detailed post-orogenic geological history spanning the Phanerozoic Eon. The juxtaposition of the terranes that compose the area was achieved in the Ediacaran Period, and post-collision was marked by intense exhumation, in which the crystalline basement reached near-surface conditions by the early to mid-Palaeozoic. Regional subsidence promoted sedimentation in the Paraná Basin until the Permian, covering and reheating much of the basement that is at present exposed. Afterwards, deposition and volcanism were mostly confined to its current limits. Regional exhumation of the shield during the Permo-Triassic exposed much of the northern portion of the basement, and the south was further affected by the opening of the South Atlantic Ocean during the Mesozoic. Little exhumation affected the Uruguayan Shield during the Cenozoic, as reflected in its modest topography. The reactivation of inherited Neoproterozoic structures influenced the development of Mesozoic basins and the present-day landscape.
Abstract: Complex ice flow history, variable bedrock topography, landform types, and drift thickness may lead to complex glacial sediment dispersal patterns that are difficult to interpret, with implications for subglacial sediment provenance and related ice sheet research, as well as for mineral (drift) exploration. This study investigates the controls of bedrock topography, drift thickness, and landforms on 3D dispersal patterns in two study areas located southeast of Lac de Gras, Northwest Territories. The two areas are situated only about 25 km apart and have a similar ice flow history (clockwise shift from SW to NW). However, study area #1 hosts kimberlites within low topographic relief, while area #2 hosts kimberlites on a small granitic hill. The distribution and type of sediment-landform assemblages, as well as drift thickness, also differ between the two areas. Sediment characteristics, matrix geochemistry, and kimberlite indicator minerals (KIM) from surficial samples (n = 51) were analyzed and compared with a sample subset (n = 2000, from 250 boreholes) from a large RC drilling dataset donated by industry. Digital elevation models and a surficial geology map were also used. Results show contrasting patterns between the two areas, despite a similar ice flow record. Area #1 has a well-developed, yet fragmented 3D dispersal train consistent with the clockwise ice flow shift record. Area #2's dispersal patterns are less clearly-defined and appear unrelated or only weakly related to the known local source within the granitic hill. We find relationships between: 1) the strength of dispersal patterns and the bedrock topography in the kimberlitic source area; and, 2) the dispersal style and 3D shape within sediment-landform landsystems. These relationships have implications for drift prospecting survey design, as well as the interpretation of dispersal train patterns.
45th. Annual Yellowknife Geoscience Forum, p. 67 abstract
Canada, Northwest Territories
geochemistry - indicator minerals
Abstract: Tracing surficial dispersal patterns of indicator minerals within glacial sediments in the main up-ice direction has greatly contributed to numerous mineral discoveries of economic value in the Northwest Territories. However, many cases have also reported perplexing scenarios of dispersal trains seemingly lacking a source, or known sources without a spatially associated dispersal train at the surface. These ‘special’ cases often hinder exploration efforts, and tend to remain poorly understood; yet these cases are becoming increasingly important to decipher as exploration moves into more complex terrains. We present an overview of our research done in the Lac de Gras area over the past few years in collaboration with the Northwest Territories Geological Survey and their partners investigating the effect of multiple ice flows, variable bedrock topography and drift thickness, and the complexities of glacial sedimentary environments on 2D and 3D mechanical (detrital) dispersion. Our research draws from surface and subsurface datasets from various sources at both the regional and local scales. We show that despite the occurrence of relatively long, continuous, surficial patterns extending in the direction of the latest-strongest ice flow event in the region, a subtle record of the time-transgressive glacial history is also frequently preserved. These records yield information about the net effect on sediment dispersion of multiple ice flow phases, bedrock geology, basal topography, and glacial depositional processes. Our findings suggest these geological factors played a key role in producing some of the most irregular and enigmatic dispersal patterns in the region. They also offer insights into how to best characterize and explain the signal (or lack thereof) from elusive buried sources of potential economic interest.
Ross, M., Kelley, S.E., Janzen, R., Stirling, R.A., Normandeau, P.X., Elliott, B.
Orphan and elusive glacial dispersal trains from kimberlites in the Lac de Gras area.
2018 Yellowknife Geoscience Forum , p. 65-66. abstract
Canada, Northwest Territories
geochemistry
Abstract: Numerous glacial dispersal trains, spatially and compositionally associated to kimberlites, have been characterized and mapped in the Lac de Gras region, Northwest Territories (NT). However, a small number of these trains have yet to be associated with a source. Additionally, a number of known sub-cropping kimberlites do not have well-defined, spatially associated, trains of indicator minerals. These issues suggest that local factors may be important in controlling the occurrence, shape, and strength of a dispersal pattern and its spatial association with a kimberlite. Identifying these factors and understanding their effect on the dispersion of indicator minerals could provide a road map for finding additional diamondiferous kimberlites in the NT and elsewhere. Here we examine contrasting dispersal trains from south and southwest of Lac de Gras, as well as situations where the source of known dispersal trains (e.g., Coppermine Train) continue to elude exploration geologists. Using both surface and subsurface datasets, we find that the bedrock geology and topography of the source area, as well as those of the dispersal area, are potential key controls on the type and shape of dispersal patterns. Even across discontinuous drift and subdued shield relief we find that bedrock topography and lithology modulated the effect of glacial dynamics on till production and provenance. These 'bedrock factors' have interacted in various ways during Quaternary glaciations, in combinations unique to each case, to generate complex dispersal patterns in three dimensions. Accounting for these factors, using both surface and subsurface data, could enhance the success of drift exploration programs and improve their outcome in the glaciated shield terrains of northern Canada.
Stirling, R.A., Kelley, S.E., Ross, M., Elliott, B., Normandeau, P.X.
Surface and subsurface till characteristics in a drumlin field south of Lac de Gras, NT; implications for drift prospecting. ( Dominion's Ekati and North Arrow)
2018 Yellowknife Geoscience Forum , p. 80. abstract
Canada, Northwest Territories
deposit - Ekati
Abstract: Successful diamond exploration is becoming increasingly challenging as the best expressed targets have been found. Areas of variable drift thickness and heterogeneous surficial deposits present several challenges to exploration. One particular aspect that is poorly understood is the effect of well-developed drumlin fields on the surface expression of dispersal trains. Our study focuses on drumlin fields and their potential effects in the expression of a dispersal pattern. Because drumlins are often stratified we hypothesize that multiple till layers of contrasting provenance, representing multiple ice-flow directions, can occur at the surface across drumlin fields due to erosional processes. This has the potential to affect analysis and interpretation of surficial till dispersion data.
To test this hypothesis, we examined data from a large RC drilling dataset donated by Dominion Diamond Ekati Corp. and North Arrow Minerals Inc. and complemented it with field-based surficial geology observations and analysis of additional surficial till samples across targeted drumlins. The surficial samples were collected at the top and on the sides of drumlins to test whether any glacial stratigraphy is expressed, especially in areas where post-glacial erosion may have exposed internal drumlin stratigraphy. Based on the RC data and available maps drift thickness within the drumlin field ranges from 1 meter in the swales between drumlins to about 20 meters on the top of the highest amplitude drumlins. Locally measured ice-flow indicators (n=11) show three distinct ice-flow directions from older to youngest: 260, 290, 305 degrees. Preliminary analysis of textural and compositional data shows variations within the till at depth as well as across the drumlin field. Ongoing work focuses on determining the relationship (or lack thereof) between till characteristics, drumlins, and ice flow history (till provenance), as well as on three-dimensional dispersal patterns of kimberlite indicator minerals and related geochemical pathfinders. This work will highlight landform feature considerations by using multiple parameters to analyze sample data in areas with complex glacial geology and high diamond potential.
Yellowknife Forum NWTgeoscience.ca, abstract volume p. 89-90.
Canada, Northwest Territories
deposit - lac de Gras
Abstract: Till dispersal patterns may appear as a consistent train of indicators extending in the direction of the latest ice-flow phase from a source, or along a direction defined by an older ice-flow phase. However, other dispersal patterns, sometimes even in the same area, may have poorly-defined, discontinuous trains, or even lack dispersal trains all together. This research investigates dispersal patterns from two sites southeast of Lac de Gras that were affected by the same ice-flow history, but show important differences in bedrock topography, till thickness, and subglacial landform assemblages. The goal is to improve our understanding of bedrock and till thickness effects on dispersal trains. New local ice-flow indicators (n=16) constrain local ice-flow history. Digital elevation models and a surficial map are used to identify surficial landforms and to loosely constrain bedrock topography. We also use a subset of KIM results from a large industry-donated RC-drilling database (n=502 from 185 boreholes) which includes information on subsurface sediment characteristics and depth-to-bedrock data, which further constrain bedrock topography. In addition, we use texture, matrix geochemistry, KIMs, and clast lithology from a smaller set of 51 surface samples to compare dispersal patterns at surface and at depth. Part of the eastern study area is characterized by a well-defined drumlin field associated with the young NW ice-flow phase, variable till thickness (0-18m), and relatively flat bedrock topography (<20m elevation change). Kimberlites WO-17/WO-20 exhibit a short, but well-defined KIM dispersal train in the direction of the last dominant flow phase (NW); the dispersal area is also characterized by thin discontinuous till. A second KIM dispersal train is also recognized in the thicker till of the drumlin field SW of WO-17/20. Based on its location relative to WO-17/20, and till geochemistry and lithology counts, this pattern is interpreted to be a palimpsest train associated to the oldest SW ice flow. The western study area, located 20km from the eastern area, is characterized by a similar ice-flow history, but its bedrock topography varies more (~70m), with thin till, generally under 4m. A known kimberlite within the western area (Big Blue) is nestled within a bedrock topographic high ~20m above the surrounding terrain. Fragmented and more elusive till anomalies occur down-ice from this source. The lack of a well-developed dispersal train associated with the kimberlite is noteworthy, and may be due to the evolution of subglacial conditions around the bedrock hill. Our current model involves initial basal sliding and erosion of the top of the kimberlite and englacial entrainment. This phase was followed by reduced local abrasion and erosion rates within the kimberlite depression, possibly related to the development of low-pressure cavities over several local depressions: an idea supported by evidence of late-stage meltwater activity. This research highlights the important role of bedrock topography and related subglacial conditions both in the source area and dispersal area, as well as the potential for enhanced preservation of palimpsest trains in drumlinized till blankets.
Abstract: Clinopyroxene is a key fractionating phase in alkaline magmatic systems, but its impact on metal enrichment processes, and the formation of REE + HFSE mineralisation in particular, is not well understood. To constrain the control of clinopyroxene on REE + HFSE behaviour in sodic (per)alkaline magmas, a series of internally heated pressure vessel experiments was performed to determine clinopyroxene-melt element partitioning systematics. Synthetic tephriphonolite to phonolite compositions were run H2O-saturated at 200?MPa, 650-825?C with oxygen fugacity buffered to log f O2 ? ?QFM + 1 or log f O2 ? ?QFM +5. Clinopyroxene-glass pairs from basanitic to phonolitic fall deposits from Tenerife, Canary Islands, were also measured to complement our experimentally-derived data set. The REE partition coefficients are 0.3-53, typically 2-6, with minima for high-aegirine clinopyroxene. Diopside-rich clinopyroxenes (Aeg5-25) prefer the MREE and have high REE partition coefficients (DEuup to 53, DSmup to 47). As clinopyroxene becomes more Na- and less Ca-rich (Aeg25-50), REE incorporation becomes less favourable, and both the VIM1 and VIIIM2 sites expand (to 0.79 Å and 1.12 Å), increasing DLREE/DMREE. Above Aeg50 both M sites shrink slightly and HREE (VIri? 0.9 Å ? Y) partition strongly onto the VIM1 site, consistent with a reduced charge penalty for REE3+ ? Fe3+ substitution. Our data, complemented with an extensive literature database, constrain an empirical model that predicts trace element partition coefficients between clinopyroxene and silicate melt using only mineral major element compositions, temperature and pressure as input. The model is calibrated for use over a wide compositional range and can be used to interrogate clinopyroxene from a variety of natural systems to determine the trace element concentrations in their source melts, or to forward model the trace element evolution of tholeiitic mafic to evolved peralkaline magmatic systems.
Geophysical Research Letters, Vol. 43, 8, pp. 3693-3697.
Mantle
Melting
Abstract: Seismic tomography models reveal two large low shear velocity provinces (LLSVPs) that identify large-scale variations in temperature and composition in the deep mantle. Other characteristics include elevated density, elevated bulk sound speed, and sharp boundaries. We show that properties of LLSVPs can be explained by the presence of small quantities (0.3-3%) of suspended, dense Fe-Ni-S liquid. Trapping of metallic liquid is demonstrated to be likely during the crystallization of a dense basal magma ocean, and retention of such melts is consistent with currently available experimental constraints. Calculated seismic velocities and densities of lower mantle material containing low-abundance metallic liquids match the observed LLSVP properties. Small quantities of metallic liquids trapped at depth provide a natural explanation for primitive noble gas signatures in plume-related magmas. Our model hence provides a mechanism for generating large-scale chemical heterogeneities in Earth's early history and makes clear predictions for future tests of our hypothesis.
Geophysical Research Letters, Vol. 43, 8, pp. 3693-3699.
Mantle
Melting
Abstract: Seismic tomography models reveal two large low shear velocity provinces (LLSVPs) that identify large-scale variations in temperature and composition in the deep mantle. Other characteristics include elevated density, elevated bulk sound speed, and sharp boundaries. We show that properties of LLSVPs can be explained by the presence of small quantities (0.3 -3%) of suspended, dense Fe-Ni-S liquid. Trapping of metallic liquid is demonstrated to be likely during the crystallization of a dense basal magma ocean, and retention of such melts is consistent with currently available experimental constraints. Calculated seismic velocities and densities of lower mantle material containing low-abundance metallic liquids match the observed LLSVP properties. Small quantities of metallic liquids trapped at depth provide a natural explanation for primitive noble gas signatures in plume-related magmas. Our model hence provides a mechanism for generating large-scale chemical heterogeneities in Earth's early history and makes clear predictions for future tests of our hypothesis.
Abstract: Some seismic models derived from tomographic studies indicate elevated shear?wave velocities (?4.7 km/s) around 120?150 km depth in cratonic lithospheric mantle. These velocities are higher than those of cratonic peridotites, even assuming a cold cratonic geotherm (i.e., 35 mW/m2 surface heat flux) and accounting for compositional heterogeneity in cratonic peridotite xenoliths and the effects of anelasticity. We reviewed various geophysical and petrologic constraints on the nature of cratonic roots (seismic velocities, lithology/mineralogy, electrical conductivity, and gravity) and explored a range of permissible rock and mineral assemblages that can explain the high seismic velocities. These constraints suggest that diamond and eclogite are the most likely high?Vs candidates to explain the observed velocities, but matching the high shear?wave velocities requires either a large proportion of eclogite (>50 vol.%) or the presence of up to 3 vol.% diamond, with the exact values depending on peridotite and eclogite compositions and the geotherm. Both of these estimates are higher than predicted by observations made on natural samples from kimberlites. However, a combination of ?20 vol.% eclogite and ~2 vol.% diamond may account for high shear?wave velocities, in proportions consistent with multiple geophysical observables, data from natural samples, and within mass balance constraints for global carbon. Our results further show that cratonic thermal structure need not be significantly cooler than determined from xenolith thermobarometry.
Geophysical Research Letters, Vol. 46, 4, pp. 2037-2044.
Mantle
melting
Abstract: Silicate melting is a major agent of thermal and chemical evolution of the Earth and other rocky planets. The melting temperature of Calcium silicate perovskite, a mineral that exists in Earth's lower mantle, is unknown over most of the pressure range that occurs in the mantle of Earth and super?Earth exoplanets. We use advanced quantum mechanical simulations to predict the melting temperature of this material. We find that the melting temperature increases with increasing pressure but at a rate that diminishes continuously. The liquid and crystal have very similar volumes in the deep portions of planetary mantles, supporting the view that crystals may float at great depth.
Journal of Geophysical Research: Solid Earth, https://doi,org/ 10.1029/2018JB016482
Mantle
anistropy
Abstract: Several theoretical studies indicate that a substantial fraction of the measured seismic anisotropy could be interpreted as extrinsic anisotropy associated with compositional layering in rocks, reducing the significance of strain?induced intrinsic anisotropy. Here we quantify the potential contribution of grain?scale and rock?scale compositional anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties and (ii) measuring velocities of synthetic seismic waves propagating through modeled strain?induced microstructures. It is shown that for typical mantle and oceanic crust subsolidus compositions, rock?scale compositional layering does not generate any substantial extrinsic anisotropy (<1%) because of the limited contrast in isotropic elastic moduli among different rocks. Quasi?laminated structures observed in subducting slabs using P and S wave scattering are often invoked as a source of extrinsic anisotropy, but our calculations show that they only generate minor seismic anisotropy (<0.1-0.2% of Vp and Vs radial anisotropy). More generally, rock?scale compositional layering, when present, cannot be detected with seismic anisotropy studies but mainly with wave scattering. In contrast, when grain?scale layering is present, significant extrinsic anisotropy could exist in vertically limited levels of the mantle such as in a mid?ocean ridge basalt?rich lower transition zone or in the uppermost lower mantle where foliated basalts and pyrolites display up to 2-3% Vp and 3-6% Vs radial anisotropy. Thus, seismic anisotropy observed around the 660?km discontinuity could be possibly related to grain?scale shape?preferred orientation. Extrinsic anisotropy can form also in a compositionally homogeneous mantle, where velocity variations associated with major phase transitions can generate up to 1% of positive radial anisotropy.
Abstract: At the Earth's surface, the majority of water resides in the oceans, while in the interior, major rock?forming minerals can incorporate significant amounts of water as hydroxyl groups (OH), likely forming another reservoir of water inside the planet. The amount of water that can be dissolved in Earth's mantle minerals, called its water storage capacity, generally decreases at higher temperatures. Over billion?year timescales, the exchange of water between Earth's interior and surface may control the surface oceans' volume change. Here, we calculated the water storage capacity in Earth's solid mantle as a function of mantle temperature. We find that water storage capacity in a hot, early mantle may have been smaller than the amount of water Earth's mantle currently holds, so the additional water in the mantle today would have resided on the surface of the early Earth and formed bigger oceans. Our results suggest that the long?held assumption that the surface oceans' volume remained nearly constant through geologic time may need to be reassessed.
Contributions to Mineralogy and Petrology, Vol. 169, pp. 52-
Europe, Spain
Deposit - Ronda
Abstract: Chemical weathering of dark-green massive peridotite, including partly serpentinized peridotite, produces a distinct and remarkable brown weathering rind when exposed to the atmosphere long enough. The structure and mineral composition of crusts on rocks from the Ronda peridotite, Spain, have been studied in some detail. The generic overall weathering reaction serpentinized peridotite + rainwater = weathering rind + runoff water describes the crust-forming process. This hydration reaction depends on water supply from the outcrop surface to the reaction front separating green peridotite from the brown crust. The reaction pauses after drying and resumes at the front after wetting. The overall net reaction transforms olivine to serpentine in a volume-conserving replacement reaction. The crust formation can be viewed as secondary serpentinization of peridotite that has been strongly altered by primary hydrothermal serpentinization. The reaction stoichiometry of the crust-related serpentinization is preserved and reflected by the composition of runoff waters in the peridotite massif. The brown color of the rind is caused by amorphous Fe(III) hydroxide, a side product from the oxidation of Fe(II) released by the dissolution of fayalite component in olivine.
The upper continental crust, an aquifer and its fluid: hydraulic and chemical dat a from 4 km depth in fractured crystalline basement rocks at the KTB test.
Journal of Structural Geology, Vol. 86, pp. 47-61.
Africa, South Africa
Deposit - Venetia
Abstract: The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3-3.1 Ga, 2.7-2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo provides an unprecedented insight into CZ ductile deformation and sheath folding. D1 juxtaposed gneisses against metasediments. D2 produced a pervasive axial planar foliation (S2) to isoclinal F2 folds. Sheared lithological contacts and S2 were refolded into regional, open, predominantly southward-verging, E-W trending F3 folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S2. Constrictional-prolate D4 shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D4 overlaps with a: 1) 2.03-2.01 Ga regional M3 metamorphic overprint; b) transpressional deformation at 2.2-1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.
Journal of Structural Geology, Vol. 86, pp. 47-61.
Africa, South Africa
deposit - Venetia
Abstract: The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3-3.1 Ga, 2.7-2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo™ provides an unprecedented insight into CZ ductile deformation and sheath folding. D1 juxtaposed gneisses against metasediments. D2 produced a pervasive axial planar foliation (S2) to isoclinal F2 folds. Sheared lithological contacts and S2 were refolded into regional, open, predominantly southward-verging, E-W trending F3 folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S2. Constrictional-prolate D4 shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D4 overlaps with a: 1) 2.03-2.01 Ga regional M3 metamorphic overprint; b) transpressional deformation at 2.2-1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.
Journal of African Earth Sciences, Vol. 137, pp. 9-21.
Africa, Botswana
deposit - Jwaneng
Abstract: Country rock at Jwaneng Diamond Mine provides a rare insight into the deformational history of the Transvaal Supergroup in southern Botswana. The ca. 235 Ma kimberlite diatremes intruded into late Archaean to Early Proterozoic, mixed, siliciclastic-carbonate sediments, that were subjected to at least three deformational events. The first deformational event (D1), caused by NW-SE directed compression, is responsible for NE-trending, open folds (F1) with associated diverging, fanning, axial planar cleavage. The second deformational event (D2) is probably progressive, involving a clockwise rotation of the principal stress to NE-SW trends. Early D2, which was N-S directed, involved left-lateral, oblique shearing along cleavage planes that developed around F1 folds, along with the development of antithetic structures. Progressive clockwise rotation of far-field forces saw the development of NW-trending folds (F2) and its associated, weak, axial planar cleavage. D3 is an extensional event in which normal faulting, along pre-existing cleavage planes, created a series of rhomboid-shaped, fault-bounded blocks. Normal faults, which bound these blocks, are the dominant structures at Jwaneng Mine. Combined with block rotation and NW-dipping bedding, a horst-like structure on the northwestern limb of a broad, gentle, NE-trending anticline is indicated. The early compressional and subsequent extensional events are consistent throughout the Jwaneng-Ramotswa-Lobatse-Thabazimbi area, suggesting that a large area records the same fault geometry and, consequently, deformational history. It is proposed that Jwaneng Mine is at or near the northernmost limit of the initial, northwards-directed compressional event.
Journal of African Earth Sciences, Vol. 137, pp. 9-21.
Africa, Botswana
deposit - Jwaneng
Abstract: Country rock at Jwaneng Diamond Mine provides a rare insight into the deformational history of the Transvaal Supergroup in southern Botswana. The ca. 235 Ma kimberlite diatremes intruded into late Archaean to Early Proterozoic, mixed, siliciclastic-carbonate sediments, that were subjected to at least three deformational events. The first deformational event (D1), caused by NW-SE directed compression, is responsible for NE-trending, open folds (F1) with associated diverging, fanning, axial planar cleavage. The second deformational event (D2) is probably progressive, involving a clockwise rotation of the principal stress to NE-SW trends. Early D2, which was N-S directed, involved left-lateral, oblique shearing along cleavage planes that developed around F1 folds, along with the development of antithetic structures. Progressive clockwise rotation of far-field forces saw the development of NW-trending folds (F2) and its associated, weak, axial planar cleavage. D3 is an extensional event in which normal faulting, along pre-existing cleavage planes, created a series of rhomboid-shaped, fault-bounded blocks. Normal faults, which bound these blocks, are the dominant structures at Jwaneng Mine. Combined with block rotation and NW-dipping bedding, a horst-like structure on the northwestern limb of a broad, gentle, NE-trending anticline is indicated. The early compressional and subsequent extensional events are consistent throughout the Jwaneng-Ramotswa-Lobatse-Thabazimbi area, suggesting that a large area records the same fault geometry and, consequently, deformational history. It is proposed that Jwaneng Mine is at or near the northernmost limit of the initial, northwards-directed compressional event.
Abstract: Simultaneous acquisition of detrital zircon Pb-Pb ages and trace element abundances from grains collected across the Indian craton, spanning ?3 b.y., reveals prominent shifts in Eu/Eu* and light and middle to heavy rare earth element ratios. These shifts correspond to a ca. 3.0-2.2 Ga interval of crustal thickening during Indian craton formation, followed by a period wherein arc magmatism occurred along thinner craton margins from ca. 1.9 to 1.0 Ga, with arc magmatism concentrated along attenuated continental margins after ca. 1.0 Ga. Similar temporal shifts in trace element concentrations are recognized in global whole-rock compilations. We propose that the post-1.0 Ga increase in juvenile magmatism reflects a switch to lateral arc terrane accretion as the primary style of continental growth over the past billion years.
The Australian Gemmologist, Vol. 26, 9-10. p. 249.
Australia
deposit - Argyle
Abstract: Many kimberlite ore bodies are relatively small, and the presence of overburden along with the complexities of interpreting geophysical data can make it challenging to intersect a kimberlite target during exploration drilling. However, the analysis of country rock drill core from the perimeter of several known kimberlites provides evidence that subtle alteration (hydration) halos around kimberlites exist, which can be detected using rapid and cost-effective spectroscopic techniques. Identification of these hydration halos, which are independent of country rock composition, may provide crucial information about the presence of undiscovered kimberlite in an exploration area, especially if kimberlite was not intersected during initial drilling. Preliminary estimates suggest that these hydration halos, which are most likely caused by kimberlite-derived hydrous fluids, can extend more than 65 meters into the country rocks, but their size strongly depends on the size of the kimberlite body. Narrow kimberlite dikes produce much smaller halos compared to large kimberlite pipes. In addition, hydration halos in carbonate rocks appear to be smaller compared to silicate rocks .
Contributions to Mineralogy and Petrology, Vol. 173, 26p. Doi.org/10.1007/s00410-018-1505-y
Europe, Greenland
carbonatite
Abstract: Petrogenetic studies of carbonatites are challenging, because carbonatite mineral assemblages and mineral chemistry typically reflect both variable pressure-temperature conditions during crystallization and fluid-rock interaction caused by magmatic-hydrothermal fluids. However, this complexity results in recognizable alteration textures and trace-element signatures in the mineral archive that can be used to reconstruct the magmatic evolution and fluid-rock interaction history of carbonatites. We present new LA-ICP-MS trace-element data for magnetite, calcite, siderite, and ankerite-dolomite-kutnohorite from the iron-rich carbonatites of the 1.3 Ga Grønnedal-Íka alkaline complex, Southwest Greenland. We use these data, in combination with detailed cathodoluminescence imaging, to identify magmatic and secondary geochemical fingerprints preserved in these minerals. The chemical and textural gradients show that a 55 m-thick basaltic dike that crosscuts the carbonatite intrusion has acted as the pathway for hydrothermal fluids enriched in F and CO2, which have caused mobilization of the LREEs, Nb, Ta, Ba, Sr, Mn, and P. These fluids reacted with and altered the composition of the surrounding carbonatites up to a distance of 40 m from the dike contact and caused formation of magnetite through oxidation of siderite. Our results can be used for discrimination between primary magmatic minerals and later alteration-related assemblages in carbonatites in general, which can lead to a better understanding of how these rare rocks are formed. Our data provide evidence that siderite-bearing ferrocarbonatites can form during late stages of calciocarbonatitic magma evolution.
Abstract: Field, petrographic and geochemical data combined with in situ zircon U-Pb LA-ICP-MS ages are documented for the São Tiago Batholith (southernmost portion of the São Francisco Craton) to understand its origin and magmatic evolution. The geologic relations indicate that the batholith is composed of granitic to granodioritic orthogneisses (L2) with tonalitic xenoliths (L1) intruded by pegmatite (L3) and metagranite (L4). L1 consists of two facies of tonalitic orthogneiss, one biotite-rich, and the other biotite-poor. The geochemical evidence, including high K2O with mantle-like chemical signature, suggests that the Bt-rich tonalitic gneiss (2816?±?30?Ma) was derived from contamination of mafic magmas by crustal-derived components. The Bt-poor tonalitic gneiss, of TTG affinity, was generated by partial melting of LILE-enriched mafic rocks, possibly from oceanic plateus in a subduction environment. L2 includes two distinct types of rocks: (i) granodioritic orthogneiss, chemically ranging from medium-pressure TTGs to potassic granitoids originated via partial melting of previous TTG crust, including L1 Bt-poor; and (ii) granitic gneiss (2664?±?4?Ma), geochemically similar to crustal-derived granites, produced by melting of the L1 Bt-rich tonalitic gneiss or mixed TTG/metasedimentary sources. L3 pegmatite (2657?±?23?Ma) results from melting of L2, whereas L4 metagranite (dikes and stocks) shows petrogenesis similar to that of the L2 granitic gneiss. Related orthogneisses occur near the São Tiago Batholith: (i) a hornblende-bearing tonalitic gneiss, and (ii) a hybrid hornblende-bearing granitic gneiss (2614?±?13?Ma), whose genesis is linked with interaction of sanukitoid and felsic potassic melts, representing the last Archean magmatic pulse of the region. The Minas strata along the Jeceaba-Bom Sucesso lineament near our study region encircle the São Tiago Archean crust, representing an irregular paleo-coastline or a micro-terrane amalgamation with the São Francisco Proto-craton, with possible subsequent dome-and-keel deformational processes. Our petrological and geochronological data reevaluate nebulous concepts in the literature about the SFC, revealing (i) a chemically and compositionally diverse crustal segment generated at the Late Archean in diverse geodynamic scenarios, and (ii) a more complex lineament than previously thought in terms of the paleogeography of the southern São Francisco Craton.
Earth and Planetary Science Letters, Vol. 430, pp. 284-295.
Asia, Tibet
Luobusa Massif Type Iib
Abstract: For more than 20 years, the reported occurrence of diamonds in the chromites and peridotites of the Luobusa massif in Tibet (a complex described as an ophiolite) has been widely ignored by the diamond research community. This skepticism has persisted because the diamonds are similar in many respects to high-pressure high-temperature (HPHT) synthetic/industrial diamonds (grown from metal solvents), and the finding previously has not been independently replicated. We present a detailed examination of the Luobusa diamonds (recovered from both peridotites and chromitites), including morphology, size, color, impurity characteristics (by infrared spectroscopy), internal growth structures, trace-element patterns, and C and N isotopes. A detailed comparison with synthetic industrial diamonds shows many similarities. Cubo-octahedral morphology, yellow color due to unaggregated nitrogen (C centres only, Type Ib), metal-alloy inclusions and highly negative View the MathML source?C13 values are present in both sets of diamonds. The Tibetan diamonds (n=3n=3) show an exceptionally large range in View the MathML source?N15 (?5.6 to +28.7‰+28.7‰) within individual crystals, and inconsistent fractionation between {111} and {100} growth sectors. This in contrast to large synthetic HPHT diamonds grown by the temperature gradient method, which have with View the MathML source?N15=0‰ in {111} sectors and +30‰+30‰ in {100} sectors, as reported in the literature. This comparison is limited by the small sample set combined with the fact the diamonds probably grew by different processes. However, the Tibetan diamonds do have generally higher concentrations and different ratios of trace elements; most inclusions are a NiMnCo alloy, but there are also some small REE-rich phases never seen in HPHT synthetics. These characteristics indicate that the Tibetan diamonds grew in contact with a C-saturated Ni-Mn-Co-rich melt in a highly reduced environment. The stable isotopes indicate a major subduction-related contribution to the chemical environment. The unaggregated nitrogen, combined with the lack of evidence for resorption or plastic deformation, suggests a short (geologically speaking) residence in the mantle. Previously published models to explain the occurrence of the diamonds, and other phases indicative of highly reduced conditions and very high pressures, have failed to take into account the characteristics of the diamonds and the implications for their formation. For these diamonds to be seriously considered as the result of a natural growth environment requires a new understanding of mantle conditions that could produce them.
Abstract: Google Earth has been part of most geoscientists' computer (and mobile) desktops for a decade, and this year Google Earth Professional has become freely available to all with a universal license key. Many users are still, however, not aware of the full potential that it can offer across a range of teaching and research areas in the geosciences. Here a pragmatic look is taken at some of the current key uses in terms of resources and applications and how they can help in research and training educational roles in the geosciences.
Geostandards and Geoanalyical Review, Nov. 17, in press available
Technology
review
Abstract: This bibliographic review covers the research contained in twenty-one scientific journals with important contributions to geoanalysis and related scientific fields (Table 1, Figure 1). The relevance of well characterised reference materials (RMs) used as calibration materials or quality control samples for precise and accurate analyses is widely known and has often been described, for example, by Jochum and Enzweiler (2014).
SIMS analyses on trace and rare earth elements in coexisting clinopyroxene and mica from minette mafic enclaves in potassic syenites crystallized under high pressure.
Contributions to Mineralogy and Petrology, Vol. 148, 6, pp. 675-688.
Eiler, J.M., Berquist, B., Bourg, I., Cartigny, P., Farquhar, J., Gagnon, A., Guo, W., Halevy, I., Hofman, A., larson, T.E., Levin, N., Schauble, E.A., Stolper, D.
Can the garnet signature in Mid Ocean Ridge Basalt (MORB) be derived from garnet pyroxenites in Mid Ocean Ridge Basalt (Mid Ocean Ridge Basalt (MORB))source regions?
Geological Society of America (GSA) Abstract Volume, Vol. 26, No. 7, ABSTRACT only p. A38.
Kent, A.J.R., Stolper, E.M., Francis, D., Woodhead, J., Frei, R., Eiler, J.
Mantle heterogeneity during the formation of the North Atlantic igneous province: constraints from trace element and Sr Nd Os O isotope - Baffin Island picrites
Geochemistry, Geophysics, Geosystems: G3, Vol. 5, pp. Q11004 10.1029/2004GC000743
Abstract: Subduction of oceanic crust generates chemical and lithological heterogeneities in the mantle. An outstanding question is the extent to which these heterogeneities contribute to subsequent magmas generated by mantle melting, but the answer differs depending on the geochemical behaviour of the elements under investigation: analyses of incompatible elements (those that preferentially concentrate into silicate melts) suggest that recycled oceanic crust is an important contributor, whereas analyses of compatible elements (those that concentrate in crystalline residues) generally suggest it is not. Recently, however, the concentrations of Mn and Ni—two elements of varying compatibility—in early-crystallizing olivines, have been used to infer that erupted magmas are mixtures of partial melts of olivine-rich mantle rocks (that is, peridotite) and of metasomatic pyroxene-rich mantle rocks (that is, pyroxenite) formed by interaction between partial melts of recycled oceanic crust and peridotite. Here, we test whether melting of peridotite alone can explain the observed trend in olivine compositions by combining new experimental data on the partitioning of Mn between olivine and silicate melt under conditions relevant to basalt petrogenesis with earlier results on Ni partitioning. We show that the observed olivine compositions are consistent with melts of fertile peridotite at various pressures—importantly, melts from metasomatic pyroxenites are not required. Thus, although recycled materials may well be present in the mantle source regions of some basalts, the Mn and Ni data can be explained without such a contribution. Furthermore, the success of modelling the Mn–Ni contents of olivine phenocrysts as low-pressure crystallization products of partial melts of peridotite over a range of pressures implies a simple new approach for constraining depths of mantle melting.
Abstract:
Decades of study have documented several orders of magnitude variation in the oxygen fugacity (fO2) of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe3+/Fe2+) can also lead to systematic variations in fO2 in the shallowest ~100 km of the mantle. Two different thermodynamic models were used to calculate fO2 vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2 decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2 and increasing pressure in this facies has little influence on fO2 (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2 (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2 reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ? Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe3+-bearing components in pyroxene and therefore to decreases in normalized fO2, whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe3+-bearing components in pyroxene and to increases in normalized fO2 (although this is counteracted to some degree by progressive partitioning of Fe3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2 inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase ? spinel ? garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations. Observed variations in FMQ-normalized fO2 of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe3+/Fe2+ or bulk O2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2 of peridotites of constant composition are likely to be superimposed on variations in fO2 that reflect differences in the whole-rock Fe3+/Fe2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.
Metasomatized lower crustal and upper mantle xenoliths from North Queensland -chemical and isotopic evidence bearing on the composition and source of fluid phase
Geochimica et Cosmochimica Acta, Vol. 53, No. 3, March pp. 649-660
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) , Annual Meeting Abstracts approximately 10 lines, Vol. 86, No. 968, March POSTER ABSTRACT p. 69
The aeromagnetic survey program of the Geological Society of Canada (GSC)- implications for kimberliteexploration.
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 27-30.
Simandl, G.J., Paradis, S., Stone, R.S., Fajber, R., Kressall, R.D., Grattan, K., Crozier, J., Simandl, L.J.
Applicablity of handheld X-ray fluroescence spectrometry in the exploration and development of carbonatite related niobium deposits: a case study of the Aley carbonatite, British Columbia, Canada.
Geochemistry: Exploration, Environment, Analysis, Vol. 14, 3, pp. 211-221.
Contributions to Mineralogy and Petrology, Vol. 171, 7, 25p.
Canada, Northwest Territories
Deposit - Lac de Gras arena
Abstract: Megacrystic (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) xenocrysts within kimberlites from Lac de Gras (Northwest Territories, Canada) contain fully crystallized melt inclusions. These ‘polymineralic inclusions’ have previously been interpreted to form by necking down of melts at mantle depths. We present a detailed petrographical and geochemical investigation of polymineralic inclusions and their host crystals to better understand how they form and what they reveal about the evolution of kimberlite melt. Genetically, the megacrysts are mantle xenocrysts with peridotitic chemical signatures indicating an origin within the lithospheric mantle (for the Cr-diopsides studied here ~4.6 GPa, 1015 °C). Textural evidence for disequilibrium between the host crystals and their polymineralic inclusions (spongy rims in Cr-diopside, kelyphite in Cr-pyrope) is consistent with measured Sr isotopic disequilibrium. The preservation of disequilibrium establishes a temporal link to kimberlite eruption. In Cr-diopsides, polymineralic inclusions contain phlogopite, olivine, chromite, serpentine, and calcite. Abundant fluid inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr2O3 and Al2O3. This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic melt + diopside ? forsterite + calcite + CO2, yielding the observed inclusion mineralogy and producing associated (CO2-rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical and chemical evidence for an origin of kimberlite that involves the reaction of high-pressure dolomitic melt with diopside-bearing mantle assemblages producing a lower-pressure melt that crystallizes a calcite-dominated assemblage in the crust.
Contributions to Mineralogy and Petrology, Vol. 171, 7, 9p.
Mantle
Carbonatite, kimberlite
Abstract: Kimberlites are rare diamond-bearing volcanic rocks that originate as melts in the Earth’s mantle. The original composition of kimberlitic melt is poorly constrained because of mantle and crustal contamination, exsolution of volatiles during ascent, and pervasive alteration during and after emplacement. One recent model (Russell et al. in Nature 481(7381):352 -356, 2012. doi:10.1038/nature10740) proposes that kimberlite melts are initially carbonatitic and evolve to kimberlite during ascent through continuous assimilation of orthopyroxene and exsolution of CO2. In high-temperature, high-pressure experiments designed to test this model, assimilation of orthopyroxene commences between 2.5 and 3.5 GPa by a reaction in which orthopyroxene reacts with the melt to form olivine, clinopyroxene, and CO2. No assimilation occurs at 3.5 GPa and above. We propose that the clinopyroxene produced in this reaction can react with the melt at lower pressure in a second reaction that produces olivine, calcite, and CO2, which would explain the absence of clinopyroxene phenocrysts in kimberlites. These experiments do not confirm that assimilation of orthopyroxene for the entirety of kimberlite ascent takes place, but rather two reactions at lower pressures (<3.5 GPa) cause assimilation of orthopyroxene and then clinopyroxene, evolving carbonatitic melts to kimberlite and causing CO2 exsolution that drives rapid ascent.
Contributions to Mineralogy and Petrology, in press available 25p.
Canada, Northwest Territories
Deposit - Lac de Gras
Abstract: Megacrystic (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) xenocrysts within kimberlites from Lac de Gras (Northwest Territories, Canada) contain fully crystallized melt inclusions. These ‘polymineralic inclusions’ have previously been interpreted to form by necking down of melts at mantle depths. We present a detailed petrographical and geochemical investigation of polymineralic inclusions and their host crystals to better understand how they form and what they reveal about the evolution of kimberlite melt. Genetically, the megacrysts are mantle xenocrysts with peridotitic chemical signatures indicating an origin within the lithospheric mantle (for the Cr-diopsides studied here ~4.6 GPa, 1015 °C). Textural evidence for disequilibrium between the host crystals and their polymineralic inclusions (spongy rims in Cr-diopside, kelyphite in Cr-pyrope) is consistent with measured Sr isotopic disequilibrium. The preservation of disequilibrium establishes a temporal link to kimberlite eruption. In Cr-diopsides, polymineralic inclusions contain phlogopite, olivine, chromite, serpentine, and calcite. Abundant fluid inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr2O3 and Al2O3. This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic melt + diopside ? forsterite + calcite + CO2, yielding the observed inclusion mineralogy and producing associated (CO2-rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical and chemical evidence for an origin of kimberlite that involves the reaction of high-pressure dolomitic melt with diopside-bearing mantle assemblages producing a lower-pressure melt that crystallizes a calcite-dominated assemblage in the crust.
Contributions to Mineralogy and Petrology, in press available 9p.
Technology
Petrology
Abstract: Kimberlites are rare diamond-bearing volcanic rocks that originate as melts in the Earth’s mantle. The original composition of kimberlitic melt is poorly constrained because of mantle and crustal contamination, exsolution of volatiles during ascent, and pervasive alteration during and after emplacement. One recent model (Russell et al. in Nature 481(7381):352-356, 2012. doi:10.1038/nature10740) proposes that kimberlite melts are initially carbonatitic and evolve to kimberlite during ascent through continuous assimilation of orthopyroxene and exsolution of CO2. In high-temperature, high-pressure experiments designed to test this model, assimilation of orthopyroxene commences between 2.5 and 3.5 GPa by a reaction in which orthopyroxene reacts with the melt to form olivine, clinopyroxene, and CO2. No assimilation occurs at 3.5 GPa and above. We propose that the clinopyroxene produced in this reaction can react with the melt at lower pressure in a second reaction that produces olivine, calcite, and CO2, which would explain the absence of clinopyroxene phenocrysts in kimberlites. These experiments do not confirm that assimilation of orthopyroxene for the entirety of kimberlite ascent takes place, but rather two reactions at lower pressures (<3.5 GPa) cause assimilation of orthopyroxene and then clinopyroxene, evolving carbonatitic melts to kimberlite and causing CO2 exsolution that drives rapid ascent.
Abstract: Origin determination is of increasing importance in the gem trade. It is possible because there is a close relationship between the geological environment of formation and the physical and chemical properties of gemstones, such as trace element and isotopic compositions, that can be measured in the laboratory using combinations of increasingly sophisticated instrumentation. Origin conclusions for ruby, sapphire, and emerald make up the bulk of demand for these services, with growing demand for alexandrite, tourmaline, and spinel. However, establishing origin with a high degree of confidence using the capabilities available today is met with varying degrees of success. Geographic origin can be determined with a high level of confidence for materials such as emerald, Paraíba-type tourmaline, alexandrite, and many rubies. For some materials, especially blue sapphire and some rubies, the situation is more difficult. The main problem is that if the geology of two deposits is similar, then the properties of the gemstones they produce will also be similar, to the point where concluding an origin becomes seemingly impossible in some cases. Origin determination currently relies on a combination of traditional gemological observations and advanced analytical instrumentation.
Potassic volcanism working group post congress excursion -volcanism in the Umbria-latium ultra alkaline district, Italy
International Conference on active volcanoes and risk mitigation, Field trip Sept. 2-, Department Scienze Della Terra, Piazza University of 06100 Perugia
Abstract: This paper is an updated overview, including many new data, of what is known about Italian alkaline-carbonatite complexes, plus a new description of a carbothermal residua-related district, and its potential for mineral deposits. The Italian carbonatite occurrences can be divided into two belts. The first is a 350 km long and 75 km wide belt along the Apennines mountain range mainly with primary extrusive carbonatites generally from monogenic volcanoes and from the Vulture volcanic complex; the second belt is 60 km long and 20 km wide in the Northern Latium region in which carbothermal residua carbonatites and fluorite mineralisation deposited by high-temperature fluids rich in CO2, SO2 and fluorine are occurring in caldera volcanoes. Several of the raw materials, such as Light Rare Earth Elements, vanadium, niobium, zirconium, fluorite and phosphate are identified as critical as well as other commodities, occur in Italian carbonatites and alkaline rocks. At the Pianciano quarry (Bracciano) fluorite-rich ore (fluor-ore = fluorite in a mineralised gangue) is actually exploited as flux for cement, but Rare Earth Elements (+ V) could be a notable by-product (300,000 metric tonnes, equivalent to 4.2% of European resources). Pyrochlore, monazite, apatite, and britholite bearing subvolcanic rocks in ejecta from the Vulture volcano are of a near-economic grade, but their geological constraints are not known. A conceptual framework of combined geological and geochemical data improves the general understanding of this regional magmatic system, aimed at laying the foundations of a future geological model disclosing unrecognised potential exploration targets. However, this paper is not intended for direct use by the exploration industry; rather it is principally aimed at mineralogists and petrologists who could develop strategies for the identification of unexposed or unrecognised deposits.
Abstract: The Mt. Vulture (Basilicata, Southern Italy) is an alkaline carbonatite volcano whose extrusive rocks are mafic, alkaline with different Na/K ratios, mainly SiO2 undersaturated, with relatively high contents of Cl, S, F, and CO2 [1]. Their composition ranges from basalts to basanites to tephrite to phono-tephrites and phonolites. Along with this magma suite have been erupted mantle xenolith bearing-carbonatitic melilitites and carbonatites [1, 2]. Holocrystalline ejecta have been studied in detail to reconstruct the composition of the subvolcanic/plutonic bodies beneath Mt. Vulture. The ejacta are haüine-bearing clinopyoxenites with variable content of olivine, amphibole and phlogopite, haüine foidolites with some nepheline and leucite, haüine-calcite-syenites, syenites, calcite melilitolites, K-feldspar bering-alvikites and a sovite [3]. There is a continuous variation in the modal and geochemical composition between clinopyroxenite and foidolite, that might be related to the chemical evolution shown by the extrusive rocks. The ejecta show an enrichment trend in LILE, LREE and HFSE consistent with fractional crystallisation evolution, from clinopyroxenites to foidolites and from foid-syenites to syenites. The foid-syeniites are rich in U, Pb, Sr, LREE and contain britholite, wholerite, Upyrochlore. The most evolved syenite however, is less enriched in REE but contains elevate content of U and HFSE. The sovite contains intercumulus alkali carbonates. A glimemerite vein in a haüine foidolite contain REE-rich apatite, shorlomite and U-pyrochlore. These findings suggest that alkaline-alogen-H2O-CO2 rich fluids can be formed during sub-volcanic/plutonic fractional crystallisation. These fluids can produce fenitisation and/or can form mineralisation enriched in REE and HFSE. The ejecta suite studied represents the intrusive complex beneath the volcano and these rock types are typical of ring complexes in alkaline carbonatite volcanoes.
Abstract: A new discovery of carbonatites at Pianciano, Ficoreto and Forcinelle in the Roman Region demonstrates that Italian carbonatites are not just isolated, mantle xenoliths-bearing, primitive diatremic rocks but also evolved sub-type fluor-calciocarbonatite (F?10 wt.%) associated with fluor ore (F?30 wt.%). New data constrain a multi-stage petrogenetic process, 1-orthomagmatic, 2-carbothermal, 3-hydrothermal. Petrography and geochemistry are conducive to processes of immiscibility and decarbonation, rather than assimilation and crystal fractionation. A CO2-rich, ultra-alkaline magma is inferred to produce immiscible melilite leucitite and carbonatite melts, at lithospheric mantle depths. At the crustal level and in the presence of massive CO2 exsolution, decarbonation reactions may be the dominant processes. Decarbonation consumes dolomite and produces calcite and periclase, which, in turn, react with silica to produce forsterite and Ca silicates (monticellite, melilite, andradite). Under carbothermal conditions, carbonate breakdown releases Sr, Ba and LREE; F and S become concentrated in residual fluids, allowing precipitation of fluorite and barite, as well as celestine and anhydrite. Fluor-calciocarbonatite is the best candidate to exsolve fluids able to deposit fluor ore, which has a smaller volume. At the hydrothermal stage, REE concentration and temperature dropping allow the formation of LREEF2+ and LREECO3+ ligands, which control the precipitation of interstitial LREE fluorcarbonate and silicates -(bastnäsite-(Ce)- Ce(CO3)F and -britholite-(Ce)- (Ce,Ca)5(SiO4,PO4)3(OH,F) . Vanadates such as wakefieldite, CeVO4, vanadinite, Pb5(VO4)3Cl and coronadite Pb(Mn4+6 Mn3+2)O16 characterise the matrix. At temperatures of ?100°C analcime, halloysite, quartz, barren calcite, and zeolites (K-Ca) precipitate in expansion fractures, veins and dyke aureoles.
Abstract: A new discovery of carbonatites at Pianciano, Ficoreto and Forcinelle in the Roman Region demonstrates that Italian carbonatites are not just isolated, mantle xenoliths-bearing, primitive diatremic rocks but also evolved sub-type fluor-calciocarbonatite (F~10 wt.%) associated with fluor ore (F~30 wt.%). New data constrain a multi-stage petrogenetic process, 1-orthomagmatic, 2-carbothermal, 3-hydrothermal. Petrography and geochemistry are conducive to processes of immiscibility and decarbonation, rather than assimilation and crystal fractionation. A CO2-rich, ultra-alkaline magma is inferred to produce immiscible melilite leucitite and carbonatite melts, at lithospheric mantle depths. At the crustal level and in the presence of massive CO2 exsolution, decarbonation reactions may be the dominant processes. Decarbonation consumes dolomite and produces calcite and periclase, which, in turn, react with silica to produce forsterite and Ca silicates (monticellite, melilite, andradite). Under carbothermal conditions, carbonate breakdown releases Sr, Ba and LREE; F and S become concentrated in residual fluids, allowing precipitation of fluorite and barite, as well as celestine and anhydrite. Fluor-calciocarbonatite is the best candidate to exsolve fluids able to deposit fluor ore, which has a smaller volume. At the hydrothermal stage, REE concentration and temperature dropping allow the formation of LREEF2+ and LREECO3+ ligands, which control the precipitation of interstitial LREE fluorcarbonate and silicates -(bastnäsite-(Ce)- Ce(CO3)F and -britholite-(Ce)- (Ce,Ca)5(SiO4,PO4)3(OH,F) . Vanadates such as wakefieldite, CeVO4, vanadinite, Pb5(VO4)3Cl and coronadite Pb(Mn4+6 Mn3+2)O16 characterise the matrix. At temperatures of =100°C analcime, halloysite, quartz, barren calcite, and zeolites (K-Ca) precipitate in expansion fractures, veins and dyke aureoles.
Abstract: Population growth and technological progress in the last 50 years have resulted in the global demand for mineral resources increasing by 400% since 1970, and it is further expected to almost double by 2050. This context forecasts a never-seen-before market for some specific mineral commodities, termed critical metals. The resource and supply flow of critical metals would be decisive for the economic well-being of economies in near future. Carbonatites are the most prospective host rocks for Rare Earth Elements (REEs), which constitute some of the most important critical elements. This special issue aims to contribute to the debate on understanding the genesis of carbonatites and their prospectivity for REEs (including exploration strategies), by presenting a wide variety of studies on carbonatites from around the globe.
Abstract: During my doctoral studies, in the late 1980s, I realised that the Italian kamafugites (kalsilite melilitites) had to be related to carbonatite magmatism. I started a detailed study of the kamafugitic sites, and I explored remote areas deep in Italy’s Apennine mountains. When I found the Polino carbonatite, I put a few drops of acid on it, and the rock reacted. I have a vivid memory of my heart beating faster. I had found it! My fellow geologists were somewhat sceptical, but the late Professor Giorgio Marinelli (1922-1993) encouraged me and predicted many new carbonatite discoveries. He was right. Overcoming my Latin temperament, I focused on the concept that carbonatites, however unusual as rocks, cannot be dismissed as simple geological oddities but require detailed and comprehensive study. I am fond of all the history that marked my latest 40 years of life, and it reminds me of the many friends and mentors that I have had, especially when I was a young researcher. Sadly, some of them are no longer with us. I am so grateful to them, and I consider it a life-changing experience to have met them
Abstract: The composition of the early Earth’s atmosphere is believed to result from significant magma outgassing during the Archaean eon. It has been widely debated whether the oxygen fugacity (fo2) of the Earth’s mantle has remained constant over the last ~3.8 Ga to levels where volatiles were mostly in their mobile form [1,2], or whether the mantle has experienced a gradual increase of its redox state [3]. Both hypotheses raise fundamental questions on the effect of composition of the early Earth’s accreting material, the origin and availability of primordial carbon in Earth’s interior, and the migration rate of CO2-rich magmas. In addition, the occurrence in nature of carbonatites (or silicate-carbonatitic rocks), diamonds and carbides indicate a dominant control of the mantle redox state on the volatile speciation over time and, maybe, on mechanisms of their formation, reaction and migration through the silicate mantle. A recent model has been developed that combines both experimental results on the fo2 of preserved carbonaceous chondrites at high pressure and thermodynamic predictions of the the temporal variation of the mantle redox state, with the CO2-bearing magmas that could form in the early asthenospheric mantle. Since any variation in melt composition is expected to cause significant changes in the physical properties (e.g., viscosity and density), the migration rate of these magmas has been determined using recent in situ viscosity data on CO2-rich melts with the falling sphere technique. Our results allow determining the composition of CO2- bearing magmas as function of the increasing mantle redox state over time, and the mechanisms and rate for exchange of carbon between mantle reservoirs.
Abstract: Knowledge of the rheology of molten materials at high pressure and temperature is required to understand magma mobility and ascent rate at conditions of the Earth's interior. We determined the viscosity of nominally anhydrous sodium carbonate (Na2CO3), an analogue and ubiquitous component of natural carbonatitic magmas, by the in situ “falling sphere” technique at 1.7, 2.4 and 4.6?GPa, at 1200 to 1700?°C, using the Paris-Edinburgh press. We find that the viscosity of liquid Na2CO3 is between 0.0028?±?0.0001?Pa•s and 0.0073?±?0.0001?Pa•s in the investigated pressure-temperature range. Combination of our results with those from recent experimental studies indicate a negligible dependence on pressure from 1?atm to 4.6?GPa, and a small compositional dependence between molten alkali metal-bearing and alkaline earth metal-bearing carbonates. Based on our results, the viscosity of Na2CO3 is consistent with available viscosity data of both molten calcite (determined at high pressure and temperature) and Na2CO3 at ambient pressure. Molten Na2CO3 is a valid experimental analogue for study of the rheology of natural and/or synthetic near-solidus carbonatitic melts. Estimated values of the mobility and ascent velocity of carbonatitic melts at upper conditions are between 70 and 300?g?cm?3•Pa?1•s?1 and 330-1450?m•year?1, respectively, when using recently proposed densities for carbonatitic melts. The relatively slow migration rate allows magma-rock interaction over time causing seismic anomalies and chemical redox exchange.
Minerals MDPI, Vol. 10, 267 doi: 10.23390/min10030267 14p. Pdf
Mantle
Melililite, carbon
Abstract: Understanding the viscosity of mantle-derived magmas is needed to model their migration mechanisms and ascent rate from the source rock to the surface. High pressure-temperature experimental data are now available on the viscosity of synthetic melts, pure carbonatitic to carbonate-silicate compositions, anhydrous basalts, dacites and rhyolites. However, the viscosity of volatile-bearing melilititic melts, among the most plausible carriers of deep carbon, has not been investigated. In this study, we experimentally determined the viscosity of synthetic liquids with ~31 and ~39 wt% SiO2, 1.60 and 1.42 wt% CO2 and 5.7 and 1 wt% H2O, respectively, at pressures from 1 to 4.7 GPa and temperatures between 1265 and 1755 °C, using the falling-sphere technique combined with in situ X-ray radiography. Our results show viscosities between 0.1044 and 2.1221 Pa•s, with a clear dependence on temperature and SiO2 content. The atomic structure of both melt compositions was also determined at high pressure and temperature, using in situ multi-angle energy-dispersive X-ray diffraction supported by ex situ microFTIR and microRaman spectroscopic measurements. Our results yield evidence that the T-T and T-O (T = Si,Al) interatomic distances of ultrabasic melts are higher than those for basaltic melts known from similar recent studies. Based on our experimental data, melilititic melts are expected to migrate at a rate ~from 2 to 57 km•yr?1 in the present-day or the Archaean mantle, respectively.
Wall, F., Niku-Paavola, V.N., Storey, C., Muller, A.,Jeffries, T.
Xenotime from carbonatite dykes at Lofdal Namibia - an extension of carbonatite REE mineralization, first dating of xenotime overgrowths on zircon.LA-ICP-MS-U-Pb
Frontiers in Mineral Sciences 2007, Joint Meeting of Mineralogical societies Held June 26-28, Cambridge, Abstract Volume p. 89-90.
Wall, F., Niku-Paavola, V.N., Storey, C., Muller, A.,Jeffries, T.
Xenotime from carbonatite dykes at Lofdal Namibia - an extension of carbonatite REE mineralization, first dating of xenotime overgrowths on zircon.LA-ICP-MS-U-Pb
Frontiers in Mineral Sciences 2007, Joint Meeting of Mineralogical societies Held June 26-28, Cambridge, Abstract Volume p. 89-90.
Wall, F., Niku-Paavola, V.N., Storey, C., Muller, A., Jeffries, T.
Xenotime - (Y) from carbonatite dykes at Lofdal, Namibia: unusually low LREE:HREE ratio in carbonatite, and the first dating of xenotime overgrowths on zircon.
Earth and Planetary Science Letters, Vol. 446, pp. 137-148.
Mantle
Coesite, subduction
Abstract: The ability to accurately constrain the secular record of high- and ultra-high pressure metamorphism on Earth is potentially hampered as these rocks are metastable and prone to retrogression, particularly during exhumation. Rutile is among the most widespread and best preserved minerals in high- and ultra-high pressure rocks and a hitherto untested approach is to use mineral inclusions within rutile to record such conditions. In this study, rutiles from three different high- and ultrahigh-pressure massifs have been investigated for inclusions. Rutile is shown to contain inclusions of high-pressure minerals such as omphacite, garnet and high silica phengite, as well as diagnostic ultrahigh-pressure minerals, including the first reported occurrence of exceptionally preserved monomineralic coesite in rutile from the Dora -Maira massif. Chemical comparison of inclusion and matrix phases show that inclusions generally represent peak metamorphic assemblages; although rare prograde phases such as titanite, omphacite and corundum have also been identified implying that rutile grows continuously during prograde burial and traps mineralogic evidence of this evolution. Pressure estimates obtained from mineral inclusions, when used in conjunction with Zr-in-rutile thermometry, can provide additional constraints on the metamorphic conditions of the host rock. This study demonstrates that rutile is an excellent repository for high- and ultra-high pressure minerals and that the study of mineral inclusions in rutile may profoundly change the way we investigate and recover evidence of such events in both detrital populations and partially retrogressed samples.
Bruno, H., Helibron, M., Strachen, R., Fowler, M., de MorrisonValeriano , C., Bersan, S., Moreira, H., Cutts, K., Dunlop, J., Almeida, R., Almeida, J., Storey, C.
Abstract: A zircon Hf isotope data set from Archean and Paleoproterozoic magmatic and metasedimentary rocks of the southern São Francisco craton (Brazil) is interpreted as evidence of accretionary and collisional plate tectonics since at least the Archean-Proterozoic boundary. During the Phanerozoic, accretionary and collisional orogenies are considered the end members of different plate tectonic settings, both involving preexisting stable continental lithosphere and consumption of oceanic crust. However, mechanisms for the formation of continental crust during the Archean and Paleoproterozoic are still debated, with the addition of magmatic rocks to the crust being explained by different geodynamic models. Hf isotopes can be used to quantify the proportion of magmatic addition into the crust: positive ?Hf values are usually interpreted as indications of magmatic input from the mantle, whereas crust-derived rocks show more negative ?Hf. We show that the crust of the amalgamated Paleoproterozoic tectonostratigraphic terranes that make up the southern São Francisco craton were generated from different proportions of mantle and crustal isotopic reservoirs. Plate tectonic processes are implied by a consistent sequence of events involving (1) the generation of juvenile subduction-related magmatic arc rocks, followed by (2) collisional orogenesis and remelting of older crust, and (3) post-collisional bimodal magmatism.
Detailed protracted crystallization history of perovskite in Orapa kimberlite.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 211-224.
Ionov, D.A., Ashchepkov, I.V., Stosch, H.G., et al.
Garnet peridotite xenoliths from the Vitim volcanic field, Baikal region:the nature of the garnet-spinel peridotite transition zone in the continentalmantle.
Journal of Petrology, Vol. 34, No. 6, pp. 1141-1175.
Subprovince accretion in the southern Superior Province or cross section through the Wawa-Quetico-Wabigoon subprovincial boundaries and Beardmore-Geraldtonbelt
Geological Association of Canada (GAC) Annual Meeting held Toronto May 1991, Guidebook, No. B6, 25p
Age and petrogenesis of two late Archean magmatic suites, northwestern Superior Province, Canada: zircon uranium-lead (U-Pb) (U-Pb) and Lu-Hf isotopic relations
Journal of Petrology, Vol. 34, No. 4, August pp. 817-
Paleomagnetism and U Pb dating of Proterozoic dykes: a new radiation swarm and an increase in post Archean crustal rotation westwards from the Kapuskasing zone.
GAC Annual Meeting Halifax May 15-19, Abstract 1p.
Halls, H.C., Davis, D.W., Stott, G.M., Ernst, R.E., Hamilton, M.A.
The Paleoproterozoic Marathon large igneous province: new evidence for a 2.1 Ga long lived mantle plume event along the southern margin of the N.A. Superior Province
Abstract: A growing number of people are entering the artisanal and small?scale mining (ASM) sector worldwide. In Madagascar, millions of individuals depend on this informal activity. Through a case study in the Alaotra?Mangoro region of Madagascar, our research aimed to understand the "bottom?up" dynamics and ripple effects of the sector, by looking at the realities for rural communities where inhabitants are both directly and indirectly affected by ASM. We were interested in community members' and miners' perceptions of the socio?economic and environmental impacts of ASM, and in identifying the factors attracting people living in one of the country's agricultural hubs to this activity. Our results show a wide diversity of push and pull factors leading people to enter the sector. Although many positive impacts of ASM exist for miners and communities within the vicinity of mines, most miner participants considered themselves worse off since starting to mine, highlighting the high risk and low probability of return of ASM. ASM's potential for local and national development will remain squandered if its negative impacts continue to go unmanaged. Accounting for local contexts and the ripple effects of ASM will be crucial in achieving safety and security for miners, and to tap into the benefits it may offer communities while minimising environmental damage.
Abstract: Over the past three decades, it has become clear that Pangea was just the most recent of several supercontinents that have amalgamated and dispersed since at least 2.0 Ga. It was fully recognized at the time that the so-called "supercontinent cycle" had a profound effect on Earth Systems, possibly one of the most significant insights since the advent of plate tectonics. In the early 1980's, Damian Nance, along with colleagues Tom Worsley and Judith Moody, were the instigators of this phase of modern thinking and since that time so many international projects and research careers have been spawned by those insights. Although many elegant papers had proposed orogenic episodicity before the acceptance of the plate tectonic paradigm, Damian and colleagues were the first to link such episodicity to a supercontinent cycle. In addition, Damian has made seminal contributions to the understanding of orogenic processes in general, and through his detailed fieldwork, to our foundational knowledge of the geology of the Avalonian belt in Maritime Canada, Paleozoic and Proterozoic complexes in Mexico, recent (Quaternary) tectonics in Greece and even more recent Beam Engine tectonics in Cornwall and the rest of the world. His body of work has had first-order implications for the interpretation of ancient orogens and the processes responsible for them. Most important of all, we have all benefited from the positive impact Damian has had on all our careers and the generosity and collegial approach to research. His influence has extended far beyond his immediate research community as a result of his co-leadership of IGCP projects and his inclusive approach to sharing and developing new avenues in science. He has inspired generations of students and his peers and his legacy is immense.
Earth and Planetary Science Letters, Vol. 449, pp. 118-126.
Gondwana, Rodinia
Subduction
Abstract: Periodic assembly and dispersal of continental fragments has been a characteristic of the solid Earth for much of its history. Geodynamic drivers of this cyclic activity are inferred to be either top-down processes related to near surface lithospheric stresses at plate boundaries or bottom-up processes related to mantle convection and, in particular, mantle plumes, or some combination of the two. Analysis of the geological history of Rodinian crustal blocks suggests that internal rifting and breakup of the supercontinent were linked to the initiation of subduction and development of accretionary orogens around its periphery. Thus, breakup was a top-down instigated process. The locus of convergence was initially around north-eastern and northern Laurentia in the early Neoproterozoic before extending to outboard of Amazonia and Africa, including Avalonia-Cadomia, and arcs outboard of Siberia and eastern to northern Baltica in the mid-Neoproterozoic (?760 Ma). The duration of subduction around the periphery of Rodinia coincides with the interval of lithospheric extension within the supercontinent, including the opening of the proto-Pacific at ca. 760 Ma and the commencement of rifting in east Laurentia. Final development of passive margin successions around Laurentia, Baltica and Siberia was not completed until the late Neoproterozoic to early Paleozoic (ca. 570-530 Ma), which corresponds with the termination of convergent plate interactions that gave rise to Gondwana and the consequent relocation of subduction zones to the periphery of this supercontinent. The temporal link between external subduction and internal extension suggests that breakup was initiated by a top-down process driven by accretionary tectonics along the periphery of the supercontinent. Plume-related magmatism may be present at specific times and in specific places during breakup but is not the prime driving force. Comparison of the Rodinia record of continental assembly and dispersal with that for Nuna, Gondwana and Pangea suggests grouping into two supercycles in which Nuna and Gondwana underwent only partial or no break-up phase prior to their incorporation into Rodinia and Pangea respectively. It was only after this final phase of assembly that the supercontinents then underwent full dispersal.
Bruno, H., Helibron, M., Strachen, R., Fowler, M., de MorrisonValeriano , C., Bersan, S., Moreira, H., Cutts, K., Dunlop, J., Almeida, R., Almeida, J., Storey, C.
Abstract: A zircon Hf isotope data set from Archean and Paleoproterozoic magmatic and metasedimentary rocks of the southern São Francisco craton (Brazil) is interpreted as evidence of accretionary and collisional plate tectonics since at least the Archean-Proterozoic boundary. During the Phanerozoic, accretionary and collisional orogenies are considered the end members of different plate tectonic settings, both involving preexisting stable continental lithosphere and consumption of oceanic crust. However, mechanisms for the formation of continental crust during the Archean and Paleoproterozoic are still debated, with the addition of magmatic rocks to the crust being explained by different geodynamic models. Hf isotopes can be used to quantify the proportion of magmatic addition into the crust: positive ?Hf values are usually interpreted as indications of magmatic input from the mantle, whereas crust-derived rocks show more negative ?Hf. We show that the crust of the amalgamated Paleoproterozoic tectonostratigraphic terranes that make up the southern São Francisco craton were generated from different proportions of mantle and crustal isotopic reservoirs. Plate tectonic processes are implied by a consistent sequence of events involving (1) the generation of juvenile subduction-related magmatic arc rocks, followed by (2) collisional orogenesis and remelting of older crust, and (3) post-collisional bimodal magmatism.
Abstract: Neoproterozoic kimberlite, ultramafic lamprophyre, and carbonatite magmatic activity was widespread across the Canadian-Greenland Shield. Models to explain the preponderance of this deeply-derived CO2-rich magmatism between 680-540 Ma range from impingement of multiple mantle plumes to rifting activity linked to the breakout of the Laurentian plate from the Rodinia supercontinent configuration. We add to the debate about the origin of kimberlite magmas and evaluate possible mantle sources of the 655 Ma ‘diamond-rich’ Renard (new SIMS U/Pb perovskite ages) and 629 Ma ‘barren’ Wemindji kimberlites on the eastern Superior craton in Quebec, Canada. Our Sr-Nd-Hf and carbon isotope data (87Sr/86Sri = 0.70241-0.70442; ?Ndi = + 0.2 to + 4.8; ?Hfi = + 0.3 to + 6.5; ?13C = ? 5.6 to ? 3.9‰) suggest a common and moderately depleted convecting upper mantle source region for both the Renard and Wemindji kimberlites, which occur 400 km apart in the interior of the Superior craton. In contrast, the low Os isotope ratios (187Os/188Osi = 0.11078-0.12620; ?Osi = ? 13.7 to ? 1.6) and unfractionated chondritic relative HSE abundances (Os, Ir, Ru, Pt, Pd, Re) indicate significant involvement of ancient refractory cratonic mantle material in kimberlite magma formation. Our model calculations suggest that for both the diamond-rich Renard and the barren Wemindji kimberlite magmas up to 30% of the Os was derived from refractory cratonic peridotites. This material might have been assimilated by originally more CO2-rich carbonated silicate melts derived from the asthenosphere. We also show that the geochemical and Sr-Nd-Hf-Os isotopic compositions of the Renard and Wemindji kimberlites do not require significant input from melts derived from olivine-poor cratonic mantle lithologies such as MARID-type veins and pyroxenites/eclogites. This contrasts with the petrogenesis of deeply-derived volatile-rich potassic magmas found along the peripheries of cratons (e.g., ultramafic lamprophyres, kamafugites, and olivine lamproites), a setting where abundant non-peridotitic components have been added to the lithospheric mantle over the course of continent evolution. Provided that CO2-rich melts, such as proto-kimberlites, occur near the solidus of volatile-fluxed peridotites, no excess mantle heat is required in their formation. This important but often overlooked constraint, together with the observation that there exist no spatial or temporal relationships between the Superior craton kimberlites and Large Igneous Provinces during the Late Neoproterozoic, suggests that kimberlite magmatic activity was tectonically controlled. In our preferred model, ubiquitous CO2-rich proto-kimberlite melts form during volatile-controlled redox melting processes at ambient mantle temperatures in a thermal boundary layer directly beneath thick cratonic lithosphere. The success rate of ‘evolving’ hybrid kimberlite magmas reaching Earth’s surface increases when tensile stresses propagate into the > 200 km thick keels of continental lithosphere. These conditions are frequently met during fast and changing plate motions associated with the assembly and breakup of supercontinents.
Genesis of ultramafic lamprophyres and carbonatites at Aillik Bay, Labrador: a consequence of incipient lithospheric thinning beneath the North Atlantic Craton
Prelevic, D., Akal, C., Foley, S.F., Romer, R.R.,Stracke, A., Van den Bogaard, P.
Ultrapotassic mafic rocks as geochemical proxies for post collisional dynamics of orogenic lithospheric mantle: the case of southwestern Anatolia, Turkey.
Peralk-Carb 2011, workshop held Tubingen Germany June 16-18, Poster
Tappe, S., Smart, K.A., Stracke, A., Romer, R.L., Prelevic, D., van den Bogaard, P.
Melt evolution beneath a rifted carton edge: 40Ar/39/Ar geochronology and Sr-Nd-Hf-Pb isotope systematics of primitive alkaline basalts and lamprophyres from the SW Baltic Shield.
Geochimica et Cosmochimica Acta, Vol. 173, pp. 1-36.
Geochimica et Cosmochimica Acta, Vol. 173, pp. 1-36.
Europe
Geochronology
Abstract: A new high-precision 40Ar/39Ar anorthoclase feldspar age of 176.7 ± 0.5 Ma (2-sigma) reveals that small-volume alkaline basaltic magmatism occurred at the rifted SW margin of the Baltic Shield in Scania (southern Sweden), at a time of global plate reorganization associated with the inception of Pangea supercontinent break-up. Our combined elemental and Sr-Nd-Hf-Pb isotope dataset for representative basanite and nephelinite samples (>8 wt.% MgO) from 16 subvolcanic necks of the 30 by 40 km large Jurassic volcanic field suggests magma derivation from a moderately depleted mantle source (87Sr/86Sri = 0.7034-0.7048; ?Ndi = +4.4 to +5.2; ?Hfi = +4.7 to +8.1; 206Pb/204Pbi = 18.8-19.5). The mafic alkaline melts segregated from mixed peridotite-pyroxenite mantle with a potential temperature of ?1400 °C at 2.7-4.2 GPa (?90-120 km depths), which places ultimate melt generation within the convecting upper mantle, provided that the lithosphere-asthenosphere boundary beneath the southern Baltic Shield margin was at ?100 km depth during Mesozoic-Cenozoic rifting. Isotopic shifts and incompatible element enrichment relative to Depleted Mantle reflect involvement of at least 20% recycled oceanic lithosphere component (i.e., pyroxenite) with some minor continent-derived sediment during partial melting of well-stirred convecting upper mantle peridotite. Although pargasitic amphibole-rich metasomatized lithospheric mantle is excluded as the main source of the Jurassic magmas from Scania, hydrous ultramafic veins (i.e., hornblendite) may have caused subtle modifications to the compositions of passing sublithospheric melts. For example, modeling suggests that the more radiogenic Hf (?Hfi = +6.3 to +8.1) and Pb (206Pb/204Pbi = 18.9-19.5) isotopic compositions of the more sodic and H2O-rich nephelinites, compared with relatively homogenous basanites (?Hfi = +4.7 to +6.1; 206Pb/204Pbi = 18.8-18.9), originate from minor interactions between rising asthenospheric melts and amphibole-rich metasomatic components. The metasomatic components were likely introduced to the lithospheric mantle beneath the southern Baltic Shield margin during extensive Permo-Carboniferous magmatic activity, a scenario that is supported by the geochemical and isotope compositions of ca. 286 Ma lamprophyres from Scania (87Sr/86Sri = 0.7040-0.7054; ?Ndi = +2.0 to +3.1; ?Hfi = +6.1 to +9.0; 206Pb/204Pbi = 17.8-18.2). Strong variations in lithosphere thickness and thermal structure across the southern Baltic Shield margin may have caused transient small-scale mantle convection. This resulted in relatively fast and focused upwellings and lateral flow beneath the thinned lithosphere, where mafic alkaline magmas formed by low degrees of decompression melting of sublithospheric mantle. Such a geodynamic scenario would allow for enriched recycled components with low melting points to be preferentially sampled from the more depleted and refractory convecting upper mantle when channeled along a destabilizing craton edge. Similar to the ‘lid effect’ in oceanic island volcanic provinces, lithospheric architecture may exert strong control on the mantle melting regime, and thus offer a simple explanation for the geochemical resemblance of continental and oceanic intraplate mafic alkaline magmas of high Na/K affinity.
Abstract: Neoproterozoic kimberlite, ultramafic lamprophyre, and carbonatite magmatic activity was widespread across the Canadian-Greenland Shield. Models to explain the preponderance of this deeply-derived CO2-rich magmatism between 680-540 Ma range from impingement of multiple mantle plumes to rifting activity linked to the breakout of the Laurentian plate from the Rodinia supercontinent configuration. We add to the debate about the origin of kimberlite magmas and evaluate possible mantle sources of the 655 Ma ‘diamond-rich’ Renard (new SIMS U/Pb perovskite ages) and 629 Ma ‘barren’ Wemindji kimberlites on the eastern Superior craton in Quebec, Canada. Our Sr-Nd-Hf and carbon isotope data (87Sr/86Sri = 0.70241-0.70442; ?Ndi = + 0.2 to + 4.8; ?Hfi = + 0.3 to + 6.5; ?13C = ? 5.6 to ? 3.9‰) suggest a common and moderately depleted convecting upper mantle source region for both the Renard and Wemindji kimberlites, which occur 400-km apart in the interior of the Superior craton. In contrast, the low Os isotope ratios (187Os/188Osi = 0.11078-0.12620; ?Osi = ? 13.7 to ? 1.6) and unfractionated chondritic relative HSE abundances (Os, Ir, Ru, Pt, Pd, Re) indicate significant involvement of ancient refractory cratonic mantle material in kimberlite magma formation. Our model calculations suggest that for both the diamond-rich Renard and the barren Wemindji kimberlite magmas up to 30% of the Os was derived from refractory cratonic peridotites. This material might have been assimilated by originally more CO2-rich carbonated silicate melts derived from the asthenosphere. We also show that the geochemical and Sr-Nd-Hf-Os isotopic compositions of the Renard and Wemindji kimberlites do not require significant input from melts derived from olivine-poor cratonic mantle lithologies such as MARID-type veins and pyroxenites/eclogites. This contrasts with the petrogenesis of deeply-derived volatile-rich potassic magmas found along the peripheries of cratons (e.g., ultramafic lamprophyres, kamafugites, and olivine lamproites), a setting where abundant non-peridotitic components have been added to the lithospheric mantle over the course of continent evolution. Provided that CO2-rich melts, such as proto-kimberlites, occur near the solidus of volatile-fluxed peridotites, no excess mantle heat is required in their formation. This important but often overlooked constraint, together with the observation that there exist no spatial or temporal relationships between the Superior craton kimberlites and Large Igneous Provinces during the Late Neoproterozoic, suggests that kimberlite magmatic activity was tectonically controlled. In our preferred model, ubiquitous CO2-rich proto-kimberlite melts form during volatile-controlled redox melting processes at ambient mantle temperatures in a thermal boundary layer directly beneath thick cratonic lithosphere. The success rate of ‘evolving’ hybrid kimberlite magmas reaching Earth’s surface increases when tensile stresses propagate into the > 200 km thick keels of continental lithosphere. These conditions are frequently met during fast and changing plate motions associated with the assembly and breakup of supercontinents.
Earth and Planetary science Letters, Vol. 466, pp. 152-167.
Mantle
Metasomatism, magma, carbonatite
Abstract: Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr-Nd-Hf-Pb-Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ??Hf??Hf coupled with View the MathML source>+5‰?7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05-3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (?3.6 to View the MathML source?2.4‰?13C for carbonatites versus ?5.7 to View the MathML source?3.6‰?13C for kimberlites) require open-system fractionation at magmatic temperatures.
Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of ‘pure’ carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16-33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly differentiated ‘cratonic carbonatites’ have only little in common with those of metasomatic agents that act on the deeper lithosphere. Consequently, carbonatite trace element systematics should only be used with caution when constraining carbon mobility and metasomatism at mantle depths. Regardless of the exact nature of carbonate-bearing melts within the mantle lithosphere, they play an important role in enrichment processes, thereby decreasing the stability of buoyant cratons and promoting rift initiation - as exemplified by the Mesozoic-Cenozoic breakup of the North Atlantic craton.
Abstract: Neoproterozoic kimberlite, ultramafic lamprophyre, and carbonatite magmatic activity was widespread across the Canadian-Greenland Shield. Models to explain the preponderance of this deeply-derived CO2-rich magmatism between 680–540 Ma range from impingement of multiple mantle plumes to rifting activity linked to the breakout of the Laurentian plate from the Rodinia supercontinent configuration. We add to the debate about the origin of kimberlite magmas and evaluate possible mantle sources of the 655 Ma ‘diamond-rich’ Renard (new SIMS U/Pb perovskite ages) and 629 Ma ‘barren’ Wemindji kimberlites on the eastern Superior craton in Quebec, Canada. Our Sr-Nd-Hf and carbon isotope data (87Sr/86Sri = 0.70241–0.70442; ?Ndi = + 0.2 to + 4.8; ?Hfi = + 0.3 to + 6.5; ?13C = ? 5.6 to ? 3.9‰) suggest a common and moderately depleted convecting upper mantle source region for both the Renard and Wemindji kimberlites, which occur 400 km apart in the interior of the Superior craton. In contrast, the low Os isotope ratios (187Os/188Osi = 0.11078–0.12620; ?Osi = ? 13.7 to ? 1.6) and unfractionated chondritic relative HSE abundances (Os, Ir, Ru, Pt, Pd, Re) indicate significant involvement of ancient refractory cratonic mantle material in kimberlite magma formation. Our model calculations suggest that for both the diamond-rich Renard and the barren Wemindji kimberlite magmas up to 30% of the Os was derived from refractory cratonic peridotites. This material might have been assimilated by originally more CO2-rich carbonated silicate melts derived from the asthenosphere. We also show that the geochemical and Sr-Nd-Hf-Os isotopic compositions of the Renard and Wemindji kimberlites do not require significant input from melts derived from olivine-poor cratonic mantle lithologies such as MARID-type veins and pyroxenites/eclogites. This contrasts with the petrogenesis of deeply-derived volatile-rich potassic magmas found along the peripheries of cratons (e.g., ultramafic lamprophyres, kamafugites, and olivine lamproites), a setting where abundant non-peridotitic components have been added to the lithospheric mantle over the course of continent evolution. Provided that CO2-rich melts, such as proto-kimberlites, occur near the solidus of volatile-fluxed peridotites, no excess mantle heat is required in their formation. This important but often overlooked constraint, together with the observation that there exist no spatial or temporal relationships between the Superior craton kimberlites and Large Igneous Provinces during the Late Neoproterozoic, suggests that kimberlite magmatic activity was tectonically controlled. In our preferred model, ubiquitous CO2-rich proto-kimberlite melts form during volatile-controlled redox melting processes at ambient mantle temperatures in a thermal boundary layer directly beneath thick cratonic lithosphere. The success rate of ‘evolving’ hybrid kimberlite magmas reaching Earth’s surface increases when tensile stresses propagate into the > 200 km thick keels of continental lithosphere. These conditions are frequently met during fast and changing plate motions associated with the assembly and breakup of supercontinents.
Journal of Petrology , Vol. 60, 6, pp. 1119--1134.
India
carbonatite
Abstract: There are disparate views about the origin of global rift- or plume-related carbonatites. The Amba Dongar carbonatite complex, Gujarat, India, which intruded into the basalts of the Deccan Large Igneous Province (LIP), is a typical example. On the basis of new comprehensive major and trace element and Sr-Nd-Pb isotope data, we propose that low-degree primary carbonated melts from off-center of the Deccan-Réunion mantle plume migrate upwards and metasomatize part of the subcontinental lithospheric mantle (SCLM). Low-degree partial melting (?2%) of this metasomatized SCLM source generates a parental carbonated silicate magma, which becomes contaminated with the local Archean basement during its ascent. Calcite globules in a nephelinite from Amba Dongar provide evidence that the carbonatites originated by liquid immiscibility from a parental carbonated silicate magma. Liquid immiscibility at crustal depths produces two chemically distinct, but isotopically similar magmas: the carbonatites (20% by volume) and nephelinites (80% by volume). Owing to their low heat capacity, the carbonatite melts solidified as thin carbonate veins at crustal depths. Secondary melting of these carbonate-rich veins during subsequent rifting generated the carbonatites and ferrocarbonatites now exposed at Amba Dongar. Carbonatites, if formed by liquid immiscibility from carbonated silicate magmas, can inherit a wide range of isotopic signatures that result from crustal contamination of their parental carbonated silicate magmas. In rift or plume-related settings, they can, therefore, display a much larger range of isotope signatures than their original asthenosphere or mantle plume source.
Abstract: Low-volume alkaline silicate and carbonate magmas are products of volatile-controlled incipient melting processes in the Earth’s mantle. Although this form of melting is ubiquitous beneath the thick and cold portions of continental lithosphere, such melts rarely reach the Earth’s surface due to a combination of their small volumes, reactive nature, and great depths of origin. In spite of being rare at surface, the impact of alkaline and carbonate magmatism on the dynamic stability of mantle lithosphere and its metal endowment may be disproportionately large, but it is difficult to grasp in the absence of spatial and temporal constraints on melt mobility. We review evidence from major alkaline and carbonatite provinces for metasomatic overprinting of the underlying continental mantle lithosphere, and evaluate how these processes influenced plate tectonic evolution in these regions. Key examples from Greenland and Africa show that metasomatic weakening of mantle lithosphere by pervasive alkaline and carbonate melts is frequently the first step in continent fragmentation ultimately leading to supercontinent dispersal. A major obstacle in identifying carbonate melt metasomatized mantle is the use of differentiated ‘surface’ carbonatite compositions as proxies for geochemical processes operating at great depths. We assess the robustness of some of the classic geochemical proxies, such as Ti/Eu and Zr/Sm, and identify new promising fingerprints of passing carbonate melts in the deep mantle lithosphere. New evidence from the Kaapvaal craton, one of world’s best endowed metallogenic provinces, shows that redox- and volatile-controlled alkaline melting events can effectively mobilize sulphide-hosted PGE and base metal budgets from eclogite components within the thick mantle lithosphere. Such precursor alkaline magmatic events, heralding the formation of major continental rifts and mantle plume impingement, can enhance the metal contents of subsequent asthenosphere-derived mafic magmas, thereby upgrading oreforming potential. However, economic metal deposits only form when geologic conditions during magma emplacement in the crust are favorable, with mantle metal budgets being less critical.
Abstract: Partial melting of Earth’s mantle generates oceanic crust and leaves behind a chemically depleted residual mantle. The time-integrated composition of this chemically depleted mantle is generally inferred from basalts produced at mid-ocean ridges. However, isotopic differences between oceanic mantle rocks and mid-ocean ridge basalts suggest that mantle and basalt composition could differ. Here we measure neodymium isotope ratios in olivine-hosted melt inclusions from lavas of the Azores mantle plume. We find neodymium isotope ratios that include the highest values measured in basalts, and suggest that melts from ultra-depleted mantle contribute to the isotopic diversity of the erupted lavas. Ultra-depleted melts have exceedingly low preservation potential during magma extraction and evolution due to progressive mixing with melts that are enriched in incompatible elements. A notable contribution of ultra-depleted melts to the Azores mantle plume therefore implies that variably depleted mantle is the volumetrically dominant component of the Azores plume. We argue that variably depleted mantle, sometimes ranging to ultra-depleted compositions, may be a ubiquitous part of most ocean island and mid-ocean ridge basalt sources. If so, Earth’s mantle may be more depleted than previously thought, which has important implications for the rate of mass exchange between crust and mantle, plume dynamics and compositional stratification of Earth’s mantle.Depleted mantle is a volumetrically dominant component of the Azores plume and possibly of oceanic basalt sources more generally, according to neodymium isotope compositions of olivine-hosted melt inclusions from lavas of the Azores mantle plume.
Chemical Geology, doi.org/10.1016/ j.chemgeo.2019. 119307
Mantle
craton
Abstract: Establishing the mode and rate of formation of the continental crust is crucial for quantifying mass exchange between Earth’s crust and mantle. The limited crustal rock record, particularly of early Archean rocks, has led to a variety of different models of continental growth. Here, we present an open-system model of silicate Earth evolution incorporating the Sm-Nd and Lu-Hf isotope systematics with the aim to constrain crustal growth during the Archean and its effect on the chemical and isotopic evolution of Earth’s crust-mantle system. Our model comprises four reservoirs: the bulk continental crust (CC), depleted upper mantle (UM), lower mantle (LM), and an isolated reservoir (IR) where recycled crust is stored transiently before being mixed with the LM. The changing abundance of isotope species in each reservoir is quantified using a series of first order linear differential equations that are solved numerically using the fourth order Runge-Kutta method at 1 Myr time steps for 4.56 Gyr (the age of the Earth). The model results show that only continuous and exponential crustal growth reproduces the present-day abundances and isotope ratios in the terrestrial reservoirs. Our preferred crustal growth model suggests that the mass of the CC by the end of Hadean (4.0 Ga) and end of Archean (2.5 Ga) was ?30% and ?75% of the present-day mass of the CC, respectively. Models proposing formation of most (?90%) of the present-day CC during the initial 1 Gyr or nearly 50-60% during the last 1 Gyr are least favorable. Significant mass exchange between crust and mantle, that is, both the formation and recycling of crust, started in the Hadean with Sm-Nd and Lu-Hf isotope evolution typical for mafic rocks. Depletion of the UM (in incompatible elements) during the early Archean is mitigated by the input of recycled crust, so that the UM maintained a near-primitive Hf-Nd isotope composition. The LM also retained a near-primitive Hf-Nd isotope composition during the Archean, but for different reasons. In contrast to the UM, the crustal return flux into the LM is transiently stored (? 1 Gyr) in an isolated reservoir (IR), which limits the mass flux into and out of the LM. The IR in our model is distinct from other mantle reservoirs and possibly related to stable crustal blocks or, alternatively, to recycled crust in the mantle that remains temporarily isolated, perhaps at the core-mantle boundary (LLSVPs).
Earth and Planetary Science Letters, Vol. 547, 14p. Pdf
Africa
LLSVP, superplume
Abstract: Many volcanic hotspots are connected via ‘plume’ conduits to thermochemical structures with anomalously low seismic velocities at the core-mantle boundary. Basaltic lavas from some of these hotspots show anomalous daughter isotope abundances for the short-lived 129I-129Xe, 146Sm-142Nd, and 182Hf-182W radioactive decay systems, suggesting that their lower mantle sources contain material that dates back to Earth-forming events during the first 100 million years in solar system history. Survival of such ‘primordial’ remnants in Earth's mantle places important constraints on the evolution and inner workings of terrestrial planets. Here we report high-precision 182W/184W measurements for a large suite of kimberlite volcanic rocks from across the African tectonic plate, which for the past 250 million years has drifted over the most prominent thermochemical seismic anomaly at the core-mantle boundary. This so-called African LLSVP, or ‘large low shear-wave velocity province’, is widely suspected to store early Earth remnants and is implicated as the ultimate source of global Phanerozoic kimberlite magmatism. Our results show, however, that kimberlites from above the African LLSVP, including localities with lower mantle diamonds such as Letseng and Karowe Orapa A/K6, lack anomalous 182W signatures, with an average W value of 0.0 ± 4.1 (2SD) for the 18 occurrences studied. If kimberlites are indeed sourced from the African LLSVP or superplume, then the extensive 182W evidence suggests that primordial or core-equilibrated mantle materials, which may contribute resolvable W excesses or deficits, are only minor or locally concentrated components in the lowermost mantle, for example in the much smaller ‘ultra-low velocity zones’ or ULVZs. However, the lack of anomalous 182W may simply suggest that low-volume kimberlite magmas are not derived from hot lower mantle plumes. In this alternative scenario, kimberlite magmas originate from volatile-fluxed ambient convecting upper mantle domains beneath relatively thick and cold lithosphere from where previously ‘stranded’ lower mantle and transition zone diamonds can be plucked.
Abstract: During the past two decades significant progress has been made in understanding the origin and evolution of kimberlites, including relationships to other diamondiferous magma types such as lamproites and aillikites. However, the association of kimberlites and carbonatites on continental shields remains poorly understood, and two opposing ideas dominate the debate. While one school of thought argues that primary carbonatite melts transform into hybrid carbonated silicate magmas akin to kimberlites by assimilation of cratonic mantle material, others use geochemical evidence to show that carbonatite magmas can evolve from near-primary kimberlite melts within the cratonic lithosphere. The 1.15 Ga Premier kimberlite pipe on the Kaapvaal craton in South Africa hosts several kimberlite and carbonatite dykes. Reconstructions of magma compositions suggest that up to 20 wt.% CO2 was lost from near-primary kimberlite melts during ascent through the cratonic lithosphere, but the carbonatite dyke compositions cannot be linked to the kimberlite melts via differentiation. Geochemical evidence, including mantle-like ?13C compositions, suggests that the co-occurring kimberlite and carbonatite dykes represent two discrete CO2-rich magma batches derived from a mixed source in the convecting upper mantle. The carbonatites probed a slightly more depleted source component in terms of Sr-Nd-Hf isotopic compositions relative to the peridotitic matrix that was more effectively tapped by the kimberlites (87Sr/86Sri = 0.70257 to 0.70316 for carbonatites vs. 0.70285 to 0.70546 for kimberlites; ?Ndi = +3.0 to +3.9 vs. +2.2 to +2.8; ?Hfi = -2.2 to +0.7 vs. -5.1 to -1.9). Platinum-group element systematics suggest that assimilation of refractory lithospheric mantle material by the carbonatite melts was negligible (<1 vol.%), whereas between 5 - 35 vol.% of digested cratonic peridotite account for the kimberlite compositions, including the low 187Os/188Os signature (?Osi = -12.7 to -4.5). The kimberlite and carbonatite dykes show similarly strong Nd-Hf isotope decoupling (??Hfi = -10.7 to -7.6 vs. -8.8 to -6.1), regardless of the variable lithospheric mantle imprints. This observation suggests a common sublithospheric origin of the negative ??Hf signature, possibly linked to ancient recycled oceanic crust components in the convecting upper mantle to transition zone sources of CO2-rich magmatism. Mesoproterozoic kimberlite and carbonatite magmatism at Premier was coeval with subduction and collision events along the southern Kaapvaal craton margin during the 1,220 -1,090 Ma Namaqua-Natal orogeny associated with Rodinia supercontinent formation. Thermochronology suggests that the entire Kaapvaal craton was affected by this collisional tectonic event, and it appears that the changing lithospheric stress-field created pathways for deep-sourced kimberlite and carbonatite magmas to reach Earth’s surface. We find that collision-induced (e.g., Premier) and continental breakup-related (e.g., Kimberley) kimberlite magmas are compositionally indistinguishable, with the inference that plate tectonic processes aid solely in the creation of magma ascent pathways without a major influence on deep mantle melting beneath cratons. It follows that on-craton kimberlite magmatism in the hinterland of collision zones is not necessarily more likely to entrain large sublithospheric diamonds than kimberlite eruptions linked to continental breakup. This implies that Premier’s world-class endowment with ‘ultradeep’ Type-II diamonds is not causally related to its setting behind an active orogenic front.
Abstract: The numerous chemical and isotopic studies of oceanic basalts have shaped our perception of mantle geochemistry over the last six decades. As partial melts of Earth’s mantle, basalts are indirect tracers of mantle composition. Because the scale of isotopic heterogeneity is smaller than the scale of melt production, melts from isotopically heterogeneous mantle ingredients mix into variable blends on their way to eruption. Basalts are therefore isotopically less variable than their mantle sources. Decrypting “the message from oceanic volcanism” thus necessitates developing strategies to see through this ubiquitous sampling bias, but also acknowledging the inherent limitations imposed by investigating mantle composition through basalts. Understanding how large the bias between melts and mantle actually is requires decoding process versus source-related causes for the isotopic variability of basalts. Ultimately, deciphering the effective range of isotopic variability in Earth’s mantle is crucial for connecting isotopic signals in basalts to different materials and thus, the geologic processes that govern silicate earth evolution. Invariably, however, basalts are weighted averages of melts from isotopically different mantle constituents. As such, their incompatible element and isotopic composition is inherently biased towards the incompatible element enriched source components. The incompatible element depleted components of Earth’s mantle must therefore range to more extreme compositions than the basalts. But although isotope data from peridotites and olivine-hosted melt inclusions have extended the bounds of mantle heterogeneity, the overall extent of incompatible element depletion and mass fraction of incompatible element depleted mantle is still elusive. Mantle depletion is driven by the rate of melt extraction, or mantle processing, and thus interconnects the geochemical and geodynamical evolution of Earth’s mantle. Better constraining mantle depletion is therefore at the root of understanding our planet’s principal mode of operation.
Tappe, S., Massuyeau, M. , Smart, K.A., Woodland, A.B., Gussone, N., Milne, S., Stracke, A.
Sheared peridotite and megacryst formation beneath the Kaapvaal Craton: a snapshot of tectonomagmetic processes across the lithosphere-asthenosphere transition.
Geological Society of London Special Publications, doi:https://dori.org/10.1144/SP513-2021-84 30p. Pdf proof
Africa, South Africa
lamproite
Abstract: Orangeites are a significant source of diamonds, yet ambiguity surrounds their status among groups of mantle-derived potassic rocks. This study reports mineralogical and geochemical data for a ca. 140 Ma orangeite dyke swarm that intersects the Bushveld Complex on the Kaapvaal craton in South Africa. The dykes comprise distinctive petrographic varieties that are linked principally by olivine fractionation, with the most evolved members containing minor amounts of primary carbonate, sanidine and andradite garnet in the groundmass. Although abundant groundmass phlogopite and clinopyroxene have compositions that are similar to those of cratonic lamproites, these phases show notable Ti-depletion, which we consider a hallmark feature of type orangeites from the Kaapvaal craton. Ti-depletion is also characteristic for the bulk rock compositions and is associated with strongly depleted Th-U-Nb-Ta contents at high Cs-Rb-Ba-K concentrations. The resultant high LILE/HFSE ratios of orangeites suggest that mantle source enrichment occurred by metasomatic processes in the proximity of ancient subduction zones. The Bushveld-intersecting orangeite dykes have strongly enriched Sr-Nd-Hf isotopic compositions (initial 87Sr/86Sr = 0.70701-0.70741; ?Nd = ?10.6 to ?5.8; ?Hf = ?14.4 to ?2.5), similar to those of other orangeites from across South Africa. Combined with the strong Ti-Nb-Ta depletion, this ubiquitous isotopic feature points to the involvement of ancient metasomatized mantle lithosphere in the origin of Kaapvaal craton orangeites, where K-rich metasomes imparted a ‘fossil’ subduction geochemical signature. Previous geochronology studies identified ancient K-enrichment events within the Kaapvaal cratonic mantle lithosphere, possibly associated with collisional tectonics during the 1.2-1.1 Ga Namaqua-Natal orogeny of the Rodinia supercontinent cycle. It therefore seems permissible that the cratonic mantle root was preconditioned for ultrapotassic magma production by tectonomagmatic events that occurred along convergent plate margins during the Proterozoic. However, reactivation of the K-rich metasomes had to await establishment of an extensional tectonic regime, such as that during the Mesozoic breakup of Gondwana, which was accompanied by widespread (1000 × 750 km) small-volume orangeite volcanism between 200 and 110 Ma. Although similarities exist between orangeites and lamproites, these and other potassic rocks are sufficiently distinct in their compositions such that different magma formation processes must be considered. In addition to new investigations of the geodynamic triggers of K-rich ultramafic magmatism, future research should more stringently evaluate the relative roles of redox effects and volatile components such as H2O-CO2-F in the petrogeneses of these potentially diamondiferous alkaline rocks.
Abstract: The cratonic lithosphere-asthenosphere boundary is commonly invoked as the site of sheared peridotite and megacryst formation, a well-recognized petrological assemblage whose genetic relationships—if any—remain poorly understood. We have undertaken a comprehensive petrology and Sr-Nd-Hf-Ca isotope study of sheared peridotite xenoliths and clinopyroxene megacrysts from the c. 1150 Ma Premier kimberlite pipe on the central Kaapvaal craton in South Africa. New textural and mineral trace element evidence suggests that strong tectonic and magmatic overprinting affected the lower cratonic mantle over a vertical distance of ?50 km from the lithosphere-asthenosphere boundary located at ?200-225 km depth. Although modification of the central Kaapvaal cratonic mantle is commonly linked to the c. 2056 Ma Bushveld large igneous event, our thermobarometry, mantle redox, and Sr-Nd-Hf-Ca isotope data support a model in which volatile-rich low-volume melts and associated high-density fluids refertilized the lithosphere base shortly before or during asthenosphere-derived kimberlite and carbonatite magmatism at around 1150 Ma. This episode of lithospheric mantle enrichment was facilitated by exceptionally strong shear movements, as are recorded in the plastically deformed peridotites. We argue that stress-driven segregation of percolating carbonated melts contributed to megacryst formation along, or in close proximity to, shear zones within the cratonic mantle lithosphere. Integration of our results from the Kaapvaal craton and modern petrological concepts allows for the identification of a lithosphere-asthenosphere transition zone between ?150 and 225 km depth. This horizon is defined by intersections of the ?40-42 mW m-2 Premier paleogeotherm with (1) CO2-H2O-present solidus curves for peridotite (upper bound), and (2) typical mantle adiabats with potential temperatures between 1315 and 1420 °C (lower bound). At Premier, the most strongly deformed sheared peridotites occur mainly between ?160 and 185 km depth, firmly within the lithosphere-asthenosphere transition zone. Contrary to many previous models, we suggest that sheared peridotite formation occurs in localized deformation zones spaced out across the entire width of the lithosphere-asthenosphere transition zone, rather than being restricted to a single thin layer at the craton base where mantle flow causes viscous drag. Hence, plate-tectonic stresses acting on the lower cratonic lithosphere may be accommodated by extensive networks of shear zones, which provide transient pathways and sinks for percolating volatile-rich melts, linking the formation of megacrysts and sheared peridotites.
Geological Society of London Special Publication 513, pp. 17-44.
Africa, South Africa
lamproites
Abstract: Orangeites are a significant source of diamonds, yet ambiguity surrounds their status among groups of mantle-derived potassic rocks. This study reports mineralogical and geochemical data for a c. 140 Ma orangeite dyke swarm that intersects the Bushveld Complex on the Kaapvaal craton in South Africa. The dykes comprise distinctive petrographic varieties that are linked principally by olivine fractionation, with the most evolved members containing minor amounts of primary carbonate, sanidine and andradite garnet in the groundmass. Although abundant groundmass phlogopite and clinopyroxene have compositions that are similar to those of cratonic lamproites, these phases show notable Ti-depletion, which we consider a hallmark feature of type orangeites from the Kaapvaal craton. Ti-depletion is also characteristic of bulk rock compositions and is associated with strongly depleted Th-U-Nb-Ta contents at high Cs-Rb-Ba-K concentrations. The resultant high large ion lithophile element/high field strength element ratios of orangeites suggest that mantle source enrichment occurred by metasomatic processes in the proximity of ancient subduction zones. The Bushveld-intersecting orangeite dykes have strongly enriched Sr-Nd-Hf isotopic compositions (initial 87Sr/86Sr = 0.70701-0.70741; ?Nd = ?10.6 to ?5.8; ?Hf = ?14.4 to ?2.5), similar to those of other orangeites from across South Africa. Combined with the strong Ti-Nb-Ta depletion, this ubiquitous isotopic feature points to the involvement of ancient metasomatized mantle lithosphere in the origin of Kaapvaal craton orangeites, where K-rich metasomes imparted a ‘fossil’ subduction geochemical signature. Previous geochronology studies identified ancient K-enrichment events within the Kaapvaal cratonic mantle lithosphere, possibly associated with collisional tectonics during the 1.2-1.1 Ga Namaqua-Natal orogeny of the Rodinia supercontinent cycle. It therefore seems permissible that the cratonic mantle root was preconditioned for ultrapotassic magma production by tectonomagmatic events that occurred along convergent plate margins during the Proterozoic. However, reactivation of the K-rich metasomes had to await establishment of an extensional tectonic regime, such as that during the Mesozoic breakup of Gondwana, which was accompanied by widespread (1000 × 750 km) small-volume orangeite volcanism between 200 and 110 Ma. Although similarities exist between orangeites and lamproites, these and other potassic rocks are sufficiently distinct in their compositions such that different magma formation processes must be considered. In addition to new investigations of the geodynamic triggers of K-rich ultramafic magmatism, future research should more stringently evaluate the relative roles of redox effects and volatile components such as H2O-CO2-F in the petrogeneses of these potentially diamondiferous alkaline rocks.
Earth and Planetary Science Letters, Vol. 441, pp. 200-210.
Mantle
Subduction
Abstract: Subduction zones are one of the most striking feature on Earth. They represent one of the two types of convergent plate boundaries, in which one tectonic plates sinks underneath another into the Earth’s mantle. Soon after the advent of the theory of plate tectonics scientists recognized that subduction zones are one of the main drivers of plate motion and mantle convection [Elsasser, 1971]. With trench motion during progressive subduction, overriding plates incorporated in subduction zones may follow the trench and/or deform internally. Such deformation is often characterized by backarc extension, which leads to opening of backarc basins, such as the Tyrrhenian Sea, the Scotia Sea, the Aegean Sea, the North Fiji Basin, and the Lau Basin.
The social licence to explore. Explorers must give due consideration to the people, environment and the socio-economic conditions of the regions they explore.
Abstract: We reconstructed the spatial distribution of eclogites in the cratonic mantle based on thermobarometry for ~ 240 xenoliths in 4 kimberlite pipes from different parts of the Slave craton (Canada). The accuracy of depth estimates is ensured by the use of a recently calibrated thermometer, projection of temperatures onto well-constrained local peridotitic geotherms, petrological screening for unrealistic temperature estimates, and internal consistency of all data. The depth estimates are based on new data on mineral chemistry and petrography of 148 eclogite xenoliths from the Jericho and Muskox kimberlites of the northern Slave craton and previously reported analyses of 95 eclogites from Diavik and Ekati kimberlites (Central Slave). The majority of Northern Slave eclogites of the crustal, subduction origin occurs at 110-170 km, shallower than in the majority of the Central Slave crustal eclogites (120-210 km). The identical geochronological history of these eclogite populations and the absence of steep suture boundaries between the central and northern Slave craton suggest the lateral continuity of the mantle layer relatively rich in eclogites. We explain the distribution of eclogites by partial preservation of an imbricated and plastically dispersed oceanic slab formed by easterly dipping Proterozoic subduction. The depths of eclogite localization do not correlate with geophysically mapped discontinuities. The base of the depleted lithosphere of the Slave craton constrained by thermobarometry of peridotite xenoliths coincides with the base of the thickened lithospheric slab, which supports contribution of the recycled oceanic lithosphere to formation of the cratonic root. Its architecture may have been protected by circum-cratonic subduction and shielding of the shallow Archean lithosphere from the destructive asthenospheric metasomatism.
Canadian Journal of Earth Sciences, Vol. 53, 1, pp. 41-58.
Canada, Northwest Territories
Deposit - Muskox
Abstract: We present petrography, mineralogy, and thermobarometry for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite includes 23% coarse peridotite, 9% porphyroclastic peridotite, 60% websterite, and 8% orthopyroxenite. Samples primarily comprise forsteritic olivine (Fo 89-94), enstatite (En 89-94), Cr-diopside, Cr-pyrope garnet, and chromite spinel. Coarse peridotites, porphyroclastic peridotites, and pyroxenites equilibrated at 650-1220 °C and 23-63 kbar (1 kbar = 100 MPa), 1200-1350 °C and 57-70 kbar, and 1030-1230 °C and 50-63 kbar, respectively. The Muskox xenoliths differ from xenoliths in the neighboring and contemporaneous Jericho kimberlite by their higher levels of depletion, the presence of a shallow zone of metasomatism in the spinel peridotite field, a higher proportion of pyroxenites at the base of the mantle column, higher Cr2O3 in all pyroxenite minerals, and weaker deformation in the Muskox mantle. We interpret these contrasts as representing small-scale heterogeneities in the bulk composition of the mantle, as well as the local effects of interaction between metasomatizing fluid and mantle wall rocks. We suggest that asthenosphere-derived pre-kimberlitic melts and fluids percolated less effectively through the less permeable Muskox mantle, resulting in lower degrees of hydrous weakening, strain, and fertilization of the peridotitic mantle. Fluids tended to concentrate and pool in the deep mantle, causing partial melting and formation of abundant pyroxenites.
Geochemistry, Geophysics, Geosystems G3, pp. 4326-
Mantle
geophysics
Abstract: Regional characterization of the continental crust has classically been performed through either geologic mapping, geochemical sampling, or geophysical surveys. Rarely are these techniques fully integrated, due to limits of data coverage, quality, and/or incompatible data sets. We combine geologic observations, geochemical sampling, and geophysical surveys to create a coherent 3-D geologic model of a 50 × 50 km upper crustal region surrounding the SNOLAB underground physics laboratory in Canada, which includes the Southern Province, the Superior Province, the Sudbury Structure, and the Grenville Front Tectonic Zone. Nine representative aggregate units of exposed lithologies are geologically characterized, geophysically constrained, and probed with 109 rock samples supported by compiled geochemical databases. A detailed study of the lognormal distributions of U and Th abundances and of their correlation permits a bivariate analysis for a robust treatment of the uncertainties. A downloadable 3-D numerical model of U and Th distribution defines an average heat production of math formula µW/m3, and predicts a contribution of math formula TNU (a Terrestrial Neutrino Unit is one geoneutrino event per 1032 target protons per year) out of a crustal geoneutrino signal of math formula TNU. The relatively high local crust geoneutrino signal together with its large variability strongly restrict the SNO+ capability of experimentally discriminating among BSE compositional models of the mantle. Future work to constrain the crustal heat production and the geoneutrino signal at SNO+ will be inefficient without more detailed geophysical characterization of the 3-D structure of the heterogeneous Huronian Supergroup, which contributes the largest uncertainty to the calculation.
Abstract: Neoproterozoic kimberlite, ultramafic lamprophyre, and carbonatite magmatic activity was widespread across the Canadian-Greenland Shield. Models to explain the preponderance of this deeply-derived CO2-rich magmatism between 680-540 Ma range from impingement of multiple mantle plumes to rifting activity linked to the breakout of the Laurentian plate from the Rodinia supercontinent configuration. We add to the debate about the origin of kimberlite magmas and evaluate possible mantle sources of the 655 Ma ‘diamond-rich’ Renard (new SIMS U/Pb perovskite ages) and 629 Ma ‘barren’ Wemindji kimberlites on the eastern Superior craton in Quebec, Canada. Our Sr-Nd-Hf and carbon isotope data (87Sr/86Sri = 0.70241-0.70442; ?Ndi = + 0.2 to + 4.8; ?Hfi = + 0.3 to + 6.5; ?13C = ? 5.6 to ? 3.9‰) suggest a common and moderately depleted convecting upper mantle source region for both the Renard and Wemindji kimberlites, which occur 400 km apart in the interior of the Superior craton. In contrast, the low Os isotope ratios (187Os/188Osi = 0.11078-0.12620; ?Osi = ? 13.7 to ? 1.6) and unfractionated chondritic relative HSE abundances (Os, Ir, Ru, Pt, Pd, Re) indicate significant involvement of ancient refractory cratonic mantle material in kimberlite magma formation. Our model calculations suggest that for both the diamond-rich Renard and the barren Wemindji kimberlite magmas up to 30% of the Os was derived from refractory cratonic peridotites. This material might have been assimilated by originally more CO2-rich carbonated silicate melts derived from the asthenosphere. We also show that the geochemical and Sr-Nd-Hf-Os isotopic compositions of the Renard and Wemindji kimberlites do not require significant input from melts derived from olivine-poor cratonic mantle lithologies such as MARID-type veins and pyroxenites/eclogites. This contrasts with the petrogenesis of deeply-derived volatile-rich potassic magmas found along the peripheries of cratons (e.g., ultramafic lamprophyres, kamafugites, and olivine lamproites), a setting where abundant non-peridotitic components have been added to the lithospheric mantle over the course of continent evolution. Provided that CO2-rich melts, such as proto-kimberlites, occur near the solidus of volatile-fluxed peridotites, no excess mantle heat is required in their formation. This important but often overlooked constraint, together with the observation that there exist no spatial or temporal relationships between the Superior craton kimberlites and Large Igneous Provinces during the Late Neoproterozoic, suggests that kimberlite magmatic activity was tectonically controlled. In our preferred model, ubiquitous CO2-rich proto-kimberlite melts form during volatile-controlled redox melting processes at ambient mantle temperatures in a thermal boundary layer directly beneath thick cratonic lithosphere. The success rate of ‘evolving’ hybrid kimberlite magmas reaching Earth’s surface increases when tensile stresses propagate into the > 200 km thick keels of continental lithosphere. These conditions are frequently met during fast and changing plate motions associated with the assembly and breakup of supercontinents.
Abstract: Neoproterozoic kimberlite, ultramafic lamprophyre, and carbonatite magmatic activity was widespread across the Canadian-Greenland Shield. Models to explain the preponderance of this deeply-derived CO2-rich magmatism between 680-540 Ma range from impingement of multiple mantle plumes to rifting activity linked to the breakout of the Laurentian plate from the Rodinia supercontinent configuration. We add to the debate about the origin of kimberlite magmas and evaluate possible mantle sources of the 655 Ma ‘diamond-rich’ Renard (new SIMS U/Pb perovskite ages) and 629 Ma ‘barren’ Wemindji kimberlites on the eastern Superior craton in Quebec, Canada. Our Sr-Nd-Hf and carbon isotope data (87Sr/86Sri = 0.70241-0.70442; ?Ndi = + 0.2 to + 4.8; ?Hfi = + 0.3 to + 6.5; ?13C = ? 5.6 to ? 3.9‰) suggest a common and moderately depleted convecting upper mantle source region for both the Renard and Wemindji kimberlites, which occur 400-km apart in the interior of the Superior craton. In contrast, the low Os isotope ratios (187Os/188Osi = 0.11078-0.12620; ?Osi = ? 13.7 to ? 1.6) and unfractionated chondritic relative HSE abundances (Os, Ir, Ru, Pt, Pd, Re) indicate significant involvement of ancient refractory cratonic mantle material in kimberlite magma formation. Our model calculations suggest that for both the diamond-rich Renard and the barren Wemindji kimberlite magmas up to 30% of the Os was derived from refractory cratonic peridotites. This material might have been assimilated by originally more CO2-rich carbonated silicate melts derived from the asthenosphere. We also show that the geochemical and Sr-Nd-Hf-Os isotopic compositions of the Renard and Wemindji kimberlites do not require significant input from melts derived from olivine-poor cratonic mantle lithologies such as MARID-type veins and pyroxenites/eclogites. This contrasts with the petrogenesis of deeply-derived volatile-rich potassic magmas found along the peripheries of cratons (e.g., ultramafic lamprophyres, kamafugites, and olivine lamproites), a setting where abundant non-peridotitic components have been added to the lithospheric mantle over the course of continent evolution. Provided that CO2-rich melts, such as proto-kimberlites, occur near the solidus of volatile-fluxed peridotites, no excess mantle heat is required in their formation. This important but often overlooked constraint, together with the observation that there exist no spatial or temporal relationships between the Superior craton kimberlites and Large Igneous Provinces during the Late Neoproterozoic, suggests that kimberlite magmatic activity was tectonically controlled. In our preferred model, ubiquitous CO2-rich proto-kimberlite melts form during volatile-controlled redox melting processes at ambient mantle temperatures in a thermal boundary layer directly beneath thick cratonic lithosphere. The success rate of ‘evolving’ hybrid kimberlite magmas reaching Earth’s surface increases when tensile stresses propagate into the > 200 km thick keels of continental lithosphere. These conditions are frequently met during fast and changing plate motions associated with the assembly and breakup of supercontinents.
Abstract: Neoproterozoic kimberlite, ultramafic lamprophyre, and carbonatite magmatic activity was widespread across the Canadian-Greenland Shield. Models to explain the preponderance of this deeply-derived CO2-rich magmatism between 680–540 Ma range from impingement of multiple mantle plumes to rifting activity linked to the breakout of the Laurentian plate from the Rodinia supercontinent configuration. We add to the debate about the origin of kimberlite magmas and evaluate possible mantle sources of the 655 Ma ‘diamond-rich’ Renard (new SIMS U/Pb perovskite ages) and 629 Ma ‘barren’ Wemindji kimberlites on the eastern Superior craton in Quebec, Canada. Our Sr-Nd-Hf and carbon isotope data (87Sr/86Sri = 0.70241–0.70442; ?Ndi = + 0.2 to + 4.8; ?Hfi = + 0.3 to + 6.5; ?13C = ? 5.6 to ? 3.9‰) suggest a common and moderately depleted convecting upper mantle source region for both the Renard and Wemindji kimberlites, which occur 400 km apart in the interior of the Superior craton. In contrast, the low Os isotope ratios (187Os/188Osi = 0.11078–0.12620; ?Osi = ? 13.7 to ? 1.6) and unfractionated chondritic relative HSE abundances (Os, Ir, Ru, Pt, Pd, Re) indicate significant involvement of ancient refractory cratonic mantle material in kimberlite magma formation. Our model calculations suggest that for both the diamond-rich Renard and the barren Wemindji kimberlite magmas up to 30% of the Os was derived from refractory cratonic peridotites. This material might have been assimilated by originally more CO2-rich carbonated silicate melts derived from the asthenosphere. We also show that the geochemical and Sr-Nd-Hf-Os isotopic compositions of the Renard and Wemindji kimberlites do not require significant input from melts derived from olivine-poor cratonic mantle lithologies such as MARID-type veins and pyroxenites/eclogites. This contrasts with the petrogenesis of deeply-derived volatile-rich potassic magmas found along the peripheries of cratons (e.g., ultramafic lamprophyres, kamafugites, and olivine lamproites), a setting where abundant non-peridotitic components have been added to the lithospheric mantle over the course of continent evolution. Provided that CO2-rich melts, such as proto-kimberlites, occur near the solidus of volatile-fluxed peridotites, no excess mantle heat is required in their formation. This important but often overlooked constraint, together with the observation that there exist no spatial or temporal relationships between the Superior craton kimberlites and Large Igneous Provinces during the Late Neoproterozoic, suggests that kimberlite magmatic activity was tectonically controlled. In our preferred model, ubiquitous CO2-rich proto-kimberlite melts form during volatile-controlled redox melting processes at ambient mantle temperatures in a thermal boundary layer directly beneath thick cratonic lithosphere. The success rate of ‘evolving’ hybrid kimberlite magmas reaching Earth’s surface increases when tensile stresses propagate into the > 200 km thick keels of continental lithosphere. These conditions are frequently met during fast and changing plate motions associated with the assembly and breakup of supercontinents.
Abstract: During the past two decades significant progress has been made in understanding the origin and evolution of kimberlites, including relationships to other diamondiferous magma types such as lamproites and aillikites. However, the association of kimberlites and carbonatites on continental shields remains poorly understood, and two opposing ideas dominate the debate. While one school of thought argues that primary carbonatite melts transform into hybrid carbonated silicate magmas akin to kimberlites by assimilation of cratonic mantle material, others use geochemical evidence to show that carbonatite magmas can evolve from near-primary kimberlite melts within the cratonic lithosphere. The 1.15 Ga Premier kimberlite pipe on the Kaapvaal craton in South Africa hosts several kimberlite and carbonatite dykes. Reconstructions of magma compositions suggest that up to 20 wt.% CO2 was lost from near-primary kimberlite melts during ascent through the cratonic lithosphere, but the carbonatite dyke compositions cannot be linked to the kimberlite melts via differentiation. Geochemical evidence, including mantle-like ?13C compositions, suggests that the co-occurring kimberlite and carbonatite dykes represent two discrete CO2-rich magma batches derived from a mixed source in the convecting upper mantle. The carbonatites probed a slightly more depleted source component in terms of Sr-Nd-Hf isotopic compositions relative to the peridotitic matrix that was more effectively tapped by the kimberlites (87Sr/86Sri = 0.70257 to 0.70316 for carbonatites vs. 0.70285 to 0.70546 for kimberlites; ?Ndi = +3.0 to +3.9 vs. +2.2 to +2.8; ?Hfi = -2.2 to +0.7 vs. -5.1 to -1.9). Platinum-group element systematics suggest that assimilation of refractory lithospheric mantle material by the carbonatite melts was negligible (<1 vol.%), whereas between 5 - 35 vol.% of digested cratonic peridotite account for the kimberlite compositions, including the low 187Os/188Os signature (?Osi = -12.7 to -4.5). The kimberlite and carbonatite dykes show similarly strong Nd-Hf isotope decoupling (??Hfi = -10.7 to -7.6 vs. -8.8 to -6.1), regardless of the variable lithospheric mantle imprints. This observation suggests a common sublithospheric origin of the negative ??Hf signature, possibly linked to ancient recycled oceanic crust components in the convecting upper mantle to transition zone sources of CO2-rich magmatism. Mesoproterozoic kimberlite and carbonatite magmatism at Premier was coeval with subduction and collision events along the southern Kaapvaal craton margin during the 1,220 -1,090 Ma Namaqua-Natal orogeny associated with Rodinia supercontinent formation. Thermochronology suggests that the entire Kaapvaal craton was affected by this collisional tectonic event, and it appears that the changing lithospheric stress-field created pathways for deep-sourced kimberlite and carbonatite magmas to reach Earth’s surface. We find that collision-induced (e.g., Premier) and continental breakup-related (e.g., Kimberley) kimberlite magmas are compositionally indistinguishable, with the inference that plate tectonic processes aid solely in the creation of magma ascent pathways without a major influence on deep mantle melting beneath cratons. It follows that on-craton kimberlite magmatism in the hinterland of collision zones is not necessarily more likely to entrain large sublithospheric diamonds than kimberlite eruptions linked to continental breakup. This implies that Premier’s world-class endowment with ‘ultradeep’ Type-II diamonds is not causally related to its setting behind an active orogenic front.
Galimov, E., Kudin, A., Skorobogatskii, V., Plotnichenko, V., Bondarev, O., Zarubin, B., Strazdovskii, V., Aronin, A., Fisenko, A., Bykov, I., Barinov, A.
Experimental corrobation of the synthesis of diamond in the cavitation process.
Doklady Physical Chemistry, Vol. 49, 3, pp. 150-153.
Iugs Subcommision on the Systematics of Igneous Rocks; Classification and Nomenclature of Volcanic Rocks, Lamprophyres, carbonatites and Melilitic Rocks; Recommendations and Suggestions.
Neues Jahr. Min., Vol. 134, No. 1, PP. 1-14.
Global
Melilite, Lamprophyre, Carbonatite, Rock Classification
Abstract: The cooling history of rift shoulders and the subsidence history of rift basins are cornerstones for reconstructing the morphotectonic evolution of extensional geodynamic provinces, assessing their role in paleoenvironmental changes and evaluating the resource potential of their basin fills. Our apatite fission track and zircon (U-Th)/He data from the Samburu Hills and the Elgeyo Escarpment in the northern and central sectors of the Kenya Rift indicate a broadly consistent thermal evolution of both regions. Results of thermal modeling support a three-phased thermal history since the early Paleocene. The first phase (~65 50?Ma) was characterized by rapid cooling of the rift shoulders and may be coeval with faulting and sedimentation in the Anza Rift basin, now located in the subsurface of the Turkana depression and areas to the east in northern Kenya. In the second phase, very slow cooling or slight reheating occurred between ~45 and 15?Ma as a result of either stable surface conditions, very slow exhumation, or subsidence. The third phase comprised renewed rapid cooling starting at ~15?Ma. This final cooling represents the most recent stage of rifting, which followed widespread flood-phonolite emplacement and has shaped the present-day landscape through rift shoulder uplift, faulting, basin filling, protracted volcanism, and erosion. When compared with thermochronologic and geologic data from other sectors of the East African Rift System, extension appears to be diachronous, spatially disparate, and partly overlapping, likely driven by interactions between mantle-driven processes and crustal heterogeneities, rather than the previously suggested north south migrating influence of a mantle plume.
The Koh-i-nur Diamond its Romance and History with Special Notes by Her Majesty the Queen. Also the Curious History of The Celebrated Pitt Diamond with Special Notes by the Ex-empress Eugenie.
Rare earth elements in phoscorites and carbonatites of the Devonian Kola alkaline province, Russia: examples from Kovdor, Khibina, Vuoriyarvi and Turiy Mys complexes.
Rare earth elements in phoscorites and carbonatites of the Devonian Kola alkaline province, Russia: examples from Kovdor, Khibina, Vuoriyarvi and Turiy Mys complexes.
Rare earth elements in phoscorites and carbonatites of the Devonian Kola alkaline province, Russia: examples from Kovdor, Khibina, Vuoriyarvi and Turiy Mys complexes.
Russian Geology and Geophysics, Vol. 56, pp. 1717-1737.
Africa, Tanzania
Deposit - Oldoinyo Lengai
Abstract: The paper is concerned with study of melt inclusions in minerals of ijolite xenoliths at Oldoinyo Lengai Volcano. Melt inclusions with different phase compositions occur in forsterite macrocrysts and in diopside, nepheline, fluorapatite, Ti-andradite, and Ti-magnetite crystals. Nepheline contains primary melt inclusions (silicate glass + gas-carbonate globule ± submicron globules ± sulfide globule ± daughter/trapped phases, represented by diopside, fluorapatite, Ti-andradite, and alumoakermanite). The gas-carbonate globule consists of a gas bubble surrounded by a fine-grained aggregate of Na-Ca-carbonates (nyerereite and gregoryite). Fluorapatite contains primary carbonate-rich melt inclusions in the core, which consist of nyerereite, gregoryite, thenardite, witherite, fluorite, villiaumite, and other phases. Their mineral composition is similar to natrocarbonatites. Primary melt inclusions (glass + gas bubble ± daughter phases) are rare in diopside and Ti-andradite. Diopside and forsterite have trails of secondary carbonate-rich inclusions. Besides the above minerals, these inclusions contain halite, sylvite, neighborite, Na-Ca-phosphate, alkali sulfates, and other rare phases. In addition, diopside contains sulfide inclusions (pyrrhotite ± chalcopy- rite ± djerfisherite ± galena ± pentlandite). The chemical compositions of silicate glasses in the melt inclusions vary widely. The glasses are characterized by high Na, K, and Fe contents and low Al contents. They have high total alkali contents (16-23 wt.% Na2O + K2O) and peralkalinity index [(Na + K)/Al] ranging from 1.1 to 7.6. The carbonate-rich inclusions in the ijolite minerals are enriched in Na, P, S, and Cl. The data obtained indicate that the parental melt in the intermediate chamber was heterogeneous and contained silicate, natrocarbonate, and sulfide components during the ijolite crystallization. According to heating experiments with melt inclusions, silicate-carbonate liquid immiscibility occurred at temperature over 580 °C.
Mineralogy and Petrology, in press available 20p. Pdf
Russia, Archangel
deposit - Grib
Abstract: In this paper, new main and trace elements and isotopic data are presented for 14 coarse-grained eclogite xenoliths from the V. Grib kimberlite pipe in the central part of the Arkhangelsk Diamondiferous Province. Based on reconstructed whole rock MgO content, this suite is divided into high-MgO and low-MgO varieties. Eclogitic groups have a similar range of variations in the trace element compositions of garnet, clinopyroxene and reconstructed whole rock. All eclogites show positive Eu anomalies in garnet and Sr anomalies in the whole rock. The negative correlation between the Mg#, Sr/Lu ratio and HREE in a whole rock points to upper and lower oceanic crustal rocks as a protolith for eclogites with high and low whole rock HREEs, respectively. Low-MgO eclogites with higher whole rock HREEs have the basaltic upper oceanic crustal protolith, whereas the protoliths of eclogites with lower whole rock HREEs could be of gabbroic composition from the lower oceanic crust. High-MgO eclogites could represent MgO-rich portions of oceanic crustal rocks: picritic/MgO basalt portions in the upper oceanic crust and troctolite portions in the lower oceanic crust. The Sr and Nd isotope compositions suggest a complex history of eclogites during their residence in the lithospheric mantle. Similarities in the Nd isotope compositions and two-point Sm-Nd isochron ages are evidence for re-equilibration of the Sm-Nd isotope system between the eclogite garnet and clinopyroxene via a pre-kimberlite thermal event at 396?±?24 Ma. The subset of clinopyroxenes from four eclogites has a Sr isotope composition that plots on the isochron at an age of 2.84 Ga, which reflects the time of the subduction event and emplacement into the lithosphere and corresponds to the time of the Belomorian Eclogite Province of Baltic Shield formation.
Abstract: Atom-probe tomography (APT) and secondary ion mass spectrometry (SIMS) provide complementary in situ element and isotope data in minerals such as zircon. SIMS measures isotope ratios and trace elements from 1–20 ?m spots with excellent accuracy and precision. APT identifies mass/charge and three-dimensional position of individual atoms (±0.3 nm) in 100 nm-scale samples, volumes up to one million times smaller than SIMS. APT data provide unique information for understanding element and isotope distribution; crystallization and thermal history; and mechanisms of mineral reaction and exchange. This atomistic view enables evaluation of the fidelity of geochemical data for zircon because it provides new understanding of radiation damage, and can test for intracrystalline element mobility. Nano-geochronology is one application of APT in which Pb isotope ratios from sub-micrometer domains of zircon provide model ages of crystallization and identify later magmatic and metamorphic reheating.
Paleomagnetism of the NeoArchean Pongola and Ventersdorp Supergroups and an appriasal of the 3.0 - 1.9 Ga apparent polar wander path of Kaapvaal Craton
Kimberlites : structural zonation and their productivity
Conference registration The Canadian Institute of Mining, Metallurgy and Petroleum (CIM) Xerox Tower Suite 1210, 3400 de Maissoneuve, Sept. 5-13, 1991 Fax 514 939-2714
Diamondiferous kimberlites in Saskatchewan, Canada: global, regional and local setting.
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 11-20.
Ackerman, L., Walker, R.J., Puchtel, I.S., Pitcher, L., Jelinek, E., Strnad, L.
Effects of melt percolation on highly siderophile elements and Os isotopes in subcontinental lithospheric mantle: a study of upper mantle profile central Europe
Geochimica et Cosmochimica Acta, Vol. 73, 8, pp. 2400-2414.
Abstract: Manually interpreting multivariate drill hole data is very time-consuming, and different geologists will produce different results due to the subjective nature of geological interpretation. Automated or semi-automated interpretation of numerical drill hole data is required to reduce time and subjectivity of this process. However, results from machine learning algorithms applied to drill holes, without reference to spatial information, typically result in numerous small-scale units. These small-scale units result not only from the presence of very small rock units, which may be below the scale of interest, but also from misclassification. A novel method is proposed that uses the continuous wavelet transform to identify geological boundaries and uses wavelet coefficients to indicate boundary strength. The wavelet coefficient is a useful measure of boundary strength because it reflects both wavelength and amplitude of features in the signal. This means that boundary strength is an indicator of the apparent thickness of geological units and the amount of change occurring at each geological boundary. For multivariate data, boundaries from multiple variables are combined and multiscale domains are calculated using the combined boundary strengths. The method is demonstrated using multi-element geochemical data from mineral exploration drill holes. The method is fast, reduces misclassification, provides a choice of scales of interpretation and results in hierarchical classification for large scales where domains may contain more than one rock type.
Abstract: All chondrites accreted ?3.5 wt.% C in their matrices, the bulk of which was in a macromolecular solvent and acid insoluble organic material (IOM). Similar material to IOM is found in interplanetary dust particles (IDPs) and comets. The IOM accounts for almost all of the C and N in chondrites, and a significant fraction of the H. Chondrites and, to a lesser extent, comets were probably the major sources of volatiles for the Earth and the other terrestrial planets. Hence, IOM was both the major source of Earth’s volatiles and a potential source of complex prebiotic molecules. Large enrichments in D and 15N, relative to the bulk solar isotopic compositions, suggest that IOM or its precursors formed in very cold, radiation-rich environments. Whether these environments were in the interstellar medium (ISM) or the outer Solar System is unresolved. Nevertheless, the elemental and isotopic compositions and functional group chemistry of IOM provide important clues to the origin(s) of organic matter in protoplanetary disks. IOM is modified relatively easily by thermal and aqueous processes, so that it can also be used to constrain the conditions in the solar nebula prior to chondrite accretion and the conditions in the chondrite parent bodies after accretion. Here we review what is known about the abundances, compositions and physical nature of IOM in the most primitive chondrites. We also discuss how the IOM has been modified by thermal metamorphism and aqueous alteration in the chondrite parent bodies, and how these changes may be used both as petrologic indicators of the intensity of parent body processing and as tools for classification. Finally, we critically assess the various proposed mechanisms for the formation of IOM in the ISM or Solar System.
Geophysics Research Letters, Vol. 44, 5, pp. 2148-2158.
Mantle
geophysics - seismics
Abstract: We explore the effect of Mg/Fe substitution on the sound velocities of iron-rich (Mg1 ? xFex)O, where x = 0.84, 0.94, and 1.0. Sound velocities were determined using nuclear resonance inelastic X-ray scattering as a function of pressure, approaching those of the lowermost mantle. The systematics of cation substitution in the Fe-rich limit has the potential to play an important role in the interpretation of seismic observations of the core-mantle boundary. By determining a relationship between sound velocity, density, and composition of (Mg,Fe)O, this study explores the potential constraints on ultralow-velocity zones at the core-mantle boundary.
Abstract: The cooling history of rift shoulders and the subsidence history of rift basins are cornerstones for reconstructing the morphotectonic evolution of extensional geodynamic provinces, assessing their role in paleoenvironmental changes and evaluating the resource potential of their basin fills. Our apatite fission track and zircon (U-Th)/He data from the Samburu Hills and the Elgeyo Escarpment in the northern and central sectors of the Kenya Rift indicate a broadly consistent thermal evolution of both regions. Results of thermal modeling support a three-phased thermal history since the early Paleocene. The first phase (~65 50?Ma) was characterized by rapid cooling of the rift shoulders and may be coeval with faulting and sedimentation in the Anza Rift basin, now located in the subsurface of the Turkana depression and areas to the east in northern Kenya. In the second phase, very slow cooling or slight reheating occurred between ~45 and 15?Ma as a result of either stable surface conditions, very slow exhumation, or subsidence. The third phase comprised renewed rapid cooling starting at ~15?Ma. This final cooling represents the most recent stage of rifting, which followed widespread flood-phonolite emplacement and has shaped the present-day landscape through rift shoulder uplift, faulting, basin filling, protracted volcanism, and erosion. When compared with thermochronologic and geologic data from other sectors of the East African Rift System, extension appears to be diachronous, spatially disparate, and partly overlapping, likely driven by interactions between mantle-driven processes and crustal heterogeneities, rather than the previously suggested north south migrating influence of a mantle plume.
Abstract: The origin of diamond-forming carbon in the Earth is unclear [1-3]; sources include subducted organic sediment and primordial mantle carbon. For example, some diamonds contain eclogitic silicate + sufide inclusions and have depleted ?13C (-10 to -30‰), enriched ?15N (+3 to +35‰) values, consistent with subducted crustal material [2-3]. However, some diamonds show mantle-like ?15N (<-5‰) and depleted ?13C values (-10 to -30‰ ) which have been cited as evidence of enstatite chondrite-like primordial C-N sources [1]. The helium isotope composition of mantle rocks are powerful tracers,of Earth’s volatile history because primordial 3He is not recycled back into the mantle. However, there are few He isotope studies of diamond fluids. The 3He/4He of garnetbearing diamondites from the Orapa mine (Botswana) range from 0.1 to 3 Ra [4-5], consistent with a recycled origin. However, our recent work has identified a suite of diamondites with 3He/4He = 0.06 to 8.2 Ra which correlates negatively with ?13C, suggesting that the subduction-related C is associated with mantle 3He/4He ratios. To unravel this complexity we are combining He, C and N isotope analyses in polycrystalline diamond from garnetbearing diamondites from the Orapa mine. These data will also be used to assess the extent to which carbon and nitrogen isotopes are decoupled during diamond-formation [3].
Abstract: The origin of the diamond-forming fluids are routinely addressed with the stable isotopes of carbon and nitrogen, where average ?13C and ?15N values are -5 ± 3‰ and -5 ± 4‰, respectively. Because these values differ from crustal sources the application of C-N stable isotopes are applied as tracers of recycled crustal volatiles into the mantle. Additionally, fluid inclusions in fast-growing diamonds provides a unique opportunity to further examine the origin of diamond-forming fluids using noble gas geochemistry. Here we combine C-N isotopes, N concentrations from the diamond with He isotopes released from trapped fluids by in vacuo crushing of mg-sized polycrystalline diamonds. The samples examined are dominantly eclogitic to websteritic abd originate from Southern Africa. ?13C values range from -4.3 to -22.2 ‰ and ?15N values from -4.9 to +23.2 ‰. These data require a significant contribution of material that is 13Cdepleted and 15N-enriched relative to mantle, akin to altered oceanic crust or deep ocean sediments. 3He/4He ratios range from typical mantle values (8.5 Ra) to those dominated by radiogenic He (< 0.1 Ra). These new data show 3He/4He correlates with 3He concentration, suggesting that the low 3He/4He are, at least in part, the result of ingrowth of radiogenic 4He in He-poor diamonds after their formation. 13C-depleted and 15N-enriched diamonds dominate the population studied here. This indicates that subducted altered oceanic crust is essential for diamondite-formation within the SCLM beneath southern Africa. However, the fluids trapped in the low ?13C diamondites (< -15 ‰) have 3He/4He ratios that indicate an origin in the convective upper asthenospheric mantle. Ergo, helium reveals what carbon and nitrogen cannot. When the carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamondformation, helium isotopes reveal an unambiguos mantle component hidden within strongly 13C-depleted diamond.
Osmium isotopes in Baffin Island and West Greenland picrites: implications for the 187 Os and 188 Os composition of the convection mantle and nature 3He/4he
Earth and Planetary Interiors, Vol. 278, 3-4, pp. 267-277.
Brown, R.J., Buisman, M.I., Fontana, G., Field, M., Mac Niocaill, C., Sparks, R.S.J., Stuart, F.M.
Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on Earth ( the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania).
Brown, R.J., Manya, S., Buisman, I., Fontana, G., Field, M., MacNiocaill, C., Sparks, R.S.J., Stuart, F.M.
Eruption of kimberlite magmas: physical volcanology, geomrphology and age of the youngest kimberlitic volcanoes known on Earth ( the Upper Pleistocene-Holocene Igwisi Hills, volcanoes, Tanzania.
Bulletin of Volcanology, Vol. 74, 7, pp. 1621-1643.
Brown, R.J., Manya, S., Buisman, I., Sparks, R.S.J., Field, M., Stuart, F.M., Fontana, G.
Physical volcanology, geomorphology, and cosmogenic 3HE dating of the youngest kimberlite volcanoes on Earth ( The Holocene Igwisi Hills, Volcanoes, Tanzania.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
Abstract: Apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated (>1000?m above sea level) but low-relief Tanzanian Craton. Coeval episodes of sedimentation documented within adjacent Paleozoic to Mesozoic basins of southern Tanzania and the Congo basin of the Democratic Republic of Congo indicate that most of the cooling in the basement rocks in Tanzania was linked to erosion. Basement samples were from an exploration borehole located within the craton and up to 2200?m below surface. Surface samples were also analyzed. AFT dates range between 317?±?33?Ma and 188?±?44?Ma. Alpha (Ft)-corrected AHe dates are between 433?±?24?Ma and 154?±?20?Ma. Modeling of the data reveals two important periods of cooling within the craton: one during the Carboniferous-Triassic (340-220?Ma) and a later, less well constrained episode, during the late Cretaceous. The later exhumation is well detected proximal to the East African Rift (70?Ma). Thermal histories combined with the estimated geothermal gradient of 9°C/km constrained by the AFT and AHe data from the craton and a mean surface temperature of 20°C indicate removal of up to 9?±?2?km of overburden since the end of Paleozoic. The correlation of erosion of the craton and sedimentation and subsidence within the Congo basin in the Paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far-field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes.
Abstract: Southern Africa is characterised by unusually elevated topography and abnormal heat flow. This can be explained by thermal perturbation of the mantle, but the origin of this is unclear. Geophysics has not detected a thermal anomaly in the upper mantle and there is no geochemical evidence of an asthenosphere mantle contribution to the Cenozoic volcanic record of the region. Here we show that natural CO2 seeps along the Ntlakwe-Bongwan fault within KwaZulu-Natal, South Africa, have C-He isotope systematics that support an origin from degassing mantle melts. Neon isotopes indicate that the melts originate from a deep mantle source that is similar to the mantle plume beneath Réunion, rather than the convecting upper mantle or sub-continental lithosphere. This confirms the existence of the Quathlamba mantle plume and importantly provides the first evidence in support of upwelling deep mantle beneath Southern Africa, helping to explain the regions elevation and abnormal heat flow.
Geochemical Perspectives Letters, Vol. 11, pp. 39-43. pdf
Africa, Botswana, South America, French Guiana
deposit - Dachine, Orapa
Abstract: Fluid inclusions trapped in fast-growing diamonds provide a unique opportunity to examine the origin of diamonds, and the conditions under which they formed. Eclogitic to websteritic diamondites from southern Africa show 13C-depletion and 15N-enrichment relative to mantle values (?13C = -4.3 to -22.2 ‰ and ?15N = -4.9 to +23.2 ‰). In contrast the 3He/4He of the trapped fluids have a strong mantle signature, one sample has the highest value so far recorded for African diamonds (8.5 ± 0.4 Ra). We find no evidence for deep mantle He in these diamondites, or indeed in any diamonds from southern Africa. A correlation between 3He/4He ratios and 3He concentration suggests that the low 3He/4He are largely the result of ingrowth of radiogenic 4He in the trapped fluids since diamond formation. The He-C-N isotope systematics can be best described by mixing between fluid released from subducted altered oceanic crust and mantle volatiles. The high 3He/4He of low ?13C diamondites reflects the high 3He concentration in the mantle fluids relative to the slab-derived fluids. The presence of post-crystallisation 4He in the fluids means that all 3He/4He are minima, which in turn implies that the slab-derived carbon has a sedimentary organic origin. In short, although carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamond-formation, helium isotopes reveal an unambiguous mantle component hidden within a strongly 13C-depleted system.
Abstract: Mineral carbonation is a form of carbon capture, utilization and storage (CCUS) that aims to transform excess CO2 into environmentally benign carbonate minerals which are geologically stable. Here, we investigated the reactivity of processed kimberlite and kimberlite ore from the Venetia Diamond Mine (South Africa). Highly reactive phases, such as brucite [Mg(OH)2], are uncommon in the samples collected from Venetia necessitating the development of new strategies for mineral carbonation. Kimberlite ore and tailings from this mine consist of a clay-rich mineral assemblage that is dominated by lizardite (a serpentine mineral) and smectites. Smectites are swelling clays that can act as a source of Mg and Ca for carbonation reactions via cation exchange, dissolution and/or direct replacement of smectites to form carbonate phases. Although carbonation of serpentine and brucite has long been a focus of CCUS in mine wastes [1], smectite carbonation has not been explored in this setting. Quantitative X-ray diffraction using Rietveld refinements coupled with Fourier-transform infrared spectroscopy indicate that smectites of stevensite-saponite composition are abundant in the Venetia samples (1.3-15.4 wt.%). Synchrotron-based X-ray fluorescence mapping correlated with scanning and transmission electron microscopy show that smectites are distributed as altered, smooth regions measuring from 1 to 20 ?m in breadth. These phases are rich in Mg and Ca and Al-poor. To better understand the behaviour/reactivity of smectites during the cation exchange process, we have used batch experiments with pure endmembers of Ca-, Mg- and Na-montmorillonite under different treatment conditions (NH4-citrate, NH4-O-acetate, NH4-Cl and Na3-citrate). After 24 hours of reaction, ICP-MS analyses reveal that the four treatments have the same efficiency for Ca and Mg exchange, while NH4-Cl and NH4- O-acetate treatments minimize calcite dissolution. Our end goals are to optimize settling time and to maximize extraction of Ca and Mg for carbonation reactions during ore processing.
45th. Annual Yellowknife Geoscience Forum, p. 4 abstract
Canada, Northwest Territories
deposit - Kelvin, Faraday
Abstract: The Kennady North Project kimberlites are located approximately 280 kilometers east-northeast of Yellowknife, in the Northwest Territories of Canada. The unusual geometry and extent of the kimberlite magmatic system is revealed by renewed exploration drilling activities by Kennady Diamonds since 2012. It has become clear that the system comprises multiple intrusive dykes within which several volcaniclastic bodies have developed, all within 11 kilometres of the Gahcho Kué kimberlite cluster and diamond mine. The detailed exploration of the entire system provides unique evidence for subterranean volcanic conduit growth processes that may have scientific and practical exploration benefits. The identified Kennady North Project volcaniclastic bodies are named Kelvin, Faraday 1, Faraday 2 and Faraday 3, and have complex geometries atypical of the more common subvertical kimberlite pipes. Rather, these pipe-like bodies are inclined between 12 and 30 degrees towards the northwest. Kelvin has sharp angular change in trend towards the north. On-going detailed petrographic studies have shown that the pipes contain layers of complex volcaniclastic units with variable volumes of xenolithic fragments, as well as coherent magmatic layers. The pipe textures include evidence for high energy magma and country rock fragmentation processes typically observed in open volcanic systems. The pipes have developed within a shallow 20 degree northwest dipping kimberlite dyke system. Detailed structural geology studies, using fault observations in oriented and unoriented drill core, have identified at least two important fault-fracture trends. The first fault-fracture system is parallel to the dyke segments, and likely related to the intrusion of the dykes and the regional stress tensor during emplacement. The second fault system is subvertical and north-south striking, parallel to the lithological layering within the metasedimentary country rock. The north-south faults match the contact geometry of the Kelvin pipe’s north-south limb exactly. The dykes have been 3-D modelled along with the pipes. Three possible renditions of the dykes have been created, based on different interpretations of dyke segment continuity. The renditions have been labelled “Optimistic”, “Realistic” and “Pessimistic”. The assumptions made have important implications for developing dyke-type mineral resources. The realistic dyke model defines dyke segments that intersect the Kelvin pipe, and those intersections match geometric trends and irregularities in the pipe shape. The coincidental geometries strongly imply that the pipe development interacted with a penecontemporaneous dyke system. The north-south faults also controlled the local trend of Kelvin pipe development, possibly by enhancing fluid permeability, alteration and brecciation along the faults, connecting from one shallow dipping dyke to the next above. Breccia bodies have been observed on similar dipping dykes at Snap Lake mine that intersect fault structures. We conclude that the pipe development geometry and process is governed by a combination of stress, structure and magmatic fluids, and speculate on the nature of the energy required for fragmentation and development of the pipe at some still unknown depth in the crust.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0621-8 16p.
Canada, Northwest Territories
deposit - Kelvin, Faraday
Abstract: The Kennady North Project kimberlites (Northwest Territories of Canada) comprises multiple shallow dipping dykes and several volcaniclastic bodies that have an unusual shallow plunging geometry and complex "pipe" shapes that are termed chonoliths. The detailed exploration of the entire system provides exceptional evidence for subterranean volcanic conduit growth processes. The possible processes leading to the development of the kimberlite bodies are discussed, with emphasis on the importance of the subsurface intrusive system geometry and the local stress tensor. Emplacement into a locally compressive stress regime (i.e. ?1 and ?2 inclined at a low angle to surface) could change the kimberlite emplacement geometries to that observed at Kennady North. Models are proposed for the development of the chonoliths, to emphasize aspects of the growth of kimberlite systems that are not well understood. The conclusions challenge or evolve current emplacement models and should influence kimberlite exploration and resource definition assumptions.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 287-318.
Geological Association of Canada Bookstore, https://gac.ca/publications/bookstore Special Paper 51, 216p. Prices 42.50 member, $75.00 non-member isbn:978-1-897095-89-8
Canada, Northwest Territories
Craton
Abstract: With its well-exposed geologic record from the Hadean Acasta gneiss complex through to Phanerozoic kimberlites, the Slave craton of northwestern Canada has long been a focus for research into early Earth evolution of both the crust and lithosphere. As a result, it has become one of the most extensively studied Archean cratons in the world. This multidisciplinary volume provides an authoritative overview of the Slave craton literally from the bottom up, integrating the nature of its lithosphere based on kimberlitic mantle samples with its upper crustal geology to provide a new model for its Archean assembly and cratonization. All aspects of Slave craton geology are covered, from the stratigraphy of its famous gold camps to the history of exploration and nature of its world-class diamondiferous kimberlite fields. Detailed and well-illustrated chapters cover its terranes and greenstone belts, magmatism, geophysical character, tectono-metamorphic evolution, and Paleoproterozoic marginal sequences. The book’s wealth of data and up-to-date bibliography provide a unique resource for understanding, researching and teaching Archean geology and subcrustal and cratonic evolution. It elegantly integrates diverse fields to provide one of the most comprehensive models for the craton and the protracted, multiphase formation of its diamond-bearing lithospheric root. (JK Note: the link above takes you to the GAC web site where Special Paper 51 can be purchased. Because the GAC only provides the abstract and a photo of the front page, I am providing a Table of Contents pdf.)
Shoshonitic and alkaline lamprophyres with elevated gold (Au) and platinum group elements (PGE) concentrations from the Kreuzeck Mountains, eastern Alps, Austria
Mineralogy and Petrology, Vol. 46, No. 1, pp. 23-42
Zirconolite and Zr Th U minerals in chromities of the Finero complex, western Alps, Italy: evidence for carbonatite type metasomatism in a subcontinental ... mantle plume.
Abstract: 40Ar/39Ar analysis showed a simultaneous (at about 490 Ma) formation of the Paleozoic picrite and basalt complexes of the West Siberian Plate basement. The petrochemistry, trace and REE geochemistry, and composition of clinopyroxene indicate the formation of the picrite of well no. 11 (Chkalov area) as a result of intraplate magmatism of the OIB type. Calculations based on the compositions of clinopyroxene allowed crystallization of minerals of porphyric picrite at 1215-1275°C and 4.5-8 kbar. In general, it has been found that the picrite basalt complexes considered were formed from enriched igneous plume systems under intraplate conditions near the active margin of the ancient ocean.
Abstract: Seismic anisotropy provides key information to map the trajectories of mantle flow and understand the evolution of our planet. While the presence of anisotropy in the uppermost mantle is well established, the existence and nature of anisotropy in the transition zone and uppermost lower mantle are still debated. Here we use three-dimensional global seismic tomography images based on a large dataset that is sensitive to this region to show the ubiquitous presence of anisotropy in the lower mantle beneath subduction zones. Whereas above the 660?km seismic discontinuity slabs are associated with fast SV anomalies up to about 3%, in the lower mantle fast SH anomalies of about 2% persist near slabs down to about 1,000-1,200?km. These observations are consistent with 3D numerical models of deformation from subducting slabs and the associated lattice-preferred orientation of bridgmanite produced in the dislocation creep regime in areas subjected to high stresses. This study provides evidence that dislocation creep may be active in the Earth’s lower mantle, providing new constraints on the debated nature of deformation in this key, but inaccessible, component of the deep Earth.
Journal of Geophysical Research Solid Earth, doi.org/10.1029/ 2017JB015349
Mantle
core mantle boundary
Abstract: The juxtaposition of a liquid iron?dominant alloy against a mixture of silicate and oxide minerals at Earth's core?mantle boundary is associated with a wide range of complex seismological features. One category of observed structures is ultralow?velocity zones, which are thought to correspond to either aggregates of partially molten material or solid, iron?enriched assemblages. We measured the phonon dispersion relations of (Mg,Fe) O magnesiowüstite containing 76 mol % FeO, a candidate ultralow?velocity zone phase, at high pressures using high?energy resolution inelastic X?ray scattering. From these measurements, we find that magnesiowüstite becomes strongly elastically anisotropic with increasing pressure, potentially contributing to a significant proportion of seismic anisotropy detected near the base of the mantle.
Contributions to Mineralogy and Petrology, Vol. 173, 26p. Doi.org/10.1007/s00410-018-1505-y
Europe, Greenland
carbonatite
Abstract: Petrogenetic studies of carbonatites are challenging, because carbonatite mineral assemblages and mineral chemistry typically reflect both variable pressure-temperature conditions during crystallization and fluid-rock interaction caused by magmatic-hydrothermal fluids. However, this complexity results in recognizable alteration textures and trace-element signatures in the mineral archive that can be used to reconstruct the magmatic evolution and fluid-rock interaction history of carbonatites. We present new LA-ICP-MS trace-element data for magnetite, calcite, siderite, and ankerite-dolomite-kutnohorite from the iron-rich carbonatites of the 1.3 Ga Grønnedal-Íka alkaline complex, Southwest Greenland. We use these data, in combination with detailed cathodoluminescence imaging, to identify magmatic and secondary geochemical fingerprints preserved in these minerals. The chemical and textural gradients show that a 55 m-thick basaltic dike that crosscuts the carbonatite intrusion has acted as the pathway for hydrothermal fluids enriched in F and CO2, which have caused mobilization of the LREEs, Nb, Ta, Ba, Sr, Mn, and P. These fluids reacted with and altered the composition of the surrounding carbonatites up to a distance of 40 m from the dike contact and caused formation of magnetite through oxidation of siderite. Our results can be used for discrimination between primary magmatic minerals and later alteration-related assemblages in carbonatites in general, which can lead to a better understanding of how these rare rocks are formed. Our data provide evidence that siderite-bearing ferrocarbonatites can form during late stages of calciocarbonatitic magma evolution.
Abstract: The Great Unconformity is a distinctive feature in the geologic record that separates more ancient rocks from younger (<540 Ma) sedimentary rocks. It commonly marks a substantial time gap in the rock record. When and why the Great Unconformity developed is much debated. We present new thermochronologic data that constrain when ancient rocks across the central Canadian Shield last cooled during exhumation to the surface before deposition of overlying sedimentary rocks that mark the Great Unconformity. These data and the geologic context indicate that the basement below the Great Unconformity erosion here was last exhumed after 650 Ma, in contrast to the pre-650 Ma timing inferred elsewhere in North America. This result is inconsistent with the notion that the Great Unconformity formed worldwide in a single erosion event.
Abstract: The Great Unconformity is a distinctive feature in the geologic record that separates more ancient rocks from younger (<540 Ma) sedimentary rocks. It commonly marks a substantial time gap in the rock record. When and why the Great Unconformity developed is much debated. We present new thermochronologic data that constrain when ancient rocks across the central Canadian Shield last cooled during exhumation to the surface before deposition of overlying sedimentary rocks that mark the Great Unconformity. These data and the geologic context indicate that the basement below the Great Unconformity erosion here was last exhumed after 650 Ma, in contrast to the pre-650 Ma timing inferred elsewhere in North America. This result is inconsistent with the notion that the Great Unconformity formed worldwide in a single erosion event.
Cobden, L., Goes, S., Ravenna, M., Styles, E., Cammarano, F., Gallagher, K., Connolly, J.
Thermochemical interpretation of 1-D seismic dat a for the lower mantle: the significance of nonadiabiatic thermal gradients and compositional heterogeneity.
Journal of Geophysical Research, Vol. 114, B 11, B11309
Abstract: The Tundulu and Kangankunde carbonatite complexes in the Chilwa Alkaline Province, Malawi, contain late-stage, apatite-rich lithologies termed quartz-apatite rocks. Apatite in these rocks can reach up to 90 modal% and displays a distinctive texture of turbid cores and euhedral rims. Previous studies of the paragenesis and rare earth element (REE) content of the apatite suggest that heavy REE (HREE)-enrichment occurred during the late-stages of crystallization. This is a highly unusual occurrence in intrusions that are otherwise light REE (LREE) enriched. In this contribution, the paragenesis and formation of the quartz-apatite rocks from each intrusion is investigated and re-evaluated, supported by new electron microprobe (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data to better understand the mechanism of HREE enrichment. In contrast to the previous work at Tundulu, we recognize three separate stages of apatite formation, comprising an “original” euhedral apatite, “turbid” apatite, and “overgrowths” of euhedral late apatite. The crystallization of synchysite-(Ce) is interpreted to have occurred subsequent to all phases of apatite crystallization. The REE concentrations and distributions in the different minerals vary, but generally higher REE contents are found in later-stage apatite generations. These generations are also more LREE-enriched, relative to apatite that formed earlier. A similar pattern of increasing LREE-enrichment and increased REE concentrations toward later stages of the paragenetic sequence is observed at Kangankunde, where two generations of apatite are observed, the second showing higher REE concentrations, and relatively higher LREE contents. The changing REE distribution in the apatite, from early to late in the paragenetic sequence, is interpreted to be caused by a combination of dissolution-reprecipitation of the original apatite and the preferential transport of the LREE complexes by F- and Cl-bearing hydrothermal fluids. Successive pulses of these fluids transport the LREE out of the original apatite, preferentially re-precipitating it on the rim. Some LREE remained in solution, precipitating later in the paragenetic sequence, as synchysite-(Ce). The presence of F is supported by the F content of the apatites, and presence of REE-fluorcarbonates. Cl is not detected in the apatite structure, but the role of Cl is suggested from comparison with apatite dissolution experiments, where CaCl2 or NaCl cause the reprecipitation of apatite without associated monazite. This study implies that, despite the typically LREE enriched nature of carbonatites, significant degrees of hydrothermal alteration can lead to certain phases becoming residually enriched in the HREE. Although at Tundulu the LREE-bearing products are re-precipitated relatively close to the REE source, it is possible that extensive hydrothermal activity in other carbonatite complexes could lead to significant, late-stage fractionation of the REE and the formation of HREE minerals.
Abstract: Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab-mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo = 92.0-92.6, Ca = 42-115 ppm), porphyroblastic orthopyroxene (En = 91.8-92.8), Cr-spinel (Cr# = 50.4-73.0, TiO2 < 0.2 wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31-47.07 wt.%) and Mg# (91.5-91.9), and low Al2O3 (0.48-0.70 wt.%), CaO (0.25-0.44 wt.%) and TiO2 (< 0.03 wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re-Os isotopic data suggest that dunites formed in the early Proterozoic (~ 2.2 Ga). These data indicate that the Lijiatun dunites were the residues of ~ 30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo = 91.6-92.6, Ca = 233-311 ppm), clinopyroxene-bearing (Mg# = 95.9-96.7, Ti/Eu = 104-838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic peridotites derived from such a lithospheric mantle wedge may be heterogeneously modified prior to their incorporation into the subduction channel, which would set up a barrier for investigation of the mass transfer from the subducted crust to the mantle wedge through orogenic peridotites.
Abstract: Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab-mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo = 92.0-92.6, Ca = 42-115 ppm), porphyroblastic orthopyroxene (En = 91.8-92.8), Cr-spinel (Cr# = 50.4-73.0, TiO2 < 0.2 wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31-47.07 wt.%) and Mg# (91.5-91.9), and low Al2O3 (0.48-0.70 wt.%), CaO (0.25-0.44 wt.%) and TiO2 (< 0.03 wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re-Os isotopic data suggest that dunites formed in the early Proterozoic (~ 2.2 Ga). These data indicate that the Lijiatun dunites were the residues of ~ 30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo = 91.6-92.6, Ca = 233-311 ppm), clinopyroxene-bearing (Mg# = 95.9-96.7, Ti/Eu = 104-838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic peridotites derived from such a lithospheric mantle wedge may be heterogeneously modified prior to their incorporation into the subduction channel, which would set up a barrier for investigation of the mas
Journal of Geophysical Research: Solid Earth, Vol. 124, 7, pp. 6504-6522.
Mantle
subduction
Abstract: Mantle wedge hybridization by crust?derived melt is a crucial mechanism responsible for arc lavas. However, how the melt?rock reactions proceed in the mantle wedge and affect melt compositions is poorly understood. Garnet peridotites from Jiangzhuang in the Sulu orogen (eastern China) host garnetite and pyroxenite veins formed by slab?mantle interactions at different melt/rock ratios. The Jiangzhuang peridotites consist mainly of garnet lherzolites and minor harzburgites and represent a fragment of the mantle wedge influenced by ultrahigh?pressure metamorphism (5.2-6.1 GPa) in the subduction channel. Petrography, major and trace element geochemistry, and in situ clinopyroxene Sr isotope values of the garnetite and pyroxenite veins reveal their derivation from interactions between mantle wedge peridotites and deeply subducted crust?derived melts. The two veins share a common metamorphic and metasomatic history and have similar mineral assemblages and compositions, enriched isotope signatures, and formation P?T conditions, indicating the same source for their reacting melts. The different mineral proportions and microtextures between the garnetite and pyroxenite veins are ascribed to different melt/rock ratios. The garnetite vein formed at relatively high melt/rock ratios (>1:1), which would likely produce hybrid slab melts with Mg?rich, high?silica adakitic signatures. In contrast, the pyroxenite vein formed at low melt/rock ratios (<1:1), and the expected hybrid slab melts would evolve into high?Mg andesites. Moreover, recycled heterogeneous garnetite and pyroxenite could contribute to the mantle sources of intraplate magmas. Therefore, slab?mantle interactions at different melt/rock ratios could be an important crustal input to lithological and geochemical heterogeneities in the mantle.
Abstract: For the celebration of the 50th anniversary of the publication of the pioneering papers that established the basis of plate tectonic, this paper was solicited to illustrate the close relation between tectonics and climate. Amongst the large spectrum of interactions that depict how tectonics modified the climate at geological time steps, we choose to illustrate two major issues: (1) How the “tryptic” climate/long?term carbon cycle/tectonics explains the extraordinary glacial episode (717-635 Ma) occurring during Neoproterozoic era? (2) How major tectonic events (i.e., the slow shrinkage of a huge epicontinental sea and the uplift of large mountains ranges in Asia and Africa) drastically changed the climate and shaped the pattern of present?day monsoons systems. This paper is the result of long?standing collaboration with many researchers from different countries.
Su, B-X., Zhang, H-F., Ying, J-F., Xiao, Y., Zhao, X-M.
Nature and processes of the lithospheric mantle beneath the western Qinling: evidence from deformed peridotitic xenoliths in Cenozoic kamafugite from Haoiti, Gansu Pro
Journal of Asian Earth Sciences, Vol. 34, 3, pp. 258-274.
Su, B-X., Zhang, H-F., Ying, J-F., Xiao, Y., Zhao, X-M.
Nature and processes of the lithospheric mantle beneath the western Qinling: evidence from deformed peridotitic xenoliths in Cenozoic kamafugite from Haoti Province.
Journal of Asian Earth Sciences, Vol. 34, pp. 258-274.
Highly heterogeneous lithospheric mantle beneath the Central Zone of the North Chin a Craton evolved from Archean mantle through diverse melt refertilization.
Abstract: Harzburgites with high modal orthopyroxene (generally >23?vol%) in Archean craton, mantle wedge and oceanic lithospheric mantle are considered to be produced by the interaction between Si-rich liquids and rocks. However, the absence of samples from continental margin hinders the recognition whether this process is prevalent. Mantle xenoliths entrained in Miocene basalts from the Thrace Basin, the margin of Eurasian continent, are dominated by harzburgites with anomalously high orthopyroxene modes. These orthopyroxene grains closely associate with spinel and occasionally with clinopyroxene. In these orthopyroxene-spinel associations, orthopyroxene grains can be up to 1?cm in diameter and display high Al2O3 contents (1.41-4.61?wt%) and Mg# values (89.6-92.4), while spinel crystals are anhedral and bud-shaped and are commonly foliated, with a wide variation in Cr# values ranging from 7.8 to 52.7. The Fe2+/Fe3+ vs. TiO2 diagram shows lots of these spinels are “magmatic” (i.e. spinel crystallized from melts). The orthopyroxene grains have LREE diverging from the modelled melting trends, indicating possible metasomatism following partial melting. They are present in elongated shape, cutting across olivine grains and also replacing olivine as surrounding rims. Fine-grained olivine is occasionally enclosed in the orthopyroxene-spinel association. We, therefore, propose that the association of orthopyroxene and spinel developed from the melt/fluid-rock interaction. These features indicate mineral phase transformation from olivine to orthopyroxene, which can be expressed by the equation: ‘Mg2SiO4 (Ol)?+?SiO2?=?Mg2Si2O6 (Opx)’. The observed Al-rich rim of spinel and bud-shaped Al-spinel, suggest sufficient amount of Al in the Si-rich liquids. The mechanism involved here is the consumption of olivine to produce orthopyroxene and spinel as in the equation: ‘Mg2SiO4 (Ol)?+?Al2O3?=?MgSiO3 (Opx)?+?MgAl2O4 (Sp)’. The Si and Al were enriched in the percolating liquids. Both the high-Cr# and low-Cr# spinels with ‘magmatic’ features imply the percolating liquids were multi-staged or inhomogeneous Cr contents in the liquids. This melt/fluid-rock interaction may account for the formation of abundant harzburgites with high orthopyroxene modes in the Eurasian continental margin. Thus, it indicates the reacting harzburgites are prevalent in the lithospheric mantle beneath oceanic crust, Archean craton and mantle wedge, as well as in the continental margin.
Abstract: Harzburgites with high modal orthopyroxene (generally >23?vol%) in Archean craton, mantle wedge and oceanic lithospheric mantle are considered to be produced by the interaction between Si-rich liquids and rocks. However, the absence of samples from continental margin hinders the recognition whether this process is prevalent. Mantle xenoliths entrained in Miocene basalts from the Thrace Basin, the margin of Eurasian continent, are dominated by harzburgites with anomalously high orthopyroxene modes. These orthopyroxene grains closely associate with spinel and occasionally with clinopyroxene. In these orthopyroxene-spinel associations, orthopyroxene grains can be up to 1?cm in diameter and display high Al2O3 contents (1.41-4.61?wt%) and Mg# values (89.6-92.4), while spinel crystals are anhedral and bud-shaped and are commonly foliated, with a wide variation in Cr# values ranging from 7.8 to 52.7. The Fe2+/Fe3+ vs. TiO2 diagram shows lots of these spinels are “magmatic” (i.e. spinel crystallized from melts). The orthopyroxene grains have LREE diverging from the modelled melting trends, indicating possible metasomatism following partial melting. They are present in elongated shape, cutting across olivine grains and also replacing olivine as surrounding rims. Fine-grained olivine is occasionally enclosed in the orthopyroxene-spinel association. We, therefore, propose that the association of orthopyroxene and spinel developed from the melt/fluid-rock interaction. These features indicate mineral phase transformation from olivine to orthopyroxene, which can be expressed by the equation: ‘Mg2SiO4 (Ol)?+?SiO2?=?Mg2Si2O6 (Opx)’. The observed Al-rich rim of spinel and bud-shaped Al-spinel, suggest sufficient amount of Al in the Si-rich liquids. The mechanism involved here is the consumption of olivine to produce orthopyroxene and spinel as in the equation: ‘Mg2SiO4 (Ol)?+?Al2O3?=?MgSiO3 (Opx)?+?MgAl2O4 (Sp)’. The Si and Al were enriched in the percolating liquids. Both the high-Cr# and low-Cr# spinels with ‘magmatic’ features imply the percolating liquids were multi-staged or inhomogeneous Cr contents in the liquids. This melt/fluid-rock interaction may account for the formation of abundant harzburgites with high orthopyroxene modes in the Eurasian continental margin. Thus, it indicates the reacting harzburgites are prevalent in the lithospheric mantle beneath oceanic crust, Archean craton and mantle wedge, as well as in the continental margin.
Abstract: Although a mantle origin of carbonatites has long been advocated, a few carbonatite bodies with crustal fingerprints have been identified. The Eppawala carbonatites in Sri Lanka are more similar to orogenic carbonatites than those formed in stable cratons and within plate rifts. They occur within the Pan-African orogenic belt and have a formation age of ca. 475 Ma newly obtained in this study with no contemporary mantle-related magmatism. These carbonatites have higher (87Sr/86Sr)i ratios (0•70479-0•70524) and more enriched Nd and Hf isotopic compositions than carbonatites reported in other parts of the world. Model ages (1•3-2•0 Ga) of both Nd and Hf isotopes [apatite ?Nd(t)?=??9•2 to ?4•7; rutile ?Hf(t)?=??22•0 to ?8•02] are in the age range of metamorphic basement in Sri Lanka, and the carbon and oxygen isotopic compositions (?13CPDB?=??2•36 to ?1•71; ?18OSMOW?=?13•91-15•13) lie between those of mantle-derived carbonatites and marble. These crustal signatures are compatible with the chemistry of accessory minerals in the carbonatites, such as Ni-free olivine and Al- and Cr-poor rutile. Modeling results demonstrate that the Eppawala carbonatite magmas originated from a mixture of basement gneisses and marbles, probably during regional metamorphism. This interpretation is supported by the occurrence of the carbonatites along, or near, the axes of synforms and antiforms where granitic gneiss and marble are exposed. Therefore, we propose that the Eppawala carbonatites constitute another rare example of a carbonatitic magma that was derived from melting of a sedimentary carbonate protolith. Our findings suggest that other orogenic carbonatites with similar features should be re-examined to re-evaluate their origin.
Gems & Gemology Lab Notes, Vol. 53, 4, pp. 476-478.
Technology
fluoresecence
Abstract: Strong phosphorescence under UV excitation is rarely seen in natural diamond and normally limited to hydrogen-rich type Ia or type IaA/Ib chameleons and type IIb diamonds (T. Hainschwang et al., "A gemological study of a collection of chameleon diamonds," Spring 2005 G&G, pp. 20-35; S. Eaton-Magaña and R. Lu, "Phosphorescence in type IIb diamonds," Diamond and Related Materials, Vol. 20, No. 7, 2011, pp. 983-989). When seen in other diamond types, an even rarer occurrence, it is shorter and less intense. Recently, the National Gemstone Testing Center (NGTC) in Beijing encountered two natural diamonds that showed extraordinarily strong blue phosphorescence and uncommon fluorescence colors under the DiamondView.
Abstract: In October 2017, a natural diamond overgrown by a thick layer of CVD synthetic diamond was identified at the Beijing laboratory of the National Gemstone Testing Center (NGTC). The round-brilliant-cut sample was near-colourless and weighed 0.11 ct. No sign of the overgrowth was observed with magnification. However, DiamondView images showed a distinct boundary in the pavilion separating layers of different luminescence: The upper layer displayed red fluorescence with greenish blue phosphorescence, while the lower portion showed deep blue fluorescence and no phosphorescence. Infrared spectroscopy revealed that the upper layer was type IIa and the lower portion was type Ia. Ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy recorded an unusual co-existence of the N3 centre at 415 nm together with absorption due to [Si-V]-defects at 737 nm. The photoluminescence (PL) spectrum confirmed a high level of [Si-V]-defects. The approximate thickness of the CVD synthetic layer was ~740 µm, which is much thicker than previously reported for such overgrowths. The presence of the N3 centre in the natural diamond layer caused this sample to be passed as natural by various screening instruments. Luminescence imaging is key to identifying such overgrowths, and should be relied upon more heavily in the screening procedures used by gemmological laboratories in the future.
Abstract: Carbonatites and related alkaline rocks host most REE resources. Phosphate minerals, e.g., apatite and monazite, commonly occur as the main REE-host in carbonatites and have been used for tracing magmatic and mineralization processes. Many carbonatite intrusions undergo metamorphic and/or metasomatic modification after emplacement; however, the effects of such secondary events are controversial. In this study, the Miaoya and Shaxiongdong carbonatite-alkaline complexes, in the South Qinling Belt of Central China, are selected to unravel their magmatic and hydrothermal remobilization histories. Both the complexes are accompanied by Nb-REE mineralization and contain apatite and monazite-(Ce) as the major REE carriers. Apatite grains from the two complexes commonly show typical replacement textures related to fluid metasomatism, due to coupled dissolution-reprecipitation. The altered apatite domains, which contain abundant monazite-(Ce) inclusions or are locally surrounded by fine-grained monazite-(Ce), have average REE concentrations lower than primary apatite. These monazite-(Ce) inclusions and fine-grained monazite-(Ce) grains are proposed to have formed by the leaching REE from primary apatite grains during fluid metasomatism. A second type of monazite-(Ce), not spatially associated with apatite, shows porous textures and zoning under BSE imaging. Spot analyses of these monazite-(Ce) grains have variable U-Th-Pb ages of 210-410 Ma and show a peak age of 230 Ma, which is significantly younger than the emplacement age (440-430 Ma) but is roughly synchronous with a regionally metamorphic event related to the collision between the North China Craton and Yangtze Block along the Mianlue suture. However, in situ LA-MC-ICP-MS analyses of those grains show that they have initial Nd values same as those of magmatic apatite and whole rock. We suggest these monazite-(Ce) grains crystallized from the early Silurian carbonatites and have been partially or fully modified during a Triassic metamorphic event, partially resetting U-Pb ages over a wide range. Mass-balance calculations, based on mass proportions and the REE contents of monazite-(Ce) and apatite, demonstrate that the quantity of metasomatized early Silurian monazite-(Ce) is far higher than the proportion of monazite-(Ce) resulting from the metasomatic alteration of the apatite. Therefore, Triassic metamorphic events largely reset the U-Th-Pb isotopic system of the primary monazite-(Ce) and apatite but only had limited or local effects on REE remobilization in the carbonatite-alkaline complexes in the South Qinling Belt. Such scenarios may be widely applicable for other carbonatite and hydrothermal systems.
Song, S., Zhang, L., Niu, Y., Su, L., Jian, P., Liu, D.
Geochronology of diamond bearing zircons from garnet peridotite in the North Qaidam UHPM belt, Northern Tibetan Plateau: a record of lithospheric subduction.
Earth and Planetary Science Letters, Vol. 234, 1-2, pp. 99-118.
Nature Geoscience, doi.org/10.1038/s41561-019-0410-y 10p pdf
Mantle
Plumes, hotspots
Abstract: The thermal and chemical state of the early Archaean deep mantle is poorly resolved due to the rare occurrences of early Archaean magnesium-rich volcanic rocks. In particular, it is not clear whether compositional heterogeneity existed in the early Archaean deep mantle and, if it did, how deep mantle heterogeneity formed. Here we present a geochronological and geochemical study on a Palaeoarchaean ultramafic-mafic suite (3.45-Gyr-old) with mantle plume signatures in Longwan, Eastern Hebei, the North China Craton. This suite consists of metamorphosed cumulates and basalts. The meta-basalts are iron rich and show the geochemical characteristics of present-day oceanic island basalt and unusually high mantle potential temperatures (1,675?°C), which suggests a deep mantle source enriched in iron and incompatible elements. The Longwan ultramafic-mafic suite is best interpreted as the remnants of a 3.45-Gyr-old enriched mantle plume. The first emergence of mantle-plume-related rocks on the Earth 3.5-3.45?billion years ago indicates that a global mantle plume event occurred with the onset of large-scale deep mantle convection in the Palaeoarchaean. Various deep mantle sources of these Palaeoarchaean mantle-plume-related rocks imply that significant compositional heterogeneity was present in the Palaeoarchaean deep mantle, most probably introduced by recycled crustal material.
Abstract: Synthetic diamonds have inspired much interest for their unique photophysical properties and versatile potential applications, but their phosphorescent phenomenon and mechanism have been paid much less attention. Here, phosphorescent diamonds with a lifetime of 5.4?s were synthesized by high-pressure and high-temperature method, and the diamonds exhibit an emission band at around 468?nm under the excitation wavelength of 230?nm. The quantum yield of the phosphorescent diamonds is about 4.7% at ambient temperature and atmosphere, which is the first report on the quantum yield of diamonds. The unique phosphorescence emission can be attributed to the radiative recombination from iron related donors and boron related acceptors.
Diamond & Related Materials, Vol. 81, pp. 161-167.
Technology
microdiamonds
Abstract: Polylactic acid (PLA)-based composites filled with 20 or 50 ?m-diameter microdiamond are synthesized by hot pressing. Through improving the interface compatibility between the filler and the matrix enabled by octadecylamine (ODA) coating on the microdiamond particles, the maximum thermal conductivity of the composites is 2.22 Wm? 1 K? 1, which is a ~ 10-fold increase in comparison with that of pure PLA. According to the analysis on the glass transmission of the composites and the surface chemistry of the fillers using DSC, FI-IR, and Raman microscopy, it is found out that ODA is connected with the -OH group on the microdiamond surface through hydrogen bonding and an interfacial structure of PLA/ODA/microdiamond is formed. Thus, the interfacial thermal transport between PLA and microdiamond is significantly improved, leading to the enhancement of the thermal conductivity of the composites. Our work presents a simple method to modify the surface chemistry of microdiamond and to improve the interface compatibility between microdiamond and PLA. The microdiamond/PLA composites with large thermal conductivity are promising thermal management materials used for modern electronic products.
Abstract: Carbonatitic magmatism plays a significant role in Earth's carbon cycle, which is also a lithoprobe of crust-mantle interaction, mantle metasomatism and partial melting. Due to different mineral assemblages and geochemical compositions, and diverse tectonic settings, the origin of carbonatite has long been debated. At subduction zones, sediments (including carbonates) are subducted into the mantle with the downgoing oceanic slab. However, the detailed mechanism of how subducted carbonates contribute to carbonatitic magmatism remains unclear. Here we present geochronological, geochemical and isotopic study on the Taohuala Mountain carbonatite at the southern margin of the Alxa Block, North China Craton. The classification of carbonatite from the Taohuala Mountain relies strongly on the observations of obvious intrusion contact relationships and flow structures in field outcrop. The Taohuala Mountain carbonatite has SiO2 ranging from 2.37 wt.% to 11.45 wt%, high CaO (45.93-53.86 wt%) and low MgO (0.51-4.39 wt%), and is characterized by enrichment of LILE (Ba, Sr), depletion of HFSE (Nb, Ta, Zr, Hf), and slightly negative Ce and Eu anomalies. Carbonates in the samples have high 87Sr/86Sr (0.70686-0.70694) and low 143Nd/144Nd (0.511635-0.511924). Remarkably, the highly fractionated ?18OVSMOW (11.83-25.92‰) indicates components of both sedimentary and mantle origin. Detailed zircon in situ U-Pb dating and oxygen isotope analysis exhibit contrast ages and ?18OVSMOW from core to rim, i.e., old ages (mainly > 800 Ma), high Th/U (mainly > 0.5) and low ?18OVSMOW (6.37-11.44‰) in cores (inherited), whereas young ages (~ 400 Ma), low Th/U (mainly < 0.01) and high ?18OVSMOW (20.04-24.54‰) in rims, suggesting that the Taohuala Mountain carbonatite may have been generated from melting of subducted sedimentary carbonates. Considering all these evidences, and that the collision along Qilian Mountains was older than the carbonatite, we propose that a large volume of sedimentary carbonates subducted and remained in the lithospheric mantle under the Alxa block during the closure of the Paleo-Qilian Ocean. Subsequently, the carbonatite was formed by melting of carbonates with minor contributions from the mantle during the breakoff or rollback of the Paleo-Asian oceanic slab.
Abstract: Carbonatites and related alkaline rocks host most REE resources. Phosphate minerals, e.g., apatite and monazite, commonly occur as the main REE-host in carbonatites and have been used for tracing magmatic and mineralization processes. Many carbonatite intrusions undergo metamorphic and/or metasomatic modification after emplacement; however, the effects of such secondary events are controversial. In this study, the Miaoya and Shaxiongdong carbonatite-alkaline complexes, in the South Qinling Belt of Central China, are selected to unravel their magmatic and hydrothermal remobilization histories. Both the complexes are accompanied by Nb-REE mineralization and contain apatite and monazite-(Ce) as the major REE carriers. Apatite grains from the two complexes commonly show typical replacement textures related to fluid metasomatism, due to coupled dissolution-reprecipitation. The altered apatite domains, which contain abundant monazite-(Ce) inclusions or are locally surrounded by fine-grained monazite-(Ce), have average REE concentrations lower than primary apatite. These monazite-(Ce) inclusions and fine-grained monazite-(Ce) grains are proposed to have formed by the leaching REE from primary apatite grains during fluid metasomatism. A second type of monazite-(Ce), not spatially associated with apatite, shows porous textures and zoning under BSE imaging. Spot analyses of these monazite-(Ce) grains have variable U-Th-Pb ages of 210-410 Ma and show a peak age of 230 Ma, which is significantly younger than the emplacement age (440-430 Ma) but is roughly synchronous with a regionally metamorphic event related to the collision between the North China Craton and Yangtze Block along the Mianlue suture. However, in situ LA-MC-ICP-MS analyses of those grains show that they have initial Nd values same as those of magmatic apatite and whole rock. We suggest these monazite-(Ce) grains crystallized from the early Silurian carbonatites and have been partially or fully modified during a Triassic metamorphic event, partially resetting U-Pb ages over a wide range. Mass-balance calculations, based on mass proportions and the REE contents of monazite-(Ce) and apatite, demonstrate that the quantity of metasomatized early Silurian monazite-(Ce) is far higher than the proportion of monazite-(Ce) resulting from the metasomatic alteration of the apatite. Therefore, Triassic metamorphic events largely reset the U-Th-Pb isotopic system of the primary monazite-(Ce) and apatite but only had limited or local effects on REE remobilization in the carbonatite-alkaline complexes in the South Qinling Belt. Such scenarios may be widely applicable for other carbonatite and hydrothermal systems.
Petrological and mineralogical characteristics of kimberlitic rocks in Bachu County, Sinkiang Uighur Chin a and comparison with some other kimberliticoccurren
Bulletin. Xian Institute Geol. and Mineral Resources, Chinese Acad. of Geol., Vol. 15, pp. 47-56
Abstract: Carbon is one of the most important elements on Earth. It is the basis of life, it is stored and mobilized throughout the Earth from core to crust and it is the basis of the energy sources that are vital to human civilization. This issue will focus on the origins of carbon on Earth, the roles played by large-scale catastrophic carbon perturbations in mass extinctions, the movement and distribution of carbon in large igneous provinces, and the role carbon plays in icehouse-greenhouse climate transitions in deep time. Present-day carbon fluxes on Earth are changing rapidly, and it is of utmost importance that scientists understand Earth's carbon cycle to secure a sustainable future.
Geochemistry and origin of the Proterozoic kimberlites, ultramafic and ultrapotassic magmatic rocks from Indravati Basin in Bastar Craton, central India.
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 94-97.
Khanna, T.C., Subba Rao, D.V., Bizimis, M., Satyanarayanan, M., Krishna, A.K., SeshaSai, V.V.
~2.1 Ga intraoceanic magmatism in the central India tectonic zone: constraints from the petrogenesis of ferropicrites in the Mahakoshal suprarcustal belt.
Abstract: Petrographic, mineral chemical and whole-rock geochemical characteristics of two newly discovered lamproitic dykes (Dyke 1 and Dyke 2) from the Sidhi Gneissic Complex (SGC), Central India are presented here. Both these dykes have almost similar sequence of mineral-textural patterns indicative of: (1) an early cumulate forming event in a deeper magma chamber where megacrystic/large size phenocrysts of phlogopites have crystallized along with subordinate amount of olivine and clinopyroxene; (2) crystallization at shallow crustal levels promoted fine-grained phlogopite, K-feldspar, calcite and Fe-Ti oxides in the groundmass; (3) dyke emplacement related quench texture (plumose K-feldspar, acicular phlogopites) and finally (4) post emplacement autometasomatism by hydrothermal fluids which percolated as micro-veins and altered the mafic phases. Phlogopite phenocrysts often display resorption textures together with growth zoning indicating that during their crystallization equilibrium at the crystal-melt interface fluctuated multiple times probably due to incremental addition or chaotic dynamic self mixing of the lamproitic magma. Carbonate aggregates as late stage melt segregation are common in both these dykes, however their micro-xenolithic forms suggest that assimilation with a plutonic carbonatite body also played a key role in enhancing the carbonatitic nature of these dykes. Geochemically both dykes are ultrapotassic (K2O/Na2O: 3.0 -9.4) with low CaO, Al2O3 and Na2O content and high SiO2 (53.3 -55.6 wt.%) and K2O/Al2O3 ratio (0.51 -0.89) characterizing them as high-silica lamproites. Inspite of these similarities, many other features indicate that both these dykes have evolved independently from two distinct magmas. In dyke 1, phlogopite composition has evolved towards the minette trend (Al-enrichment) from a differentiated parental magma having low MgO, Ni and Cr content; whereas in dyke 2, phlogopite composition shows an evolutionary affinity towards the lamproite trend (Al-depletion) and crystallized from a more primitive magma having high MgO, Ni and Cr content. Whole-rock trace-elements signatures like enriched LREE, LILE, negative Nb-Ta and positive Pb anomalies; high Rb/Sr, Th/La, Ba/Nb, and low Ba/Rb, Sm/La, Nb/U ratios in both dykes indicate that their parental magmas were sourced from a subduction modified garnet facies mantle containing phlogopite. From various evidences it is proposed that the petrogenesis of studied lamproitic dykes stand out to be an example for the lamproite magma which attained a carbonatitic character and undergone diverse chemical evolution in response to parental melt composition, storage at deep crustal level and autometasomatism.
Noble gas isotopes in minerals from phoscorites and carbonatites in Kovdor and Seblyavr ultramafic alkaline complexes ( Kola alkaline province NW Russia).
Periodico di Mineralogia, (in english), Vol. LXX11, 1. April, pp. 135-146.
Abstract: This study reports major, trace, rare earth and platinum group element compositions of lava flows from the Vempalle Formation of Cuddapah Basin through an integrated petrological and geochemical approach to address mantle conditions, magma generation processes and tectonic regimes involved in their formation. Six flows have been identified on the basis of morphological features and systematic three-tier arrangement of vesicular-entablature-colonnade zones. Petrographically, the studied flows are porphyritic basalts with plagioclase and clinopyroxene representing dominant phenocrystal phases. Major and trace element characteristics reflect moderate magmatic differentiation and fractional crystallization of tholeiitic magmas. Chondrite-normalized REE patterns corroborate pronounced LREE/HREE fractionation with LREE enrichment over MREE and HREE. Primitive mantle normalized trace element abundances are marked by LILE-LREE enrichment with relative HFSE depletion collectively conforming to intraplate magmatism with contributions from sub-continental lithospheric mantle (SCLM) and extensive melt-crust interaction. PGE compositions of Vempalle lavas attest to early sulphur-saturated nature of magmas with pronounced sulphide fractionation, while PPGE enrichment over IPGE and higher Pd/Ir ratios accord to the role of a metasomatized lithospheric mantle in the genesis of the lava flows. HFSE-REE-PGE systematics invoke heterogeneous mantle sources comprising depleted asthenospheric MORB type components combined with plume type melts. HFSE-REE variations account for polybaric melting at variable depths ranging from garnet to spinel lherzolite compositional domains of mantle. Intraplate tectonic setting for the Vempalle flows with P-MORB affinity is further substantiated by (i) their origin from a rising mantle plume trapping depleted asthenospheric MORB mantle during ascent, (ii) interaction between plume-derived melts and SCLM, (iii) their rift-controlled intrabasinal emplacement through Archean-Proterozoic cratonic blocks in a subduction-unrelated ocean-continent transition zone (OCTZ). The present study is significant in light of the evolution of Cuddapah basin in the global tectonic framework in terms of its association with Antarctica, plume incubation, lithospheric melting and thinning, asthenospheric infiltration collectively affecting the rifted margin of eastern Dharwar Craton and serving as precursors to supercontinent disintegration.
Yellowknife Forum NWTgeoscience.ca, abstract Volume p. 5.
Canada, Northwest Territories
deposit - Curiousity
Abstract: The Curiosity Property, located in the Slave Province to the southwest of Contwoyto Lake, is situated ~25 kilometers north of the Ekati Diamond Mine’s mineral rights. This newly acquired property hosts a known diamondiferous kimberlite, called “LI-201”, which was originally discovered in a 1997 diamond drill campaign. Multiple attempts have been made over the past twenty years to delineate the extent of the body using an assortment of traditional exploration methods, yet LI-201 continues to remain poorly understood in terms of its overall dimensions and diamond-bearing potential. As part of a ten-day exploration program in August 2019, 275 geochemical till samples and 170 biogeological samples were collected. Geochemical sampling along 100-meter spaced fences that are down-ice and approximately perpendicular to the main ice-flow direction were collected in an attempt to further prioritize key geophysical targets in the project area surrounding LI-201. In the vicinity of LI-201, geochemical and biogeological samples were collected as a pilot study in an attempt to investigate the potential microbial community’s response to the presence of kimberlite and to determine if a discernable relationship exists between soil geochemistry and microbial populations. Despite the inconclusive understanding of the kimberlitic body, historical samples of LI-201 show apparent geochemical endowment and bode well for the prospectivity of the project area as a whole. Currently, efforts are being made to compile, verify, and interpret historical data, in addition to integrating newly collected data and interpretations. At the time of presenting, only preliminary geochemical results will be available; microbiological results are pending. In the future, findings from this study will be used to assess the effectiveness of the microbiological method as a means of detecting the known footprint of LI-201, which may also offer insights to the true footprint of the kimberlitic body.
Yellowknife Forum NWTgeoscience.ca, abstract Volume p. 5.
Canada, Northwest Territories
deposit - Curiousity
Abstract: The Curiosity Property, located in the Slave Province to the southwest of Contwoyto Lake, is situated ~25 kilometers north of the Ekati Diamond Mine’s mineral rights. This newly acquired property hosts a known diamondiferous kimberlite, called “LI-201”, which was originally discovered in a 1997 diamond drill campaign. Multiple attempts have been made over the past twenty years to delineate the extent of the body using an assortment of traditional exploration methods, yet LI-201 continues to remain poorly understood in terms of its overall dimensions and diamond-bearing potential. As part of a ten-day exploration program in August 2019, 275 geochemical till samples and 170 biogeological samples were collected. Geochemical sampling along 100-meter spaced fences that are down-ice and approximately perpendicular to the main ice-flow direction were collected in an attempt to further prioritize key geophysical targets in the project area surrounding LI-201. In the vicinity of LI-201, geochemical and biogeological samples were collected as a pilot study in an attempt to investigate the potential microbial community’s response to the presence of kimberlite and to determine if a discernable relationship exists between soil geochemistry and microbial populations. Despite the inconclusive understanding of the kimberlitic body, historical samples of LI-201 show apparent geochemical endowment and bode well for the prospectivity of the project area as a whole. Currently, efforts are being made to compile, verify, and interpret historical data, in addition to integrating newly collected data and interpretations. At the time of presenting, only preliminary geochemical results will be available; microbiological results are pending. In the future, findings from this study will be used to assess the effectiveness of the microbiological method as a means of detecting the known footprint of LI-201, which may also offer insights to the true footprint of the kimberlitic body.
The case for Greenfields Renaissance.... not specific to diamonds but overview ' increases in commodity prices alone are not enough for poor quality deposits'
Contributions to Mineralogy and Petrology, 176, 16p. Pdf
Mantle
geothermobarometry
Abstract: The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25-4.5 GPa and 1100-1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ? 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10-100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule (lnDNigrt/olv;NigrtNiolv), the Ca mole fraction in garnet (XCagrt; Ca/(Ca?+?Fe?+?Mg)), and the Cr mole fraction in garnet (XCrgrt; Cr/(Cr?+?Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as: T(?C)=?8254.568((XCagrt×3.023)+(XCrgrt×2.307)+(lnDNigrtolv?2.639))?273±55 where DNigrt/olv=NigrtNiolv, Ni is in ppm, XCagrt = Ca/(Ca?+?Fe?+?Mg) in garnet, and XCrgrt= Cr/(Cr?+?Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541-554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.
Contributions to Mineralogy and Petrology, Vol. 176, 10.1007/s0041 0-020-01768-z 21p. Pdf
Mantle
geothermometry
Abstract: The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541-554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3-7 GPa, and 1100-1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene (acpxCaCrTs), and Cr/(Cr?+?Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure: P(GPa)=11.03+(?T(K) ln(acpxCaCrTs)×0.001088)+(1.526×ln(Cr#cpxT(K))) where Cr#cpx=(CrCr+Al), acpxCaCrTs=Cr?0.81?Cr#cpx?(Na+K), with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene. Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4-6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2-7 GPa, and 900-1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere?asthenosphere boundary compared with the original NT00 calibration.
Abstract: Major and trace element geochemistry of eclogitic mineral inclusions from Chinese diamonds are reported in this study, for the first time. Bulk major element compositions of mantle eclogite, estimated from diamond inclusions, are very close to that of MORB. All the analyzed samples exhibit evident positive Eu anomalies. Estimated bulk trace element compositions of mantle eclogite are generally parallel to that of MORB, but with deviations like enrichment in LILE and depletion in HFSE. It is proposed that the formation of mantle eclogite could be closely related to recycling of ancient oceanic crust. Other processes like (1) metasomatism by incompatible trace element rich melts; or (2) remelting and interaction with mantle peridotite, may also be involved. Coexisting of olivine with eclogitic mineral inclusions in a same diamond host, and evident trace element variations in some mineral inclusions show that some diamonds were formed by disequilibrium growth.
Development of a Built in scanning near field microscope head for an atomic force microscope system and its application to natural polycrystalline diamonds
International Mineralogical Association 19th. General Meeting, held Kobe, Japan July 23-28 2006, Abstract p. 114.
Physics and Chemistry of Minerals, Vol. 45, 3, pp. 237-247.
Technology
xenolths
Abstract: We determined elastic constants of a single-crystal chromian spinel at temperatures from ?15 to 45 °C through the Rectangular Parallelepiped Resonance method. The sample is a natural chromian spinel, which was separated from a mantle xenolith. Elastic constants at an ambient temperature (T = 24.0 °C) are C 11 = 264.8(1.7) GPa, C 12 = 154.5(1.8) GPa and C 44 = 142.6(0.3) GPa. All the elastic constants decrease linearly with increasing temperature. The temperature derivatives are dC 11/dT = ?0.049(2) GPa/°K, dC 12/dT = ?0.019(1) GPa/°K and dC 44/dT = ?0.020(1) GPa/°K. As an implication of the elastic constants, we applied them to the correction of a fluid inclusion geobarometry, which utilizes residual pressure of fluid inclusion as a depth scale. Before entrainment by a magma, the fluid inclusions must have the identical fluid density in constituent minerals of a xenolith. It has been, however, pointed out that fluid density of fluid inclusions significantly varies with host mineral species. The present study elucidates that elastic constants and thermal expansion coefficients cannot explain the difference in fluid density among mineral species. The density difference would reflect the difference in the degree of plastic deformation in the minerals.
Journal of Asian Earth Sciences, Vol. 188, 26p. Pdf
Russia, Siberia
carbonatite
Abstract: The Cambrian Kharly alkaline plutonic complex composed mainly of foidolite and nepheline syenite makes up a small intrusive field in the Sangilen Plateau in Tuva (southern Siberia). The rocks show large ranges of major oxides (38-58 wt% SiO2; 1-18 wt% Na2O + K2O; 11-28 wt% Al2O3; 1.5-20 wt% CaO; 0.1-8 wt% MgO; 2-12 wt% Fe2O3) controlled by variable percentages of minerals: clinopyroxenes, calcic amphiboles, micas, nepheline and feldspars. Alkaline rocks are cut by carbonatite veins composed of predominant calcite coexisting with femic minerals (10-15% of aegirine-ferrosalite-hedenbergite, sodic-calcic amphiboles, ferrobiotite, Ti-garnet), Na-K feldspar and nepheline (up to 15-20%), fluorapatite (up to 20-25%), Sr-apatite, and accessory carbocernaite, titanite, Ti-magnetite and ilmenite. Carbonatites (4057-8859 ppm Sr, 426-1901 ppm Ba (Sr/Ba ? 2), 290-980 ppm REE + Y, 2 to 100 ppm Zr, and 0.5 to 15 ppm Nb) possibly originated at high (?500-650 °C) temperatures as a result of liquid immiscibility. The isotope systematics of rocks and minerals (?Nd(t) from ~2.9 to 6.5; 207Pb/206Pbin = 0.89; 208Pb/206Pbin = 2.15; 87Sr/86Sr(t) = 0.70567-0.70733, ?18OV-SMOW ? 7.2-19.5‰, and ?13CV-PDB from ?6.0 to ?1.4‰) suggest mixing of PREMA and EM 1 material during magma generation and crustal contamination of the evolving melts. The rocks bear signatures of interaction with “magmatic-equilibrated” fluids or heated meteoric waters. LILE/HFSE ratios indicate mixed magma sources that involved the material of IAB and OIB, as well as a crustal component, possibly, due to interaction of a mantle plume with rock complexes on the active continental margin.
Geological Society of London, Special Publication no. 446 on line available
Global
Diatreme model
Abstract: We report here a growth model for phreatomagmatic maar-diatreme volcanoes with respect to the number of eruptions documented in the tephra beds of maar tephra rings and the upper bedded diatreme facies. We show that the number of tephra beds in large diatremes is larger than that in maar tephra rings. Base surges that lack sufficient momentum to scale high maar crater walls deposit their tephra only inside the crater. Thus the total number of eruptions at large maar-diatreme volcanoes will be larger than the number recorded in maar tephra rings. As many maar-diatreme volcanoes erupt dominantly accidental clasts, an incremental mathematical model was applied to study the growth of diatremes. The model is based only on the ejection of distinct amounts of accidental clasts per unit eruption and the chosen number of eruptions is assumed to be identical. The incremental growth of cone-shaped diatremes follows cube-root functions with respect to diameter and depth and slows down with ongoing eruptions. In nature, small and large maar-diatreme volcanoes are formed and filled syn-eruptively, mostly by tephra, depending on the duration and quantity of magma involved in phreatomagmatic eruptions. In our opinion, this mathematical model is the only current method able to model the growth of diatremes.
Geological Society of London, Special Publication: Monogenetic volcanism, No. 446, pp. 29-59.
Technology
diatremes
Abstract: We report here a growth model for phreatomagmatic maar-diatreme volcanoes with respect to the number of eruptions documented in the tephra beds of maar tephra rings and the upper bedded diatreme facies. We show that the number of tephra beds in large diatremes is larger than that in maar tephra rings. Base surges that lack sufficient momentum to scale high maar crater walls deposit their tephra only inside the crater. Thus the total number of eruptions at large maar-diatreme volcanoes will be larger than the number recorded in maar tephra rings. As many maar-diatreme volcanoes erupt dominantly accidental clasts, an incremental mathematical model was applied to study the growth of diatremes. The model is based only on the ejection of distinct amounts of accidental clasts per unit eruption and the chosen number of eruptions is assumed to be identical. The incremental growth of cone-shaped diatremes follows cube-root functions with respect to diameter and depth and slows down with ongoing eruptions. In nature, small and large maar-diatreme volcanoes are formed and filled syn-eruptively, mostly by tephra, depending on the duration and quantity of magma involved in phreatomagmatic eruptions. In our opinion, this mathematical model is the only current method able to model the growth of diatremes.
Geological Society of London, Special Publication no. 446 on line available
Global
Diatreme model
Abstract: We report here a growth model for phreatomagmatic maar-diatreme volcanoes with respect to the number of eruptions documented in the tephra beds of maar tephra rings and the upper bedded diatreme facies. We show that the number of tephra beds in large diatremes is larger than that in maar tephra rings. Base surges that lack sufficient momentum to scale high maar crater walls deposit their tephra only inside the crater. Thus the total number of eruptions at large maar-diatreme volcanoes will be larger than the number recorded in maar tephra rings. As many maar-diatreme volcanoes erupt dominantly accidental clasts, an incremental mathematical model was applied to study the growth of diatremes. The model is based only on the ejection of distinct amounts of accidental clasts per unit eruption and the chosen number of eruptions is assumed to be identical. The incremental growth of cone-shaped diatremes follows cube-root functions with respect to diameter and depth and slows down with ongoing eruptions. In nature, small and large maar-diatreme volcanoes are formed and filled syn-eruptively, mostly by tephra, depending on the duration and quantity of magma involved in phreatomagmatic eruptions. In our opinion, this mathematical model is the only current method able to model the growth of diatremes.
Geological Society of London, Special Publication: Monogenetic volcanism, No. 446, pp. 29-59.
Technology
diatremes
Abstract: We report here a growth model for phreatomagmatic maar-diatreme volcanoes with respect to the number of eruptions documented in the tephra beds of maar tephra rings and the upper bedded diatreme facies. We show that the number of tephra beds in large diatremes is larger than that in maar tephra rings. Base surges that lack sufficient momentum to scale high maar crater walls deposit their tephra only inside the crater. Thus the total number of eruptions at large maar-diatreme volcanoes will be larger than the number recorded in maar tephra rings. As many maar-diatreme volcanoes erupt dominantly accidental clasts, an incremental mathematical model was applied to study the growth of diatremes. The model is based only on the ejection of distinct amounts of accidental clasts per unit eruption and the chosen number of eruptions is assumed to be identical. The incremental growth of cone-shaped diatremes follows cube-root functions with respect to diameter and depth and slows down with ongoing eruptions. In nature, small and large maar-diatreme volcanoes are formed and filled syn-eruptively, mostly by tephra, depending on the duration and quantity of magma involved in phreatomagmatic eruptions. In our opinion, this mathematical model is the only current method able to model the growth of diatremes.
IN: Nemeth, K., Carrasco-Nunez, G., Aranda-Gomez, J.J., Smith, I.E.M. eds. Monogenetic volcanism GSL Special Volume, Vol 446, 31p. Pdf * note date
Europe, Germany , United States, Australia, Mexico
maars
Abstract: We report here a growth model for phreatomagmatic maar-diatreme volcanoes with respect to the number of eruptions documented in the tephra beds of maar tephra rings and the upper bedded diatreme facies. We show that the number of tephra beds in large diatremes is larger than that in maar tephra rings. Base surges that lack sufficient momentum to scale high maar crater walls deposit their tephra only inside the crater. Thus the total number of eruptions at large maar-diatreme volcanoes will be larger than the number recorded in maar tephra rings. As many maar-diatreme volcanoes erupt dominantly accidental clasts, an incremental mathematical model was applied to study the growth of diatremes. The model is based only on the ejection of distinct amounts of accidental clasts per unit eruption and the chosen number of eruptions is assumed to be identical. The incremental growth of cone-shaped diatremes follows cube-root functions with respect to diameter and depth and slows down with ongoing eruptions. In nature, small and large maar-diatreme volcanoes are formed and filled syn-eruptively, mostly by tephra, depending on the duration and quantity of magma involved in phreatomagmatic eruptions. In our opinion, this mathematical model is the only current method able to model the growth of diatremes.
Abstract: The paper reports the results of an experimental study of phase relations and distribution of elements in silicate melt-salt melt systems (carbonate, phosphate, fluoride, chloride), silicate melt I - silicate melt II, and fluid-magmatic systems in the presence of alkali metal fluorides. Extraction of a number of ore elements (Y, REE, Sr, Ba, Ti, Nb, Zr, Ta, W, Mo, Pb) by salt components was studied in liquid immiscibility processes within a wide temperature range of 800-1250°? and pressure of 1-5.5 kbar. It is shown that partition coefficients are sufficient for concentration of ore elements in amounts necessary for the genesis of ore deposits. In a fluid-saturated trachyrhyolite melt, the separation into two silicate liquids has been determined. The partition coefficients of a number of elements (Sr, La, Nb, Fe, Cr, Mo, K, Rb, Cs) between phases L1 and L2 have been obtained. The interaction processes of a heterophase fluid in the granite (quartz)-ore mineral-heterophase fluid (Li, Na, K-fluoride) system were studied at 650-850°C and P = 1 kbar. The formation of the phase of a highly alkaline fluid-saturated silicate melt concentrating Ta and Nb is shown as a result of the interaction of the fluid with rock and ore minerals.
International Geology Review, Vol. 58, 12, pp. 1461-1480.
India
Carbonatite
Abstract: The South Indian Granulite Terrane (SGT) is a collage of Archaean to Neoproterozoic age granulite facies blocks that are sutured by an anastomosing network of large-scale shear systems. Besides several Neoproterozoic carbonatite complexes emplaced within the Archaean granulites, there are also smaller Paleoproterozoic (2.4 Ga, Hogenakkal) carbonatite intrusions within two NE-trending pyroxenite dikes. The Hogenakkal carbonatites, further discriminated into sövite and silicate sövite, have high Sr and Ba contents and extreme light rare earth element (LREE) enrichment with steep slopes typical of carbonatites. The C- and O-isotopic ratios [?13CVPDB = ?6.7 to ?5.8‰ and ?18OVSMOW = 7.5-8.7‰ except a single 18O-enriched sample (?18O = 20.0‰)] represent unmodified mantle compositions. The ?Nd values indicate two groupings for the Hogenakkal carbonatites; most samples show positive ?Nd values, close to CHUR (?Nd = ?0.35 to 2.94) and named high-?Nd group while the low-?Nd group samples show negative values (?5.69 to ?8.86), corresponding to depleted and enriched source components, respectively. The 87Sr/86Sri ratios of the two groups also can be distinguished: the high-?Nd ones have low 87Sr/86Sri ratios (0.70161-0.70244) while the low-?Nd group shows higher ratios (0.70247-0.70319). We consider the Nd-Sr ratios as primary and infer derivation from a heterogeneous mantle source. The emplacement of the Hogenakkal carbonatites may be related to Paleoproterozoic plume induced large-scale rifting and fracturing related to initiation of break-up of the Neoarchean supercontinent Kenorland.
Abstract: A team of researchers recently discovered an ancient relic hidden within Earth: a tectonic plate resting beneath the southern Indian Ocean. Scientists have found other tectonic plates that sank below Eurasia and North America, but here Simmons et al. describe the unique structure of this newly discovered slab, which they named the Southeast Indian Slab (SEIS). The slab has at least one feature scientists have rarely seen before: It maintains its slab-like structure all the way from the upper mantle near Earth’s crust down to the region where the mantle meets the planet’s superheated core. The Farallon plate beneath North America is a well-known example of this—but it was expected to exist and sank much more recently than the SEIS. In addition, not only does the SEIS traverse the entire mantle, but it also becomes more vertical along one end, so much so that it stands almost vertically between the crust and core along the eastern edge, whereas the western portion is more horizontal. Researchers can make out structures beneath Earth’s crust by examining the speed at which seismic waves generated by earthquakes and similar Earth-shattering events—known as P and S waves—travel through Earth. Here the researchers used wave data from 12,607 seismic events dating back to the 1960s, collected by 7783 seismic stations around the world, to develop the model that identified the ancient slab. Once this tectonic slab was identified, the team looked at the region’s tectonic history over millions of years to determine where and when this plate was on the surface. They determined that the slab was once along the eastern portion of the early supercontinent of Gondwana. Then, sometime during the Triassic or Jurassic period, which stretched from 250 million years ago to 145 million years ago, the slab plunged underneath another plate. They further concluded that the subduction, or the sinking of the Southeast Indian Slab beneath another plate, terminated around 130 to 140 million years ago in the Mesozoic era, around the same time that the tectonic plates under eastern Gondwana began to separate and split up the continent. Tectonic plates usually sink down into the mantle at a rate of about 1 centimeter per year or more; they don’t necessarily melt but instead bunch up at the base of the mantle and eventually assimilate or become undetectable as their temperature increases. However, if the researchers accurately estimated the timing of their newly discovered slab’s subduction, this slab must have stalled in a transition zone before descending deeper down into the mantle, allowing the slab to persist in the mantle longer than any other known plate.
New age determinations of central Colorado Plateau laccoliths, Utah:recognizing disturbed K-Ar systematics and re-evaluating tectonomagmaticrelationships.
Geological Society of America Bulletin, Vol. 194, No. 12, December pp. 1547-1560.
Abstract: The petrochemistry of carbonatites of three formation types were studied: (1) ultrahigh-pressure garnet-containing carbonatites (UHPC) of the Caledonian sheet (Tromsö, Norway); (2) rocks of the carbonatite-lkaline-ultrabasic Kovdor massif (the Kola Peninsula); and (3) rocks of the carbonatite-alkaline-gabbroid Tikshozero massif (north of Karelia). The samples of carbonatites were examined and tested with a microprobe; the microelements were determined using the ICP-MS technique at the Institute of Microelectronics Technology and High Purity Materials (Chernogolovka). The carbonatites of the Kovdor and Tikshozero massifs are characterized by similar negative REE trends, with a degree of REE enrichment of the Tikshozero carbonatites. The UHPC from Tromsö are different from those of the Kovdor and Tikshozero massifs in the negative trend along with lower concentrations of light REEs. The Tromsö UHPC are similar to the carbonatites of the Kovdor and Tikshozero massifs in the trend and concentrations of heavy REEs. The carbonatites of the Fennoscandian shield of various formation times and types are characterized by the geochemical similarity to those in different regions of the world with the sources associated to mantle plumes. This similarity might be caused by the formation of the mantle carbonated magmas of carbonatite-containing igneous complexes from a mantle source enriched under either mantle metasomatism or plume-lithosphere interaction, with similar mechanisms of formation. The appearance of the formations as such within a wide time interval points to the long-term occurrence of a superplume at the Fennoscandian shield and to permanent activation of the related processes of magma formation.
Abstract: Impact diamonds are technical material with valuable mechanical properties. Despite of a quite long story from their discovery and huge diamond storages at the Popigai astrobleme (Siberia, Russia) they were not involved into industrial production, first of all because of remoteness of objects, complexity of extraction and economically more favourable synthesis of technical diamonds in the seventies of the past century. However, due to the high hardness of impact diamonds and also to the high demand of new carbon materials, including nanomaterials, the interest towards this type of natural diamonds is significantly increased in the recent years. Although the mentioned Popigai astrobleme is situated in a remote part of Russia it has been studied in more details. At the same time, the less known Kara giant meteorite crater (Pay-Khoy, Russia) is situated essentially closer to the industrial infrastructure of the European part of Russia. This astrobleme, similarly to Popigai, is enriched in impact diamonds as well. But, till recent years it was not deeply studied using modern analytical methods. During our studies in 2015 and 2017 at the territory of the Kara meteorite crater we have distinguished and described 5 varieties of impactites - bulk melt impactites which form cover-like and thick dike bodies; melt ultrahigh-pressure vein bodies and at least 3 types of suevites formed after specific sedimentary target rocks. These varieties have typomorphic features regarding the crystallinity and mineral composition. It was found that all of them have high concentration of microdiamonds formed by high-pressure high temperature pyrolysis mechanism from precursor materials like coal and organic relicts. Using a set of modern mineralogical methods we have found two principal types of diamond morphologies within the Kara impactites - sugar-like after coal diamonds and diamond paramorphs after organic relicts. The Kara diamonds have several accompanying carbon substances including newly formed graphite, glass-like carbon and probably carbyne. The studied diamondiferous Kara impactites provide an essentially novel knowledge of impact processes in sedimentary targets.
Geochemistry, Geophysics, Geosystems, Vol. 20, 2, pp. 952-973.
United States, New Mexico
xenoliths
Abstract: Elemental and isotopic compositions of volatile species such as halogens, noble gases, hydrogen, and carbon can be used to trace the evolution of these species in the Earth. Halogens are important tracers of subduction recycling of surface volatiles into the mantle: however, there is only limited understanding of halogens in the mantle. Here we provide new halogen data of mantle xenoliths from intraplate settings. The mantle xenoliths show a wide range of halogen elemental ratios, which are expected to be related to later processes after the xenoliths formed. A similar primary halogen component is present in the xenoliths sampled from different localities. This suggests that the mantle has the uniform halogen composition over a wide scale. The halogen composition in the convecting mantle is expected to have remained constant over more than 2 billion years, despite subduction of iodine?rich halogens. We used mass balance calculations to gain understanding into evolution rate of I/Cl ratio in the mantle. Calculations suggest that, in order to maintain the I/Cl ratio of the mantle over 2 Gyr, the I/Cl ratio of the subducted halogens must be no more than several times higher than the present?day mantle value.
Abstract: The Cretaceous Juína Kimberlite Province (JKP, 95-92 Ma) is located in the southwest of the Amazonian Craton, northwest of Mato Grosso, Brazil. Here we present new geochemical and isotopic data of garnet (n=187) and zircon (n=25) megacrysts collected from the KS2 kimberlite. The magmatic zircon megacrysts have U-Pb ages of 92.1 ± 0.7 Ma. The chondrite-normalized rare earth element (REE) patterns (LREE
Africa, South Africa, Europe, Greenland, China, Russia, Siberia, Canada, South America, Brazil
subduction, metasomatism
Abstract: To investigate halogen heterogeneity in the subcontinental lithospheric mantle (SCLM), we measured the concentrations of Cl, Br, and I in kimberlites and their mantle xenoliths from South Africa, Greenland, China, Siberia, Canada, and Brazil. The samples can be classified into two groups based on halogen ratios: a high-I/Br group (South Africa, Greenland, Brazil, and Canada) and a low-I/Br group (China and Siberia). The halogen compositions were examined with the indices of crustal contamination using Sr and Nd isotopes and incompatible trace elements. The results indicate that the difference between the two groups was not due to different degrees of crustal contamination but from the contributions of different mantle sources. The low-I/Br group has a similar halogen composition to seawater-influenced materials such as fluids in altered oceanic basalts and eclogites and fluids associated with halite precipitation from seawater. We conclude that the halogens of the high-I/Br group are most likely derived from a SCLM source metasomatized by a fluid derived from subducted serpentinite, whereas those of the low-I/Br group are derived from a SCLM source metasomatized by a fluid derived from seawater-altered oceanic crust. The SCLM beneath Siberia and China could be an important reservoir of subducted, seawater-derived halogens, while such role of SCLM beneath South Africa, Greenland, Canada, and Brazil seems limited.
Diamond & Related Materials, Vol. 83, pp. 104-108.
Technology
luminescence
Abstract: The luminescence of triboplasma during diamond polishing was investigated. The main luminescence in the ultraviolet range came from N2 molecules in the air. The colors of the visible range of triboplasma were the same as those observed in the photoluminescence images, excited by the ultraviolet light. The color of the triboplasma luminescence was green for type Ib diamond, which was mainly from the H3 center. The blue luminescence for type IIa diamond was mainly from Band A. The correlation between the diamond temperature and periphery speed indicate that that the mechanical abrasion component also increased linearly. However the polishing rate showed a threshold at the periphery speed of 26?km/h which corresponds well with the threshold of the triboplasma generation. These results imply that the electrical and optical energy of the triboplasma excited the defect level at the diamond surface and enhanced the chemical polishing rate of the diamond.
Journal of African Earth Sciences, Vol. 100, pp. 20-41.
Africa, Namibia
Geothermometry
Abstract: Intracontinental deformation accommodated along major lithospheric scale shear zone systems and within associated extensional basins has been well documented within West, Central and East Africa during the Late Cretaceous. The nature of this deformation has been established by studies of the tectonic architecture of sedimentary basins preserved in this part of Africa. In southern Africa, where the post break-up history has been dominated by major erosion, little evidence for post-break-up tectonics has been preserved in the onshore geology. Here we present the results of 38 new apatite fission track analyses from the Damara region of northern Namibia and integrate these new data with our previous results that were focused on specific regions or sections only to comprehensively document the thermo-tectonic history of this region since continental break-up in the Early Cretaceous. The apatite fission track ages range from 449 ± 20 Ma to 59 ± 3 Ma, with mean confined track lengths between 14.61 ± 0.1 ?m (SD 0.95 ?m) to 10.83 ± 0.33 ?m (SD 2.84 ?m). The youngest ages (c. 80–60 Ma) yield the longest mean track lengths, and combined with their spatial distribution, indicate major cooling during the latest Cretaceous. A simple numerical thermal model is used to demonstrate that this cooling is consistent with the combined effects of heating caused by magmatic underplating, related to the Paraná-Etendeka continental flood volcanism associated with rifting and the opening of the South Atlantic, and enhanced erosion caused by major reactivation of major lithospheric structures within southern Africa during a key period of plate kinematic change that occurred in the South Atlantic and SW Indian ocean basins between 87 and 56 Ma. This phase of intraplate tectonism in northern Namibia, focused in discrete structurally defined zones, is coeval with similar phases elsewhere in Africa and suggests some form of trans-continental linkage between these lithospheric zones.
Abstract: The last of a group of dense minerals that make up much of Earth’s crust and upper mantle has been found tucked inside a meteorite that slammed into Australia 135 years ago. The newly discovered mineral, a variety of majorite, is potentially abundant in sinking tectonic plates and could help illuminate the behavior of the deep Earth, its discoverers say. ach identical component of this mineral contains 32 magnesium atoms, 32 silicon atoms and 96 oxygen atoms arranged in a distorted cube. Natural samples of MgSiO3 tetragonal garnet, the mineral’s scientific moniker, had eluded scientists since the mineral was first artificially produced in 1985. aotaka Tomioka, a mineralogist at the Kochi Institute for Core Sample Research in Japan, and colleagues discovered 0.5-micrometer-wide grains of the mineral in a slice of the 19th century meteorite. While many minerals found in meteorites form when slamming into Earth, the new mineral formed in space when two asteroids collided at a relative speed of about 2 kilometers per second, the researchers report online March 25 in Science Advances. ne challenge remains for the researchers: As discoverers of the mineral, they now get to name it.
Abstract: The oxygen fugacity of the upper mantle is 3-4 orders of magnitude higher than that of the lower mantle and this has been attributed to Fe2 + disproportionating into Fe3 + plus Fe0 at pressures > 24 GPa. The upper mantle might therefore have been expected to have evolved to more oxidizing compositions through geological time, but it appears that the oxygen fugacity of the upper mantle has remained constant for the last 3.5 billion years. Thus, it indicates that the mantle has been actively buffered from the accumulation of Fe3 +, and that this is linked to oxidation of diamond to carbonate coupled with reduction of Fe3 + to Fe2 +. When subducted plates penetrate into the lower mantle, compensational upwelling transports bridgmanite into the transition zone, where it breaks down to ringwoodite and majorite, releasing the ferric iron. The system returns to equilibrium through oxidation of diamond. Early in Earth history, diamond may have been enriched at the base of the transition zone in the Magma Ocean, because it is denser than peridotite melts at depths shallower than 660 km, and it is more buoyant below. Ongoing oxidation of diamond forms carbonate, leading to relatively high carbonate concentrations in the source of ocean island basalts.
Earth and Planetary Science Letters, Vol. 494, 1, pp. 92-98.
Mantle
water
Abstract: In this study, we present new experimental constraints on the phase stability and thermal equation of state of an important hydrous phase, ?-AlOOH, using synchrotron X-ray diffraction up to 142 GPa and 2500 K. Our experimental results have shown that ?-AlOOH remains stable at the whole mantle pressure-temperature conditions above the D? layer yet will decompose at the core-mantle boundary because of a dramatic increase in temperature from the silicate mantle to the metallic outer core. At the bottom transition zone and top lower mantle, the formation of ?-AlOOH by the decomposition of phase Egg is associated with a ?2.1-2.5% increase in density (?) and a ?19.7-20.4% increase in bulk sound velocity (V?). The increase in ? across the phase Egg to ?-AlOOH phase transition can facilitate the subduction of ?-AlOOH to the lower mantle. Compared to major lower-mantle phases, ?-AlOOH has the lowest ? but greatest V?, leading to an anomalous low ? /V? ratio which can help to identify the potential presence of ?-AlOOH in the region. More importantly, water released from the breakdown of ?-AlOOH at the core-mantle boundary could lower the solidus of the pyrolitic mantle to cause partial melting and/or react with Fe in the region to form the low-velocity FeO2Hx phase. The presence of partial melting and/or the accumulation of FeO2Hx phase at the CMB could be the cause for the ultra-low velocity zone. ?-AlOOH is thus an important phase to transport water to the lowermost mantle and helps to understand the origin of the ultra-low velocity zone.
Earth and planetary Science Letters, Vol. 579, 117343, 11p. Pdf
Australia
geophysics- seismics
Abstract: Plate tectonics, including rifting, subduction, and collision processes, was likely to have been different in the past due to the secular cooling of the Earth. The northeastern part of the West Australian Craton (WAC) has a complex Archean and Paleoproterozoic tectonic history; therefore, it provides an opportunity to study how subduction and collision processes evolved during the emergence of plate tectonics, particularly regarding the assembly of Earth's first supercontinent, Columbia. Because the northeastern boundary of the WAC and the southwestern boundary of the North Australian Craton (NAC) are covered by the Phanerozoic Canning Basin, the regional tectonic evolution has remained enigmatic, including how many tectonic elements were assembled and what may have driven rifting and subsequent collision events. Here, we use new passive-source seismic modeling to identify a seismically distinct segment of the lithosphere, the Percival Lakes Province, which lies east of the Pilbara Craton and is separated by two previously unknown southeast-trending lithosphere scale Paleoproterozoic sutures. We interpret that the northeastern suture, separates the Percival Lakes Province from the NAC, records the amalgamation of the WAC with the NAC. The southwestern suture separates the PLP from the reworked northeastern margin of the Pilbara Craton, including the East Pilbara Terrane and the Rudall Province. A significant upper mantle dipping structure was identified in the southwestern suture, and we interpret it to be a relic of subduction that records a previously unknown Paleoproterozoic collision that pre-dated the amalgamation of the WAC and NAC. By comparing our findings with previously documented dipping features, we show that the Paleoproterozoic collisions are seismically distinguishable from their Phanerozoic counterparts.
The importance of crystal chemistry on REE partitioning between mantle minerals ( garnet, clinopyroxene, orthopyroxene, and olivine) and basaltic melts.
Earth and Planetary Science Letters, Vol. 454, pp. 103-112.
Mantle, Africa, Morocco
Melting
Abstract: Pyroxenites are often documented among exhumed mantle rocks, and can be found in most tectonic environments, from supra-subduction to sub-continental and sub-oceanic mantle. In particular, websterites, i.e. orthopyroxene-clinopyroxene bearing pyroxenites, are found in parallel layers in most orogenic and ophiolitic peridotites. Their formation is often ascribed to melt infiltration and melt-rock reaction processes accompanied by variable amount of deformation. One outstanding question is whether the ubiquitous occurrence of layered websterites in exhumed rocks is generally linked to the exhumation process or truly represents large-scale melt infiltration processes at depth prior to exhumation. These two hypotheses can be distinguished by comparing the exhumation and formation ages of the websterites. However, determination of the layered websterite formation age is challenging. Here we present a novel approach to constrain the formation age of websterite layers using samples from the Lherz massif (France), where layered websterites and lherzolites have formed through melt-rock reaction. By combining high-resolution REE variations, isotope model ages, and diffusive re-equilibration timescales using REE closure temperatures across the websterite layers, we constrain a minimum age and a maximum age for the formation of layered websterites. We show that layered websterites in Lherz formed 1,500-1,800 Ma ago, and are thus clearly disconnected from the process of exhumation at 104 Ma. Multiple generations of layered websterites commonly found in ultramafic massifs, along with the evidence for ancient melt-rock reaction in Lherz, indicate that melt-rock reactions can happen episodically or continuously in the mantle and that layered websterites found in exhumed mantle rocks record ubiquitous melt infiltration processes in the mantle.
Earth and Planetary Science Letters, Vol. 506, pp. 38-52.
Mantle
kimberlite genesis
Abstract: Low-degree partial melts from deeply subducted, carbonated ocean crust are carbonatite liquids with ?35-47 wt% CO2. Their reactions with the overlying mantle regulate the slab-mantle interaction and carbon transport in the deep upper mantle but have not been investigated systematically. Here we present new multi-anvil experiments and parameterized phase relation models to constrain the fate of slab-derived carbonatite melts in the upper mantle. The experiments were conducted at 7 GPa/1400 °C and 10 GPa/1450 °C, and used starting compositions mimicking the ambient mantle infiltrated by variable carbonatite fluxes (0-45 wt%) from the slab surface. Kimberlitic melts (CO2 = 14-32 wt%, SiO2 = 15-33 wt%, and MgO = 20-29 wt%) were produced from experiments with 5.8-25.6 wt% carbonatite influxes. Experimental phase relations demonstrate a reactive melting process in which the carbonatite influx increases in proportion by dissolution of olivine, orthopyroxene, garnet and precipitation of clinopyroxene. This manifests a feasible mechanism for slab-derived carbonatite melts to efficiently transport in the ambient mantle through high-porosity channels. The melt and mineral fractions from this study and previous phase equilibria experiments in peridotite + O systems were empirically parameterized as functions of temperature (900-2000 °C), pressure (3-20 GPa), and bulk compositions (e.g., CO2 = 0.9-17.1 wt% and Na2O + K2O = 0.27-2.51 wt%). Applications of the phase relation models to prescribed melting processes indicate that reactive melting of a carbonatite-fluxed mantle source could produce kimberlitic melts with diverse residual lithologies under various melting conditions. However, reactive melting at the slab-mantle interface can only commence when the slab-released carbonatite melt conquers the carbonation freezing front, i.e., the peridotite solidi suppressed by infiltration of CO2-rich melts in an open system. Depending on temperatures and local influxes, reactive melting and carbonation/redox freezing can occur simultaneously above the slab-mantle interface, yielding heterogeneous lithologies and redox conditions as well as various time-scales of carbon transport in Earth's mantle.
Abstract: Earth’s status as the only life-sustaining planet is a result of the timing and delivery mechanism of carbon (C), nitrogen (N), sulfur (S), and hydrogen (H). On the basis of their isotopic signatures, terrestrial volatiles are thought to have derived from carbonaceous chondrites, while the isotopic compositions of nonvolatile major and trace elements suggest that enstatite chondrite-like materials are the primary building blocks of Earth. However, the C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which rules out volatile delivery by a chondritic late veneer. In addition, if delivered during the main phase of Earth’s accretion, then, owing to the greater siderophile (metal loving) nature of C relative to N, core formation should have left behind a subchondritic C/N ratio in the BSE. Here, we present high pressure-temperature experiments to constrain the fate of mixed C-N-S volatiles during core-mantle segregation in the planetary embryo magma oceans and show that C becomes much less siderophile in N-bearing and S-rich alloys, while the siderophile character of N remains largely unaffected in the presence of S. Using the new data and inverse Monte Carlo simulations, we show that the impact of a Mars-sized planet, having minimal contributions from carbonaceous chondrite-like material and coinciding with the Moon-forming event, can be the source of major volatiles in the BSE.
Earth and Planetary Letters, Vol. 550, 116549, 13p.
Global, United States, Wyoming, Canada, Northwest Territories, Europe, Baltic, India
geothermometry
Abstract: Cratonic lithosphere is believed to have been chemically buoyant and mechanically resistant to destruction over billions of years. Yet the absence of cratonic roots at some Archean terrains casts doubt on the craton stability and longevity on a global scale. As unique mantle-derived melts at ancient continents, silica-poor, kimberlitic melts are ideal tools to constrain the temporal variation of lithosphere thickness and the processes affecting the lithosphere root. However, no reliable thermobarometer exists to date for strongly silica-undersaturated, mantle-derived melts. Here we develop a new thermobarometer for silica-poor, CO2-rich melts using high-temperature, high-pressure experimental data. Our barometer is calibrated based on a new observation of pressure-dependent variation of Al2O3 in partial melts saturated with garnet and olivine, while our thermometer is calibrated based on the well-known olivine-melt Mg-exchange. For applications to natural magmas, we also establish a correction scheme to estimate their primary melt compositions. Applying this liquid-based thermobarometer to the estimated primary melt compositions for a global kimberlite dataset, we show that the equilibration depths between primary kimberlite melts and mantle peridotites indicate a decrease of up to ?150 km in cratonic lithosphere thickness globally during the past ?2 Gyr. Together with the temporal coupling between global kimberlite frequency and cold subduction flux since ?2 Gyr ago, our results imply a causal link between lithosphere thinning and supply of CO2-rich melts enhanced by deep subduction of carbonated oceanic crusts. While hibernating at the lithosphere root, these melts chemically metasomatize and rheologically weaken the rigid lithosphere and consequently facilitate destruction through convective removal in the ambient mantle or thermo-magmatic erosion during mantle plume activities.
Earth and Planetary Science Letters, Vol. 496, pp. 80-88.
Mantle
perovskite, hotspots
Abstract: Mineralogical studies indicate that two major phase transitions occur near the depth of 660 km in the Earth's pyrolitic mantle: the ringwoodite (Rw) to perovskite (Pv) + magnesiowüstite (Mw) and the majorite (Mj) to perovskite (Pv) phase transitions. Seismological results also show a complicated phase boundary structure at this depth in plume regions. However, previous geodynamical modeling has mainly focused on the effects of the Rw-Pv+Mw phase transition on plume dynamics and has largely neglected the effects of the Mj-Pv phase transition. Here, we develop a 3-D regional spherical geodynamic model to study the combined influence of these two phase transitions on plume dynamics. Our results show the following: (1) A double phase boundary occurs in the high-temperature center of the plume, corresponding to the double reflections in seismic observations. Other plume regions feature a single, flat uplifted phase boundary, causing a gap of high seismic velocity anomalies. (2) Large amounts of relatively low-temperature plume materials can be trapped in the transition zone due to the combined effects of phase transitions, forming a complex truncated cone shape. (3) The Mj-Pv phase transition greatly enhances the plume penetration capability through 660-km phase boundary, which has a significant influence on the plume dynamics. Our results provide new insights which can be used to better constrain the 660-km discontinuity variations, seismic wave velocity structure and plume dynamics in the mantle transition zone. The model can also help to estimate the mantle temperature and Clapeyron slopes at the 660 km phase boundary.
Repeated kimberlite magmatism beneath Yakutia and its relationship to Siberian flood volcanism: insights from in situ U-Pb and Sr-Nd perovskite isotope analysis.
Earth and Planetary Science Letters, Vol. 404, Oct. pp. 283-295.
Abstract: Comprehensive studies of zircon xenocrysts from kimberlites of the Kuoika field (northeastern Siberian craton) and several kimberlite fields of the eastern Anabar shield, along with data compilation on the age of kimberlite-hosting terranes, reveal details of the evolution of the northern Siberian craton. The age distribution and trace element characteristic of zircons from the Kuoika field kimberlites (Birekte terrane) provide evidence of significant basic and alkaline-carbonatite magmatism in northern Siberia in the Paleozoic and Mesozoic periods. The abundance of 1.8-2.1 Ga zircons in both the Birekte and adjacent Hapchan terranes (the latter hosting kimberlites of the eastern Anabar shield) supports the Paleoproterozoic assembly and stabilization of these units in the Siberian craton and the supercontinent Columbia. The abundance of Archean zircons in the Hapchan terrane reflects the input of an ancient source other than the Birekte terrane and addresses the evolution of the terrane to west (Magan and Daldyn terranes of the Anabar shield). The present study has also revealed the oldest known remnant of the Anabar shield crust, whose 3.62 Ga age is similar to that of the other ancient domain of Siberia, the Aldan shield. The first Hf isotope data for the Anabar shield coupled with the U-Pb systematics indicate three stages of crustal growth (Paleoproterozoic, Neoarchean and Paleoarchean) and two stages of the intensive crustal recycling in the Paleoproterozoic and Neoarchean. Intensive reworking of the existing crust at 2.5-2.8 Ga and 1.8-2.1 Ga is interpreted to provide evidence for the assembly of Columbia. The oldest Hf model age estimation provides a link to Early Eoarchean (3.7-3.95 Ga) and possibly to Hadean crust. Hence, some of the Archean cratonic segments of the Siberian craton could be remnants of the Earth's earliest continental crust.
Abstract: The characteristics of the sub-continental lithospheric mantle (SCLM) post-date the Siberian plume event (250 Ma) is still unclear; nearly all published data for mantle xenoliths are from a single kimberlite erupt before he Siberian plume (Udachnaya). We report major elements of the whole rock, trace elements data of clinopyroxene and Re-Os isotope and PGE concentration of mantle xenoliths from the Obnazhennaya kimberlite pipe (160 Ma). The Obnazhennaya mantle xenoliths, including spinel harzburgites, spinel dunites, spinel lherzolites, spinel-garnet lherzolite. The spinel harzburgites and dunites have refractory compositions, with 0.23-1.35 wt.% Al2O3, 0.41-3.11 wt.% CaO and 0.00-0.09 wt.% TiO2. Clinopyroxenes in harzburgites and dunites have lower Na2O but higher Cr2O3 contents. Modeling of the Y and Yb contents in clinopyroxenes indicates that the spinel harzburgites and dunites have been subjected to ca. 12-17% degrees of partial melting. The spinel harzburgites and dunites have 187Os/188Os of 0.11227-0.11637, giving a TRD age of 1.6-2.2 Ga. This suggests that old cratonic mantle still existed beneath the Obnazhennaya. In contrast, the lherzolites (both spinel- and spinel-garnet-) have more fertile compositions, containing 2.16-6.55 wt.% Al2O3, 2.91-7.55 wt.% CaO and 0.04-0.15 wt.% TiO2. Both spinel and spinelgarnet lherzolites have more radiogenic 187Os/188Os ratios (0.11931-0.17627), enriched P-PGEs. The higher Al2O3 and Os content and depleted IPGE character of these lherzolites suggest that they were not juvenile mantle accreted by Siberian mantle plume but the refertilized ancient mantle. Therefore, our result suggest that the cratonic mantle beneath the Obnazhennaya has not been replaced by juvenile mantle during the Siberian mantle plume.
Abstract: The characteristics of the sub-continental lithospheric mantle (SCLM) post-date the Siberian plume event (250 Ma) is still unclear; nearly all published data for mantle xenoliths are from a single kimberlite erupt before he Siberian plume (Udachnaya). We report major elements of the whole rock, trace elements data of clinopyroxene and Re-Os isotope and PGE concentration of mantle xenoliths from the Obnazhennaya kimberlite pipe (160 Ma). The Obnazhennaya mantle xenoliths, including spinel harzburgites, spinel dunites, spinel lherzolites, spinel-garnet lherzolite. The spinel harzburgites and dunites have refractory compositions, with 0.23-1.35 wt.% Al2O3, 0.41-3.11 wt.% CaO and 0.00-0.09 wt.% TiO2. Clinopyroxenes in harzburgites and dunites have lower Na2O but higher Cr2O3 contents. Modeling of the Y and Yb contents in clinopyroxenes indicates that the spinel harzburgites and dunites have been subjected to ca. 12-17% degrees of partial melting. The spinel harzburgites and dunites have 187Os/188Os of 0.11227-0.11637, giving a TRD age of 1.6-2.2 Ga. This suggests that old cratonic mantle still existed beneath the Obnazhennaya. In contrast, the lherzolites (both spinel- and spinel-garnet-) have more fertile compositions, containing 2.16-6.55 wt.% Al2O3, 2.91-7.55 wt.% CaO and 0.04-0.15 wt.% TiO2. Both spinel and spinelgarnet lherzolites have more radiogenic 187Os/188Os ratios (0.11931-0.17627), enriched P-PGEs. The higher Al2O3 and Os content and depleted IPGE character of these lherzolites suggest that they were not juvenile mantle accreted by Siberian mantle plume but the refertilized ancient mantle. Therefore, our result suggest that the cratonic mantle beneath the Obnazhennaya has not been replaced by juvenile mantle during the Siberian mantle plume.
Abstract: A comprehensive, internally consistent U-Pb and Lu-Hf isotope data set for 93 mantle-derived zircons from the Yakutian kimberlite province confirms and further refines the four major episodes of kimberlite magmatism on the Siberian craton: 421-409?Ma (Late Silurian-Early Devonian), 358-353?Ma (Late Devonian-Early Carboniferous), 226-218?Ma (Late Triassic), and 161-144?Ma (Middle-Late Jurassic). The relatively narrow, constant range of ?Hf values between +2 and +10 for both the Paleozoic and Mesozoic mantle-derived zircons (and by inference kimberlites) suggests that the volatile-rich magmas were repeatedly sourced from the convecting upper mantle beneath the Siberian craton. This finding is in keeping with the narrow and constant range of ?Nd values for groundmass perovskites from the Yakutian kimberlite province between +1.8 and +5.5 between 420 and 150?Ma. Our preferred model implies that the convecting upper mantle beneath the Yakutian kimberlite province ‘recovered’ rapidly back to ambient conditions shortly after the giant plume-related flood volcanic event that produced the Siberian Traps at 250?Ma. Although close spatial relationships exist between kimberlites and flood basalts on the Siberian craton during both the Paleozoic and Mesozoic, exact timing of the igneous events and the isotopic compositions of the diverse deep-sourced melting products rule out any direct genetic links.Besides the highly economic kimberlite-hosted diamond deposits of Late Devonian age (e.g., Mir and Udachnaya), the Siberian craton also contains significant Mesozoic placer diamond deposits (e.g., along the Anabar river), for which lamproite sources have been suggested recently. Our study shows that mantle-derived zircon megacryst fragments from the Ebelyakh placer deposit have Late Triassic ages of ca. 224?Ma. Their long-term depleted Hf isotopic compositions (+8.5 ?Hf) suggest that the alluvial diamonds were sourced from asthenosphere-derived Triassic kimberlites rather than from lithospheric mantle derived isotopically enriched lamproites.
Abstract: A comprehensive, internally consistent U-Pb and Lu-Hf isotope data set for 93 mantle-derived zircons from the Yakutian kimberlite province confirms and further refines the four major episodes of kimberlite magmatism on the Siberian craton: 421-409?Ma (Late Silurian-Early Devonian), 358-353?Ma (Late Devonian-Early Carboniferous), 226-218?Ma (Late Triassic), and 161-144?Ma (Middle-Late Jurassic). The relatively narrow, constant range of ?Hf values between +2 and +10 for both the Paleozoic and Mesozoic mantle-derived zircons (and by inference kimberlites) suggests that the volatile-rich magmas were repeatedly sourced from the convecting upper mantle beneath the Siberian craton. This finding is in keeping with the narrow and constant range of ?Nd values for groundmass perovskites from the Yakutian kimberlite province between +1.8 and +5.5 between 420 and 150?Ma. Our preferred model implies that the convecting upper mantle beneath the Yakutian kimberlite province ‘recovered’ rapidly back to ambient conditions shortly after the giant plume-related flood volcanic event that produced the Siberian Traps at 250?Ma. Although close spatial relationships exist between kimberlites and flood basalts on the Siberian craton during both the Paleozoic and Mesozoic, exact timing of the igneous events and the isotopic compositions of the diverse deep-sourced melting products rule out any direct genetic links. Besides the highly economic kimberlite-hosted diamond deposits of Late Devonian age (e.g., Mir and Udachnaya), the Siberian craton also contains significant Mesozoic placer diamond deposits (e.g., along the Anabar river), for which lamproite sources have been suggested recently. Our study shows that mantle-derived zircon megacryst fragments from the Ebelyakh placer deposit have Late Triassic ages of ca. 224?Ma. Their long-term depleted Hf isotopic compositions (+8.5 ?Hf) suggest that the alluvial diamonds were sourced from asthenosphere-derived Triassic kimberlites rather than from lithospheric mantle derived isotopically enriched lamproites.
Abstract: The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterization of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, to unravel the timing and nature of mantle metasomatism, and its link to the formation of low-volume melts (e.g. kimberlites) and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognized. The first type comprises lherzolites, harzburgites and dunites, some phlogopite-bearing, which occur from ?100-170?km depth. They form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to c. 150?Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet breakdown, texturally evident by pyroxene-spinel assemblages occupying former coarse grains and compositionally evident by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, heavy rare earth elements) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidizing melts percolated upwards and metasomatized the shallower lithospheric mantle, which is composed of phlogopite-bearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene-breakdown. This is the second type of lithology, which occurs at ?90-110?km depth and is inferred to have highly depleted protoliths. This type is compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. Whereas low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidizing mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590-550?Ma extended to >210?km depth, and suggests loss of ?40?km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval (?90-110?km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton.
Abstract: A comprehensive, internally consistent U-Pb and Lu-Hf isotope data set for 93 mantle-derived zircons from the Yakutian kimberlite province confirms and further refines the four major episodes of kimberlite magmatism on the Siberian craton: 421-409?Ma (Late Silurian-Early Devonian), 358-353?Ma (Late Devonian-Early Carboniferous), 226-218?Ma (Late Triassic), and 161-144?Ma (Middle-Late Jurassic). The relatively narrow, constant range of ?Hf values between +2 and +10 for both the Paleozoic and Mesozoic mantle-derived zircons (and by inference kimberlites) suggests that the volatile-rich magmas were repeatedly sourced from the convecting upper mantle beneath the Siberian craton. This finding is in keeping with the narrow and constant range of ?Nd values for groundmass perovskites from the Yakutian kimberlite province between +1.8 and +5.5 between 420 and 150?Ma. Our preferred model implies that the convecting upper mantle beneath the Yakutian kimberlite province ‘recovered’ rapidly back to ambient conditions shortly after the giant plume-related flood volcanic event that produced the Siberian Traps at 250?Ma. Although close spatial relationships exist between kimberlites and flood basalts on the Siberian craton during both the Paleozoic and Mesozoic, exact timing of the igneous events and the isotopic compositions of the diverse deep-sourced melting products rule out any direct genetic links. Besides the highly economic kimberlite-hosted diamond deposits of Late Devonian age (e.g., Mir and Udachnaya), the Siberian craton also contains significant Mesozoic placer diamond deposits (e.g., along the Anabar river), for which lamproite sources have been suggested recently. Our study shows that mantle-derived zircon megacryst fragments from the Ebelyakh placer deposit have Late Triassic ages of ca. 224?Ma. Their long-term depleted Hf isotopic compositions (+8.5 ?Hf) suggest that the alluvial diamonds were sourced from asthenosphere-derived Triassic kimberlites rather than from lithospheric mantle derived isotopically enriched lamproites.
Contributions to Mineralogy and Petrology, Vol. 174, 23p.
Europe, Greenland
metasomatism
Abstract: We report highly siderophile element (HSE) abundances and Re-Os isotope compositions, obtained by isotope dilution induc-tively coupled plasma mass spectrometry, of olivine separates from a suite of multiply metasomatised peridotite xenoliths entrained in kimberlites from SW Greenland. Combined with petrographic and compositional observations on accessory base metal sulphides (BMS), the results reveal new insights into the chemical, physical and mineralogical effects of multi-stage rifting and associated melt percolation on the Archaean lithospheric mantle. Refertilised lherzolites are dominated by rare to frequent small (tens of µm) BMS inclusions in olivine, whereas modally metasomatised phlogopite-bearing lherzolite and wehrlites have higher proportions of more Ni-rich BMS, including abundant large interstitial grains (hundreds of µm). The olivine separates display depleted HSE systematics with Primitive Upper Mantle (PUM)-normalised Pd/Ir of 0.014-0.62, and have both depleted and enriched 187 Os/ 188 Os (0.1139-0.2724) relative to chondrite that are not correlated with 187 Re/ 188 Os. Four out of ten olivine separates retain similarly depleted Os corresponding to Re-depletion model ages of 2.1-1.8 Ga. They may reflect Palaeoproterozoic refertilisation (lherzolitisation) during Laurentia plate assembly, with re-introduction of clinopyroxene and Os-rich BMS into the originally refractory mantle lithosphere by asthenosphere-derived basaltic melts, followed by recrystallisation and occlusion in olivine. Unradiogenic Os is observed regardless of lithology, including from peridotites that contain abundant interstitial BMS. This reflects addition of Os-poor BMS (<< 1 ppm) during more recent wehrlitisation and phlogopite-introduction, and control of the Os isotopic signature by older Os-rich BMS that precipitated from the basaltic melt. Depletions in compatible HSE (< 0.5 × PUM for Ru, Ir, Os) in all, but one olivine separate reflect nugget effects (amount of depleted vs. metasomatic BMS inclusions) and/or loss due to sulphide dissolution into oxidising small-volume melts that invaded the lithosphere during recurrent rifting, the latter supported by similar depletions in published bulk peridotite data. Combined, these multiple metasomatic events destroyed all vestiges of Mesoarchaean or older inheritance in the olivine separates investigated here, and highlight that caution is needed when interpreting Proterozoic Os model ages in terms of Proterozoic lithosphere stabilisation.
Contributions to Mineralogy and Petrology, Vol. 175, 22p. Pdf.
Russia
deposit - Obnazhennaya
Abstract: The petrology, mineral major and trace-element concentrations, and garnet oxygen isotopic composition of low-MgO (11-16 wt%) eclogites from the Obnazhennaya kimberlite, Siberian craton, are used to infer their petrogenesis. These eclogites contain two types of compositionally distinct garnet: granular coarse garnet, and garnet exsolution (lamellae and fine-grained garnet) in clinopyroxene. The former record higher temperatures at lower pressures than the latter, which record the last stage of equilibrium at moderate pressure-temperature conditions 2.3-3.7 GPa and 855-1095 °C in the upper mantle at the time of entrainment. Although derived from the garnet stability field, these rocks have low-pressure cumulate protoliths containing plagioclase, olivine, and clinopyroxene as reflected by pronounced positive Eu and Sr anomalies in all eclogites, and low heavy rare earth element (HREE) contents in both minerals and reconstructed bulk rocks for a number of samples. Major elements, transition metals, and the HREE compositions of the reconstructed whole rocks are analogous to modern oceanic gabbro cumulates. Despite geochemical signatures supporting an oceanic crust origin, mantle-like ?18O of the garnets (5.07-5.62‰) for most samples indicates that the protoliths either did not interact with seawater or have coincidently approximately normal igneous values. Some of the eclogite xenoliths have lower SiO2 contents and depleted light REE ((Nd/Yb)N?
Abstract: This study concerns the geochemical characteristics of mantle xenoliths from the upper-Jurassic Obnazhennaya kimberlite pipe (Kuoika field, Yakutian kimberlite province, the north-east of Siberian craton). The so-called magnesian xenolith group (Sp, Sp-Grt, Grt lherzolites, olivine websterites and websterites) was distinguished, the rocks of the group are assumed to be of the same genesis based on transitions in modal mineral composition and a change in the composition of minerals. The chemical composition (CaO, MgO) of most depleted harzburgites, as well as part of the lherzolites of the magnesian group coincide with the restites obtained by experimental melting, which suggested their residue origin. Narrow variations in the composition of olivine (Mg # - 91-92; NiO - 0.35-0.45 wt.%) and orthopyroxene (Mg # - 92-93) for Obnazhennaya peridotites also support this hypothesis. In terms of chemical composition, olivines coincide with the “mantle trend” of olivines from the lithospheric mantle. Nevertheless, garnets from the peridotites consistently change their composition in the direction of decreasing Cr2O3, CaO and Mg # values from Grt, Sp-Grt lherzolites to Grt websterites. The garnet composition from Obnazhennaya peridotites differs from Udachnaya peridotites, for which the residue hypothesis assumed. They are similar in composition to garnets from Beni-Bousera garnet pyroxenites, as well as to garnets from deformed lherzolites of the Udachnaya pipe, which suggests crystallization of garnets from the melt and the effect of metasomatic processes. The formation of orogenic massifs is a multi-stage process, many authors suggest that pyroxenite veins in mafic complexes are cumulative in origin and show signs of metasomatic processes (in particular, enrichment with aluminum, calcium and chromium, increased REE concentrations in garnet). So peridotite cumulative origin and further metasomatic transformations were suggested.
Abstract: The petrology, mineral major and trace element concentrations, and garnet oxygen isotopic composition of low-MgO (11-16 wt.%) eclogites from the Obnazhennaya kimberlite, Siberian craton, are used to infer their petrogenesis. These eclogites equilibrated at moderate pressure-temperature conditions 2.3-3.7 GPa and 855- 1095?C at the time of entrainment. Although derived from the garnet stability field, these rocks have low-pressure cumulate protoliths containing plagioclase, olivine, and clinopyroxene as reflected by pronounced positive Eu and Sr anomalies in all eclogites, and low heavy rare earth element (HREE) contents in both minerals and reconstructed bulk rocks for a number of samples. Major elements, transition metals, and the HREE compositions of the reconstructed whole rocks are analogous to modern oceanic gabbro cumulates. Despite geochemical signatures supporting an oceanic crust origin, mantle-like ?18O of the garnets (5.07-5.62 ‰ ) for most samples indicates that the protoliths either did not interact with seawater or have coincidently approximately normal igneous values. Some of the eclogite xenoliths have lower SiO2 contents and depleted light REE ((Nd/Yb)N ? 1) compared to modern oceanic gabbros, suggesting that they experienced partial melting. Positively inclined middle to heavy-REE patterns ((Dy/Yb)N ?1) of the reconstructed bulk rocks mostly result from repeated partial melting in the eclogite stability field, based on melting model calculations. We therefore suggest that the Obnazhennaya low-MgO eclogites may represent the gabbroic section of subducted or foundered basaltic crust that underwent continued partial melting processes at high pressures where garnet was the main residual phase.
Abstract: Magnetic data are sensitive to both the induced magnetization in rock units caused by the present earth's magnetic field and the remanent magnetization acquired by rock units in past geologic time. Susceptibility is a direct indicator of the magnetic mineral content, whereas remanent magnetization carries information about the formation process and subsequent structural movement of geologic units. The ability to recover and use total magnetization, defined as the vectorial sum of the induced and remanent magnetization, therefore enables us to take full advantage of magnetic data. The exploration geophysics community has achieved significant advances in inverting magnetic data affected by remanent magnetization. It is now feasible to invert any magnetic data set for total magnetization. We provide an overview of the state of the art in magnetization inversion and demonstrate the informational value of inverted magnetization through a set of case studies from mineral exploration problems. We focus on the methods that recover either the magnitude of the total magnetization or the total magnetization vector itself.
Geochemical Perspectives Letters, Vol. 17, pp. 11-15. pdf
Mantle
carbonatites
Abstract: Carbonatite, an unusual carbonate-rich igneous rock, is known to be sourced from the mantle which provides insights into mantle-to-crust carbon transfer. To constrain further the Ca isotopic composition of carbonatites, investigate the behaviour of Ca isotopes during their evolution, and constrain whether recycled carbonates are involved in their source regions, we report ?44/42Ca for 47 worldwide carbonatite and associated silicate rocks using a refined analytical protocol. Our results show that primary carbonatite and associated silicate rocks are rather homogeneous in Ca isotope compositions that are comparable to ?44/42Ca values of basalts, while non-primary carbonatites show detectable ?44/42Ca variations that are correlated to ?13C values. Our finding suggests that Ca isotopes fractionate during late stages of carbonatite evolution, making it a useful tool in the study of carbonatite evolution. The finding also implies that carbonatite is sourced from a mantle source without requiring the involvement of recycled carbonates.
Journal of African Sciences, Vol. 184, 104308, 12p. Pdf
Africa, Tanzania
deposit - Mbalizi
Abstract: The Mbalizi carbonatite is located in the middle of the Paleoproterozoic Ubendian Mobile Belt and the western branch of East Africa Rift, southwestern Tanzania. Calcite, dolomite, phlogopite, pyrochlore and apatite are found in the sample. Mineral chemistry studies have shown that the carbonatite phlogopite is linked to mantle-derived magmatism. The apatite is fluorapatite, means they are of magmatic origin. The analyses on two crystals of pyrochlore show high concentrations of Nb2O5, and therefore the Nb-oxide is classified as pyrochlore subspecies. Three types of zircon have been obtained from the Mbalizi carbonatite, including xenocrysts zircon, igneous zircon and metamorphic zircon. Zircon in-situ LA-ICP-MS U-Pb dating in this contribution indicates that the Mbalizi carbonatite was crystallized at ca. 116.0 ± 1.8 Ma. The ?Hf(t) values of igneous zircon ranging from ?13.9 to +5.7, indicates that the carbonatite parental magma was originated from the sub-continental lithospheric mantle, and evolves toward HIMU and EM. The whole-rock Sr-Nd isotopic data suggest more contribution of the HIMU and EM? material. We propose that the complex evolutionary history of the Ubendian Mobile Belt has stored the subduction oceanic crust which has the EM? and HIMU components, forming the compositional heterogeneity mantle beneath the Ubendian Mobile Belt. At 116.0 ± 1.8 Ma, with the extension stress field, deep faults cause the pressure reduction, resulting in reactive of the upwelling of the HIMU and EM? components. This provides the metamorphic conditions to induce the isotopic resetting and may result in large scatter of initial 176Hf/177Hf ratios of carbonatite melts.
Zhang, H-F., Nakamura, E., Sun, M., Kobayashi,K., Zhang, J., Yang, J-F., Tang, Y-J.
Transformation of subcontinental lithospheric mantle through peridotite melt reaction: evidence from a highly fertile mantle xenolith from the North Chin a Craton.
International Geology Review, Vol. 49, 7, July pp. 658-679.
Zhang, H-F., Goldstein, S.L., Zhou, X-H., Sun, M., Zheng, J-P., Cai, Y.
Evolution of subcontinental lithospheric mantle beneath eastern China: Re-Os isotopic evidence from mantle xenoliths in Paleozoic kimberlites and Mesozoic basalts
Contributions to Mineralogy and Petrology, Vol. 155, pp. 271-293.
The origin of Mengyin and Fuxian Diamondiferous kimberlites from the North Chin a craton: implications for Paleozoic subducted oceanic slab mantle interact
Journal of Asian Earth Sciences, Vol. 37, 5-6, pp. 425-437.
Li, L-M., Sun, M., Wang, Y., Xing, G., Zhao, G., Cai, K., Zhang, Y.
Geochronological and geochemical study of Paleproterozoic gneissic granites and clinopyroxenite xenolths from NW Fujian: implications for crustal evol.
Journal of Asian Earth Sciences, Vol. 41, 2, pp. 204-212.
Earth and Planetary Science Letters, Vol. 503, pp. 108-117.
Africa, South Africa, Zimbabwe
geophysics - seismic
Abstract: The depths of the 410 km (d410) and 660 km (d660) discontinuities beneath southern Africa, which is presumably underlain by the lower-mantle African Superswell, are imaged in 1? radius consecutive circular bins using over 6400 P-to-S receiver functions (RFs) recorded by 130 seismic stations over a 27 yr period. When the IASP91 standard Earth model is utilized for moveout correction and time-depth conversion, a normal mantle transition zone (MTZ) thickness of 246 ± 7 km is observed, suggesting that the Superswell has no discernible effect on mantle transition zone temperature. Based on the negligible disparity of the mean MTZ thicknesses between on (246 ± 6 km) and off (246 ± 8 km) cratonic regions, we conclude that the deep Archean cratonic keels possess limited influence on MTZ thermal structure. The apparently shallower-than-normal MTZ discontinuities and the parallelism between the d410 and d660 are mostly the results of upper mantle high wave speed anomalies probably corresponding to a thick lithosphere with a mean thickness of about 245 km beneath the Kaapvaal and 215 km beneath the Zimbabwe cratons. In contradiction to conclusions from some of the previous studies, the resulting spatial distribution of the stacking amplitudes of the P-to-S converted phases at the discontinuities is inconsistent with the presence of an excessive amount of water in the MTZ and atop the d410.
Geochimica et Cosmochimica Acta, Vol. 303, pp. 92-109, pdf
Mantle
melting
Abstract: The role of water on melting in the mantle wedge is still debated due to large uncertainty on the estimates of H2O flux beneath arcs. B has been proven as an effective proxy for water flux because B and H2O show similar chemical behaviors during subduction. The Habahe mafic dikes from the Chinese Altai were emplaced within a narrow area (<20?km from south to north) during the northward subduction of the Junggar Ocean in the middle Paleozoic. These dikes have been classified into four types with distinct geochemical and Sr-Nd-Hf-Pb isotopic compositions, which originated from mantle sources metasomatized by different subduction components, including melts from subducted sediments (Type-I, Type-IV), fluids from subducted sediments (Type-II), and melts from subducted oceanic crust (Type-III). We present B content and isotope data for the Habahe mafic dikes to investigate the influence of subduction components on melting in the mantle wedge. Type-I and -III mafic dikes all have negative ?11B values (?7.7‰ to ?5.0‰) with variable B contents (3.65-13.4?ppm) and B/Nb ratios (2.10-7.39), indicating B isotopically light features for the subducted sediments and oceanic crust. Type-II mafic dikes have lower B contents (3.97-9.90?ppm) and higher B/Nb ratios (7.07-14.4) than Type-I mafic dikes, with a wide range of ?11B values from ?7.8‰ to ?2.7‰. This suggests that their mantle source may have been metasomatized by fluids from subducted serpentinite besides fluids from subducted sediments. Type-IV mafic dikes have higher B contents (17.0-27.5?ppm) and B/Nb ratios (25.0-40.8), and heavier B isotopic compositions (?11B?=??2.9‰ to +3.5‰) than Type-I mafic dikes. This indicates involvement of fluids from the slab serpentinite in metasomatism of their mantle source in addition to melts from the subducted sediments. The Habahe mafic dikes show wide range of B/Nb ratios, suggesting that different amounts of water were added into their mantle sources. These dikes exhibit variable Zr/Yb and Nb/Yb ratios, and constantly low TiO2/Yb, indicating their formation through different degrees melting of depleted mantle sources. Their Zr/Yb and Nb/Yb ratios are negatively correlated with B/Nb, which reflects elevation of the melting degree of their mantle sources as increasing water input. Similar trends are also observed in basalts from global arcs and their major and trace elements correlate well with B/Nb ratios. Thus, water flux should play an important role on melting in the mantle wedge and control magma compositions of the arcs.
Sun, N., Wei, W., Han, S., Song, J., Li, X., Duan, Y., Prakapenka, V.B., Mao, Z.
Phase transition and thermal equations of state of (Fe, Al) -bridgmanite and post perovskite: implication for the chemical heterogeneity at the lowermost mantle.
Earth Planetary Science Letters, Vol. 490, pp. 161-169.
Earth and Planetary Science Letters, Vol. 490, 1, pp. 161-169.
Mantle
geothermometry
Abstract: In this study, we have determined the phase boundary between Mg0.735Fe0.21Al0.07Si0.965O3-Bm and PPv and the thermal equations of state of both phases up to 202 GPa and 2600 K using synchrotron X-ray diffraction in laser heated diamond anvil cells. Our experimental results have shown that the combined effect of Fe and Al produces a wide two-phase coexistence region with a thickness of 26 GPa (410 km) at 2200 K, and addition of Fe lowers the onset transition pressure to 98 GPa at 2000 K, consistent with previous experimental results. Furthermore, addition of Fe was noted to reduce the density (?) and bulk sound velocity () contrasts across the Bm-PPv phase transition, which is in contrast to the effect of Al. Using the obtained phase diagram and thermal equations of state of Bm and PPv, we have also examined the effect of composition variations on the ? and
profiles of the lowermost mantle. Our modeling results have shown that the pyrolitic lowermost mantle should be highly heterogeneous in composition and temperature laterally to match the observed variations in the depth and seismic signatures of the D? discontinuity. Normal mantle in a pyrolitic composition with ?10% Fe and Al in Bm and PPv will lack clear seismic signature of the D? discontinuity because the broad phase boundary could smooth the velocity contrast between Bm and PPv. On the other hand, Fe-enriched regions close to the cold slabs may show a seismic signature with a change in the velocity slope of the D? discontinuity, consistent with recent seismic observations beneath the eastern Alaska. Only regions depleted in Fe and Al near the cold slabs would show a sharp change in velocity. Fe in such regions could be removed to the outer core by strong core-mantle interactions or partitions together with Al to the high-pressure phases in the subduction mid ocean ridge basalts. Our results thus have profound implication for the composition of the lowermost mantle.
Zhang, Y., Bi, H., Yu, L., Sun, S., Qui, J., Xu, C., Wang, H., Wang, R.
Evidence for metasomatic mantle carbonatitic magma extrusion in Mesoproterozoic ore hosting dolomite rocks in the middle Kunyang rift, central Yunnan China.
Progress in Natural Science, Vol. 18, 8, pp. 965-974.
Abstract: The Paleo-Mesoproterozoic Zhongtiao aulacogen in the North China Craton and Cuddapah basin in the Indian Craton, have both been interpreted as intra-continental rift formed by a mantle plume that led to the breakup of Columbia supercontinent, but the mechanism has not been completely deciphered. In this paper, the mechanism of the Zhongtiao aulacogen and Cuddapah basin related to initial breakup of Columbia has been evaluated with 2D elastic finite element models of the North China Craton and the Indian Craton. The trajectories of the horizontal maximum principal compressive stress of the best-fit model fit well with the trends of dyke swarms in the North China Craton and the Indian Craton. When the other three models generated were compared with the best-fit model, it can be found that a mantle plume beneath the Zhongtiao and Cuddapah areas played the most vital role in developing the Zhongtiao aulacogen, Cuddapah basin and initial breakup of Columbia supercontinent. The boundary subduction forces, including the northern margin of the NCC, the northwest and southwest margins of the Indian Craton are indispensable factors for the rifting and breakup, whereas the mechanical properties have little influence on these modeling results. The initial breakup of Columbia supercontinent might have been resulted from the coupling between a mantle plume upwelling and some plate tectonic forces.
Abstract: Earth’s inner core (IC) is less dense than pure iron, indicating the existence of light elements within it1. Silicon, sulfur, carbon, oxygen and hydrogen have been suggested to be the candidates2,3, and the properties of iron-light-element alloys have been studied to constrain the IC composition4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19. Light elements have a substantial influence on the seismic velocities4,5,6,7,8,9,10,11,12,13, the melting temperatures14,15,16,17 and the thermal conductivities18,19 of iron alloys. However, the state of the light elements in the IC is rarely considered. Here, using ab initio molecular dynamics simulations, we find that hydrogen, oxygen and carbon in hexagonal close-packed iron transform to a superionic state under the IC conditions, showing high diffusion coefficients like a liquid. This suggests that the IC can be in a superionic state rather than a normal solid state. The liquid-like light elements lead to a substantial reduction in the seismic velocities, which approach the seismological observations of the IC20,21. The substantial decrease in shear-wave velocity provides an explanation for the soft IC21. In addition, the light-element convection has a potential influence on the IC seismological structure and magnetic field.
Isotopic and Geochemical Systematics in Tertiary Recent Basalts from Southeastern Australia and Implications for the Evolution of the Subcontinental Lithosphere.
Geochimica et Cosmochimica ACTA., Vol. 49, No. 10, PP. 2051-2067.
Use of geochemistry as a guide to platinum group element potential of mafic ultramafic rocks- examples the West Pilbara block and Halls Creek Mobile Zone:
Precambrian Research, Vol. 50, No. 102, April pp. 1-35
Edwards, D., Rock, N.M.S., Taylor, W.R., Griffin, B.J., Sun, S-S.
The Aries Diamondiferous kimberlite pipe, central Kimberley block, westernAustralia: mineralogy, petrology and geochem. of the pipe rock and indicators
Proceedings of Fifth International Kimberlite Conference held Araxa June 1991, Servico Geologico do Brasil (CPRM) Special, pp. 82-84
Carboniferous and Triassic eclogites in the Western Dabie Mountains east central Chin a: evidence for protracted convergence of the North and South Chin a Blocks.
Journal of Metamorphic Geology, Vol. 20, 9, pp. 873-886.
Abstract: The Bayan Obo deposit in Inner Mongolia, North China Craton (NCC) is the largest rare-earth element (REE) resource in the world. Due to the complex element and mineral compositions and the activity of several geological events, the ore-forming mechanism is still controversial. Previous models are reviewed here to provide information for further investigation on the Bayan Obo deposit. In this study, we summarize all different types of Fe-REE-Nb mineralization using field observations and microscope work, in which we recognize 9 types of Fe-REE-Nb ores in the Bayan Obo ore district. By compiling and re-evaluating a large number of published geochemical data, this paper provides solid evidence that the Bayan Obo deposit formed through interaction between sedimentary rocks and carbonatite magmatism. From the results of our review, it can be conjectured that the formation of iron ores was originated from sedimentation (Pt1), whereas the formation of REE mineralized dolomite might be related to interaction and reaction between the carbonatite magmas and/or associated fluids with sedimentary carbonate rocks, with the REE-bearing carbonatite magmas having undergone intense fractionation enrichment process. The C-O-S-Fe-Mg isotopes indicate that the REE-Nb mineralization was derived from metasomatism (fenitic alteration) of sedimentary carbonate. A new model is proposed for this unique REE-Nb mineralization, which is related to the subduction of Siberian Craton beneath the North China Craton since Early Paleozoic period. We interpret that the Bayan Obo Fe-REE-Nb ore deposits and their massive barren host, H8 dolomite, were generated as a result of interaction of fluids expelled from a subcontinental lithospheric mantle (SCLM)-derived carbonatite magma with sedimentary carbonates.
Abstract: The Bayan Obo ore deposit in Inner Mongolia, North China, the largest-known rare earth element (REE) deposit in the world, is closely associated with carbonatite dykes. Scarce zircon grains, with a wide range of ages and diverse origins, have been extracted from the Wu dyke, a REE-enriched calcitic carbonatite dyke 2?km from the East Ore Body of the Bayan Obo deposit. Three zircon populations were identified based on ages and trace element compositions: 1) Captured zircons with Paleoproterozoic and Archean ages. These zircons have REE patterns and moderate Th/U ratios similar to zircon with silicate inclusions from basement igneous rocks, which have been recognized as contaminants from wall rocks. 2) Carbonatite magmatic zircons with Mesoproterozoic ages. These zircons have high to extremely high Th/U ratios (13-1600), a characteristic signature of the Bayan Obo deposit. Two zircon grains yielded concordant 206Pb/238U ages (1.27?±?0.11?Ga???1.42?±?0.18?Ga) and 208Pb/232Th age (1.26?±?0.20?Ga) with calcite inclusions, indicating that the Wu dyke was emplaced at ca. 1.34?Ga, which coincides with a worldwide generation of Mesoproterozoic kimberlites, lamprophyres, carbonatites, and anorogenic magmatism. 3) Hydrothermal zircons with Caledonian and Triassic ages. The Caledonian zircon has 206Pb/238U age of 381?±?4?Ma and 208Pb/232Th age of 367?±?14?Ma with dolomite inclusion. These evidences are consistent with multiple stages of mineralization, Mesoproterozoic calcite carbonatite magmatism interacted by protracted fluxing of subduction-released Caledonian fluids during the closure of the Palaeo-Asian Ocean, coupled with interaction with the mantle wedge and metasomatism of overlying sedimentary carbonate.
Abstract:
Carbonatitic magmatism plays a significant role in outgassing carbon from mantle and the formation of rare earth element (REE), rare metal (e.g., Nb and Th) and other types of deposits. The mechanism of REE mineralization associated with carbonatite have been widely studied. However, it is hard to understand U-Nb mineralization without Th enrichment associated with carbonatite. Here we report a carbonatite-hosted U-Nb deposit in Huayangchuan, located in the north Qinling Orogenic Belt. Field observation, mineralogy and geochemical analysis on a suite of drillhole samples were conducted to decipher the mineralization mechanism and its relationship with carbonatite. Huayangchuan carbonatite samples mainly consist of calcite and augite with small volume of accessory minerals (e.g., allanite, fluorapatite, barite and celestite). Betafite [(Ca,U)2(Ti,Nb,Ta)2O6(OH)] is the major ore-bearing mineral in Huayangchuan deposit. The carbonatite shows high CaO, low MgO and alkali contents, which should be products to be differentiated from primary carbonatite (high MgO and alkali contents). The immiscibility and crystallization processes could explain the high CaO/(CaO + MgO + FeO) ratios and the enrichment of LILE. Numerical modeling also indicates positive ?18OSMOW (7.29 to 15.53‰) and negative ?13CPDB (?5.26 to ?10.08‰) shifts are induced by reduced sediments assimilation from source consistent with there being enriched Sr-Nd and low Mg isotopic compositions. LA-ICP-MS zircon U-Pb dating of Huayangchuan carbonatite yielded Triassic ages of 229 ± 3 Ma, which corresponds to the post-collision stage of Qinling Orogen during the middle-late Triassic. We then proposed that the recycling of subducted sediments and later re-melting of those materials in shallow mantle generated the Huayangchuan carbonatite and subsequently formed the Huayangchuan deposit. Fluorine concentration decrease, caused by fluorapatite crystallization, ultimately resulted in betafite mineralization.
Geochimica et Cosmochimica Acta, Vol. 312, pp. 158-179. pdf
Europe, Norway
deposit - Spitzbergen
Abstract: Enrichments in light REE without concomitant enrichments in high-field-strength elements in mantle peridotites are usually attributed to inputs from carbonate-rich melts and referred to as ‘carbonatite’ metasomatism as opposed to interaction with evolved silicate melts. Alternatively, both enrichment types are ascribed to percolating volatile-bearing mafic liquids whose chemical signatures evolve from ‘silicate’ to ‘carbonatite’. Here we compare these models for peridotites in which these enrichment types are combined, as may be common in the mantle. We report new Ca-Sr-Nd isotope and chemical data for lherzolite and harzburgite xenoliths from Spitsbergen that were metasomatized, first, by silicate, then by carbonate-rich melts that formed carbonate-bearing pockets replacing earlier minerals. Seven crushed samples were treated with acetic acid that dissolved carbonates formed in the latest event, but not silicates. The leachates (acid-removed carbonates making up 0.6-1.4% of total sample mass) contain much more Sr than the residues after leaching (277-2923 vs. 16-60 ppm), have a greater overall 87Sr/86Sr range (0.7049-0.7141 vs. 0.7036-0.7055) and higher 87Sr/86Sr in each sample than the residues. The leachates have lower ?44/40Ca range (0.17-0.68‰) than the residues (0.78-1.00‰), as well as lower ?44/40Ca than the residues in all samples but one. By and large, the carbonates are out of Ca-Sr isotope equilibrium with the host peridotites implying that the older silicate and younger carbonatite metasomatism were produced by different parental melts, thus supporting the existence of distinctive carbonate-rich metasomatic media in the lithospheric mantle, possibly including recycled materials. The ?44/40Ca in the leachates (i.e. carbonates, 0.17-0.68‰) are well below bulk silicate Earth (BSE) estimates (0.94 ± 0.05‰) and ?44/40Ca in non-metasomatized melt-depleted mantle. Yet, ?44/40Ca in the non-leached whole rock (WR) carbonate-bearing samples (0.75-0.95‰) fall within, or are only slightly lower than, the BSE range. The 87Sr/86Sr range in these WR samples (0.7030-0.7112) includes very high values for peridotites with large aggregates of dolomite and Mg-calcite. It appears that both carbonatite and silicate metasomatism may produce ?44/40Ca values lower than the BSE such that Ca-isotope data cannot robustly tell apart these two enrichment types, yet carbonatite metasomatism may yield the lowest ?44/40Ca. Carbonates, even at small mass fractions, are significant hosts of Sr in the WR Spitsbergen peridotites (8-51 wt.% of Sr mass) because of very high Sr concentrations, but add little to WR Ca balance (3-12 wt.%). As a result, high Sr content and 87Sr/86Sr ratios may be indices (though not definitive proofs) of carbonatite metasomatism in mantle rocks.
Fluids in deeply subducted continental crust: petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu Orogen, eastern China.
Geochimica et Cosmochimica Acta, Vol. 72, 13, July 1, pp. 3200-3228.
Abstract: The oxygen fugacity of the upper mantle is 3-4 orders of magnitude higher than that of the lower mantle and this has been attributed to Fe2 + disproportionating into Fe3 + plus Fe0 at pressures > 24 GPa. The upper mantle might therefore have been expected to have evolved to more oxidizing compositions through geological time, but it appears that the oxygen fugacity of the upper mantle has remained constant for the last 3.5 billion years. Thus, it indicates that the mantle has been actively buffered from the accumulation of Fe3 +, and that this is linked to oxidation of diamond to carbonate coupled with reduction of Fe3 + to Fe2 +. When subducted plates penetrate into the lower mantle, compensational upwelling transports bridgmanite into the transition zone, where it breaks down to ringwoodite and majorite, releasing the ferric iron. The system returns to equilibrium through oxidation of diamond. Early in Earth history, diamond may have been enriched at the base of the transition zone in the Magma Ocean, because it is denser than peridotite melts at depths shallower than 660 km, and it is more buoyant below. Ongoing oxidation of diamond forms carbonate, leading to relatively high carbonate concentrations in the source of ocean island basalts.
Abstract: Carbonatite is a magmatic rock with high carbonate and low silicate contents, which mostly originate in the mantle. It is therefore of critical importance to understand the behavior of carbon in the mantle, and consequently deep carbon recycling. However, the formation of carbonatite is largely unresolved. In particular, the source of carbonatite the carbonate remains obscure. Previous studies showed that the solidus of carbonated mantle peridotite was lower than the Earth’s geotherm in the Archean and the Early Proterozoic era, before ~1.4 Ga ago. Therefore, the mantle should have been severely decarbonated early in Earth’s history. This is consistent with the low carbon abundance in the asthenospheric mantle (~100 ppm), as indicated by low carbonate concentrations in mid-ocean ridge basalts. Consequently, carbonate in young mantle must have been mostly obtained in the post-Archean era by two processes. These are either oxidation of diamond in the mantle or recycling of sedimentary carbonates through plate subduction. Here we show that the Sr and Nd isotope variations in carbonatite may be plausibly explained by mixing of three endmembers, (1) recycled sedimentary carbonates, (2) depleted mantle, and (3) a low Sr and Nd isotopes endmember. The low Sr, Nd carbonate reservoirs for carbonatites of different ages plot roughly on the evolution line of the primitive mantle, suggesting that they were successively released from a well-preserved, non-carbonate mantle source. The preferred candidate for this endmember is carbonate formed through oxidation of diamond by ferric ion released through decomposition of bridgmanite, which is carried up from the lower mantle via background upwelling, compensational to the volume of oceanic slabs penetrating into the lower mantle1.
Journal of Asian Earth Sciences, Vol. 214, 104772, 8p. Pdf
Mantle
carbon
Abstract: The temperature of the upper mantle was a principal factor controlling the style of plate tectonics and influencing magmatism and metamorphism on Earth over geological history. Recent studies emphasized that Earth’s tectonic style has transited into the modern plate tectonics since the late Neoproterozoic, which is characterized by a global network of plate boundaries with deep and cold oceanic plate subduction. However, the consequence of the establishment of modern plate tectonics to Earth’s mantle temperature and deep carbon cycle has not been fully understood. Here we apply statistical analysis on the geochemical data of continental igneous rocks and identify an increased magnitude of nephelinitic volcanism at the end of the Ediacaran. Nephelinitic rocks, a silica-undersaturated high-alkaline rock group, are mostly formed by low-degree melting of carbonated mantle sources. We link their widespread emergence with an enhanced mantle cooling event and a dramatically increased flux of crustal carbonates transporting to the mantle. The rapid cooling of the mantle was ascribed to the onset of modern-style plate tectonics with global-scale cold oceanic and continental subduction since the late Neoproterozoic. The declined upper-mantle temperature could not only favor the low-degree melting but also allow the subduction of carbonates into the deep mantle without decarbonation at shallow depth. Considering the high oxygen fugacity feature of the nephelinitic rocks and some other high-alkaline volcanism, the establishment of modern plate tectonics and thereafter enhanced mantle cooling and deep carbon cycle might contribute to the high-level atmospheric oxygen content during the Phanerozoic.
Contribution to Mineralogy and Petrology, Vol. 177, 19 , 18p. Pdf
Global
garnet
Abstract: Garnet is a common U-bearing mineral in various magmatic and metamorphic rocks with a high U-Pb closure temperature (>?850 °C), rendering it a potentially valuable U-Pb geochronometer. However, a high U (>?10 ppm) garnet reference material that suits both quadrupole and/or multi-collector inductively coupled plasma mass spectrometry (ICP-MS) is yet to be established. This study evaluates a potential reference material for in situ garnet U-Pb analysis with anomalously high U content from the Prairie Lake alkaline complex, Canada. The PL57 garnet, occurring in a calcite ijolite, has high TiO2 (6.5-15.0 wt%, average 12.7 wt%) and Fe2O3 (17.1-21.3 wt%) contents and is a member of the andradite (26-66 mol.%)-morimotoite (18-41 mol.%)-schorlomite (16-35 mol.%) solid solution series. Four samples were dated by U-Pb ID-TIMS to assess reproducibility. Twelve TIMS analyses produced concordant, equivalent results. Garnet PL57 yielded a concordant age of 1156.2?±?1.2 Ma (2?, n?=?10, MSWD?=?1.0), based on ten analyses with two results discarded due to possible mineral inclusions (if included, the concordia age is 1156.6?±?1.8 Ma; n?=?12, MSWD?=?2.0). PL57 had 27-76 ppm (average 41 ppm) U with Th/U of 0.51-0.68 (average 0.63). The total common Pb content ranged from 0.4 to 3.9 pg (average 1.1 pg). Laser ablation coupled with ICP-MS and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging provide direct evidence that U is incorporated and homogeneously distributed within the garnet lattice rather than as defects or pore spaces. Published garnet samples and standards were then tested by calibrating the Willsboro, Mali, Qicun, and Tonglvshan garnet against PL57, which gave accurate ages within the recommended values. Case studies of garnet from the Archean Musselwhite orogenic gold deposit in Canada and the Cenozoic Changanchong and Habo skarn deposits in China yield reliable ages. This suggests that PL57 is a robust U-Pb isotope reference material. The limited variations of U and Pb isotopic ratios, together with the high U concentration and extremely low initial common Pb, make PL57 an ideal calibration and monitor reference material for in situ measurements.
Abstract: Lamproites are commonly found in post-collisional or intracontinental environments and characterized by unique elemental and radiogenic isotopic signatures that signify derivation from the subcontinental lithospheric mantle. An improved understanding on their genesis is important regarding the dynamics of the Earth’s mantle lithosphere, and requires knowledge in identifying source components and magmatic processes. In order to better constrain the mechanism producing the geochemical diversity of lamproites, we measure the elemental and Mg
