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SDLRC - Scientific Articles all years by Author - Gr+
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
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Abstract: This Tajno alkaline massif (together with the nearby E?k and Pisz intrusions) occurs beneath a thick Mesozoic- Cenozoic sedimentary cover. It has first been recognized by geophysical (magnetic and gravity) investigations, then directly by deep drilling (12 boreholes down to 1800 m). The main rock types identified as clinopyroxenites, syenites, carbonatites, have been cut by later multiphase volcanic /subvolcanic dykes. This massif was characterized as a differentiated ultramafic, alkaline and carbonatite complex, quite comparable to the numerous massifs of the Late Devonian Kola Province of NW Russia [1,2]. Recent geochronological data (U-Pb on zircon from an albitite and Re-Os on pyrrhotite from a carbonatite) indicate that the massif was emplaced at ca. 348 Ma (Early Carboniferous). All the rocks, but more specifically the carbonatites, are enriched in Sr, Ba and LREE, like many carbonatites worldwide but depleted in high field strength elements (Ti, Nb, Ta, Zr). The initial 87Sr/86Sr (0.70370 to 0.70380) and ?Nd(t) (+3.3 to +0.7) isotopic compositions of carbonatites plot in the depleted quadrant of the Nd-Sr diagram, close to “FOcal ZOne” (FOZO) deep mantle domain [1]. The Pb isotopic data (206Pb/204Pb <18.50) do not point to an HIMU (high U/Pb) source. The ranges of C and O stable isotopic compositions of the carbonatites are quite large; some data plot in (or close to) the “Primary Igneous Carbonatite” box while others extend to much higher, typically crustal ?18O and ?13C values.
Abstract: This manuscript presents results of the newest petrographic, mineralogical and bulk chemical, as well as H, C and O stable isotope study of carbonatites and associated silicate rocks from the Tajno Massif (NE Poland). The Tajno Intrusion is a Tournaisian-Visean ultramafic-alkaline-carbonatite body emplaced within the Paleoproterozoic rocks of the East European Craton (EEC). Carbonatites of the Tajno Massif can be subdivided into the calciocarbonatite (calcite), ferrocarbonatite (ankerite), and breccias with an ankerite-fluorite matrix. Due to location at the cratonic margin and abundance in the REE, Tajno classifies (Hou et al., 2015) as the carbonatite-associated REE deposit (CARD), and more precisely as the Dalucao-Style orebody (the breccia-hosted orebody). High Fe2O3 (13.8 wt%), MnO (2.1 wt%), total REE (6582 ppm), Sr (43895 ppm), Ba (6426 ppm), F (greater than10000 ppm) and CO2 contents points for the involvement of the slab - including pelagic metalliferous sediments - in the carbonatites formation. Spatial relations and Sr isotope composition ((87Sr/86Sr)i = 0.7043-0.7048; Wiszniewska et al., 2020) of alkali clinopyroxenite and syenite suggest that these are products of differentiation of the magma, generated by the initial melting of the SCLM due to influx of F-rich fluids from subducted marine sediments. Carbonatites Sr isotope composition ((87Sr/86Sr)i = 0.7037-0.7038), and Ba/Th (16-20620) and Nb/Y (0.01-6.25) ratios, link their origin with a more advanced melting of the SCLM, triggered by CO2-rich fluids from the subducted AOC and melts from sediments. The Tajno Massif - and coeval mafic-alkaline intrusions - age, high potassic composition, and location along the craton margin nearly parallel the Variscan deformation front, are suggesting Variscan subduction beneath the EEC. The oxygen isotope compositions of clinopyroxene (?18O value = 5.2‰) and alkali feldspar (?18O value = 5.7‰), from alkali clinopyroxenite and foid syenite, respectively, are consistent with mantle-derived magmas. Isotopic compositions of carbonatites and breccias (carbonate ?18O = 8.7‰ to 10.7‰; ?13C = -4.8‰ to ?0.4‰) span from values of primary carbonatites to carbonatites affected by a fractionation or sedimentary contamination. The highest values (?18O = 10.7‰; ?13C = -0.4‰) were reported for breccia cut by numerous veins confirming post-magmatic hydrothermal alteration. The lowest carbonate ?18O (9.3‰ to 10.7‰) and ?13C (?5.0‰ to ?3.8‰) values are reported for veins in alkali clinopyroxenites, whereas the highest ?18O (11.2‰) and ?13C (?1.2‰ to ?1.1‰) values are for veins in syenites and trachytes. Isotopic composition of veins suggests hydrothermal origin, and interaction with host mantle-derived rocks, as well as country rocks. In silicate rocks of the Tajno Massif, fluid influx leads to the development of Pb, Zn, Cu, Ag, Au sulfide mineralization-bearing stockwork vein system, with carbonate, silicate and fluorite infilling the veins. Bulk-rock contents of molybdenum (925 ppm), rhenium (905 ppb) and palladium (29 ppb) are notable. The Re-rich molybdenite association with galena, pyrite and Th-rich bastnäsite in carbonate veins is similar as in Mo deposits associated with carbonatites, implying the mantle source of Mo and Re.
Abstract: The problem of the existence of the asthenosphere for old Precambrian cratons is still discussed. In order to study the seismic lithosphere-asthenosphere boundary (LAB) beneath the Baltic Shield, we used records of nine local earthquakes with magnitudes ranging from 2.7 to 5.9. To model the LAB, original data were corrected for topography and Moho depth using a reference model with a 46-km-thick crust. For two northern events at Spitsbergen and Novaya Zemlya, we observe a low-velocity layer, 60-70-km-thick asthenosphere, and the LAB beneath Barents Sea was found at depth of c. 200 km. Sections for other events show continuous first arrivals of P-waves with no evidence for "shadow zone" in the whole range of registration, which could either be interpreted as the absence of the asthenosphere beneath the central part of the Baltic Shield, or that the LAB in this area occurs deeper (>200 km). The relatively thin low-velocity layer found beneath southern Sweden, 15 km below the Moho, could be interpreted as small-scale lithospheric heterogeneities, rather than asthenosphere. Differentiation of the lower lithosphere velocities beneath the Baltic Shield could be interpreted as regional heterogeneity or as anisotropy of the Baltic Shield lithosphere, with high velocities approximately in the east-west direction, and slow velocities approximately in the south-north direction.
Valentine, G.A., Graettinger, A.H, Macorps, E., Ross, P-S., White, J.D.L., Dohring, E., Sonder, I.
Experiments with vertically and laterally migrating subsurface explosions with applications to the geology of phreatomagmatic and hydrothermal explosion craters and diatremes.
Abstract: The Rae craton is an important part of the Canadian Shield and was amalgamated to the Slave craton at ?? 1.9 Ga [1]. Recent geophysical and geochemical data indicate a protracted geodynamic history [1, 2]. Even though the oxidation state of the Earth’s mantle has an important influence of fluid compositions and melting behavior, no data on the oxidation state of the Rae’s mantle are available. The aims of this study were to 1) determine the oxidation state (ƒO2) of the lithosphere beneath the Rae craton, 2) link these results to potential metasomatic overprints and 3) compare the geochemical evolution with the Slave craton. We studied 5 peridotite xenoliths from Pelly Bay (central craton) and 22 peridotites from Somerset Island (craton margin). Pelly Bay peridotites give T < 905°C and depths of ??80- 130 km. Garnets have depleted or “normal” REE patterns, the latter samples recording fO2 values ??0.5 log units higher. The deeper samples are more enriched and oxidised. Peridotites from Somerset Island record T ??825-1190°C, a ?logfO2 ranging from ?? FMQ - FMQ-3.6 from a depth interval of ??100-150 km. Garnets exhibit two REE signatures - sinusoidal and “normal” - indicating an evolutionary sequence of increasing metasomatic re-enrichment and a shift from fluid to melt dominated metasomatism. Compared to the Slave craton, the Rae mantle is more reduced at ??80km but becomes up to 2 log units more oxidised (up to ??FMQ-1) at ??100-130 km. Similar oxidising conditions can be found >140 km in the Slave mantle [3]. Especially under Somerset Island, the lithospheric mantle has contrasting fO2 and metasomatic overprints in the same depth range, which may represent juxtaposed old and rejuvenated domains [2].
Nature Scientific Reports, 10.1038/s41598-021-83261-6 11p. Pdf
Canada, Northwest Territories
metasomatism
Abstract: We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~?100 km depth could have locally destabilised any pre-existing diamond or graphite.
Abstract: We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~?100 km depth could have locally destabilised any pre-existing diamond or graphite.
Geochimica et Cosmochimica Acta, in press available, doi.org/101016 /j.gca.2020.07.013 45p. Pdf
Canada, Northwest Territories
deposit - Lac de Gras
Abstract: Whether hydrogen incorporated in nominally anhydrous mantle minerals plays a role in the strength and longevity of the thick cratonic lithosphere is a matter of debate. In particular, the percolation of hydrogen-bearing melts and fluids could potentially add hydrogen to the mantle lithosphere, weaken its olivines (the dominant mineral in mantle peridotite), and cause delamination of the lithosphere's base. The influence of metasomatism on hydrogen contents of cratonic mantle minerals can be tested in mantle xenoliths from the Slave Craton (Canada) because they show extensive evidence for metasomatism of a layered cratonic mantle. Minerals from mantle xenoliths from the Diavik mine in the Lac de Gras kimberlite area located at the center of the Archean Slave craton were analyzed by FTIR for hydrogen contents. The 18 peridotites, two pyroxenites, one websterite and one wehrlite span an equilibration pressure range from 3.1 to 6.6 GPa and include samples from the shallow (? 145 km), oxidized ultra-depleted layer; the deeper (?145-180 km), reduced less depleted layer; and an ultra-deep (? 180 km) layer near the base of the lithosphere. Olivine, orthopyroxene, clinopyroxene and garnet from peridotites contain 30 - 145, 110 - 225, 105 - 285, 2 - 105 ppm H2O, respectively. Within each deep and ultra-deep layer, correlations of hydrogen contents in minerals and tracers of metasomatism (for example light over heavy rare-earth-element ratio (LREE/HREE), high-field-strength-element (HFSE) content with equilibration pressure) can be explained by a chromatographic process occurring during the percolation of kimberlite-like melts through garnet peridotite. The hydrogen content of peridotite minerals is controlled by the compositions of the evolving melt and of the minerals and by mineral/melt partition coefficients. At the beginning of the process, clinopyroxene scavenges most of the hydrogen and garnet most of the HFSE. As the melt evolves and becomes enriched in hydrogen and LREE, olivine and garnet start to incorporate hydrogen and pyroxenes become enriched in LREE. The hydrogen content of peridotite increases with decreasing depth, overall (e.g., from 75 to 138 ppm H2O in the deep peridotites). Effective viscosity calculated using olivine hydrogen content for the deepest xenoliths near the lithosphere-asthenosphere boundary overlaps with estimates of asthenospheric viscosities. These xenoliths cannot be representative of the overall cratonic root because the lack of viscosity contrast would have caused basal erosion of lithosphere. Instead, metasomatism must be confined in narrow zones channeling kimberlite melts through the lithosphere and from where xenoliths are preferentially sampled. Such localized metasomatism by hydrogen-bearing melts therefore does not necessarily result in delamination of the cratonic root.
Geochimica et Cosmochimica Acta, Vol. 286, pp. 29-83. pdf
Canada, Northwest Territories
xenoliths
Abstract: Whether hydrogen incorporated in nominally anhydrous mantle minerals plays a role in the strength and longevity of the thick cratonic lithosphere is a matter of debate. In particular, the percolation of hydrogen-bearing melts and fluids could potentially add hydrogen to the mantle lithosphere, weaken its olivines (the dominant mineral in mantle peridotite), and cause delamination of the lithosphere's base. The influence of metasomatism on hydrogen contents of cratonic mantle minerals can be tested in mantle xenoliths from the Slave Craton (Canada) because they show extensive evidence for metasomatism of a layered cratonic mantle. Minerals from mantle xenoliths from the Diavik mine in the Lac de Gras kimberlite area located at the center of the Archean Slave craton were analyzed by FTIR for hydrogen contents. The 18 peridotites, two pyroxenites, one websterite and one wehrlite span an equilibration pressure range from 3.1 to 6.6 GPa and include samples from the shallow (?145?km), oxidized ultra-depleted layer; the deeper (?145-180?km), reduced less depleted layer; and an ultra-deep (?180?km) layer near the base of the lithosphere. Olivine, orthopyroxene, clinopyroxene and garnet from peridotites contain 30-145, 110-225, 105-285, 2-105?ppm H2O, respectively. Within each deep and ultra-deep layer, correlations of hydrogen contents in minerals and tracers of metasomatism (for example light over heavy rare-earth-element ratio (LREE/HREE), high-field-strength-element (HFSE) content with equilibration pressure) can be explained by a chromatographic process occurring during the percolation of kimberlite-like melts through garnet peridotite. The hydrogen content of peridotite minerals is controlled by the compositions of the evolving melt and of the minerals and by mineral/melt partition coefficients. At the beginning of the process, clinopyroxene scavenges most of the hydrogen and garnet most of the HFSE. As the melt evolves and becomes enriched in hydrogen and LREE, olivine and garnet start to incorporate hydrogen and pyroxenes become enriched in LREE. The hydrogen content of peridotite increases with decreasing depth, overall (e.g., from 75 to 138?ppm H2O in the deep peridotites). Effective viscosity calculated using olivine hydrogen content for the deepest xenoliths near the lithosphere-asthenosphere boundary overlaps with estimates of asthenospheric viscosities. These xenoliths cannot be representative of the overall cratonic root because the lack of viscosity contrast would have caused basal erosion of lithosphere. Instead, metasomatism must be confined in narrow zones channeling kimberlite melts through the lithosphere and from where xenoliths are preferentially sampled. Such localized metasomatism by hydrogen-bearing melts therefore does not necessarily result in delamination of the cratonic root.
Hydrothermal oxygen diffusion in diopside at 1 KB, 900-1200C, a comparison with O diffusion in forsterite, and constraints on OI disequil. in peridotitenodules
Terra, Abstracts of Experimental mineralogy, petrology and, Vol. 2, December abstracts p. 72
Abstract: Gem-quality diamonds have been found in several alluvial deposits across central and southern Borneo. Borneo has been a known source of diamonds for centuries, but the location of their primary igneous source remains enigmatic. Many geological models have been proposed to explain their distribution, including: the diamonds were derived from a local diatreme; they were brought to the surface through ophiolite obduction or exhumation of UHP metamorphic rocks; they were transported long distances southward via major Asian river systems; or, they were transported from the Australian continent before Borneo was rifted from its northwestern margin in the Late Jurassic. To assess these models, we conducted a study of the provenance of heavy minerals from Kalimantan's Cempaka alluvial diamond deposit. This involved collecting U Pb isotopic data, fission track and trace element geochemistry of zircon as well as major element geochemical data of spinels and morphological descriptions of zircon and diamond. The results indicate that the Cempaka diamonds were likely derived from at least two sources, one which was relatively local and/or involved little reworking, and the other more distal which records several periods of reworking. The distal diamond source is interpreted to be diamond-bearing pipes that intruded the basement of a block that: (1) rifted from northwest Australia (East Java or SW Borneo) and the diamonds were recycled into its sedimentary cover, or: (2) were emplaced elsewhere (e.g. NW Australia) and transported to a block (e.g. East Java or SW Borneo). Both of these scenarios require the diamonds to be transported with the block when it rifted from NW Australia in the Late Jurassic. The local source could be diamondiferous diatremes associated with eroded Miocene high-K alkaline intrusions north of the Barito Basin, which would indicate that the lithosphere beneath SW Borneo is thick (~ 150 km or greater). The ‘local’ diamonds could also be associated with ophiolitic rocks that are exposed in the nearby Meratus Mountains.
Mineralogy and Petrology, Vol. 110, 2, pp. 399-420.
Indonesia
Megacrysts
Abstract: Zircon megacrysts (± gem corundum) appear in basalt fields of Indo-Pacific origin over a 12,000 km zone (ZIP) along West Pacific continental margins. Age-dating, trace element, oxygen and hafnium isotope studies on representative zircons (East Australia-Asia) indicate diverse magmatic sources. The U-Pb (249 to 1 Ma) and zircon fission track (ZFT) ages (65 to 1 Ma) suggest thermal annealing during later basalt transport, with?1 to 203 Ma gaps between the U-Pb and ZFT ages. Magmatic growth zonation and Zr/Hf ratios (0.01-0.02) suggest alkaline magmatic sources, while Ti—in—zircon thermometry suggests that most zircons crystallized within ranges between 550 and 830 °C. Chondrite-normalised multi-element plots show variable enrichment patterns, mostly without marked Eu depletion, indicating little plagioclase fractionation in source melts. Key elements and ratios matched against zircons from magmatic rocks suggest a range of ultramafic to felsic source melts. Zircon O-isotope ratios (?18O in the range 4 to 11‰) and initial Hf isotope ratios (?Hf in the range +2 to +14) encompass ranges for both mantle and crustal melts. Calculated Depleted Mantle (TDM 0.03-0.56 Ga) and Crustal Residence (0.20-1.02 Ga) model ages suggest several mantle events, continental break-ups (Rodinia and Gondwana) and convergent margin collisions left imprints in the zircon source melts. East Australian ZIP sites reflect prolonged intraplate magmatism (~85 Ma), often during times of fast-migrating lithosphere. In contrast, East Asian-Russian ZIP sites reflect later basaltic magmatism (<40 Ma), often linked to episodes of back-arc rifting and spreading, slow-migrating lithosphere and slab subduction.
Cathodluminescence of colored diamonds by transmissionelectronmicroscopy
Geological Association of Canada (GAC)/Mineralogical Association of Canada (MAC) Vancouver 90 Program with Abstracts, Held May 16-18, Vol. 15, p. A50. Abstract
Abstract: The Yilgarn Craton and its northern margin contain a variety of petrogenetically poorly defined small-volume alkaline ultramafic rocks of Proterozoic age. This study documents the petrography, mineral and bulk-rock geochemistry and Nd-Hf-Sr-Pb isotope compositions of a selected suite of these rocks. They comprise ~2.03-2.06 Ga ultramafic lamprophyres (UML) and carbonatites from the Eastern Goldfields Superterrane (EGS), ~0.86 Ga UML from Norseman, and orangeites from the Earaheedy Basin, including samples from Jewill (~1.3 Ga), Bulljah (~1.4 Ga) and Nabberu (~1.8-1.9 Ga). The Proterozoic UML and carbonatites from the EGS and Norseman display very consistent chondritic to superchondritic Nd-Hf isotope compositions and trace-element ratios similar to modern OIBs, which are indicative of a common mantle source across this wide alkaline province. These Nd-Hf isotope compositions overlap with the evolution trends of global kimberlites through time, thus suggesting that this mantle source could be deep and ancient as that proposed for kimberlites. Conversely, the orangeites located in the Earaheedy Basin along the northern margin of the Yilgarn Craton display trace element signatures similar to subduction-related calc-alkaline magmas. Taken together with their highly enriched Sr-Nd-Hf isotope compositions, these characteristics indicate an ancient lithospheric mantle source, which was probably metasomatised by subduction-related fluids. As the ages of the Bulljah and Jewill orangeites overlap with the breakup of the Columbia supercontinent, it is proposed that orangeite magmatism was triggered by changes in plate stress conditions associated with this event. This study provides a comprehensive picture of the genesis of Proterozoic alkaline magmatism in the Yilgarn Craton, highlighting the complex tectono-magmatic evolution of this lithospheric block after its assembly in the Archean.
Doklady Earth Sciences, Vol. 465, 2, pp. 1297-1301.
Russia, Siberia
Deposit - Malokuonapskaya
Abstract: The U-Pb (SHRIMP) age was determined for zircons collected from 26 observation and sampling sites of diamonds and index minerals in the northeastern Siberian Platform. This part of the region hosts 15 low-diamondiferous Paleozoic and Mesozoic kimberlite fields, excluding the near economic Triassic Malokuonapskaya pipe in the Kuranakh field. Four epochs of kimberlite formation (Silurian, Late Devonian to Early Carboniferous, Middle to Late Triassic, and Middle to Late Jurassic) of the Siberian Platform, including its northeastern part, are confirmed as a result of our studies. Most observation points, including economic Quaternary diamond placers, contain Middle to Late Triassic zircons, which confirms the abundant Late Triassic volcanism in this region. The positive correlation of diamonds and major index minerals of kimberlites (mostly, garnets) at some observation sites indicates the possible Triassic age of the predictable diamondiferous kimberlites.
Abstract: Although irrefutable evidence for the presence of signs of diamondiferous kimberlite on the Taimyr Peninsula were obtained in the 1930s, it was only in 2020 that a macrodiamond (>1 mm) was first discovered in Eastern Taimyr. This was a colorless laminar crystal of a transitional shape from an octahedron to a rhombododecahedron. According to the set of features, the crystal is rare and atypical of the known primary and alluvial deposits of the Siberian Diamond Province. The find of this diamond indicates the presence of primary sources and the need for medium-scale geological survey and exploration over a large area from Anabar Bay (Pronchishchev Ridge) to the west to the Kiryaka-Tas and Tulai-Kiryaka highlands and to the northeast to Tsvetkov Cape.
Abstract: Modern plate tectonics may have gotten under way as early as 3.2 billion years ago, about 400 million years earlier than scientists thought. That, in turn, suggests that the movement of large pieces of Earth’s crust could have played a role in making the planet more hospitable to life. Geologist Alec Brenner of Harvard University and his colleagues measured the magnetic orientations of iron-bearing minerals in the Honeyeater Basalt, a layer of rock that formed between 3.19 billion and 3.18 billion years ago. The basalt is part of the East Pilbara Craton, an ancient bit of continent in Western Australia that includes rocks as old as 3.5 billion years. This craton, the researchers found, was on the move between 3.35 billion and 3.18 billion years ago, drifting around the planet at a rate of at least 2.5 centimeters per year. That’s a speed comparable to modern plate motions, the team reports April 22 in Science Advances.
Joint seismic geodynamic mineral physical modeling of African geodynamics: a reconciliation of deep mantle convection with surface geophysical constraints.
Earth and Planetary Science Letters, Vol. 295, 3-4, pp. 329-341.
A Tertiary asthenospheric flow beneath the southern French Massif Central indicated by upper mantle seismic anisotropy and related to west mediterranean extension.
Earth and Planetary Science Letters, Vol. 202, 1, pp.31-47.
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.
South African Journal of Geology, Vol. 121, pp. 271-286.
Africa, Mozambique
geodynamics
Abstract: Major, trace, radiogenic isotope and stable isotope data from lavas along the northeastern coast of Mozambique are described. The whole rock composition data demonstrate that the rocks are dominantly andesitic with compositions typical of calc-alkaline volcanic rocks from arc environments. SHRIMP U/Pb data from zircons indicate that the zircons are xenocrystic, having ages of between 500 Ma and 660 Ma, with the age of the lava constrained by Rb/Sr data at ~184 Ma. Strontium, Nd and Pb radiogenic isotope data support an interpretation of extensive mixing between a Karoo age basaltic magma (dolerite) from Antarctica and continental crust similar in composition to the Mozambique basement. Oxygen isotope data also imply a significant crustal contribution to the lavas. Possible tectonic settings for the lavas are at the margin of a plume or from a locally restricted compressional setting during Gondwana breakup processes.
Grantham, G.H., Manhica, A.D.S.T., Armstrong, R.A., Kruger, F.J., Loubser, M.
New SHRIMP, Rb/Sr and Sm/Nd isotope and whole rock chemical dat a from central Mozambique and western Dronning Maud Land: implications for eastern Kalahari
Journal of African Earth Sciences, Vol. 59, 1, pp.74-100.
Abstract: The Cretaceous High Arctic Large Igneous Province (HALIP) in Canada, although dominated by tholeiites (135-90?Ma), contains two main groups of alkaline igneous rocks. The older alkaline rocks (?96?Ma) scatter around major fault and basement structures. They are represented by the newly-defined Fulmar Suite alkaline basalt dykes and sills, and include Hassel Formation volcanics. The younger alkaline group is represented by the Wootton Intrusive Complex (92.2-92.7?Ma), and the Audhild Bay Suite (83-73?Ma); both emplaced near the northern coast of Ellesmere Island. Fulmar Suite rocks resemble EM-type ocean island basalts (OIB) and most show limited crustal contamination. The Fulmar Suite shows increases of P2O5 at near-constant Ba-K-Zr-Ti that are nearly orthogonal to predicted fractionation- or melting-related variations; which we interpret as the result of melting composite mantle sources containing a regionally widespread apatite-bearing enriched component (P1). Low-P2O5 Fulmar Suite variants overlap compositionally with enriched HALIP tholeiites, and fall on common garnet lherzolite trace element melting trajectories, suggesting variable degrees of melting of a geochemically similar source. High-P2O5 Hassel Formation basalts are unusual among Fulmar rocks, because they are strongly contaminated with depleted lower crust; and because they involve a high-P2O5-Ba-Eu mantle component (P2), similar to that seen in alkali basalt dykes from Greenland. The P2 component may have contained Ba-Eu-rich hawthorneite and/or carbonate minerals as well as apatite, and may typify parts of the Greenlandic sub-continental lithospheric mantle (SCLM). Mafic alkaline Audhild Bay Suite (ABS) rocks are volcanic and hypabyssal basanites, alkaline basalts and trachy-andesites, and resemble HIMU ocean island basalts in having high Nb, low Zr/Nb and low 87Sr/86Sri. These mafic alkaline rocks are associated with felsic alkaline lavas and syenitic intrusions, but crustally-derived rhyodacites and rhyolites also exist. The Wootton Intrusive Complex (WIC) contains geochemically similar plutonic rocks (alkali gabbros, diorites and anatectic granites), and may represent a more deeply eroded, slightly older equivalent of the ABS. Low-P2O5 ABS and WIC alkaline mafic rocks have flat heavy rare-earth (HREE) profiles suggesting shallow mantle melting; whereas High-P2O5 variants have steep HREE profiles indicating deeper separation from garnet-bearing residues. Some High-P2O5 mafic ABS rocks seem to contain the P1 and P2 components identified in Fulmar-Hassel rocks, whereas other samples trend towards possible High-P2O5+Zr (PZr) and High-P2O5+K2O (PK) components. We argue that the strongly alkaline northern Ellesmere Island magmas sampled mineralogically heterogeneous veins or metasomes in Greenlandic-type SCLM, which contained trace phases like apatite, carbonates, hawthorneite, zircon, mica or richterite. The geographically more widespread apatite-bearing component (P1), could have formed part of a heterogeneous plume or upwelling mantle current that also generated HALIP tholeiites when melted more extensively, but may also have resided in the SCLM as relics of older events. Rare HALIP alkaline rocks with high K-Rb-U-Th fall on mixing paths implying strong local contamination from either Sverdrup Basin sedimentary rocks or granitic upper crust. However, the scarcity of potassic alkaline HALIP facies, together with the other trace element and isotopic signatures, provide little support for an ubiquitous fossil sedimentary subduction zone component in the HALIP mantle source.
Abstract: The Cretaceous High Arctic Large Igneous Province (HALIP) in Canada, although dominated by tholeiites (135-90?Ma), contains two main groups of alkaline igneous rocks. The older alkaline rocks (?96?Ma) scatter around major fault and basement structures. They are represented by the newly-defined Fulmar Suite alkaline basalt dykes and sills, and include Hassel Formation volcanics. The younger alkaline group is represented by the Wootton Intrusive Complex (92.2-92.7?Ma), and the Audhild Bay Suite (83-73?Ma); both emplaced near the northern coast of Ellesmere Island. Fulmar Suite rocks resemble EM-type ocean island basalts (OIB) and most show limited crustal contamination. The Fulmar Suite shows increases of P2O5 at near-constant Ba-K-Zr-Ti that are nearly orthogonal to predicted fractionation- or melting-related variations; which we interpret as the result of melting composite mantle sources containing a regionally widespread apatite-bearing enriched component (P1). Low-P2O5 Fulmar Suite variants overlap compositionally with enriched HALIP tholeiites, and fall on common garnet lherzolite trace element melting trajectories, suggesting variable degrees of melting of a geochemically similar source. High-P2O5 Hassel Formation basalts are unusual among Fulmar rocks, because they are strongly contaminated with depleted lower crust; and because they involve a high-P2O5-Ba-Eu mantle component (P2), similar to that seen in alkali basalt dykes from Greenland. The P2 component may have contained Ba-Eu-rich hawthorneite and/or carbonate minerals as well as apatite, and may typify parts of the Greenlandic sub-continental lithospheric mantle (SCLM). Mafic alkaline Audhild Bay Suite (ABS) rocks are volcanic and hypabyssal basanites, alkaline basalts and trachy-andesites, and resemble HIMU ocean island basalts in having high Nb, low Zr/Nb and low 87Sr/86Sri. These mafic alkaline rocks are associated with felsic alkaline lavas and syenitic intrusions, but crustally-derived rhyodacites and rhyolites also exist. The Wootton Intrusive Complex (WIC) contains geochemically similar plutonic rocks (alkali gabbros, diorites and anatectic granites), and may represent a more deeply eroded, slightly older equivalent of the ABS. Low-P2O5 ABS and WIC alkaline mafic rocks have flat heavy rare-earth (HREE) profiles suggesting shallow mantle melting; whereas High-P2O5 variants have steep HREE profiles indicating deeper separation from garnet-bearing residues. Some High-P2O5 mafic ABS rocks seem to contain the P1 and P2 components identified in Fulmar-Hassel rocks, whereas other samples trend towards possible High-P2O5+Zr (PZr) and High-P2O5+K2O (PK) components. We argue that the strongly alkaline northern Ellesmere Island magmas sampled mineralogically heterogeneous veins or metasomes in Greenlandic-type SCLM, which contained trace phases like apatite, carbonates, hawthorneite, zircon, mica or richterite. The geographically more widespread apatite-bearing component (P1), could have formed part of a heterogeneous plume or upwelling mantle current that also generated HALIP tholeiites when melted more extensively, but may also have resided in the SCLM as relics of older events. Rare HALIP alkaline rocks with high K-Rb-U-Th fall on mixing paths implying strong local contamination from either Sverdrup Basin sedimentary rocks or granitic upper crust. However, the scarcity of potassic alkaline HALIP facies, together with the other trace element and isotopic signatures, provide little support for an ubiquitous fossil sedimentary subduction zone component in the HALIP mantle source.
Abstract: The Cretaceous High Arctic Large Igneous Province (HALIP) in Canada, although dominated by tholeiites (135-90?Ma), contains two main groups of alkaline igneous rocks. The older alkaline rocks (?96?Ma) scatter around major fault and basement structures. They are represented by the newly defined Fulmar Suite alkaline basalt dykes and sills, and include Hassel Formation volcanic rocks. The younger alkaline group is represented by the Wootton Intrusive Complex (92•2-92•7?Ma), and the Audhild Bay Suite (83-73?Ma), both emplaced near the northern coast of Ellesmere Island. Fulmar Suite rocks resemble EM-type ocean island basalts (OIB) and most show limited crustal contamination. The Fulmar Suite shows increases of P2O5 at near-constant Ba-K-Zr-Ti that are nearly orthogonal to predicted fractionation- or melting-related variations, which we interpret as the result of melting composite mantle sources containing a regionally widespread apatite-bearing enriched component (P1). Low-P2O5 Fulmar Suite variants overlap compositionally with enriched HALIP tholeiites, and fall on common garnet lherzolite trace element melting trajectories, suggesting variable degrees of melting of a geochemically similar source. High-P2O5 Hassel Formation basalts are unusual among Fulmar rocks, because they are strongly contaminated with depleted lower crust; and because they involve a high-P2O5-Ba-Eu mantle component (P2), similar to that seen in alkali basalt dykes from Greenland. The P2 component may have contained Ba-Eu-rich hawthorneite and/or carbonate minerals as well as apatite, and may typify parts of the Greenlandic sub-continental lithospheric mantle (SCLM). Mafic alkaline Audhild Bay Suite (ABS) rocks are volcanic and hypabyssal basanites, alkaline basalts and trachy-andesites, and resemble HIMU ocean island basalts in having high Nb, low Zr/Nb and low 87Sr/86Sri. These mafic alkaline rocks are associated with felsic alkaline lavas and syenitic intrusions, but crustally derived rhyodacites and rhyolites also exist. The Wootton Intrusive Complex (WIC) contains geochemically similar plutonic rocks (alkali gabbros, diorites and anatectic granites), and may represent a more deeply eroded, slightly older equivalent of the ABS. Low-P2O5 ABS and WIC alkaline mafic rocks have flat heavy rare earth element (HREE) profiles suggesting shallow mantle melting; whereas High-P2O5 variants have steep HREE profiles indicating deeper separation from garnet-bearing residues. Some High-P2O5 mafic ABS rocks seem to contain the P1 and P2 components identified in Fulmar-Hassel rocks, whereas other samples trend towards possible High-P2O5 + Zr (PZr) and High-P2O5 + K2O (PK) components. We argue that the strongly alkaline northern Ellesmere Island magmas sampled mineralogically heterogeneous veins or metasomes in Greenlandic-type SCLM, which contained trace phases such as apatite, carbonates, hawthorneite, zircon, mica or richterite. The geographically more widespread apatite-bearing component (P1) could have formed part of a heterogeneous plume or upwelling mantle current that also generated HALIP tholeiites when melted more extensively, but may also have resided in the SCLM as relics of older events. Rare HALIP alkaline rocks with high K-Rb-U-Th fall on mixing paths implying strong local contamination from either Sverdrup Basin sedimentary rocks or granitic upper crust. However, the scarcity of potassic alkaline HALIP facies, together with the other trace element and isotopic signatures, provides little support for a ubiquitous fossil sedimentary subduction-zone component in the HALIP mantle source.
Geophysical Research abstracts, EGU, EGU2019-9348, 1p. Pdf
Canada
geodynamics
Abstract: We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~?100 km depth could have locally destabilised any pre-existing diamond or graphite.
Abstract: We report on pipe-like bodies and dikes of carbonate rocks related to sodic alkaline intrusions and amphibole mantle peridotites in the Ivrea zone (European Southern Alps). The carbonate rocks have bulk trace-element concentrations typical of low-rare earth element carbonatites interpreted as cumulates of carbonatite melts. Faintly zoned zircons from these carbonate rocks contain calcite inclusions and have trace-element compositions akin to those of carbonatite zircons. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon dating yields concordant ages of 187 ± 2.4 and 192 ± 2.5 Ma, coeval with sodic alkaline magmatism in the Ivrea zone. Cross-cutting relations, ages, as well as bulk and zircon geochemistry indicate that the carbonate rocks are carbonatites, the first ones reported from the Alps. Carbonatites and alkaline intrusions are comagmatic and were emplaced in the nascent passive margin of Adria during the Early Jurassic breakup of Pangea. Extension caused partial melting of amphibole-rich mantle domains, yielding sodic alkaline magmas whose fractionation led to carbonatite-silicate melt immiscibility. Similar occurrences in other rifts suggest that small-scale, sodic and CO2-rich alkaline magmatism is a typical result of extension and decompression-driven reactivation of amphibole-bearing lithospheric mantle during passive continental breakup and the evolution of magma-poor rifts.
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.
Symposium on critical and strategic materials, British Columbia Geological Survey Paper 2015-3, held Nov. 13-14, pp. 211-218.
Technology
Rare earths
Abstract: Quantitative Evaluation of Materials by Scanning electron microscopy (QEMSCAN®) was used to assess carbonatite indicator minerals in fl uvial sediments from the drainage area of the Aley carbonatite, in north-central British Columbia. QEMSCAN® is a viable method for rapid detection and characterization of carbonatite indicator minerals with minimal processing other than dry sieving. Stream sediments from directly above, and up to 11 km downstream, of the carbonatite deposit were selected for this indicator mineral study. The geology of the Aley carbonatite is described by Mäder (1986), Kressal et al. (2010), McLeish (2013), Mackay and Simandl (2014), and Chakhmouradian et al. (2015). Traditional indicator mineral exploration methods use the 0.25-2.0 mm size fraction of unconsolidated sediments (Averill, 2001, 2014; McCurdy, 2006, 2009; McClenaghan, 2011, 2014). Indicator minerals are detectable by QEMSCAN® at particle sizes smaller than those used for hand picking (<0.25 mm). Pre-concentration (typically by shaker table) is used before heavy liquid separation, isodynamic magnetic separation, optical identifi cation using a binocular microscope, and hand picking (McClenaghan, 2011). Following additional sieving, the 0.5-1 and 1-2 mm fractions are hand picked for indicator minerals while the 0.25-0.5 mm fraction is subjected to paramagnetic separation before hand picking (Averill, 2001; McClenaghan, 2011). Hand picking indicator minerals focuses on monomineralic grains, and composite grains may be lost during processing. Composite grains are diffi cult and time consuming to hand pick and characterize using optical and Scanning Electron Microscopy (SEM) methods. A single grain mount can take 6-12 hours to chemically analyse (Layton- Matthews et al., 2014). Detailed sample analysis using the QEMSCAN® Particle Mineral Analysis routine allows for 5-6 samples to be analyzed per day. When only mineral identifi cation and mineral concentrations and counts are required, the use of a Bulk Mineral Analysis routine reduces the analysis time from ~4 hours to ~30 minutes per sample.
Journal of Geochemical Exploration, Vol. 165, pp. 159-173.
Canada, British Columbia
Geochemistry - carbonatites
Abstract: This orientation survey indicates that Quantitative Evaluation of Materials by Scanning electron microscopy (QEMSCAN®) is a viable alternative to traditional indicator mineral exploration approaches which involve complex processing followed by visual indicator mineral hand-picking with a binocular microscope. Representative polished smear sections of the 125-250 ?m fraction (dry sieved and otherwise unprocessed) and corresponding Mozley C800 table concentrates from the drainages of three carbonatites (Aley, Lonnie, and Wicheeda) in the British Columbia Alkaline Province of the Canadian Cordillera were studied. Polished smear sections (26 × 46 mm slide size) contained an average of 20,000 exposed particles. A single section can be analyzed in detail using the Particle Mineral Analysis routine in approximately 3.5-4.5 h. If only mineral identification and mineral concentrations are required, the Bulk Mineral Analysis routine reduces the analytical time to 30 min. The most useful carbonatite indicator minerals are niobates (pyrochlore and columbite), REE-fluorocarbonates, monazite, and apatite. Niobate minerals were identified in the 125-250 ?m fraction of stream sediment samples more than 11 km downstream from the Aley carbonatite (their source) without the need for pre-concentration. With minimal processing by Mozley C800, carbonatite indicator minerals were detected downstream of the Lonnie and Wicheeda carbonatites. The main advantages of QEMSCAN® over the traditional indicator mineral exploration techniques are its ability to: 1) analyze very small minerals, 2) quickly determine quantitative sediment composition and mineralogy by both weight percent and mineral count, 3) establish mineral size distribution within the analyzed size fraction, and 4) determine the proportions of monomineralic (liberated) grains to compound grains and statistically assess mineral associations in compound grains. One of the key advantages is that this method permits the use of indicator minerals based on their chemical properties. This is impossible to accomplish using visual identification.
in: Ferbey, T. Plouffe, A., Hickein, A.S. eds. Indicator minerals in tills and stream sediments of the Canadian Cordillera. Geological Association of Canada Special Paper,, Vol. 50, pp. 175-190.
Canada, British Columbia
carbonatite - Aley, Lonnie, Wicheeda
Abstract: This volume consists of a series of papers of importance to indicator minerals in the Canadian Cordillera. Topics include the glacial history of the Cordilleran Ice Sheet, drift prospecting methods, the evolution of survey sampling strategies, new analytical methods, and recent advances in applying indicators minerals to mineral exploration. This volume fills a notable knowledge gap on the use of indicator minerals in the Canadian Cordillera. We hope that the volume serves as a user guide, encouraging the wider application of indicator minerals by the exploration community.
Mechanisms to Explain the Loss of Heavy Minerals from Upper paleozoic Tillites of South Africa and Australia and the Late Precambrian Tillites of Australia.
Viljoen, K.S., Harris, J.W., Richardson, S.H., Gray, K.
Trace element chemistry of peridotitic garnets in diamonds from the Premier ( Cullinan) and Finsch kimberlites, South Africa: contrasting styles of mantle metasomatism.
Canadian Journal of Earth Sciences, Vol. 52, 11, pp. 980-989.
Canada, Nunavut, Baffin Island
Geomorphology
Abstract: Speckle tracking of ALOS PALSAR fine beam data from 2007-2011 are used to determine the surface motion of major ice masses on Baffin Island and Bylot Island in the southern Canadian Arctic Archipelago. Glacier velocities are low overall, with peaks of ?100 m a?1 and means of ?20-60 m a?1 common along the main trunk of many outlet glaciers. Peak velocities on Penny and Bylot Island ice caps tend to occur near the mid-sections of their primary outlet glaciers, while the fastest velocities on all other glaciers usually occur near their termini due to relatively large accumulation areas draining through narrow outlets. Estimates of ice thickness at the fronts of tidewater-terminating glaciers are combined with the velocity measurements to determine a regional dynamic discharge rate of between ?17 Mt a?1 and ?108 Mt a?1, with a mid-point estimate of ?55 Mt a?1, revising downward previous approximations. These velocities can be used as inputs for glacier flow models, and provide a baseline dataset against which future changes in ice dynamics can be detected.
Journal of Geophysical Letters, Vol. 47, e2020GL087222
Mantle
water
Abstract: The amount of water trapped in the Earth's interior has a strong effect on the evolution and dynamics of the planet, which ultimately controls the occurrence of earthquakes and volcanic eruptions. However, the distribution of water inside the Earth is not yet well understood. To study the Earth's deep interior, we make use of changes in the Earth's magnetic field to detect variations in electrical conductivity inside the planet. Electrical conductivity is a characteristic of a rock that varies with temperature and water content. Here, we present a novel methodology to estimate the amount of water in different regions of Earth's mantle. Our analysis suggests the presence of small amounts of water in the mantle underneath Europe, whereas larger amounts are expected beneath North America and northern Asia.
Abstract: Magnesium and oxygen are critical elements in the solid Earth and hydrosphere. A better understanding of the combined behavior of Mg and O isotopes will refine their use as a tracer of geochemical processes and Earth evolution. In this study, the Mg-isotope compositions of garnet and omphacite separated from well-characterized xenolithic eclogites from the Roberts Victor kimberlite pipe (South Africa) have been measured by solution multi-collector ICP-MS. The reconstructed whole-rock ?26Mg values of Type I (metasomatized) eclogites range from ? 0.61‰ to ? 0.20‰ (Type IA) and from ? 0.60‰ to ? 0.30‰ (Type IB) (mean ? 0.43‰ ± 0.12‰), while ?26Mg of Type IIA (fresh, least metasomatized) eclogites ranges from ? 1.09‰ to ? 0.17‰ (mean ? 0.69‰ ± 0.41‰); a Type IIB (fresh, least metasomatized) has ?26Mg of ? 0.37‰. Oxygen-isotope compositions of garnet were analyzed in situ by SIMS (CAMECA 1280) and cross-checked by laser fluorination. Garnets have ?18O of 6.53‰ to 9.08‰ in Type IA, 6.14‰ to 6.65‰ in Type IB, and 2.34‰ to 2.91‰ in Type IIB. The variation of ?26Mg and ?18O in Type IA and IB eclogites is consistent with the previously proposed model for the evolution of these samples, based on major and trace elements and radiogenic isotopes. In this model, the protoliths (Type II eclogites) were metasomatized by carbonatitic to kimberlitic melts/fluids to produce first Type IA eclogites and then Type IB. Metasomatism has changed the O-isotope compositions, but the Mg-isotope compositions of Type IA are mainly controlled by the protoliths; those of Type IB eclogites reflect mixing between the protoliths and the kimberlitic melt/fluid. The combination of a large range of ?26Mg and low ?18O in Type II eclogites cannot be explained easily by seawater alteration of oceanic crust, interaction of carbonate/silicate sediments with oceanic crust, or partial melting of mafic rocks.
Abstract: Widespread Miocene (24-8 Ma) ultrapotassic rocks and their entrained xenoliths provide information on the composition, structure, and thermal state of the sub-continental lithospheric mantle in southern Tibet during the India-Asia continental collision. The ultrapotassic rocks along the Lhasa block delineate two distinct lithospheric domains with different histories of depletion and enrichment. The eastern ultrapotassic rocks (89°E-92°E) reveal a depleted, young, and fertile lithospheric mantle (87Sr/86Srt = 0.704-0.707 [t is eruption time]; Hf depleted-mantle model age [TDM] = 377-653 Ma). The western ultrapotassic rocks (79°E-89°E) and their peridotite xenoliths (81°E) reflect a refractory harzburgitic mantle refertilized by ancient metasomatism (lavas: 87Sr/86Srt = 0.714-0.734; peridotites: 87Sr/86Srt = 0.709-0.716). These data integrated with seismic tomography suggest that upwelling asthenosphere was diverted away from the deep continental root beneath the western Lhasa block, but rose to shallower depths beneath a thinner lithosphere in the eastern part. Heating of the lithospheric mantle by the rising asthenosphere ultimately generated the ultrapotassic rocks with regionally distinct geochemical signatures reflecting the different nature of the lithospheric mantle.
Geostandards and Geoanalytical Research, in press available, 16p.
Australia
deposit - Mud Tank
Abstract: Zircon megacrysts from the Mud Tank carbonatite, Australia, are being used in many laboratories as a reference material for LA?ICP?MS U?Pb dating and trace element measurement, and LA?MC?ICP?MS determination of Hf isotopes. We summarise a database of > 10000 analyses of Mud Tank zircon (MTZ), collected from 2000 to 2018 during its use as a secondary reference material for simultaneous U?Pb and trace element analysis, and for Hf?isotope analysis. Trace element mass fractions are highest in dark red?brown stones and lowest in colourless and gem?quality ones. Individual unzoned grains can be chemically homogeneous, while significant variations in trace element mass fraction are associated with oscillatory zoning. Chondrite?normalised trace element patterns are essentially parallel over large mass fraction ranges. A Concordia age of 731.0 ± 0.2 Ma (2s, n = 2272) is taken as the age of crystallisation. Some grains show lower concordant to mildly discordant ages, probably reflecting minor Pb loss associated with cooling and the Alice Springs Orogeny (450-300 Ma). Our weighted mean 176Hf/177Hf is 0.282523 ± 10 (2s, n = 9350); the uncertainties on this ratio reflect some heterogeneity, mainly between grains. A few analyses suggest that colourless grains have generally lower 176Hf/177Hf. MTZ is a useful secondary reference material for U?Pb and Hf?isotope analysis, but individual grains need to be carefully selected using CL imaging and tested for homogeneity, and ideally should be standardised by solution analysis.
Geostandards and Geoanalysis Research, doi.org/10.1111/GGR.12419 34p. Pdf
Australia
geochemistry
Abstract: To promote a more efficient and transparent geochemistry data ecosystem, a consortium of Australian university research laboratories called the AuScope Geochemistry Network (AGN) assembled to build a collaborative platform for the express purpose of preserving, disseminating, and collating geochronology and isotopic data. In partnership with geoscience-data-solutions company Lithodat Pty Ltd, the open, cloud-based AusGeochem platform (https://ausgeochem.auscope.org.au) was developed to simultaneously serve as a geosample registry, a geochemical data repository, and a data analysis tool. Informed by method-specific groups of geochemistry experts and established international data reporting practices, community-agreed database schemas were developed for rock and mineral geosample metadata and secondary ion mass spectrometry U-Pb analysis, with additional models for laser ablation inductively-coupled mass spectrometry U-Pb and Lu-Hf, Ar-Ar, fission-track and (U-Th-Sm)/He under development. Collectively, the AusGeochem platform provides the geochemistry community with a new, dynamic resource to help facilitate FAIR (Findable, Accessible, Interoperable, Reusable) data management, streamline data dissemination and advanced quantitative investigations of Earth system processes. By systematically archiving detailed geochemical (meta-)data in structured schemas, intractably large datasets comprising thousands of analyses produced by numerous laboratories can be readily interrogated in novel and powerful ways. These include rapid derivation of inter-data relationships, facilitating on-the-fly data compilation, analysis, and visualisation.
Abstract: The primordial crust on the Earth formed from the crystallization of the surface magma ocean during the Hadean. However, geological surveys have found no evidence of rocks dating back to more than 4 Ga on the Earth's surface, suggesting the Hadean crust was lost due to some processes. We investigated the subduction of one of the possible candidates for the primordial crust, anorthosite and KREEP crust similar to the Moon, which is also considered to have formed from the crystallization of the magma ocean. Similar to the present Earth, the subduction of primordial crust by subduction erosion is expected to be an effective way of eliminating primordial crust from the surface. In this study, the subduction rate of the primordial crust via subduction channels is evaluated by numerical simulations. The subduction channels are located between the subducting slab and the mantle wedge and are comprised of primordial crust materials supplied mainly by subduction erosion. We have found that primordial anorthosite and KREEP crust of up to ?50 km thick at the Earth's surface was able to be conveyed to the deep mantle within 0.1-2 Gy by that mechanism.
Physics and Chemistry of Minerals, in press available 9p.
Technology
Garnet morphology
Abstract: We present a systematic experimental study on the phase transition, lattice microstrain, and order-disorder of cations for garnets in the majorite-pyrope system. Polycrystalline gem-quality garnets were synthesized at high pressure and high temperature using a Kawai-type multi-anvil apparatus. A phase transition from a cubic to tetragonal structure is clearly observed for garnets with the majorite content of more than 74 mol % through X-ray diffraction (XRD) and Raman scattering studies. Microstrain of garnets, evaluated with the Williamson-Hall plot on XRD profiles, shows a nonlinear dependence of the garnet compositions. The variation of the XRD peak broadening suggests the lattice microstrain of these garnets may be associated with the local structural heterogeneities due to the substitution of different cations via the coupled substitution (Mg2+ + Si4+ = 2Al3+) in the garnet structure. The width variation of Raman scattering peaks indicates that cation disorder occurs in the garnet structure for intermediate compositions. It is found that intermediate garnets and end-members have a minimum of microstrain, while those between end-members and intermediate compositions possess a larger microstrain.
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: The thermoelastic parameters of Ca3Cr2Si3O12 uvarovite garnet were examined in situ at high pressure up to 13 GPa and high temperature up to 1100 K by synchrotron radiation energy-dispersive X-ray diffraction within a 6-6-type multi-anvil press apparatus. A least-square fitting of room T data to a third-order Birch-Murnaghan (BM3) EoS yielded K0 = 164.2 ± 0.7 GPa, V0 = 1735.9 ± 0.3 Å3 (K’0 fixed to 4.0). P-V-T data were fitted simultaneously by a modified HT-BM3 EoS, which gave the isothermal bulk modulus K0 = 163.6 ± 2.6 GPa, K’0 = 4.1 ± 0.5, its temperature derivative (?K0,T/?T)P = -0.014 ± 0.002 GPa K?1, and the thermal expansion coefficients a0 = 2.32 ± 0.13 ×10?5 K?1 and b0 = 2.13 ± 2.18 ×10?9 K?2 (K’0 fixed to 4.0). Our results showed that the Cr3+ enrichment in natural systems likely increases the density of ugrandite garnets, resulting in a substantial increase of mantle garnet densities in regions where Cr-rich spinel releases chromium through a metasomatic reaction.
Abstract: The thermoelastic parameters of Ca3Cr2Si3O12 uvarovite garnet were examined in situ at high pressure up to 13 GPa and high temperature up to 1100 K by synchrotron radiation energy-dispersive X-ray diffraction within a 6-6-type multi-anvil press apparatus. A least-square fitting of room T data to a third-order Birch-Murnaghan (BM3) EoS yielded K0 = 164.2 ± 0.7 GPa, V0 = 1735.9 ± 0.3 Å3 (K’0 fixed to 4.0). P-V-T data were fitted simultaneously by a modified HT-BM3 EoS, which gave the isothermal bulk modulus K0 = 163.6 ± 2.6 GPa, K’0 = 4.1 ± 0.5, its temperature derivative (?K0,T/?T)P = -0.014 ± 0.002 GPa K?1, and the thermal expansion coefficients a0 = 2.32 ± 0.13 ×10?5 K?1 and b0 = 2.13 ± 2.18 ×10?9 K?2 (K’0 fixed to 4.0). Our results showed that the Cr3+ enrichment in natural systems likely increases the density of ugrandite garnets, resulting in a substantial increase of mantle garnet densities in regions where Cr-rich spinel releases chromium through a metasomatic reaction.
Abstract: The effect of aluminum (Al) on the elasticity of majorite-pyrope garnets was investigated by means of ultrasonic interferometry measurements on well-fabricated polycrystalline specimens. Both velocities and elastic moduli increase almost linearly with increasing Al content within analytical uncertainty. No significant variation of the velocities and elastic moduli is observed across the tetragonal-to-cubic phase transition at majorite with the pyrope content up to 26 mol% along the majorite-pyrope system. The elasticity variation of majorite-pyrope garnets is largely dominated by the Al content, while the phase transition as a result of cation ordering/disordering of Mg and Si via substitution of Al on octahedral sites cannot significantly affect elastic properties. Seismic velocity variations of a garnet-bearing mantle transition zone are therefore dominated by garnet composition (e.g., Al, Fe, Ca, and Na) rather than the tetragonal-to-cubic phase transition because of cation ordering/disordering.
Physics and Chemistry of Minerals, Vol. 48, 10, 6p. Pdf s00269-021-Q1163-5
Mantle
bridgmanite
Abstract: Phase D is proposed to be the most important hydrous phase at the upper part of the lower mantle, and it has been shown to coexist with bridgmanite (Brg), the most abundant mineral and main host for Al2O3 in the lower mantle. The concentration of Al in Phase D could significantly increase the thermal stability field of Phase D, therefore, partitioning of Al between Brg and Phase D is of particular importance to constrain water distribution in the deep mantle. Here, we performed high P-T experiments in MgO-Al2O3-SiO2-H2O system to investigate the partitioning of Al between Brg and Phase D up to 32 GPa and 1350 °C. Our results indicated that Al distributes strongly into Phase D relative to Brg and the partition coefficient slightly decreases with increasing temperature. Al-bearing Phase D exhibits a very high thermal stability region, but it completely decomposed around 28 GPa and 1350 °C, at which point Brg coexisted with a large amount of melt. The depth?~?850 km (28 GPa) is thus proposed to be the second choke point for hydrous minerals. This may shed new lights on several important geophysical observations in subduction zones.
Abstract: Several interstellar environments produce 'anomalous microwave emission', with brightness-peaks at tens-of-gigahertz frequencies. The emission's origins are uncertain - rapidly-spinning nano-particles could emit electric-dipole radiation, but polycyclic aromatic hydrocarbons proposed as the carrier are now found not to correlate with Galactic signals. The difficulty is to identify co-spatial sources over long lines of sight. Here we identify anomalous microwave emission in three proto-planetary discs. These are the only known systems that host hydrogenated nano-diamonds, in contrast to very common detection of polycyclic aromatic hydrocarbons. Spectroscopy locates the nano-diamonds close to the host-stars, at physically-constrained temperatures. Developing disc models, we reproduce the emission with diamonds 0.75-1.1 nanometres in radius, holding less than or equal to 1-2 per cent of the carbon budget. The microwave-emission:stellar-luminosity ratios are approximately constant, allowing nano-diamonds to be ubiquitous but emitting below detection thresholds in many star-systems. This can unify the findings with similar-sized diamonds found within solar system meteorites. As nano-diamond spectral absorption is seen in interstellar sightlines, these particles are also a candidate for generating galaxy-scale anomalous microwave emission.
Abstract: Plate tectonics may have gotten a pretty early start in Earth’s history. Most estimates put the onset of when the large plates that make up the planet’s outer crust began shifting at around 3 billion years ago. But a new study in the Sept. 22 Science that analyzes titanium in continental rocks asserts that plate tectonics began 500 million years earlier. Nicolas Greber, now at the University of Geneva, and colleagues suggest that previous studies got it wrong because researchers relied on chemical analyses of silicon dioxide in shales, sedimentary rocks that bear the detritus of a variety of continental rocks. These rocks’ silicon dioxide composition can give researchers an idea of when continental rocks began to diverge in makeup from oceanic rocks as a result of plate tectonics.But weathering can wreak havoc on the chemical makeup of shales. To get around that problem, Greber’s team turned to a new tool: the ratios of two titanium isotopes, forms of the same element that have different masses. The proportion of titanium isotopes in the rocks is a useful stand-in for the difference in silicon dioxide concentration between continental and oceanic rocks, and isn’t so easily altered by weathering. Those data helped the team estimate that continental rocks — and therefore plate tectonics — were already going strong by 3.5 billion years ago.
Moscow University Geology Bulletin, Vol. 72, 5, pp. 299-304.
Mantle
perovskite
Abstract: Semi-empirical and quantum chemical studies of Al atom energy in CaSiO3 and MgSiO3 with the perovskite-type structure at pressures and temperatures of the Earth’s mantle are reported. The phase diagram for CaSiO3 is reproduced and refined. Probable mechanisms of Al incorporation in the structures studied are considered. According to the results of the calculations, Al is preferably incorporated into MgSiO3, rather than into CaSiO3. Evaluation of the isomorphic capacity of perovskite phases in relation to Al shows that the Al content in MgSiO3 may reach 2.4 mol % at 120 GPa and 2400 K. CaSiO3 cannot be a source of Al atoms in the Earth’s mantle.
Phase relationships of hydrous alkalic magmas at high pressures: production of nepheline hawaiitic to mugearitic liquids by amphibole dominated fractional
Hutchinson, D.R., Lee, M.W., Behrendt, J., Cannon, W.F., Green
Variations in the reflectivity of the Moho transition zone beneath The midcontinent Rift System of North America. Results from true amplitude Glimpcedata
Journal of Geophysical Research, Vol. 97, No. B4, April 10, pp. 4721-4738
Chemical Reviews, Vol. 120, 4, 10.1021/ acs.chemrev.9b00578 50p. Pdf
Global
HPHT, CVD, synthetics
Abstract: Nitrogen is ubiquitous in both natural and laboratory-grown diamond, but the number and nature of the nitrogen-containing defects can have a profound effect on the diamond material and its properties. An ever-growing fraction of the supply of diamond appearing on the world market is now lab-grown. Here, we survey recent progress in two complementary diamond synthesis methods: high pressure high temperature (HPHT) growth and chemical vapor deposition (CVD), how each is allowing ever more precise control of nitrogen incorporation in the resulting diamond, and how the diamond produced by either method can be further processed (e.g., by implantation or annealing) to achieve a particular outcome or property. The burgeoning availability of diamond samples grown under well-defined conditions has also enabled huge advances in the characterization and understanding of nitrogen-containing defects in diamond alone and in association with vacancies, hydrogen, and transition metal atoms. Among these, the negatively charged nitrogen-vacancy (NV-) defect in diamond is attracting particular current interest in account of the many new and exciting opportunities it offers for, for example, quantum technologies, nanoscale magnetometry, and biosensing.
Abstract: Large (> 100 mm3), relatively pure (type II) and low birefringence single crystal diamond can be produced by high pressure high temperature (HPHT) synthesis. In this study we examine a HPHT sample of good crystalline perfection, containing less than 1 ppb (part per billion carbon atoms) of boron impurity atoms in the {001} growth sector and only tens of ppb of nitrogen impurity atoms. It is shown that the boundaries between {111} and {113} growth sectors are decorated by negatively charged nitrogen vacancy centres (NV?): no decoration is observed at any other type of growth sector interface. This decoration can be used to calculated the relative {111} and {113} growth rates. The bulk (001) sector contains concentrations of luminescent point defects (excited with 488 and 532 nm wavelengths) below 1011 cm?3 (10?3 ppb). We observe the negatively charged silicon-vacancy (SiV?) defect in the bulk {111} sectors along with a zero phonon line emission associated with a nickel defect at 884 nm (1.40 eV). No preferential orientation is seen for either NV? or SiV? defects, but the nickel related defect is oriented with its trigonal axis along the <111> sector growth direction. Since the NV? defect is expected to readily re-orientate at HPHT diamond growth temperatures, no preferential orientation is expected for this defect but the lack of preferential orientation of SiV? in {111} sectors is not explained.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 266.
Global
Fluoresence
Abstract: Photoluminescence (PL) and phosphorescence underpin many of the discrimination techniques used to separate natural from synthetic diamond. PL is at the heart of many new quantum technologies based on color centers in lab-grown diamonds. In HPHT synthetic diamond, the phosphorescence observed is explained in terms of donor-acceptor pair recombination. The thermal activation of electrons to neutral boron acceptors shows that boron plays a key role in the phosphorescence process. However, there are a number of things we struggle to explain. For example, the phosphorescence peak positions are not fully explained, and there is no conclusive link between the emission and charge transfer involving the substitutional nitrogen donor. Secondly, the origin of the phosphorescence observed in some synthetic diamond samples grown by the CVD process is unclear. Although we now have evidence for unintentional boron impurity incorporation at stop-start growth boundaries in some CVD syn- thetic samples, it is possible that some of the observed phosphorescence does not involve boron impurities. In this paper we report on the results of combined fluorescence, phosphorescence, thermoluminescence, and quantitative charge transfer investigations undertaken on both HPHT and CVD synthetic diamond, with the objective of identifying which defects are involved in the fluorescence and phosphorescence processes.
Geochemistry: Exploration, Environment, Analysis, Vol. 19, pp. 414-430.
Canada, British Columbia
geochemistry
Abstract: Using Rock Canyon Creek REE-F-Ba deposit as an example, we demonstrate the need for verifying inherited geochemical data. Inherited La, Ce, Nd, and Sm data obtained by pressed pellet XRF, and La and Y data obtained by aqua regia digestion ICP-AES for 300 drill-core samples analysed in 2009 were compared to sample subsets reanalysed using lithium metaborate-tetraborate (LMB) fusion ICP-MS, Na2O2 fusion ICP-MS, and LMB fusion-XRF. We determine that LMB ICP-MS and Na2O2 ICP-MS accurately determined REE concentrations in SY-2 and SY-4, and provided precision within 10%. Fusion-XRF was precise for La and Nd at concentrations exceeding ten times the lower detection limit; however, accuracy was not established because REE concentrations in SY-4 were below the lower detection limit. Analysis of the sample subset revealed substantial discrepancies for Ce concentrations determined by pressed pellet XRF in comparison to other methods due to Ba interference. Samarium, present in lower concentrations than other REE compared, was underestimated by XRF methods relative to ICP-MS methods. This may be due to Sm concentrations approaching the lower detection limits of XRF methods, elemental interference, or inadequate background corrections. Aqua regia dissolution ICP-AES results, reporting for La and Y, are underestimated relative to other methods.
Abstract: Several interstellar environments produce 'anomalous microwave emission', with brightness-peaks at tens-of-gigahertz frequencies. The emission's origins are uncertain - rapidly-spinning nano-particles could emit electric-dipole radiation, but polycyclic aromatic hydrocarbons proposed as the carrier are now found not to correlate with Galactic signals. The difficulty is to identify co-spatial sources over long lines of sight. Here we identify anomalous microwave emission in three proto-planetary discs. These are the only known systems that host hydrogenated nano-diamonds, in contrast to very common detection of polycyclic aromatic hydrocarbons. Spectroscopy locates the nano-diamonds close to the host-stars, at physically-constrained temperatures. Developing disc models, we reproduce the emission with diamonds 0.75-1.1 nanometres in radius, holding less than or equal to 1-2 per cent of the carbon budget. The microwave-emission:stellar-luminosity ratios are approximately constant, allowing nano-diamonds to be ubiquitous but emitting below detection thresholds in many star-systems. This can unify the findings with similar-sized diamonds found within solar system meteorites. As nano-diamond spectral absorption is seen in interstellar sightlines, these particles are also a candidate for generating galaxy-scale anomalous microwave emission.
Coexisting Garnets and Ilmenites Synthesized at High Pressure pressures from Pyrolite and Olivine Basanite and Their Significance for Kimberlitic Assemblages.
Contributions to Mineralogy and Petrology, Vol. 50, PP. 217-229.
Ultramafic Xenoliths from Lake Bulletinen Merri and Mt. Leura, South East Australia, and Their Bearing on the Evolution of The Continental Upper Mantle.
Proceedings of Third International Kimberlite Conference, TERRA COGNITA, ABSTRACT VOLUME., Vol. 2, No. 3, P. 230, (abstract.).
high pressure experimental calibration of the olivine ortho pyroxene spinel oxygen geobarometer-implications for the oxidation state of the upper mantle
Contributions to Mineralogy and Petrology, Vol. 107, No. 1, pp. 27-40
Mineral chem. of silicate and oxide phases from fertile peridotite equilibrated with a C-O-H fluid phase- a low fO2 dat a set- evaluation of mineralbarometers, therM.
Proceedings of Fifth International Kimberlite Conference held Araxa June 1991, Servico Geologico do Brasil (CPRM) Special, pp. 417-419
Tian, W., Chen, B., Ireland, T.R., Green, D.H., Suzuki, K., Chu, Z.
Petrology and geochemistry of dunites, chromitites and mineral inclusions from the Gaositai Alaskan type complex, North Chin a craton: mantle source characters
Physics and Chemistry of Minerals, Vol. 42, 2, pp. 95-122.
Mantle
Peridotite
Abstract: For over 50 years, the use of high-pressure piston/cylinder apparatus combined with an increasing diversity of microbeam analytical techniques has enabled the study of mantle peridotite compositions and of magmas derived by melting in the upper mantle. The experimental studies have been guided by the petrology and geochemistry of peridotites from diverse settings and by the remarkable range of mantle-derived magma types. Recent experimental study using FTIR spectroscopy to monitor water content of minerals has shown that fertile lherzolite (MORB-source upper mantle) at ~1,000 °C can store ~200 ppm H2O in defect sites in nominally anhydrous minerals (olivine, pyroxenes, garnet and spinel). Water in excess of 200 ppm stabilizes amphibole (pargasite) at P < 3 GPa up to the lherzolite solidus. However, at P > 3 GPa, water in excess of 200 ppm appears as an aqueous vapour phase and this depresses the temperature of the upper mantle solidus. Provided the uppermost mantle (lithosphere) has H2O < 4,000 ppm, the mantle solidus has a distinctive P, T shape. The temperature of the vapour-undersaturated or dehydration solidus is approximately constant at 1,100 °C at pressures up to ~3 GPa and then decreases sharply to ~1,010 °C. The strongly negative dT/dP of the vapour-undersaturated solidus of fertile lherzolite from 2.8 to 3 GPa provides the basis for understanding the lithosphere/asthenosphere boundary. Through upward migration of near-solidus hydrous silicate melt, the asthenosphere becomes geochemically zoned with the ‘enriched’ intraplate basalt source (>500 ppm H2O) overlying the ‘depleted’ MORB source (~200 ppm H2O). From the study of primitive MOR picrites, the modern mantle potential temperature for MORB petrogenesis is ~1,430 °C. The intersection of the 1,430 °C adiabat with the vapour-saturated lherzolite solidus at ~230 km suggests that upwelling beneath mid-ocean ridges begins around this depth. In intraplate volcanism, diapiric upwelling begins from shallower depths and lower temperatures within the asthenosphere and the upwelling lherzolite is enriched in water, carbonate and incompatible elements. Magmas including olivine melilitites, olivine nephelinites, basanites, alkali picrites and tholeiitic picrites are consequences of increasing melt fraction and decreasing pressure at melt segregation. Major element, trace element and isotopic characteristics of island chain or ‘hot-spot’ magmas show that they sample geochemically distinct components in the upper mantle, differing from MORB sources. There is no evidence for higher-temperature ‘hot-spot’ magmas, relative to primitive MORB, but there is evidence for higher water, CO2 and incompatible element contents. The distinctive geochemical signatures of ‘hot-spot’ magmas and their ‘fixed’ position and long-lived activity relative to plate movement are attributed to melt components derived from melting at interfaces between old, oxidised subducted slabs (suspended beneath or within the deeper asthenosphere) and ambient, reduced mantle. In convergent margin volcanism, the inverted temperature gradients inferred for the mantle wedge above the subducting lithosphere introduce further complexity which can be explored by overlaying the phase relations of appropriate mantle and crustal lithologies. Water and carbonate derived from the subducted slab play significant roles, magmas are relatively oxidised, and distinctive primary magmas such as boninites, adakites and island arc ankaramites provide evidence for fluxing of melting in refractory harzburgite to lherzolite by slab-derived hydrous adakitic melt and by wedge-derived carbonatite.
Physics and Chemistry of Minerals, Vol. 42, pp. 95-102.
Mantle
Peridotite
Abstract: For over 50 years, the use of high-pressure piston/cylinder apparatus combined with an increasing diversity of microbeam analytical techniques has enabled the study of mantle peridotite compositions and of magmas derived by melting in the upper mantle. The experimental studies have been guided by the petrology and geochemistry of peridotites from diverse settings and by the remarkable range of mantle-derived magma types. Recent experimental study using FTIR spectroscopy to monitor water content of minerals has shown that fertile lherzolite (MORB-source upper mantle) at ~1,000 °C can store ~200 ppm H2O in defect sites in nominally anhydrous minerals (olivine, pyroxenes, garnet and spinel). Water in excess of 200 ppm stabilizes amphibole (pargasite) at P < 3 GPa up to the lherzolite solidus. However, at P > 3 GPa, water in excess of 200 ppm appears as an aqueous vapour phase and this depresses the temperature of the upper mantle solidus. Provided the uppermost mantle (lithosphere) has H2O < 4,000 ppm, the mantle solidus has a distinctive P, T shape. The temperature of the vapour-undersaturated or dehydration solidus is approximately constant at 1,100 °C at pressures up to ~3 GPa and then decreases sharply to ~1,010 °C. The strongly negative dT/dP of the vapour-undersaturated solidus of fertile lherzolite from 2.8 to 3 GPa provides the basis for understanding the lithosphere/asthenosphere boundary. Through upward migration of near-solidus hydrous silicate melt, the asthenosphere becomes geochemically zoned with the ‘enriched’ intraplate basalt source (>500 ppm H2O) overlying the ‘depleted’ MORB source (~200 ppm H2O). From the study of primitive MOR picrites, the modern mantle potential temperature for MORB petrogenesis is ~1,430 °C. The intersection of the 1,430 °C adiabat with the vapour-saturated lherzolite solidus at ~230 km suggests that upwelling beneath mid-ocean ridges begins around this depth. In intraplate volcanism, diapiric upwelling begins from shallower depths and lower temperatures within the asthenosphere and the upwelling lherzolite is enriched in water, carbonate and incompatible elements. Magmas including olivine melilitites, olivine nephelinites, basanites, alkali picrites and tholeiitic picrites are consequences of increasing melt fraction and decreasing pressure at melt segregation. Major element, trace element and isotopic characteristics of island chain or ‘hot-spot’ magmas show that they sample geochemically distinct components in the upper mantle, differing from MORB sources. There is no evidence for higher-temperature ‘hot-spot’ magmas, relative to primitive MORB, but there is evidence for higher water, CO2 and incompatible element contents. The distinctive geochemical signatures of ‘hot-spot’ magmas and their ‘fixed’ position and long-lived activity relative to plate movement are attributed to melt components derived from melting at interfaces between old, oxidised subducted slabs (suspended beneath or within the deeper asthenosphere) and ambient, reduced mantle. In convergent margin volcanism, the inverted temperature gradients inferred for the mantle wedge above the subducting lithosphere introduce further complexity which can be explored by overlaying the phase relations of appropriate mantle and crustal lithologies. Water and carbonate derived from the subducted slab play significant roles, magmas are relatively oxidised, and distinctive primary magmas such as boninites, adakites and island arc ankaramites provide evidence for fluxing of melting in refractory harzburgite to lherzolite by slab-derived hydrous adakitic melt and by wedge-derived carbonatite.
Abstract: Primary or parental magmas act as probes to infer eruption and source temperatures for both mid-ocean ridge (MOR) and ‘hot-spot’ magmas (tholeiitic picrites). The experimental petrogenetic constraints (‘inverse’ experiments) argue for no significant temperature differences between them. However, there are differences in major, minor and trace elements which characterise geochemical, not thermal, anomalies beneath ‘hot-spots’. We suggest that diapiric upwelling from interfaces (redox contrasts) between old subducted slab and normal MOR basalt source mantle is the major reason for the observed characteristics of island chain or ‘hot-spot’ volcanism. Intraplate basalts also include widely distributed volcanic centres containing lherzolite xenoliths, i.e. mantle-derived magmas. Inverse experiments on olivine basalt, alkali olivine basalt, olivine basanite, olivine nephelinite, olivine melilitite and olivine leucitite (lamproite) determined liquidus phases as a function of pressure, initially under anhydrous and CO2-absent conditions. Under C- and H-absent conditions, only tholeiites to alkali olivine basalts had Ol + Opx ± Cpx as high-pressure liquidus phases. Addition of H2O accessed olivine basanites at 2.5-3 GPa, ~1,200 °C, but both CO2 and H2O were necessary to obtain saturation with Ol, Opx, Cpx and Ga at 2.5-3.5 GPa for olivine nephelinite and olivine melilitite. The forward and inverse experimental studies are combined to formulate a petrogenetic grid for intraplate, ‘hot-spot’ and MOR magmatism within the plate tectonics paradigm. The asthenosphere is geochemically zoned by slow upward migration of incipient melt. The solidus and phase stabilities of lherzolite with very small water contents (<3,000 ppm) determine the thin plate behaviour of the oceanic lithosphere and thus the Earth’s convection in the form of plate tectonics. There is no evidence from the parental magmas of MOR and ‘hot-spots’ to support the ‘deep mantle thermal plume’ hypothesis. The preferred alternative is the presence of old subducted slabs, relatively buoyant and oxidised with respect to MORB source mantle and suspended or upwelling in or below the lower asthenosphere (and thus detached from overlying plate movement).
Journal of Metamorphic Geology, doi.org.10.1111/ jmg.12484
Global
geochemistry
Abstract: This essay in honour of Mike Brown addresses aspects of chemical equilibrium and equilibration in rocks, with a focus on the role that chemical potentials play. Chemical equilibrium is achieved by diffusive flattening of chemical potential gradients. The idea of equilibration volume is developed, and the way equilibration volumes may evolve along a pressure-temperature path is discussed. The effect of the environment of an equilibration volume is key to understanding the evolution of the equilibration volume with changing conditions. The likely behaviour of equilibration volumes is used to suggest why preservation of equilibrium mineral assemblages and mineral compositions from metamorphism tends to occur. This line of logic then provides the conceptual support to conventional equilibrium thermodynamic approaches to studying rocks, using, for example, thermobarometry and pseudosections.
Trace element partitioning between mica and amphibole bearing garnet lherzolite and hydrous basanitic melt: 1. experimental results and the investigation controls
Contributions to Mineralogy and Petrology, Online, available
Adam, J., Oberti, R., Camara, F., Green, T.H., Rushmer, T.
The effect of water on equilibrium relations between clinopyroxenes and basanitic magmas: tracing water and non- volatile incompatible elements in the Earth's mantle.
Dorfman, S.M., Potapkin, V., Lv, M., Greenberg, E., Kupenko, I., Chumakov, A.I., Bi, W., Alp, E.E., Liu, J., Magrez, A., Dutton, S.E., Cava, R.J., McCammon, C.A., Gillet, P.
American Mineralogist, Vol. 105, pp. 1030-1039. pdf
Mantle
redox
Abstract:
Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mössbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mössbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10-50% Fe/total cations, 0-25% Al/total cations, 12-100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mössbauer reference library for bridgmanite and demonstrate the effects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at ~70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mössbauer sites with center shift ~1 mm/s and quadrupole splitting 2.4-3.1 and 3.9 mm/s at ~70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the effects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to ~20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle.
Nature Communications, doi.org/10.10.1038 /s41467-021-21761-9 8p. Pdf
Mantle
diamond inclusions
Abstract: The stable forms of carbon in Earth’s deep interior control storage and fluxes of carbon through the planet over geologic time, impacting the surface climate as well as carrying records of geologic processes in the form of diamond inclusions. However, current estimates of the distribution of carbon in Earth’s mantle are uncertain, due in part to limited understanding of the fate of carbonates through subduction, the main mechanism that transports carbon from Earth’s surface to its interior. Oxidized carbon carried by subduction has been found to reside in MgCO3 throughout much of the mantle. Experiments in this study demonstrate that at deep mantle conditions MgCO3 reacts with silicates to form CaCO3. In combination with previous work indicating that CaCO3 is more stable than MgCO3 under reducing conditions of Earth’s lowermost mantle, these observations allow us to predict that the signature of surface carbon reaching Earth’s lowermost mantle may include CaCO3.
Abstract: Diamonds are the deepest accessible “fragments” of Earth, providing records of deep geological processes. Absolute ages for diamond formation are crucial to place these records in the correct time context. Diamond ages are typically determined by dating inclusions, assuming that they were formed simultaneously with their hosts. One of the most widely used mineral inclusions for dating diamond is garnet, which is amenable to Sm-Nd geochronology and is common in lithospheric diamonds. By investigating worldwide garnet-bearing diamonds, we provide crystallographic evidence that garnet inclusions that were previously considered to be syngenetic may instead be protogenetic, i.e., they were formed before the host diamond, raising doubts about the real significance of many reported diamond “ages.” Diffusion modeling at relevant pressures and temperatures, however, demonstrates that isotopic resetting would generally occur over geologically short time scales. Therefore, despite protogenicity, the majority of garnet-based ages should effectively correspond to the time of diamond formation. On the other hand, our results indicate that use of large garnet inclusions (e.g., >100 ?m) and diamond hosts formed at temperatures lower than ?1000 °C is not recommended for diamond age determinations.
Abstract: Wehrlite and pyroxenite xenoliths and megacrysts from the Jericho kimberlite were analyzed by ?XRF and EBSD, and for major elements, trace elements, and isotopes (Pb-Sr- O) in major phases. Thermobarometry places these samples at 60 - 180 km and 600 - 1200 ??C. While modes and textures vary, many samples have olivine-olivine grain boundaries with straight edges and 120° angle junctions, indicating granoblastic recrystallisation, while clinopyroxene and orthopyroxene are complexly intergrown. Clinopyroxene twins and subgrains recording orientations distinct from the encapsulating grain were detected using EBSD and are inferred to represent recent modification processes. Several distinct garnet compositions were measured, with multiple thin garnet rims in some samples suggesting possible successive stages of garnet crystallisation. Complex chromium zoning in garnet is detected by ?XRF in several samples (fig.1). Pb-Pb ages for most samples are similar to the age of kimberlite entrainment (173 Ma), but the shallowest pyroxenite sample preserves the most radiogenic Pb composition, intercecting concordia at 0.7 - 1.1 Ga, and is the only sample with ?18O above the mantle range (6.2±0.1 ‰). The deepest sample has the lowest ?18O (5.5±0.1 ‰) and radiogenic 87Sr/86Sr similar to MARID rocks (0.709±1 ‰). These results suggest the Jericho lithosphere experienced several melt/fluid injection events that modified substantial portions of the sampled section soon before kimberlite entrainment.
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.
Abstract: The terminal Mesoproterozoic was a period of widespread tectonic convergence globally, culminating in the amalgamation of the Rodinia supercontinent. However, in Laurentia, long-lived orogenesis on its eastern margin was punctuated by short-lived extension that generated the Midcontinent Rift ca. 1110-1090 Ma. Whereas this cratonic rift basin is typically considered an isolated occurrence, a series of new depositional ages demonstrate that multiple cratonic basins in northern Laurentia originated around this time. We present a Re-Os isochron date of 1087.1 ± 5.9 Ma from organic-rich shales of the Agu Bay Formation of the Fury and Hecla Basin, which is one of four closely spaced cratonic basins spanning from northeastern Canada to northwestern Greenland known as the Bylot basins. This age is identical, within uncertainty, to ages from the Midcontinent Rift and the Amundsen Basin in northwestern Canada. These ages imply that the late Mesoproterozoic extensional episode in Laurentia was widespread and likely linked to a common origin. We propose that significant thermal anomalies and mantle upwelling related to supercontinent assembly centered around the Midcontinent Rift influenced the reactivation of crustal weaknesses in Arctic Laurentia beginning ca. 1090 Ma, triggering the formation of a series of cratonic basins.
International Journal of Earth Sciences, Vol. 107, 3, pp. 787-810.
Mantle
geochemistry
Abstract: Archean and Proterozoic subcontinental lithospheric mantle (SLM) is compared using 83 similarly incompatible element ratios (SIER; minimally affected by % melting or differentiation, e.g., Rb/Ba, Nb/Pb, Ti/Y) for >3700 basalts from ten continental flood basalt (CFB) provinces representing nine large igneous provinces (LIPs). Nine transition metals (TM; Fe, Mn, Sc, V, Cr, Co, Ni, Cu, Zn) in 102 primitive basalts (Mg# = 0.69-0.72) from nine provinces yield additional SLM information. An iterative evaluation of SIER values indicates that, regardless of age, CFB transecting Archean lithosphere are enriched in Rb, K, Pb, Th and heavy REE(?); whereas P, Ti, Nb, Ta and light REE(?) are higher in Proterozoic-and-younger SLM sources. This suggests efficient transfer of alkali metals and Pb to the continental lithosphere perhaps in association with melting of subducted ocean floor to form Archean tonalite-trondhjemite-granodiorite terranes. Titanium, Nb and Ta were not efficiently transferred, perhaps due to the stabilization of oxide phases (e.g., rutile or ilmenite) in down-going Archean slabs. CFB transecting Archean lithosphere have EM1-like SIER that are more extreme than seen in oceanic island basalts (OIB) suggesting an Archean SLM origin for OIB-enriched mantle 1 (EM1). In contrast, OIB high U/Pb (HIMU) sources have more extreme SIER than seen in CFB provinces. HIMU may represent subduction-processed ocean floor recycled directly to the convecting mantle, but to avoid convective homogenization and produce its unique Pb isotopic signature may require long-term isolation and incubation in SLM. Based on all TM, CFB transecting Proterozoic lithosphere are distinct from those cutting Archean lithosphere. There is a tendency for lower Sc, Cr, Ni and Cu, and higher Zn, in the sources for Archean-cutting CFB and EM1 OIB, than Proterozoic-cutting CFB and HIMU OIB. All CFB have SiO2 (pressure proxy)-Nb/Y (% melting proxy) relationships supporting low pressure, high % melting resembling OIB tholeiites, but TM concentrations do not correlate with % melting. Thus, the association of layered intrusion (plutonic CFB) TM deposits with Archean terranes does not appear related to higher metal concentrations or higher percentages of melting in Archean SLM. Other characteristics of these EM1-like magmas (e.g., S2 or O2 fugacity) may lead to element scavenging and concentration during differentiation to form ore deposits.
Abstract: Eocene paleoclimate reconstructions are rarely accompanied by parallel estimates of CO2 from the same locality, complicating assessment of the equilibrium climate response to elevated CO2. We reconstruct temperature, precipitation, and CO2 from latest middle Eocene (ca. 38 Ma) terrestrial sediments in the posteruptive sediment fill of the Giraffe kimberlite in subarctic Canada. Mutual climatic range and oxygen isotope analyses of botanical fossils reveal a humid-temperate forest ecosystem with mean annual temperatures (MATs) more than 17 °C warmer than present and mean annual precipitation ?4× present. Metasequoia stomatal indices and gas-exchange modeling produce median CO2 concentrations of ?630 and ?430 ppm, respectively, with a combined median estimate of ?490 ppm. Reconstructed MATs are more than 6 °C warmer than those produced by Eocene climate models forced at 560 ppm CO2. Estimates of regional climate sensitivity, expressed as ?MAT per CO2 doubling above preindustrial levels, converge on a value of ?13 °C, underscoring the capacity for exceptional polar amplification of warming and hydrological intensification under modest CO2 concentrations once both fast and slow feedbacks become expressed.
45th. Annual Yellowknife Geoscience Forum, p. 65 abstract
Canada, Northwest Territories
deposit - Giraffe
Abstract: Several Lac de Gras kimberlite pipes host thick accumulations of stratified post-eruptive lacustrine sediment and peat. Given the range of Lac de Gras kimberlite emplacement ages, these fills - though rare - provide a unique sedimentary archive of paleoenvironments during the sustained Early Cenozoic “greenhouse” interval, in a high-latitude region otherwise devoid of Phanerozoic sediment cover. Extensive exploration drilling has provided a valuable window into this unique sedimentary record, which would have otherwise remained covered by Quaternary glacial deposits. Our focus to date has been multidisciplinary study of the Giraffe pipe sediment fill: an ~80 m-thick sequence of post-eruptive lacustrine silt overlain by peat, which paints a remarkable picture of a humid-temperate Middle Eocene forest ecosystem on the Canadian Shield. Post-eruptive chronology is provided by interbedded distal tephra horizons, likely sourced from Alaska, that have been dated by glass fission-track and zircon U-Pb techniques. Paleoclimate proxies derived from pollen, wood cellulose oxygen isotopes, and biomarkers converge on reconstructed mean annual temperatures >17 °C warmer than present, with mean winter temperatures above freezing, and mean annual precipitation ~4x present. Two independent reconstructions of CO2 from well preserved conifer foliage suggest that this warming occurred under relatively modest atmospheric CO2 concentrations of 430-630 ppm. These findings provide direct field-based evidence for dramatic past arctic warming at CO2 concentrations that were well within the range of projections under “business-as-usual” emissions scenarios, underscoring the capacity for exceptional polar amplification of climate change under modest CO2 concentrations once both fast and slow feedbacks processes become expressed. Our studies at Giraffe pipe also highlight the scientific value of archived exploration drill core in the Lac de Gras kimberlite field, particularly with respect to pipes that are unremarkable for the purpose of diamond exploration.
Oxygen isotope evidence for the origin of pyroxenites in the Beni Bousera peridotite massif, North Morocco: derivation from subducted oceaniclithosphere
Earth and Planetary Science Letters, Vol. 102, No. 3/4, March pp. 289-301
Earth and Planetary Science Letters, Vol. 453, pp. 146-156.
Canada, Ontario
Gravity
Abstract: The Earth's surface was depressed under the weight of ice during the last glaciations. Glacial Isostatic Adjustment (GIA) induces the slow recession of the trough that is left after deglaciation and is responsible for a contemporary uplift rate of more than 1 cm/yr around Hudson Bay. The present-day residual depression, an indicator of still-ongoing GIA, is difficult to identify in the observed topography, which is predominantly sensitive to crustal heterogeneities. According to the most widespread GIA models, which feature a viscosity of on top of the lower mantle, the trough is approximately 100 m deep and cannot explain the observed gravity anomalies across North America. These large anomalies are therefore usually attributed to subcontinental density heterogeneities in the tectosphere or to slab downwelling in the deep mantle.
Earth Planetary Science Letters, Vol. 487, pp. 210-220.
Africa
tectonics
Abstract: Rifts are often associated with ancient traces of hotspots, which are supposed to participate to the weakening of the lithosphere. We investigated the expected past trajectories followed by three hotspots (Afar, East-Africa and Lake-Victoria) located around the Red Sea. We used a hotspot reference frame to compute their location with respect to time, which is then compared to mantle tomography interpretations and geological features. Their tracks are frequently situated under continental crust, which is known to strongly filter plume activity. We looked for surface markers of their putative ancient existence, such as volcanism typology, doming, and heat-flow data from petroleum wells. Surface activity of the East-Africa hotspot is supported at 110 Ma, 90 Ma and 30 Ma by uplift, volcanic activity and rare gas isotopic signatures, reminiscent of a deep plume origin. The analysis of heat-flow data from petroleum wells under the Arabian plate shows a thermal anomaly that may correspond to the past impact of the Afar hotspot. According to derived hotspot trajectories, the Afar hotspot, situated (at 32 Ma) 1000 km north-east of the Ethiopian-Yemen traps, was probably too far away to be accountable for them. The trigger of the flood basalts would likely be linked to the East-Africa hotspot. The Lake-Victoria hotspot activity appears to have been more recent, attested only by Cenozoic volcanism in an uplifted area. Structural and thermal weakening of the lithosphere may have played a major role in the location of the rift systems. The Gulf of Aden is located on inherited Mesozoic extensional basins between two weak zones, the extremity of the Carlsberg Ridge and the present Afar triangle, previously impacted by the East-Africa hotspot. The Red Sea may have opened in the context of extension linked to Neo-Tethys slab-pull, along the track followed by the East Africa hotspot, suggesting an inherited thermal weakening.
The Tornquist Zone, north east inclining lithospheric transition at the south western margin of the Baltic Shield: revealed through a nonlinear teleseismic tomographic inversion.
Tectonophysics, Vol. 416, 1-4, April 5, pp. 151-166.
Enigmatic relationship between silicic and volcanic and plutonic rocks: geophysical evidence for silicic crustal melt in the continents: where. What kind, and how much?
Gregoire, M., Tinguely, C., Bell, D.R., Le Roex, A.P.
Spinel lherzolite xenoliths from the Premier kimberlite ( Kaapvaal craton) South Africa: nature and evolution of the shallow upper mantle beneath Bushveld Complex.
On the iron isotope heterogeneity of lithospheric mantle xenoliths: implications for mantle metasomatism, the origin of basalts and the iron isotope composition of the Earth.
Contributions to Mineralogy and Petrology, Vol. 165, 6, pp. 1243-1258.
Abstract: Pyroxenites and garnet pyroxenites are mantle heterogeneities characterized by a lower solidus temperature than the enclosing peridotites; it follows that they are preferentially involved during magma genesis. Constraining their origin, composition, and the interactions they underwent during their subsequent evolution is therefore essential to discuss the sources of magmatism in a given area. Pyroxenites could represent either recycling of crustal rocks in mantle domains or mantle originated rocks (formed either by olivine consuming melt-rock reactions or by crystal fractionation). Petrological and geochemical (major and trace elements, Sr-Nd and O isotopes) features of xenoliths from various occurrences (French Massif-Central, Jordan, Morocco and Cameroon) show that these samples represent cumulates crystallized during melt percolation at mantle conditions. They formed in mantle domains at pressures of 1-2 GPa during post-collisional magmatism (possibly Hercynian for the French Massif-Central, and Panafrican for Morocco, Jordan and Cameroon). The thermal re-equilibration of lithospheric domains, typical of the late orogenic exhumation stages, is also recorded by the samples. Most of the samples display a metasomatic overprint that may be either inherited or likely linked to the recent volcanic activity that occurred in the investigated regions. The crystallization of pyroxenites during late orogenic events has implications for the subsequent evolution of the mantle domains. The presence of large amounts of mantle pyroxenites in old orogenic regions indeed imparts peculiar physical and chemical characteristics to these domains. Among others, the global solidus temperature of the whole lithospheric domain will be lowered; in turn, this implies that old orogenic regions are refertilized zones where magmatic activity would be enhanced.
Abstract: It is now accepted that significant volumes of pyroxenites are generated in the subduction factory and remain trapped in the mantle. In ophiolites and orogenic massifs the geometry of pyroxenite layers and their relationships with the host peridotite can be observed directly. Since a large part of what is known about the upper mantle structure is derived from the analysis of seismic waves, it is crucial to integrate pyroxenites in the interpretations. We modeled the seismic properties of a peridotitic mantle rich in pyroxenite layers in order to determine the impact of layering on the seimsic properties. To do so, EBSD data on deformed and undeformed pyroxenites from the Cabo Ortegal complex (Spain) and the Trinity ophiolite (California, USA) respectively are combined with either A or B-type olivine fabrics in order to model a realistic pyroxenite-rich upper mantle. Consideration of pyroxeniterich domains within the host mantle wall rock is incorporated in the calculations using the Schoenberg and Muir group theory [1]. This quantification reveals the complex dependence of the seismic signal on the deformational state and relative abundance of each mineral phase. The incorporation of pyroxenites properties into geophysical interpretations in understanding the lithospheric structure of subduction zones will lead to more geologically realistic models.
Abstract: Mantle lithologies in orogenic massifs and xenoliths commonly display strikingly different Hf- and Nd-isotope compositions compared to oceanic basalts. While the presence of pyroxenites has long been suggested in the source region of mantle-derived magmas, very few studies have reported their combined HfNd isotope compositions. We here report the first LuHf data along with ReOs data and S concentrations on the Cabo Ortegal Complex, where the pyroxenite-rich Herbeira massif has been interpreted as remnants of a delaminated arc root. The pyroxenites, chromitites and their host harzburgites show a wide range of whole-rock 187Re/188Os and 187Os/188Os (0.16-1.44), indicating that Re was strongly mobilized, partly during hydrous retrograde metamorphism but mostly during supergene alteration that preferentially affected low-Mg#, low Cu/S pyroxenites. Samples that escaped this disturbance yield an isochron age of 838 ± 42 Ma, interpreted as the formation of Cabo Ortegal pyroxenites. Corresponding values of initial 187Os/188Os (0.111-0.117) are relatively unradiogenic, suggesting limited contributions of slab-derived Os to primitive arc melts such as those parental to these pyroxenites. This interpretation is consistent with radiogenic Os in arc lavas being mostly related to crustal assimilation. Paleoproterozoic to Archean Os model ages confirm that Cabo Ortegal pyroxenites record incipient volcanic arc magmatism on the continental margin of the Western African Craton, as notably documented by zircon UPb ages of 2.1 and 2.7 Ga. LuHf data collected on clinopyroxene and amphibole separates and whole-rock samples are characterized by uncorrelated 176Lu/177Hf and 176Hf/177Hf (0.2822-0.2855), decoupled from Nd-isotope compositions. This decoupling is ascribed to diffusional disequilibrium during melt-peridotite interaction, in good agreement with the results of percolation-diffusion models simulating the interaction of an arc melt with an ancient melt-depleted residue. These models notably show that HfNd isotopic decoupling such as recorded by Cabo Ortegal pyroxenites and peridotites (??Hf(i) up to +97) is enhanced during melt-peridotite interaction by slow diffusional re-equilibration and can be relatively insensitive to chromatographic fractionation. Finally, we discuss the hypothesis that arc-continent interaction may provide preferential conditions for such isotopic decoupling and propose that its ubiquitous recognition in peridotites reflects the recycling of sub-arc mantle domains derived from ancient, reworked SCLM.
Journal of Petrology, https://doi.org/ 10.1093/petrology /egaa040 50p. Pdf
Africa, South Africa
kimberlites
Abstract: Eclogite xenoliths, together with garnet pyroxenites and some mafic garnet granulites, found in kimberlites located along the southern margin of the Kaapvaal craton in southern Africa have been analysed by electron microprobe and mass spectrometry techniques to determine their geochemical characteristics. The majority of eclogites are bimineralic with garnet and omphacitic clinopyroxene in subequal proportions, with rutile as the main accessory phase; a few contain kyanite. Based on K2O in clinopyroxene and Na2O in garnet, the eclogites can be classified as Group II eclogites, and the majority are high-Ca in character. Garnet pyroxenites comprise garnet clinopyroxenites and garnet websterites. Major and trace element concentrations and isotope ratios of reconstituted bulk rock compositions of the eclogites and garnet pyroxenites allow constraints to be placed on depth of origin and likely protolith history. Calculated Fe–Mg exchange equilibration temperatures for the eclogites range from 815 to 1000?°C, at pressures of 1·7?±?0·4?GPa as determined by REE partitioning, indicating that they were sampled from depths of 50–55?km; i.e. within the lower crust of the Namaqua–Natal Belt. The garnet pyroxenites show slightly lower temperatures (686–835?°C) at similar pressures of equilibration. Initial 143Nd/144Nd and 87Sr/86Sr ratios (calculated to time of kimberlite emplacement) of both lithologies overlap the field for lower crustal samples from the Namaqua–Natal Belt. Further evidence for a crustal origin is found in the similar REE patterns shown by many of the associated garnet granulite xenoliths. Garnet pyroxenites are interpreted to have a similar origin as the associated eclogites but with the mafic protolith having insufficient Na (i.e. low modal plagioclase) to allow for development of omphacitic pyroxene. Metamorphism of the mafic protoliths to these eclogites and garnet pyroxenites is inferred to have occurred during crustal shortening and thickening associated with the collision of the Namaqua–Natal Belt with the Kaapvaal craton at 1–1·2?Ga.
Abstract: Eclogite xenoliths, together with garnet pyroxenites and some mafic garnet granulites, found in kimberlites located along the southern margin of the Kaapvaal craton in southern Africa have been analysed by electron microprobe and mass spectrometry techniques to determine their geochemical characteristics. The majority of eclogites are bimineralic with garnet and omphacitic clinopyroxene in subequal proportions, with rutile as the main accessory phase; a few contain kyanite. Based on K2O in clinopyroxene and Na2O in garnet, the eclogites can be classified as Group II eclogites, and the majority are high-Ca in character. Garnet pyroxenites comprise garnet clinopyroxenites and garnet websterites. Major and trace element concentrations and isotope ratios of reconstituted bulk rock compositions of the eclogites and garnet pyroxenites allow constraints to be placed on depth of origin and likely protolith history. Calculated Fe-Mg exchange equilibration temperatures for the eclogites range from 815 to 1000?°C, at pressures of 1•7?±?0•4?GPa as determined by REE partitioning, indicating that they were sampled from depths of 50-55?km; i.e. within the lower crust of the Namaqua-Natal Belt. The garnet pyroxenites show slightly lower temperatures (686-835?°C) at similar pressures of equilibration. Initial 143Nd/144Nd and 87Sr/86Sr ratios (calculated to time of kimberlite emplacement) of both lithologies overlap the field for lower crustal samples from the Namaqua-Natal Belt. Further evidence for a crustal origin is found in the similar REE patterns shown by many of the associated garnet granulite xenoliths. Garnet pyroxenites are interpreted to have a similar origin as the associated eclogites but with the mafic protolith having insufficient Na (i.e. low modal plagioclase) to allow for development of omphacitic pyroxene. Metamorphism of the mafic protoliths to these eclogites and garnet pyroxenites is inferred to have occurred during crustal shortening and thickening associated with the collision of the Namaqua-Natal Belt with the Kaapvaal craton at 1-1•2?Ga.
Abstract: Carbonatites are carbonate-dominated igneous rocks derived by low-degree partial melting of metasomatized mantle, although the geodynamic processes responsible for their emplacement into the crust are disputed. Current models favor either reactivation of lithospheric structures in response to plate movements, or the impingement of mantle plumes. Geochronology provides a means of testing these models, but constraining the age of carbonatites and related metasomatic events is rarely straightforward. We use in situ U-Th-Pb analysis of monazite by SHRIMP to constrain the emplacement age and hydrothermal history of the rare earth element-bearing Gifford Creek Carbonatite Complex in Western Australia, which has been linked to plume magmatism at ca. 1075 Ma. Monazite in carbonatites and related metasomatic rocks (fenites) from the carbonatite complex dates the initial emplacement of the carbonatite at 1361 ± 10 Ma (n = 22, MSWD = 0.91). The complex was subjected to multiple stages of magmatic/hydrothermal overprinting from ca. 1300 Ma to 900 Ma during later regional tectonothermal events. Carbonatite emplacement at ca. 1360 Ma appears to be an isolated igneous event in the region, and occurred about 300 million years before intrusion of the ca. 1075 Ma Warakurna large igneous province, thus precluding a genetic connection. The Gifford Creek Carbonatite Complex occurs within a major crustal suture, and probably formed in response to reactivation of this suture during plate reorganization. Our study demonstrates the veracity of monazite geochronology in determining the magmatic and hydrothermal histories of a carbonatite complex, critical for evaluating competing geodynamic models for carbonatites. The approach involving in situ SHRIMP U-Th-Pb dating of monazite from a wide spectrum of rocks in a carbonatite complex is best suited to establishing the intrusive age and hydrothermal history of carbonatites.
Abstract: Carbonatites are carbonate-dominated igneous rocks derived by low-degree partial melting of metasomatized mantle, although the geodynamic processes responsible for their emplacement into the crust are disputed. Current models favor either reactivation of lithospheric structures in response to plate movements, or the impingement of mantle plumes. Geochronology provides a means of testing these models, but constraining the age of carbonatites and related metasomatic events is rarely straightforward. We use in situ U-Th-Pb analysis of monazite by SHRIMP to constrain the emplacement age and hydrothermal history of the rare earth element-bearing Gifford Creek Carbonatite Complex in Western Australia, which has been linked to plume magmatism at ca. 1075 Ma. Monazite in carbonatites and related metasomatic rocks (fenites) from the carbonatite complex dates the initial emplacement of the carbonatite at 1361 ± 10 Ma (n = 22, MSWD = 0.91). The complex was subjected to multiple stages of magmatic/hydrothermal overprinting from ca. 1300 Ma to 900 Ma during later regional tectonothermal events. Carbonatite emplacement at ca. 1360 Ma appears to be an isolated igneous event in the region, and occurred about 300 million years before intrusion of the ca. 1075 Ma Warakurna large igneous province, thus precluding a genetic connection. The Gifford Creek Carbonatite Complex occurs within a major crustal suture, and probably formed in response to reactivation of this suture during plate reorganization. Our study demonstrates the veracity of monazite geochronology in determining the magmatic and hydrothermal histories of a carbonatite complex, critical for evaluating competing geodynamic models for carbonatites. The approach involving in situ SHRIMP U-Th-Pb dating of monazite from a wide spectrum of rocks in a carbonatite complex is best suited to establishing the intrusive age and hydrothermal history of carbonatites.
Geochimica et Cosmochimica Acta, Vol. 282, pp. 297-323.
Africa, East Africa
carbonatites
Abstract: Rare earth element (REE) enrichments in carbonatites are often described as resulting from late magmatic-hydrothermal or supergene processes. However, magmatic pre-enrichment linked to the igneous processes at the origin of carbonatites are likely to contribute to the REE fertilisation. Experimental constraints reveals that immiscibility processes between carbonate and silicate melts can lead to both REE enrichments and depletions in carbonatites making the magmatic processes controlling REE enrichments unclear.
We link REE contents of carbonatites to the magmatic stage at which carbonatites are separated from silicate magma in their course of differentiation. We present results of experiments made at pressure and temperature conditions of alkaline magmas and associated carbonatites differentiation (0.2-1.5 GPa; 725-975?°C; FMQ to FMQ?+?2.5), simultaneously addressing crystal fractionation of alkaline magmas and immiscibility between carbonate (calcio-carbonate type) and silicate melts (nephelinite to phonolite type). The experimental data shows that the degree of differentiation, controlling the chemical composition of alkaline melts, is a key factor ruling the REE concentration of the coexisting immiscible carbonate melts. In order to predict carbonate melt REE enrichments during alkaline magma differentiation, we performed a parameterisation of experimental data on immiscible silicate and carbonate melts, based exclusively on the silica content, the alumina saturation index and the alkali/alkaline-earth elements ratio of silicate melts. This parameterisation is applied to more than 1600 geochemical data of silicate magmas from various alkaline provinces (East African Rift, Canary and Cape Verde Islands) and show that REE concentrations of their potential coeval carbonatite melts can reach concentration ranges similar to those of highly REE enriched carbonatites (?REE?>?30 000?ppm) by immiscibility with phonolitic/phono-trachytic melt compositions, while more primitive alkaline magmas can only be immiscible with carbonatites that are not significantly enriched in REE.
India's changing place in global Proterozoic reconstructions: a review of geochronologic constraints and paleomagnetic poles from the Dharwar Bundelk hand and Marwar
Journal of Geodynamics, Vol. 50, 3-4, pp. 224-242.
Sanatmaria-Perez, D., Ruiz-Fuertes, J., Pena-Alvarez, M., Chulia-Jordan, R., Marquerno, T., Zimmer, D., Guterrez-Cano, V., Macleod, S., Gregoryanz, E., Popescue, C., Rodriguez-Herandez, P., Munoz, A.
Abstract: Calcium carbonate is a relevant constituent of the Earth’s crust that is transferred into the deep Earth through the subduction process. Its chemical interaction with calcium-rich silicates at high temperatures give rise to the formation of mixed silicate-carbonate minerals, but the structural behavior of these phases under compression is not known. Here we report the existence of a dense polymorph of Ca5(Si2O7)(CO3)2 tilleyite above 8 GPa. We have structurally characterized the two phases at high pressures and temperatures, determined their equations of state and analyzed the evolution of the polyhedral units under compression. This has been possible thanks to the agreement between our powder and single-crystal XRD experiments, Raman spectroscopy measurements and ab-initio simulations. The presence of multiple cation sites, with variable volume and coordination number (6-9) and different polyhedral compressibilities, together with the observation of significant amounts of alumina in compositions of some natural tilleyite assemblages, suggests that post-tilleyite structure has the potential to accommodate cations with different sizes and valencies.
Earth and Planetary Science Letters, Vol. 401, pp. 132-147.
Africa, South Africa
metasomatism
Abstract: The Karoo igneous rocks (174-185 Ma) of southern Africa represent one of the largest continental flood basalt provinces on Earth. Available evidence indicates that Karoo magmas either originated in the asthenosphere and were extensively modified by interaction with the lithospheric mantle prior to emplacement in the upper crust; or were produced by partial melting of enriched mantle lithosphere. However, no direct evidence of interaction by Karoo melts (or their precursors) with lithospheric mantle rocks has yet been identified in the suites of mantle xenoliths sampled by post-Karoo kimberlites in southern Africa. Here we report U-Pb ages for lindsleyite-mathiasite (LIMA) titanate minerals (crichtonite series) from three metasomatised, phlogopite and clinopyroxene-rich peridotite xenoliths from the ?84 Ma Bultfontein kimberlite (Kimberley, South Africa), located in the southern part of the Karoo magmatic province. The LIMA minerals appear to have formed during metasomatism of the lithospheric mantle by fluids enriched in HFSE (Ti, Zr, Hf, Nb), LILE (K, Ba, Ca, Sr) and LREE. LIMA U-Pb elemental and isotopic compositions were measured in situ by LA-ICP-MS methods, and potential matrix effects were evaluated by solution-mode analysis of mineral separates. LIMA minerals from the three samples yielded apparent U-Pb ages of , and (). A single zircon grain extracted from the ?190 Ma LIMA-bearing sample produced a similar U-Pb age of , within uncertainty of the LIMA ages. These data provide the first robust evidence of fluid enrichment in the lithospheric mantle beneath the Kimberley region at ?180-190 Ma, and suggest causation of mantle metasomatism by Karoo melts or their precursor(s). The results further indicate that U-Pb dating of LIMA minerals provides a new, accurate tool for dating metasomatic events in the lithospheric mantle.
Abstract: The trace element composition of olivine is becoming increasingly important in petrological studies due to the ubiquity of olivine in the Earth's upper mantle and in primitive magmatic rocks. The LA-ICP-MS method allows for the routine analysis of trace elements in olivine to sub-ppm levels, but a major drawback of this method is the lack of knowledge about possible downhole fractionation effects when non matrix-matched calibration is used. In this contribution, we show that matrix-matched (i.e., olivine-based) calibration is preferable for small laser spot sizes (<100??m) due to significant laser-induced inter-element fractionation between olivine and commonly used silicate glass calibration materials, e.g., NIST SRM 612, GSD-1G and BHVO-2G. As a result, we present two Mg-rich natural olivine standards (355OL and SC-GB) that have been characterized by independent methods (EPMA, solution ICP-MS), and by LA-ICP-MS in four different laboratories. These natural olivines have been used 1) as primary standards for the matrix-matched calibration of olivine samples for most elements of interest (e.g., Li, Na, Al, P, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn), and 2) as secondary standards to assess the accuracy of results. Comparison of olivine- and silicate glass-calibrated results for natural peridotitic olivine reveals that matrix-matched calibration is essential when using small laser spot sizes (<100??m) in order to mitigate downhole fractionation effects for certain elements, especially Na, P, Mn, Co, Ni and Zn. If matrix-matched calibration is not feasible, we recommend that spot sizes of ?100??m, laser fluence of ?4.0?J/cm2, and total laser shot counts of ?250 (e.g., 5?Hz repetition rate for 50?s) are used in order to minimize fractionation effects between olivine and silicate glass calibration materials. We demonstrate the applicability of matrix-matched calibration on olivine from a suite of different mantle peridotite xenoliths sampled by kimberlites and alkali basalts from on-craton and off-craton localities.
Journal of African Earth Sciences, Vol. 146, pp. 28-47.
Africa, Cameroon
craton
Abstract: Field, microstructural, and anisotropy of magnetic susceptibility (AMS, magnetic fabrics) studies assessed the Pan-African deformational history and strain geometry at the southern margin of the Central African Fold Belt (CAFB) against the older, cratonic basement of the Congo Shield (CS). Reflected light microscopy and thermomagnetic studies supported the identification of magnetic minerals. Data cover a low angle thrust margin (Mbengis-Sangmelima area) in the east and high angle shear zones cutting the margin (Kribi area) in the west, at the Atlantic coast. In the CS basement units, magnetic anisotropy is generally higher than in the low grade Pan-African units. In the latter, early D1/D2 shortening produced a flat-lying magnetic foliation parallel with the regional trend of the belt, a shallow magnetic lineation, and mostly oblate fabrics. Subsequent D3 deformation is only of local importance in the Mbengis-Sangmelima area. The magnetic lineation shows distinct maxima in NNE-SSW direction, parallel with the low angle tectonic transport direction. In the Kribi area, the NNE-SSW trending Kribi-Campo shear zone (KCSZ) affected both older rocks and Pan-African high grade metapelites of the Yaoundé unit together with their basal thrust. The early planar fabric (S1) was overprinted during D2 folding under relatively high T conditions, and subsequent D3 wrenching. Magnetic fabrics document a progressive change from oblate towards prolate ellipsoids towards the KCSZ. Magnetic foliations with medium to steep dips curve into the N-S to NE-SW orientation of the KCSZ, lineations follow the same trend with shallow to medium plunges. This fabric implies that the KCSZ is a Pan-African strike-slip shear zone with a subordinate component of compression. Strike-slip tectonics in the west (KCSZ) and thrusting in the east imply N-S to NE-SW convergence during Pan-African terrane assembly against the present northern margin of the CS. In addition, the KCSZ may separate the CS from the São Francisco Craton in Brazil and thus be the northern part of a link connecting the CAFB to the West Congo Belt in the south. This putative Pan-African link separated the São Francisco Craton from the Congo Shield prior to Mesozoic Gondwana break-up.
Dyke Swarms of the World: a modern perspective, Srivastava et al. eds. Springer , pp. 263-314.
Africa, West Africa, South America
geochronology
Abstract: Eight different generations of dolerite dykes crosscutting the Paleoproterozoic basement in West Africa and one in South America were dated using the high precision U-Pb TIMS method on baddeleyite. Some of the individual dykes reach over 300 km in length and they are considered parts of much larger systems of mafic dyke swarms representing the plumbing systems for large igneous provinces (LIPs). The new U-Pb ages obtained for the investigated swarms in the southern West African Craton (WAC) are the following (oldest to youngest): 1791?±?3 Ma for the N010° Libiri swarm, 1764?±?4 Ma for the N035° Kédougou swarm, 1575?±?5 for the N100° Korsimoro swarm, ~1525-1529 Ma for the N130° Essakane swarm, 1521?±?3 Ma for the N90° Sambarabougou swarm, 915?±?7 Ma for the N070° Oda swarm, 867?±?16 Ma for the N355° Manso swarm, 202?±?5 Ma and 198?±?16 Ma for the N040° Hounde swarm, and 200?±?3 Ma for the sills in the Taoudeni basin. The last ones are related to the Central Atlantic Magmatic Province (CAMP) event. The Hounde swarm is oblique to the dominant radiating CAMP swarm and may be linked with the similar-trending elongate Kakoulima intrusion in Guinea. In addition, the N150° Käyser swarm (Amazonian craton, South America) is dated at 1528?±?2 Ma, providing a robust match with the Essakane swarm in a standard Amazonia-West African craton reconstruction, and resulting in a combined linear swarm >1500 km by >1500 km in extent. The Precambrian LIP barcode ages of c. 1790, 1765-1750, 1575, 1520, 915. 870 Ma for the WAC are compared with the global LIP record to identify possible matches on other crustal blocks, with reconstruction implications. These results contribute to the refinement of the magmatic ‘barcode’ for the West African and Amazonian cratons, representing the first steps towards plausible global paleogeographic reconstructions involving the West African and Amazonian cratons.
Abstract: A series of precise nondestructive analytical methods (Raman spectroscopy, cathodoluminescence, and EBSD—electron backscatter diffraction) has been employed to investigate the internal textures of kyanite porphyroblasts from diamondiferous and diamond?free ultrahigh?pressure metamorphic rocks (Kokchetav massif, Northern Kazakhstan). Such internal kyanite characteristics as twinning, radial fibrous pattern, and spotty zoning were identified by means of Raman and cathodoluminescence imaging, whereas an intergrowth of two kyanite crystals was distinguished only by Raman imaging. The EBSD analysis recorded an ~10-25° changing of orientations along the elongation in the investigated kyanite porphyroblasts. The absence of a radial fibrous pattern and a spotty zoning on the EBSD maps indicates that these textures are not related to variations in crystallographic orientation. The absence of clear zoning patterns (cores, mantles, and rims) on the Raman, cathodoluminescence, or EBSD maps of the kyanite porphyroblasts indicates the rapid single?stage formation of these porphyroblasts near the peak metamorphic conditions and the lack of recrystallization processes. The obtained results provide important implications for deciphering of mineral internal textures, showing that the data obtained by cathodoluminescence mapping can be clearly reproduced by Raman imaging, with the latter method occasionally being even more informative. This observation is of significant importance for the study of minerals that are unexposed on a thin section surface or Fe? and Ni?rich minerals that do not show luminescence emission. The combination of the Raman spectroscopic, cathodoluminescence, and EBSD techniques may provide better spatial resolution for distinguishing different domains and textural peculiarities of mineral than the selective application of individual approaches.
van den Heuvel, Q., Matveev, S., Drury, M., Gress, M., Chinn, I., Davies, G.
Genesis of diamond inclusions: an integrated cathodluminescence ( CL) and electron backscatter diffraction (EBSD) study on eclogitic and peridotitic inclusions and their diamond host.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 6564 Abstract
Abstract: The diamondiferous Letlhakane kimberlites are part of the Orapa kimberlite cluster (˜ 93.1 Ma) in north-eastern Botswana, located on the edge of the Zimbabwe Craton, close to the Proterozoic Magondi Mobile Belt. Here we report the first Re-Os ages of six individual eclogitic sulphide inclusions (3.0 to 35.7?g) from Letlhakane diamonds along with their rhenium, osmium, iridium and platinum concentrations, and carbon isotope, nitrogen content and N-aggregation data from the corresponding growth zones of the host diamonds. For the first time, Re-Os data will be compared to Sm-Nd ages of individual eclogitic silicate inclusions recovered from the same diamonds using a Triton Plus equipped with four 1013? amplifiers. The analysed inclusion set currently encompasses pairs of individual sulphides from two diamonds (LK040 sf4 & 5, LK113 sf1 & 2) and two sulphide inclusions from separate diamonds (LK048, LK362). Ongoing work will determine the Sm-Nd ages and element composition of multiple individual eclogitic garnets (LK113/LK362, n=4) and an eclogitic clinopyroxene (LK040) inclusion. TMA ages of the six sulphides range from 1.06 to 2.38 Ga (± 0.1 to 0.54 Ga) with Re and Os contents between 7 and 68 ppb and 0.03 and 0.3 ppb, respectively. The host diamond growth zones have low nitrogen abundances (21 to 43 ppm N) and high N-aggregation (53 to 90% IaB). Carbon isotope data suggests the involvement of crustal carbon (?13C between -19.3 to -22.7 ± 0.2 per mill) during diamond precipitation. Cathodoluminescence imaging of central plates from LK040 and LK113 displays homogenous internal structure with no distinct zonation. The two sulphide inclusions from LK040 define an 'isochron' of 0.92 ± 0.23 Ga (2SD) with initial 187Os/188Os = 1.31 ± 0.24. Sulphides from LK113 have clear imposed diamond morphology and indicate diamond formation at 0.93 ± 0.36 Ga (2SD) with initial 187Os/188Os = 0.69 ± 0.44. The variation in the initial 187Os/188Os does not justify including these inclusions (or any from other diamonds) on the same isochron and implies an extremely heterogeneous diamond crystallisation environment that incorporated recycled Os. C1-normalized osmium, iridium and platinum (PGE) compositions from the analysed sulphide inclusions display enrichment in Ir (3.4 to 33) and Pt (2.3 to 28.1) in comparison to eclogitic xenolith data from Orapa that are depleted relative to chondrite. The Re-Os isochrons determined in this study are within error of previously reported ages from the adjacent (˜40km) Orapa diamond mine (1.0 to 2.9 Ga) based on sulphide inclusions and a multi-point 990 ± 50 Ma (2SD) isochron for composite (n=730) silicate inclusions. Together with additional new Sm-Nd isochron age determinations from individual silicate inclusions from Letlhakane (2.3 ± 0.02 (n = 3); 1.0 ± 0.14 (n = 4) and 0.25 ± 0.04 Ga (n = 3), all 2SE) these data suggest a phase of Mesoproterozoic diamond formation as well as Neoarchean/Paleoproterozoic and Mesozoic diamond growth, in punctuated events spanning >2.0 Ga.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 5540 Abstract
Africa, Botswana
Deposit - Letlhakane
Abstract: The diamondiferous Letlhakane kimberlites are part of the Orapa kimberlite cluster (˜ 93.1 Ma) in north-eastern Botswana, located on the edge of the Zimbabwe Craton, close to the Proterozoic Magondi Mobile Belt. Here we report the first Re-Os ages of six individual eclogitic sulphide inclusions (3.0 to 35.7?g) from Letlhakane diamonds along with their rhenium, osmium, iridium and platinum concentrations, and carbon isotope, nitrogen content and N-aggregation data from the corresponding growth zones of the host diamonds. For the first time, Re-Os data will be compared to Sm-Nd ages of individual eclogitic silicate inclusions recovered from the same diamonds using a Triton Plus equipped with four 1013? amplifiers. The analysed inclusion set currently encompasses pairs of individual sulphides from two diamonds (LK040 sf4 & 5, LK113 sf1 & 2) and two sulphide inclusions from separate diamonds (LK048, LK362). Ongoing work will determine the Sm-Nd ages and element composition of multiple individual eclogitic garnets (LK113/LK362, n=4) and an eclogitic clinopyroxene (LK040) inclusion. TMA ages of the six sulphides range from 1.06 to 2.38 Ga (± 0.1 to 0.54 Ga) with Re and Os contents between 7 and 68 ppb and 0.03 and 0.3 ppb, respectively. The host diamond growth zones have low nitrogen abundances (21 to 43 ppm N) and high N-aggregation (53 to 90% IaB). Carbon isotope data suggests the involvement of crustal carbon (?13C between -19.3 to -22.7 ± 0.2 per mill) during diamond precipitation. Cathodoluminescence imaging of central plates from LK040 and LK113 displays homogenous internal structure with no distinct zonation. The two sulphide inclusions from LK040 define an 'isochron' of 0.92 ± 0.23 Ga (2SD) with initial 187Os/188Os = 1.31 ± 0.24. Sulphides from LK113 have clear imposed diamond morphology and indicate diamond formation at 0.93 ± 0.36 Ga (2SD) with initial 187Os/188Os = 0.69 ± 0.44. The variation in the initial 187Os/188Os does not justify including these inclusions (or any from other diamonds) on the same isochron and implies an extremely heterogeneous diamond crystallisation environment that incorporated recycled Os. C1-normalized osmium, iridium and platinum (PGE) compositions from the analysed sulphide inclusions display enrichment in Ir (3.4 to 33) and Pt (2.3 to 28.1) in comparison to eclogitic xenolith data from Orapa that are depleted relative to chondrite. The Re-Os isochrons determined in this study are within error of previously reported ages from the adjacent (˜40km) Orapa diamond mine (1.0 to 2.9 Ga) based on sulphide inclusions and a multi-point 990 ± 50 Ma (2SD) isochron for composite (n=730) silicate inclusions. Together with additional new Sm-Nd isochron age determinations from individual silicate inclusions from Letlhakane (2.3 ± 0.02 (n = 3); 1.0 ± 0.14 (n = 4) and 0.25 ± 0.04 Ga (n = 3), all 2SE) these data suggest a phase of Mesoproterozoic diamond formation as well as Neoarchean/Paleoproterozoic and Mesozoic diamond growth, in punctuated events spanning >2.0 Ga.
Abstract: Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. Here we report Sm-Nd isotope data from 26 individual garnet inclusions from 26 harzburgitic diamonds from Venetia, South Africa. Garnet inclusions and host diamonds comprise two compositional suites formed under markedly different conditions and define two isochrons, one Archaean (2.95?Ga) and one Proterozoic (1.15?Ga). The Archaean diamond suite formed from relatively cool fluid-dominated metasomatism during rifting of the southern shelf of the Zimbabwe Craton. The 1.8 billion years younger Proterozoic diamond suite formed by melt-dominated metasomatism related to the 1.1?Ga Umkondo Large Igneous Province. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic.
Geophysical Research Abstracts www.researchgate.net, Vol. 20, EGU2018-128291p. Abstract
Africa, South Africa
diamond inclusions
Abstract: Changing recycling budgets of surface materials and volatiles by subduction of tectonic plates influence the compositions of Earth’s major reservoirs and affect climate throughout geological time. Fluids play a key role in processes governing subduction recycling, but quantifying the exact fate of volatiles introduced into the mantle at ancient and recent destructive plate boundaries remains difficult. Here, we report on the role of fluids and the fate of volatiles and other elements at two very different tectonic settings: 1) at subduction settings, and 2) within the subcontinental lithospheric mantle (SCLM). We will show how olivine-hosted melt inclusions from subduction zones and mineral inclusions in diamond from the SCLM are used to reveal how changing tectonic settings influence volatile cycles with time. Melt inclusions from the complex Italian post-collisional tectonic setting are used to identify changing subduction recycling through time. The use of CO2 in deeply trapped melt inclusions instead of in lavas or volcanic gases provides a direct estimate of deep recycling, minimizing possible effects of contamination during transfer through the crust. The aim is to distinguish if increased recycling of sediments from the down-going plate at continental subduction settings results in increased deep CO2 recycling or if the increased CO2 flux results from crustal degassing of the overriding plate. Both processes likely affected climate through Earth history but could thus far not be discriminated. The study of mineral inclusions and their host diamonds from the SCLM can link changes in the cycling of carbon-rich fluids and the time and process through which the carbon redistribution took place. We use Sm-Nd isotope techniques to date the mineral inclusions and use the carbon isotope data of the host diamonds to investigate the growth conditions. I will present case-studies of peridotitic and eclogitic diamonds from three mines in Southern Africa.
Abstract: The diamondiferous Jwaneng kimberlite cluster (~240 Ma) is located on the NW rim of the Archaean Kaapvaal Craton in central Botswana. Previous studies report eclogitic diamond formation in the late Archean (2.9 Ga) and in the Middle Proterozoic (1.5 Ga) involving different mantle and sedimentary components [1;2;3]. Here we report newly acquired Sm- Nd ages of individual eclogitic pyrope-almandine and omphacite inclusions along with their major element data and nitrogen data from the diamond hosts to re-examine Jwaneng’s diamond formation ages. The Sm-Nd isotope analyses were performed via TIMS using 1013? resistors [4]. An initial suite of three pyropealmandine and 14 omphacite inclusions yield 143Nd/144Nd from 0.51102±7 to 0.5155±5. 147Sm/144Nd vary from 0.024 to 0.469. Major element data defines two inclusion populations: (1) seven omphacites with high Mg#, high Cr# and one pyropealmandine with low-Ca define an isochron age of 1.93±0.16 Ga with ?Ndi= +3.5; (2) seven omphacites with low Mg#, low Cr# and two pyrope-almandines with low-Ca define an isochron age of 0.82±0.06 Ga with ?Ndi= +3.7. Nitrogen contents of corresponding diamond host growth zones in Group (1) are ? 50 at.ppm whereas Group (2) range between 50 to 700 at.ppm with N-aggregation > 70 %B. Additional data used to define “co-genetic” inclusion suites include Sr-isotopes and trace elements of the inclusions and carbon isotopes of the diamond hosts. Re-Os data of coexisting sulphide inclusions from the same silicate-bearing diamonds further validates the ages and indicates more periods of diamond formation at Jwaneng than previously assumed. The integrated data indicate the possibility of an extensive Paleoproterozoic diamond-forming event in southern Africa.
Africa, Zimbabwe. Australia, Canada, Northwest Territories, South Africa, Botswana
deposit - Murowa, Argyle, Diavik, Venetia, Orapa
Abstract: FTIR maps of diamond plates, cut through the centre of growth, contain abundant information about changing defect concentrations from core to rim. These data can, in principle, be interpreted in terms of the variation in conditions of diamond growth and the temperatures experienced by the diamond during the period of mantle residence between growth and exhumation. Many diamonds show multiple growth zones that can be observed by cathodoluminescence. Importantly, the combination of nitrogen concentration and nitrogen aggregation measured by FTIR can be used to determine whether the growth zones are of similar or very different ages (Kohn et al., 2016). In this study, we use automated fitting of several thousand individual spectra within each FTIR map to define a model temperature for each pixel using the Python program, QUIDDIT. We then use a two-stage aggregation model to constrain potential temperature-time histories for each diamond. To take full advantage of the temperature history recorded by zoned diamonds, radiometric ages of inclusions are required. If the growth ages of each zone and the date of exhumation are well-known, then a model temperature can be calculated for each zone. The combination of zone-specific ages and improved quality and processing of FTIR spectra is able to provide unique new insights into the thermal history of diamondbearing lithospheric mantle. For the first time we will be able to use the N defects in diamonds to work out whether a particular location in the lithosphere has heated or cooled over long periods of geological time. The implications for the mechanism of formation of lithosphere will be discussed. We will illustrate the approach using examples of zoned diamonds from Murowa (Zimbabwe), Argyle (Australia), Diavik (Canada), Venetia (South Africa) and Orapa (Botswana).
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0634-3 12p.
Africa, Botswana
deposit - Jwaneng
Abstract: To fully understand the implications of the compositional information recorded by inclusions in diamond it is vital to know if their growth was syn- or protogenetic and the extent to which they have equilibrated with diamond forming agents. The current paradigm is that the majority of inclusions in diamond are syngenetic but recently this assumption has been questioned. This study presents an integrated cathodoluminescence (CL) and electron backscatter diffraction (EBSD) study of 8 diamonds containing eclogitic inclusions: 19 pyrope-almandine garnets, 12 omphacitic clinopyroxenes, 4 sulphides, 1 coesite and 1 rutile from the Jwaneng diamond mine, Botswana. Diamond plates were sequentially polished to expose inclusions at different levels and CL imaging and EBSD were performed to constrain the relationship between diamond and inclusion growth. Despite complex growth and resorption, individual diamonds are single crystals with a homogeneous crystallographic orientation. All individual inclusions have homogeneous crystallographic orientation and no resolvable compositional zonation. The combined CL and EBSD data suggest that epitaxial inclusion-diamond growth is rare (none of 24 inclusions) and that the imposition of cubo-octahedral faces on inclusions does not necessarily result in epitaxy. Individual diamonds contain inclusions that record evidence of both syngentic and protogenetic relationships with the host diamond and in one case an inclusion appears syngenetic to the diamond core but protogenetic to the growth zone that surrounds 70% of the inclusion. These findings emphasise that inclusions in diamonds have multiple modes of origin and that in order to validate the significance of geochronological studies, further work is needed to establish that there is rapid chemical equilibration of protogenetic inclusions with diamond forming agents at mantle temperatures.
Geochimica et Cosmochimica Acta, 10.1016/j.gca.2020.10.010 35p. Pdf
Africa, Botswana
deposit - Jwaneng
Abstract: Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds’ geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different ‘mechanisms’ of diamond formation and define two isochrons, one Paleoproterozoic (1.8 Ga) and one Neoproterozoic (0.85 Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (> 100’s km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (< 10 km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (?13C < -21.1‰) provides further indication of a recycled origin for surface-derived carbon in some diamonds from Jwaneng. Determining Earth’s long-term deep carbon cycle using diamonds, however, requires an understanding of the nature and scale of specific diamond-forming events.
Geochimica et Cosmochimica Acta, Vol. 293, pp. 1-17. pdf
Africa, Botswana
deposit - Jwaneng
Abstract: Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds’ geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different ‘mechanisms’ of diamond formation and define two isochrons, one Paleoproterozoic (1.8?Ga) and one Neoproterozoic (0.85?Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (>100?s km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (<10?km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (?13C??21.1‰) provides further indication of a recycled origin for surface-derived carbon in some diamonds from Jwaneng. Determining Earth’s long-term deep carbon cycle using diamonds, however, requires an understanding of the nature and scale of specific diamond-forming events.
Abstract: Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 ± 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 ± 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (~ 2.0 to 1.7 Ga), Meso- (~ 1.2 to 1.1 Ga) and Neoproterozoic (~ 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics. Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich "fibrous" and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.
Appendix to previous Lithos article in March 2021, 11p. Pdf
Africa, Botswana
deposit - Damtshaa, Orapa
Abstract: Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 ± 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 ± 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (~ 2.0 to 1.7 Ga), Meso- (~ 1.2 to 1.1 Ga) and Neoproterozoic (~ 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics. Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich “fibrous” and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.
Abstract: Recent studies of mineral diversity and distribution lead to the prediction of >1563 mineral species on Earth today that have yet to be described-approximately one fourth of the 6394 estimated total mineralogical diversity. The distribution of these "missing" minerals is not uniform with respect to their essential chemical elements. Of 15 geochemically diverse elements (Al, B, C, Cr, Cu, Mg, Na, Ni, P, S, Si, Ta, Te, U, and V), we predict that approximately 25% of the minerals of Al, B, C, Cr, P, Si, and Ta remain to be described - a percentage similar to that predicted for all minerals. Almost 35% of the minerals of Na are predicted to be undiscovered, a situation resulting from more than 50% of Na minerals being white, poorly crystallized, and/or water soluble, and thus easily overlooked. In contrast, we predict that fewer than 20% of the minerals of Cu, Mg, Ni, S, Te, U, and V remain to be discovered. In addition to the economic value of most of these elements, their minerals tend to be brightly colored and/or well crystallized, and thus likely to draw attention and interest. These disparities in percentages of undiscovered minerals reflect not only natural processes, but also sociological factors in the search, discovery, and description of mineral species.
Geochemical Perspective Letters, Vol. 10, pp. 43-48. doi:10.7185/ geochemlet.1904
Mantle
xenoliths
Abstract: Heat flow studies suggest that the lower crust has low concentrations of heat-producing elements. This could be due to either (i) greater fractions of basaltic rock at depth or (ii) metamorphic depletion of radioactive elements from rocks with more evolved (andesitic to granodioritic) compositions. However, seismic data suggest that lower crust is not predominantly basaltic, and previous studies (using Pb and Sr isotopes) have shown that lower crustal rocks have experienced significant losses of U and Rb. This loss, however, is poorly constrained for K, which is inferred to be the most important source of radioactive heat in the earliest crust. Our high precision Ca isotope measurements on a suite of granulite facies rocks and minerals from several localities show that significant losses of K (~60 % to >95 %) are associated with high temperature metamorphism. These results support models whereby reduction of heat production from the lower crust, and consequent stabilisation of continental cratons in the Precambrian, are largely due to high temperature metamorphic processes. Relative changes in whole rock K/Ca suggest that 20-30 % minimum (granitic) melt removal can explain the K depletions.
Geochimica et Cosmochimica Acta, Vol. 238, pp. 477-495.
Mantle
carbon
Abstract: Constraining carbon (C) fractionation between silicate magma ocean (MO) and core-forming alloy liquid during early differentiation is essential to understand the origin and early distribution of C between reservoirs such as the crust-atmosphere, mantle, and core of Earth and other terrestrial planets. Yet experimental data at high pressure (P)-temperature (T) on the effect of other light elements such as sulfur (S) in alloy liquid on alloy-silicate partitioning of C and C solubility in Fe-alloy compositions relevant for core formation is lacking. Here we have performed multi-anvil experiments at 6-13?GPa and 1800-2000?°C to examine the effects of S and Ni on the solubility limit of C in Fe-rich alloy liquid as well as partitioning behavior of C between alloy liquid and silicate melt (). The results show that C solubility in the alloy liquid as well as decreases with increasing in S content in the alloy liquid. Empirical regression on C solubility in alloy liquid using our new experimental data and previous experiments demonstrates that C solubility significantly increases with increasing temperature, whereas unlike in S-poor or S-free alloy compositions, there is no discernible effect of Ni on C solubility in S-rich alloy liquid. Our modelling results confirm previous findings that in order to satisfy the C budget of BSE, the bulk Earth C undergoing alloy-silicate fractionation needs to be as high as those of CI-type carbonaceous chondrite, i.e., not leaving any room for volatility-induced loss of carbon during accretion. For Mars, on the other hand, an average single-stage core formation at relatively oxidized conditions (1.0 log unit below IW buffer) with 10-16?wt% S in the core could yield a Martian mantle with a C budget similar to that of Earth’s BSE for a bulk C content of ?0.25-0.9?wt%. For the scenario where C was delivered to the proto-Earth by a S-rich differentiated impactor at a later stage, our model calculations predict that bulk C content in the impactor can be as low as ?0.5?wt% for an impactor mass that lies between 9 and 20% of present day Earth’s mass. This value is much higher than 0.05-0.1?wt% bulk C in the impactor predicted by Li et al. (Li Y., Dasgupta R., Tsuno K., Monteleone B., and Shimizu N. (2016) Carbon and sulfur budget of the silicate Earth explained by accretion of differentiated planetary embryos. Nat. Geosci.9, 781-785) because C-solubility limit of 0.3?wt% in a S-rich alloy predicted by their models is significantly lower than the experimentally derived C-solubility of ?1.6?wt% for the relevant S-content in the core of the impactor.
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.
Geochimica et Cosmochimica Acta, Vol. 251. pp. 87-115.
Mantle
nitrogen
Abstract: Nitrogen, the most dominant constituent of Earth’s atmosphere, is critical for the habitability and existence of life on our planet. However, its distribution between Earth’s major reservoirs, which must be largely influenced by the accretion and differentiation processes during its formative years, is poorly known. Sequestration into the metallic core, along with volatility related loss pre- and post-accretion, could be a critical process that can explain the depletion of nitrogen in the Bulk Silicate Earth (BSE) relative to the primitive chondrites. However, the relative effect of different thermodynamic parameters on the alloy-silicate partitioning behavior of nitrogen is not well understood. Here we present equilibrium partitioning data of N between alloy and silicate melt () from 67 new high pressure (P?=?1-6?GPa)-temperature (T?=?1500-2200?°C) experiments under graphite saturated conditions at a wide range of oxygen fugacity (logfO2????IW ?4.2 to ?0.8), mafic to ultramafic silicate melt compositions (NBO/T?=?0.4 to 2.2), and varying chemical composition of the alloy melts (S and Si contents of 0-32.1?wt.% and 0-3.1?wt.%, respectively). Under relatively oxidizing conditions (??IW ?2.2 to ?0.8) nitrogen acts as a siderophile element ( between 1.1 and 52), where decreases with decrease in fO2 and increase in T, and increases with increase in P and NBO/T. Under these conditions remains largely unaffected between S-free conditions and up to ?17?wt.% S content in the alloy melt, and then drops off at >?20?wt.% S content in the alloy melt. Under increasingly reduced conditions (?IW ?2.2), N becomes increasingly lithophile ( between 0.003 and 0.5) with decreasing with decrease in fO2 and increase in T. At these conditions, fO2 along with Si content of the alloy under the most reduced conditions (?IW -3.0), is the controlling parameter with T playing a secondary role, while, P, NBO/T, and S content of the alloy have minimal effects. A multiple linear least-squares regression parametrization for based on the results of this study and previous studies suggests, in agreement with the experimental data, that fO2 (represented by Si content of the alloy melt and FeO content of the silicate melt), followed by T, has the strongest control on . Based on our modeling, to match the present-day BSE N content, impactors that brought N must have been moderately to highly oxidized. If N bearing impactors were reduced, and/or there was significant disequilibrium core formation, then the BSE would be too N-rich and another mechanism for N loss, such as atmospheric loss, would be required.
Geochimica et Cosmochimica Acta, Vol. 251, pp. 87-115.
Mantle
nitrogen
Abstract: Nitrogen, the most dominant constituent of Earth’s atmosphere, is critical for the habitability and existence of life on our planet. However, its distribution between Earth’s major reservoirs, which must be largely influenced by the accretion and differentiation processes during its formative years, is poorly known. Sequestration into the metallic core, along with volatility related loss pre- and post-accretion, could be a critical process that can explain the depletion of nitrogen in the Bulk Silicate Earth (BSE) relative to the primitive chondrites. However, the relative effect of different thermodynamic parameters on the alloy-silicate partitioning behavior of nitrogen is not well understood. Here we present equilibrium partitioning data of N between alloy and silicate melt () from 67 new high pressure (P?=?1-6?GPa)-temperature (T?=?1500-2200?°C) experiments under graphite saturated conditions at a wide range of oxygen fugacity (logfO2????IW ?4.2 to ?0.8), mafic to ultramafic silicate melt compositions (NBO/T?=?0.4 to 2.2), and varying chemical composition of the alloy melts (S and Si contents of 0-32.1?wt.% and 0-3.1?wt.%, respectively). Under relatively oxidizing conditions (??IW ?2.2 to ?0.8) nitrogen acts as a siderophile element ( between 1.1 and 52), where decreases with decrease in fO2 and increase in T, and increases with increase in P and NBO/T. Under these conditions remains largely unaffected between S-free conditions and up to ?17?wt.% S content in the alloy melt, and then drops off at >?20?wt.% S content in the alloy melt. Under increasingly reduced conditions (?IW ?2.2), N becomes increasingly lithophile ( between 0.003 and 0.5) with decreasing with decrease in fO2 and increase in T. At these conditions, fO2 along with Si content of the alloy under the most reduced conditions (?IW -3.0), is the controlling parameter with T playing a secondary role, while, P, NBO/T, and S content of the alloy have minimal effects. A multiple linear least-squares regression parametrization for based on the results of this study and previous studies suggests, in agreement with the experimental data, that fO2 (represented by Si content of the alloy melt and FeO content of the silicate melt), followed by T, has the strongest control on . Based on our modeling, to match the present-day BSE N content, impactors that brought N must have been moderately to highly oxidized. If N bearing impactors were reduced, and/or there was significant disequilibrium core formation, then the BSE would be too N-rich and another mechanism for N loss, such as atmospheric loss, would be required.
Earth and Planetary Science Letters, doi.org/10.1016/ j.epsl.2020. 116720 19p. Pdf
Africa, South Africa
deposit - Bellsbank
Abstract: The nature of the deep calcium geochemical cycle through time is unresolved, in part due to the dearth of information about the calcium isotope composition of Archean recycled oceanic crust. Remnants of such ancient oceanic crust are preserved in the form of cratonic mantle eclogites, brought to surface as xenoliths in kimberlite magma eruptions. The ? 44 / 40Ca of fresh mantle-derived eclogite xenoliths (i.e., garnet and omphacite mineral separates) from the Bellsbank kimberlite on the Kaapvaal craton in South Africa are presented here in combination with their trace element compositions, garnet Fe3+ contents and ?18O values. The studied Bellsbank eclogite xenoliths have geochemical compositions that indicate oceanic crustal protoliths, with bulk Al2O3 from 15 to 27 wt.%, Eu anomalies from 0.8 to 2.6 and, significantly, garnet ?18O values from +2.7 to +6.2‰. Garnet Fe3+/?Fe contents yield logfO2(?FMQ) values between -4.0 and -1.2 for a depth range of 110-180 km, recording strong redox heterogeneity of the eclogite component within the Archean Kaapvaal mantle lithosphere. Reconstructed bulk eclogite MgO contents correlate negatively with fO2, suggesting that the redox compositions are related to magmatic differentiation during oceanic crust formation, excluding secondary metasomatic overprints. These data may thus emphasize that Archean basaltic oceanic crust had a similarly variable redox composition to modern MORB-type crust. Reconstructed bulk ? 44 / 40Ca values for the Bellsbank eclogites range from +0.28 to +1.56‰. Although some of the xenoliths have ? 44 / 40Ca values that overlap with the average mantle composition and modern MORB (+0.94 ± 0.1 and +0.83 ± 0.05‰), half of our dataset shows excursions to more extreme Ca isotopic compositions. Both higher and lower ? 44 / 40Ca relative to mantle compositions are recorded by the eclogites, with a general negative correlation with ?18O suggestive of seawater-alteration of oceanic crust. The combined low ? 44 / 40Ca (+0.28‰) and ?18O (+3.4‰) measured for one eclogite xenolith may record a subtle imprint by carbonate-rich mantle melts, which are known to contain isotopically light calcium contributed by recycled sediments. In contrast, the high ? 44 / 40Ca of up to +1.56‰ for some eclogite xenoliths, coupled with strong LREE depletion, can be explained by calcium isotope fractionation during partial melting. The protracted history of recycled oceanic crust as probed by cratonic mantle eclogites is recorded by their highly variable ? 44 / 40Ca-?18O-fO2 signatures. Whereas some of this heterogeneity can be linked to processes that operated on the Archean ocean floor such as seawater-alteration of basaltic crust, other sources of compositional variability are introduced by loss and addition of melts during subduction recycling and mantle residence. The observed ? 44 / 40Ca complexity of ancient recycled oceanic crust components at the scale of a single mantle-derived eclogite xenolith suite implies that mantle plume sourced intraplate magmas should reveal similarly strong calcium isotope variations contributed by apparently essential recycled crust components - as observed in the global oceanic island basalt record.
Chemical composition, volatile components, and trace elements in melts of the Karymskii volcanic centre, Kamchatka and Golovnin a volcano, Kunashir Island....
Vladykin, N.V., Deep Seated Magmatism: its sources and plumes, pp. 104-127.
Evolution of deep crustal roots of the Arhangelsk Diamondiferous province: evidences from crustal xenoliths and xenocrysts from Devonian kimberlite pipes.
Abstract: The model, which will incorporate 227 million surface wave measurements, could help with everything from earthquake characterization to neutrino geosciences.
Abstract: Bolide impact is a ubiquitous geological process in the Solar System, which produced craters and basins filled with impact melt sheets on the terrestrial planets. However, it remains controversial whether these sheets were able to undergo large-scale igneous differentiation, or not. Here, we report on the discovery of large discrete bodies of melanorites that occur throughout almost the entire stratigraphy of the 1.85-billion-year-old Sudbury Igneous Complex (SIC) - the best exposed impact melt sheet on Earth - and use them to reaffirm that conspicuous norite-gabbro-granophyre stratigraphy of the SIC is produced by fractional crystallization of an originally homogeneous impact melt of granodioritic composition. This implies that more ancient and compositionally primitive Hadean impact melt sheets on the Earth and other terrestrial planets also underwent large-volume igneous differentiation. The near-surface differentiation of these giant impact melt sheets may therefore have contributed to the evolution and lithological diversity of the proto-crust on terrestrial planets.
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
Mineral chemistry and zircon geochronology of xenocrysts and altered mantle and crustal xenoliths from the Aries micaceous kimberlite: constraints age..
Downes, P.J., Demeny, A., Czuppon, G., Jacques, A.L., Verrall, M., Sweetapple, M., Adams, D., McNaughton, N.J., Gwalani, L.G., Griffin, B.J.
Stable H-C-O isotope and trace element geochemistry of the Cummins Range carbonatite complex, Kimberley region Western Australia: implications for hydrothermal REE mineralization, carbonatite evolution and mantle source regions.
Downes, P.J., Demeny, A., Czuppon, G., Jaques, A.L., Verrall, M., Sweetapple, M., Adams, D., McNaughton, N.J., Gwalani, L.G., Griffin, B.J.
Stable H-C-O isotope and trace element geochemistry of the Cummins Range carbonatite complex, Kimberley region western Australia: implications for hydrothermal REE mineralization, carbonatite evolution and mantle source regions.
Heterogeneity in the thermal state of the lower crust and upper mantle beneath eastern Australia
Australian Society of Exploration Geophysicists and Geological Society of Australia, 8th. Exploration Conference in the Bulletin., Vol. 22, No. 2, June pp. 295-298
Variations in trapping temperatures and trace elements in peridotite-suite inclusions African diamonds- evidence 2 inclusion suites implications lithosphere stratigr
Contributions to Mineralogy and Petrology, Vol. 110, No. 1, March pp. 1-15
Condie, K.C., Cox, J., O'Reilly, S.Y., Griffin, W.L., Kerrich, R.
Definition of high field strength and rare elements in mantle and lower crustal xenoliths from the SE United States: the role of grain boundary phases.
Geochimica et Cosmochimica Acta, Vol. 68, 19, pp. 3919-3942.
Zircon crystal morphology, trace element signatures and Hf isotope composition as a tool for petrogenetic modelling: examples from eastern Australian granitoids.
Mineral chemistry and zircon geochronology of xenocrysts and altered mantle and crustal xenoliths from the Aries micaceous kimberlite: constraints age..
Mechanism and timing of lithospheric modification and replacement beneath the eastern North Chin a Craton: peridotitic xenoliths from the 100 Ma Fuxin basalts
Geochimica et Cosmochimica Acta, In press, available
Mechanism and timing of lithospheric modification and replacement beneath the eastern North Chin a Craton: peridotitic xenoliths from the 100 Ma Fuxin basalts...
Geochimica et Cosmochimica Acta, Vol. 71, 21, pp. 5303-5225.
Babu, E.V.S.S.K., Griffin, W.L., Mukherjee, A., O'Reilly, S.Y., Belousova, E.A.
Combined U Pb and Lu Hf analysis of megacrystic zircons from the Kalyandurg 4 kimberlite pipe, S. India: implications for the emplacement age and HF isotopic..
Griffin, W.L., Kobussen, A.F., Babu, E.V.S.S.K., O'Reilly, S.Y., Norris, R., Sengupta, P.
A translithospheric suture in the vanished 1 Ga lithospheric root of South India: evidence from contrasting lithospheric sections in the Dharwar Craton.
Griffin, W.L., Kobussen, A.F., Babu, E.V.S.S.K., O'Reilly, S.Y., Norris, R., Sengupta, P.
A translithospheric suture in the vanished 1 Ga lithospheric root of South India: evidence from contrasting lithosphere sections in the Dharwar craton.
Persistence of mantle lithospheric Re-Os signature during asthenospherization of the subcontinental lithospheric mantle: insights in situ sulphides....
Contributions to Mineralogy and Petrology, Vol. 159, 3, pp. 315-330.
Kahoui, M., Kemainsky, F.V., Griffin, W.L., Belousova, E., Mahdjoub, Y., Chabane, M.
Detrital pyrope garnets from the El Kseibat area, Algeria: a glimpse into the lithospheric mantle beneath the north-eastern edge of the West African Craton.
Journal of African Earth Sciences, In press available, 46p.
In situ U Pb dating and Sr Nd isotopic analysis of perovskite: constraints on the age and petrogenesis of the Kuruman kimberlite province, Kaapvaal Craton, South Africa.
Howell, D., Piazolo, S., Dobson, D.P., Wood, I.G., Jones, A.P., Watte, N., Frost, D.J., Fisher, D., Griffin, W.L.
Quantitative characterization of plastic deformation of single diamond crystals: a high pressure high temperature (HPHT) experimental deformation study combines with electron backscatter diffraction.
Diamond and Related Materials, Vol. 30, pp. 20-30.
Kahoui, M., Kaminsky, F.V., Griffin, W.L., Belousova, E., Mahdjoub, Y., Chabane, M.
Detrital pyrope garnets from the El Kseibat area, Algeria: a glimpse into lithospheric mantle beneath the north eastern edge of the west African Craton.
Journal of African Earth Sciences, Vol. 63, Feb. pp. 1-11.
Gonzalez-Jimienez, J.M., Marchesi, C., Griffin, W.L., Gutierrez-Narbona, R., Lorand, J-P., O'Reilly, S.Y., Garrido, C.J., Gervilla, F., Pearson, N.J., Hidas, K.
Transfer of Os isotopic signatures from peridotite to chromitite in the subcontinental mantle: insights from in situ analysis of platinum-group and base metal minerals (Ojen peridotite massif, southern Spain.
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.
Kaminsky, F.V., Kahoui, M.,Mahdjoub, Y., Belousova, E., Griffin, W.L.,O'Reilly, S.Y.
Pyrope garnets from the Eglab Shield, Algeria: look inside the Earth's mantle in the West African Craton and suggestions about primary sources of diamond and indicator minerals.
Vladykin, N.V. ed. Deep seated magmatism, its sources and plumes, Russian Academy of Sciences, pp. 73-103.
U Pb and Lu Hf isotopes in detrital zircon from Neoproterozoic sedimentary rocks in the northern Yangtze block: implications for Precambrian crust evolution.
Abstract: The Os-isotope compositions of sulphides in mantle xenoliths hosted by Late Miocene alkali basalts from the Sviyaginsky volcano, Russian Far East, reveal the presence of Archaean-Proterozoic subcontinental lithospheric mantle beneath the Khanka massif. Their TMA and TRD model ages reveal similar peaks at 1.1 and 0.8 Ga suggesting later thermotectonic events in the subcontinental lithospheric mantle, whereas TRD model ages range back to 2.8 ± 0.5 (2?) Ga. The events recognized in the subcontinental lithospheric mantle are consistent with those recorded in the crust of the Khanka massif. The sulphide Os-isotope data show that the subcontinental lithospheric mantle beneath the Khanka massif had formed at least by the Mesoproterozoic, and was subsequently metasomatized by juvenile crustal-growth events related to the evolution of the Altaids. The Khanka massif is further proposed to have tectonic affinity to the Siberia Craton and should originate from it accordingly.
Yang, J.S., Wirth, R., Wiedenbeck, M., Griffin, W.L., Meng, F.C., Chen, S.Y., Bai, W.J., Xu, X.X., Makeeyev, A.B., Bryanchaniniova, N.I.
Diamonds and highly reduced minerals from chromitite of the Ray-Iz ophiolite of the Polar Urals: deep origin of podiform chromitites and ophiolitic diamonds.
Abstract: This ending talk, focused on the ongoing cooperative research of Prof. Griffin and his team at Macquarie University and Shefa Yamim, since January 2014, highlighting unique corundum species characteristics. Preliminary results of this research were presented in the IGS Annual Meeting of 2015, whereas this year Prof. Griffin has shared innovative findings only microscopically tracked within titanium-rich corundum aggregates. One of the more abundant minerals is Tistarite (Ti2O3), previously known only as a single grain in a primitive type of meteorite (!). An article has been submitted to a scientific journal detailing this first terrestrial occurrence. Several other minerals are common in meteorites, but unknown or extremely rare on Earth. About half of these minerals are unknown to science, and will be described as new minerals in the scientific literature. The first of these is a Titanium-Aluminium-Zirconium oxide, informally known as TAZ; it will be submitted to the International Mineralogical Association for recognition as a new mineral, ShefaTAZite. Using state of the art technologies such as Thermal Ionisation Mass Spectrometry (TIMS) and Electron Microscopy Facility (EMF) that has three scanning electron microscopes, all with EBSD capability, and a transmission electron microscope - Prof. Griffin revealed spectacular imagery of minerals and rare compounds associated with titanium rich corundum aggregates.
Abstract: This talk shortly reviewed geological and geochemical mechanisms of the deep lithosphere, a layer composed of the Earth's crust and uppermost mantle at a depth range of 100-150km below the surface. Definition of these processes at depth, reflects on surface recovery of gem and heavy minerals, of which metallic minerals were stressed out. Prof. O'reilley has also referred to Shefa Yamim's exploration area in northern Israel where the eruption of gem-bearing volcanic rocks appears to be related to a major lithospheric suture (the Dead Sea Transform) and related faulting. The Dead Sea Transform is a 1000km plate boundary stretching out from Turkey in the north to Eilat Bay in the south. As such, it is a preferred pathway for magma emplacement crystalizing in volcanic bodies that are being surveyed by Shefa Yamim as Primary Sources for gem and heavy minerals.
Abstract: Perovskite (CaTiO3) has become a very useful mineral for dating kimberlite eruptions, as well as for constraining the compositional evolution of a kimberlitic magma and its source. Despite the undeniable potential of such an approach, no similar study had been done in Angola, the fourth largest diamond producer in Africa. Here we present the first work of in situ U-Pb geochronology and Sr-Nd isotope analyses of perovskite in six Angolan kimberlites, supported by a detailed petrographic and geochemical study of their perovskite populations. Four types of perovskite were identified, differing in texture, major- and trace-element composition, zoning patterns, type of alteration and the presence or absence of inclusions. Primary groundmass perovskite is classified either as anhedral, Na-, Nb- and LREE-poor perovskite (Ia); or euhedral, strongly zoned, Na-, Nb- and LREE-rich perovskite (Ib). Secondary perovskite occurs as reaction rims on ilmenite (IIa) or as high Nb (up to 10.6 wt% Nb2O5) perovskite rims on primary perovskite (IIb). The occurrence of these four types within the Mulepe kimberlites is interpreted as an evidence of a complex, multi-stage process that involved mingling of compositionally different melts. U-Pb dating of these perovskites yielded Lower Cretaceous ages for four of the studied kimberlites: Mulepe 1 (116.2 ± 6.5 Ma), Mulepe 2 (123.0 ± 3.6 Ma), Calonda (119.5 ± 4.3 Ma) and Cat115 (133 ± 10 Ma). Kimberlite magmatism occurred in NE Angola likely due to reactivation of deep-seated translithospheric faults (> 300 km) during the break-up of Gondwana. Sr-Nd isotope analyses of four of these kimberlites indicate that they are Group I kimberlites, which is consistent with the petrological observations.
Abstract: Perovskite (CaTiO3) has become a very usefulmineral for dating kimberlite eruptions, aswell as for constraining the compositional evolution of a kimberlitic magma and its source. Despite the undeniable potential of such an approach, no similar study had been done in Angola, the fourth largest diamond producer in Africa. Here we present the firstwork of in situ U-Pb geochronology and Sr-Ndisotope analyses of perovskite in six Angolan kimberlites, supported by a detailed petrographic and geochemical study of their perovskite populations. Four types of perovskitewere identified, differing in texture,major- and trace-element composition, zoning patterns, type of alteration and the presence or absence of inclusions. Primary groundmass perovskite is classified either as anhedral, Na-, Nb- and LREE-poor perovskite (Ia); or euhedral, strongly zoned, Na-, Nb- and LREE-rich perovskite (Ib). Secondary perovskite occurs as reaction rims on ilmenite (IIa) or as high Nb (up to 10.6 wt% Nb2O5) perovskite rims on primary perovskite (IIb). The occurrence of these four types within the Mulepe kimberlites is interpreted as an evidence of a complex, multi-stage process that involved mingling of compositionally different melts. U-Pb dating of these perovskites yielded Lower Cretaceous ages for four of the studied kimberlites: Mulepe 1 (116.2±6.5Ma),Mulepe 2 (123.0±3.6Ma), Calonda (119.5±4.3 Ma) and Cat115 (133±10Ma). Kimberlite magmatism occurred in NE Angola likely due to reactivation of deep-seated translithospheric faults (N300 km) during the break-up of Gondwana. Sr-Nd isotope analyses of four of these kimberlites indicate that they are Group I kimberlites, which is consistent with the petrological observations.
Abstract: Large peridotite massifs are scattered along the 1500?km length of the Yarlung-Zangbo Suture Zone (southern Tibet, China), the major suture between Asia and Greater India. Diamonds occur in the peridotites and chromitites of several massifs, together with an extensive suite of trace phases that indicate extremely low fO2 (SiC, nitrides, carbides, native elements) and/or ultrahigh pressures (UHP) (diamond, TiO2 II, coesite, possible stishovite). New physical and isotopic (C, N) studies of the diamonds indicate that they are natural, crystallized in a disequilibrium, high-T environment, and spent only a short time at mantle temperatures before exhumation and cooling. These constraints are difficult to reconcile with previous models for the history of the diamond-bearing rocks. Possible evidence for metamorphism in or near the upper part of the Transition Zone includes the following: (1) chromite (in disseminated, nodular and massive chromitites) containing exsolved pyroxenes and coesite, suggesting inversion from a high-P polymorph of chromite; (2) microstructural studies suggesting that the chromitites recrystallized from fine-grained, highly deformed mixtures of wadsleyite and an octahedral polymorph of chromite; (3) a new cubic Mg-silicate, with the space group of ringwoodite but an inverse-spinel structure (all Si in octahedral coordination); (4) harzburgites with coarsely vermicular symplectites of opx + Cr-Al spinel ± cpx; reconstructions suggest that these are the breakdown products of majoritic garnets, with estimated minimum pressures to?>?13?GPa. Evidence for a shallow pre-metamorphic origin for the chromitites and peridotites includes the following: (1) trace-element data showing that the chromitites are typical of suprasubduction-zone (SSZ) chromitites formed by magma mixing or mingling, consistent with Hf-isotope data from magmatic (375?Ma) zircons in the chromitites; (2) the composition of the new cubic Mg-silicate, which suggests a low-P origin as antigorite, subsequently dehydrated; (3) the peridotites themselves, which carry the trace element signature of metasomatism in an SSZ environment, a signature that must have been imposed before the incorporation of the UHP and low-fO2 phases. A proposed P-T-t path involves the original formation of chromitites in mantle-wedge harzburgites, subduction of these harzburgites at c. 375?Ma, residence in the upper Transition Zone for >200 Myr, and rapid exhumation at c. 170-150?Ma or 130-120?Ma. Os-isotope data suggest that the subducted mantle consisted of previously depleted subcontinental lithosphere, dragged down by a subducting oceanic slab. Thermomechanical modeling shows that roll-back of a (much later) subducting slab would produce a high-velocity channelized upwelling that could exhume the buoyant harzburgites (and their chromitites) from the Transition Zone in?10 Myr. This rapid upwelling, which may explain some characteristics of the diamonds, appears to have brought some massifs to the surface in forearc or back-arc basins, where they provided a basement for oceanic crust. This model can reconcile many apparently contradictory petrological and geological datasets. It also defines an important, previously unrecognized geodynamic process that may have operated along other large suture zones such as the Urals.
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.
Abstract: The minimum oxygen fugacity (fO2) of Earth's upper mantle probably is controlled by metal saturation, as defined by the iron-wüstite (IW) buffer reaction (FeO ? Fe + O). However, the widespread occurrence of moissanite (SiC) in kimberlites, and a suite of super-reduced minerals (SiC, alloys, native elements) in peridotites in Tibet and the Polar Urals (Russia), suggest that more reducing conditions (fO2 = 6-8 log units below IW) must occur locally in the mantle. We describe pockets of melt trapped in aggregates of corundum crystals ejected from Cretaceous volcanoes in northern Israel which contain high-temperature mineral assemblages requiring extremely low fO2 (IW < -10). One abundant phase is tistarite (Ti2O3), previously known as a single grain in the Allende carbonaceous chondrite (Mexico) and believed to have formed during the early evolution of the solar nebula. It is associated with other reduced phases usually found in meteorites. The development of super-reducing conditions in Earth's upper mantle may reflect the introduction of CH4 + H2 fluids from the deep mantle, specifically related to deep-seated volcanic plumbing systems at plate boundaries.
Abstract: Magnesium and oxygen are critical elements in the solid Earth and hydrosphere. A better understanding of the combined behavior of Mg and O isotopes will refine their use as a tracer of geochemical processes and Earth evolution. In this study, the Mg-isotope compositions of garnet and omphacite separated from well-characterized xenolithic eclogites from the Roberts Victor kimberlite pipe (South Africa) have been measured by solution multi-collector ICP-MS. The reconstructed whole-rock ?26Mg values of Type I (metasomatized) eclogites range from ? 0.61‰ to ? 0.20‰ (Type IA) and from ? 0.60‰ to ? 0.30‰ (Type IB) (mean ? 0.43‰ ± 0.12‰), while ?26Mg of Type IIA (fresh, least metasomatized) eclogites ranges from ? 1.09‰ to ? 0.17‰ (mean ? 0.69‰ ± 0.41‰); a Type IIB (fresh, least metasomatized) has ?26Mg of ? 0.37‰. Oxygen-isotope compositions of garnet were analyzed in situ by SIMS (CAMECA 1280) and cross-checked by laser fluorination. Garnets have ?18O of 6.53‰ to 9.08‰ in Type IA, 6.14‰ to 6.65‰ in Type IB, and 2.34‰ to 2.91‰ in Type IIB. The variation of ?26Mg and ?18O in Type IA and IB eclogites is consistent with the previously proposed model for the evolution of these samples, based on major and trace elements and radiogenic isotopes. In this model, the protoliths (Type II eclogites) were metasomatized by carbonatitic to kimberlitic melts/fluids to produce first Type IA eclogites and then Type IB. Metasomatism has changed the O-isotope compositions, but the Mg-isotope compositions of Type IA are mainly controlled by the protoliths; those of Type IB eclogites reflect mixing between the protoliths and the kimberlitic melt/fluid. The combination of a large range of ?26Mg and low ?18O in Type II eclogites cannot be explained easily by seawater alteration of oceanic crust, interaction of carbonate/silicate sediments with oceanic crust, or partial melting of mafic rocks.
Abstract: Mineralogical studies and U-Pb dating have been carried out on rutile included in peridotitic and eclogitic garnets from the Internatsionalnaya pipe, Mirny field, Siberian craton. We also describe a unique peridotitic paragenesis (rutile + forsterite + enstatite + Cr-diopside + Cr-pyrope) preserved in diamond from the Mir pipe, Mirny field. Compositions of rutile from the heavy mineral concentrates of the Internatsionalnaya pipe and rutile inclusions in crustal almandine-rich garnets from the Mayskaya pipe (Nakyn field), as well as from a range of different lithologies, are presented for comparison. Rutile from cratonic mantle peridotites shows characteristic enrichment in Cr, in contrast to lower-Cr rutile from crustal rocks and off-craton mantle. Rutile with Cr2O3 > 1.7 wt% is commonly derived from cratonic mantle, while rutiles with lower Cr2O3 may be both of cratonic and off-cratonic origin. New analytical developments and availability of standards have made rutile accessible to in situ U-Pb dating by laser ablation ICP-MS. A U-Pb age of 369 ± 10 Ma for 9 rutile grains in 7 garnets from the Internatsionalnaya pipe is consistent with the accepted eruption age of the pipe (360 Ma). The equilibrium temperatures of pyropes with rutile inclusions calculated using Ni-in-Gar thermometer range between ~ 725 and 1030 °C, corresponding to a depth range of ca ~ 100-165 km. At the time of entrainment in the kimberlite, garnets with Cr-rich rutile inclusions resided at temperatures well above the closure temperature for Pb in rutile, and thus U-Pb ages on mantle-derived rutile most likely record the emplacement age of the kimberlites. The synthesis of distinctive rutile compositions and U-Pb dating opens new perspectives for using rutile in diamond exploration in cratonic areas.
Abstract: The petrogenesis of kimberlites commonly is obscured by interaction with hydrothermal fluids, including deuteric (late-magmatic) and/or groundwater components. To provide new constraints on the modification of kimberlite rocks during overprinting by such fluids and on the fractionation of kimberlite magmas during crystallisation, we have undertaken a detailed petrographic and geochemical study of a hypabyssal sample (BK) from the Bultfontein kimberlite (Kimberley, South Africa).
Abstract: Mineralogical studies and U-Pb dating have been carried out on rutile included in peridotitic and eclogitic garnets from the Internatsionalnaya pipe, Mirny field, Siberian craton. We also describe a unique peridotitic paragenesis (rutile + forsterite + enstatite + Cr-diopside + Cr-pyrope) preserved in diamond from the Mir pipe, Mirny field. Compositions of rutile from the heavy mineral concentrates of the Internatsionalnaya pipe and rutile inclusions in crustal almandine-rich garnets from the Mayskaya pipe (Nakyn field), as well as from a range of different lithologies, are presented for comparison. Rutile from cratonic mantle peridotites shows characteristic enrichment in Cr, in contrast to lower-Cr rutile from crustal rocks and off-craton mantle. Rutile with Cr2O3 > 1.7 wt% is commonly derived from cratonic mantle, while rutiles with lower Cr2O3 may be both of cratonic and off-cratonic origin. New analytical developments and availability of standards have made rutile accessible to in situ U-Pb dating by laser ablation ICP-MS. A U-Pb age of 369 ± 10 Ma for 9 rutile grains in 6 garnets from the Internatsionalnaya pipe is consistent with the accepted eruption age of the pipe (360 Ma). The equilibrium temperatures of pyropes with rutile inclusions calculated using Ni-in-Gar thermometer range between ~ 725 and 1030 °C, corresponding to a depth range of ca ~ 100-165 km. At the time of entrainment in the kimberlite, garnets with Cr-rich rutile inclusions resided at temperatures well above the closure temperature for Pb in rutile, and thus U-Pb ages on mantle-derived rutile most likely record the emplacement age of the kimberlites. The synthesis of distinctive rutile compositions and U-Pb dating opens new perspectives for using rutile in diamond exploration in cratonic areas.
Abstract: Microinclusions of high-density fluids (HDF's) occur in cloudy diamonds from the Mir and Internatsionalnaya kimberlite pipes (Malobotuobia kimberlite field, Siberian platform). These HDFs are of typical high-Mg carbonatitic composition; a few diamonds contain microinclusions that define a low-Mg carbonatitic to silicic trend. The observed variations are interpreted as resulted from mixing of two contrasting fluids derived from the partial melting mainly of carbonated peridotite (the high-Mg carbonatitic HDFs) and eclogite (silica-rich HDFs and HDFs with high Ca/(Ca + Mg + Fe)). Immiscibility of carbonatitic and silica-rich fluids provides a possible mechanism for the co-existence of the observed HDFs but needs further proof. The uniform carbon isotope composition of cloudy diamonds with high-Mg carbonatitic microinclusions from both kimberlite pipes implies a single peridotitic source.
Geochimica et Cosmochimica Acta, in press available 45p.
Africa, Russia
Geochronology
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243‰ to ?0.204‰ with an average of ?0.225 ± 0.037‰ (2?, n = 19), and ?56Fe from ?0.038‰ to 0.060‰ with an average of ?0.003 ± 0.068‰ (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (?±0.05‰ for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Abstract: Kimberlites are not only the most economically important source of diamonds; they also carry unique information encapsulated in rock fragments entrained as the magma traverses the whole thickness of the lithosphere. The Nurbinskaya pipe in the Siberian kimberlite province (Russia) is one of several intruded along the Vilyui Rift, a major terrane boundary. The pipe contains three populations of mantle-derived zircon xenocrysts: Archean (mean age 2709 ± 9 Ma), Devonian (mean age 371 ± 2.3 Ma), and a subset of grains with evidence of brittle deformation and rehealing, and a range of ages between 370 and 450 Ma. The Hf-isotope, O-isotope and trace-element signatures of the last group provide a link between the Archean and Devonian events, indicating at least three episodes of magmatic activity and zircon crystallization in the lithosphere beneath the pipe. The emplacement of the Nurbinskaya pipe ca 370 Ma ago was only the youngest activity in a magma plumbing system that has been periodically reactivated over at least 2.7 billion years, controlled by the lithosphere-scale structure of the Vilyui Rift.
Geochimica et Cosmochimica Acta, Vol. 200, pp. 167-185.
Africa, South Africa, Russia
Metasomatism
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243 to ?0.204 with an average of ?0.225 ± 0.037 (2?, n = 19), and ?56Fe from ?0.038‰ o 0.060 with an average of ?0.003 ± 0.068 (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (±0.05 for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Abstract: Widespread Miocene (24-8 Ma) ultrapotassic rocks and their entrained xenoliths provide information on the composition, structure, and thermal state of the sub-continental lithospheric mantle in southern Tibet during the India-Asia continental collision. The ultrapotassic rocks along the Lhasa block delineate two distinct lithospheric domains with different histories of depletion and enrichment. The eastern ultrapotassic rocks (89°E-92°E) reveal a depleted, young, and fertile lithospheric mantle (87Sr/86Srt = 0.704-0.707 [t is eruption time]; Hf depleted-mantle model age [TDM] = 377-653 Ma). The western ultrapotassic rocks (79°E-89°E) and their peridotite xenoliths (81°E) reflect a refractory harzburgitic mantle refertilized by ancient metasomatism (lavas: 87Sr/86Srt = 0.714-0.734; peridotites: 87Sr/86Srt = 0.709-0.716). These data integrated with seismic tomography suggest that upwelling asthenosphere was diverted away from the deep continental root beneath the western Lhasa block, but rose to shallower depths beneath a thinner lithosphere in the eastern part. Heating of the lithospheric mantle by the rising asthenosphere ultimately generated the ultrapotassic rocks with regionally distinct geochemical signatures reflecting the different nature of the lithospheric mantle.
Geochimica et Cosmochimica Acta, Vol. 200, pp. 167-185.
Africa, South Africa, Russia
Craton, Peridotite
Abstract: We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134-186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in ?26Mg and ?56Fe of garnet peridotites from these two cratons. ?26Mg of whole rocks varies from ?0.243‰ to ?0.204‰ with an average of ?0.225 ± 0.037‰ (2?, n = 19), and ?56Fe from ?0.038‰ 0.060 with an average of ?0.003 ± 0.068‰ (2?, n = 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show ?26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation. The precision of the Mg and Fe isotope data (?±0.05‰ for ?26Mg and ?56Fe, 2?) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest ?26Mg and ?56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy ?26Mg and much lighter ?56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.
Abstract: Detailed petrography, microstructure, and geochemistry of garnet pyroxenite xenoliths in Holocene basanite tuffs from maars at Lakes Bullenmerri and Gnotuk (western Victoria, southeastern Australia) have been used to track their igneous and metamorphic history, enabling the reconstruction of the thermal-tectonic evolution of the lithospheric mantle. The exsolution of orthopyroxene and garnet and rare spinel, plagioclase, and ilmenite from complex clinopyroxene megacrysts suggests that the xenoliths originally were clinopyroxene-dominant cumulates associated with minor garnet, orthopyroxene, or spinel. The compositions of exsolved phases and their host clinopyroxene were reintegrated using measured modal proportions to show that the primary clinopyroxene was enriched in Al2O3 (5.53-13.63 wt%) and crystallized at ~1300-1500 °C and 16-30 kbar. These cumulates then underwent extensive exsolution, recrystallization, and reaction during cooling, and finally equilibrated at ~950-1100 °C and 12-18 kbar before entrainment in the basanites. Rare earth element (REE) thermobarometry of garnets and coexisting clinopyroxenes preserves evidence of an intermediate stage (1032 °C and 21 kbar). These results imply that the protoliths of the garnet pyroxenite formed at a range of depths from ~50 to 100 km, and then during or shortly after cooling, they were tectonically emplaced to higher levels (~40-60 km; i.e., uplifted by at least 10-20 km) along the prevailing geotherm. This uplift may have been connected with lithosphere-scale faulting during the Paleozoic orogeny, or during Mesozoic-Cenozoic rifting of eastern Australia.
Abstract: The petrogenesis of kimberlites is commonly obscured by interaction with hydrothermal fluids, including deuteric (late-magmatic) and/or groundwater components. To provide new constraints on the modification of kimberlite rocks during fluid interaction and the fractionation of kimberlite magmas during crystallisation, we have undertaken a detailed petrographic and geochemical study of a hypabyssal sample (BK) from the Bultfontein kimberlite (Kimberley, South Africa). Sample BK consists of abundant macrocrysts (> 1 mm) and (micro-) phenocrysts of olivine and lesser phlogopite, smaller grains of apatite, serpentinised monticellite, spinel, perovskite, phlogopite and ilmenite in a matrix of calcite, serpentine and dolomite. As in kimberlites worldwide, BK olivine grains consist of cores with variable Mg/Fe ratios, overgrown by rims that host inclusions of groundmass phases (spinel, perovskite, phlogopite) and have constant Mg/Fe, but variable Ni, Mn and Ca concentrations. Primary multiphase inclusions in the outer rims of olivine and in Fe-Ti-rich (‘MUM’) spinel are dominated by dolomite, calcite and alkali carbonates with lesser silicate and oxide minerals. Secondary inclusions in olivine host an assemblage of Na-K carbonates and chlorides. The primary inclusions are interpreted as crystallised alkali-Si-bearing Ca-Mg-rich carbonate melts, whereas secondary inclusions host Na-K-rich C-O-H-Cl fluids. In situ Sr-isotope analyses of groundmass calcite and perovskite reveal similar 87Sr/86Sr ratios to perovskite in the Bultfontein and the other Kimberley kimberlites, i.e. magmatic values. The ?18O composition of the BK bulk carbonate fraction is above the mantle range, whereas the ?13C values are similar to those of mantle-derived magmas. The occurrence of different generations of serpentine and occasional groundmass calcite with high 87Sr/86Sr, and elevated bulk carbonate ?18O values indicate that the kimberlite was overprinted by hydrothermal fluids, which probably included a significant groundwater component. Before this alteration the groundmass included calcite, monticellite, apatite and minor dolomite, phlogopite, spinel, perovskite and ilmenite. Inclusions of groundmass minerals in olivine rims and phlogopite phenocrysts show that olivine and phlogopite also belong to the magmatic assemblage. We therefore suggest that the crystallised kimberlite was produced by an alkali-bearing, phosphorus-rich, silica-dolomitic melt. The alkali-Si-bearing Ca-Mg-rich carbonate compositions of primary melt inclusions in the outer rims of olivine and in spinel grains with evolved compositions (MUM spinel) support formation of these melts after fractionation of abundant olivine, and probably other phases (e.g., ilmenite and chromite). Finally, the similarity between secondary inclusions in kimberlite olivine of this and other worldwide kimberlites and secondary inclusions in minerals of carbonatitic, mafic and felsic magmatic rocks, suggests trapping of residual Na-K-rich C-O-H-Cl fluids after groundmass crystallisation. These residual fluids may have persisted in pore spaces within the largely crystalline BK groundmass and subsequently mixed with larger volumes of external fluids, which triggered serpentine formation and localised carbonate recrystallisation.
Abstract: Carbon isotope compositions and the distribution of nitrogen and hydrogen in diamonds from 18 eclogites from Nurbinskaya kimberlites were studied in situ in polished plates. Cathodoluminescence images show that most of the diamonds have complex growth structures with distinctive core, intermediate and rim zones. In some diamonds the cores display dissolution features, and intermediate growth zones are separated from the cores by narrow rounded oscillatory zones. At least three crystals show interrupted multistage diamond growth; variations in ?13C of 2–3‰ occur across the contacts between distinct zones. Generally, ?13C within the diamond cores varies only by 1–2‰, in rare cases up to 3.3‰. ?13C values are usually lower in the intermediate zones and drop further towards the rims by up to 3‰. High-resolution SIMS profiles show that variations in ?13C across the diamond growth zones are sharp with no evidence of diffusive relaxation. Diamonds with predominantly tangential octahedral growth have a wide range in ?13C from ? 15.2‰ up to 9.0‰ (± 0.4‰), and their nitrogen (N) contents vary between 30 and 1500 at. ppm. Six diamonds show little internal variation along the isotopic profiles with changes in ?13C of only 0.3–0.9‰ around mean values ranging from ? 6‰ to ? 3‰. Five crystals are isotopically heavy, with relatively homogeneous ?13C up to 9‰. FTIR data show markedly different N concentrations and nitrogen aggregation states between major growth zones. This implies that the diamonds in eclogitic xenoliths from Nyurbinskaya pipe grew in multiple and interrupted growth events, probably from fluids enriched in K and H. The wide variations of ?13C in the studied eclogitic diamonds and identification of their anomalously positive ?13C values, combined with the wide range of high ?18O in garnets from the diamondiferous xenoliths of the Nyurbinskaya pipe, which are mostly outside of the mantle range, suggest a crustal contribution to the parental mantle-related fluids forming diamonds in these xenoliths and indicate the complex metasomatic evolution of the lithospheric mantle beneath the Nakynsky kimberlite field.
Geological Society of London, Chapter 2, pp. 27-48.
Australia, Victoria
xenoliths
Abstract: Detailed petrography, microstructure, and geochemistry of garnet pyroxenite xenoliths in Holocene basanite tuffs from maars at Lakes Bullenmerri and Gnotuk (western Victoria, southeastern Australia) have been used to track their igneous and metamorphic history, enabling the reconstruction of the thermal-tectonic evolution of the lithospheric mantle. The exsolution of orthopyroxene and garnet and rare spinel, plagioclase, and ilmenite from complex clinopyroxene megacrysts suggests that the xenoliths originally were clinopyroxene-dominant cumulates associated with minor garnet, orthopyroxene, or spinel. The compositions of exsolved phases and their host clinopyroxene were reintegrated using measured modal proportions to show that the primary clinopyroxene was enriched in Al2O3 (5.53–13.63 wt%) and crystallized at ~1300–1500 °C and 16–30 kbar. These cumulates then underwent extensive exsolution, recrystallization, and reaction during cooling, and finally equilibrated at ~950–1100 °C and 12–18 kba
Abstract: It is now accepted that significant volumes of pyroxenites are generated in the subduction factory and remain trapped in the mantle. In ophiolites and orogenic massifs the geometry of pyroxenite layers and their relationships with the host peridotite can be observed directly. Since a large part of what is known about the upper mantle structure is derived from the analysis of seismic waves, it is crucial to integrate pyroxenites in the interpretations. We modeled the seismic properties of a peridotitic mantle rich in pyroxenite layers in order to determine the impact of layering on the seimsic properties. To do so, EBSD data on deformed and undeformed pyroxenites from the Cabo Ortegal complex (Spain) and the Trinity ophiolite (California, USA) respectively are combined with either A or B-type olivine fabrics in order to model a realistic pyroxenite-rich upper mantle. Consideration of pyroxeniterich domains within the host mantle wall rock is incorporated in the calculations using the Schoenberg and Muir group theory [1]. This quantification reveals the complex dependence of the seismic signal on the deformational state and relative abundance of each mineral phase. The incorporation of pyroxenites properties into geophysical interpretations in understanding the lithospheric structure of subduction zones will lead to more geologically realistic models.
Abstract: Moissanite (SiC) occurs in mantle and mantle-generated rocks from different tectonic settings. SiC is stable only at low oxygen fugacity (ƒO2) ?IW. Israeli SiC is assiociated with corundum, Fe globules, native V and other phases in Cretaceous pyroclastic rocks from Mt Carmel and associated alluvial deposits[1]. The SiC grains contain inclusions of Si metal, FeSi2, FeTiSi2, FeAlSi2 and CaSi2+xSi2-x, which were liquids before being trapped during SiC crystallization. SiC has been found included in corundum, associated with Fe-Ti silicides, connecting the formation of SiC, reduced melts in corundum and conrundum itself. All grains are of the 6H polytype. ?13C ranges from - 32.1 to -24.5‰ and ?30Si from -0.68 to +1.42‰. These SiC grains are one product of the interaction of basaltic magma and mantle methane in a volcanic plumbing system. SiC crystallized from metallic melts that became immiscible during the reduction of the magma. Its low ?13C may reflect Rayleigh fractionation under reduced conditions; the variation in Si isotopes may reflect fractionation between SiC and immiscible metallic melts. SiC samples from the Udachnaya and Mir kimberlite pipes contain inclusions of Si metal, FeSi2, FeSi, FeTiSi2, Si(N,O). The SiC has ?13C ranging from -28.5 to -24.8‰, and ?30Si from -1.72 to +1.42‰. SiC from harzburgites, chromitites and pyroxenites of the Tibetan Zedang ophiolites have inclusions of Si metal and unmixed Fe-Ni-Ti-Si alloy. Their ?13C ranges from -30.6 to -24.7‰ and ?30Si from -0.85 to +1.26‰. SiC samples from these different settings show very similar characteristics, implying that they may be formed in similar mantle conditions, where the flux of mantle methane gradually reduces magmas and interacts with them to produce different reduced phases at different stages.
European Journal of Mineralogy, Vol. 29, 4, pp. 557-570.
Europe, Israel
mineralogy
Abstract: Ultrahigh-pressure (UHP) materials (e.g., diamond, high-pressure polymorph of chromite) and super-reduced (SuR) phases (e.g., carbides, nitrides, silicides and native metals) have been identified in chromitites and peridotites of the Tibetan and Polar-Urals ophiolites. These unusual assemblages suggest previously unrecognized fluid- or melt-related processes in the Earth’s mantle. However, the origin of the SuR phases, and in particular their relationships with the UHP materials in the ophiolites, are still enigmatic. Studies of a recently recognized SuR mineral system from Cretaceous volcanics on Mt Carmel, Israel, suggest an alternative genesis for the ophiolitic SuR phases. The Mt Carmel SuR mineral system (associated with Ti-rich corundum xenocrysts) appears to reflect the local interaction of mantle-derived CH4 ± H2 fluids with basaltic magmas in the shallow lithosphere (depths of ?30-100 km). These interactions produced desilication of the magma, supersaturation in Al2O3 leading to rapid growth of corundum, and phase assemblages requiring local oxygen fugacity (fO2) gradually dropping to ?11 log units below the iron-wüstite (IW) buffer. The strong similarities between this system and the SuR phases and associated Ti-rich corundum in the Tibetan and Polar-Urals ophiolites suggest that the ophiolitic SuR suite probably formed by local influx of CH4 ± H2 fluids within previously subducted peridotites (and included chromitites) during their rapid exhumation from the deep upper mantle to lithospheric levels. In the final stages of their ascent, the recycled peridotites and chromitites were overprinted by a shallow magmatic system similar to that observed at Mt Carmel, producing most of the SuR phases and eventually preserving them within the Tibetan and Polar-Urals ophiolites.
Abstract: In order to develop a 4D understanding of the architecture of the entire lithosphere, it is necessary to embrace integration of multi-disciplinary, multi-scale data in a GIS environment. An holistic understanding has evolved whereby geologic, geochemical and geophysical signals are consistent with a subcontinental lithospheric mantle (SCLM) dominated by a mosaic of domains of Archean ancestry, variably overprinted by subsequent tectonothermal events. Pristine Archean SCLM is mostly highly depleted (high Mg#), low density, high velocity and highly resistive, and preserves intact Archean crust. There is a first order relationship between changes to these signals and the degree of tectonothermal overprint (by melts, fluids). Continental crust is comprised largely of reconstituted Archean components, variably diluted by juvenile addition, symptomatic of the various overprinting events. These events impart crustal fabrics and patterns dictated by SCLM architecture, influenced by the free surface and crust-mantle decoupling.
Abstract: Magnesian ilmenite is a common kimberlite indicator mineral, although its use in diamond exploration is still controversial. Complex crystallisation and replacement processes have been invoked to explain the wide compositional and textural ranges of ilmenite found in kimberlites. This work aims to shed light on these processes, as well as their implications for diamond exploration. Petrographic studies were combined for the first time with both major- and trace-element analyses to characterise the ilmenite populations found in xenoliths and xenocrysts in two Angolan kimberlites (Congo-Kasai craton). A multi-stage model describes the evolution of ilmenite in these pipes involving: i) crystallisation of ferric and Mg-rich ilmenite either as metasomatic phases or as megacrysts, both in crustal and in metasomatised mantle domains; ii) kimberlite entrainment and xenolith disaggregation producing at least two populations of ilmenite nodules differing in composition; iii) interaction of both types with the kimberlitic magma during eruption, leading to widespread replacement by Mg-rich ilmenite along grain boundaries and fractures. This process produced similar major-element compositions in ilmenites regardless of their primary (i.e., pre-kimberlitic) origin, although the original enrichment in HFSE (Zr, Hf, Ta, Nb) observed in Fe3 +-rich xenocrysts is preserved. Finally (iv) formation of secondary Mn-ilmenite by interaction with a fluid of carbonatitic affinity or by infiltration of a late hydrothermal fluid, followed in some cases by subsolidus alteration in an oxidising environment. The complexities of ilmenite genesis may lead to misinterpretation of the diamond potential of a kimberlite during the exploration stage if textural and trace-element information is disregarded. Secondary Mg-enrichment of ilmenite xenocrysts is common and is unrelated to reducing conditions that could favour diamond formation/preservation in the mantle. Similarly, Mn-rich ilmenite should be disregarded as a diamond indicator mineral, unless textural studies can prove its primary origin.
Abstract: Podiform chromitites occur in mantle peridotites of the Late Triassic Puerto Nuevo Ophiolite, Baja California Sur State, Mexico. These are high-Cr chromitites [Cr# (Cr/Cr + Al atomic ratio = 0.61-0.69)] that contain a range of minor- and trace-elements and show whole-rock enrichment in IPGE (Os, Ir, Ru). That are similar to those of high-Cr ophiolitic chromitites crystallised from melts similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction-zone mantle wedges. Crystallisation of these chromitites from S-undersaturated melts is consistent with the presence of abundant inclusions of platinum-group minerals (PGM) such as laurite (RuS2)-erlichmanite (OsS2), osmium and irarsite (IrAsS) in chromite, that yield TMA ? TRD model ages peaking at ~ 325 Ma. Thirty-three xenocrystic zircons recovered from mineral concentrates of these chromitites yield ages (2263 ± 44 Ma to 278 ± 4 Ma) and Hf-O compositions [?Hf(t) = ? 18.7 to + 9.1 and 18O values < 12.4‰] that broadly match those of zircons reported in nearby exposed crustal blocks of southwestern North America. We interpret these chromitite zircons as remnants of partly digested continental crust or continent-derived sediments on oceanic crust delivered into the mantle via subduction. They were captured by the parental melts of the chromitites when the latter formed in a supra-subduction zone mantle wedge polluted with crustal material. In addition, the Puerto Nuevo chromites have clinopyroxene lamellae with preferred crystallographic orientation, which we interpret as evidence that chromitites have experienced high-temperature and ultra high-pressure conditions (< 12 GPa and ~ 1600 °C). We propose a tectonic scenario that involves the formation of chromitite in the supra-subduction zone mantle wedge underlying the Vizcaino intra-oceanic arc ca. 250 Ma ago, deep-mantle recycling, and subsequent diapiric exhumation in the intra-oceanic basin (the San Hipólito marginal sea) generated during an extensional stage of the Vizcaino intra-oceanic arc ca. 221 Ma ago. The TRD ages at ~ 325 Ma record a partial melting event in the mantle prior to the construction of the Vizcaino intra-oceanic arc, which is probably related to the Permian continental subduction, dated at ~ 311 Ma.
Abstract: According to present views, the crustal terranes of the Anabar province of the Siberian craton were initially independent blocks, separated from the convecting mantle at 3.1 (Daldyn terrane), 2.9 (Magan terrane) and 2.5?Ga (Markha terrane) (Rosen, 2003, 2004; Rosen et al., 1994, 2005, 2009). Previous studies of zircons in a suite of crustal xenoliths from kimberlite pipes of the Markha terrane concluded that the evolution of the crust of the Markha terrane is very similar to that of the Daldyn terrane. To test this conclusion we present results of U-Pb and Hf-isotope studies on zircons in crustal xenoliths from the Zapolyarnaya kimberlite pipe (Upper Muna kimberlite field), located within the Daldyn terrane, and the Botuobinskaya pipe (Nakyn kimberlite field) in the center of the Markha terrane. The data on xenoliths from the Botuobinskaya kimberlite pipe record tectonothermal events at 2.94, 2.8, 2.7 and 2?Ga. The event at 2?Ga caused Pb loss in zircons from a mafic granulite. U-Pb dating of zircons from the Zapolyarnaya pipe gives an age of 2.7?Ga. All zircons from the studied crustal xenoliths have Archean Hf model ages ranging from 3.65 to 3.11?Ga. This relatively narrow range suggests that reworking of the ancient crust beneath the Nakyn and Upper Muna kimberlite fields was minor, compared with the Daldyn and Alakit-Markha fields (Shatsky et al., 2016). This study, when combined with dating of detrital zircons, implies that tectonic-thermal events at 2.9-2.85, 2.75-2.7 and 2.0-1.95?Ga occurred everywhere on the Anabar tectonic province, and could reflect the upwelling of superplumes at 2.9, 2.7 and 2?Ga. The presence of the same tectonic-thermal events in the Daldyn and Markha terranes (Rosen et al., 2006a,b) supports the conclusion that the identification of the Markha terrane as a separate unit is not valid.
Abstract: he meta-analysis of about 13,000 analyses of scandium content in garnet grains shows that, below the spinel-garnet transition, this phase carries about three-quarters of the Sc budget of the mantle, indicating its control on Sc mobility. The Sc content of garnets in mafic rocks is low, due to a dilution effect resulting from their high modal content in garnet. Garnets from ultramafic rocks exhibit a wider range of Sc concentrations. We assess the relative influence of thermobarometry, crystal chemistry and fluid-related events on the distribution of Sc in garnet from such rocks to improve the tracking of geochemical processes in the mantle. Pressure and temperature of equilibration in the mantle are second-order factors influencing the Sc content of garnet, while crystal-chemistry, in particular and , is the main parameter controlling the compatibility of Sc. Scandium is incorporated in both X and Y sites of Cr-Ca-rich garnets, resulting in a behaviour intermediate between rare-earth elements, incorporated in the X site, and trivalent transition elements, occupying the Y site. This affinity for both sites results in a mild compatibility of Sc in the garnet stability field of the mantle; hence Sc concentration in garnet increases with melt extraction and can be reduced by silicate-melt metasomatism. In contrast, metasomatism by volatile-rich fluids increases the Sc concentration in garnet. The control of garnet on the compatibility of Sc in deep lithospheric rocks demonstrates the potential of using Sc to track the conditions of formation of magmas and their residual rocks, as well as the origin and nature of metasomatic fluids.
Mineralogy and Petrology, in press available, 14p.
Europe, Israel
metasomatism
Abstract: Oxygen fugacity (fO2) is a key parameter of Earth’s mantle, because it controls the speciation of the fluids migrating at depth; a major question is whether the sublithospheric mantle is metal-saturated, keeping fO2 near the Iron-Wustite (IW) buffer reaction. Cretaceous basaltic pyroclastic rocks on Mt. Carmel, Israel erupted in an intraplate environment with a thin, hot lithosphere. They contain abundant aggregates of hopper-shaped crystals of Ti-rich corundum, which have trapped melts with phenocryst assemblages (Ti2O3, SiC, TiC, silicides, native V) requiring extremely low fO2. These assemblages are interpreted to reflect interaction between basaltic melts and mantle-derived fluids dominated by CH4 + H2. Similar highly reduced assemblages are found associated with volcanism in a range of tectonic situations including subduction zones, major continental collisions, intraplate settings, craton margins and the cratons sampled by kimberlites. This distribution, and the worldwide similarity of ?13C in mantle-derived SiC and associated diamonds, suggest a widespread process, involving similar sources and independent of tectonic setting. We suggest that the common factor is the ascent of abiotic (CH4 + H2) fluids from the sublithospheric mantle; this would imply that much of the mantle is metal-saturated, consistent with observations of metallic inclusions in sublithospheric diamonds (e.g. Smith et al. 2016). Such fluids, perhaps carried in rapidly ascending deep-seated magmas, could penetrate high up into a depleted cratonic root, establishing the observed trend of decreasing fO2 with depth (e.g. Yaxley et al. in Lithos 140:142-151, 2012). However, repeated metasomatism (associated with the intrusion of silicate melts) will raise the FeO content near the base of the craton over time, developing a carapace of oxidizing material that would prevent the rise of CH4-rich fluids into higher levels of the subcontinental lithospheric mantle (SCLM). Oxidation of these fluids would release CO2 and H2O to drive metasomatism and low-degree melting both in the carapace and higher in the SCLM. This model can explain the genesis of cratonic diamonds from both reduced and oxidized fluids, the existence of SiC as inclusions in diamonds, and the abundance of SiC in some kimberlites. It should encourage further study of the fine fractions of heavy-mineral concentrates from all types of explosive volcanism.
Abstract: Cr-pyrope xenocrysts and associated inclusions of crichtonite-group minerals from the Internatsionalnaya kimberlite pipe were studied to provide new insights into processes in the lithospheric mantle beneath the Mirny kimberlite field, Siberian craton. Pyropes are predominantly of lherzolitic paragenesis (Cr2O3 2-6?wt%) and have trace-element spectra typical for garnets from fertile mantle (gradual increase in chondrite-normalized values from LREE to MREE-HREE). Crichtonite-group minerals commonly occur as monomineralic elongated inclusions, mostly in association with rutile, Mg-ilmenite and Cr-spinel within individual grains of pyrope. Sample INT-266 hosts intergrowth of crichtonite-group mineral and Cl-apatite, while sample INT-324 contains polymineralic apatite- and dolomite-bearing assemblages. Crichtonite-group minerals are Al-rich (1.1-4.5?wt% Al2O3), moderately Zr-enriched (1.3-4.3?wt% ZrO2), and are Ca-, Sr-, and occasionally Ba-dominant in terms of A-site occupancy; they also contain significant amounts of Na and LREE. T-estimates and chemical composition of Cr-pyropes imply that samples represent relatively low-T peridotite assemblages with ambient T ranging from 720 to 820°?. Projected onto the 35?mW/m2 cratonic paleogeotherm for the Mirny kimberlite field (Griffin et al., 1999b. Tectonophysics 310, 1-35), temperature estimates yield a P range of ~34-42?kbar (~110-130?km), which corresponds to a mantle domain in the uppermost part of the diamond stability field. The presence of crichtonite-group minerals in Cr-pyropes has petrological and geochemical implications as evidence for metasomatic enrichment of some incompatible elements in the lithospheric mantle beneath the Mirny kimberlite field. The genesis of Cr-pyropes with inclusions of crichtonite-group minerals is attributed to the percolation of Ca-Sr-Na-LREE-Zr-bearing carbonate-silicate metasomatic agents through Mg- and Cr-rich depleted peridotite protoliths. The findings of several potentially new members of the crichtonite group as inclusions in garnet extend existing knowledge on the compositions and occurrences of exotic titanates stable in the lithospheric mantle.
Griffin, W.L., Huang, J-X., Thomassot, E., Gain, S.E.M., Toledo, V., O'Reilly, S.Y.
Super-reducing conditions in ancient and modern volcanic systems: sources and behaviour of carbon-rich fluids in the lithospheric mantle ( Mt. Carmel).
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0575-x 14p.
Abstract: According to present views, the crustal terranes of the Anabar province of the Siberian craton were initially independent blocks, separated from the convecting mantle at 3.1 (Daldyn terrane), 2.9 (Magan terrane) and 2.5?Ga (Markha terrane) (Rosen, 2003, 2004; Rosen et al., 1994, 2005, 2009). Previous studies of zircons in a suite of crustal xenoliths from kimberlite pipes of the Markha terrane concluded that the evolution of the crust of the Markha terrane is very similar to that of the Daldyn terrane. To test this conclusion we present results of U-Pb and Hf-isotope studies on zircons in crustal xenoliths from the Zapolyarnaya kimberlite pipe (Upper Muna kimberlite field), located within the Daldyn terrane, and the Botuobinskaya pipe (Nakyn kimberlite field) in the center of the Markha terrane. The data on xenoliths from the Botuobinskaya kimberlite pipe record tectonothermal events at 2.94, 2.8, 2.7 and 2?Ga. The event at 2?Ga caused Pb loss in zircons from a mafic granulite. U-Pb dating of zircons from the Zapolyarnaya pipe gives an age of 2.7?Ga. All zircons from the studied crustal xenoliths have Archean Hf model ages ranging from 3.65 to 3.11?Ga. This relatively narrow range suggests that reworking of the ancient crust beneath the Nakyn and Upper Muna kimberlite fields was minor, compared with the Daldyn and Alakit-Markha fields (Shatsky et al., 2016). This study, when combined with dating of detrital zircons, implies that tectonic-thermal events at 2.9 -2.85, 2.75 -2.7 and 2.0 -1.95?Ga occurred everywhere on the Anabar tectonic province, and could reflect the upwelling of superplumes at 2.9, 2.7 and 2?Ga. The presence of the same tectonic-thermal events in the Daldyn and Markha terranes (Rosen et al., 2006a,b) supports the conclusion that the identification of the Markha terrane as a separate unit is not valid.
Abstract: Carbonates in fresh hypabyssal kimberlites worldwide have been studied to understand their origin [i.e. primary magmatic (high T) versus deuteric (‘low T’) versus hydrothermal/alteration (‘low T’)] and identify optimal strategies for petrogenetic studies of kimberlitic carbonates. The approach presented here integrates detailed textural characterisation, cathodoluminescence (CL) imaging, in situ major- and trace-element analysis, as well as in situ Sr-isotope analysis. The results reveal a wide textural diversity. Calcite occurs as fine-grained groundmass, larger laths, segregations, veins or as a late crystallising phase, replacing olivine or early carbonates. Different generations of carbonates commonly coexist in the same kimberlite, each one defined by a characteristic texture, CL response and composition (e.g., variable Sr and Ba concentrations). In situ Sr isotope analysis revealed a magmatic signature for most of the carbonates, based on comparable 87Sr/86Sr values between these carbonates and the coexisting perovskite, a robust magmatic phase. However, this study also shows that in situ Sr isotope analysis not always allow distinction between primary (i.e., magmatic) and texturally secondary carbonates within the same sample. Carbonates with a clear secondary origin (e.g., late-stage veins) occasionally show the same moderately depleted 87Sr/86Sr ratios of primary carbonates and coexisting perovskite (e.g., calcite laths-shaped crystals with 87Sr/86Sr values identical within uncertainty to those of vein calcite in the De Beers kimberlite). This complexity emphasises the necessity of integrating detailed petrography, geochemical and in situ Sr isotopic analyses for an accurate interpretation of carbonate petrogenesis in kimberlites. Therefore, the complex petrogenesis of carbonates demonstrated here not only highlights the compositional variability of kimberlites, but also raises concerns about the use of bulk-carbonate C-O isotope studies to characterise the parental melt compositions. Conversely, our integrated textural and in situ study successfully identifies the most appropriate (i.e. primary) carbonates for providing constraints on the isotopic parameters of parental kimberlite magmas.
Abstract: Xenocrystic zircons from Cretaceous pyroclastic vents on Mt. Carmel, N. Israel, document two major periods of earlier mafic magmatism: Permo-Triassic (285-220?Ma) and Jurassic (200-160?Ma). Related alluvial deposits also contain these zircon populations. However, most alluvial zircons are Cretaceous (118-80?Ma) or younger, derived from Miocene to Pliocene volcanic episodes. The Permo-Triassic-Jurassic zircons are typically large and glassy; they have irregular shapes and a wide variety of internal zoning patterns. They appear to have grown in the interstitial spaces of coarse-grained rocks; many show evidence of recrystallization, including brecciation and rehealing by chemically similar zircon. Grains with relict igneous zoning have mantle-like ?18O (5.5?±?1.0‰), but brecciation leads to lower values (mean 4.8‰, down to 3.1‰). Hf-isotope compositions lie midway between the Chondritic Uniform Reservoir (CHUR) and Depleted Mantle (DM) reservoirs; Hf model ages suggest that the source region separated from DM in Neoproterozoic time (1500-1000?Ma). Most Cretaceous zircons have 176Hf/177Hf similar to those of the older zircons, suggesting recrystallization and/or Pb loss from older zircons in the Cretaceous thermal event. The Permo-Jurassic zircons show trace-element characteristics similar to those crystallized from plume-related magmas (Iceland, Hawaii). Calculated melts in equilibrium with them are characterized by strong depletion in LREE and P, large positive Ce anomalies, variable Ti anomalies, and high and variable Nb, Ta, Th and U, consistent with the fractionation of monazite, zircon, apatite and Ti-bearing phases. We suggest that these coarse-grained zircons crystallized from late differentiates of mafic magmas, ponded near the crust-mantle boundary (ca 30?km depth), and were reworked repeatedly by successively younger igneous/metasomatic fluids. The zircon data support a published model that locates a fossil Neoproterozoic plume head beneath much of the Arabia-Levant region, which has been intermittently melted to generate the volcanic rocks of the region. The Cretaceous magmas carry mantle xenoliths derived from depths up to 90?km, providing a minimum depth for the possible plume head. Post-Cretaceous magmatism, as recorded in detrital zircons, shows distinct peaks at 30?Ma, 13?Ma, 11.4?±?0.1?Ma (a major peak; n?=?15), 9-10?Ma and 4?Ma, representing the Lower and Cover Basalts in the area. Some of these younger magmas tapped the same mantle source as the Permian-Jurassic magmatism, but many young zircons have Hf-isotope compositions extending up to DM values, suggesting derivation of magmas from deeper, more juvenile sources.
Geochemical Perspectives Letters, Vol. 7, pp. 1-2.
Mantle
moissanite
Abstract: Ballhaus et al. (2017) use electric-discharge experiments to argue that lightning strikes could produce ultra-high pressure (UHP) and super-reduced (SuR) phases "identical to those found in 'high-pressure' ophiolites" and that thus there is "not sufficient evidence to challenge long-established models of ophiolite genesis", specifically for the UHP processing of Tibetan ophiolites. However, the authors produced no evidence for UHP phases in their experiments. There are pertinent observations, relevant to the authors’ assertions, in the literature regarding the relationship between the UHP and SuR assemblages in the Tibetan peridotites. Their conclusions are not consistent with this evidence.
Abstract: Pyroxenites provide important information on mantle heterogeneity and can be used to trace mantle evolution. New major and trace element and Sr-, Nd-, and Hf-isotope analyses of minerals and whole-rock samples of garnet websterites entrained in basanite tuffs in Bullenmerri and Gnotuk maars, southeastern Australia, are here combined with detailed petrographic observations to constrain the sources and genesis of the pyroxenites, and to trace the dynamic evolution of the lithospheric mantle. Most garnet websterites have high MgO and Cr2O3 contents, relatively flat light rare earth element (LREE) patterns ([La/Nd]CN?=?0•77-2•22) and ocean island basalt-like Sr-, Nd-, and Hf-isotope compositions [87Sr/86Sr?=?0•70412-0•70657; ?Nd(t)?=?-0•32 to +4•46; ?Hf(t)=+1•69 to +18•6] in clinopyroxenes. Some samples show subduction-related signatures with strong enrichments in large ion lithophile elements and LREE, and negative anomalies in high field strength elements, as well as high 87Sr/86Sr (up to 0•709), and decoupled Hf- and Nd-isotope compositions [?Nd(t)?=?-3•28; ?Hf(t) =?+11•6). These data suggest that the garnet pyroxenites represent early crystallization products of mafic melts derived from a convective mantle wedge. Hf model ages and Sm-Nd mineral isochrons suggest that these pyroxenites record at least two stages of evolution. The initial formation stage corresponds to the Paleozoic subduction of the proto-Pacific plate beneath southeastern Australia, which generated hydrous tholeiitic melts that crystallized clinopyroxene-dominated pyroxenites at ?1420-1450°C and ?75?km depth in the mantle wedge. The second stage corresponds to Eocene (c. 40?Ma) back-arc lithospheric extension, which led to uplift of the former mantle-wedge domain to 40-60?km depths, and subsequent cooling to the ambient geotherm (?950-1100°C). Extensive exsolution and recrystallization of garnet and orthopyroxene (±?ilmenite) from clinopyroxene megacrysts accompanied this stage. The timing of these mantle events coincides with vertical tectonism in the overlying crust.
Geostandards and Geoanalytical Research, http://orchid.org/0000-0002-2701-4635 80p.
Asia, Sri Lanka
geochronology
Abstract: Here we document a detailed characterization of two zircon gemstones, GZ7 and GZ8. Both stones had the same mass at 19.2 carats (3.84 g) each; both came from placer deposits in the Ratnapura district, Sri Lanka. The U-Pb data are in both cases concordant within the uncertainties of decay constants and yield weighted mean ²??Pb/²³?U ages (95% confidence uncertainty) of 530.26 Ma ± 0.05 Ma (GZ7) and 543.92 Ma ± 0.06 Ma (GZ8). Neither GZ7 nor GZ8 have been subjected to any gem enhancement by heating. Structure?related parameters correspond well with the calculated alpha doses of 1.48 × 10¹? g?¹ (GZ7) and 2.53 × 10¹? g?¹ (GZ8), respectively, and the (U-Th)/He ages of 438 Ma ± 3 Ma (2s) for GZ7 and 426 Ma ± 9 Ma (2s) for GZ8 are typical of unheated zircon from Sri Lanka. The mean U concentrations are 680 ?g g?¹ (GZ7) and 1305 ?g g?¹ (GZ8). The two zircon samples are proposed as reference materials for SIMS (secondary ion mass spectrometry) U-Pb geochronology. In addition, GZ7 (Ti concentration 25.08 ?g g?¹ ± 0.18 ?g g?¹; 95% confidence uncertainty) may prove useful as reference material for Ti?in?zircon temperature estimates.
Contributions to Mineralogy and Petrology, Vol. 173, 16p. Doi.org/10.1007/s00410-018-1513-y
Russia
deposit - Udachnaya
Abstract: A xenolith of bimineralic eclogite from the Udachnaya kimberlite pipe provides a snapshot of interaction between mantle rocks and diamond-forming fluids/melts. The major-element composition of the eclogite is similar to that of N-MORB and/or oceanic gabbros, but its trace-element pattern shows the effects of mantle metasomatism, which resulted in diamond formation. The diamonds are clustered in alteration veins that crosscut primary garnet and clinopyroxene. The diamonds contain microinclusions of a fluid/melt dominated by carbonate and KCl. Compared to the worldwide dataset, the microinclusions in these diamonds fall in middle of the range between saline fluids and low-Mg carbonatitic melts. The fluid/melt acted as a metasomatic agent that percolated through ancient eclogitic rocks stored in the mantle. This interaction is consistent with calculated partition coefficients between the rock-forming minerals and diamond-forming fluid/melt, which are similar to experimentally-determined values. Some differences between the calculated and experimental values may be due to the low contents of water and silicates in the chloride-carbonate melt observed in this study, and in particular its high contents of K and LILE. The lack of nitrogen aggregation in the diamonds implies that the diamond-forming metasomatism took place shortly before the eruption of the kimberlite, and that the microinclusions thus represent saline carbonate-rich fluids circulating in the basement of lithospheric mantle (150-170 km depth).
Abstract: The continental lithosphere is not forever; some cratons have lost their original roots during the course of their evolution. Yet, it is not clear whether gravitational instability of dense lower crust is the primary driver of decratonization. This is addressed here with emphasis being placed on the North China Craton (NCC), because it represents one of the best examples of craton-root disruption in the world, and a place where models can be tested. If lower-crustal delamination was the trigger for decratonization, we would expect a clear contrast in crustal structure and composition between disturbed (rootless) and intact cratons. However, the eastern (disturbed) and western (intact) parts of the NCC show virtually identical physical structure and composition (a thin mafic lower crust and a predominantly intermediate composition overall) although the crust in the disturbed part is thinner than in the intact craton. This suggests that delamination of the lower crust was not a viable mechanism of craton-root disruption in the NCC case. Indeed, the crust beneath the NCC largely resembles those of stable Archean cratons worldwide. Therefore the delamination, if it occurred, may have taken place much earlier (Archean) than previously thought, rather than in the Mesozoic. Delamination may have been a common phenomenon in the early evolution of cratons, probably due to relatively higher mantle temperatures in the Archean Eon.
Abstract: The new mineral species carmeltazite, ideally ZrAl2Ti4O11, was discovered in pockets of trapped melt interstitial to, or included in, corundum xenocrysts from the Cretaceous Mt Carmel volcanics of northern Israel, associated with corundum, tistarite, anorthite, osbornite, an unnamed REE (Rare Earth Element) phase, in a Ca-Mg-Al-Si-O glass. In reflected light, carmeltazite is weakly to moderately bireflectant and weakly pleochroic from dark brown to dark green. Internal reflections are absent. Under crossed polars, the mineral is anisotropic, without characteristic rotation tints. Reflectance values for the four COM wavelengths (Rmin, Rmax (%) (? in nm)) are: 21.8, 22.9 (471.1); 21.0, 21.6 (548.3), 19.9, 20.7 (586.6); and 18.5, 19.8 (652.3). Electron microprobe analysis (average of eight spot analyses) gave, on the basis of 11 oxygen atoms per formula unit and assuming all Ti and Sc as trivalent, the chemical formula (Ti3+3.60Al1.89Zr1.04Mg0.24Si0.13Sc0.06Ca0.05Y0.02Hf0.01)?=7.04O11. The simplified formula is ZrAl2Ti4O11, which requires ZrO2 24.03, Al2O3 19.88, and Ti2O3 56.09, totaling 100.00 wt %. The main diffraction lines, corresponding to multiple hkl indices, are (d in Å (relative visual intensity)): 5.04 (65), 4.09 (60), 2.961 (100), 2.885 (40), and 2.047 (60). The crystal structure study revealed carmeltazite to be orthorhombic, space group Pnma, with unit-cell parameters a = 14.0951 (9), b = 5.8123 (4), c = 10.0848 (7) Å, V = 826.2 (1) Å3, and Z = 4. The crystal structure was refined to a final R1 = 0.0216 for 1165 observed reflections with Fo > 4?(Fo). Carmeltazite exhibits a structural arrangement similar to that observed in a defective spinel structure. The name carmeltazite derives from Mt Carmel (“CARMEL”) and from the dominant metals present in the mineral, i.e., Titanium, Aluminum and Zirconium (“TAZ”). The mineral and its name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification (2018-103).
Abstract: Hibonite (CaAl12O19) is a constituent of some refractory calcium-aluminum inclusions (CAIs) in carbonaceous meteorites, commonly accompanied by grossite (CaAl4O7) and spinel. These phases are usually interpreted as having condensed, or crystallized from silicate melts, early in the evolution of the solar nebula. Both Ca-Al oxides are commonly found on Earth, but as products of high-temperature metamorphism of pelitic carbonate rocks. We report here a unique occurrence of magmatic hibonitegrossite-spinel assemblages, crystallized from Ca-Al-rich silicate melts under conditions [high-temperature, very low oxygen fugacity (fO2)] comparable to those of their meteoritic counterparts. Ejecta from Cretaceous pyroclastic deposits on Mt Carmel, N. Israel, include aggregates of hopper/skeletal Ti-rich corundum, which have trapped melts that crystallized at fO2 extending from 7 log units below the iron-wustite buffer (?IW = -7; SiC, Ti2O3, Fe-Ti silicide melts) to ?IW ? -9 (native V, TiC, and TiN). The assemblage hibonite + grossite + spinel + TiN first crystallized late in the evolution of the melt pockets; this hibonite contains percentage levels of Zr, Ti, and REE that reflect the concentration of incompatible elements in the residual melts as corundum continued to crystallize. A still later stage appears to be represented by coarse-grained (centimeter-size crystals) ejecta that show the crystallization sequence: corundum + Liq ? (low-REE) hibonite ? grossite + spinel ± krotite ? Ca4Al6F2O12 + fluorite. V0 appears as spheroidal droplets, with balls up to millimeter size and spectacular dendritic intergrowths, included in hibonite, grossite, and spinel. Texturally late V0 averages 12 wt% Al and 2 wt% Mn. Spinels contain 10-16 wt% V in V0-free samples, and <0.5 wt% V in samples with abundant V 0. Ongoing paragenetic studies suggest that the fO2 evolution of the Mt Carmel magmatic system reflects the interaction between OIB-type mafic magmas and mantle-derived CH4+H2 fluids near the crust-mantle boundary. Temperatures estimated by comparison with 1 atm phase-equilibrium studies range from ca. 1500 °C down to 1200-1150 °C. When fO2 reached ca. ?IW = -7, the immiscible segregation of Fe,Ti-silicide melts and the crystallization of SiC and TiC effectively desilicated the magma, leading to supersaturation in Al2O3 and the rapid crystallization of corundum, preceding the development of the hibonite-bearing assemblages. Reports of Ti-rich corundum and SiC from other areas of explosive volcanism suggest that these phenomena may be more widespread than presently realized, and the hibonite-grossite assemblage may serve as another indicator to track such activity. This is the first reported terrestrial occurrence of krotite (CaAl2O4), and of at least two unknown Zr-Ti oxides.
Geochimica et Cosmochimica Acta, in press available, 19p.
Africa, South Africa, Canada, South America, Brazil
deposit - Lac de Gras, Paranaiba
Abstract: Kimberlites are the deepest melts produced on Earth that are erupted at the surface and can therefore provide unique insights into the composition and evolution of the mantle. Radiogenic isotopes provide ambiguous evidence for the occurrence of recycled crustal material in kimberlite sources. Oxygen isotopes can fractionate significantly only in the shallow crust, and thus represent a powerful tracer of subducted material in the sources of kimberlite. To constrain the oxygen isotope composition of kimberlite melts, we have examined olivine grains in eleven Cretaceous to Eocene archetypal kimberlites from southern Africa, Lac de Gras (Canada) and Alto Paranaiba (Brazil), which exhibit radiogenic isotope evidence for recycled crustal material in their sources including highly radiogenic Pb isotopes and Nd-Hf isotope compositions deviating below the mantle array. Olivine grains are commonly zoned between a mantle-derived xenocrystic core and one or more magmatic overgrowths, i.e. occasional internal zones, ubiquitous rims and rare rinds (moving outward from the core). The oxygen isotope composition of different olivine zones was determined in situ within separated olivine grains by secondary ion mass spectrometry (SIMS) after point selection using back-scattered electron (BSE) images combined with major and minor element analyses. With the exception of a few cores, the ?18O values of different olivine zones do not deviate from typical mantle olivine values of 5.18?±?0.28‰ (Mattey et al., 1994). There are no correlations between oxygen isotopes and major/minor element compositions for internal zones and rims from individual localities or in the entire dataset. This indicates that the oxygen isotope composition of kimberlite melts is not affected by melt differentiation to the point of olivine rim crystallisation. However, olivine rinds from the Koala kimberlite (Canada) display an inverse correlation between ?18O and Mn-Ca concentrations, with ?18O values extending below the mantle range, which is probably due to carbonate fractionation, CO2 degassing and/or assimilation of serpentine-rich material after kimberlite emplacement in the upper crust. The mantle-like ?18O composition of olivine internal zones and rims suggests that assimilation of mantle material and liberation of a CO2-rich phase during ascent in the mantle do not significantly modify the original ?18O signature of kimberlite melts. Modelling of oxygen isotope fractionation shows that up to 15 wt% of CO2 can be lost by kimberlites en route to the upper crust. Our results combined with mass balance calculations indicate that only a limited amount (<5-10 wt%) of recycled crustal material could occur in the source of kimberlites from southern Africa, Lac de Gras and Alto Paranaiba, or that the recycled material had an oxygen isotope composition similar to the mantle.
Abstract: Although hydrogen is the most abundant element in the solar system, the mechanisms of exchange of this element between the deep interior and surface of Earth are still uncertain. Hydrogen has profound effects on properties and processes on microscopic-to-global scales. Here we report the discovery of the first hydride (VH2) ever reported in nature. This phase has been found in the ejecta of Cretaceous pyroclastic volcanoes on Mt Carmel, N. Israel, which include abundant xenoliths containing highly reduced mineral assemblages. These xenoliths were sampled by their host magmas at different stages of their evolution but are not genetically related to them. The xenoliths are interpreted as the products of extended interaction between originally mafic magmas and CH4+H2 fluids, derived from a deeper, metal-saturated mantle. The last stages of melt evolution are recorded by coarse-grained aggregates of hibonite (CaAl12O19) + grossite (CaAl4O7) + V-rich spinels ± spheroidal to dendritic inclusions of metallic vanadium (V0), apparently trapped as immiscible metallic melts. The presence of V0 implies low oxygen fugacities and suggests crystallization of the aggregates in a hydrogen-rich atmosphere. The presence of such reducing conditions in the upper mantle has major implications for the transport of carbon, hydrogen and other volatile species from the deep mantle to the surface.
Geostandards and Geoanalytical Research, in press available, 16p.
Australia
deposit - Mud Tank
Abstract: Zircon megacrysts from the Mud Tank carbonatite, Australia, are being used in many laboratories as a reference material for LA?ICP?MS U?Pb dating and trace element measurement, and LA?MC?ICP?MS determination of Hf isotopes. We summarise a database of > 10000 analyses of Mud Tank zircon (MTZ), collected from 2000 to 2018 during its use as a secondary reference material for simultaneous U?Pb and trace element analysis, and for Hf?isotope analysis. Trace element mass fractions are highest in dark red?brown stones and lowest in colourless and gem?quality ones. Individual unzoned grains can be chemically homogeneous, while significant variations in trace element mass fraction are associated with oscillatory zoning. Chondrite?normalised trace element patterns are essentially parallel over large mass fraction ranges. A Concordia age of 731.0 ± 0.2 Ma (2s, n = 2272) is taken as the age of crystallisation. Some grains show lower concordant to mildly discordant ages, probably reflecting minor Pb loss associated with cooling and the Alice Springs Orogeny (450-300 Ma). Our weighted mean 176Hf/177Hf is 0.282523 ± 10 (2s, n = 9350); the uncertainties on this ratio reflect some heterogeneity, mainly between grains. A few analyses suggest that colourless grains have generally lower 176Hf/177Hf. MTZ is a useful secondary reference material for U?Pb and Hf?isotope analysis, but individual grains need to be carefully selected using CL imaging and tested for homogeneity, and ideally should be standardised by solution analysis.
Abstract: Earth Scientists have two ways of examining and mapping the structure and composition of the subcontinental lithospheric mantle (SCLM): geophysical surveys, and studies of mantle samples from volcanic rocks or exposed terranes. Interpretation of both types of data requires an understanding of some basic strengths and limitations of each approach.
Abstract: Scandium is often considered as immobile during chemical weathering, based on its low solubility. In contrast to other conservative (i.e. relatively immobile) elements incorporated into accessory minerals resistant to weathering (e.g. zirconium, thorium or niobium), the scarcity of scandium minerals indicates that the processes accounting for scandium's immobilisation are distinctive. However, the evolution of scandium speciation during weathering is unknown, limiting the understanding of the processes controlling its dynamics in the critical zone. Exceptional scandium concentrations in east Australian laterites provide the possibility of unravelling these mechanisms. We follow scandium speciation through thick lateritic profiles (> 30 m) using a multiscale mineralogical and spectroscopic approach involving electron microprobe, laser-ablation--inductively coupled plasma mass spectrometry, selective leaching and X-ray absorption near-edge structure spectroscopy, complemented by mass-transfer calculations. We show that the initial reservoir of scandium contained in the parent rock is preserved under reducing conditions occurring in the lowest horizons of the profiles. The dissolution of scandium-bearing clinopyroxene generates smectitic clays that immobilise and concentrate scandium. It is subsequently trapped in the lateritic duricrust by goethite. Scandium mobilisation appears in this horizon and increases upward as a result of the dissolution of goethite, possibly assisted by dissolved organic matter, and the precipitation of hematite. Molecular-scale analyses demonstrate that changes in speciation govern scandium dynamics, with substitution in smectitic clays and adsorption on iron oxyhydroxides playing a crucial role in scandium immobility in the saprolite and lower lateritic duricrust. The higher affinity of scandium for goethite relative to hematite drives scandium mobilisation in the upper lateritic duricrust, leading to its concentration downward in the lower lateritic duricrust. These successive mechanisms illustrate how the unique complexity of the critical zone leads to scandium concentrations that may form new types of world-class scandium deposits. Comparison with conservative elements and with rare-earth elements, expected to have similar geochemical properties, emphasizes the unique behaviour of scandium in the critical zone. While scandium remains immobile during the early stages of weathering, intense and long-term alteration processes, observed in lateritic contexts, lead to scandium mobilisation. This study highlights the dependence of scandium mobility on weathering conditions.
Microscopy and Microanalysis ( M&M)Co. Conference, Sept. 9, posters 1 p. each
Europe, Israel
deposit - Kishon
Abstract: The Microscopy and Microanalysis (M&M) conference in Portland Oregon, USA is one of the biggest microscopy conferences in the world and this year it hosted its largest meeting in history with over 3,300 participants, up to 20 parallel sessions and over 600 posters. The two posters were presented by Sarah E.M. Gain who is from the University of Western Australia where she trains and supports researchers in Microscopy, Characterisation and Microanalysis. Sarah discussed some of the unique gem material collected from Shefa Gems’ exploration activity in the Kishon Mid Reach and Rakefet Magmatic Complex, analysed using a range of microscopy and microanalysis techniques. She also discussed the scientific importance of this material.The first poster looked at hibonite (a Ca-Al-oxide) with inclusions of vanadium metal. The second poster looked at, Cr corundum (ruby), which is unusual due to the extremely high Cr levels and the inclusions of Cr metal.
Abstract: Archean cratons are underlain by highly depleted subcontinental lithospheric mantle (SCLM). However, there are extensive evidences that Archean SCLM has been extensively refertilized by metasomatic processes, with the addition of Fe, Ca, and Al to depleted protoliths. The distribution of sub-calcic Cr-rich garnets in the SCLM beneath the Siberian craton suggests (1) sub-calcic garnets and diamonds are metasomatic phases in the cratonic SCLM; (2) the distribution of both phases is laterally heterogeneous on relatively small scales and related to ancient structural controls [1]. Re-Os isotopic compositions of twenty six sulfide inclusions in lherzolitic Cr-pyropes from Upper Muna kimberlites have been determined by laser ablation MCICPMS. Most analysed sulfides (~92%) have very low Re/Os ratios (<0.07), and their Re-depletion ages (TRD) form three major peaks: 3.4-2.8, 2.2-1.8 and 1.4-1.2 Ga (±0.03 Ga, mean 2s analytical uncertainty). One sulfide give the oldest TRD age at 4 Ga. Our data suggest that refertilization of the highly depleted SCLM and the introduction of Cr-pyrope garnet occurred in several episodes. The oldest age of ca 4 Ga indicate on the beginning of the formation of the depleted SCLM of the Siberian Craton in Hadean time [2].
Abstract: Mantle lithologies in orogenic massifs and xenoliths commonly display strikingly different Hf- and Nd-isotope compositions compared to oceanic basalts. While the presence of pyroxenites has long been suggested in the source region of mantle-derived magmas, very few studies have reported their combined HfNd isotope compositions. We here report the first LuHf data along with ReOs data and S concentrations on the Cabo Ortegal Complex, where the pyroxenite-rich Herbeira massif has been interpreted as remnants of a delaminated arc root. The pyroxenites, chromitites and their host harzburgites show a wide range of whole-rock 187Re/188Os and 187Os/188Os (0.16-1.44), indicating that Re was strongly mobilized, partly during hydrous retrograde metamorphism but mostly during supergene alteration that preferentially affected low-Mg#, low Cu/S pyroxenites. Samples that escaped this disturbance yield an isochron age of 838 ± 42 Ma, interpreted as the formation of Cabo Ortegal pyroxenites. Corresponding values of initial 187Os/188Os (0.111-0.117) are relatively unradiogenic, suggesting limited contributions of slab-derived Os to primitive arc melts such as those parental to these pyroxenites. This interpretation is consistent with radiogenic Os in arc lavas being mostly related to crustal assimilation. Paleoproterozoic to Archean Os model ages confirm that Cabo Ortegal pyroxenites record incipient volcanic arc magmatism on the continental margin of the Western African Craton, as notably documented by zircon UPb ages of 2.1 and 2.7 Ga. LuHf data collected on clinopyroxene and amphibole separates and whole-rock samples are characterized by uncorrelated 176Lu/177Hf and 176Hf/177Hf (0.2822-0.2855), decoupled from Nd-isotope compositions. This decoupling is ascribed to diffusional disequilibrium during melt-peridotite interaction, in good agreement with the results of percolation-diffusion models simulating the interaction of an arc melt with an ancient melt-depleted residue. These models notably show that HfNd isotopic decoupling such as recorded by Cabo Ortegal pyroxenites and peridotites (??Hf(i) up to +97) is enhanced during melt-peridotite interaction by slow diffusional re-equilibration and can be relatively insensitive to chromatographic fractionation. Finally, we discuss the hypothesis that arc-continent interaction may provide preferential conditions for such isotopic decoupling and propose that its ubiquitous recognition in peridotites reflects the recycling of sub-arc mantle domains derived from ancient, reworked SCLM.
Contributions to Mineralogy and Petrology, in press available, 21p. Pdf
Africa, South Africa
deposit - Benfontein
Abstract: Primary carbonates in kimberlites are the main CO2 carriers in kimberlites and thus can be used to constrain the original carbon and oxygen-isotope composition of kimberlite melts and their deep mantle sources. However, the contribution of syn- and post-emplacement processes to the modification of the C-O-isotope composition of kimberlites is yet to be fully constrained. This study aims to shed new light on this topic through a detailed textural, compositional (major and trace elements), and in situ C-O-Sr isotopic characterisation of carbonates in the Benfontein kimberlite sills (Kimberley, South Africa). Our multi-technique approach not only reveals the petrographic and geochemical complexity of carbonates in kimberlites in unprecedented detail, but also allows identification of the processes that led to their formation, including: (1) magmatic crystallisation of Sr-rich calcite laths and groundmass; (2) crystallisation of late groundmass calcite from hydrothermal fluids; and (3) variable degrees of crustal contamination in carbonate-rich diapirs and secondary veins. These diapirs most likely resulted from a residual C-O-H fluid or carbonate melt with contributions from methane-rich fluids from the Dwyka shale wall rock, leading to higher 87Sr/86Sr and ?18O, but lower ?13C values than in pristine magmatic calcite. Before coalescing into the diapiric segregations, these fluids/melts also variably entrained early formed calcite laths and groundmass phases. Comparison between in situ and bulk-carbonate analyses confirms that O isotopic analyses of bulk carbonates from kimberlite rocks are not representative of the original isotopic signature of the kimberlite magma, whereas bulk C-isotope compositions are similar to those of the pristine magmatic carbonates. Calcite laths and most groundmass grains at Benfontein preserve isotopic values (?18O?=?6-8‰ and ?13C?=???4 to ??6‰), similar to those of unaltered carbonatites worldwide, which, therefore, probably correspond to those of their parental melts. This narrow range suggests kimberlite derivation from a mantle source with little contribution from recycled crustal material unless the recycled material had isotopic composition indistinguishable from typical mantle values.
Earth and Planetary Science Letters, Vol. 544, 116365 14p. Pdf
Australia
carbonatite
Abstract: Petrological and geochemical studies have revealed the contribution of garnet pyroxenites in basalt petrogenesis. However, whether primary mantle melts are produced with such signature or acquired it subsequently remains somewhat controversial. We here integrate new major-, trace-element and Sr-Nd-Hf isotopic compositions of garnet pyroxenite xenoliths in Holocene alkali basalts from Lakes Bullenmerri and Gnotuk, Southeastern Australia, to relate their petrogenesis to mantle-wedge melt circulation and subsequent lithospheric evolution. Results show that the clinopyroxenites have lower MgO and Cr2O3 contents than the associated websterites, and range in compositions from depleted LREE patterns and highly radiogenic Nd and Hf isotopic signatures in relatively low-MgO samples (Type 1), to enriched REE patterns with negative HFSE anomalies, unradiogenic Nd and Hf isotopes, and extremely radiogenic Sr-isotopic ratios in samples with higher MgO (Type 2). Such compositional variabilities suggest that these pyroxenites represent segregates from melts derived from a recycled oceanic lithosphere with a potential contribution from pelagic sediments. Variable LREE contents and isotopic compositions between those of Type 1 and 2 clinopyroxenites are observed in amphibole-bearing samples (Type 3), which are interpreted as Type 1-like protoliths metasomatized by the basaltic and carbonatitic melts, possibly parental to Type 2 clinopyroxenites. The lithosphere beneath Southeastern Australia thus has received variable melt contributions from a heterogeneous mantle-wedge source, which notably includes a subducted oceanic slab package that has retained its integrity during subduction. On this basis, we suggest that the compositional heterogeneity and temporal evolution of the subsequent Southeastern Australian basaltic magmatism were probably affected by the presence of pyroxenite fragments in the basalt source and formed by the tectonic reactivation of this lithosphere during Cenozoic rifting. This interpretation is notably consistent with a trend of Nd-Pb isotopes towards EMII in Older Volcanic Provinces (OVP basalts) and limited Sr-Nd-Pb isotopic variations towards HIMU in the Newer Volcanic Provinces (NVP basalts, including the host lavas), which also exhibit low SiO2, high FeO and high CaO/Al2O3 commonly interpreted as due to pyroxenite contributions. Therefore, the identification of a subduction signature in these rift-related lavas attests to a "lithospheric memory" of earlier subduction episodes (as documented by the xenoliths), rather than a reflection of contemporaneous subduction tectonics.
Geochimica et Cosmochimica Acta, in press available, doi.org/101016 /j.gca.2020.07.013 45p. Pdf
Canada, Northwest Territories
deposit - Lac de Gras
Abstract: Whether hydrogen incorporated in nominally anhydrous mantle minerals plays a role in the strength and longevity of the thick cratonic lithosphere is a matter of debate. In particular, the percolation of hydrogen-bearing melts and fluids could potentially add hydrogen to the mantle lithosphere, weaken its olivines (the dominant mineral in mantle peridotite), and cause delamination of the lithosphere's base. The influence of metasomatism on hydrogen contents of cratonic mantle minerals can be tested in mantle xenoliths from the Slave Craton (Canada) because they show extensive evidence for metasomatism of a layered cratonic mantle. Minerals from mantle xenoliths from the Diavik mine in the Lac de Gras kimberlite area located at the center of the Archean Slave craton were analyzed by FTIR for hydrogen contents. The 18 peridotites, two pyroxenites, one websterite and one wehrlite span an equilibration pressure range from 3.1 to 6.6 GPa and include samples from the shallow (? 145 km), oxidized ultra-depleted layer; the deeper (?145-180 km), reduced less depleted layer; and an ultra-deep (? 180 km) layer near the base of the lithosphere. Olivine, orthopyroxene, clinopyroxene and garnet from peridotites contain 30 - 145, 110 - 225, 105 - 285, 2 - 105 ppm H2O, respectively. Within each deep and ultra-deep layer, correlations of hydrogen contents in minerals and tracers of metasomatism (for example light over heavy rare-earth-element ratio (LREE/HREE), high-field-strength-element (HFSE) content with equilibration pressure) can be explained by a chromatographic process occurring during the percolation of kimberlite-like melts through garnet peridotite. The hydrogen content of peridotite minerals is controlled by the compositions of the evolving melt and of the minerals and by mineral/melt partition coefficients. At the beginning of the process, clinopyroxene scavenges most of the hydrogen and garnet most of the HFSE. As the melt evolves and becomes enriched in hydrogen and LREE, olivine and garnet start to incorporate hydrogen and pyroxenes become enriched in LREE. The hydrogen content of peridotite increases with decreasing depth, overall (e.g., from 75 to 138 ppm H2O in the deep peridotites). Effective viscosity calculated using olivine hydrogen content for the deepest xenoliths near the lithosphere-asthenosphere boundary overlaps with estimates of asthenospheric viscosities. These xenoliths cannot be representative of the overall cratonic root because the lack of viscosity contrast would have caused basal erosion of lithosphere. Instead, metasomatism must be confined in narrow zones channeling kimberlite melts through the lithosphere and from where xenoliths are preferentially sampled. Such localized metasomatism by hydrogen-bearing melts therefore does not necessarily result in delamination of the cratonic root.
Journal of Petrology, 10.1093/petrology/egaa079 110p. Pdf
China
xenoliths
Abstract: Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ? 90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880 ? 970 oC), and low-Mg# (70 ? 80) plagioclase-bearing websterites with low equilibration temperatures (550 ? 835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr-Nd (?Nd?=?+11.41, 87Sr/86Sr = ?0.7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ?0.703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g., FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr-Nd isotopic ratios (?Nd = -14.20 ? -16.74, 87Sr/86Sr?=?0.7070 ? 0.7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr?=?0.706-0.711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian ocean and the Late Mesozoic lithospheric extension of eastern Asia.
Geochimica et Cosmochimica Acta, Vol. 286, pp. 29-83. pdf
Canada, Northwest Territories
xenoliths
Abstract: Whether hydrogen incorporated in nominally anhydrous mantle minerals plays a role in the strength and longevity of the thick cratonic lithosphere is a matter of debate. In particular, the percolation of hydrogen-bearing melts and fluids could potentially add hydrogen to the mantle lithosphere, weaken its olivines (the dominant mineral in mantle peridotite), and cause delamination of the lithosphere's base. The influence of metasomatism on hydrogen contents of cratonic mantle minerals can be tested in mantle xenoliths from the Slave Craton (Canada) because they show extensive evidence for metasomatism of a layered cratonic mantle. Minerals from mantle xenoliths from the Diavik mine in the Lac de Gras kimberlite area located at the center of the Archean Slave craton were analyzed by FTIR for hydrogen contents. The 18 peridotites, two pyroxenites, one websterite and one wehrlite span an equilibration pressure range from 3.1 to 6.6 GPa and include samples from the shallow (?145?km), oxidized ultra-depleted layer; the deeper (?145-180?km), reduced less depleted layer; and an ultra-deep (?180?km) layer near the base of the lithosphere. Olivine, orthopyroxene, clinopyroxene and garnet from peridotites contain 30-145, 110-225, 105-285, 2-105?ppm H2O, respectively. Within each deep and ultra-deep layer, correlations of hydrogen contents in minerals and tracers of metasomatism (for example light over heavy rare-earth-element ratio (LREE/HREE), high-field-strength-element (HFSE) content with equilibration pressure) can be explained by a chromatographic process occurring during the percolation of kimberlite-like melts through garnet peridotite. The hydrogen content of peridotite minerals is controlled by the compositions of the evolving melt and of the minerals and by mineral/melt partition coefficients. At the beginning of the process, clinopyroxene scavenges most of the hydrogen and garnet most of the HFSE. As the melt evolves and becomes enriched in hydrogen and LREE, olivine and garnet start to incorporate hydrogen and pyroxenes become enriched in LREE. The hydrogen content of peridotite increases with decreasing depth, overall (e.g., from 75 to 138?ppm H2O in the deep peridotites). Effective viscosity calculated using olivine hydrogen content for the deepest xenoliths near the lithosphere-asthenosphere boundary overlaps with estimates of asthenospheric viscosities. These xenoliths cannot be representative of the overall cratonic root because the lack of viscosity contrast would have caused basal erosion of lithosphere. Instead, metasomatism must be confined in narrow zones channeling kimberlite melts through the lithosphere and from where xenoliths are preferentially sampled. Such localized metasomatism by hydrogen-bearing melts therefore does not necessarily result in delamination of the cratonic root.
American Mineralogist, Vol. 105, pp. 1609-1621. pdf
Europe, Israel
deposit - Mt. Carmel
Abstract: Titanium diboride (TiB2) is a minor but common phase in melt pockets trapped in the corundum aggregates that occur as xenoliths in Cretaceous basaltic volcanoes on Mt. Carmel, north Israel. These melt pockets show extensive textural evidence of immiscibility between metallic (Fe-Ti-C-Si) melts, Ca-Al-Mg-Si-O melts, and Ti-(oxy)nitride melts. The metallic melts commonly form spherules in the coexisting oxide glass. Most of the observed TiB2 crystallized from the Fe-Ti-C silicide melts and a smaller proportion from the oxide melts. The parageneses in the melt pockets of the xenoliths require fO2 ? ?IW-6, probably generated through interaction between evolved silicate melts and mantle-derived CH4+H2 fluids near the crust-mantle boundary. Under these highly reducing conditions boron, like carbon and nitrogen, behaved mainly as a siderophile element during the separation of immiscible metallic and oxide melts. These parageneses have implications for the residence of boron in the peridotitic mantle and for the occurrence of TiB2 in other less well-constrained environments such as ophiolitic chromitites.
Contributions to Mineralogy and Petrology, Vol. 175, 106, 21p. Pdf
Russia
deposit - Nyurbinskaya
Abstract: We present a new dataset on the composition of high-density fluids (HDFs) in cloudy (n?=?25), coated (n?=?10) and cuboid (n?=?10) diamonds from the Nyurbinskaya kimberlite pipe. These diamonds represent different populations each showing distinct growth histories. The cores of coated diamonds display multiple growth stages and contrasting sources of carbon. Fibrous coats and cuboid diamonds have similar carbon isotopes and nitrogen systematics, suggesting their formation in the last metasomatic events related to kimberlite magmatism, as is common for most such diamonds worldwide. The HDFs in most of these diamonds span a wide range from low-Mg carbonatitic to hydrous silicic compositions. The major- and trace-element variations suggest that the sources for such HDFs range in composition between the depleted mantle and more fertile mantle reservoirs. Hydrous-silicic HDFs could originate from a 13C-enriched source, which originates through subduction of crustal metasedimentary material. Percolation of such HDFs through carbonated eclogites and peridotites facilitates the formation of cuboid diamonds and fibrous coats in the mantle section beneath the corresponding area of the Siberian craton. Cloudy diamonds represent an apparently older population, reflecting continuous diamond formation predominantly from high-Mg carbonatitic HDFs that caused discrete episodes of diamond precipitation. Their high Mg# and enrichment in incompatible elements support a metasomatized peridotitic source for these HDFs.
Minerals MDPI, Vol. 10, 1118, doi:10.3390/ min10121118 10p. Pdf
Europe, Israel
deposit - Mt. Carmel
Abstract: Here, we describe two new minerals, kishonite (VH2) and oreillyite (Cr2N), found in xenoliths occurring in pyroclastic ejecta of small Cretaceous basaltic volcanoes exposed on Mount Carmel, Northern Israel. Kishonite was studied by single-crystal X-ray diffraction and was found to be cubic, space group Fm3¯m, with a = 4.2680(10) Å, V = 77.75(3) Å3, and Z = 4. Oreillyite was studied by both single-crystal X-ray diffraction and transmission electron microscopy and was found to be trigonal, space group P3¯1m, with a = 4.7853(5) Å, c = 4.4630(6) Å, V = 88.51 Å3, and Z = 3. The presence of such a mineralization in these xenoliths supports the idea of the presence of reduced fluids in the sublithospheric mantle influencing the transport of volatile species (e.g., C, H) from the deep Earth to the surface. The minerals and their names have been approved by the Commission of New Minerals, Nomenclature and Classification of the International Mineralogical Association (No. 2020-023 and 2020-030a).
Abstract: The mantle beneath the Western Dharwar Craton of the Indian shield comprises a suite of refractory and fertile peridotites and mafic granulites. Detailed petrographic studies coupled with new mineral analysis and geothermobarometric estimations permit to decipher the thermal architecture and get an insight into the evolution of this ancient craton. The refractory rocks are coarse grained harzburgites/dunites, whereas the more fertile ones are at times, porphyroclastic lherzolites. Both show a similar range of equilibration temperatures and pressures indicating intermixing between the two at various levels. The peridotites contain undeformed interstitial REE-enriched clinopyroxene, phlogopite, apatite and carbonates recording post-kinematic modal and cryptic metasomatic events in the Precambrian cratonic lithosphere. Xenoliths of mafic granulite contain layers of clinopyroxenite which also vein the granulite. The P-T range of the granulites overlaps that of the ultramafic rocks. This study in combination with previous investigations reveals a distinct change in the thermal architecture of the craton from a warm/hot geotherm in the Proterozoic to a highly perturbed, still hotter geotherm of the Palaeocene. The Cenozoic thermotectonic rifting episodes heated, refertilized and thinned the bulk of the cratonic lithosphere beneath the Western Dharwar Craton, which has witnessed the most re-activation among cratons of the Indian shield. The waning of the Deccan Traps volcanism in Palaeocene time saw the reworking of ancient cratonic lithosphere and its replacement by non-cratonic, juvenile mantle and magmatic accretions, indicated by compound xenoliths. Differing petrological and geochemical characteristics of refractory xenoliths and fertile lherzolites serve to constrain the relative timing and composition of non-cratonic lithosphere. By the end of the Palaeocene the Western Dharwar Craton was characterised by a thermal high, an attenuated continental lithosphere (60-80 km), and a thin crust (<10- ~ 21 km), reflecting the decratonization of at least the western part of the Western Dharwar Craton.
American Mineralogist , in press available 33p. Pdf
Europe, Israel
deposit - Mt. Carmel
Abstract: Titanium diboride (TiB2) is a minor but common phase in melt pockets trapped in the corundum aggregates that occur as xenoliths in Cretaceous basaltic volcanoes on Mt. Carmel, north Israel. These melt pockets show extensive textural evidence of immiscibility between metallic (Fe-Ti-C-Si) melts, Ca-Al-Mg-Si-O melts, and Ti-(oxy)nitride melts. The metallic melts commonly form spherules in the coexisting oxide glass. Most of the observed TiB2 crystallized from the Fe-Ti-C silicide melts and a smaller proportion from the oxide melts. The parageneses in the melt pockets of the xenoliths require fO2 ? ?IW-6, probably generated through interaction between evolved silicate melts and mantle-derived CH4+H2 fluids near the crust-mantle boundary. Under these highly reducing conditions boron, like carbon and nitrogen, behaved mainly as a siderophile element during the separation of immiscible metallic and oxide melts. These parageneses have implications for the residence of boron in the peridotitic mantle and for the occurrence of TiB2 in other less well-constrained environments such as ophiolitic chromitites.
Lithos, doi.org/10.1016/ jlithos.2020.105880, 26p. Pdf
Africa, South Africa
deposit - Bultfontein
Abstract: The metasomatised continental mantle may play a key role in the generation of some ore deposits, in particular mineral systems enriched in platinum-group elements (PGE) and Au. The cratonic lithosphere is the longest-lived potential source for these elements, but the processes that facilitate their pre-concentration in the mantle and their later remobilisation to the crust are not yet well-established. Here, we report new results on the petrography, major-element, and siderophile- and chalcophile-element composition of native Ni, base metal sulphides (BMS), and spinels in a suite of well-characterised, highly metasomatised and weakly serpentinised peridotite xenoliths from the Bultfontein kimberlite in the Kaapvaal Craton, and integrate these data with published analyses. Pentlandite in polymict breccias (failed kimberlite intrusions at mantle depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite in phlogopite peridotites and Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel is an insignificant host for all elements except Zn, and BMS and native Ni account for typically <25% of the bulk-rock PGE and Au. High bulk-rock Te/S suggest a role for PGE-bearing tellurides, which, along with other compounds of metasomatic origin, may host the missing As, Ag, Cd, Sb, Te and, in part, Bi that are unaccounted for by the main assemblage. The close spatial relationship between BMS and metasomatic minerals (e.g., phlogopite, ilmenite) indicates that the lithospheric mantle beneath Bultfontein was resulphidised by metasomatism after initial melt depletion during stabilisation of the cratonic lithosphere. Newly-formed BMS are markedly PGE-poor, as total PGE contents are <4.2 ppm in pentlandite from seven samples, compared to >26 ppm in BMS in other peridotite xenoliths from the Kaapvaal craton. This represents a strong dilution of the original PGE abundances at the mineral scale, perhaps starting from precursor PGE alloy and small volumes of residual BMS. The latter may have been the precursor to native Ni, which occurs in an unusual Ni-enriched zone in a harzburgite and displays strongly variable, but overall high PGE abundances (up to 81 ppm). In strongly metasomatised peridotites, Au is enriched relative to Pd, and was probably added along with S. A combination of net introduction of S, Au +/? PGE from the asthenosphere and intra-lithospheric redistribution, in part sourced from subducted materials, during metasomatic events may have led to sulphide precipitation at ~80-120 km beneath Bultfontein. This process locally enhanced the metallogenic fertility of this lithospheric reservoir. Further mobilisation of the metal budget stored in these S-rich domains and upwards transport into the crust may require interaction with sulphide-undersaturated melts that can dissolve sulphides along with the metals they store.
Abstract: Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ?90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880-970 oC), and low-Mg# (70-80) plagioclase-bearing websterites with low equilibration temperatures (550-835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr-Nd (?Nd = +11•41, 87Sr/86Sr = ?0•7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ?0•703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g., FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr-Nd isotopic ratios (?Nd = -14•20- -16•74, 87Sr/86Sr = 0•7070-0•7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr = 0•706-0•711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian Ocean and the Late Mesozoic lithospheric extension of eastern Asia.
AGU Research Letter, 10.1029/2021GL092570 11p. Pdf
Africa, South Africa
geophysics - magnetotellurics
Abstract: Measuring the composition of the Earth’s mantle is important for understanding mantle processes like plate tectonics, but is surprisingly difficult. Our most accurate information comes from mantle rocks, called xenoliths, that have been brought to the surface during volcanic eruptions. However, these rocks only come from a handful of places. We tend to expect that the rest of the mantle has the same composition as the xenoliths but this might be incorrect. We tested whether xenolith compositions really are representative of the broader mantle by comparing them with compositions interpreted from electrical conductivity models of the mantle. We carried out this comparison in the Kimberley region, South Africa, because it has excellent xenolith and electrical conductivity data. Our results show that xenolith compositions do seem to be broadly representative but there are two important differences: Hydrous minerals found in some xenoliths may not be spatially extensive depending on temperature, and the water contents of some other minerals are different from the broader region. This means that the compositions of xenoliths are at least partly controlled by local processes. Electrical conductivity data may be more useful for measuring some aspects of the composition of the broader mantle, especially its water content.
Abstract: Titanium oxynitrides (Ti(N,O,C)) are abundant in xenolithic corundum aggregates in pyroclastic ejecta of Cretaceous volcanoes on Mount Carmel, northern Israel. Petrographic observations indicate that most of these nitrides existed as melts, immiscible with coexisting silicate and Fe-Ti-C silicide melts; some nitrides may also have crystallized directly from the silicide melts. The TiN phase shows a wide range of solid solution, taking up 0-10 wt% carbon and 1.7-17 wt% oxygen; these have crystallized in the halite (fcc) structure common to synthetic and natural TiN. Nitrides coexisting with silicide melts have higher C/O than those coexisting with silicate melts. Analyses with no carbon fall along the TiN-TiO join in the Ti-N-O phase space, implying that their Ti is a mixture of Ti3+ and Ti2+, while those with 1-3 at.% C appear to be solid solutions between TiN and Ti0.75O. Analyses with >10 at% C have higher Ti2+/Ti3+, reflecting a decrease in fO2. Oxygen fugacity was 6 to 8 log units below the iron-wüstite buffer, at or below the Ti2O3-TiO buffer. These relationships and coexisting silicide phases indicate temperatures of 1400-1100 °C. Ti oxynitrides are probably locally abundant in the upper mantle, especially in the presence of CH4-H2 fluids derived from the deeper metal-saturated mantle.
Abstract: A 150-200 km thick, cold (35-45 mWm?2), melt-depleted lithospheric keel characterised the eastern cratons of the Indian shield at the end of the Precambrian. Differing chemical- and isotopic-characteristics, and ages of the crust and mantle rocks reveal the decoupling of the crust and mantle beneath the cratons, beginning at 2.45 Ga, in the Bastar craton. The Pan-African event was more pervasive and brought about widespread reworking in most of the cratons of the shield. Major-, trace- and rare-earth elements combined with Sr, Nd and Hf isotope data suggest a heterogenous SCLM beneath southern India. The trace element signatures of xenoliths and the presence of majoritic garnet inclusions in diamond suggest that some kimberlites were derived from the mantle transition zone. Mesoproterozoic (1.2-1.4 Ga) modal and cryptic refertilisation by asthenosphere-derived, low-degree carbonated melts led to the generation of the fluids responsible for the metasomatic transformation of the source rocks. The western craton of the shield has witnessed more severe reactivation than the eastern due to the frequent interaction of the Indian plate with mantle plumes. One plume caused major igneous activity during the late Cretaceous, synchronous with crustal attenuation, rifting and the ridge-jump at 66 Ma, in the Indian Ocean. By the end of the Palaeocene the geotherm of the western craton had risen from 50 to 55 mWm?2 in the Proterozoic to a peak 80-90 mWm?2. This increase in heat flow not only modulated the mantle thermal regime, but led to a net loss of more than 100 km of lithosphere and to destabilisation of the craton. After this thermal event, the lithosphere preserves a thickness of barely 60-80 km, and a thin crust (10-21 km) beneath the continental margin in the west. These changes decratonized the western part of the shield and the transitional region further west in the Indian Ocean where the continental ridges are almost devoid of crustal sections and the lithosphere is ~60 km thick. The waning of the Deccan Traps (65 Ma) magmatism was marked by alkaline intrusive activity along the western margin of the shield, probably derived from the SCLM in response to the rise of the mantle plume. Low degree (2-3%) partial melting of a modally and cryptically metasomatized source may have been involved in the generation of alkaline magmas from a depleted mantle source variously contaminated by an enriched endmember.
American Mineralogist, Vol. 106, pp. 1420-1437. pdf
Europe, Israel
deposit - Mount Carmel
Abstract: Xenocrysts and xenoliths in Upper Cretaceous pyroclastics on Mount Carmel (northern Israel) represent a series of similar magma-fluid systems at different stages of their evolution, recording a continuous decrease in oxygen fugacity (fO2) as crystallization proceeded. Corundum coexisting with Fe-Mg-Cr-Al spinels, other Fe-Mg-Al-Na oxides, and Fe-Ni alloys in apparent cumulates crystallized at fO2 values near the iron-wüstite (IW) buffer (fO2 = IW±1) and is zoned from high-Cr cores to lower-Cr rims, consistent with fractional crystallization trends. The reconstructed parental melts of the cumulates are Al-Cr-Fe-Mg oxides with ca. 2 wt% SiO2. Corundum in other possible cumulates that contain Cr-Fe (Fe 45 wt%) alloys has low-Cr cores and still lower-Cr rims. Corundum coexisting with Cr0 (fO2 = IW-5) in some possible cumulates has low-Cr cores, but high-Cr rims (to >30% Cr2O3). These changes in zoning patterns reflect the strong decrease in the melting point of Cr2O3, relative to Al2O3, with decreasing fO2. The electron energy loss spectroscopy (EELS) analyses show that all Cr in corundum that coexists with Cr0 is present as Cr3+. This suggests that late in the evolution of these reduced melts, Cr2+ has disproportionated via the reaction 3Cr2+(melt) ? 2Cr3+(Crn) + Cr0. The most Cr-rich corundum crystallized together with ?-alumina phases including NaAl11O17 (diaoyudaoite) and KAl11O17 (kahlenbergite) and ??-alumina phases; residual melts crystallized a range of (K,Mg)2(Al,Cr)10O17 phases with the kahlenbergite structure. The parental melts of these assemblages appear to have been Al-Cr-K-Na-Mg oxides, which may be related to the Al-Cr-Fe-Mg oxide melts mentioned above, through fractional crystallization or liquid immiscibility. These samples are less reduced (fO2 from IW to IW-5) than the assemblages of the trapped silicate melts in the more abundant xenoliths of corundum aggregates (fO2 = IW-6 to IW-10). They could be considered to represent an earlier stage in the fO2 evolution of an “ideal” Mt. Carmel magmatic system, in which mafic or syenitic magmas were fluxed by mantle-derived CH4+H2 fluids. This is a newly recognized step in the evolution of the Mt. Carmel assemblages and helps to understand element partitioning under highly reducing conditions.
Journal of Geophysical Research: Solid Earth , preprint available 40p. Pdf
Africa
kimberlites
Abstract: The tectonic history of Southern Africa includes Archean formation of cratons, multiple episodes of subduction and rifting and some of the world's most significant magmatic events. These processes left behind a compositional trail that can be observed in xenoliths and measured by geophysical methods. The abundance of kimberlites in southern Africa makes it an ideal place to test and calibrate mantle geophysical interpretations that can then be applied to less well-constrained regions. Magnetotellurics (MT) is a particularly useful tool for understanding tectonic history because electrical conductivity is sensitive to temperature, bulk composition, accessory minerals and rock fabric. We produced three-dimensional MT models of the southern African mantle taken from the SAMTEX MT dataset, mapped the properties of $\sim36000$ garnet xenocrysts from Group I kimberlites, and compared the results. We found that depleted regions of the mantle are uniformly associated with high electrical resistivities. The conductivity of fertile regions is more complex and depends on the specific tectonic and metasomatic history of the region, including the compositions of metasomatic fluids or melts and the emplacement of metasomatic minerals. The mantle beneath the $\sim 2.05$ Ga Bushveld Complex is highly conductive, probably caused by magmas flowing along a lithospheric weakness zone and precipitating interconnected, conductive accessory minerals such as graphite and sulfides. Kimberlites tend to be emplaced near the edges of the cratons where the mantle below 100 km depth is not highly resistive. Kimberlites avoid strong mantle conductors, suggesting a systematic relationship between their emplacement and mantle composition.
Nature Communications, 10.10.1038/s4167-021-266668-z 14p. Pdf
Mantle
olivine
Abstract: Oxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities. Here we provide an alternative view for the origin of light oxygen-isotope signatures in mantle-derived magmas using kimberlites, carbonate-rich magmas that assimilate mantle debris during ascent. Olivine grains in kimberlites are commonly zoned between a mantle-derived core and a magmatic rim, thus constraining the compositions of both mantle wall-rocks and melt phase. Secondary ion mass spectrometry (SIMS) analyses of olivine in worldwide kimberlites show a remarkable correlation between mean oxygen-isotope compositions of cores and rims from mantle-like 18O/16O to lower ‘crustal’ values. This observation indicates that kimberlites entraining low-18O/16O olivine xenocrysts are modified by assimilation of low-18O/16O sub-continental lithospheric mantle material. Interaction with geochemically-enriched domains of the sub-continental lithospheric mantle can therefore be an important source of apparently ‘crustal’ signatures in mantle-derived magmas.
Abstract: Xenolithic corundum aggregates in Cretaceous mafic pyroclastics from Mount Carmel contain pockets of silicate melts with mineral assemblages [SiC (moissanite), TiC, Ti2O3 (tistarite), Fe-Ti-Zr silicides/phosphides] indicative of magmatic temperatures and oxygen fugacity (fO2) at least 6 log units below the iron-wüstite buffer (?IW ? -6). Microstructural evidence indicates that immiscible, carbon-rich metallic (Fe-Ti-Zr-Si-P) melts separated during the crystallization of the silicate melts. The further evolution of these metallic melts was driven by the crystallization of two main ternary phases (FeTiSi and FeTiSi2) and several near-binary phases, as well as the separation of more evolved immiscible melts. Reconstructed melt compositions fall close to cotectic curves in the Fe-Ti-Si system, consistent with trapping as metallic liquids. Temperatures estimated from comparisons with experimental work range from ?1500 °C to ca. 1150 °C; these probably are maximum values due to the solution of C, H, P, and Zr. With decreasing temperature (T), the Si, Fe, and P contents of the Fe-Ti-Si melts increased, while contents of Ti and C decreased. The increase in Si with declining T implies a corresponding decrease in fO2, probably to ca. ?IW-9. The solubility of P in the metallic melts declined with T and fO2, leading to immiscibility between Fe-Ti-Si melts and (Ti,Zr)-(P,Si) melts. Decreasing T and fO2 also reduced the solubility of C in the liquid metal, driving the continuous crystallization of TiC and SiC during cooling. The lower-T metallic melts are richer in Cr, and to some extent V, as predicted by experimental studies showing that Cr and V become more siderophile with decreasing fO2. These observations emphasize the importance of melt-melt immiscibility for the evolution of magmas under reducing conditions. The low fO2 and the abundance of carbon in the Mt. Carmel system are consistent with a model in which differentiating melts were fluxed by fluids that were dominated by CH4+H2, probably derived from a metal-saturated sublithospheric mantle. A compilation of other occur-rences suggests that these phenomena may commonly accompany several types of explosive volcanism.
Science Advances, doi.10.1126/sciadv.abj1325 1p. Pdf
Mantle
subduction
Abstract: Earth’s carbon cycle is strongly influenced by subduction of sedimentary material into the mantle. The composition of the sedimentary subduction flux has changed considerably over Earth’s history, but the impact of these changes on the mantle carbon cycle is unclear. Here, we show that the carbon isotopes of kimberlite magmas record a fundamental change in their deep-mantle source compositions during the Phanerozoic Eon. The 13C/12C of kimberlites before ~250 Ma preserves typical mantle values, whereas younger kimberlites exhibit lower and more variable ratios-a switch coincident with a recognized surge in kimberlite magmatism. We attribute these changes to increased deep subduction of organic carbon with low 13C/12C following the Cambrian Explosion when organic carbon deposition in marine sediments increased significantly. These observations demonstrate that biogeochemical processes at Earth’s surface have a profound influence on the deep mantle, revealing an integral link between the deep and shallow carbon cycles.
Abstract: We report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite (Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O isotopes. Type II eclogites are the protoliths of the highly metasomatized Type I. Garnet and clinopyroxene in Type II eclogites have ?66Zn from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed whole-rock ?66Zn is from 0.24‰ to 0.54‰, which is higher than typical mantle values (0.16-0.20‰). Their heavy Zn- and light Mg- and O-isotope compositions (?26Mg = -1.1‰ to -0.14‰, ?18O = 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low ?18O may be inherited from parental melts derived from low-?18O mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high ?66Zn and low ?26Mg reflect diffusion-driven Zn-Mg-isotope exchange between peridotites and the parental melts during their emplacement in the deep lithosphere. Type I eclogites have reconstructed whole-rock ?66Zn from 0.03‰ to 0.43‰ and garnet ?18O from 6‰ to 9.1‰ but show more scatter in inter-mineral Zn-isotope fractionation than Type II, reflecting incomplete equilibration during later metasomatism by carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus suggests that the Roberts Victor eclogites might have crystallized from deep-seated melts at mantle depths.
Journal of Geophysical Research: Solid Earth, doi: 10.1029/2021JB023105
Africa
geophysics
Abstract: The present-day composition of Earth's tectonic plates results from past geological processes. We can learn about Earth's composition from deep rock samples that are carried to the surface during volcanic eruptions and by probing its physical properties, like electrical conductivity, with geophysics. In southern Africa, there are extensive deep rock samples, which have been brought to the surface by kimberlite volcanoes that also host diamonds, and also extensive geophysical data. In this article, we compare the rock compositions with electrical conductivity to learn more about Earth's composition. Our results show that the oldest parts of the plates, which retain compositions similar to their initial composition, appear resistive. On the other hand, regions that have been intruded by deep fluids or molten rock can be resistive or conductive, depending on the types of minerals that were formed during the intrusion. The kimberlite volcanoes mostly erupted through the edges of the most resistive parts of the plates and did not erupt through the conductors. These results will help us to make more accurate interpretations about the composition of parts of the Earth where we do not have deep rock samples.
Abstract: The thermochemical structure of the subcontinental mantle holds information on its origin and evolution that can inform energy and mineral exploration strategies, natural hazard mitigation and evolutionary models of Earth. However, imaging the fine-scale thermochemical structure of continental lithosphere remains a major challenge. Here we combine multiple land and satellite datasets via thermodynamically constrained inversions to obtain a high-resolution thermochemical model of central and southern Africa. Results reveal diverse structures and compositions for cratons, indicating distinct evolutions and responses to geodynamic processes. While much of the Kaapvaal lithosphere retained its cratonic features, the western Angolan-Kasai Shield and the Rehoboth Block have lost their cratonic keels. The lithosphere of the Congo Craton has been affected by metasomatism, increasing its density and inducing its conspicuous low-topography, geoid and magnetic anomalies. Our results reconcile mantle structure with the causes and location of volcanism within and around the Tanzanian Craton, whereas the absence of volcanism towards the north is due to local asthenospheric downwellings, not to a previously proposed lithospheric root connecting with the Congo Craton. Our study offers improved integration of mantle structure, magmatism and the evolution and destruction of cratonic lithosphere, and lays the groundwork for future lithospheric evolutionary models and exploration frameworks for Earth and other terrestrial planets.
Abstract: We report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite (Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O isotopes. Type II eclogites are the protoliths of the highly metasomatized Type I. Garnet and clinopyroxene in Type II eclogites have ?66Zn from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed whole-rock ?66Zn is from 0.24‰ to 0.54‰, which is higher than typical mantle values (0.16-0.20‰). Their heavy Zn- and light Mg- and O-isotope compositions (?26Mg = ?1.1‰ to ?0.14‰, ?18O = 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low ?18O may be inherited from parental melts derived from low-?18O mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high ?66Zn and low ?26Mg reflect diffusion-driven Zn-Mg-isotope exchange between peridotites and the parental melts during their emplacement in the deep lithosphere. Type I eclogites have reconstructed whole-rock ?66Zn from 0.03‰ to 0.43‰ and garnet ?18O from 6‰ to 9.1‰ but show more scatter in inter-mineral Zn-isotope fractionation than Type II, reflecting incomplete equilibration during later metasomatism by carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus suggests that the Roberts Victor eclogites might have crystallized from deep-seated melts at mantle depths.
Journal of Geophysical Research, 10.1029/2021JB023105, 28p.
Africa, South Africa
geophysics - magnetotellurics
Abstract: The present-day composition of Earth's tectonic plates results from past geological processes. We can learn about Earth's composition from deep rock samples that are carried to the surface during volcanic eruptions and by probing its physical properties, like electrical conductivity, with geophysics. In southern Africa, there are extensive deep rock samples, which have been brought to the surface by kimberlite volcanoes that also host diamonds, and also extensive geophysical data. In this article, we compare the rock compositions with electrical conductivity to learn more about Earth's composition. Our results show that the oldest parts of the plates, which retain compositions similar to their initial composition, appear resistive. On the other hand, regions that have been intruded by deep fluids or molten rock can be resistive or conductive, depending on the types of minerals that were formed during the intrusion. The kimberlite volcanoes mostly erupted through the edges of the most resistive parts of the plates and did not erupt through the conductors. These results will help us to make more accurate interpretations about the composition of parts of the Earth where we do not have deep rock samples.
Age correlation of endogenic processes of the Slave (Canada) and Middle Peri Dneiper (Ukraine) cratons in connection with diamond bearing ability problems.
Gems & Gemology, abstracts Mineralogical Journal (Ukraine) Vol. 26, 1, pp. 18-23. *** in English, Vol. 41, 2, Summer p. 194. abstract only
Doklady Earth Sciences, Vol. 466, 2, Feb. pp. 135-137.
Russia
Carbonatite
Abstract: The first data on variations of the isotope composition and element ratios of carbon, nitrogen, and argon in carbonatites of different generations and ultrabasic rocks of the Guli massif obtained by the method of step crushing are reported. It is shown that early carbonatite differs significantly from the later ones by the concentration of highly volatile components, as well as by the isotope compositions of carbon (CO2), argon, and hydrogen (H2O). The data obtained allow us to conclude that the mantle component predominated in the fluid at the early stages of formation of rocks of the Guli massif, whereas the late stages of carbonatite formation were characterized by an additional fluid source, which introduced atmospheric argon, and most likely a high portion of carbon dioxide with isotopically heavy carbon.
Abstract: We have analyzed 141 grains of pyrope from Neogene sediments in a quarry of construction materials, in the Kenkeme River catchment, along its left-side tributary (Chakiya River), about 60 km northwest of Yakutsk city. The mineral chemistry patterns of pyropes are typical of Jurassic-Cretaceous barren kimberlites, like the pipes of Obnazhennaya or Muza, but are uncommon to diamondiferous Middle Paleozoic kimberlites. The results allow identifying the magmatic event and placing time constraints on kimberlite magmatism in the southeastern flank of the Vilui basin, which was part of the Late Jurassic-Early Cretaceous tectonic-magmatic event in northeastern Asia.
Abstract: Super Deep Diamonds (SDD) are known to form at depths between ~300 and ~1000 km in the Earth’s mantle [1]. These diamonds as well as their minerals, melts and fluid inclusions are rare natural materials from deep Earth. The aim of this study is to indentify and characterize mineral inclusions in diamonds from Juína, Mato Grosso, Brazil, and hence classify them as SDD (or not). Twelve diamonds from four different mining sites of Juína were selected according to their inclusions using an Estereo Microscope. The main diamond features were based on crystallographic faces, shape, degrees of resportion, crystal state and intergrowing [2]. Diamond samples are transparent, colorless and present octahedro, octahedro-tetrahexahedral and tetrahexahedral habits. Some diamonds show trigons with positive and negative relief, and hexagons with negative relief. Four diamonds are heavily resorbed and were classified as "unknowing habits", as their shapes are distorced and fragmented. Moreover, three samples show abrasion on the vertices of the quartenary axes, and the others have distinct degrees of resorption. Some crystals present intergorwth, such as contact twins (macle) in {111} or aggregates. All diamonds have mineral inclusions of different colors. Most inclusions are black and could be carbon spots, oxides or even silicates, such as olivine. Other inclusions are yellow to red, which might indicate garnet. In addition, blue inclusions were observed, and could be sulphides. The next steps consists of Fourier Transform Infrared (FTIR) to determine diamond nitrogen impurities, and Micro-Raman spectroscopy and X-Ray Diffraction analyses using Synchrotron radiation to determine in situ the chemical composition of mineral inclusions.
Abstract: The Rosário-6 is a non-diamondiferous hypabyssal kimberlite located above the Rio de la Plata craton and near the south-eastern edge of the Paraná Basin, in southern Brazil. It is petrographically an inequigranular texture, macrocrystal kimberlite, fresh and the groundmass exhibits a microporphyritic texture and round megacrysts of olivine, which are derived from disaggregated mantle xenoliths. Olivine is also present as macrocrysts, microphenocrysts and in the groundmass together with phlogopite and apatite. These microphenocrysts are immersed in a groundmass of olivine, monticellite, phlogopite, CaTiO3-perovskite, apatite, Mg-chromite and Mg-ulvöspinel and melilite. A mesostasis assemblage of phlogopite, melilite, soda melilite, akermanite and calcium carbonate is segregated from the groundmass. Its geochemical signature is similar to those of transitional kimberlites of Kaapvaal Craton, South Africa, and the U-Pb ages of ~ 128 Ma on perovskite reveal that Rosário-6 kimberlite post-dates the main pulse of volcanism in the Paraná-Etendeka Large Igneous Province (LIP). The high Ti content of some minerals, such as Mg-chromite, Mg-ulvöspinel, phlogopite and melilite, and the presence of perovskite suggest a Ti-rich source. The petrographic, geochemical and isotopic data indicate that the Rosário-6 kimberlite source is a depleted mantle metasomatized by H2O-rich fluids, CO2-rich and silicate melts derived from the recycling of an ancient subducted oceanic plate (eclogite) before the South Atlantic opening. Although several authors indicate the influence of Tristan da Cunha plume for the generation of alkaline magmatism associated to the Paraná-Etendeka flood basalts, our data demonstrates that Tristan da Cunha plume has no chemical contribution to the generation of Rosário-6 kimberlite, except by its thermal influence.
The Forming and Distribution of Kiberlites in the Eastern Part of the Siberian Platform in Relation with its Deep Structural Peculiar Charact Ertistics.
Izv. Akad. Nauk Geol. Ser., No. 3, MARCH PP. 54-65.
Abstract: Cesanite (Ca2Na3(OH)(SO4)3), a rare mineral, has been found in a few places restricted to a geothermal field and caves. We report the new occurrence of cesanite in quite different geological site—within sulfate-rich melt inclusions in chloride segregations from kimberlites of Udachnaya-East pipe (Siberia). Two halite generations: ?esanite free and ?esanite-bearing, were distinguished in concentrically zonal segregations according to the results of the mineral and sulfate melt inclusion study by micro-Raman spectroscopy and SEM-EDS. We have applied the Raman spectroscopy and first principles calculations to understand structural and vibrational properties of cesanite daughter mineral in polyphase sulfate inclusions. Polarized spectra provided additional information on the overlapped components of the spectral profile. The Raman spectra of cesanite in the range of OH stretching vibrations are reported for the first time. The study aims to clarify the source of the Na-S-Cl-enrichment in the Udachnaya-East pipe, which is highly discussed.
Abstract: In the present study, four samples of natural melilites were characterized using electron microprobe analysis, powder X-ray diffraction, FTIR, and Raman spectroscopy, and their thermodynamic properties were measured with a high-temperature heat-flux Tian-Calvet microcalorimeter. The enthalpies of formation from the elements were determined to be: -3796.3 ± 4.1 kJ/mol for Ca1.8Na0.2(Mg0.7Al0.2Fe2+0.1?)Si2O7, -3753.6 ± 5.2 kJ/mol for Ca1.6Na0.4(Mg0.5Al0.4Fe2+0.1?)Si2O7, -3736.4 ± 3.7 kJ/mol for Ca1.6Na0.4(Mg0.4Al0.4Fe2+0.2?)Si2O7, and -3929.2 ± 3.8 kJ/mol for Ca2(Mg0.4Al0.6)[Si1.4Al0.6O7]. Using the obtained formation enthalpies and estimated entropies, the standard Gibbs free energies of formation of these melilites were calculated. Finally, the enthalpies of the formation of the end-members of the isomorphic åkermanite-gehlenite and åkermanite-alumoåkermanite series were derived. The obtained thermodynamic properties of melilites of different compositions can be used for quantitative modeling of formation conditions of these minerals in related geological and industrial processes.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 277-8.
Global
gemstones
Abstract: Most colored gemstones form near the earth’s surface in a wide range of different environments; for example, they can crystallize from igneous magmas or hydrothermal solutions, or via the recrystallization of preexisting minerals during metamorphism. The specific environment determines the types of gem minerals that form, as well as their physical and chemical properties. Field studies of colored gem deposits provide the basis for the scientific understanding of natural gemstone formation and, in turn, the basis for criteria for gem identification. Gem deposits are of scientific interest because they represent unusual geologic and geochemical conditions; for example, emeralds are rare because they require beryllium and chromium (and/or vanadium), which generally travel in very different geochemical circles. Scientists study gem deposits by collecting rock and mineral samples in the field, mapping geological formations and structures, documenting the environment in which the gems occur, and examining the collected samples back in the laboratory. Such examination yields information on the chemical, temperature, and pressure conditions of gem formation, the associated minerals (often found as distinctive inclusions in the gems themselves), and the age of the deposit. Determining the origin of a gem deposit usually requires a small amount of very specific data. The results are published in publicly available peerreviewed publications. Such field studies provide clues that can be used to explore for similar types of gem deposits. Challenges include the remoteness of locations that have not been previously studied by geologists, the small size of deposits that precludes study by large mining companies, and the rarity of the gems themselves. There is much left to do in gem deposit research. For example, despite its growing popularity as a gemstone, there are few studies of gem spinel deposits, especially cobalt-blue spinel (figure 1), for which only one deposit has been studied. To date we know little about what factors control spinel genesis and color. Recently there has been another reason to study gem deposits: gem fingerprinting, in which modern methods are used to obtain characteristic information. This information is then compared to information obtained from stones from known localities to estimate where a stone with no locality information originated. Modern fingerprinting methods analyze the chemistry of the stones (using electron probe microanalysis, isotopic analysis, laser ablation-inductively coupled plasma-mass spectrometry) and/or their solid and fluid inclusions. We know that the chemistry of the stones must reflect the chemistry of the host rock environment; for example, the chromophore in emerald from Lened in Canada is vanadium, and not the typical chromium, because there are no chromium-bearing rocks in the area. With respect to solid inclusions, rubies from Aappaluttoq in Greenland have phlogopite mica inclusions because they recrystallized in a rock at pressures and temperatures where phlogopite is the stable potassium-bearing phase. An example of diagnostic fluid inclusions is the three-phase variety seen in Colombian emeralds (and now also observed elsewhere). New is the use of ICP-MS on fluid inclusions to define part of the fluid assemblage from which the stones were formed; this tells us about the environment of formation, but also may assist in defining a fingerprint for the stone. Where scientific studies require only very specific data, the more data available from stones of known origin, and the more representative those stones are of the full range of compositions and inclusions found in a specific deposit or country of origin, the more accurate the estimation should be. Unfortunately, these data are generally not made public, so every lab doing fingerprinting is essentially working independently, and there is no way to know how accurate their data and the resulting country- or deposit-oforigin estimates are. We also note that a serious problem in origin determination is that some of the best gemstones will be lacking diagnostic inclusions altogether, which then restricts the tools and observations can be used.
A cathodluminescence study of calcite dolomite microstructures and Cal-Dol geothermometry in highly metamorphosed carbonatites: an example from the Fir carbonatites, east central British Columbia, Canada.
GAC-MAC 2013: GS2: Igneous and Metamorphic Petrology and Volcanology, abstract only
The effect of amphibolite facies metamorphism on the U-Th-Pb geochronology of accessory minerals from meta-carbonatites and associated meta-alkaline rocks.
Russia, Kola Peninsula, Europe, Finland, Canada, British Columbia
deposit - Khibina, Fir, Siilinjarvi
Abstract: Phosphorus is the central ingredient in fertilizer that allows modern agriculture to feed the world’s population. This element, also critical in a host of industrial applications, is a nonrenewable resource that is sourced primarily from the phosphatic mineral apatite, hosted in sedimentary and igneous ores. World phosphate resources are estimated by the U.S. Geological Survey at ca. 300,000 Mt, of which 95% are sedimentary and 5% are igneous. Current known USGS reserve estimates are sufficient for a maximum of 200 to 300 years; the exploration and discovery of new resources, enhanced mining technologies, and new technologies aimed at the recovery and recycling of P from sewage and agricultural runoff will all contribute to extending P production. Igneous ores are generally associated with Phanerozoic carbonatites and silica-deficient alkalic intrusions that typically average 5 to 15 wt % P2O5, which can be beneficiated to high-grade concentrates of at least 30 wt % P2O5 with few contaminants. Carbonatites are typically the smallest and youngest parts of a carbonatite-alkaline rock complex that formed during fractional crystallization of a calcic parental alkaline silicate melt, or from liquid immiscibility of a carbonate-rich nephelinite that underwent magmatic fractionation and differentiation during ascent from the mantle source. Fluorapatite generally crystallizes early, near the liquidus, and over a small temperature interval below the apatite saturation temperature that varies strongly with temperature, SiO2 and CaO concentrations, and the aluminosity of the melt. Carbonatite-alkaline rock complexes commonly possess a concentric, zonal structure thought to reflect caldera volcanism. Pathfinder elements in soils, sediments, tills, and vegetation include Nb, rare earth elements (REEs), P, Ba, Sr, F, U, and Th, and in water, F, Th, and U are indicators. Remote sensing techniques with the ability to identify minerals rich in CO3, REEs, and Fe2+ that are characteristic of carbonatites are also important exploration tools that may provide vectors to ore. Sedimentary phosphorite is a marine bioelemental sedimentary rock that contains >18 wt % P2O5. While small peritidal phosphorites formed in Precambrian coastal environments, economically significant upwelling-related phosphorite did not accumulate until the late Neoproterozoic and continued through the Phanerozoic. Coastal upwelling delivered deep, P-rich waters to continental shelves and in epeiric seas to drive phosphogenesis and form the largest phosphorites on Earth. High-grade deposits formed as a result of hydraulic concentration of phosphate grains to form granular beds with minimal gangue. The amalgamation of these beds into decameter-thick, stratiform ore zones is generally focused along the maximum flooding surface, which is a primary exploration target in upwelling-related phosphorite. In addition to P, other elements concentrated in igneous and sedimentary phosphorites are Se, Mo, Zn, Cu, and Cr, which are important agricultural micronutrients. Other saleable by-products include U and REEs. The U concentration in sedimentary phosphorite is generally between 50 and 200 ppm, but can be as high as 3,000 ppm, making it an increasingly important source of U for the nuclear industry. The concentration of REEs in some sedimentary phosphorites is comparable to the world’s richest igneous and Chinese clay-type REE deposits. The source of the dissolved P in upwelling ocean water is ultimately derived from the chemical weathering of continental rocks, the process that links igneous and sedimentary phosphorites through time and space. The covarying temporal relationship of igneous and sedimentary deposits suggests that plate tectonics and the concentration of apatite in a progressively more felsic crust underpins the feedback processes regulating the biogeochemical cycling of P. Critical to the generation of greenfield exploration targets is the recognition that large P deposits emerged in the late Neoproterozoic. The geological environments conducive for exploration can be constrained from an understanding of ore-forming processes by the use of complementary petrological techniques, including fieldwork, petrography, sedimentology, sequence stratigraphy, and geochemistry.
Abstract: Baffin Island's Lake Harbour Group (LHG), a Paleoproterozoic granulite facies metasedimentary sequence rich in carbonates, contains occurrences of the gemstones sapphire (corundum), spinel (including vivid blue, cobalt-enriched spinel), and lapis lazuli (haüyne-bearing rock). Most occurrences of these gem minerals are uniquely metasedimentary (carbonates and calc-silicate rock), while a few spinel occurrences formed from metasomatic reactions between Si-Al-rich rock (syenogranite or gneiss) and marble. The metasedimentary corundum, spinel, and haüyne occurrences have similar protoliths: primarily dolomitic marls with a high Al/Si relative abundance (interpreted as sandy mud to clay siliciclastic fraction in the protolith). Kimmirut-type sapphire deposits formed via a multi-step metamorphic process under three different and specific P-T conditions. Lapis lazuli formation required the presence of evaporites to provide Na and possibly S for the blue mineral haüyne. In addition to high Al/Si calc-silicate rocks, spinel also occurs in impure dolomitic marbles with very low K/Al. Potential for Kimmirut-type sapphire deposits is expected to be restricted to metacarbonate sequences proximal to the thrust fault separating the LHG from the Narsajuaq Arc, where retrograde upper amphibolite facies mineralization is most pervasive. Spinel and Kimmirut-type sapphire deposits are expected to be found in dolomitic marble sequences rich in calc-silicate layers. The potential occurrence of lapis lazuli is more difficult to predict but deposits could be identified thanks to large geographical footprints and their color. Similar gem occurrences or deposits to those in the LHG may be found in other metacarbonate-bearing terranes with similar metamorphic conditions (and for Kimmirut-type sapphire, a similar metamorphic history). Aerial hyperspectral and photographic surveys are well-suited to gemstone exploration on southern Baffin Island thanks to excellent rock exposure with minimal sediment or plant/lichen cover. Spectral mapping of dolomite-, diopside-, phlogopite-, and scapolite-rich domains in LHG metacarbonate sequences using airborne hyperspectral data is expected to provide exploration targets. Remote sensing exploration could be used in other metacarbonate-bearing, upper amphibolite to granulite facies metamorphic terranes found in polar climates, arid climates, or at high elevation in mountainous regions where such rocks are well exposed with minimal vegetative cover.
Africa, Madagascar, Zambia, Asia, Sri Lanka, South America, Colombia
emerald
Abstract: The great challenge of geographic origin determination is to connect the properties and features of individual gems to the geology of their deposits. Similar geologic environments can produce gems with similar gemological properties, making it difficult to find unique identifiers. Over the last two decades, our knowledge of corundum and emerald deposit formation has improved significantly. The mineral deposits are classically separated into primary and secondary deposits. Primary corundum deposits are subdivided into two types based on their geological environment of formation: (1) magmatic and (2) metamorphic. Magmatic deposits include gem corundum in alkali basalts as in eastern Australia, and sapphire in lamprophyre and syenite as in Montana (United States) and Garba Tula (Kenya), respectively. Metamorphic deposits are divided into two subtypes (1) metamorphic deposits sensu stricto (in marble; mafic and ultramafic rocks, or M-UMR), and (2) metamorphic-metasomatic deposits characterized by high fluid-rock interaction and metasomatism (i.e., plumasite or desilicated pegmatites in M-UMR and marble, skarn deposits, and shear zonerelated deposits in different substrata, mainly corundum-bearing Mg-Cr-biotite schist). Examples of the first subtype include the ruby deposits in marble from the Mogok Stone Tract or those in M-UMR from Montepuez (Mozambique) and Aappaluttoq (Greenland). The second subtype concerns the sapphire from Kashmir hosted by plumasites in M-UMR. Secondary corundum deposits (i.e., present-day placers) result from the erosion of primary corundum deposits. Here, corundum is found in the following types of deposits: eluvial (derived by in situ weathering or weathering plus gravitational movement), diluvial (scree or talus), colluvial (deposited at the base of slopes by rainwash, sheetwash, slow continuous downslope creep, or a combination of these processes), and alluvial (deposited by rivers). Today, most sapphires are produced from gem placers related to alkali basalts, as in eastern Australia or southern Vietnam, while placers in metamorphic environments, such as in Sri Lanka (Ratnapura, Elahera) and Madagascar (Ilakaka), produce the highest-quality sapphires. The colluvial Montepuez deposit in Mozambique provides a huge and stable supply of clean and very high-quality rubies. Primary emerald deposits are subdivided into two types based on their geological environment of formation: (1) tectonic-magmatic-related (Type I) and (2) tectonic-metamorphic-related (Type II). Several subtypes are defined and especially Type IA, hosted in M-UMR, which accounts for about 70% of worldwide production (Brazil, Zambia, Russia, and others). It is characterized by the intrusion of pegmatites or quartz veins in M-UMR accompanied by huge hydrothermal fluid circulation and metasomatism with the formation of emerald-bearing desilicated pegmatite (plumasite) and biotite schist. Type IB in sedimentary rocks (China, Canada, Norway, Kazakhstan, and Australia) and Type IC in granitic rocks (Nigeria) are of minor importance. The subtype Type IIA of metamorphic deposits is related to hydrothermal fluid circulation at high temperature, in thrust fault and/or shear zones within M-UMR of volcano-sedimentary series, such as at the Santa Terezinha de Goiás deposit in Brazil. The subtype Type IIB is showcased by the Colombian emerald deposits located in the Lower Cretaceous black shales of the Eastern Cordillera Basin. These are related to the circulation of hydrothermal basinal fluids in black shales, at 300330°C, that dissolved evaporites in (1) thrust and tear faults for the deposits of the western emerald zone (Yacopi, Coscuez, Muzo, Peñas Blancas, Cunas, and La Pita mines) and (2) a regional evaporite level intercalated in the black shales or the deposits of the eastern emerald zone (Gachalá, Chivor, and Macanal mining districts). Secondary emerald deposits are unknown because emerald is too fragile to survive erosion and transport in rivers.
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.
Abstract: The magnetic field protects Earth's surface from deadly cosmic radiation and provides clues about the planet's interior. Tarduno et al. found that some of the oldest minerals on Earth, Jack Hills zircons, preserved a record of a magnetic field over 4 billion years ago (see the Perspective by Aubert). Earth's magnetic field appears to have been fully operational a mere few hundred million years after the planet formed. This suggests an early start for plate tectonics and an ancient cosmic radiation shield that was important for habitability.
Abstract: Plate tectonics on Earth are linked to the dynamics of the interior today. However, the interior dynamics in the distant past are a far greater mystery because of the subduction of surface rock. Wang et al. analyzed 3.5-billion-year-old rocks in China and discovered the oldest geochemical evidence of mantle plume magmatism along with high mantle temperatures. The rocks also appear to record chemical heterogeneity and may be evidence of convection in the deep mantle 3.5 billion years ago.
Abstract: The carbon allotropes of diamond and graphene have different types of bonding that lead to their exceptional properties. Bakharev et al. pull off the impressive trick of making a monolayer carbon film that is diamond-like in its bonding. The authors accomplish this by attaching fluorine atoms to the carbon film, creating “F-diamane.” Diamane is a long-sought-after, but challenging to make, material that should have useful properties. F-diamane may find use in a variety of applications, from microelectronics as a semiconductor to a seed material for growing single-crystal diamond films.
Physics of the Earth and Planetary Interiora, 13p. Pdf
Mantle
melting
Abstract: Physical properties of silicate melts play a key role for global planetary dynamics, controlling for example volcanic eruption styles, mantle convection and elemental cycling in the deep Earth. They are significantly modified by structural changes at the atomic scale due to external parameters such as pressure and temperature or due to chemistry. Structural rearrangements such as 4- to 6-fold coordination change of Si with increasing depth may profoundly influence melt properties, but have so far mostly been studied at ambient temperature due to experimental difficulties. In order to investigate the structural properties of silicate melts and their densification mechanisms at conditions relevant to the deep Earth's interior, we studied haplo basaltic glasses and melts (albite-diopside composition) at high pressure and temperature conditions in resistively and laser-heated diamond anvil cells using X-ray absorption near edge structure spectroscopy. Samples were doped with 10 wt of Ge, which is accessible with this experimental technique and which commonly serves as a structural analogue for the network forming cation Si. We acquired spectra on the Ge K edge up to 48 GPa and 5000 K and derived the average Ge-O coordination number , and bond distance as functions of pressure. Our results demonstrate a continuous transformation from tetrahedral to octahedral coordination between ca. 5 and 30 GPa at ambient temperature. Above 1600 K the data reveal a reduction of the pressure needed to complete conversion to octahedral coordination by ca. 30 . The results allow us to determine the influence of temperature on the Si coordination number changes in natural melts in the Earth's interior. We propose that the complete transition to octahedral coordination in basaltic melts is reached at about 40 GPa, corresponding to a depth of ca. 1200 km in the uppermost lower mantle. At the core-mantle boundary (2900 km, 130 GPa, 3000 K) the existence of non-buoyant melts has been proposed to explain observed low seismic wave velocity features. Our results highlight that the melt composition can affect the melt density at such extreme conditions and may strongly influence the structural response.
Earth Science Informatics, Vol. 14, pp. 521-528. pdf
Mantle
tectonics
Abstract: The spinel group minerals provide useful information regarding the geological environment in which the host rocks were formed, constituting excellent petrogenetic indicators, and guides in the search for mineral deposits of economic interest. In this article, we present the Spinel Web, a web application to visualize the chemical composition of spinel group minerals. Spinel Web integrates most of the diagrams commonly used for analyzing the chemical characteristics of the spinel group minerals. It incorporates parallel coordinates and a 3D representation of the spinel prisms. It also provides coordinated views and appropriate interactions for users to interact with their datasets. Spinel Web also supports semi-automatic categorization of the geological environment of formation through a standard Web browser.
Yelisseyev, A., Khrenov, A., Afanasiev, V., Pustavarov, V., Gromilov, S., Panchenko, A., Poikilenko, N., Litasov, K.
Luminesence of impact diamonds from the Popigai astrobleme.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Diamond and Related Materials, Vol. 58, pp. 69-77.
Russia
Deposit - Popigai
Abstract: Impact diamonds (IDs) from the Popigai crater are aggregates of nanoparticulate graphite and cubic and hexagonal diamonds. IDs demonstrate broad-band emissions at 3.05, 2.8, 2.3 and 2.0 eV, which are associated with structural defects and are similar to those in detonation ultra-dispersed diamonds and CVD diamond films. A doublet with components at 1.7856 and 1.7892 eV in some ID samples is related to R1,2 lines of Cr3 + ions in corundum inclusions. The presence of N3, H3, NV0 and NV? vibronic systems in some of the ID samples shows that (i) there is nitrogen impurity and (ii) samples underwent high temperature annealing that promoted vacancies and nitrogen diffusion and defect aggregation. The luminescence decay fits with a sum of two exponential components: lifetime of the fast one is in the 5 to 9 ns range. Parameters of the traps responsible for broad thermoluminescence peaks at 148, 180, 276 and 383 K were estimated.
Diamonds & Related Materials, Vol. 101, 107640, 13p. Pdf
Russia
Popigai
Abstract: The special features of impact diamonds are the orientation of the nanosized grains relative to each other, the presence of hexagonal diamond (lonsdaleite, L) in a large part of the samples and the increased wear resistance. Using Raman spectroscopy and XRD, two groups of translucent samples of Popigai impact diamonds (PIDs) were selected: with and without lonsdaleite and the effect of lonsdaleite on the optical properties of the samples was studied. In all L-containing PIDs there is a strong absorption band of about 1230 cm-1 in the one-phonon region, in the mid-IR. The absorption edge is blurred and described by the Urbach rule. The estimated value of Eg ~4 eV for L is consistent with the first principles calculations. Impurity nitrogen is found only in L-free PIDs: There are signals from nitrogen-vacancy complexes in the photoluminescence (PL) spectra. Variations in the number of nitrogen atoms (N = 1 to 4) in the structure of these centers indicate significant variations in the parameters of PID annealing. L-containing PIDs are characterized by large strains in the lattice and, as a consequence, there are problems with the defect diffusion. The narrow lines in PL spectra, uncommon for diamond, can be the result of several orders of magnitude higher concentrations of impurities in PIDs formed during the solid-phase transition. The broadened peaks of 180, 278 and 383 K are distinguishable in the curves of thermostimulated luminescence (TSL) for L-free PIDs, but in the presence of L the TSL glow becomes continuous as in natural IaA-type diamonds with platelets. In general, lonsdaleite deteriorates the optical properties of impact diamonds and makes it difficult to create certain types of impurity-vacancy complexes for different applications.
Abstract: Yakutites are coarse (up to 15 mm or larger) aggregates dispersed for more than 500 km around the Popigai meteorite crater. They share many features of similarity with impact diamonds found inside the crater, in elemental and phase compositions, texture, and optical properties as revealed by X-ray photoelectron spectroscopy, X-ray diffraction, and optical spectroscopy (Raman, absorption, luminescence and microscopic) studies. The N3 vibronic system appearing in the luminescence spectra of Popigai impact diamonds (PIDs) indicates a presence of nitrogen impurity and a high-temperature annealing of diamonds that remained in the crater after solid-phase conversion from graphite. Yakutites lack nitrogen-vacancy centers as signatures of annealing, which may indicate quenching at the time of ejection. Thus, both PIDs and yakutites originated during the Popigai impact event and yakutites were ejected to large distances.
Abstract: Moissanites were found in tagamites of the Popigai meteorite crater along with impact diamonds. We have studied 55 samples including 49 individual polytypes and six intergrowths. The numbers of 6H, 15R, 4H, 6H/15R, and 6H/4H polytypes are 82, 7, 5, 4, and 2%, respectively. By the assemblage of polytypes, the moissanites of the Popigai astrobleme are distinct from kimberlite moissanites, as well as from synthetic SiC, which is characterized by the absence of the 4H polytype and the presence of more diverse inclusions (including Fe-bearing). The Popigai astrobleme is one of few objects with reliable natural moissanite. Technogenic contamination is excluded, since any researcher can find this mineral in tagamites.
Abstract: For the first time, 60 large diamond aggregates were found inside the Popigai meteorite crater during washing of alluvial deposits along the Dogoi river crossing the crater. These aggregates are similar in appearance to yakutites from the placers of Northern Yakutia (YPY), and we regard them as yakutites from the Popigai crater (YPC). The structure and optical properties of Popigai impact diamonds from the impact melt rocks (tagamites) in the crater (PIDT) and yakutites YPC/YPY were compared in detail. In all these cases, a polycrystalline structure consisting of nanoscale grains of cubic and twinned cubic diamond (lonsdaleite) was found. This is the result of a solid-phase graphite-diamond transition due to an impact event 35?million?years ago. The diamond aggregates show the following features: a red shift of the short-wave edge of the transmission, broadening of the diamond Raman peaks, signals from other diamond polytypes and numerous inclusions of other minerals in the Raman spectra, and a dominant broadband photoluminescence (PL). PL in the N3 system associated with N3V centers in PIDT diamonds indicates a high-temperature annealing of these aggregates with resulting aggregation of impurities during the prolonged cooling of large impact melt pockets and pools. It is assumed that some of the impact diamonds were ejected from the crater during the impact event and experienced rapid cooling. Some of these diamonds fell back into the crater (YPC yakutites), others have been deposited outside the crater and displaced during erosion (YPY yakutites). Difference in size and shape between the PIDTs and yakutites YPC/YPY is due to the difference in size of original graphite flakes or aggregates and/or due to the fundamentally different technologies of diamond extraction.
Mayer, B., Jung, S., Romer, R.,Pfander, J., Klugel, A., Pack, A., Groner, E.
Amphibole in alkaline basalts from intraplate settings: implications for the petrogenesis of alkaline lavas from the metasomatised lithospheric mantle.
Contributions to Mineralogy and Petrology, Vol. 167, 3, pp. 1-22.
Journal of Volcanology and Geothermal Research, Vol. 357, pp. 68-91.
Mantle
volcanism
Abstract: Volcanic stratigraphy is a fundamental component of geological mapping in volcanic areas as it yields the basic criteria and essential data for identifying the spatial and temporal relationships between volcanic products and intra/inter-eruptive processes (earth-surface, tectonic and climatic), which in turn provides greater understanding of the geological evolution of a region. Establishing precise stratigraphic relationships in volcanic successions is not only essential for understanding the past behaviour of volcanoes and for predicting how they might behave in the future, but is also critical for establishing guidelines for exploring economic and energy resources associated with volcanic systems or for reconstructing the evolution of sedimentary basins in which volcanism has played a significant role. Like classical stratigraphy, volcanic stratigraphy should also be defined using a systematic methodology that can provide an organised and comprehensive description of the temporal and spatial evolution of volcanic terrain. This review explores different methods employed in studies of volcanic stratigraphy, examines four case studies that use differing stratigraphic approaches, and recommends methods for using systematic volcanic stratigraphy based on the application of the concepts of traditional stratigraphy but adapted to the needs of volcanological environment.
Liu, Y-C., Deng, L-P., Gu, X-F., Groppo, C., Rolfo, F.
Application of Ti in zircon and Zr in rutile thermometers to constrain high temperature metamorphism in eclogites from the Dabie Orogen, central China.
European Journal of Mineralogy, Vol. 31, pp. 889-803. pdf
China
eclogite
Abstract: A combined study of detailed petrographic observation, mineral chemistry analysis and phase equilibrium modeling indicates that the high-temperature eclogites from the Dabie orogen, central China, experienced two episodes of anatexis: the first is phengite dehydration melting during the exhumation of deeply subducted slices, and the second is heating melting related to the post-orogenic collapse. Petrographic evidence and clues of the anatectic events include biotite + plagioclase + garnet ± amphibole intergrowth in matrix and biotite + plagioclase intergrowth within amphibole porphyroblast. Pressure-temperature (P-T) pseudosection and modal variation diagram indicate that the biotite + plagioclase + garnet ± amphibole in matrix was formed by the reactions phengite + clinopyroxene + quartz = melt + sanidine + garnet + plagioclase and later melt + sanidine + garnet = biotite + plagioclase, while the biotite + plagioclase intergrowths within poikiloblastic amphibole were formed by the reaction amphibole + muscovite + epidote = biotite + plagioclase + melt. In addition, the combination of petrological observations and P-T estimates suggests that the first melting event occurred at the late Triassic, while the second is related to the early Cretaceous mountain-root removal and subsequent asthenospheric upwelling and heat input. As the P-T paths of high-temperature/ultrahigh-pressure rocks have high probabilities to cross-cut phengite-melting curves, phengite melting during decompression may be a common process in these rocks. Moreover, the coexistence of multiple episodes of anatexis in a single tectonic slice suggests caution when identifying and dating partial melting in high-temperature/(ultra)high-pressure rocks.
Tichomirowa, M., Grosche, G., Gotze, J., Belyatsky, B.V., Savva, E.V., Keller, J., Todt, W.
The mineral isotope composition of two Precambrian carbonatite complexes from the Kola Alkaline Province - alteration versus primary magmatic signatures.
Abstract: Carbonate-rich ultramafic lamprophyres (aillikites) and associated rocks characteristically occur during the early stages of thinning and rifting of cratonic mantle lithosphere, prior to the eruption of melilitites, nephelinites and alkali basalts. It is accepted that they require volatile-rich melting conditions, and the presence of phlogopite and carbonate in the source, but the exact source rock assemblages are debated. Melts similar to carbonate-rich ultramafic lamprophyres (aillikites) have been produced by melting of peridotites in the presence of CO2 and H2O, whereas isotopes and trace elements appear to favor distinct phlogopite-bearing rocks.
Olivine macrocrysts in aillikites are usually rounded and abraded, so that it is debated whether they are phenocrysts or mantle xenocrysts. We have analyzed minor and trace element composition in olivines from the type aillikites from Aillik Bay in Labrador, Canada. We characterize five groups of olivines: [1] mantle xenocrysts, [2] the main phenocryst population, and [3] reversely zoned crystals interpreted as phenocrysts from earlier, more fractionated, magma batches, [4] rims on the phenocrysts, which delineate aillikite melt fractionation trends, and [5] rims around the reversely zoned olivines. The main phenocryst population is characterized by mantle-like Ni (averaging 3400 ?g g? 1) and Ni/Mg at Mg# of 88-90, overlapping with phenocrysts in ocean island basalts and Mediterranean lamproites. However, they also have low 100 Mn/Fe of 0.9-1.3 and no correlation between Ni and other trace elements (Sc, Co, Li) that would indicate recycled oceanic or continental crust in their sources. The low Mn/Fe without high Ni/Mg, and the high V/Sc (2-5) are inherited from phlogopite in the source that originated by solidification of lamproitic melts at the base of the cratonic lithosphere in a previous stage of igneous activity. The olivine phenocryst compositions are interpreted to result from phlogopite and not high modal pyroxene in the source. The presence of kimberlites and ultramafic lamprophyres of Mesozoic age in Greenland indicates the persistence of a steep edge to the cratonic lithosphere at a time when this had been removed from the western flank in Labrador.
Geochimica et Cosmochimica Acta, in press available 49p. Pdf
Africa, South Africa
deposit - Benfontein
Abstract: Olivine chemistry has been widely used to track the petrogenesis of mafic and ultramafic magmas from their mantle source to eruption at the surface. A major challenge in these studies is deciphering crystal growth versus diffusion controlled zoning. Here we report a multi-element approach using high-precision electron microprobe techniques to evaluate crystal growth versus diffusion in kimberlitic olivine from the Benfontein kimberlite, South Africa. These results have implications for both the petrogenesis of kimberlite magmas and the understanding of crystal growth and diffusion-based zoning in igneous olivine in general. The Benfontein olivine contain multiple phosphorous (P)-rich and P-poor zones. Core zones are characterized by homogenous low-P (<78?ppm) concentrations, consistent with xenocrystic origins. Gradational changes in Fo, Ni, Cr and other minor/trace elements at core-margins are similarly characterized by constant low-P concentrations that are indistinguishable from the central regions of the core. Olivine P-maps effectively outline the original xenocryst core, whereas gradational margins are interpreted as diffusion controlled zones related to early-stage equilibration of xenocrystic olivine with proto-/kimberlite melt. Multiple P-poor (100-150?ppm) and P-rich (200-450?ppm) concentric, oscillatory zones with inclusions of kimberlitic oxide phases are observed surrounding the low-P xenocrystic cores. Oxide phases change from chromite in the inner zones to ilmenite in the intermediate zones to magnetite-rich spinel in the outer zones of the olivine. The P-zoning corresponds with changes in Fo content implying that stages of crystal growth was preserved by both fast and slow diffusing elements rather than diffusion processes. Elements compatible with olivine (±chromite) crystallization (i.e., Ni and Cr) display a constant decrease across all zones, suggesting that magma mixing is unlikely a controlling process for P-zoning. We interpret P-rich zones to result from stages of solute trapping related of rapid disequilibrium growth driven by extrinsic factors such as changes in pressure-temperature during kimberlite evolution. In contrast, P-poor zones represent stages of equilibrium crystal growth. The outer olivine zones are characterized by an increase in Fo contents up to Fo96, and in conjunction with a change to more Fe3+-rich oxides, suggest late stage increase in fO2. Correlated Fo and P changes in the Benfontein olivine suggest that major element zonation represents an example where crystal growth-induced Fo zoning has been preserved in olivine. Furthermore, P-rich olivine zones preserve evidence for concentric growth rather than common dendritic structures seen in other occurrences. These results have implications for understanding the effect of magma dynamics and changes in pressure-temperature-fO2 conditions on olivine growth in igneous rocks.
Abstract: The permafrost zone is expected to be a substantial carbon source to the atmosphere, yet large-scale models currently only simulate gradual changes in seasonally thawed soil. Abrupt thaw will probably occur in <20% of the permafrost zone but could affect half of permafrost carbon through collapsing ground, rapid erosion and landslides. Here, we synthesize the best available information and develop inventory models to simulate abrupt thaw impacts on permafrost carbon balance. Emissions across 2.5?million?km2 of abrupt thaw could provide a similar climate feedback as gradual thaw emissions from the entire 18?million?km2 permafrost region under the warming projection of Representative Concentration Pathway 8.5. While models forecast that gradual thaw may lead to net ecosystem carbon uptake under projections of Representative Concentration Pathway 4.5, abrupt thaw emissions are likely to offset this potential carbon sink. Active hillslope erosional features will occupy 3% of abrupt thaw terrain by 2300 but emit one-third of abrupt thaw carbon losses. Thaw lakes and wetlands are methane hot spots but their carbon release is partially offset by slowly regrowing vegetation. After considering abrupt thaw stabilization, lake drainage and soil carbon uptake by vegetation regrowth, we conclude that models considering only gradual permafrost thaw are substantially underestimating carbon emissions from thawing permafrost.
Symposium on critical and strategic materials, British Columbia Geological Survey Paper 2015-3, held Nov. 13-14, pp. 69-74.
Canada, Quebec
Alkalic
Abstract: The Crevier alkaline intrusion is in the Grenville Province, north of the Lac Saint-Jean region of Québec (Fig. 1). It covers ~25 km2 (Bergeron, 1980) and intrudes charnockitic suites in the allochthon belt defi ned by Rivers et al. (1989). This intrusion has a U-Pb zircon age of 957.5 ± 2.9 Ma (Groulier et al., 2014) and is oriented N320°, along the axis of crustal weakness known as the Waswanipi-Saguenay corridor (Bernier and Moorhead, 2000). This corridor is related to the Saguenay graben, which hosts the Saint-Honoré (Niobec) Nb-Ta-REE deposit and Montviel REE deposit. The age of the Saint-Honoré carbonatite was estimated at 584 to 650 Ma (K-Ar whole rock; Vallée and Dubuc, 1970; Boily and Gosselin, 2004). The Montviel intrusion has a U-Pb zircon age of 1894 ± 3.5 Ma (David et al., 2006; Goutier, 2006). These crystallization ages are very different and cannot be related to a single event for the injection of alkaline intrusions. As mapped by Bergeron (1980), the Crevier alkaline intrusion is broadly composed of syenite and carbonatite rocks (Fig. 2). The Nb- Ta mineralization consists of pyrochlore hosted by a nepheline syenite dike swarm in the centre of the intrusion. The highest REE concentrations, up to 729 ppm La and 1465 ppm Ce, are at the edge of the Crevier alkaline intrusion (Niotaz sud showing; Fig. 2).
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.
Contributions to Mineralogy and Petrology, Vol. 171, pp. 107-
Mantle
Subduction
Abstract: This experimental study simulates the interaction of hotter, deeper hydrous mantle melts with shallower, cooler depleted mantle, a process that is expected to occur in the upper part of the mantle wedge. Hydrous reaction experiments (~6 wt% H2O in the melt) were conducted on three different ratios of a 1.6 GPa mantle melt and an overlying 1.2 GPa harzburgite from 1060 to 1260 °C. Reaction coefficients were calculated for each experiment to determine the effect of temperature and starting bulk composition on final melt compositions and crystallizing assemblages. The experiments used to construct the melt-wall rock model closely approached equilibrium and experienced <5% Fe loss or gain. Experiments that experienced higher extents of Fe loss were used to critically evaluate the practice of “correcting” for Fe loss by adding iron. At low ratios of melt/mantle (20:80 and 5:95), the crystallizing assemblages are dunites, harzburgites, and lherzolites (as a function of temperature). When the ratio of deeper melt to overlying mantle is 70:30, the crystallizing assemblage is a wehrlite. This shows that wehrlites, which are observed in ophiolites and mantle xenoliths, can be formed by large amounts of deeper melt fluxing though the mantle wedge during ascent. In all cases, orthopyroxene dissolves in the melt, and olivine crystallizes along with pyroxenes and spinel. The amount of reaction between deeper melts and overlying mantle, simulated here by the three starting compositions, imposes a strong influence on final melt compositions, particularly in terms of depletion. At the lowest melt/mantle ratios, the resulting melt is an extremely depleted Al-poor, high-Si andesite. As the fraction of melt to mantle increases, final melts resemble primitive basaltic andesites found in arcs globally. An important element ratio in mantle lherzolite composition, the Ca/Al ratio, can be significantly elevated through shallow mantle melt-wall rock reaction. Wall rock temperature is a key variable; over a span of <80 °C, reaction with deeper melt creates the entire range of mantle lithologies from a depleted dunite to a harzburgite to a refertilized lherzolite. Together, the experimental phase equilibria, melt compositions, and reaction coefficients provide a framework for understanding how melt-wall rock reaction occurs in the natural system during melt ascent in the mantle wedge.
Contributions to Mineralogy and Petrology, Vol. 174, 22p. Pdf
Mantle
subduction, melting
Abstract: To investigate the first melts of the mantle wedge in subduction zones and their relationship to primitive magmas erupted at arcs, the compositions of low degree melts of hydrous garnet lherzolite have been experimentally determined at 3.2 GPa over the temperature range of 925-1150 °C. Two starting compositions with variable H2O contents were studied; a subduction-enriched peridotite containing 0.61% Na2O, 0.16 K2O% (wt%) with 4.2 wt% H2O added (Mitchell and Grove in Contrib Mineral Petrol 170:13, 2015) and an undepleted mantle peridotite (Hart and Zindler in Chem Geol 57:247-267, 1986) with 14.5% H2O added (Till et al. in Contrib Mineral Petrol 163:669-688, 2012). Saturating phases include olivine, orthopyroxene, clinopyroxene, garnet and rutile. Melting extent is tracked from near solidus (~?5 wt%) to 25 wt%, which is close to or beyond the point where clinopyroxene and garnet are exhausted. The beginning of melting is a peritectic reaction where 0.54 orthopyroxene?+?0.17 clinopyroxene?+?0.13 garnet react to produce 1.0 liquid?+?0.88 olivine. The melt production rate near the solidus is 0.1 wt% °C?1 and increases to 0.3 wt% °C?1 over the experimentally studied interval. These values are significantly lower than that observed for anhydrous lherzolite (~?1 wt% °C?1). When melting through this reaction is calculated for a metasomatized lherzolite source, the rare earth element characteristics of the melt are similar to melts of an eclogite, as well as those observed in many subduction zone magmas. Moreover, since rutile is stable up to?~?8 wt% melting, the first melts of a hydrous lherzolite source could also show strong high field strength element depletions as is observed in many subduction zone lavas. The silicate melts measured at the lowest temperatures and melting extents (10 wt%) are high silica andesites (56-60 wt% SiO2) and contain very low Ca/Al and high alkalis. These deep low degree andesitic melts, if added to experimentally produced hydrous liquids from melting (20-25 wt%) of harzburgite residues at shallow pressures (1.0-1.2 GPa, Mitchell and Grove in Contrib Mineral Petrol 170:13, 2015), can match the compositional characteristics of primitive natural basaltic andesite and magnesian andesite lavas found globally in arcs. In addition to a silicate melt phase, there is a small amount of silicate dissolved in the H2O supercritical fluid that coexists with the silicate liquid and solids in our experiments. The composition of this fluid is in equilibrium with the Mg-rich minerals and it is granitic. The results presented here are used to develop a model for producing hydrous arc magmas. We hypothesize that mantle wedge melting produced by the flux of hydrous fluid from the slab occurs over a range of depths that begins at the base of the mantle wedge and ends at shallow mantle depths. These melts ascend and remain isolated until they mix in the shallow, hottest part of the mantle wedge. In this melting scenario, the metasomatic “slab melt” contributions to arc magmas is small (~?5 wt%), but its effect on the alkali, REE and incompatible trace element budget of the derivative magmas is large and able to reproduce the trace elemental characteristics of the primitive andesites. Higher proportions of slab or sediment melt do not resemble primitive high magnesian arc andesites and basaltic andesites.
Abstract: Plate reconstruction studies show that the Neotethys Ocean was closing due to the convergence of Africa and Eurasia toward the end of the Cretaceous. The period around 75 Ma reflects the onset of continental collision between the two plates as convergence continued to be taken up mostly by subduction of the Neotethys slab beneath Eurasia. The Owen transform plate boundary in the northeast accommodated the fast northward motion of the Indian plate relative to the African plate. The rest of the plate was surrounded by mid-ocean ridges. Africa was experiencing continent-wide rifting related to northeast-southwest extension. We aim to quantify the forces and paleostresses that may have driven this continental extension. We use the latest plate kinematic reconstructions in a grid search to estimate horizontal gravitational stresses (HGSs), plate boundary forces, and the plate's interaction with the asthenosphere. The contribution of dynamic topography to HGSs is based on recent mantle convection studies. We model intraplate stresses and compare them with the strain observations. The fit to observations favors models where dynamic topography amplitudes are smaller than 300 m. The results also indicate that the net pull transmitted from slab to the surface African plate was low. To put this into context, we notice that available tectonic reconstructions show fragmented subduction zones and various colliding micro-continents along the northern margin of the African plate around this time. We therefore interpret a low net pull as resulting from either a small average slab length or from the micro-continents' resistance to subduction.
Proceedings of the Royal Society B: Biological sciences, Vol. 284. no 1856, pp.
Mantle
pangea
Abstract: The break-up of the supercontinent Pangaea around 180 Ma has left its imprint on the global distribution of species and resulted in vicariance-driven speciation. Here, we test the idea that the molecular clock dates, for the divergences of species whose geographical ranges were divided, should agree with the palaeomagnetic dates for the continental separations. Our analysis of recently available phylogenetic divergence dates of 42 pairs of vertebrate taxa, selected for their reduced ability to disperse, demonstrates that the divergence dates in phylogenetic trees of continent-bound terrestrial and freshwater vertebrates are consistent with the palaeomagnetic dates of continental separation.
Cawthorn, R.G., Maske, S., de Wit, M., Groves, D.I., Cassidy, K.
Mineralogical geochemical indicators of the formation conditions of apatite bearing carbonatites of the Arbarastakh Massif,Southern Yakutia (USSR).(Russian)
Abstract: The origin of dolomite that hosts the Bayan Obo REE-Nb-Fe deposit (57.4 Mt.@6% REE2O3, 2.16 [email protected]% Nb2O5, and >1500 Mt.@35% iron oxides) has been controversial for decades, but it is integral to understanding of the genesis of this giant deposit. In this study, based on the textures and in situ major and trace element composition of its carbonates, the dolomite was proved to be initially generated from magnesio-ferro?carbonatite melts. It subsequently experienced magmatic-hydrothermal alteration and recrystallization in a low strain environment, caused by calcio?carbonatitic fluids, with formation of finer-grained dolomite, interstitial calcite and increasing amounts of associated fluorocarbonates. Available stable isotope analyses indicate that the recrystallized ore-hosting dolomite has higher ?13C and ?18O ratios compared to its igneous coarse-grained precursor. Rayleigh fractionation during the recrystallization process, rather than crustal contamination, played a major role in the highly-variable stable isotope composition of carbonates in the dolomite. Low-T alteration increased variability with apparently random increases in ?18O within carbonates. The REE, Ba and Sr were added simultaneously with the elevated (La/Yb)cn from magnesio-ferro?carbonatite melts to calcio?carbonatitic fluids, and to carbonatite-derived aqueous fluids, through which extensive fluorine metasomatism and alkali alteration overlapped the recrystallization of the ore-hosting dolomite. Therefore, the multi-stage REE mineralization at Bayan Obo is closely related to metasomatism by calcio?carbonatitic fluids of previously-emplaced intrusive magnesio-ferro?carbonatite bodies during late evolution of the Bayan Obo carbonatite complex. Then, the ore-hosting dolomitic carbonatite was subjected to compressive tectonics during a Paleozoic subduction event, and suffered intense, largely brittle, deformation, which partially obscured the earlier recrystallization process. The complex, multi-stage evolution of the ore-hosting dolomite is responsible for the uniqueness, high grade and giant size of the Bayan Obo deposit, the world's largest single REE resource with million tonnes of REE oxides.
Gwalani, L.G., Rogers, K.A., Demeny, A., Groves, D.L., Ramsay, R., Beard, A., Downes, P.J., Eves, A.
The Yungul carbonatite dykes associated with the epithermal fluorite deposit at Speewah, Kimberley, Australia: carbon and oxygen isotope constraints origin
Mineralogy and Petrology, Vol. 98, 1-4, pp. 123-141.
Environmental Science and Technology, Vol. 49, 13, pp. 7589-7596.
Canada, Northwest Territories
Deposit - Diavik
Abstract: Mining-related perchlorate [ClO4(-)] in the receiving environment was investigated at the operating open-pit and underground Diavik diamond mine, Northwest Territories, Canada. Samples were collected over four years and ClO4(-) was measured in various mine waters, the 560 km(2) ultraoligotrophic receiving lake, background lake water and snow distal from the mine. Groundwaters from the underground mine had variable ClO4(-) concentrations, up to 157 ?g L(-1), and were typically an order of magnitude higher than concentrations in combined mine waters prior to treatment and discharge to the lake. Snow core samples had a mean ClO4(-) concentration of 0.021 ?g L(-1) (n=16). Snow and lake water Cl(-)/ClO4(-) ratios suggest evapoconcentration was not an important process affecting lake ClO4(-) concentrations. The multiyear mean ClO4(-) concentrations in the lake were 0.30 ?g L(-1) (n = 114) in open water and 0.24 ?g L(-1) (n = 107) under ice, much below the Canadian drinking water guideline of 6 ?g L(-1). Receiving lake concentrations of ClO4(-) generally decreased year over year and ClO4(-) was not likely [biogeo]chemically attenuated within the receiving lake. The discharge of treated mine water was shown to contribute mining-related ClO4(-) to the lake and the low concentrations after 12 years of mining were attributed to the large volume of the receiving lake.
Lecuyer, C., Gruau, G., Anhaeusser, C.R., Fourcade, S.
The origin of fluids and the effects of metamorphism on the primary chemical compositions of Barberton komatiites: new evidence from geochemistry, isotopes
Economic Geology Research Unit, University of the Witwatersrand, Inf. Circular No. 272, 32p
Lithos, doi.org/10.1016/ j.lithos.2020.105889 13p. Pdf
Canada, Northwest Territories
deposit - Diavik A-154
Abstract: Ambient Moho temperatures and lower crustal heat production are surprisingly poorly constrained in cratons. Here we address these problems using 15 lower crustal xenoliths from the Diavik A-154 kimberlite, Slave craton, Canada. Iron?magnesium exchange geothermometry on small biotite and amphibole inclusions in garnet indicates that the Slave craton lower crust was at a temperature of ?500 °C at the time of kimberlite eruption (~55 Ma). The ambient lower crustal temperature was likely lower than 500 °C because the thermometers record the closure temperature of diffusional Fe2+-Mg exchange between touching mineral pairs. New measurements of K, U and Th concentrations in the constituent minerals, together with xenolith modes, allow reconstruction of the heat-producing element (HPE) K, U, and Th budget of the Slave craton lower crust. Metasedimentary granulites have an average heat production of 0.29 ± 0.01 ?W/m3 (n = 3) whereas mafic granulites have an average heat production of 0.13 ± 0.03 ?W/m3 (n = 12). Our new data clearly show that plagioclase abundance in both lithologies has a major influence on overall lower crustal heat production, being an important reservoir of all three HPE. Combining the heat production of mafic and metasedimentary granulites in their observed 80:20 proportions results in an average heat production value for the Slave craton lower crust of 0.16 ± 0.03 ?W/m3. Using these heat production estimates, modeled Moho temperatures beneath Diavik of ~450-470 °C are broadly consistent with maximum lower crustal temperatures indicated by geothermometry. The low HPE contents predicted for cratonic lower crust must result in lower temperatures in the deep crust and mantle lithosphere, and in turn higher estimates for the thickness of mantle lithosphere. This effect becomes larger as the thickness of the low-HPE lower crustal layer increases. In the specific case of the central Slave craton, we find that model estimates of the diamond potential of the mantle lithosphere, as judged by the proportion of lithospheric mantle in the diamond stability field, are not strongly affected by small variations in lower crustal heat production and Moho temperature.
The thermochemical conditions of the Diavik lower crust: a kimberlite-hosted xenolith study.
2018 Yellowknife Geoscience Forum , p. 25-26. abstract
Canada, Northwest Territories
deposit - Diavik
Abstract: Thermochemical variables such as lower crustal heat production and Moho temperatures in cratonic regions offer critical insight in constraining the thermal and geodynamic evolution of the lithosphere. In this study, 15 lower crustal granulite xenoliths erupted via the A154N kimberlite at the Diavik mine in the NWT, Canada were studied to quantify the thermal properties of the local Moho and the effects of different heat production models on geotherm models. We quantitatively constrain the thermal properties of the local Moho and the effects of different heat production models on ancient Moho temperatures, the effects of crustal thickness on Moho temperatures, and potential lower crustal compositions. We evaluate the effect of these parameters on total lithospheric thickness estimates. In order to test the accuracy of deep crust thermal calculations, we estimated the ambient temperature of the lower crust at the time of kimberlite eruption through garnet-biotite Fe-Mg exchange geothermometry (Ferry & Spear, 1978). Rim compositions from touching garnet-biotite pairs were used in the calculations and yielded temperatures of 524 ± 77°C (n=20). These represent a maximum estimate of the ambient lower crustal temperature as the closure temperature of garnet-biotite Fe-Mg exchange between garnet and biotite may be higher than the ambient temperature. The primary objective of this study is to quantify lower crustal heat production and its effects on the thermal architecture of cratons. The concentrations of the main heat-producing elements (HPEs) U, Th, and K were quantified via LA-ICP-MS and EPMA in multiple mineral phases per xenolith. By combining these measurements with mineral modes, we derived reconstructed bulk-rock HPE concentrations that were utilized to calculate a range of lower crustal heat production values. This method is preferred over whole-rock analyses as 1) kimberlite is generally enriched in HPEs (Tappe et al. 2013) and can bias trace-element data for their xenoliths and 2) data on individual minerals allows for theoretical lower crustal compositions to be calculated on an idealized basis. A lower crust comprising exclusively mafic granulite (garnet, plagioclase, clinopyroxene ± orthopyroxene) provides a lower bound to heat production (0.07 ± 0.04 W/m3) whereas a lower crust made exclusively of high-grade metasedimentary rocks yields an upper bound (0.42 ± 0.08 W/m3). Both endmembers are present as xenoliths in the A154N kimberlite but mafic granulites predominate following the worldwide trend (Rudnick, 1992). We model the lower crust comprising 20% metasedimentary granulites and 80 % depleted mafic granulites, in accordance with the present xenolith collection. Using this preferred crustal model, we calculate an average heat production of 0.12 ± 0.05 W/m3) for the lower crust beneath Lac de Gras. Utilizing heat flow measurements (Russell et thickness estimates (Mareschal et al. 2004) in conjunction with these HPE determinations, the Moho temperature underlying A-154N can be calculated to be 502 ± 10°C. Using these values along with available mantle xenolith thermobaromtetry (Hasterok & Chapman, 2011) the geotherm is extrapolated to present a mantle potential temperature of 1365°C, at 200 km (FITPLOT, Mather et al, 2011).
Abstract: Lower crustal heat production is poorly constrained due to the relative inaccessibility of lower crustal samples and their inherent complexity. To obtain the requisite information, the current project conducts spatially resolved geochemical analyses on minerals in 15 lower crustal xenoliths erupted via the Diavik A-154 kimberlite of the Northwest Territories, Canada. The aims are to: 1) conduct geothermometric measurements on lower crustal minerals, 2) construct a heatproducing element budget of the lower crust of the Slave craton, and 3) test the validity of these measurements in a parameter space relevant to geodynamic modeling and diamond exploration. The Diavik lower crustal xenolith suite comprises two main lithologies, mafic granulite (garnet-plagioclase-clinopyroxene ± orthopyroxene) and metasedimentary granulite (garnetplagioclase- orthopyroxene ± quartz ± K-feldspar ± kyanite), which are present in proportions of approximately 80:20, respectively. Application of mineral-pair, iron-magnesium exchange geothermometers (garnet-biotite, garnet-amphibole, and garnet-clinopyroxene) to these xenoliths indicates that the lower crust was at a maximum temperature of roughly 500 °C at the time of kimberlite eruption (~ 55 Ma). The actual temperature of the lower crust is likely lower than 500 °C as the geothermometers probably record the closure temperature of diffusional Fe2+-Mg exchange between touching mineral pairs rather than the ambient temperature of the rocks prior to their entrainment in the kimberlite magma. Heat-producing element (HPE) concentration measurements show that the lower crustal heat production of the Slave craton is likely 0.14 ± 0.02 ?W/m3, which is lower than most values in the literature but broadly comparable to some geophysical estimates. This estimate is the result of (20:80) bimodal mixing of idealized lower crustal endmembers: a metasedimentary lower crust (0.37 ± 0.06 ?W/m3) and a mafic lower crust (0.08 ± 0.01 ?W/m3). These endmembers were iii calculated via a reconstructed bulk rock calculation utilizing trace element concentrations of constituent lower crustal minerals and idealized lithologies from the lower crustal xenoliths. Using these heat production estimates and other crustal parameters such as continental heat flux, mantle heat flux, crustal thickness, and crustal thermal conductivity, I modeled a Moho temperature for the Slave craton of 425 °C, which is consistent with maximum lower crustal temperature estimate given by geothermometry. Adjusting the lower crustal heat production in the geotherm modeling program FITPLOT changes the temperature of the Moho in a similar fashion to the calculated models; however, the diamond propensity of the mantle lithosphere (partially a function of Moho temperature and heat production) does not appear to be strongly affected by a changing Moho temperature and is more strongly controlled by the conditions of the mantle P-T array.
Aravanis, T., Chen, J., Fuechsle, M., Grujic, M., Johnston, P., Kok, Y., Magaraggia, R., Mann, A., Mann, L., McIntoshm S., Rheinberger, G., Saxey, D., Smalley, M., van Kann, F., Walker, G., Winterflood, J.
Abstract: The minerals exploration industry’s demand for a highly precise airborne gravity gradiometer has driven development of the VK1TM Airborne Gravity Gradiometer, a collaborative effort by Rio Tinto and the University of Western Australia. VK1TM aims to provide gravity gradient data with lower uncertainty and higher spatial resolution than current commercial systems. In the recent years of VK1TM development, there have been significant improvements in hardware, signal processing and data processing which have combined to result in a complete AGG system that is approaching competitive survey-ready status. This paper focuses on recent improvements. Milestone-achieving data from recent lab-based and moving-platform trials will be presented and discussed, along with details of some advanced data processing techniques that are required to make the most use of the data.
Abstract: We examine the partial melting and the cooling history of a ~5?km section of mantle lithosphere preserved in the Donjek massif, part of a Permian ophiolite in the northern Cordillera of Yukon, Canada. The mantle rocks are depleted spinel harzburgite containing <3% clinopyroxene displaying steep rare-earth element (REE) chondrite-normalized profiles and low (Gd/Yb)n (0.02 to 0.07) compared to most other ophiolites. The REE patterns of clinopyroxene can be modeled as 16-20% partial melts of typical depleted mid-ocean ridge (MOR) mantle. The REE exchange between coexisting ortho- and clinopyroxene preserves temperatures (TREE) of 1150-1360?°C, some of the highest values recorded in ophiolites and abyssal peridotites, and show a positive correlation with CaMg exchange (solvus) temperatures (TBKN) of 900-970?°C. The harzburgite represents lithosphere formed at an initial melting temperature of ~ 1350?°C that cooled at rate of 10?1 to 10?4?°C/year as deduced by TREE values with cation diffusion and grain size data. The TREE temperatures and cooling rates for the Donjek massif show a regular systematic variation with depth from the crust-mantle transition along a trend similar to the Samail ophiolite of Oman, consistent with conductive heat transfer beneath a cool lower crust. High near-solidus temperatures and the cooling rates in the massif were a consequence of rapid obduction against oceanic crust along either a transform or low angle detachment soon after melt extraction. Final emplacement of the ophiolite as klippen on underlying continental crust occurred ~ 40?m.y. later.
Uranium-lead-zirconium and Rubidium-strontium Mineral Dating of eclogites and Their Country Rocks. Example: Munchberg Gneiss Massif, Northeast bavaria.
Earth and Planetary Science Letters, Vol. 42, PP. 35-44.
Geophysical Research Letters, Vol. 46, 15, pp. 8731-8740.
Mantle
bridgmanite
Abstract: The lower mantle encompasses the largest region of the Earth's interior and is mainly composed of the perovskite?structured mineral (Mg,Fe,Al)(Al,Si)O3 bridgmanite. Its properties, therefore, control both the diffusive transport of elements and solid state flow in the lower mantle, which will be strongly influenced by point defects. We have identified and quantified defects in bridgmanite that arise from the replacement of silicon by aluminum and result in the creation of a vacant oxygen site. These oxygen defects are also found to form clusters in the structure, which in other perovskite structured minerals have been shown to strongly affect physical properties. As defect formation and ordering is dependent on composition and pressure, strong variations in physical properties may be expected within the upper 300 km of the lower mantle.
Structural geology and geochronology of subduction complexes along the margin of Gondwanaland: new dat a from the Antarctic Peninsula and southernmostAndes
Geological Society of America (GSA) Bulletin, Vol. 104, No. 11, November pp. 1497-1514
Abstract: Data- and knowledge-driven techniques are used to produce regional Au prospectivity maps of a portion of Melville Peninsula, Northern Canada using geophysical and geochemical data. These basic datasets typically exist for large portions of Canada's North and are suitable for a "greenfields" exploration programme. The data-driven method involves the use of the Random Forest (RF) supervised classifier, a relatively new technique that has recently been applied to mineral potential modelling while the knowledge-driven technique makes use of weighted-index overlay, commonly used in GIS spatial modelling studies. We use the location of known Au occurrences to train the RF classifier and calculate the signature of Au occurrences as a group from non-occurrences using the basic geoscience dataset. The RF classification outperformed the knowledge-based model with respect to prediction of the known Au occurrences. The geochemical data in general were more predictive of the known Au occurrences than the geophysical data. A data-driven approach such as RF for the production of regional Au prospectivity maps is recommended provided that a sufficient number of training areas (known Au occurrences) exist.
Structural emplacement of RADARSAT-1 principal component imagery and its potential application to kimberlitic exploration in the Buffalo Head Hills area.
Alberta Energy and Utilities Board and Alberta Geological Survey, Report 2001-03, 47p.
Classification of distinct eruptive phases of the Diamondiferous Star kimberlite, Sask. based on statistical treatment whole rock geochemical analyses.
Classification of distinct eruptive phases of the Diamondiferous Star kimberlite, Saskatchewan, Canada based on statistical treatment of whole rock geochemical analyses.
Grunsky, E.C., Kjarsgaard, B.A., Kurzlaukis, S., Seller, M.
The use of statistical methods applied to multi-element geochemistry for phase discrimination in kimberlites - examples from the Star and Whiskey kimberlites.
GAC/MAC joint annual meeting, Vol. 36, p. 1. abstract
Integration of structural, gravity and magnetic dat a using the weights of evidence method as a tool for kimberlite exploration in the Buffalo Head Hills, northern central Alberta Canada
Geochemistry: Exploration, Environment, Analysis, http://doi.org/10.1144/ geochem2019-031 16p. Pdf
Canada, Nunavut, Australia
geochemistry
Abstract: Multi-element geochemical surveys of rocks, soils, stream/lake/floodplain sediments and regolith are typically carried out at continental, regional and local scales. The chemistry of these materials is defined by their primary mineral assemblages and their subsequent modification by comminution and weathering. Modern geochemical datasets represent a multi-dimensional geochemical space that can be studied using multivariate statistical methods from which patterns reflecting geochemical/geological processes are described (process discovery). These patterns form the basis from which probabilistic predictive maps are created (process validation). Processing geochemical survey data requires a systematic approach to effectively interpret the multi-dimensional data in a meaningful way. Problems that are typically associated with geochemical data include closure, missing values, censoring, merging, levelling different datasets and adequate spatial sample design. Recent developments in advanced multivariate analytics, geospatial analysis and mapping provide an effective framework to analyse and interpret geochemical datasets. Geochemical and geological processes can often be recognized through the use of data discovery procedures such as the application of principal component analysis. Classification and predictive procedures can be used to confirm lithological variability, alteration and mineralization. Geochemical survey data of lake/till sediments from Canada and of floodplain sediments from Australia show that predictive maps of bedrock and regolith processes can be generated. Upscaling a multivariate statistics-based prospectivity analysis for arc-related Cu-Au mineralization from a regional survey in the southern Thomson Orogen in Australia to the continental scale, reveals a number of regions with a similar (or stronger) multivariate response and hence potentially similar (or higher) mineral potential throughout Australia.
Grunsky, EC., Kjarsgaard, B.A., Kurzlaukis, S., Seller, M., Knight, R., Moroz, M.
Classification of whole rock geochemistry based on statistical treatment of whole rock geochemical analyses and portable XRF analyses at the Attawapiskat kimberlite field of Ontario.
Geological Survey of Canada, Scientific Presentation 15,, 1 sheet 10.4095/292446
Lithospheric structure, evolution and diamond prospectivity of the Rehoboth Terrane and western Kaapvaal Craton, southern Africa: constraints from broadband
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.
Exploration and discovery of the Chidliak kimberlite province, Baffin Island, Nunavut: Canada's newest diamond district.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 209-227.
Tracing kimberlitic indicators to their kimberlite source at Chidliak, Nunavut, Canada, re-visited: the unexpected accuracy of a simplified Mahalanobis-distance approach.
Abstract: The evaluation of advanced stage diamond projects is materially constrained by the time and cost involved in bulk sampling (or trial mining) campaigns that serve to demonstrate the grade and value of (macro)diamonds in a deposit. However, comparatively inexpensive assay data for (micro)diamonds may also be used to estimate (macro)diamond grade, by way of geostatistical extrapolation or modelling of total diamond content curves and diamond size frequency distributions. Geoscientists at SRK (Canada) Inc. compiled publicly available technical disclosure related to micro/macrodiamond sampling campaigns completed since early-2004 and developed a model-independent benchmarking approach to estimate in-situ (macro)diamond grades based on microdiamond assay results - a one-page summary of that work is available here: here Our ongoing investigation of microdiamond data has developed a focus on the occurrence of "low-count" and "high-count" microdiamond assay results. In this VKC talk we contrast "normal"-count with "low"-count results (Snap Lake vs FALC and others) and appeal to diamond-bearing mantle xenoliths to explain occasional "high"-count results. Real-world examples are used to cover topics like microdiamond sample sizes and (attained) resolution thresholds in the range 1 part in 1010 to 1012. The talk closes out with an examination of the microdiamond dataset for the Tuwawi pipe (northern Baffin Island).
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.
Cr Ca and related characteristics of peridotitic garnets from the central Slave and central Kaapvaal craton roots, with implications - carbon in peridotite
GAC Annual Meeting Halifax May 15-19, Abstract 1p.
U-Pb geochronology and Sr/Nd isotope compositions of groundmass perovskite from the newly discovered Jurassic Chidliak kimberlite field, Baffin Island, Canada.
Earth and Planetary Science Letters, Vol. 415, April pp. 183-189.
Mineralogy and Petrology, June 14, DOI:10.1007/ s0710-018 -0578-7, 12p.
Canada, Nunavut
deposit - Chidliak
Abstract: Exploration for diamond-bearing kimberlites in the Chidliak project area by Peregrine Diamonds has generated a grid-like till sampling pattern across four discrete areas of interest totalling 402 km2 that is densely populated with research-grade compositional data for 10,743 mantle-derived Cr-pyrope garnets. The available dataset is well suited to statistical analysis, in part due to the relatively unbiased spatial coverage. Previous workers showed empirically that the TiO2 and Mn thermometry (Ti-TMn) attributes of Cr-pyrope populations at the Chidliak project may serve as source-specific “fingerprints”. In this work, we employ a simplified version of the multivariate Mahalanobis distance technique to formally examine the variability of, and differences between, Ti-TMn attributes of Cr-pyrope subpopulations recovered from a Laurentide-age glaciated terrain that also contains 30 known kimberlites within the four areas of interest. We show the simplified Mahalanobis distance approach enables accurate discrimination of Cr-pyrope subpopulations with subtly to distinctly different Ti-TMn attributes, and permits proper demarcation of their respective kimberlite source(s), specifically in areas with straightforward glacial histories. Redistribution and blending of Cr-pyrope subpopulations from known kimberlite sources is also observed, and typifies areas at Chidliak with complex late-glacial histories. Our results support <1 km horizontal scale subtle to obvious variability in the proportions of TiO2-rich and high-temperature (> 1100 °C) Cr-pyropes between closely spaced kimberlite source(s) and also between physically adjacent magma batches within single kimberlite pipes. The local scale variability is attributed to protokimberlite fluid or melt interacting with, and metasomatizing discrete conduits within, the ambient diamond-facies peridotitic mantle at times closely preceding eruption of kimberlite magma batches at Chidliak.
Abstract: In this experimental study we obtained new mineral/melt (DF = cmineral/cmelt) partitioning data for fluorine in a bimineralic hydrous eclogite under Earth's upper mantle conditions (4-6 GPa, 1460-1550 °C). Omphacitic clinopyroxene displays mineral/melt partition coefficients between DF = 0.056 ± 0.005 and DF = 0.074 ± 0.001. Garnet partition coefficients are consistently lower with an average partition coefficient of DF = 0.016 ± 0.003. We found that omphacitic clinopyroxene is the dominant nominally fluorine-free phase in subducted oceanic crust and hence omphacite is expected to be the major fluorine carrier during subduction of crust into the deeper mantle. Together with previously obtained partitioning data we propose that the oceanic crust can host more fluorine per mass unit than the underlying depleted oceanic mantle. If the majority of entrained fluorine is recycled into Earth's transition zone it is possible that the fluorine is either incorporated into high-pressure transition zone phases or released during high-pressure phase transformations and forming fluorine-rich small degree partial melts. Both scenarios are supported by elevated fluorine concentration in ocean island basalts, kimberlites, and lamproites. Combining the fluorine partitioning data with water partitioning data yields a plausible process to generate lamproitic magmas with a high F/H2O ratio. The enrichment of fluorine relative to H2O is triggered by multiple episodes of small degree melting that deplete the residual more in H2O than in fluorine, caused by the approximately three times smaller mineral-melt partition coefficients of H2O.
Abstract: Metasomatic processes are responsible for many of the heterogeneities found in the upper mantle. To better understand the metasomatism in the lithospheric mantle and to illustrate the differences between metasomatism caused by hydrous silicate and carbonate-rich melts, we performed various interaction experiments: (1) Reactions between hydrous eclogite-derived melts and peridotite at 2.2-2.5 GPa and 900-1000 °C reproduce the metasomatism in the mantle wedge above subduction zones. (2) Reactions between carbonate-rich melts and peridotite at 2.5 GPa and 1050-1000 °C, and at 6 GPa and 1200-1250 °C simulate metasomatism of carbonatite and ultramafic silicate-carbonate melts in different regions of cratonic lithosphere. Our experimental results show that partial melting of hydrous eclogite produces hydrous Si- and Al-rich melts that react with peridotite and form bi-mineralic assemblages of Al-rich orthopyroxene and Mg-rich amphibole. We also found that carbonate-rich melts with different compositions react with peridotite and form new metasomatic wehrlitic mineral assemblages. Metasomatic reactions caused by Ca-rich carbonatite melt consume the primary peridotite and produce large amounts of metasomatic clinopyroxene; on the other hand, metasomatism caused by ultramafic silicate-carbonate melts produces less clinopyroxene. Furthermore, our experiments show that ultramafic silicate-carbonate melts react strongly with peridotite and cause crystallization of large amounts of metasomatic Fe-Ti oxides. The reactions of metasomatic melts with peridotite also change the melt composition. For instance, if the carbonatite melt is not entirely consumed during the metasomatic reactions, its melt composition may change dramatically, generating an alkali-rich carbonated silicate melt that is similar in composition to type I kimberlites.
Abstract: Carbonatites are rare igneous carbonate-rich rocks. Most carbonatites contain a large number of accessory oxide, sulfide, and silicate minerals. Baddeleyite (ZrO2) and zircon (ZrSiO4) are common accessory minerals in carbonatites and because these minerals host high concentrations of U and Th, they are often used to determine the ages of formation of the carbonatite. In an experimental study, we constrain the stability fields of baddeleyite and zircon in Ca-rich carbonate melts with different silica concentrations. Our results show that SiO2-free and low silica carbonate melts crystallize baddeleyite, whereas zircon only crystallizes in melts with higher concentration of SiO2. We also find that the zirconsilicate baghdadite (Ca3ZrSi2O9) crystallizes in intermediate compositions. Our experiments indicate that zircon may not be a primary mineral in a low-silica carbonatite melt and care must be taken when interpreting zircon ages from low-silica carbonatite rocks.
Earth and Planteray Science Letters, Vol. 482, pp. 236-244.
Mantle
transition zone
Abstract: The Earth's mantle contains significant amounts of volatile elements, such as hydrogen (H), carbon (C) and the halogens fluorine (F), chlorine (Cl) and bromine (Br) and iodine (I). There is a wealth of knowledge about the global cycling of H and C, but there is only scant data on the concentrations of halogens in different Earth reservoirs and on the behavior of halogens during recycling in subduction zones. Here we focus on the storage potential of F in deeper parts of the Earth's mantle. The transition zone is a region in the Earth's mantle (410-660 km) known for its high water storage capacity, as the high pressure polymorphs of olivine, wadsleyite and ringwoodite are known to be able to incorporate several per-cent of water. In order to assess potential fractionation between water and F in the transition zone of the Earth's mantle, we set out to investigate the storage capacity of the halogen F in wadsleyite and olivine at transition zone conditions. Experiments were performed in a simplified mantle composition at temperatures from 1400?°C to 1900?°C and pressures from 17 up to 21 GPa in a multi anvil apparatus. The results show that F can shift the olivine-wadsleyite transition towards higher pressure. We find that F has an opposing effect to water, the latter of which extends the transition zone towards lower pressure. Moreover, the F storage capacity of wadsleyite is significantly lower than previously anticipated. F concentrations in wadsleyite range from to independent of temperature or pressure. The F storage capacity in wadsleyite is even lower than the F storage capacity of forsterite under transition zone conditions, and the latter can incorporate F under these conditions. Based on our data we find that the transition zone cannot be a reservoir for F as it is assumed to be for water. Furthermore, we argue that during subduction of a volatile-bearing slab, fractionation of water from F will occur, where water enters preferentially the transition zone and F remains in the peridotite of the lowermost upper mantle.
Abstract: Saline (Cl-rich) fluids potentially play an important role as metasomatic agents in the lithospheric mantle. Natural evidence for deep saline fluids exists as inclusions within diamonds and within groundmass minerals in kimberlites. Previous experimental studies have investigated melting relations in the chloride-carbonate-silicate system at upper mantle conditions, but a systematic experimental study of how saline fluids react with the lithospheric mantle is still lacking. Here, we present high-pressure, high-temperature (HP-HT) reaction experiments between a saline fluid and different mantle rocks (lherzolite, harzburgite, eclogite) at conditions corresponding to the lower cratonic lithosphere. Experiments were performed over a P-T range of 3-6 GPa and 1050-1300 °C using a multi-anvil apparatus. Preliminary results show that the interaction between saline fluid and mantle rocks is very reactive, compared to reactions with silico-carbonate melts. The reaction between saline fluid and lherzolite at 4 GPa and 1200 °C leads to extensive melting. The restite consists mainly of olivine and garnet, whereas pyroxenes are only observed as rare inclusions within garnet. In contrast, reactions between saline fluid and eclogite at 4 GPa and 1200 °C also lead to melting, but the melt is more enriched in Si. The restite consists exclusively of garnet. The experimental results demonstrate how saline fluids react with different components of the lithospheric mantle and support evolutionary models of high density fluids within diamonds.
Abstract: Diamond etching in high-temperature ambient-pressure experiments has been performed aimed to assess possible postimpact effects on diamonds in impact craters, for the case of the Popigai crater in Yakutia (Russia). The experiments with different etchants, including various combinations of silicate melts, air, and inert gases, demonstrated the diversity of microstructures on {111} diamond faces: negative or positive trigons, as well as hexagonal, round, or irregularly shaped etch pits and striation. The surface features obtained after etching experiments with kimberlitic diamonds are similar to those observed on natural impact diamonds with some difference due to the origin of the latter as a result of a martensitic transformation of graphite in target rocks. Extrapolated to natural impact diamonds, the experimental results lead to several inferences: (1) Diamond crystals experienced natural oxidation and surface graphitization during the pressure decrease after the impact event, while the molten target rocks remained at high temperatures. (2) Natural etching of diamonds in silicate melts is possible in a large range of oxidation states controlled by O2 diffusion. (3) Impact diamonds near the surface of molten target rocks oxidized at the highest rates, whereas those within the melt were shielded from the oxidizing agents and remained unchanged.
Doklady Earth Sciences, Vol. 489, 2, pp. 1449-1452 .pdf
Russia
diamond morphology, CLIPPIR
Abstract: The primary results are presented on the dissolution of plane-faced diamond crystals of octahedral habit in a Fe-Ni-S melt under 3.5 GPa and 1400°C. It was found that the dissolution resulted in the transformation of plane-faced into curve-faced individuals of morphological features characteristic for kimberlite diamonds. It was concluded that the diamond forms as such might have formed in reduced domains of the Earth’s mantle before becoming involved in the kimberlite magma.
Africa, South Africa, China, United States, Canada, South America
diamond inclusions
Abstract: We present the first evidence for inclusions of ice-VII in diamonds from southern Africa, China, North- and South-America [1]. Combining synchrotron X-ray diffraction, - X-ray fluorescence and IR spectroscopy, we show the presence of ice-VII as inclusions in diamonds that have formed at depth > 410 km to about 800 km in the Earth's mantle. What is now crystalline ice-VII, a high pressure polymorph of water-ice, was component of an aqueous fluid entrapped in the diamonds that were growing in the deep mantle. Because of the confinement by the host diamonds, the inclusions retain high pressures. The same holds for inclusions of magnesian calcite, halite, and ilmenite found in the same diamond specimens. These inclusions reflect the presence of aqueous and carbonaceous fluids in the mantle transition zone and the shallow lower mantle. Using their current residual pressures and the equations of state, we can reconstruct their recovery paths [2,3]. Further, we can use the intersection of modelled recovery paths to better constrain the encapsulation pressure and temperature of these inclusions in diamonds.
Abstract: The North China Craton (NCC) is an atypical ancient landmass that suffered lithospheric destruction. Previous studies suggest that the eastern part of the lithospheric mantle of the NCC has been thinned and modified in the Mesozoic. However, the initiation time and mechanism of the destruction remain controversial. Mafic magmatismcould provide a unique windowinto deciphering the lithospheric mantle composition and its evolution. Here we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan, located in the southeastern margin of the NCC. Zircon U-Pb dating yielded an average age of 174 ± 14 Ma, representing the first reported Jurassic basalts in the eastern NCC. The Lan'gan basalts are enriched in light rare earth elements (LREE) and large ion lithosphile elements (LILE). Sr-Nd-Pb-Hf isotopic compositions (87Sr/86Sr(t) = 0.70646-0.70925, ?Nd(t) = ?2.1 to ?4.9, 206Pb/204Pb(t) = 17.14-18.12, 207Pb/204Pb(t) = 15.28-15.61, 208Pb/204Pb(t) = 37.82-38.67, and zircon ?Hf(t) = ?17 to ?21) are enriched compared to depleted mantle. The presence of primary amphibole indicates that the magma source of the basalts was water enriched. These observations suggest that, the lithospheric mantle of the eastern NCC were significantly refertilized, likely by slab derived fluids/melts fromthe Paleo-Pacific subduction. Owing to the Paleo-Pacific subduction, the lithosphericmantle of the eastern NCCwere reduced in viscosity and intensity, and finally promoted partialmelting in a limited scale to generate the investigated alkaline basalts. Hence, the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick, and that large-scale destruction had not initiated in the early Jurassic beneath this region.
Abstract: The North China Craton (NCC) is an atypical ancient landmass that suffered lithospheric destruction. Previous studies suggest that the eastern part of the lithospheric mantle of the NCC has been thinned and refertilized in the Mesozoic. However, the initiation time and mechanism of the destruction remain controversial. Mafic magmatism could provide a unique window into deciphering the lithospheric mantle composition and its evolution. Here we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan, located in the southeastern margin of the NCC. Zircon UPb dating yielded an average age of 174?±?14?Ma, representing the first reported Jurassic basalts in the eastern NCC. The Lan'gan basalts are enriched in light rare earth elements (LREE) and large ion lithosphile elements (LILE). Sr-Nd-Pb-Hf isotopic compositions (87Sr/86Sr(t)?=?0.70646-0.70925, ?Nd(t)?=??2.1 to ?4.9, 206Pb/204Pb(t)?=?17.14-18.12, 207Pb/204Pb(t)?=?15.28-15.61, 208Pb/204Pb(t)?=?37.82-38.67, and zircon ?Hf(t)?=??17 to ?21) are slightly enriched compared to depleted mantle. The presence of primary amphibole indicates that the magma source of the basalts was water enriched. These observations suggest that, the lithospheric mantle of the eastern NCC were significantly refertilized, likely by slab derived fluids/melts from the Paleo-Pacific subduction. Owing to the Paleo-Pacific subduction, the lithospheric mantle of the eastern NCC were reduced in viscosity and intensity, and finally promoted partial melting in a limited scale to generate the investigated alkaline basalts. Hence, the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick, and that large-scale destruction had not initiated in the early Jurassic beneath this region.
Contributions to Mineralogy and Petrology, Vol. 171, 7, 14p.
China
Geothermometry
Abstract: The large equilibrium Mg isotope fractionation between clinopyroxene and garnet observed in eclogites makes it a potential high-precision geothermometer, but calibration of this thermometer by natural samples is still limited. Here, we report Mg isotopic compositions of eclogite whole rocks as well as Mg and O isotopic compositions of clinopyroxene and garnet separates from 16 eclogites that formed at different temperatures from the Dabie orogen, China. The whole-rock ?26Mg values vary from ?1.20 to +0.10 ‰. Among them, 11 samples display limited ?26Mg variations from ?0.36 to ?0.17 ‰, similar to those of their protoliths. The mineral separates exhibit very different ?26Mg values, from ?0.39 to +0.39 ‰ for clinopyroxenes and from ?1.94 to ?0.81 ‰ for garnets. The clinopyroxene -garnet Mg isotope fractionation (?26Mgclinopyroxene -garnet = ?26Mgclinopyroxene -?26Mggarnet) varies from 1.05 to 2.15 ‰. The clinopyroxene -garnet O isotope fractionation (?18Oclinopyroxene -garnet = ?18Oclinopyroxene -?18Ogarnet) varies from ?1.01 to +0.98 ‰. Equilibrium Mg isotope fractionation between clinopyroxene and garnet in the investigated samples is selected based on both the ?26Mgclinopyroxene versus ?26Mggarnet plot and the state of O isotope equilibrium between clinopyroxene and garnet. The equilibrium ?26Mgclinopyroxene -garnet and corresponding temperature data obtained in this study, together with those available so far in literatures for natural eclogites, are used to calibrate the clinopyroxene -garnet Mg isotope thermometer. This yields a function of ?26Mgclinopyroxene -garnet = (0.99 ± 0.06) × 106/T 2, where T is temperature in Kelvin. The refined function not only provides the best empirically calibrated clinopyroxene -garnet Mg isotope thermometer for precise constraints of temperatures of clinopyroxene- and garnet-bearing rocks, but also has potential applications in high-temperature Mg isotope geochemistry.
Diamond & Related Materials, Vol. 89, pp. 322-329.
Russia, India
deposit - Mir, Panna
Abstract: The optical features of milky type IaB diamonds were studied systematically by non-destructive approaches including FTIR, photoluminescence (PL), and cathodoluminescence (CL) spectroscopy. From 97 type IaB diamonds ranging from 0.2?ct to ~100?ct submitted to GIA's New York laboratory for screening, we found that all the milky type IaB diamonds consistently displayed the hydrogen-related defect with an absorption line at 3107?cm?1, and ~96% of them were accompanied by a weaker line at 3085.4?cm?1, which is undetectable in most non-milky diamonds. Most of the diamond samples display no platelet defect or a very tiny residual platelet peak with a position at larger wavenumber in milky diamonds than in non-milky counterparts. “Amber center” with a weak but sharp absorption line at 4168.8?cm?1 has been observed in ~73% of the milky diamonds and ~24% of the non-milky ones. Photoluminescence (PL) results reveal that several defects with ZPLs at 490.7, 536, 575.9 and 612.4?nm are more common in milky type IaB diamonds than non-milky ones. A zero-phonon line (ZPL) at 536?nm has been confirmed by PL mapping and CL spectra as a product of plastic deformation, and it might be linked with the H4 center (N4V2 defect). A ZPL at 490.7?nm could be related to a nitrogen-vacancy complex. The defects present more often in milky IaB diamonds are generally associated with plastic deformation. The presence of a hydrogen-related peak at 3085.4?cm?1 and a 536?nm center would help effectively distinguish IaB diamonds with subtle milky areas from their non-milky counterparts.
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.
Abstract: Mantle metasomatism by percolating melts/fluids can significantly modify the geochemical and mineralogical compositions of the sub-continental lithospheric mantle (SCLM). We present a detailed study of water contents and Li concentrations and isotopic compositions in mantle minerals from a suite of peridotite xenoliths entrained by a Cenozoic Strombolian volcano in the southern French Massif Central (FMC). Wide ranging clinopyroxene trace element distributions (e.g., (La/Yb)N from 0.25 to 22.21; Ti/Eu ratios from 453 to 4892) suggest that the SCLM has undergone metasomatism by carbonatitic melts/fluids or melts/fluids related to subducted materials. Two amphibole-bearing samples exhibit depletion of light rare earth elements (LREE; (La/Yb)N = 0.26 and 0.30, respectively) in amphiboles, similar to that in co-existing clinopyroxenes; these samples indicate that amphiboles grew during a separate modally metasomatic event predating the cryptic metasomatism accounting for LREE enrichment and negative HFSE anomalies in other samples. Mineral Li concentrations are similar to those in the normal mantle, with inter-mineral Li partitioning nearly equilibrated and intragranular Li distributions nearly homogeneous. However, negative ?7Li values of pyroxenes in some samples (as low as ? 8.8‰ in clinopyroxene of sample MC38) can be attributed to diffusive exchange with a small-volume melt of moderate Li concentration and light Li isotopic composition, originally associated with a recycled component. Preservation of the currently observed large inter-mineral Li isotopic variations indicates that melt percolation occurred shortly before entrainment of the peridotite xenoliths by the host magma. Mineral water contents vary from 41 to 428 ppm in clinopyroxenes and from 28 to 152 ppm in orthopyroxenes, and their roughly negative co-variation with coexisting olivine Fo contents imply that partial melting was the main control over mineral water content variations in most samples. Varied water contents in LREE-enriched metasomatized samples indicate the involvement of metasomatic agents of different origins. The aqueous agent responsible for generation of amphiboles in two samples did not produce a notable increase in the water contents of coexisting nominally anhydrous minerals.
Ultra high pressure metamorphism and multistage exhumation of eclogite of the Luotian dome, North Dabie Complex Zone: evidence from mineral inclusions -texture
Journal of Asian Earth Sciences, Vol. 42, 4, pp. 607-617.
Liu, Y-C., Deng, L-P., Gu, X-F., Groppo, C., Rolfo, F.
Application of Ti in zircon and Zr in rutile thermometers to constrain high temperature metamorphism in eclogites from the Dabie Orogen, central China.
European Journal of Mineralogy, Vol. 31, pp. 889-803. pdf
China
eclogite
Abstract: A combined study of detailed petrographic observation, mineral chemistry analysis and phase equilibrium modeling indicates that the high-temperature eclogites from the Dabie orogen, central China, experienced two episodes of anatexis: the first is phengite dehydration melting during the exhumation of deeply subducted slices, and the second is heating melting related to the post-orogenic collapse. Petrographic evidence and clues of the anatectic events include biotite + plagioclase + garnet ± amphibole intergrowth in matrix and biotite + plagioclase intergrowth within amphibole porphyroblast. Pressure-temperature (P-T) pseudosection and modal variation diagram indicate that the biotite + plagioclase + garnet ± amphibole in matrix was formed by the reactions phengite + clinopyroxene + quartz = melt + sanidine + garnet + plagioclase and later melt + sanidine + garnet = biotite + plagioclase, while the biotite + plagioclase intergrowths within poikiloblastic amphibole were formed by the reaction amphibole + muscovite + epidote = biotite + plagioclase + melt. In addition, the combination of petrological observations and P-T estimates suggests that the first melting event occurred at the late Triassic, while the second is related to the early Cretaceous mountain-root removal and subsequent asthenospheric upwelling and heat input. As the P-T paths of high-temperature/ultrahigh-pressure rocks have high probabilities to cross-cut phengite-melting curves, phengite melting during decompression may be a common process in these rocks. Moreover, the coexistence of multiple episodes of anatexis in a single tectonic slice suggests caution when identifying and dating partial melting in high-temperature/(ultra)high-pressure rocks.
Geophysical Journal International, Vol. 202, pp. 142-162.
Canada, Alberta
Geophysics - seismics
Abstract: We analyse continuous recordings from 23 broadband seismic stations near Alberta, the southwestern sector of the Western Canada Sedimentary Basin. Noise correlation tomo-graphy based on vertical-component seismograms reveals below-average shear velocities at shallow and middle crustal depths in central Alberta, spanning across Proterozoic accreted terranes and Archean microcontinents. This observation likely results from extensive plate convergence and crustal melting during the Proterozoic eon. The overall correlation between the crustal velocities and presumed basement domains is lower than expected, however. In the lower crust, the main pattern of shear velocities is relatively concordant with the reported domain boundaries and key Precambrian structures appear to be intact. The shear velocities beneath the Loverna Block, the largest constituent of the Hearne craton, are 10?per?cent higher than the regional average. This prominent northeast striking seismic anomaly is moderately correlated with the regional heat flow and potentially represents the remnant core of the Archean Hearne province. The associated high velocities extend into the western part of the Medicine Hat Block, a possible Archean microcontinent with a debatable origin, and contribute to a strong east-west structural gradient in the lower crust. The presence and the continuity of this anomalous structure imply extensive communications among the various basement domains in southern Alberta during the assembly of the North American continent.
Journal of Geophysical Research, Vol. 120, 10, pp. 6993-7012.
Canada, Alberta
Geophysics - seismics LVZs
Abstract: Crustal low-velocity zones (LVZs) have been reported in active orogens such as the Himalayas and the Andes but rarely in stable cratonic regions. In this study, we provide compelling evidence for a significant midcrustal LVZ beneath eastern-central Alberta, an integral part of the Precambrian Canadian Shield covered by thick Phanerozoic sedimentary deposits. This 200?km wide, over 10?km thick midcrustal LVZ is well resolved by shear velocity inversions using P-to-S receiver functions from more than 4600 earthquakes. It is generally overlain by a high-velocity upper crust in the depth range of 8-15?km, especially in western-central Alberta, which coincides with the previously documented Winagami reflection sequence. We interpret the LVZ to be of granitic composition, potentially in connection with the crystallization of partially molten crust during the Paleoproterozoic eon. In addition to the Precambrian tectonic history of western Laurentia, which featured plate convergence conducive to crustal melting, our crustal model is further supported by (1) a moderate spatial correlation between the LVZ and heat flow, and (2) shear velocities consistent with that of granite. The well preserved Winagami reflection sequence and the LVZ are potential evidence of distinct episodes of magmatism and crust modification in the Precambrian basement of the Western Canada Sedimentary Basin. The existence of a broad crustal LVZ suggests extensive subduction, orogenesis, and crustal melting during the Precambrian assembly of the North American craton.
Abstract: The Western Canada Sedimentary Basin marks a boundary zone between the Precambrian North American craton and the Phanerozoic Cordillera. Its crystalline basement has documented more than 3 billion years of evolution history of western Laurentia. Here we conduct a high?resolution survey of the mantle P and S wave velocities using finite?frequency tomography. Our models show pronounced eastward increases of 4% P and 6% S wave velocities beneath the foreland region, which define a sharp seismic Cordillera?Craton boundary. In the cratonic region, distinctive high? (>2%) velocity anomalies representing depleted mantle lithospheres are well correlated with major Precambrian crustal domains. The largest lithosphere thickness contrast coincides with the Snowbird Tectonic Zone, where the Hearne province extends down to ~300 km, nearly 100 km deeper than the Proterozoic terranes in northern Alberta. In the latter region, a pronounced cylindrical negative velocity anomaly extends subvertically from 75 to ~300?km depth, which potentially results from significant tectonothermal modifications during subduction and/or plume activities. At the basin scale, mantle velocities show no apparent correlations with surface heat flux, suggesting a minimum mantle contribution to the regional thermal variability. Furthermore, the long?wavelength isostatic gravity correlates negatively with the velocities, which confirms that the melt extraction from Precambrian cratons is responsible for the formation of highly depleted mantle lithospheres. Moreover, our model reveals the increased concentrations of kimberlites and lamproites near the zones of high horizontal velocity gradients. The distinct spatial pattern may reflect either preferential formation or eruption of potentially diamondiferous rocks at lithospheric weak zones near the western margin of Laurentia.
Abstract: The Late Cretaceous kimberlites in northern Alberta, Canada, intruded into the Paleoproterozoic crust and represent a nonconventional setting for the discovery of diamonds. Here, we examined the origin of kimberlite magmatism using a multidisciplinary approach. A new teleseismic survey reveals a low-velocity (-1%) corridor that connects two deep-rooted (>200 km) quasi-cylindrical anomalies underneath the Birch Mountains and Mountain Lake kimberlite fields. The radiometric data, including a new U-Pb perovskite age of 90.3 ± 2.6 Ma for the Mountain Lake intrusion, indicate a northeast-trending age progression in kimberlite magmatism, consistent with the (local) plate motion rate of North America constrained by global plate reconstructions. Taken together, these observations favor a deep stationary (relative to the lower mantle) source region for kimberlitic melt generation. Two competing models, mantle plume and slab subduction, can satisfy kinematic constraints and explain the exhumation of ultradeep diamonds. The plume hypothesis is less favorable due to the apparent age discrepancy between the oldest kimberlites (ca. 90 Ma) and the plume event (ca. 110 Ma). Alternatively, magma generation may have been facilitated by decompression of hydrous phases (e.g., wadsleyite and ringwoodite) within the mantle transition zone in response to thermal perturbations by a cold slab. The three-dimensional lithospheric structures largely controlled melt migration and intrusion processes during the Late Cretaceous kimberlite magmatism in northern Alberta.
Abstract: Porphyritic olivine kimberlitic breccia, discovered in the Dörbed Banner of Inner Mongolia, Western China, is referred to as Longtou Shan Kimberlite in our study. This kimberlite occurs as a pipe in the Halahuogete Formation of Bayan Obo Group. Zircon U–Pb ages of Longtou Shan Kimberlite reveals a Mesoproterozoic age of ~1,552 Ma, constraining the deposition age of Halahuogete Formation to the Mesoproterozoic. Compared with Mesoproterozoic kimberlite of the ancient landmass, it can be inferred that the North China Craton is a member of the Ur ancient continent of the Columbia supercontinent. Furthermore, according to the tectonic background of the Bayan Obo Group, we raise this possibility that “Bayan Obo Aulacogen” should be renamed the “Bayan Obo Continental Rift.”
Abstract: The highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au) exist in solid solution in accessory base-metal sulfides (BMS) as well as nano-to-micron scale minerals in rocks of the subcontinental lithospheric mantle (SCLM). The latter include platinum-group minerals (PGM) and gold minerals, which may vary widely in morphology, composition and distribution. The PGM form isolated grains often associated with larger BMS hosted in residual olivine, located at interstices in between peridotite-forming minerals or more commonly in association with metasomatic minerals (pyroxenes, carbonates, phosphates) and silicate glasses in some peridotite xenoliths. The PGM found inside residual olivine are mainly Os-, Ir- and Ru-rich sulfides and alloys. In contrast, those associated with metasomatic minerals or silicate glasses of peridotite xenoliths consist of Pt, Pd, and Rh bonded with semimetals like As, Te, Bi, and Sn. Nanoscale observations on natural samples along with the results of recent experiments indicate that nucleation of PGM is mainly related with the uptake of HSE by nanoparticles, nanominerals or nanomelts at high temperature (> 900?°C) in both silicate and/or sulfide melts, regardless of the residual or metasomatic origin of their host minerals. A similar interpretation can be assumed for gold minerals. Our observations highlight that nanoscale processes play an important role on the ore-forming potential of primitive mantle-derived magmas parental to magmatic-hydrothermal deposits enriched in noble metals. The metal inventory in these magmas could be related with the physical incorporation of HSE-bearing nanoparticles or nanomelts during processes of partial melting of mantle peridotite and melt migration from the mantle to overlying continental crust.
Abstract: Pre-magmatic continental extension often precedes the major magmatic expulsion of large igneous provinces (LIPs). However, the cause-and-effect relationship between pre-magmatic rifting and the extrusion of large amount of magma is controversial. It remains unclear whether magmatism arises as a consequence of passive rifting or whether it is related to active upwelling of the mantle. In addition, the relationship between the pre-magmatic stages and the final breakup, with the onset of conjugate passive margins, is ambiguous. In this study, we compiled available data from six LIPs (Central Atlantic, Karoo, Parana-Etendeka, Deccan, North Atlantic, and Afar igneous provinces) that successively occurred during the fragmentation of Pangea and found that pre-magmatic rift trends may show a high obliquity or even be orthogonal with respect to the future passive margins. We conclude that syn-magmatic rifts should not be directly correlated, both structurally and dynamically, to the ancient pre-magmatic rift phase. Furthermore, following the breakup of a supercontinent, seafloor spreading usually initiates within volcanic passive margins (VPMs) and then propagates away to create non-volcanic passive margins (NVPMs) as a consequence of the consumption and cooling of a sub-lithospheric positive thermal anomaly. Major transform faults often exist between VPMs and NVPMs, acting as a mechanical barrier to mantle melting and magmatism transportation.
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.
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Mantle
NAMs Nominally Anhydrous Minerals
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Low water contents in diamond mineral inclusions: proto-genetic origin in a dry cratonic lithosphere.
Earth and Planetary Science Letters, Vol. 433, pp. 125-132.
Russia, Africa
Kaapvaal and Siberian SCLMs
Abstract: The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., , , and ), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.
Abstract: Carbonates in subducting oceanic slabs can survive beyond slab dehydration and be transferred into the deep mantle. Such deep carbon cycling plays a critical role in generating carbonatitic to alkaline melts. However, whether and how this process has influenced the lithospheric mantle still remains enigmatic. To address these issues, here we provide a detailed petrographic, in-situ chemical and Sr isotopic study on two mantle xenoliths (a wehrlite and a melt pocket-bearing peridotite) entrained by the Changle Miocene basalts from the eastern China. The Changle wehrlite contains carbonate melt inclusions and apatites and is merely enriched in clinopyroxene relative to the lherzolites. The clinopyroxenes are characterized by high (La/Yb)N (4.7-41) and low Ti/Eu (873-2292) ratios and equilibrated with carbonated silicate melt-like compositions. These petrographic and chemical features indicate that the wehrlite was formed by reaction between peridotite and carbonated silicate melts. On the other hand, the Changle melt pocket-bearing peridotite is suggested to have been produced by in-situ melting/breakdown of amphiboles of an amphibole-rich dunite. Low olivine Fo (~89), presence of amphiboles with high (La/Yb)N (~50) and low Ti/Eu (~1070) ratios suggest that such amphibole-rich dunite would have been formed by reaction of peridotite with hydrous alkaline basaltic melts from a carbonated mantle. Our data, combined with previously reported data of the Changle lherzolite xenoliths, unravel a series of mantle metasomatisms by carbonatitic to alkaline melts from carbonated mantle sources. The consistently high 87Sr/86Sr ratios (up to 0.7036) of the clinopyroxenes in both the wehrlites and lherzolites indicate the carbonate components in the mantle sources were derived from the stagnant Pacific slab within the Mantle Transition Zone. This study provides a fresh perspective on the role of deep carbon cycling from subducted oceanic slabs in chemical modification of intracontinental lithospheric mantle through reaction with different types of melts.
The Canadian Mineralogist, Vol. 54, pp. 1285-1291.
China, Mongolia
carbonatite - Bayan Obo
Abstract: Fluorcalciopyrochlore, ideally (Ca,Na)2Nb2O6F, cubic, is a new mineral species (IMA2013-055) occurring in the Bayan Obo Fe-Nb-REE deposit, Inner Mongolia, People's Republic of China. The mineral is found in a dolomite-type niobium rare-earth ore deposit. Associated minerals are dolomite, aegirine, riebeckite, diopside, fluorite, baryte, phlogopite, britholite-(Ce), bastnäsite-(Ce), zircon, magnetite, pyrite, fersmite, columbite-(Fe), monazite-(Ce), rutile, and others. Crystals mostly form as octahedra {111}, dodecahedra {110}, and cubes {100}, or combinations thereof, and generally range in size from 0.01 to 0.3 mm. It is brownish-yellow to reddish-orange in color with a light yellow streak. Crystals of fluorcalciopyrochlore are translucent to transparent with an adamantine to greasy luster on fractured surfaces. It has a conchoidal fracture. No parting or cleavage was observed. The Mohs hardness is 5, and the calculated density is 4.34(1) g/cm3. The empirical formula is (Ca1.14Na0.74Ce0.06Sr0.03Th0.01Fe0.01Y0.01La0.01Nd0.01)?2.02(Nb1.68Ti0.29Zr0.02Sn0.01)?2.00O6.00(F0.92O0.08)?1.00 on the basis of 7(O,F) anions pfu. The simplified formula is (Ca,Na)2Nb2O6F. The strongest four reflections in the X-ray powder-diffraction pattern [d in Å (I) hkl] are: 6.040 (9) 1 1 1, 3.017 (100) 2 2 2, 2.613 (17) 0 0 4, 1.843 (29) 0 4 4, and 1.571 (15) 2 2 6. The unit-cell parameters are a 10.4164(9) Å, V 1130.2(2) Å3, Z = 8. The structure was solved and refined in space group FdEmbedded Image m with R = 0.05. The type material is deposited in the Geological Museum of China, Beijing, People's Republic of China, catalogue number M12182.
Abstract: The Mg isotopic compositions of garnet and clinopyroxene mineral separates and whole rocks from 21 xenolithic eclogites (11 low-MgO eclogites and 10 high-MgO eclogites) from the Koidu kimberlite complex, erupted within the Archean Man Shield, Sierra Leone, West Africa, provide new evidence bearing on the origin of cratonic eclogites. Garnet and clinopyroxene in both low-MgO and high-MgO eclogites generally record equilibrium inter-mineral Mg isotope partitioning, with ?26Mg varying from -2.15‰ to -0.46‰ in garnets and from -0.49‰ to +0.35‰ in clinopyroxenes. Bulk ?26Mg values (-1.38‰-to +0.05-), constructed from garnet and clinopyroxene data, are similar to results from rock powders (-1.60‰ to +0.17‰), suggesting that kimberlite infiltration has had negligible influence on the Mg isotopic compositions of the xenoliths. The ?26Mg values of low-MgO eclogites (-0.80‰ to +0.05‰) exceed the range of mantle peridotite xenoliths (-0.25- ± 0.04-), consistent with the eclogite’s derivation from recycled altered oceanic crust. Similarly variable ?26Mg values in high-MgO eclogites (-0.95‰ to -0.13‰), together with their high MgO and low FeO contents, suggest that high-MgO eclogites were produced by Mg-Fe exchange between partially molten low-MgO eclogites and surrounding peridotites. Our study shows that cratonic xenolithic eclogites preserve a record of Mg isotopic compositions produced by low-pressure, surficial isotope fractionations. The recycling of oceanic crust therefore increases the Mg isotope heterogeneity of the mantle.-
Guarino, V., Wu, F-Y., Lustrino, M., Melluso, L.,Brotzu, P., De Barros Gomes, C., Ruberti, E., Tassarini, C.C.G., Svisero, D.P.
U Pb ages, Sr Nd isotope geochemistry, and petrogenesis of kimberlites, kamafugites and phlogopite picrites of the Alto Paranaiba Igneous Province, Brazil.
Guarino, V., Wu, F-Y., Lustrino, M., Melluso, L., Brotzu, P., Barros Gomes, C.de, Ruberti, E., Tassarini, C.C.G., Svisero, D.P.
U-Pb ages, Sr, Nd isotope geochemistry, and petrogenesis of kimberlites, kamafugites and phlogopte-picrites of the Alto Paranaiba Igneous Province, Brazil.
Abstract: The Late Cretaceous Itatiaia complex is made up of nepheline syenite grading to peralkaline varieties, quartz syenite and granite, emplaced in the metamorphic rocks of the Serra do Mar, SE Brazil. The nepheline syenites are characterized by assemblages with alkali feldspar, nepheline, Fe-Ti oxides, clinopyroxene, amphibole, apatite and titanite, while the peralkaline nepheline syenites have F-disilicates (rinkite, wöhlerite, hiortdahlite, låvenite), britholite and pyrophanite as the accessory phases. The silica-oversaturated rocks have alkali feldspar, plagioclase, quartz, amphibole, clinopyroxene and Fe-Ti oxides; the chevkinite-group minerals are the featured accessory phases and are found with allanite, fluorapatite, fluorite, zircon, thorite, yttrialite, zirconolite, pyrochlore and yttrocolumbite. The major- and trace-element composition of the Itatiaia rocks have variations linked to the amount of accessory phases, have smooth, enriched chondrite-normalized rare-earth element (REE) distribution patterns in the least-evolved nepheline syenites and convex patterns in the most-evolved nepheline syenites. The REE distribution patterns of the quartz syenites and granites show a typical pattern caused by fractional crystallization of feldspar and amphibole, in an environment characterized by relatively high oxygen fugacity (>NiNiO buffer) and high concentrations of H2O and F, supporting the crystallization of hydrous phases, fluorite and F-disilicates. The removal of small amounts of titanite in the transition from the least-evolved to the most-evolved nepheline syenites stems from petrogenetic models involving REE, and is shown to be a common feature of the magmatic evolution of many other syenitic/ trachytic/ phonolitic complexes of the Serra do Mar and elsewhere.
International Journal of Earth Sciences, Vol. 106, 6, pp. 1963-1989.
South America, Brazil
carbonatite - Catalao II
Abstract: The Catalão II carbonatitic complex is part of the Alto Paranaíba Igneous Province (APIP), central Brazil, close to the Catalão I complex. Drill-hole sampling and detailed mineralogical and geochemical study point out the existence of ultramafic lamprophyres (phlogopite-picrites), calciocarbonatites, ferrocarbonatites, magnetitites, apatitites, phlogopitites and fenites, most of them of cumulitic origin. U–Pb data have constrained the age of Catalão I carbonatitic complex between 78 ± 1 and 81 ± 4 Ma. The initial strontium, neodymium and hafnium isotopic data of Catalão II (87Sr/86Sri= 0.70503–0.70599; ?Ndi= ?6.8 to ?4.7; 176Hf/177Hf = 0.28248–0.28249; ?Hfi= ?10.33 to ?10.8) are similar to the isotopic composition of the Catalão I complex and fall within the field of APIP kimberlites, kamafugites and phlogopite-picrites, indicating the provenance from an old lithospheric mantle source. Carbon isotopic data for Catalão II carbonatites (?13C = ?6.35 to ?5.68 ‰) confirm the mantle origin of the carbon for these rocks. The origin of Catalão II cumulitic rocks is thought to be caused by differential settling of the heavy phases (magnetite, apatite, pyrochlore and sulphides) in a magma chamber repeatedly filled by carbonatitic/ferrocarbonatitic liquids (s.l.). The Sr–Nd isotopic composition of the Catalão II rocks matches those of APIP rocks and is markedly different from the isotopic features of alkaline-carbonatitic complexes in the southernmost Brazil. The differences are also observed in the lithologies and the magmatic affinity of the igneous rocks found in the two areas, thus demonstrating the existence of regional-scale heterogeneity in the mantle sources underneath the Brazilian platform.
Abstract: New in-situ U-Pb ages and Sr-Nd-Hf isotopic data on mineral phases of the Sung Valley and Jasra ultramafic-alkaline-(carbonatite) intrusions (Shillong Plateau, India) shed new light on the petrogenetic processes of volcanism in north-eastern India during the Cretaceous. Perovskites of Sung Valley dunite, ijolite and uncompahgrite yielded U-Pb ages of 109.1?±?1.6, 104.0?±?1.3 and 101.7?±?3.6?Ma, respectively. A U-Pb age of 106.8?±?1.5?Ma was obtained on zircons of a Sung Valley nepheline syenite. Perovskite of a Jasra clinopyroxenite yielded an age of 101.6?±?1.2?Ma, different from the U-Pb age of 106.8?±?0.8?Ma on zircon of Jasra syenites. The variation in Sr-Nd-Hf isotopic compositions [initial 87Sr/86Sr?=?0.70472 to 0.71080; ?Nd i?=??10.85 to +0.86; ?Hf i?=??7.43 to +1.52] matches the bulk-rock isotopic composition of the different rock units of Sung Valley and Jasra. Calcite and apatite in the carbonatites, the perovskite in a dunite, and the bulk-rock carbonatites of Sung Valley intrusion have the lowest initial 87Sr/86Sr and ?Nd, taken to be the best proxies of the mantle source composition, which is dominated by components derived from the lithospheric mantle. The alkaline intrusions of north-eastern India are significantly younger than the Sylhet tholeiitic magmatism. The silicate rocks of both intrusions have isotopic composition trending to that of the underlying Shillong crust, indicating the effects of fractional crystallization and low-pressure crustal contamination during the emplacement of the various intrusive magma pulses.
Abstract: General information is presented on ten agpaitic occurrences located in southern Brazil and at the border between Brazil and Paraguay. All the Brazilian agpaitic rocks are Late Cretaceous in age, whereas the Paraguayan ones are older than Early Triassic. The most significant occurrence is Poços de Caldas, the largest alkaline massif in South America. In general, these agpaitic rocks contain mineral assemblages that indicate presence of typical halogen-bearing Na-Ca-HFSE phases, eudialyte-, rinkite- and wöhlerite-group minerals being the most frequent ones. However, these associations are indeed more complex in terms of composition, with accessory phases in some cases consisting of various minerals, including U-Th oxides/silicates, Nb oxides, REE-Sr-Ba bearing carbonates-fluorocarbonates-phosphates-silicates and Zr-Na rich silicates. They usually form late magmatic stage to hydrothermal/deuteric assemblages linked with coarse and fine-grained, mainly silica-undersaturated evolved rocks. Data also indicate significant differences in type, amount and composition of agpaitic minerals in all investigated occurrences.
Mineralogy and Petrology, 10.1007/s00710-022-00777-z 20p. Pdf
India
deposit - Shilong Plateau
Abstract: A comprehensive mineralogical, geochemical and isotopic review of six ultramafic-alkaline-carbonatite magmatic intrusions of the Shillong Plateau (Sung Valley, Jasra, Swangkre-Rongjeng, and Mawpyut) and Mikir Hills (Samchampi-Samteran and Barpung) is presented here, using the published data. These intrusions emplaced ca. 115-102 Ma ago, thus are significantly younger than the tholeiitic flood basalts erupted in Rajmahal-Sylhet province (ca. 118-115 Ma). The intrusive lithologies vary from ultramafic (dunites, clinopyroxenites, melilitolites) to mafic (ijolites, gabbros sensu lato, shonkinites), to felsic (syenites, nepheline syenites) and carbonatites (mostly calcite-rich varieties). The volcanic-subvolcanic facies (lamprophyres, phonolites) are not abundant. The range of chemical compositions of the magmatic phases in the various assemblages is notable; the intrusive rocks are thus the result of crystallization of magmas from variably evolved, independent liquid-lines-of descent, generally of alkaline/strongly alkaline lineages and sodic-to-potassic in affinity. The large variations of the Sr-Nd isotopic ratios of the silicate intrusive rocks (sensu lato) suggest a role of shallow-level crustal contamination during their formation. The carbonatites of the Sung Valley and Samchampi-Samteran have different isotope ratios than the associated silicate rocks, have some isotopic affinity with the Group I tholeiitic basalts of Rajmahal Traps and have an ultimate genesis in a carbonate-bearing lithospheric mantle.
South African Journal of Geology, Vol. 121, pp. 227-236.
Africa, South Africa
geomorphology
Abstract: Rocky reaches of the southeast African coastline are characterized mainly by log-spiral and headland-bound bays. Extensive fieldwork was carried out to investigate both documented and new exposures of Cretaceous beds on the southern KwaZulu-Natal and upper Eastern Cape (Transkei) coasts. Our results suggest that geological inheritance plays an important role in the contemporary rocky coast geomorphology. We offer evidence that post-Gondwana break-up mass flow channels play an important role in the present southeast African coastline morphology. Mass flow channels contain fills of incompetent Cretaceous rocks which are being preferentially eroded by prevailing marine and fluvial processes to form headland-bound embayment landforms. This study has identified an important geomorphic process for the development of the current southeast African coastline.
Abstract: The Earth’s magnetic field is powered by energy supplied by the slow cooling and freezing of the liquid iron core. Efforts to determine the thermal and chemical history of the core have been hindered by poor knowledge of the properties of liquid iron alloys at the extreme pressures and temperatures that exist in the core. This obstacle is now being overcome by high-pressure experiments and advanced mineral physics computations. Using these approaches, updated transport properties for FeSiO mixtures have been determined at core conditions, including electrical and thermal conductivities that are higher than previous estimates by a factor of two to three. Models of core evolution with these high conductivities suggest that the core is cooling much faster than previously thought. This implies that the solid inner core formed relatively recently (around half a billion years ago), and that early core temperatures were high enough to cause partial melting of the lowermost mantle. Estimates of core-mantle boundary heat flow suggest that the uppermost core is thermally stratified at the present day.
Mineralogy and Petrology, in press available, 13p.
India
Deposit - Wajrakur
Abstract: Although Ti-rich garnets are commonly encountered in the groundmass of many alkaline igneous rocks, they are comparatively rare in kimberlites. Here we report on the occurrence of Ti-rich garnets in the groundmass of the P-15 and KL-3 kimberlites from the diamondiferous Wajrakarur field in the Eastern Dharwar craton of southern India. These garnets contain considerable Ti (11.7-23.9 wt.% TiO2), Ca (31.3-35.8 wt.% CaO), Fe (6.8-15.5 wt.% FeOT) and Cr (0.04-9.7 wt.% Cr2O3), but have low Al (0.2-5.7 wt.% Al2O3). In the case of the P-15 kimberlite they display a range in compositions from andradite to schorlomite, with a low proportion of grossular (andradite(17.7-49.9)schorlomite(34.6-49.5)-grossular(3.7-22.8)-pyrope(1.9-10.4)). A few grains also contain significant chromium and represent a solid solution between schorlomite and uvarovite. The Ti-rich garnets in the KL-3 kimberlite, in contrast, are mostly schorlomitic (54.9?90.9 mol %) in composition. The Ti-rich garnets in the groundmass of these two kimberlites are intimately associated with chromian spinels, perhaps suggesting that the garnet formed through the replacement of spinel. From the textural evidence, it appears unlikely that the garnets could have originated through secondary alteration, but rather seem to have formed through a process in which early magmatic spinels have reacted with late circulating, residual fluids in the final stages of crystallization of the kimberlite magma. Raman spectroscopy provides evidence for low crystallinity in the spinels which is likely to be a result of their partial transformation into andradite during their reaction with a late-stage magmatic (kimberlitic) fluid. The close chemical association of these Ti-rich garnets in TiO2-FeO-CaO space with those reported from ultramafic lamprophyres (UML) is also consistent with results predicted by experimental studies, and possibly implies a genetic link between kimberlite and UML magmas. The occurrence of Ti-rich garnets of similar composition in the Swartruggens orangeite on the Kaapvaal craton in South Africa, as well as in other kimberlites with an orangeitic affinity (e.g. the P-15 kimberlite on the Eastern Dharwar craton in southern India), is inferred to be a reflection of the high Ca- and high Ti-, and the low Al-nature, of the parent magma (i.e. Group II kimberlites).
Continuous gradations among primary carbonatitic, kimberlitic, melilititic, basaltic, picritic and komatiitic melts in equilibrium with garnet lherzolite at 3-8 GPa.
Melting phase relations of simplified carbonated peridotite at 12-26 GPa in the system CaO-MgO-SiO2-CO2 and CaO-MgO-Al2O3-Sio2-CO2: highly calcic magmas Earth
Melting phase equilibration temperatures of model carbonated peridotite from 8 to 12 Gpa in the system CaO-MgO-Al2O3-SiO2-CO2 and kimberlitic liquids in the Earth's upper mantle.
Abstract: Experimentally determined major and trace element partition coefficients between majoritic garnet, clinopyroxene, and carbon dioxide-rich liquid are reported at 10 GPa and 1800 °C in a model carbonated peridotite composition in the system CaO-MgO-Al2O3-SiO2-CO2. Besides majoritic garnet, the liquid coexists with forsterite, orthopyroxene, and clinopyroxene, making melting phase relations invariant at fixed pressure and temperature conditions. Partition coefficients span a wide range of values - for instance, Sr, Nb, Ba, La, and Ce are highly incompatible in majoritic garnet, while Ca, Y, Nb, and Ho are moderately incompatible, and Lu, Si, Al, and Mg are compatible. Strong fractionation of light rare earth elements (e.g., La, Ce, Nd, Sm) and high field strength elements (e.g., Nb, Ta, Zr, Hf, Th) is seen between majoritic garnet and liquid. The experimentally determined partitioning values are used to calculate compositions of melts in equilibrium with majoritic garnet inclusions in diamonds from select localities in Brazil and Guinea. The calculated melts largely straddle those between documented carbonatites, kimberlites, and alkali basalts, low-degree mantle melting products from carbonated peridotite. This resemblance firmly suggests that majoritic garnet inclusions in diamonds from Brazil and Guinea can simply be interpreted as precipitates from such melts, thereby offering an alternative to the hypothesis that the element chemistry of such inclusions in diamonds can largely, and sometimes only, be ascribed to subducted oceanic crust, and further that, fusion of this crust may limit the terrestrial 'carbon recycling' at depths much beyond corresponding to those of Earth's transition zone.
Journal of Volcanology and Geothermal Research, Vol 304, pp. 1-10.
Mantle
Magmatism
Abstract: For a typical poroelastic shallow crustal magma chamber, about 0.1% of the mafic magma and about 4% of the felsic magma are erupted and/or injected during magma-chamber rupture. Magma chambers with volumes of the order of several tens to several hundred cubic kilometres, as are common, are thus sufficiently large to supply magma to small or moderate eruptions. For large eruptions, however, a much higher percentage of the magma volume must be squeezed out of the chamber. For an ordinary poroelastic chamber, the excess pressure in the chamber falls exponentially during the eruption. For a large eruption to be possible, however, the excess pressure must be essentially maintained until the very end of the eruption. Here I show that caldera collapse can maintain the excess pressure through forced magma-chamber volume reduction, in which case a resulting large eruption would be the consequence (not the cause) of the collapse. I also show that ring-fault dip partly controls the size and intensity (volumetric flow or effusion rate) of the eruption. If the ring-fault dips inward (a normal fault), the displacement is ‘stable’, the volumetric flow rate (intensity) remains essentially constant during the collapse, and the magma chamber remains active following the collapse. By contrast, if the ring-fault dips outward (a reverse fault), the displacement is ‘unstable’, the volumetric flow rate normally increases dramatically during the collapse, and the magma chamber may be entirely destroyed during the collapse.
Pilidou, SA., Priestley, K., Gudmundsson, O., Debayle, E.
Upper mantle S-wave speed heterogeneity and anisotropy beneath the North Atlantic from regional surface wave tomography: the Iceland and Azores plumes.
Geophysical Journal International, Vol. 159, 3, pp. 1057-1076.
Journal of South American Earth Sciences, Vol. 108, 103255. 17p. Pdf
South America, Brazil
lineaments, tectonics
Abstract: The effect of previous structures inheritance is known to be important in the development of tectonic rifts. A series of overlapping structures generally can be represented by lineaments marking the successive tectonic events. We studied the NNE structural lineaments corridor in the central region of the Ribeira Belt. We used a digital elevation model (DEM) and new and previous fieldwork data to investigate the structural control of such lineaments and their relevance for the Brazilian continental margin. Our results suggest that the NNE direction is a crustal weakness zone characterising corridors of intense ductile and brittle deformation which was recurrently reactivated. Aligned NNE Neoproterozoic-Ordovician ductile and brittle structures as foliations, shear zones, lithological boundaries, and fractures filled by pegmatitic veins coincide with the lineaments. During the Cretaceous rift, a transtensional sinistral regime generated NNE T-fractures filled by mafic dykes. In the Cenozoic, the NNE direction is represented by transfer and domino faults developed within a mega accommodation zone in an intracontinental rift system. Our results suggest that the NNE direction was active in this region throughout the Phanerozoic and has high relevance for the structural development of the continental margin of southeastern Brazil.
Abstract: The interaction between hydrous fluids and melts with dry pre-existing mantle rocks alters the physocochemical porperties of the deep lithosphere. Here we present new insight into mantle metasomatism based on petrology, geochemistry, and Rare Earth Element (REE) distribution modelling using mantle xenoliths from various eruption centres in the Cenozoic Tariat volcanic field, Mongolia. These centres include the Horgo, Tsagan, Zala, Haer and Shavaryn-Tsaram lava flows that vary in composition and age between alkali basalts to trachybasalts to tephrite basanites and 4 ka to 1.5 Ma, respectively. Our sample suite contains xenolith from the lower crust and underlying mantle with a size range of individual xenoliths between 3 cm and 8 cm. Based on the clinopyroxene REE concentration pattern, the investigated xenoliths can be divided in two groups, characterized by LREE depletion (Group 1) and enrichment (Group 2) relative to primitive mantle. Group 1 xenoliths display well-preserved deformation textures and are considered to represent the sub-continental lithosphere prior to Cenozoic rejuvenation. In contrast, Group 2 samples are marked by partial annealing of pre-existing textures. REE distribution modelling between clinopyroxene and inferred chemically enriched basaltic melt suggests that the observed REE pattern do not reconcile with a simple mixing model but reflect chromatographic fractionation during reactive melt flow. In addition, the clinopyroxene core-rim REE variation in some of the xenoliths suggests interaction with at least one other melt of distinct chemical composition.
Abstract: H2O strongly influences physical properties of the mantle and its ability to melt or convect and can trace recycling of surface reservoirs down to the deep mantle1,2. This makes knowledge of water content in the Earth's interior and its evolution through time crucial to understanding global geodynamics. Komatiites (MgO-rich ultramafic magmas) result from high-degree mantle melting at high pressures3 and thus are excellent probes of H2O contents in the deep mantle. A significant excess of H2O over elements of similar geochemical behavior during mantle melting (e.g. Ce) was recently found in melt inclusions in the most Mg-rich olivine in 2.7 Ga old komatiites from Canada4 and Zimbabwe5. These data were taken as evidence for a deep hydrated mantle reservoir, probably the transition zone, in the Neoarchean time. In this paper we confirm the mantle source of this H2O by measurement of deuterium to hydrogen ratios in these melt inclusions and present similar data for 3.3 Ga old komatiites from the Barberton Greenstone Belt. Using hydrogen isotopes, we show that the mantle sources of these melts contained excess H2O which implies that a deep mantle hydrated reservoir has been present in the Earth's interior at least since the Paleoarchean. The reconstructed initial hydrogen isotope composition of komatiites is significantly more depleted in deuterium than all surface reservoirs and typical mantle but resembles that in dehydrated subducted slabs. Together with a significant excess of chlorine and a temporal trend of Pb/Ce in the mantle sources of komatiites, these results argue that lithosphere recycling into the deep mantle, arguably via subduction, started before 3.3 Ga. (a un-reviewed version of the manuscript accepted for publication in Nature magazine).
Abstract: Crystalline basement rocks of southwestern Montana have been subjected to multiple tectonothermal events since ?3.3 Ga: the Paleoproterozoic Big Sky/Great Falls orogeny, Mesoproterozoic extension associated with Belt-Purcell basin formation, Neoproterozoic extension related to Rodinia rifting, and the late Phanerozoic Sevier-Laramide orogeny. We investigated the long-term (>1 Ga), low-temperature (erosion/burial within 10 km of the surface) thermal histories of these tectonic events with zircon and apatite (U-Th)/He thermochronology. Data were collected across nine sample localities (n = 55 zircon and n = 26 apatite aliquots) in the northern and southern Madison ranges, the Blacktail-Snowcrest arch, and the Tobacco Root uplift. Our zircon (U-Th)/He data show negative trends between single aliquot date and effective uranium (a radiation damage proxy), which we interpreted with a thermal history model that considers the damage-He diffusivity relationship in zircon. Our model results for these basement ranges show substantial cooling from temperatures above 400°C to near surface conditions between 800 and 510 Ma. Subsequent Phanerozoic exhumation culminated by ?75 Ma. Late Phanerozoic cooling is coincident with along-strike Sevier belt thin-skinned thrusting in southeastern Idaho, and older than exhumation in basement-involved uplifts of the Wyoming Laramide province. Our long-term, low-temperature thermal record for these southwestern Montana basement ranges shows that: (a) these basement blocks have experienced multiple episodes of upper crustal exhumation and burial since Archean time, possibly influencing Phanerozoic thrust architecture and (b) the late Phanerozoic thick-skinned thrusting recorded by these rocks is among the earliest thermochronologic records of Laramide basement-involved shortening and was concomitant with Sevier belt thin-skinned thrusting.
Anais da Academia Brasileira de Ciencas, Vol. 79, pp. 321-332.
South America, Brazil
microdiamonds
Abstract: The origin of diamonds from Serra do Espinhaço in Diamantina region (State of Minas Gerais) and in Chapada Diamantina, Lençóis region (State of Bahia) remains uncertain, even taking into account the ample research carried out during the last decades. The lack of typical satellite minerals in both districts makes a kimberlitic source for these diamonds uncertain. In mid 18th century the occurrence of a metamorphosed igneous rock composed of martite, sericite and tourmaline was described in Diamantina region and named hematitic phyllite, considered by some researchers as a possible diamond source. Similar rocks were found in Lençóis and examined petrographically and their heavy mineral concentration was investigated by means of scanning electron microscopy (SEM). Petrographic analyses indicated an igneous origin for these rocks and SEM analyses showed the discovery of microdiamonds. Geochronological studies using the Ar/Ar technique in muscovites yielded minimum ages of 1515+/-3 Ma, which may correlate with 1710+/-12 Ma from U-Pb method in igneous zircons from the hematitic phyllites. Both rock types also have the same mineral and chemical composition which leads to the conclusion that the intrusive rocks were protolith of the hematitic phyllites. This first discovery of microdiamonds in intrusive rocks opens the possibility of new investigation models for diamond mineralization in Brazilian Proterozoic terrains.
Earth Planetary Science Letters, Vol. 487, pp. 210-220.
Africa
tectonics
Abstract: Rifts are often associated with ancient traces of hotspots, which are supposed to participate to the weakening of the lithosphere. We investigated the expected past trajectories followed by three hotspots (Afar, East-Africa and Lake-Victoria) located around the Red Sea. We used a hotspot reference frame to compute their location with respect to time, which is then compared to mantle tomography interpretations and geological features. Their tracks are frequently situated under continental crust, which is known to strongly filter plume activity. We looked for surface markers of their putative ancient existence, such as volcanism typology, doming, and heat-flow data from petroleum wells. Surface activity of the East-Africa hotspot is supported at 110 Ma, 90 Ma and 30 Ma by uplift, volcanic activity and rare gas isotopic signatures, reminiscent of a deep plume origin. The analysis of heat-flow data from petroleum wells under the Arabian plate shows a thermal anomaly that may correspond to the past impact of the Afar hotspot. According to derived hotspot trajectories, the Afar hotspot, situated (at 32 Ma) 1000 km north-east of the Ethiopian-Yemen traps, was probably too far away to be accountable for them. The trigger of the flood basalts would likely be linked to the East-Africa hotspot. The Lake-Victoria hotspot activity appears to have been more recent, attested only by Cenozoic volcanism in an uplifted area. Structural and thermal weakening of the lithosphere may have played a major role in the location of the rift systems. The Gulf of Aden is located on inherited Mesozoic extensional basins between two weak zones, the extremity of the Carlsberg Ridge and the present Afar triangle, previously impacted by the East-Africa hotspot. The Red Sea may have opened in the context of extension linked to Neo-Tethys slab-pull, along the track followed by the East Africa hotspot, suggesting an inherited thermal weakening.
Abstract: Deep crustal and mantle rocks are exhumed in core complex mode of extension in three types of structures: metamorphic core complexes, oceanic core complexes and magma poor passive margins. Using available analogue and numerical models and their comparison with natural examples, the present paper reviews the mechanical processes involved in these different types of extensional setting. Three main aspects are considered: i) the primary role of lithosphere rheology, ii) the lithosphere-scale patterns of progressive deformation that lead to the exhumation of deep metamorphic or mantle rocks and iii) the initiation and development of detachment zones. Crustal core complexes develop in continental lithospheres whose Moho temperature is higher than 750 °C with “upper crust-dominated” strength profiles. Contrary to what is commonly believed, it is argued from analogue and numerical models that detachments that accommodate exhumation of core complexes do not initiate at the onset of extension but in the course of progressive extension when the exhuming ductile crust reaches the surface. In models, convex upward detachments result from a rolling hinge process. Mantle core complexes develop in either the oceanic lithosphere, at slow and ultra-slow spreading ridges, or in continental lithospheres, whose initial Moho temperature is lower than 750 °C, with “sub-Moho mantle-dominated” strength profiles. It is argued that the mechanism of mantle exhumation at passive margins is a nearly symmetrical necking process at lithosphere scale without major and permanent detachment, except if strong strain localization could occur in the lithosphere mantle. Distributed crustal extension, by upper crust faulting above a décollement along the ductile crust increases toward the rift axis up to crustal breakup. Mantle rocks exhume in the zone of crustal breakup accommodated by conjugate mantle shear zones that migrate with the rift axis, during increasing extension.
Chorowicz, J., Guezlane, M., Rudant, J., Vidal, G.
Use of MOMS-1 dat a for geological mapping of the Aswa lineament(East African Rift)
National Technical Information Service, In ESA Proceedings 4th. International Colloquium in Spectral Signatures in Remote, N89 10382/4 4p. April 1988 Entire Conference $ 49.95
Mass transport mechanism between the upper and lower mantle in numerical simulations of thermochemical mantle convection with multicomponent phase changes.
Earth and Planetary Science Letters, Vol. 230, 1-2, pp. 11-27.
Arabian Journal of Geosciences, Vol. 8, no. 11, pp. 9373-9388.
India
Deposit - Narayanpet
Abstract: In the present study, an attempt was made to analyse the reflectance spectra of kimberlites to evaluate its potential as key in remote sensing based spatial mapping. The spectral profiles of kimberlite samples were collected within the visible-near infrared-shortwave infrared (VNIR-SWIR) electromagnetic domain. In this regard, we analysed the reflectance spectra of three kimberlite pipes (having variable mineralogy) of Narayanpet kimberlite field (NKF) based on the comparative analysis of spectral features of kimberlite samples with the spectral features of their dominant constituent minerals. The relative abundances of each of the constituent minerals were confirmed using semiquantitative mineralogical data from X-ray diffraction analysis. This was supplemented with petrographical data as reference. We found that the absorption features imprinted in the reflectance spectra of kimberlites were mineralogically sensitive. These spectral features were imprinted by spectral features of serpentine, olivine, and calcite depending on the relative dominance of these minerals in kimberlites. With regard to understand the spectral behaviour of weathered residue of kimberlite for targeting buried kimberlite, we also attempted a comparative analysis of spectral profiles of in-situ soil developed above the pipes with the spectra of respective kimberlites in NKF area. While comparing aforementioned spectra, it was observed that the spectral signatures of NKF kimberlites were broadly translated to the in-situ soil. Further, we compared the spectral profiles of selected NKF kimberlites with the spectra of three distinct kimberlite pipes of Wajrakarur kimberlite field (WKF) characterised with similar mineralogy with respect to the selected NKF pipes. Relative dominance of constituent minerals (i.e., serpentine, olivine, calcite, etc.) in these pipes was taken as reference to identify the mineralogical similarity of the pipes of both the field. It was observed that the spectral profiles of NKF and WKF kimberlites were highly correlated with regard to wavelength of diagnostic absorption features. Finally, we also made an attempt to understand the effect of spectral mixing, in spectral separation of kimberlites and associated granite-granodiorite gneiss (i.e., Dharwar Gneiss). It was seen that the spectral contrast of kimberlite and gneiss was dependent on the relative size of the pipe with respect to pixel or ground sampling diameter of spectral data acquisition. Study confirmed the diagnostic nature of reflectance spectra of pipes along with their mineralogical sensitiveness and spatial integrity. It also highlighted how spectral mixing can influence the spectral feature based remote detection of kimberlites.
Journal of Earth System Science, Vol. 129, 1, 102 10p. Pdf
India
deposit - Pipe 6
Abstract: The Wajrakarur kimberlite Pipe 6 in Eastern Dharwar Craton, is hardly explored using latest ground-based geophysical techniques. The present study uses the Very Low Frequency Electromagnetic (VLF-EM) method for understanding the aerial extension, depth and geometry of the kimberlite pipe. The VLF-EM data have been analyzed using Fraser filtering of in-phase component, 3D Euler deconvolution of Fraser filtered in-phase data, radially average power spectrum (RAPS) of VLF data (raw data) and 2D inversion of VLF data (raw data). The Fraser filtered in-phase grid anomaly map has witnessed as an effective tool for mapping extension of the kimberlite pipe. The maxima of Fraser filtered in-phase component has been observed as a key parameter to delineate the conducting bodies. The high apparent current density in Karous-Hjelt (K-H) pseudo section locate relatively conducting body possibly associated with kimberlite pipe. Two depth interfaces at about 15 and 32 m have been delineated using RAPS. 3D Euler solution indicate dyke-like structure associated with kimberlite pipe having depth solutions ranging from 6 to 40 m with mode of depth 17 m in the study area. 2D resistivity sections indicate that causative bodies are in the depth range of 15-50 m. The results of VLF-EM study are well validated using geological borehole data over the study area reported by Geological Survey of India.
The International Achives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. XLII-5 12p. Pdf
India, Madhya Pradesh
ASTER, lineament
Abstract: In the present study, we have prepared the thematic evidence layers for identifying the potential zones of kimberlite emplacement in parts of Chhatarpur district, Madhya Pradesh. These thematic layers or evidence layers are geological structure, alteration zones, lineament density, surface alteration and geomorphic anomaly and these layers are prepared from the remote sensing data. As orientation of the geological structures (i.e fault system) and their density have the major role in the emplacement of kimberlite; both of these evidence layers are integrated using "AND" Boolean Logical Operator. On the other hand, two evidential layers regarded as the proxy to indicate the "surface expressions on kimberlite (i.e. alteration zones and geomorphic anomaly) are combined using "OR" operator as either of these two surface expression is indicative of kimberlite. Consequently, conjugate evidence layers on the surface expressions of kimberlite are integrated with the causative evidence layers of kimberlite emplacement using "AND" operator to identify the potential zones of diamond occurrences. Potential zones of kimberlite are overlaid on the residual gravity anomaly map derived from space-based gravity model of European Improved Gravity of Earth by New Technique (EIGEN6C4) to relate potential zones of kimberlite with the similar structural alignment (delineated in the residual gravity map) of known occurrence of kimberlite. We also have carried out indicator mineral survey around these potential zones and some of the kimberlite specific indicator minerals are identified in the stream sediments within these potential zones.
Abstract: The physical manifestations of plate tectonics on the modern Earth are relatively well-understood, but the nature and timing of its onset remains enigmatic, with the geodynamic regime(s) that operated during the Archaean hotly debated. This absence of a consistent geodynamic framework within which regional-scale observations can be placed limits our understanding of Archaean assemblages and associated mineral deposits. To engage with the Archaean geodynamics discussion, this thesis focuses on ultramafic-mafic complexes in the Lewisian Gneiss Complex (LGC) of the North Atlantic Craton and Johannesburg Dome of the Kaapvaal Craton. Globally, such complexes have been the subject of wide-ranging interpretations that have disparate implications for Archaean geodynamic regimes. Throughout this thesis, it is demonstrated that confidently constraining element mobility is of paramount importance when aiming to constrain the origin of Archaean ultramafic rocks, with a variety of geochemical proxies shown to be susceptible to element mobility. Notably, high field strength element anomalies - a geochemical proxy commonly used to fingerprint subduction-related magmatism - are here shown to be highly susceptible to element mobility, with the role of subduction as an Archaean geodynamic process potentially overestimated as a result. Such mobility can, however, be constrained and a primary geochemistry obtained using the integrated approach utilised here, whereby detailed petrography, bulk-rock geochemistry and mineral chemistry are examined using the context provided by rigorous field geology. Using this approach, the ultramafic-mafic complexes in the LGC are here interpreted as recording two temporally and petrogenetically distinct phases of Archaean magmatism. One group of complexes likely represents an early ultramafic-mafic crust that pre-dates the tonalite-trondhjemite-granodiorite (TTG) magmas, while a second group of complexes are interpreted as representing several layered intrusions that were emplaced into TTG. The ultramafic-mafic complexes in the Johannesburg Dome are considered to represent intrusive and extrusive remnants of an Archaean greenstone belt, contradicting a recently proposed hypothesis whereby the complexes are interpreted as fragments of an Archaean ophiolite. When combined with similar opposition to other proposed Archaean ophiolite occurrences in other cratons, this contradiction is potentially significant to the Archaean geodynamics debate, raising questions as to the validity of a > 3.6 Ga onset for modern-style plate tectonics.
Abstract: Diamonds from Juina, Brazil, are well-known examples of superdeep diamond crystals formed under sublithospheric conditions and evidence would indicate their origins lie as deep as the Earth's mantle transition zone and the Lower Mantle. Detailed characterization of these minerals and of inclusions trapped within them may thus provide precious minero-petrogenetic information on their growth history in these inaccessible environments. With the aim of studying non-destructively the structural defects in the entire crystalline volume, two diamond samples from this locality, labelled JUc4 and BZ270, respectively, were studied in transmission mode by means of X-ray Diffraction Topography (XRDT) and micro Fourier Transform InfraRed Spectroscopy (µFTIR). The combined use of these methods shows a good fit between the mapping of spatial distribution of extended defects observed on the topographic images and the µFTIR maps corresponding to the concentration of N and H point defects. The results obtained show that both samples are affected by plastic deformation. In particular, BZ270 shows a lower content of nitrogen and higher deformation, and actually consists of different, slightly misoriented grains that contain sub-grains with a rounded-elongated shape. These features are commonly associated with deformation processes by solid-state diffusion creep under high pressure and high temperature.
The American Economic Review, Vol. 97, 5, pp. 1978-1993.
Global
Economics
Abstract: This paper studies the relationship between civil war and the value of firms in a poor, resource abundant country using microeconomic data for Angola. We focus on diamond mining firms and conduct an event study on the sudden end of the conflict, marked by the death of the rebel movement leader in 2002. We find that the stock market perceived this event as “bad news” rather than "good news" for companies holding concessions in Angola, as their abnormal returns declined by 4 percentage points. The event had no effect on a control portfolio of otherwise similar diamond mining companies. This finding is corroborated by other events and by the adoption of alternative methodologies. We interpret our findings in the light of conflict-generated entry barriers, government bargaining power and transparency in the licensing process.
American Mineralogist, Vol. 105, pp. 1662-1671. pdf
Mantle
melting
Abstract: Zircon is the most frequently used mineral for dating terrestrial and extraterrestrial rocks. However, the system of zircon in mafic/ultramafic melts has been rarely explored experimentally and most existing models based on the felsic, intermediate and/or synthetic systems are probably not applicable for prediction of zircon survival in terrestrial shallow asthenosphere. In order to determine the zircon stability in such natural systems, we have performed high-temperature experiments of zircon dissolution in natural mid-ocean ridge basaltic and synthetic haplobasaltic melts coupled with in situ electron probe microanalyses of the experimental products at high current.
Taking into account the secondary fluorescence effect in zircon glass pairs during electron microprobe analysis, we have calculated zirconium diffusion coefficient necessary to predict zircon survival in asthenospheric melts of tholeiitic basalt composition. The data imply that typical 100 micron zircons dissolve rapidly (in 10 hours) and congruently upon the reaction with basaltic melt at mantle pressures. We observed incongruent (to crystal ZrO2 and SiO2 in melt) dissolution of zircon in natural mid-ocean ridge basaltic melt at low pressures and in haplobasaltic melt at elevated pressure. Our experimental data raise questions about the origin of zircons in mafic and ultramafic rocks, in particular, in shallow oceanic asthenosphere and deep lithosphere, as well as the meaning of the zircon-based ages estimated from the composition of these minerals. Large size zircon megacrysts in kimberlites, peridotites, alkali basalts and other magmas suggest the fast transport and short interaction between zircon and melt.The origin of zircon megacrysts is likely related to metasomatic addition of Zr into mantle as any mantle melting episode should obliterate them.
Abstract: Experimental data reveal that Earth’s mantle melts more readily than previously thought, and may have remained mushy until two to three billion years ago.
Earth and Planetary Science Letters, Vol. 533, 11p. Pdf
Mantle
carbon
Abstract: Knowledge of the effect of water on the density of carbonate melts is fundamental to constrain their mobility in the Earth's interior and the exchanges of carbon between deep and surficial reservoirs. Here we determine the density of hydrous MgCO3 and CaMg(CO3)2 melts (10 wt% H2O) from 1.09 to 2.98 GPa and 1111 to 1763 K by the X-ray absorption method in a Paris-Edinburgh press and report the first equations of state for hydrous carbonate melts at high pressure. Densities range from 2.26(3) to 2.50(3) g/cm3 and from 2.34(3) to 2.48(3) g/cm3 for hydrous MgCO3 and CaMg(CO3)2 melts, respectively. Combining the results with density data for the dry counterparts from classical Molecular Dynamic (MD) simulations, we derive the partial molar volume (, ) and compressibility of H2O and CO2 components at crustal and upper mantle conditions. Our results show that in alkaline carbonate melts is larger and less compressible than at the investigated conditions. Neither the compressibility nor depend on carbonate melt composition within uncertainties, but they are larger than those in silicate melts at crustal conditions. in alkaline earth carbonate melts decreases from 25(1) to 16.5(5) cm3/mol between 0.5 and 4 GPa at 1500 K. Contrastingly, comparison of our results with literature data suggests strong compositional effects on , that is also less compressible than in transitional melts (e.g., kimberlites) and carbonated basalts. We further quantify the effect of hydration on the mobility of carbonate melts in the upper mantle and demonstrate that 10 wt% H2O increases the mobility of MgCO3 melts from 37 to 67 g.cm?3.Pa?1s?1 at 120 km depth. These results suggest efficient carbonate melt extraction during partial melting and fast migration of incipient melts in the shallow upper mantle.
Geochemical Perspectives Letters, Vol. 4, pp. 1-6.
Africa, South Africa, Zimbabwe
metasomatism, geochronology
Abstract: Metasomatism, the chemical alteration of rocks by a variety of melts and fluids, has formed a key concept in studies of the Earth’s mantle for decades. Metasomatic effects are often inferred to be far-reaching and yet the evidence for their occurrence is usually based upon individual hand specimens or suites of rocks that display considerable heterogeneity. In rare cases, however, we are offered insights into larger-scale chemical modifications that occur in the mantle. Here we utilise the Lu–Hf systematics of zircon megacrysts erupted in kimberlite magmas to discern two temporally and compositionally discrete metasomatic events in the mantle beneath southern Africa, each having an influence extending over an area exceeding one million km2. These data provide unambiguous evidence for metasomatic processes operating at continental scales and seemingly unperturbed by the age and composition of the local lithospheric mantle. The most recent of these events may be associated with the major Jurassic-Karoo magmatism in southern Africa.
Science Advances, doi.10.1126/sciadv.abj1325 1p. Pdf
Mantle
subduction
Abstract: Earth’s carbon cycle is strongly influenced by subduction of sedimentary material into the mantle. The composition of the sedimentary subduction flux has changed considerably over Earth’s history, but the impact of these changes on the mantle carbon cycle is unclear. Here, we show that the carbon isotopes of kimberlite magmas record a fundamental change in their deep-mantle source compositions during the Phanerozoic Eon. The 13C/12C of kimberlites before ~250 Ma preserves typical mantle values, whereas younger kimberlites exhibit lower and more variable ratios-a switch coincident with a recognized surge in kimberlite magmatism. We attribute these changes to increased deep subduction of organic carbon with low 13C/12C following the Cambrian Explosion when organic carbon deposition in marine sediments increased significantly. These observations demonstrate that biogeochemical processes at Earth’s surface have a profound influence on the deep mantle, revealing an integral link between the deep and shallow carbon cycles.
Abstract: We reconstruct the history of topographic growth in southern Africa on both sides of the Orange River valley from an integrated analysis of erosion surfaces, crustal deformation and climate change. First, we propose an inventory of erosion surfaces observed in the study area and classify them according to their most likely formative process, i.e. chemical weathering or mechanical erosion. Among the various land units observed we define a new class of landform: the pedivalley, which corresponds to a wide valley with a flat erosional floor. In the Orange River valley, we mapped three low-relief erosion surfaces, each bevelling a variety of lithologies. The oldest and most elevated is (1) a stripped etchplain evolving laterally into (2) a stepped pediplain bearing residual inselbergs; (3) a younger pediplain later formed in response to a more recent event of crustal deformation. These are all Cenozoic landforms: the etchplain is associated with a late Palaeocene to middle Eocene weathering event, and the two pediplains are older than the middle Miocene alluvial terraces of the Orange River. Landscape evolution was first driven by slow uplift (10 m/Ma), followed by a second interval of uplift involving a cumulative magnitude of at least 200 m. This event shaped the transition between the two pediplains and modified the drainage pattern. A final phase of uplift (magnitude: 60 m) occurred after the Middle Miocene and drove the incision of the lower terraces of the Orange River. Climate exerted a major control over the denudation process, and involved very humid conditions responsible for lateritic weathering, followed by more arid conditions, which promoted the formation of pedivalleys. Collectively, these produce pediplains.
Abstract: There are two types of emerged relief on the Earth: high elevation areas (mountain belts and rift shoulders) in active tectonic settings and low elevation domains (anorogenic plateaus and plains) characteristic of the interior of the continents i.e. 70% of the Earth emerged relief. Both plateaus and plains are characterized by large erosional surfaces, called planation surfaces that display undulations with middle (several tens of kilometres) to very long (several thousands of kilometres) wavelengths, i.e. characteristic of lithospheric and mantle deformations respectively. Our objective is here (1) to present a new method of characterization of the very long and long wavelength deformations using planation surfaces with an application to Central Africa and (2) to reconstruct the growth of the very long wavelength relief since 40 Ma, as a record of past mantle dynamics below Central Africa. (i) The African relief results from two major types of planation surfaces, etchplains (weathering surfaces by laterites) and pediplains/pediments. These planation surfaces are stepped along plateaus with different elevations. This stepping of landforms records a local base level fall due to a local tectonic uplift. (ii) Central Africa is an extensive etchplain-type weathering surface - called the African Surface - from the uppermost Cretaceous (70 Ma) to the Middle Eocene (45 Ma) with a paroxysm around the Early Eocene Climatic Optimum. Restoration of this surface in Central Africa suggests very low-elevation planation surfaces adjusted to the Atlantic Ocean and Indian Ocean with a divide located around the present-day eastern branch of the East African Rift. (iii) The present-day topography of Central Africa is younger than 40 -30 Ma and records very long wavelength deformations (1000 -2000 km) with (1) the growth of the Cameroon Dome and East African Dome since 34 Ma, (2) the Angola Mountains since 15 -12 Ma increasing up to Pleistocene times and (3) the uplift of the low-elevation (300 m) Congo Basin since 10 -3 Ma. Some long wavelength deformations (several 100 km) also occurred with (1) the low-elevation Central African Rise since 34 Ma and (2) the Atlantic Bulge since 20 -16 Ma. These very long wavelength deformations record mantle dynamics, with a sharp increase of mantle upwelling around 34 Ma and an increase of the wavelength of the deformation and then of mantle convection around 10 -3 Ma.
Abstract: The rifted continental margins of Mozambique provide excellent examples of continental passive margins with a significant structural variability associated with magmatism and inheritance. Despite accumulated knowledge, the tectonic structure and nature of the crust beneath the South Mozambique Coastal Plain (SMCP) are still poorly known. This study interprets high-resolution seismic reflection data paired with data from industry-drilled wells and proposes a structural model of the Limpopo transform margin in a magma-rich context. Results indicate that the Limpopo transform margin is characterized by an ocean-continent transition that links the Beira-High and Natal valley margin segments and represents the western limit of the continental crust, separating continental volcano-sedimentary infilled grabens from the oceanic crust domain. These basins result from the emplacement of the Karoo Supergroup during a Permo-Triassic tectonic event, followed by an Early Jurassic tectonic and magmatic event. This latter led to the establishment of steady-state seafloor spreading at ca.156 Ma along the SMCP. A Late Jurassic to Early Cretaceous event corresponds to formation of the Limpopo transform fault zone. Which accommodated the SSE-ward displacement of Antarctica with respect to Africa. We define a new type of margin: the magma-rich transform margin, characterized by the presence of voluminous magmatic extrusion and intrusion coincident with the formation and evolution of the transform margin. The Limpopo transform fault zone consists of several syn-transfer and -transform faults rather than a single transform fault. The intense magmatic activity was associated primarily with mantle dynamics, which controlled the large-scale differential subsidence along the transform margin.
Academia.edu, Supplementary material app. 1 and 2, both 10p.
Asia, Kalimantan
deposit - Kalimantan
Abstract: Diamonds in alluvial deposits in Southeast Asia are not accompanied by indicator minerals suggesting primary kimberlite or lamproite sources. The Meratus Mountains in Southeast Borneo (Province Kalimantan Selatan, Indonesia) provide the largest known deposit of these so-called “headless” diamond deposits. Proposals for the origin of Kalimantan diamonds include the adjacent Meratus ophiolite complex, ultra-high pressure (UHP) metamorphic terranes, obducted subcontinental lithospheric mantle and undiscovered kimberlite-type sources. Here we report results from detailed sediment provenance analysis of diamond-bearing Quaternary river channel material and from representative outcrops of the oldest known formations within the Alino Group, including the diamond-bearing Campanian–Maastrichtian Manunggul Formation. Optical examination of surfaces of diamonds collected from artisanal miners in the Meratus area (247 stones) and in West Borneo (Sanggau Area, Province Kalimantan Barat; 85 stones) points toward a classical kimberlite-type source for the majority of these diamonds. Some of the diamonds host mineral inclusions suitable for deep single-crystal X-ray diffraction investigation. We determined the depth of formation of two olivines, one coesite and one peridotitic garnet inclusion. Pressure of formation estimates for the peridotitic garnet at independently derived temperatures of 930–1250 °C are between 4.8 and 6.0 GPa. Sediment provenance analysis includes petrography coupled to analyses of detrital garnet and glaucophane. The compositions of these key minerals do not indicate kimberlite-derived material. By analyzing almost 1400 zircons for trace element concentrations with laser ablation ICP-MS (LA-ICP-MS) we tested the mineral's potential as an alternative kimberlite indicator. The screening ultimately resulted in a small subset of ten zircons with a kimberlitic affinity. Subsequent U–Pb dating resulting in Cretaceous ages plus a detailed chemical reflection make a kimberlitic origin unfavorable with respect to the regional geological history. Rather, trace elemental analyses (U, Th and Eu) suggest an eclogitic source for these zircons. The age distribution of detrital zircons allows in general a better understanding of collisional events that formed the Meratus orogen and identifies various North Australian Orogens as potential Pre-Mesozoic sediment sources. Our data support a model whereby the majority of Kalimantan diamonds were emplaced within the North Australian Craton by volcanic processes. Partly re-deposited into paleo-collectors or residing in their primary host, these diamond-deposits spread passively throughout Southeast Asia by terrane migration during the Gondwana breakup. Terrane amalgamation events largely metamorphosed these diamond-bearing lithologies while destroying the indicative mineral content. Orogenic uplift finally liberated their diamond-content into new, autochthonous placer deposits.
Earth and Planetary Science Letters, Vol. 434, pp. 129-140.
Technology
Petrology - experimental
Abstract: Transitional melts, intermediate in composition between silicate and carbonate melts, form by low degree partial melting of mantle peridotite and might be the most abundant type of melt in the asthenosphere. Their role in the transport of volatile elements and in metasomatic processes at the planetary scale might be significant yet they have remained largely unstudied. Their molecular structure has remained elusive in part because these melts are difficult to quench to glass. Here we use FTIR, Raman, 13C and 29Si NMR spectroscopy together with First Principle Molecular Dynamic (FPMD) simulations to investigate the molecular structure of transitional melts and in particular to assess the effect of CO2 on their structure. We found that carbon in these glasses forms free ionic carbonate groups attracting cations away from their usual ‘depolymerising’ role in breaking up the covalent silicate network. Solution of CO2 in these melts strongly modifies their structure resulting in a significant polymerisation of the aluminosilicate network with a decrease in NBO/Si of about 0.2 for every 5 mol% CO2 dissolved. This polymerisation effect is expected to influence the physical and transport properties of transitional melts. An increase in viscosity is expected with increasing CO2 content, potentially leading to melt ponding at certain levels in the mantle such as at the lithosphere-asthenosphere boundary. Conversely an ascending and degassing transitional melt such as a kimberlite would become increasingly fluid during ascent hence potentially accelerate. Carbon-rich transitional melts are effectively composed of two sub-networks: a carbonate and a silicate one leading to peculiar physical and transport properties.
Earth and Planetary Science Letters, Vol. 533, 11p. Pdf
Mantle
carbon
Abstract: Knowledge of the effect of water on the density of carbonate melts is fundamental to constrain their mobility in the Earth's interior and the exchanges of carbon between deep and surficial reservoirs. Here we determine the density of hydrous MgCO3 and CaMg(CO3)2 melts (10 wt% H2O) from 1.09 to 2.98 GPa and 1111 to 1763 K by the X-ray absorption method in a Paris-Edinburgh press and report the first equations of state for hydrous carbonate melts at high pressure. Densities range from 2.26(3) to 2.50(3) g/cm3 and from 2.34(3) to 2.48(3) g/cm3 for hydrous MgCO3 and CaMg(CO3)2 melts, respectively. Combining the results with density data for the dry counterparts from classical Molecular Dynamic (MD) simulations, we derive the partial molar volume (, ) and compressibility of H2O and CO2 components at crustal and upper mantle conditions. Our results show that in alkaline carbonate melts is larger and less compressible than at the investigated conditions. Neither the compressibility nor depend on carbonate melt composition within uncertainties, but they are larger than those in silicate melts at crustal conditions. in alkaline earth carbonate melts decreases from 25(1) to 16.5(5) cm3/mol between 0.5 and 4 GPa at 1500 K. Contrastingly, comparison of our results with literature data suggests strong compositional effects on , that is also less compressible than in transitional melts (e.g., kimberlites) and carbonated basalts. We further quantify the effect of hydration on the mobility of carbonate melts in the upper mantle and demonstrate that 10 wt% H2O increases the mobility of MgCO3 melts from 37 to 67 g.cm?3.Pa?1s?1 at 120 km depth. These results suggest efficient carbonate melt extraction during partial melting and fast migration of incipient melts in the shallow upper mantle.
Researchgate , DOI: 10.1017/ 9781108677950.007 26p. Pdf
mantle
carbon
Abstract: Significant investment in new capacities for experimental research at high temperatures and pressures have provided new levels of understanding about the physical properties of carbon in fluids and melts, including its viscosity, electrical conductivity, and density. This chapter reviews the physical properties of carbon-bearing melts and fluids at high temperatures and pressures and highlights remaining unknowns left to be explored. The chapter also reviews how the remote sensing of the inaccessible parts of the Earth via various geophysical techniques - seismic shear wave velocity, attenuation, and electromagnetic signals of mantle depths - can be reconciled with the potential presence of carbon-bearing melts or fluids.
Abstract: Fluorine (50-650 ppm), bromine (0.03-0.3 ppm), iodine (0.03-0.4 ppm), boron (20-100 ppm) and nitrogen (5-45 ppm) concentrations are elevated in antigorite-serpentinites associated with the Tso Morari ultrahigh-pressure unit (Himalayas) exhumed from >100 km depth in the mantle wedge. These fluid-mobile elements are likely released with fluids from subducted marine sediments on the Indian continental margin to hydrate overlying forearc serpentinites at shallow depths. Of these, F and B appear to remain in serpentinites during the lizardite-antigorite transition. Our results demonstrate serpentinites as transient reservoirs of halogens, B, and N to at least 100 km depth in the mantle wedge, and likely deeper in colder slabs, providing a mechanism for their transport to the deeper mantle.
Journal of Metamorphic Geology, doi:10.1111/jmg.12599
Canada, Quebec
cratons
Abstract: Dating the onset of continental collision is fundamental in defining orogenic cycles and their effects on regional tectonics and geodynamic processes through time. Part of the Palaeoproterozoic Trans?Hudson Orogen, the Southeastern Churchill Province (SECP) is interpreted to result from the amalgamation of Archean to Palaeoproterozoic crustal blocks (amalgamated as the central Core Zone) that diachronically collided with the margins of the North Atlantic and Superior cratons, resulting in two bounding transpressive orogens: the Torngat and New Quebec Orogens. The SECP exposes mainly gneissic middle to lower orogenic crust in which deformation and amphibolite to granulite facies metamorphism and anatexis overprinted the early geological features classically used to constrain the timing of collisional events. To enable improved tectonic models for the development of the SECP, and the Trans?Hudson as a whole, we investigated granulite facies supracrustal sequences from the Tasiuyak Complex (TC) accretionary prism and the western margin of the North Atlantic Craton-that is, Saglek Block (upper plate)-using a multi?chronometer approach coupled with trace element geochemistry. In particular, the use of garnet Lu-Hf geochronology provides an important minimal time constraint for crustal thickening and collision. Garnet growth in the TC is constrained at 1885 ± 12 Ma (Lu-Hf), indistinguishable from U-Pb age of prograde monazite at 1873 ± 5 Ma. Zircon growth during melt crystallization occurred at 1848 ± 12 Ma. Garnet from the overriding Saglek Block is dated at 2567 ± 4.4 Ma (Lu-Hf) and indicates that gneissic rocks from the upper plate did not record the metamorphic imprint of the Torngat Orogeny. The diachronicity of the integrated metamorphic record across the strike of the SECP is explained by the location of terrane boundaries, consistent with the westward growth of the Churchill plate margin through sequential amalgamation of narrow crustal blocks during accretionary tectonics from c. 1.9 to 1.8 Ga.
Journal of Metamorphic Geology, Vol. 39, 8, 31p. Pdf
Canada
geochronology
Abstract: Dating the onset of continental collision is fundamental in defining orogenic cycles and their effects on regional tectonics and geodynamic processes through time. Part of the Palaeoproterozoic Trans-Hudson Orogen, the Southeastern Churchill Province (SECP) is interpreted to result from the amalgamation of Archean to Palaeoproterozoic crustal blocks (amalgamated as the central Core Zone) that diachronically collided with the margins of the North Atlantic and Superior cratons, resulting in two bounding transpressive orogens: the Torngat and New Quebec Orogens. The SECP exposes mainly gneissic middle to lower orogenic crust in which deformation and amphibolite to granulite facies metamorphism and anatexis overprinted the early geological features classically used to constrain the timing of collisional events. To enable improved tectonic models for the development of the SECP, and the Trans-Hudson as a whole, we investigated granulite facies supracrustal sequences from the Tasiuyak Complex (TC) accretionary prism and the western margin of the North Atlantic Craton—that is, Saglek Block (upper plate)—using a multi-chronometer approach coupled with trace element geochemistry. In particular, the use of garnet Lu-Hf geochronology provides an important minimal time constraint for crustal thickening and collision. Garnet growth in the TC is constrained at 1885 ± 12 Ma (Lu-Hf), indistinguishable from U-Pb age of prograde monazite at 1873 ± 5 Ma. Zircon growth during melt crystallization occurred at 1848 ± 12 Ma. Garnet from the overriding Saglek Block is dated at 2567 ± 4.4 Ma (Lu-Hf) and indicates that gneissic rocks from the upper plate did not record the metamorphic imprint of the Torngat Orogeny. The diachronicity of the integrated metamorphic record across the strike of the SECP is explained by the location of terrane boundaries, consistent with the westward growth of the Churchill plate margin through sequential amalgamation of narrow crustal blocks during accretionary tectonics from c. 1.9 to 1.8 Ga.
Da Silva Filho, A.F., Guimaraes, I.P., Thompson, R.N.
Shoshonitic and ultrapotassic Proterozoic intrusive suites in the Cachoeirinha-Saigueiro belt, northeast Brasil: a transition collisional to post-collisional magmatism
Precambrian Research, Vol. 62, No. 3, June pp. 323-342
Earths Oldest Rocks, researchgate.com Chapter 28, 20p. Pdf available
Canada, Manitoba
craton
Abstract: This chapter describes the Assean Lake Complex (ALC) at ancient crust at the Northwestern margin of the Superior Craton, Manitoba, and Canada. An initial tectonic model for the Assean Lake area indicated that a regionally extensive high-strain zone running through the lake marks the suture between Archean high-grade crustal terranes of the Superior Craton to the southeast and Paleoproterozoic rocks of the Trans-Hudson Orogen to the northwest. Detailed geologic remapping combined with isotopic and geochemical studies led to a re-interpretation of the crust immediately north of the Assean Lake high-strain zone as Mesoarchean. The study area straddles the boundary between the Archean Superior Craton and the ca.1.90-1.84 Ga arc and marginal basin rocks of the Trans-Hudson Orogen, which represent the remains of ca. 1.83-1.76 Ga ocean closure and orogeny. It is indicated that the gneisses of the Split Lake Block consist primarily of meta-igneous protoliths of gabbroic to granitic composition. Tonalite and granodiorite are the most volumetrically dominant, but an anorthosite dome is also present in the northeast. Mapping, isotopic, and age data combined with high-resolution aero-magnetic data indicate that the Mesoarchean ALC is a crustal slice up to 10 km wide, and has a strike length of at least 50 km.
Journal of Contemporary African Studies, Vol. 38, 1, pp. 39-54.
Africa, Zimbabwe
deposit - Chiadzwa
Abstract: The Chiadzwa diamonds attracted widespread attention due to human rights violations and illegal smuggling. When diamonds were discovered in 2006, thousands of artisanal miners descended on the diamond fields. In response, the government unleashed the army and police in brutal crackdowns to drive artisanal miners off the diamond fields. This militarisation of diamond fields and extraction was followed by forced displacement of the Chiadzwa people. This article examines the lived, everyday experiences of the displaced Chiadzwa people. Findings reveal that displacements dislocated the livelihoods and socialities of the people. Displacements also exacerbated people's vulnerability to livelihood shocks, insecurity, and poverty. In relocating people the government adopted a ‘top-down’ approach which triggered contestations and conflicts with the people who felt alienated from their ancestral land and excluded from diamond wealth. Consequently, sabotage, resistance and subversion were commonplace in the relocation process. These socio-political ‘tactics’ should be viewed as ‘weapons of the weak’.
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: Mobilistic plate-tectonic interpretation of Precambrian orogens requires that two conjoined crustal blocks may derive from distant portions of the globe. Nonetheless, many proposed Precambrian cratonic juxtapositions are broadly similar to those of younger times (so-called “strange attractors”), raising the specter of bias in their construction. We evaluated the possibility that the Congo and Kalahari cratons (Africa) were joined together prior to their amalgamation along the Damara-Lufilian-Zambezi orogen in Cambrian time by studying diabase dikes of the Huila-Epembe swarm and sills in the southern part of the Congo craton in Angola and in Namibia. We present geologic, U-Pb geochronologic, and paleomagnetic evidence showing that these two cratons were directly juxtaposed at ca. 1.1 Ga, but in a slightly modified relative orientation compared to today. Recurring persistence in cratonic connections, with slight variations from one supercontinent to the next, may signify a style of supercontinental transition similar to the northward motion of Gondwana fragments across the Tethys-Indian oceanic tract, reuniting in Eurasia.
Abstract: Mobilistic plate-tectonic interpretation of Precambrian orogens requires that two conjoined crustal blocks may derive from distant portions of the globe. Nonetheless, many proposed Precambrian cratonic juxtapositions are broadly similar to those of younger times (so-called “strange attractors”), raising the specter of bias in their construction. We evaluated the possibility that the Congo and Kalahari cratons (Africa) were joined together prior to their amalgamation along the Damara-Lufilian-Zambezi orogen in Cambrian time by studying diabase dikes of the Huila-Epembe swarm and sills in the southern part of the Congo craton in Angola and in Namibia. We present geologic, U-Pb geochronologic, and paleomagnetic evidence showing that these two cratons were directly juxtaposed at ca. 1.1 Ga, but in a slightly modified relative orientation compared to today. Recurring persistence in cratonic connections, with slight variations from one supercontinent to the next, may signify a style of supercontinental transition similar to the northward motion of Gondwana fragments across the Tethys-Indian oceanic tract, reuniting in Eurasia.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 24.
Canada, Northwest Territories
Deposit - Ekati
Abstract: How would a large open pit mine on caribou range (e.g., the Ekati Diamond Mine in the Bathurst caribou’s summer range) have influenced caribou? A traditional knowledge study on the cumulative impacts on the Bathurst caribou herd qualitatively described how mining activities might have influenced the herd (Mackenzie et al. 2013): caribou migration routes deflected away from the mines probably due to seeing mining activities or hearing the noises; and skinny caribou or abnormal smells and materials in caribou meat, liver, or the hide linings probably related to changes in caribou forage and quality of water and air. In other words, the potential influences of mining operations on caribou were most likely through altering what caribou can see, hear, smell (e.g., dusts and fine particle matter < 2.5 ?m (PM2.5) in the air, and from acidity in the soil), and taste (e.g., dust on foliage, vegetation composition change). Boulanger et al. (2012) estimated the size of a zone of influence (ZOI) of the Ekati-Diavik mining complex in the Bathurst caribou summer range, using caribou presence dataset. They also explored the mechanisms of ZOI using the spatial distribution of the total suspended particles, which was simulated with an atmospheric transport and dispersion model (Rescan, 2006). While these studies have added to our understanding of the possible impacts of mining operations on caribou, knowledge gaps remain. One outstanding gap is the lack of direct measurements about the caribou relevant environmental changes caused by mining operations. For example, exactly from how far away can caribou clearly see the vehicles driving on a mining road, or the buildings and the elevated waste piles in a camp? From how far away might caribou hear the noise caused by mining operations? To what spatial extent had the dusts and PM2.5 from mining operations influenced the tundra ecosystems? And how the dusts and PM2.5 from mining operations might have influenced caribou forage quality? Potentially these questions can be answered by in-situ measurements and satellite remote sensing. For example, studies have showed that it is possible to remotely sense PM2.5 distribution using twice-daily MODIS data at a spatial resolution of 1 km (Lyapustin et al., 2011; Chudnovsky et al., 2013; Hu et al., 2014). The objective of this study is thus to quantitatively measure these changes around the Ekati Diamond Mine, by means of in-situ surveys and satellite remote sensing. We conducted field surveys at more than 100 sites around the Ekati Diamond Mine during August 14-23, 2015, a collaborative effort of the NWT CIMP project entitled “Satellite Monitoring for Assessing Resource Development’s Impact on Bathurst Caribou (SMART)”, and the Dominion Diamond Ekati Corporation. In this presentation, we will report preliminary results and lessons learned from our first year’s study.
Abstract: The Republic of Liberia in West Africa is underlain mostly by Precambrian rocks of Archaean (Liberian) age in the west and of Proterozoic (Eburnean) age in the east. By analogy with similar terranes elsewhere in the world, and in West Africa in particular, the geology of Liberia is favourable for the occurrence of deposits of a wide range of metals and industrial minerals, including gold, iron ore, diamonds, base metals, bauxite, manganese, fluorspar, kyanite and phosphate. Known gold deposits, mostly orogenic in style, occur widely and are commonly associated with north-east-trending regional shear zones. Gold mining commenced at the New Liberty deposit in western Liberia in 2015, while significant gold resources have also been identified at several other sites in both Archaean and Proterozoic terranes. Liberia has large resources of itabirite-type iron ores, most of which are located in the Liberian terrane, and was the largest producer in Africa prior to the onset of civil war in 1989. Production of iron ore is currently restricted to a single mine, Yekepa, in the Nimba Range. Other important deposits, some of them previously mined, include Bong, the Western Cluster, Putu and Goe Fantro. There is a long history of alluvial diamond production in western and central Liberia, together with more than 160 known occurrences of kimberlite. Most of the known kimberlites occur in three clusters of small pipes and abundant dykes, located at Kumgbor, Mano Godua and Weasua, close to the border with Sierra Leone. Many of these are considered to be part of a single province that includes Jurassic age diamondiferous kimberlites in Sierra Leone and Guinea. Deposits and occurrences of a wide range of other metals and industrial minerals are also known. Several of these have been worked on a small scale in the past, mainly by artisanal miners, but most are poorly known in detail with sub-surface information available at only a few localities. By comparison with most other countries in West Africa, the geology of Liberia is poorly known and there has been very little systematic exploration carried out for most commodities other than gold, iron ore and diamonds since the 1960s and 1970s. Further detailed field and laboratory investigations using modern techniques are required to properly evaluate the potential for the occurrence of economic deposits of many minerals and metals in a variety of geological settings. Digital geological, geochemical, geophysical and mineral occurrence datasets, including new national airborne geophysical survey data, provide a sound basis for the identification of new exploration targets, but in almost every part of the country there is a need for new and more detailed geological surveys to underpin mineral exploration.
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: Stable (C and O) isotope data from carbonates are one of the most important methods used to infer genetic processes in carbonatites. However despite their ubiquitous use in geological studies, it is suspected that carbonates are susceptible to dissolution-reprecipitation and isotopic resetting, especially in shallow intrusions, and may not be the best records of either igneous or hydrothermal processes. Apatite, however, should be much less susceptible to these resetting problems but has not been used for O isotope analysis. In this contribution, a novel bulk-carbonatite method for the analysis of O isotopes in the apatite PO4 site demonstrates a more robust record of stable isotope values. Analyses of apatite from five carbonatites with magmatic textures establishes a preliminary Primary Igneous Apatite (PIA) field of ?18O = + 2.5 to + 6.0‰ (VSMOW), comparable to Primary Igneous Carbonatite (PIC) compositions from carbonates. Carbonate and apatite stable isotope data are compared in 10 carbonatite samples from Songwe Hill, Malawi. Apatite is heavy rare earth element (HREE) enriched at Songwe and, therefore, oxygen isotope analyses of this mineral are ideal for understanding HREE-related mineralisation in carbonatites. Carbonate C and O isotope ratios show a general trend, from early to late in the evolution, towards higher ?18O values (+ 7.8 to + 26.7‰, VSMOW), with a slight increase in ?13C (? 4.6 to ? 0.1‰, VPDB). Oxygen isotope ratios from apatite show a contrary trend, decreasing from a PIA field towards more negative values (+ 2.5 to ? 0.7‰, VSMOW). The contrasting results are interpreted as the product of the different minerals recording fluid interaction at different temperatures and compositions. Modelling indicates the possibility of both a CO2 rich fluid and mixing between meteoric and deuteric waters. A model is proposed where brecciation leads to depressurisation and rapid apatite precipitation. Subsequently, a convection cell develops from a carbonatite, interacting with surrounding meteoric water. REE are likely to be transported in this convection cell and precipitate owing to decreasing salinity and/or temperature.
Abstract: Compared to all published data from carbonatites and granitoids, the fluorapatite compositions in the Songwe Hill carbonatite, determined by EPMA and LA ICP-MS, have the highest heavy (H)REE concentration of any carbonatite apatite described so far. A combination of this fluorapatite and the REE fluorocarbonates, synchysite-(Ce) and parisite-(Ce), which are the other principal REE bearing minerals at Songwe, gives a REE deposit with a high proportion of Nd and a higher proportion of HREE (Eu-Lu including Y) than most other carbonatites. Since Nd and HREE are currently the most sought REE for commercial applications, the conditions that give rise to this REE profile are particularly important to understand. Multiple apatite crystallisation stages have been differentiated texturally and geochemically at Songwe and fluorapatite is divided into five different types (Ap-0-4). While Ap-0 and Ap-1 are typical of apatite found in fenite and calcite-carbonatite, Ap-2, -3 and -4 are texturally atypical of apatite from carbonatite and are progressively HREE-enriched in later paragenetic stages. Ap-3 and Ap-4 exhibit anhedral, stringer-like textures and their REE distributions display an Y anomaly. These features attest to formation in a hydrothermal environment and fluid inclusion homogenisation temperatures indicate crystallisation occurred between 200-350 °C. Ap-3 crystallisation is succeeded by a light (L)REE mineral assemblage of synchysite-(Ce), strontianite and baryte. Finally, late-stage Ap-4 is associated with minor xenotime-(Y) mineralisation and HREE-enriched fluorite. Fluid inclusions in the fluorite constrain the minimum HREE mineralisation temperature to approximately 160 °C. A model is suggested where sub-solidus, carbonatite-derived, (carbo)-hydrothermal fluids remobilise and fractionate the REE. Chloride or fluoride complexes retain LREE in solution while rapid precipitation of apatite, owing to its low solubility, leads to destabilisation of HREE complexes and substitution into the apatite structure. The LREE are retained in solution, subsequently forming synchysite-(Ce). This model will be applicable to help guide exploration in other carbonatite complexes.
Journal of African Earth Sciences, Vol. 134, pp. 10-23.
Africa, Malawi
carbonatite - Songwe Hill
Abstract: Songwe Hill, Malawi, is one of the least studied carbonatites but has now become particularly important as it hosts a relatively large rare earth deposit. The results of new mapping, petrography, geochemistry and geochronology indicate that the 0.8 km diameter Songwe Hill is distinct from the other Chilwa Alkaline Province carbonatites in that it intruded the side of the much larger (4 x 6 km) and slightly older (134.6 ± 4.4 Ma) Mauze nepheline syenite and then evolved through three different carbonatite compositions (C1–C3). Early C1 carbonatite is scarce and is composed of medium–coarse-grained calcite carbonatite containing zircons with a U–Pb age of 132.9 ± 6.7 Ma. It is similar to magmatic carbonatite in other carbonatite complexes at Chilwa Island and Tundulu in the Chilwa Alkaline Province and others worldwide. The fine-grained calcite carbonatite (C2) is the most abundant stage at Songwe Hill, followed by a more REE- and Sr-rich ferroan calcite carbonatite (C3). Both stages C2 and C3 display evidence of extensive (carbo)-hydrothermal overprinting that has produced apatite enriched in HREE (<2000 ppm Y) and, in C3, synchysite-(Ce). The final stages comprise HREE-rich apatite fluorite veins and Mn-Fe-rich veins. Widespread brecciation and incorporation of fenite into carbonatite, brittle fracturing, rounded clasts and a fenite carapace at the top of the hill indicate a shallow level of emplacement into the crust. This shallow intrusion level acted as a reservoir for multiple stages of carbonatite-derived fluid and HREE-enriched apatite mineralisation as well as LREE-enriched synchysite-(Ce). The close proximity and similar age of the large Mauze nepheline syenite suggests it may have acted as a heat source driving a hydrothermal system that has differentiated Songwe Hill from other Chilwa carbonatites.
Journal of Metamorphic Geology, Vol. 38, pp. 593-627.
Australia
geochronology
Abstract: The final assembly of the Mesoproterozoic supercontinent Nuna was marked by the collision of Laurentia and Australia at 1.60 Ga, which is recorded in the Georgetown Inlier of NE Australia. Here, we decipher the metamorphic evolution of this final Nuna collisional event using petrostructural analysis, major and trace element compositions of key minerals, thermodynamic modelling, and multi?method geochronology. The Georgetown Inlier is characterised by deformed and metamorphosed 1.70-1.62 Ga sedimentary and mafic rocks, which were intruded by c. 1.56 Ga old S?type granites. Garnet Lu-Hf and monazite U-Pb isotopic analyses distinguish two major metamorphic events (M1 at c. 1.60 Ga and M2 at c. 1.55 Ga), which allows at least two composite fabrics to be identified at the regional scale—c. 1.60 Ga S1 (consisting in fabrics S1a and S1b) and c. 1.55 Ga S2 (including fabrics S2a and S2b). Also, three tectono?metamorphic domains are distinguished: (a) the western domain, with S1 defined by low?P (LP) greenschist facies assemblages; (b) the central domain, where S1 fabric is preserved as medium?P (MP) amphibolite facies relicts, and locally as inclusion trails in garnet wrapped by the regionally dominant low?P amphibolite facies S2 fabric; and (c) the eastern domain dominated by upper amphibolite to granulite facies S2 foliation. In the central domain, 1.60 Ga MP-medium?T (MT) metamorphism (M1) developed within the staurolite-garnet stability field, with conditions ranging from 530-550°C at 6-7 kbar (garnet cores) to 620-650°C at 8-9 kbar (garnet rims), and it is associated with S1 fabric. The onset of 1.55 Ga LP-high?T (HT) metamorphism (M2) is marked by replacement of staurolite by andalusite (M2a/D2a), which was subsequently pseudomorphed by sillimanite (M2b/D2b) where granite and migmatite are abundant. P-T conditions ranged from 600 to 680°C and 4-6 kbar for the M2b sillimanite stage. 1.60 Ga garnet relicts within the S2 foliation highlight the progressive obliteration of the S1 fabric by regional S2 in the central zone during peak M2 metamorphism. In the eastern migmatitic complex, partial melting of paragneiss and amphibolite occurred syn? to post? S2, at 730-770°C and 6-8 kbar, and at 750-790°C and 6 kbar, respectively. The pressure-temperature-deformation-time paths reconstructed for the Georgetown Inlier suggest a c. 1.60 Ga M1/D1 event recorded under greenschist facies conditions in the western domain and under medium?P and medium?T conditions in the central domain. This event was followed by the regional 1.56-1.54 Ga low?P and high?T phase (M2/D2), extensively recorded in the central and eastern domains. Decompression between these two metamorphic events is ascribed to an episode of exhumation. The two?stage evolution supports the previous hypothesis that the Georgetown Inlier preserves continental collisional and subsequent thermal perturbation associated with granite emplacement.
Abstract: Jeffbenite, ideally Mg3Al2Si3O8, previously known as tetragonal-almandine-pyrope-phase (‘TAPP’), has been characterized as a new mineral from an inclusion in an alluvial diamond from São Luiz river, Juina district of Mato Grosso, Brazil. Its density is 3.576 g/cm3 and its microhardness is ?7. Jeffbenite is uniaxial (-) with refractive indexes ??=?1.733(5) and ??=?1.721(5). The crystals are in general transparent emerald green. Its approximate chemical formula is (Mg2.62Fe2+0.27)(Al1.86Cr0.16)(Si2.82Al0.18)O12 with very minor amounts of Mn, Na and Ca. Laser ablation ICP-MS showed that jeffbenite has a very low concentration of trace elements. Jeffbenite is tetragonal with space group I4¯2d, cell edges being a?=?6.5231(1) and c?=?18.1756(3) Å. The main diffraction lines of the powder diagram are [d (in Å), intensity, hkl]: 2.647, 100, 2 0 4; 1.625, 44, 3 2 5; 2.881, 24, 2 1 1; 2.220, 19, 2 0 6; 1.390, 13, 4 2 4; 3.069, 11, 2 0 2; 2.056, 11, 2 2 4; 1.372, 11, 2 0 12. The structural formula of jeffbenite can be written as (M1)(M2)2(M3)2(T1)(T2)2O12 with M1 dominated by Mg, M2 dominated by Al, M3 dominated again by Mg and both T1 and T2 almost fully occupied by Si. The two tetrahedra do not share any oxygen with each other (i.e. jeffbenite is classified as an orthosilicate). Jeffbenite was approved as a new mineral by the IMA Commission on New Minerals and Mineral Names with the code IMA 2014-097. Its name is after Jeffrey W. Harris and Ben Harte, two world-leading scientists in diamond research. The petrological importance of jeffbenite is related to its very deep origin, which may allow its use as a pressure marker for detecting super-deep diamonds. Previous experimental work carried out on a Ti-rich jeffbenite establishes that it can be formed at 13 GPa and 1700 K as maximum P-T conditions.
Isotopic disequilibration temperatures (Samarium/neodymium, Rubidium-Strontium) between mineral phases of coarse grained, low temperature garnet peridotites from Kimberley Floors.
Proceedings of Fifth International Kimberlite Conference, Vol. 1, pp. 354-365.
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.
Abstract: The formation of tectonic magmatic-related emerald deposits necessarily invokes a mixing model of Be-rich granitic rocks and Cr and/or V-rich surrounding rocks. However, there has been continuing debate on the deposit genesis, with the essential controversy being the relative significance of magma versus metamorphism in mineralizing as well as the key triggers for emerald deposition. The Dayakou emerald deposit genetically related to the Cretaceous granitic magmatism and hosted within the Neoproterozoic metasedimentary rocks is an ideal study case to probe into the above outstanding issue. In this paper, three hydrothermal mineralization and related alteration stages have been recognized in Dayakou, comprised of the greisenization and early emerald mineralization in high-temperature hydrothermal condition (stage-I; peak at 380 °C to 480 °C), the silicification and main emerald mineralization in medium-high temperature fluid (stage-II; peak at 300 °C to 360 °C) and the late carbonate alteration and scheelite mineralization (stage-III). Analysis results of fluid inclusion and C-H-O isotopes of emeralds and associated minerals suggest that ore-forming fluids belong to the H2O-NaCl ± CO2 system with minor H2S, CH4, and N2, exsolved from the Cretaceous granites and gradually interacted with the surrounding metamorphic rocks. We combine the new data with those reported in earlier studies to further propose a genesis scenario for the Dayakou deposit, in which Be-bearing fluids originally exsolved from peraluminous melts and fluoride complexes may be an effective transport proxy for Be in hydrothermal fluids. Fluid boiling during fluid ascent leads to the significant fractionation and enrichment of elements and the escape of volatiles (e.g., HF, H2O, CO2) in ore system. Meanwhile, sustained fluid-rock interaction (e.g., greisenization) increasingly extracts Cr, V and Ca into fluids to facilitate mineral precipitation, wherein the crystallization of fluoride minerals would cause the destabilization of Be-F complexes. Our study indicates that fluid boiling and fluid-rock interactions are the primary triggers for emerald deposition.
Abstract: Bastnäsite is the main ore mineral in many carbonatite-related rare earth element (REE) deposits, which account for ?51% of rare-earth oxide reserves worldwide. However, the occurrence, geochemistry, and genetic significance of bastnäsite has not been methodically investigated. The Cenozoic Mianning-Dechang (MD) REE belt in Sichuan Province, SW China, contains the Maoniuping, Dalucao, Lizhuang, and Muluozhai deposits as well as numerous smaller REE occurrences. Individual deposits within the belt contain different types of bastnäsite-bearing ore, which provides a unique opportunity to explore in detail the common mechanisms controlling the formation of bastnäsite-rich REE deposits. Here, we present detailed results from field observations and petrographic, geochemical, and fluid inclusion studies of bastnäsite from the main MD deposits. Calcite, fluorite, and barite form stable mineral assemblages that are commonly overprinted by bastnäsite. Homogenization temperatures of fluid inclusions in bastnäsite of ?150-270?°C (Dalucao and Lizhuang deposits) and 155-210?°C (Maoniuping deposit) are systematically lower than those of fluid inclusions in gangue minerals. Therefore, the petrographic studies and homogenization temperatures both show that large-scale crystallization of bastnäsite took place during the later stage of the hydrothermal system. The bastnäsite, relatively geochemically homogeneous within all of the MD deposits, is enriched in Ba (293-8425?ppm), Th (16.4-2527?ppm), and U (4.19-92.7?ppm), and relatively depleted in high field strength elements such as Nb (0.15-17.4?ppm), Ta (0.06-6.48?ppm), Zr (0.71-31.1?ppm), Hf (0.62-5.65?ppm), and Ti (<60?ppm), the same to carbonatite, and ore veins. In comparison, the samples from the study area show an increase in average REE contents from syenites to carbonatites to ore veins (i.e., bastnäsite-bearing ores) and finally to bastnäsite. Lanthanum and Ce were commonly substituted by Th, U, Sc, Ba, and Sr supplied by more evolved hydrothermal fluids. Combining the present results with existing data, we present a three-stage model for the formation of carbonatite-related REE deposits. First, partial melting of metasomatized sub-continental lithospheric mantle, fluxed by REE- and CO2-rich fluids, forms the parental carbonatite-syenite magma. Second, Sr, Ba, and REEs are strongly partitioned into carbonatite melts during liquid immiscibility in the carbonatite-syenite magmatic system. Third, hydrothermal fluids exsolved from the crystalizing syenite and carbonatite magmas form ore veins with early gangue minerals and later bastnäsite overgrowths. Consequently, barite, calcite, and fluorite assemblages are a valuable guide in REE exploration.
Geochemistry of late Mesozoic adakites from the Sulu belt, China: magma genesis and implications for crustal recycling beneath continental collisional orogens.
Lower crustal melting via magma underplating: elemental Sr Nd Pb isotopic constraints from late Mesozoic intermediate felsic volcanic rocks in NE Chin a block.
Contributions to Mineralogy and Petrology, Vol. 174, 5, 27p.
Africa, Ethiopia
xenoliths
Abstract: The behavior of sub-continental lithospheric mantle (SCLM) in extensional settings, up to successful rifting, plays an important role in geodynamics and in the global carbon cycle, yet the underlying processes and rates of lithosphere destruction remain poorly constrained. We determined platinum-group element (PGE: Os, Ir, Ru, Pt, and Pd) abundances and Re-Os-isotope systematics for well-characterized mantle xenoliths hosted in Cenozoic basalts from the northwestern plateau (Gundeweyn area) and southern rift zone (Dillo and Megado areas) of Ethiopia to provide new insights on the nature and timing of processes leading to the formation and transformation of the off-cratonic lithospheric mantle beneath the East Africa rift system (EARS). The whole-rock PGE concentrations are highly variable, with total PGE abundances ranging from 6.6 to 12.6 ppb for Gundeweyn, 11.5 to 23.3 ppb for Dillo, and 9.9 to 19.4 ppb for Megado mantle xenoliths. The 187Os/188Os ratios of the whole-rock mantle xenoliths vary from 0.1180 to 0.1287 for Gundeweyn, 0.1238 to 0.1410 for Dillo and 0.1165 to 0.1277 for Megado, compared to 0.130 for the Afar plume and???0.14 for the Kenya plume, with Re depletion ages up to 1.45 Ga for Gundeweyn, 0.64 Ga for Dillo, and 1.65 Ga for Megado mantle xenoliths. The regional differences between refertilizing agents recorded in mantle xenoliths from the plateau area and the rift systems reflect distinct tectonomagmatic settings: (1) low PGE abundances, with some retention of low 187Os/188Os in Gundeweyn peridotites, are ascribed to scavenging by early small-volume oxidizing melts, generated in the convecting mantle ahead of the arrival of the Afar plume. (2) Percolation of late-stage silicate/basaltic melts, associated with the arrival of hot mantle plume and lithosphere thinning in the rift setting, locally led to refertilization and sulfide precipitation and partial replenishment of the PGE (Dillo), with convecting mantle-like 187Os/188Os. Local enclaves of older, cryptically metasomatised mantle with unradiogenic Os (Megado) attest to the heterogeneous nature of melt-peridotite interaction at this stage (pervasive vs. focused melt flow). Highly depleted abundances of the compatible PGE are characteristic of SCLM affected by incipient rifting and percolation of oxidizing melts, here associated with the Afar and Kenya plume beneath the East Africa rift, and may be precursors to advanced degrees of lithosphere destruction/transformation.
Abstract: We report new ?13C ?values data and N?content and N?aggregation state values for microdiamonds recovered from peridotites and chromitites of the Luobusa ophiolite (Tibet) and chromitites of the Ray?Iz ophiolite in the Polar Urals (Russia). All analyzed microdiamonds contain significant nitrogen contents (from 108 up to 589 ± 20% atomic ppm) with a consistently low aggregation state, show identical IR spectra dominated by strong absorption between 1130 cm?1 and 1344 cm?1, and hence characterize Type Ib diamond. Microdiamonds from the Luobusa peridotites have ?13C ?PDB?values ranging from ?28.7‰ to ?16.9‰, and N?contents from 151 to 589 atomic ppm. The ?13C and N?content values for diamonds from the Luobusa chromitites are ?29‰ to ?15.5‰ and 152 to 428 atomic ppm, respectively. Microdiamonds from the Ray?Iz chromitites show values varying from ?27.6 ‰ to ?21.6 ‰ in ?13C and from 108 to 499 atomic ppm in N. The carbon isotopes values bear similar features with previously analyzed metamorphic diamonds from other worldwide localities, but the samples are characterized by lower N?contents. In every respect, they are different from diamonds occurring in kimberlites and impact craters. Our samples also differ from the few synthetic diamonds; we also analyzed showing enhanced ?13C ?variability and less advanced aggregation state than synthetic diamonds. Our newly obtained N?aggregation state and N?content data are consistent with diamond formation over a narrow and rather cold temperature range (i.e. <950°C), and in a short residence time (i.e. within several million years) at high temperatures in the deep mantle.
Abstract: Swat Valley has become an important source of emeralds, including recently discovered trapiche-type crystals. In this study, emerald samples from Swat were examined by standard gemological testing, UV-Vis-NIR, FTIR, Raman analysis, EDXRF, and LA-ICP-MS. The study found three-phase hexagonal inclusions consisting of water, gaseous carbon dioxide and nitrogen, and a magnesite crystal. The gaseous mixture in two-phase inclusions is characteristic in both trapiche-type (CO2 + N2) and non-trapiche samples (CO2 + N2 + CH4). Mineral inclusions of hematite, magnetite, rutile, graphite, and siderite are reported for the first time. Regular non-trapiche-type Swat emeralds contain high chromium (avg. 7471 ppmw), alkali metal (avg. 21040 ppmw), magnesium (avg. 34263 ppmw), and iron (avg. 9265 ppmw), as well as scandium (avg. 633 ppmw). Infrared spectra show that the absorption of type II H2O is stronger than that of type I H2O. Logarithm plots of trace elements appear to be diagnostic. Based on Raman spectroscopy, the trapiche-type emeralds’ colorless core, light green hexagonal growth zone area, and green rim are emerald, while the six black arms are a mixture of hematite and graphite.
Geochimica et Cosmochimica Acta, in press available. 45p.
Mantle
subduction
Abstract: Archean peridotite xenoliths in the ?2.52 Ga Zhulagou diorite (Yinshan Block, North China Craton) show chemical and Li isotopic evidence for metasomatism above an ancient subduction zone. The peridotite xenoliths are composed of olivine + orthopyroxene + amphibole + phlogopite + serpentine. The peridotite xenoliths have low whole-rock Mg# (80-81) and low Mg# (81-84) in olivine, indicating that they are cumulates that formed near the crust-mantle boundary. Petrological observations, mineral trace element data and isotopic ages show that the sequence of hydrous minerals is amphibole-serpentine-phlogopite. SIMS U-Pb dating of zircon from peridotites yielded an upper intercept age at ?2.53 Ga, and a U-Pb lower intercept age at ?1.8 Ga. The age of ?2.53 Ga is interpreted to date the crystallization of zircon from the metasomatized mantle melt that formed the Zhulagou cumulate peridotite. Rb-Sr mineral isochrons date phlogopite formation at ?1760 Ma, consistent with the lower intercept age of zircon. Pargasitic amphibole from the Zhulagou peridotites has fractionated REE, pronounced depletions of Nb, Ta, Zr and Ti, and heavy ?7Li (?+14‰) and light ?11B (?-11‰). Combined with slightly depleted mantle whole rock ?Nd (?+1.3) and high zircon ?18O (+5.6 to +7.0‰), the amphibole composition reflects that the peridotite xenoliths formed from melts that carried the geochemical and isotopic fingerprint typical for a metasomatized mantle wedge above a subduction zone. The Zhulagou peridotite xenoliths have the highest ?7Li values (?+12‰) recorded in Archean peridotites. Isotopically heavy Li and light B in olivine, orthopyroxene, and amphibole from the peridotite xenoliths show that Li and B may decouple during partial melting or fluid release from the subducted slab. The decoupling of Li and B may have a variety of reasons, including different host minerals for Li and B in the source and different protoliths in the subducted slab. The Li and B isotopic record on the recycling of ancient material demonstrates that modern-style subduction operated already in the late Archean.
Geochimica et Cosmochimica Acta, in press available pdf 45p.
China
craton
Abstract: Archean peridotite xenoliths in the ?2.52?Ga Zhulagou diorite (Yinshan Block, North China Craton) show chemical and Li isotopic evidence for metasomatism above an ancient subduction zone. The peridotite xenoliths are composed of olivine?+?orthopyroxene?+?amphibole?+?phlogopite?+?serpentine. The peridotite xenoliths have low whole-rock Mg# (80-81) and low Mg# (81-84) in olivine, indicating that they are cumulates that formed near the crust-mantle boundary. Petrological observations, mineral trace element data and isotopic ages show that the sequence of hydrous minerals is amphibole-serpentine-phlogopite. SIMS U-Pb dating of zircon from peridotites yielded an upper intercept age at ?2.53?Ga, and a U-Pb lower intercept age at ?1.8?Ga. The age of ?2.53?Ga is interpreted to date the crystallization of zircon from the metasomatized mantle melt that formed the Zhulagou cumulate peridotite. Rb-Sr mineral isochrons date phlogopite formation at ?1760?Ma, consistent with the lower intercept age of zircon. Pargasitic amphibole from the Zhulagou peridotites has fractionated REE, pronounced depletions of Nb, Ta, Zr and Ti, and heavy ?7Li (?+14‰) and light ?11B (?-11‰). Combined with slightly depleted mantle whole rock ?Nd (?+1.3) and high zircon ?18O (+5.6 to +7.0‰), the amphibole composition reflects that the peridotite xenoliths formed from melts that carried the geochemical and isotopic fingerprint typical for a metasomatized mantle wedge above a subduction zone. The Zhulagou peridotite xenoliths have the highest ?7Li values (?+12‰) recorded in Archean peridotites. Isotopically heavy Li and light B in olivine, orthopyroxene, and amphibole from the peridotite xenoliths show that Li and B may decouple during partial melting or fluid release from the subducted slab. The decoupling of Li and B may have a variety of reasons, including different host minerals for Li and B in the source and different protoliths in the subducted slab. The Li and B isotopic record on the recycling of ancient material demonstrates that modern-style subduction operated already in the late Archean.
Abstract: Natural emeralds from 11 mining areas were studied using an infrared spectrometer. The results showed different spectroscopic characteristics for emerald from different mine regions. Infrared absorption is mainly attributed to the vibration of Si-O lattice, channel water, alkaline cations, and molecules such as CO2, [Fe2(OH)4]2+, etc. Both near-infrared and mid-infrared spectra showed that the differences in band positions, intensities, and shapes are related to the mixed ratio of the two types of channel water. Accordingly, emerald and its mining regions can be divided into 3 types: H2O I, H2O II, and transition I-II. Furthermore, the study indicates that the relative amounts of the two different orientations of channel water molecules are mainly affected by the presence of (Mg + Fe)2+ in the host rock or in the mineralizing fluid. Therefore, the mineralization environment type (alkali-poor, alkali-rich, and transitional types) of emerald can be preliminarily identified from IR spectroscopy. This can be useful for determining the origin of emeralds.
Geochimica et Cosmochimica Acta, Vol. 270, pp. 95-111.
China, Russia
metasomatism, melting
Abstract: The eastern part of Asia between the North China and Siberian cratons contains orogenic belts formed by the Paleo-Asian and Pacific subduction and older continental blocks. A fundamental question regarding these and all mobile belts is the fate of the continental lithospheric mantle (CLM) during their formation, i.e. whether, or to what extent the CLM may be formed, replaced or affected during orogeny. Insights into these processes can be obtained from mantle xenoliths hosted by Cenozoic basalts in the Proterozoic Khanka block in the far eastern Russia between NE China and the Pacific coast of Asia. We report petrographic, chemical, and Os-Sr-Nd isotope data for spinel peridotite xenoliths at two Khanka sites: Sviyagin and Podgelban. The modal abundances and chemical compositions suggest that the peridotites are residues of low to moderate degrees of melt extraction from fertile mantle. They show an 187Os/188Os vs. 187Re/188Os correlation with an apparent 1.9?Ga age; the 187Os/188Os ratios are positively correlated with Al2O3 and other melt extraction indices. These results provide the first robust CLM age constraints for the eastern Central Asian Orogenic Belt (CAOB). The ages suggest that the ancient CLM of the Khanka block may be roughly coeval with reworked CLM at Hannuoba (North China craton), and that it persisted through the Phanerozoic orogenies. Moreover, despite the proximity to Phanerozoic subduction zones, the Khanka CLM shows little post-melting enrichment, e.g. the clinopyroxenes are typically LREE-depleted and have Sr-Nd isotope ratios typical of the MORB mantle. We posit that the metasomatism of the CLM, earlier proposed for North China xenolith suites and ascribed to the effects of Pacific or older subduction and related mantle upwelling, may not be widespread in the CAOB. In general, Proterozoic blocks composed of residual peridotites may be more common in the CLM of the SE Siberia and northern China, and possibly other orogenic belts, than previously thought.
Geochimica et Cosmochimica Acta, Vol. 270, pp. 95-111.
Russia
peridotites
Abstract: The eastern part of Asia between the North China and Siberian cratons contains orogenic belts formed by the Paleo-Asian and Pacific subduction and older continental blocks. A fundamental question regarding these and all mobile belts is the fate of the continental lithospheric mantle (CLM) during their formation, i.e. whether, or to what extent the CLM may be formed, replaced or affected during orogeny. Insights into these processes can be obtained from mantle xenoliths hosted by Cenozoic basalts in the Proterozoic Khanka block in the far eastern Russia between NE China and the Pacific coast of Asia. We report petrographic, chemical, and Os-Sr-Nd isotope data for spinel peridotite xenoliths at two Khanka sites: Sviyagin and Podgelban. The modal abundances and chemical compositions suggest that the peridotites are residues of low to moderate degrees of melt extraction from fertile mantle. They show an 187Os/188Os vs. 187Re/188Os correlation with an apparent 1.9?Ga age; the 187Os/188Os ratios are positively correlated with Al2O3 and other melt extraction indices. These results provide the first robust CLM age constraints for the eastern Central Asian Orogenic Belt (CAOB). The ages suggest that the ancient CLM of the Khanka block may be roughly coeval with reworked CLM at Hannuoba (North China craton), and that it persisted through the Phanerozoic orogenies. Moreover, despite the proximity to Phanerozoic subduction zones, the Khanka CLM shows little post-melting enrichment, e.g. the clinopyroxenes are typically LREE-depleted and have Sr-Nd isotope ratios typical of the MORB mantle. We posit that the metasomatism of the CLM, earlier proposed for North China xenolith suites and ascribed to the effects of Pacific or older subduction and related mantle upwelling, may not be widespread in the CAOB. In general, Proterozoic blocks composed of residual peridotites may be more common in the CLM of the SE Siberia and northern China, and possibly other orogenic belts, than previously thought.
Abstract: There are a few studies reported in the literature describing the conversion of intrinsic defects but the involvement of nitrogen-interstitials in diamond has not been reported so far. In this paper, a detailed study on the conversion of nitrogen-interstitials in diamond during the irradiation and further annealing were presented by the micro-photoluminescence spectra. The results indicated that the interstitials were immobile until 300?°C and then escaped from the nitrogen capture, followed by migration and recombination with vacancies in the structure of nitrogen-vacancy and vacancy centers.
Guo, S., Ye, K., Wu, Y., Chen, Y., Yang, Y., Zhang, L., Liu, J., Mao, Q., Ma, Y.
A potential method to confirm the previous existence of lawsonite in eclogite: the mass imbalance of Sr and LREEs in multi stage epidote ( Ganghe, Dabie UHP terrane).
Journal of Metamorphic Gology, Vol. 31, 4, pp. 415-435.
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.
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.
Reviews in Economic Geology, Vol. 18, pp. 115-136.
China
REE deposits
Abstract: China is the world’s leading rare earth element (REE) producer and hosts a variety of deposit types. Carbonatite-related REE deposits, the most significant deposit type, include two giant deposits presently being mined in China, Bayan Obo and Maoniuping, the first and third largest deposits of this type in the world, respectively. The carbonatite-related deposits host the majority of China’s REE resource and are the primary supplier of the world’s light REE. The REE-bearing clay deposits, or ion adsorption-type deposits, are second in importance and are the main source in China for heavy REE resources. Other REE resources include those within monazite or xenotime placers, beach placers, alkaline granites, pegmatites, and hydrothermal veins, as well as some additional deposit types in which REE are recovered as by-products. Carbonatite-related REE deposits in China occur along craton margins, both in rifts (e.g., Bayan Obo) and in reactivated transpressional margins (e.g., Maoniuping). They comprise those along the northern, eastern, and southern margins of the North China block, and along the western margin of the Yangtze block. Major structural features along the craton margins provide first-order controls for REE-related Proterozoic to Cenozoic carbonatite alkaline complexes; these are emplaced in continental margin rifts or strike-slip faults. The ion adsorption-type REE deposits, mainly situated in the South China block, are genetically linked to the weathering of granite and, less commonly, volcanic rocks and lamprophyres. Indosinian (early Mesozoic) and Yanshanian (late Mesozoic) granites are the most important parent rocks for these REE deposits, although Caledonian (early Paleozoic) granites are also of local importance. The primary REE enrichment is hosted in various mineral phases in the igneous rocks and, during the weathering process, the REE are released and adsorbed by clay minerals in the weathering profile. Currently, these REE-rich clays are primarily mined from open-pit operations in southern China. The complex geologic evolution of China’s Precambrian blocks, particularly the long-term subduction of ocean crust below the North and South China blocks, enabled recycling of REE-rich pelagic sediments into mantle lithosphere. This resulted in the REE-enriched nature of the mantle below the Precambrian cratons, which were reactivated and thus essentially decratonized during various tectonic episodes throughout the Proterozoic and Phanerozoic. Deep fault zones within and along the edges of the blocks, including continental rifts and strike-slip faults, provided pathways for upwelling of mantle material.
SEG Reviews In Economic Geology Chapter 6, Vol. 18, pp. 115-136.
China
REE
Abstract: China is the world’s leading rare earth element (REE) producer and hosts a variety of deposit types. Carbonatite- related REE deposits, the most significant deposit type, include two giant deposits presently being mined in China, Bayan Obo and Maoniuping, the first and third largest deposits of this type in the world, respectively. The carbonatite-related deposits host the majority of China’s REE resource and are the primary supplier of the world’s light REE. The REE-bearing clay deposits, or ion adsorption-type deposits, are second in importance and are the main source in China for heavy REE resources. Other REE resources include those within monazite or xenotime placers, beach placers, alkaline granites, pegmatites, and hydrothermal veins, as well as some additional deposit types in which REE are recovered as by-products. Carbonatite-related REE deposits in China occur along craton margins, both in rifts (e.g., Bayan Obo) and in reactivated transpressional margins (e.g., Maoniuping). They comprise those along the northern, eastern, and southern margins of the North China block, and along the western margin of the Yangtze block. Major structural features along the craton margins provide first-order controls for REE-related Proterozoic to Cenozoic carbonatite alkaline complexes; these are emplaced in continental margin rifts or strike-slip faults. The ion adsorption-type REE deposits, mainly situated in the South China block, are genetically linked to the weathering of granite and, less commonly, volcanic rocks and lamprophyres. Indosinian (early Mesozoic) and Yanshanian (late Mesozoic) granites are the most important parent rocks for these REE deposits, although Caledonian (early Paleozoic) granites are also of local importance. The primary REE enrichment is hosted in various mineral phases in the igneous rocks and, during the weathering process, the REE are released and adsorbed by clay minerals in the weathering profile. Currently, these REE-rich clays are primarily mined from open-pit operations in southern China. The complex geologic evolution of China’s Precambrian blocks, particularly the long-term subduction of ocean crust below the North and South China blocks, enabled recycling of REE-rich pelagic sediments into mantle lithosphere. This resulted in the REE-enriched nature of the mantle below the Precambrian cratons, which were reactivated and thus essentially decratonized during various tectonic episodes throughout the Proterozoic and Phanerozoic. Deep fault zones within and along the edges of the blocks, including continental rifts and strike-slip faults, provided pathways for upwelling of mantle material.
Abstract: A colour-changing garnet exhibits the "alexandrite effect", whereby its colour changes from green in the presence of daylight to purplish red under incandescent light. This study examines this species of garnets as well as the causes of the colour change by using infrared and ultraviolet visible (UV-Vis) spectroscopy. The infrared spectra show that the colour-changing garnets in this paper belong to the solid solution of pyrope-spessartine type. CIE1931 XYZ colour matching functions are used to calculate the colour parameters influencing garnet colour-changing under different light sources. The UV-Vis spectra show two zones of transmittance, in the red region at 650-700 nm and the blue-green region at 460-510 nm. As they exhibit the same capacity to transmit light, the colour of the gem is determined by the external light source. The absorption bands of Cr3+ and V3+ at 574 nm in the UV-Vis spectra are the main cause of the change in colour. With the increase in the area of peak absorption, the differences in the chroma and colour of the garnet gradually increase in daylight and incandescent light, and it exhibits a more prominent colour-changing effect.
Abstract: A modeling approach combining the entropy method and color difference formula is proposed in order to quantitatively evaluate diamond scintillation. The images of 66 diamonds were captured from 0° to 105° rotation at 15° intervals. The color difference of corresponding pixels in adjacent rotation angle images was calculated using a MatLab r2014a program, which indicated the diamond’s color change due to its scintillation. A threshold (10) was determined to divide the color difference into seven color difference intervals, the percentage of which indicated the color-change area. The color difference and the percentage were comprehensively analyzed using the entropy method to evaluate diamond scintillation objectively and quantitatively. Lightness was the main factor affecting the diamond scintillation while chroma and hue also significantly affected it.
Post collisional, potassic and ultrapotassic magmatism of the northern Tibetan Plateau: constraints on characteristics of the mantle source, geodynamic uplift
Abstract: Continental intraplate basalts (15.42-0.16 Ma) from Abaga-Dalinuoer volcanic field (ADVF) in central Inner Mongolia of eastern China, as a part of Cenozoic volcanic province along eastern margin of the Eurasian continent, provide a good opportunity to explore potential links between deep subduction of the Pacific slab and continental intraplate volcanism. In this study, we report an integrated dataset of whole-rock K-Ar ages, major and trace elements and Sr-Nd-Pb isotopes, and olivine major and minor elements for the Abaga-Dalinuoer basalts (ADBs), and propose that mantle source lithology of the ADB magmas may consist of both pyroxenite and peridotite. The ADBs display low SiO2 (42.3-50.2 wt.%), high MgO (7.3-11.4 wt.%) and moderate K2O + Na2O (3.8-6.4 wt.%), and can be subdivided into basanites, alkali basalts and tholeiitic basalts that are all characterized by ocean island basalt (OIB)-like rare earth elements (REE) and enrichment in both large ion lithosphile elements (LILE) and high field strength elements (HFSE). Olivine phenocrysts have higher Ni and Fe/Mn and lower Mn, Ca and Ca/Fe relative to those from peridotite melts, but exhibit clearly lower Ni contents (< 2500 ppm) compared with expected Ni range (> 3000 ppm) for olivines crystallized from olivine-free pyroxenite melts. Estimated compositions of the ADB primary magmas, together with olivine compositions, suggest an iron-rich mantle source related with silica-deficient pyroxenite that is most likely derived from deeply subducted Pacific oceanic crust. Additionally, peridotite and recent subducted sediments are also required to account for high Ni and MgO in primary magmas together with their trace elements and Sr-Nd-Pb isotope systematics. We suggest that a mixed pyroxenite-peridotite source lithology can better match observed whole-rock and olivine signatures in the ADB, compared with either peridotite only or olivine-free pyroxenite only source lithology. In our model, pyroxenite melts would either react with or mechanically mix with peridotite, and the proportion of pyroxenite melts may range from 30% to 45% for mechanical mixing scenario. A continuous spectrum from tholeiitic to alkali melts revealed by melt-peridotite reaction experiment can explain formation of primary magmas of basanites, alkali basalts and tholeiitic basalts by increasing melting degree of a similar mantle source. Relatively higher 206Pb/204Pb of the ADB may suggest more significant role of recent (< 0.5 Ga) subducted Pacific oceanic materials, in contrast to other Cenozoic basalts in eastern China (e.g., Changbai basalts) that exhibit varying contributions from ancient (> 1.5 Ga) subducted continental sediments. We emphasize that coupled geochemical and geodynamic links (i.e., subduction polarity) between deeply subducted Pacific slab and continental intraplate volcanism in eastern China may exist, which directly support the involvement of deeply subducted Pacific materials in petrogenesis of the ADB. From the perspective of plate motion kinetics, decompression partial melting of upwelling fragmented Pacific slab (silica-deficient pyroxenite + recent subducted sediments) may be triggered by rollback of deeply subducted Pacific slab during Late Cenozoic times. Continental intraplate volcanism in the ADVF generally started with termination of opening of the Japan Sea, suggesting that deep subduction of the Pacific slab may have been an important geodynamic mechanism responsible for tectono-magmatic evolution of northeastern Asia. We suggest that the ADBs have the potential to shed light on genetic links between continental intraplate volcanism and deep subduction of the Pacific slab in geochemical and geodynamic processes.
Abstract: Widespread Miocene (24-8 Ma) ultrapotassic rocks and their entrained xenoliths provide information on the composition, structure, and thermal state of the sub-continental lithospheric mantle in southern Tibet during the India-Asia continental collision. The ultrapotassic rocks along the Lhasa block delineate two distinct lithospheric domains with different histories of depletion and enrichment. The eastern ultrapotassic rocks (89°E-92°E) reveal a depleted, young, and fertile lithospheric mantle (87Sr/86Srt = 0.704-0.707 [t is eruption time]; Hf depleted-mantle model age [TDM] = 377-653 Ma). The western ultrapotassic rocks (79°E-89°E) and their peridotite xenoliths (81°E) reflect a refractory harzburgitic mantle refertilized by ancient metasomatism (lavas: 87Sr/86Srt = 0.714-0.734; peridotites: 87Sr/86Srt = 0.709-0.716). These data integrated with seismic tomography suggest that upwelling asthenosphere was diverted away from the deep continental root beneath the western Lhasa block, but rose to shallower depths beneath a thinner lithosphere in the eastern part. Heating of the lithospheric mantle by the rising asthenosphere ultimately generated the ultrapotassic rocks with regionally distinct geochemical signatures reflecting the different nature of the lithospheric mantle.
Abstract: Carbonates in subducting oceanic slabs can survive beyond slab dehydration and be transferred into the deep mantle. Such deep carbon cycling plays a critical role in generating carbonatitic to alkaline melts. However, whether and how this process has influenced the lithospheric mantle still remains enigmatic. To address these issues, here we provide a detailed petrographic, in-situ chemical and Sr isotopic study on two mantle xenoliths (a wehrlite and a melt pocket-bearing peridotite) entrained by the Changle Miocene basalts from the eastern China. The Changle wehrlite contains carbonate melt inclusions and apatites and is merely enriched in clinopyroxene relative to the lherzolites. The clinopyroxenes are characterized by high (La/Yb)N (4.7-41) and low Ti/Eu (873-2292) ratios and equilibrated with carbonated silicate melt-like compositions. These petrographic and chemical features indicate that the wehrlite was formed by reaction between peridotite and carbonated silicate melts. On the other hand, the Changle melt pocket-bearing peridotite is suggested to have been produced by in-situ melting/breakdown of amphiboles of an amphibole-rich dunite. Low olivine Fo (~89), presence of amphiboles with high (La/Yb)N (~50) and low Ti/Eu (~1070) ratios suggest that such amphibole-rich dunite would have been formed by reaction of peridotite with hydrous alkaline basaltic melts from a carbonated mantle. Our data, combined with previously reported data of the Changle lherzolite xenoliths, unravel a series of mantle metasomatisms by carbonatitic to alkaline melts from carbonated mantle sources. The consistently high 87Sr/86Sr ratios (up to 0.7036) of the clinopyroxenes in both the wehrlites and lherzolites indicate the carbonate components in the mantle sources were derived from the stagnant Pacific slab within the Mantle Transition Zone. This study provides a fresh perspective on the role of deep carbon cycling from subducted oceanic slabs in chemical modification of intracontinental lithospheric mantle through reaction with different types of melts.
Abstract: Peridotites occur as lensoid bodies within the Mesoarchaean orthogneiss in the Akia terrane of Southern West Greenland. The Ulamertoq peridotite body is the largest of these peridotites hosted within the regional orthogneiss. It consists mainly of olivine, orthopyroxene, and amphibole-rich ultramafic rocks exhibiting metamorphic textural and chemical features. Chromitite layers from different localities in Ulamertoq show contrasting characteristics. In one locality, zoned chromites are hosted in orthopyroxene-amphibole peridotites. Compositional zonation in chromites is evident with decreasing Cr and Fe content from core to rim, while Al and Mg increase. Homogeneous chromites from another locality are fairly uniform and Fe-rich. The mineral chemistry of the major and accessory phases shows metamorphic signatures. Inferred temperature conditions suggest that the zoned chromites, homogeneous chromites, and their hosts are equilibrated at different metamorphic conditions. In this paper, various mechanisms during the cumulus to subsolidus stages are explored in order to understand the origin of the two contrasting types of chromites.
Abstract: Peridotites occur as lensoid bodies within the Mesoarchaean orthogneiss in the Akia terrane of Southern West Greenland. The Ulamertoq peridotite body is the largest of these peridotites hosted within the regional orthogneiss. It consists mainly of olivine, orthopyroxene, and amphibole-rich ultramafic rocks exhibiting metamorphic textural and chemical features. Chromitite layers from different localities in Ulamertoq show contrasting characteristics. In one locality, zoned chromites are hosted in orthopyroxene-amphibole peridotites. Compositional zonation in chromites is evident with decreasing Cr and Fe content from core to rim, while Al and Mg increase. Homogeneous chromites from another locality are fairly uniform and Fe-rich. The mineral chemistry of the major and accessory phases shows metamorphic signatures. Inferred temperature conditions suggest that the zoned chromites, homogeneous chromites, and their hosts are equilibrated at different metamorphic conditions. In this paper, various mechanisms during the cumulus to subsolidus stages are explored in order to understand the origin of the two contrasting types of chromites.
Abstract: Peridotites occur as lensoid bodies within the Mesoarchaean orthogneiss in the Akia terrane of Southern West Greenland. The Ulamertoq peridotite body is the largest of these peridotites hosted within the regional orthogneiss. It consists mainly of olivine, orthopyroxene, and amphibole-rich ultramafic rocks exhibiting metamorphic textural and chemical features. Chromitite layers from different localities in Ulamertoq show contrasting characteristics. In one locality, zoned chromites are hosted in orthopyroxene-amphibole peridotites. Compositional zonation in chromites is evident with decreasing Cr and Fe content from core to rim, while Al and Mg increase. Homogeneous chromites from another locality are fairly uniform and Fe-rich. The mineral chemistry of the major and accessory phases shows metamorphic signatures. Inferred temperature conditions suggest that the zoned chromites, homogeneous chromites, and their hosts are equilibrated at different metamorphic conditions. In this paper, various mechanisms during the cumulus to subsolidus stages are explored in order to understand the origin of the two contrasting types of chromites.
Abstract: A titanian clinohumite-bearing dunite was recently found in the Ulamertoq ultramafic body within the 3.0 Ga Akia Terrane of southern West Greenland. Titanian clinohumite occurs as disseminated and discrete grains. Titanian clinohumite contains relatively high amounts of fluorine, reaching up to 2.4 wt.%. The high-Fo content of olivine (Fo93) coupled with low Cr/(Cr + Al) ratio of orthopyroxene implies that the dunite host is not of residual origin after melt extraction by partial melting of the primitive mantle. Olivine grains are classified into two types based on abundances of opaque mineral inclusions: (1) dusty inclusion-rich and (2) clear inclusion-free olivines. Opaque inclusions in coarse-grained olivines are mainly magnetite. Small amounts of ilmenite are also present around titanian clinohumite grains. The observed mineral association indicates partial replacement of titanian clinohumite to ilmenite (+magnetite) and olivine following the reaction: titanian clinohumite = ilmenite + olivine + hydrous fluid. The coexistence of F-bearing titanian clinohumite, olivine, and chromian chlorite indicates equilibration at around 800-900 °C under garnet-free conditions (<2 GPa). Petrological and mineralogical characteristics of the studied titanian clinohumite-bearing dunite are comparable to deserpentinized peridotites derived from former serpentinites. This study demonstrates the importance of considering the effects of hydration/dehydration processes for the origin of ultramafic bodies found in polymetamorphic Archaean terranes.
The Canadian Mineralogist, Vol. 54, pp. 1285-1291.
China, Mongolia
carbonatite - Bayan Obo
Abstract: Fluorcalciopyrochlore, ideally (Ca,Na)2Nb2O6F, cubic, is a new mineral species (IMA2013-055) occurring in the Bayan Obo Fe-Nb-REE deposit, Inner Mongolia, People's Republic of China. The mineral is found in a dolomite-type niobium rare-earth ore deposit. Associated minerals are dolomite, aegirine, riebeckite, diopside, fluorite, baryte, phlogopite, britholite-(Ce), bastnäsite-(Ce), zircon, magnetite, pyrite, fersmite, columbite-(Fe), monazite-(Ce), rutile, and others. Crystals mostly form as octahedra {111}, dodecahedra {110}, and cubes {100}, or combinations thereof, and generally range in size from 0.01 to 0.3 mm. It is brownish-yellow to reddish-orange in color with a light yellow streak. Crystals of fluorcalciopyrochlore are translucent to transparent with an adamantine to greasy luster on fractured surfaces. It has a conchoidal fracture. No parting or cleavage was observed. The Mohs hardness is 5, and the calculated density is 4.34(1) g/cm3. The empirical formula is (Ca1.14Na0.74Ce0.06Sr0.03Th0.01Fe0.01Y0.01La0.01Nd0.01)?2.02(Nb1.68Ti0.29Zr0.02Sn0.01)?2.00O6.00(F0.92O0.08)?1.00 on the basis of 7(O,F) anions pfu. The simplified formula is (Ca,Na)2Nb2O6F. The strongest four reflections in the X-ray powder-diffraction pattern [d in Å (I) hkl] are: 6.040 (9) 1 1 1, 3.017 (100) 2 2 2, 2.613 (17) 0 0 4, 1.843 (29) 0 4 4, and 1.571 (15) 2 2 6. The unit-cell parameters are a 10.4164(9) Å, V 1130.2(2) Å3, Z = 8. The structure was solved and refined in space group FdEmbedded Image m with R = 0.05. The type material is deposited in the Geological Museum of China, Beijing, People's Republic of China, catalogue number M12182.
Journal of Geophysical Research, Vol. 120, 10, pp. 6698-6721.
India
Geophysics - craton
Abstract: Magnetotelluric-derived two-dimensional lithospheric resistivity structure of the western Dharwar craton (WDC) and adjoining Coorg block indicates isolated low-resistivity zones in the crust and three striking upper mantle conductive features within the highly resistive Archean lithosphere. The crustal conductors in the WDC show good spatial correlation with the exposed supracrustal rocks conformable with the relic schist belt channels having conductive mineral grains. Conductive zones within the Coorg crust might be related to the relatively young (933?Ma) metamorphic processes in the area and/or possible fluids derived from the Cretaceous passage of Reunion plume in the proximity of Coorg area. A near-vertical conductive structure extending from the lower crust into the upper mantle coincides with the transition zone between Coorg and WDC. This is interpreted as the suture zone between the two tectonic blocks and provides evidence for the individuality of the two Archean terrains. An anomalous upper mantle conductive zone found beneath the craton nucleus may indicate a modified cratonic lithosphere. This could have been derived due to the collision between Coorg and WDC and possibly survived by the subsequent multiple episodes of melt and fluid infiltration processes experienced in the region. Thick (~190?km) and preserved lithosphere is mapped at the eastern segment of WDC. Resistive lithosphere of ~125?km thickness is imaged for the Coorg block.
Physics of the Earth and Planetary Interiors, Vol. 306, 106543, 9p. Pdf
India
geophysics -seismic
Abstract: The south Indian shield, primarily consisting of Archean cratons and Cretaceous-Tertiary Deccan Volcanic Province (DVP), has undergone several major tectonic episodes during its evolution. The Deccan volcanism at Cretaceous-Tertiary (~65 Ma) is the last major tectono-thermal event, which influenced a substantial part of the south Indian shield. To understand the influence of the Deccan volcanism on the evolution of the south Indian shield, we study the crustal seismic structure of the ~65 Ma Deccan Volcanic Province and the adjacent ~2.6 Ga Eastern Dharwar Craton (EDC), which forms the basement of the volcanic terrain. We calculate teleseismic receiver functions for 18 broadband seismic stations along a ~1000 km long seismological profile that cut across both the EDC and DVP. The analysis and modelling, using H-Vp/Vs stacking and generalized neighbourhood algorithm inversion of the receiver functions show distinct crustal structure (crustal thickness, average composition, shear wave velocity variation, nature of crust-mantle boundary, etc.) across the EDC and DVP. The results clearly indicate that the crustal structure is heterogeneous beneath the DVP compared to a relatively uniform structure below the EDC. Using results from this study along with earlier results, we infer that the present Eastern Dharwar Craton terrain is not affected by any tectono-thermal event for a long geological time, including the Deccan volcanism. Whereas, the present Deccan Volcanic Province is highly affected by the Reunion mantle plume-crust interaction.
Diamonds from the Behradih kimberlite pipe, Bastar craton, India: a reconnaissance study.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 309-316.
Petrology of lamproites from the Nuapada lamproite field, Bastar craton, India.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 137-165.
Abstract: All living things contain carbon in some form, as it is the primary component of macromolecules including proteins, lipids, nucleic acids (RNA and DNA), and carbohydrates. As a matter of fact, it is the backbone of all organic (chemistry) compounds forming different kinds of bonds. Carbon: The Black, the Gray and the Transparent is not a complete scientific history of the material, but a book that describes key discoveries about this old faithful element while encouraging broader perspectives and approaches to its research due to its vast applications. All allotropes of carbon are described in this book, along with their properties, uses, and methods of procurement or manufacturing. Black carbon is represented by coal, gray carbon is represented by graphite, and transparent carbon is represented by diamond.
Abstract: The graphite-to-diamond transformation under shock compression has been of broad scientific interest since 1961. The formation of hexagonal diamond (HD) is of particular interest because it is expected to be harder than cubic diamond and due to its use in terrestrial sciences as a marker at meteorite impact sites. However, the formation of diamond having a fully hexagonal structure continues to be questioned and remains unresolved. Using real-time (nanosecond), in situ x-ray diffraction measurements, we show unequivocally that highly oriented pyrolytic graphite, shock-compressed along the c axis to 50 GPa, transforms to highly oriented elastically strained HD with the (100)HD plane parallel to the graphite basal plane. These findings contradict recent molecular dynamics simulation results for the shock-induced graphite-to-diamond transformation and provide a benchmark for future theoretical simulations. Additionally, our results show that an earlier report of HD forming only above 170 GPa for shocked pyrolytic graphite may lead to incorrect interpretations of meteorite impact events.
Geological Society of America Annual Meeting, Vol. 50, 4, 1p. Abstract
United States, Kansas
kimberlite
Abstract: The Critical Zone is the realm where rocks meet life. This study examines the physicochemical interactions that occur when interbedded limestone-shale systems and kimberlitc eruptive materials weather to form soils. Fast weathering with extensive soil loss has been a major environmental concern in the Flint Hills. Knowledge of soil formation processes, rates of formation and loss and understanding how these processes differ in different systems are critical for managing soil as a resource. The kimberlites of Riley County, KS, are CO2-rich igneous rocks that are high in Mg and Fe; they are compositionally distinct from the Paleozoic limestones and shales found throughout the rest of the region. Bulk composition and mineralogy of the soils overlying these different bedrock types have been analyzed using X-Ray Fluorescence (XRF), X-Ray diffraction of <2mm soil fraction and <2µm soil clay fraction, bulk elemental extraction, and particle size analyses. Results show that the kimberlitic soils have higher concentrations of Fe, Mg, Ca, K and some trace elements (e.g. Ti, Ni, Cu). The weathering products differ mineralogically as well, e.g. lizardite is abundant in kimberlitic soils and absent from the limestone terrane. As a result, kimberlite-sourced soils have significantly different physical properties than the thin limestone-sourced soils surrounding them. Particle size analysis shows that the limestone-shale soils have a higher proportion of silt-sized particles whereas the kimberlitic soils have more clay (10.55% vs. 8.06%) and significantly more sand (36.12% vs. 14.83%). Mineralogy was determined for all <2mm fractions and for some <2µm soil clay fraction to understand the association and mobility of these major and trace elements in the respective soils. Interestingly some of the similarities between the kimberlite and limestone-shale soils suggests that loess/wind-blown sediment is making a significant contribution to the soil profile. Kimberlite-sourced and limestone-shale-sourced soils produce different weathering products and could potentially have agricultural significance in terms of ionic and nutrient mobility.
Abstract: Trapped-charge dating methods including luminescence and electron spin resonance dating have high potential as low temperature (< 100 °C) thermochronometers. Despite an early proof of concept almost 60 years ago, it is only in the past two decades that thermoluminescence (TL), electron-spin-resonance (ESR), and optically stimulated luminescence (OSL), have begun to gain momentum in geological thermochronometry and thermometry applications. Here we review the physics of trapped-charge dating, the studies that led to its development and its first applications for deriving palaeo-temperatures and/or continuous cooling histories. Analytical protocols, which enable the derivation of sample specific kinetic parameters over laboratory timescales, are also described. The key limitation of trapped-charge thermochronometry is signal saturation, which sets an upper limit of its application to < 1 Ma, thus restricting it to rapidly exhuming terrains (> 200 °C Ma? 1), or elevated-temperature underground settings (> 30 °C). Despite this limitation, trapped-charge thermochronometry comprises a diverse suite of versatile methods, and we explore potential future applications and research directions.
Abstract: Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth’s mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source1, 2. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes3, 4, 5, 6 while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes7, 8, 9. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5?mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone10. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth’s history.
Abstract: How the Earth's continental crust was formed in the Hadean eon is a subject of considerable debates [1-4]. For example, shallow hydrous peridotites [2,5], in particular the Hadean Earth's serpentinites [6], are potentially important ingredients in the creation of the continental ptoto-crust, but the mechanisms of this formation remain elusive. In this work, experiments to explore serpentinite-basalt interaction under conditions of the Hadean Earth were conducted. Kinetic runs lasting 0.5 to 48 hours at 0.2 to 1.0 GPa and 1250 to 1300°C reveal dehydration of serpentinite and release of a Si-Al-Na-K-rich aqueous fluid. For the first time, generation of heterogeneous hydrous silicic melts (56 to 67 wt% SiO2) in response to the fluid-assisted fertilisation and the subsequent partial melting of the dehydrated serpentinite has been discovered. The melts produced at 0.2 GPa have compositions similar to those of the bulk continental crust [2,3]. These new findings imply that the Earth's sialic proto-crust may be generated via fluid-assisted melting of serpentinized peridotite at shallow depths (?7 km) that do not require plate subduction during the Hadean eon. Shallow serpentinite dehydration and melting may be the principal physico-chemical processes affecting the earliest lithosphere.
Making Earth's continental crust from serpentinite and basalt.
Abstract: We have studied mineral-hosted melt, crystal and fluid inclusions from two ugandite, one mafurite and two katungite samples from the Toro-Ankole volcanic province in the East African Rift, which is the archetypal location for kamafugitic rocks. A main finding of our study is the presence of orthopyroxene as inclusions in an early generation of olivine from all three types of kamafugites, suggesting interaction of a carbonate-rich metasomatic agent with lithospheric peridotite mantle that may have caused almost complete dissolution of orthopyroxene. This process was preceded, accompanied or followed by the formation of phlogopite-clinopyroxene veins resulting from interaction of F-rich and low H2O/CO2 metasomatic fluids with the mantle rocks, which then became the source of the Toro-Ankole kamafugites. Pressure-temperature (P-T) estimates suggest that the parental kamafugitic melts last equilibrated with their source rocks at ?16?±?8?kbar and ?1160?±?130°C. This implies that they could have originated significantly below the solidus of dry, carbonated peridotite, but above the solidus of phlogopite-bearing clinopyroxenite. We conclude that the Toro-Ankole kamafugites originated by very low degrees of partial melting at moderately oxidized conditions (?FMQ = +2•2?±?0•4?atm log units, where FMQ is fayalite-magnetite-quartz buffer) under a high geothermal gradient of 60-80?mW?m?2, in response to lithospheric extension and probable association with an adjacent mantle plume. We estimate that differentiation of parental ugandite, mafurite and katungite magmas could have occurred at depths <12?km in the T range 1150-850°C. Laboratory-heated, homogenized melt inclusions trapped by a second generation of olivine and clinopyroxene are characterized by remarkable silica-undersaturation, compared with mid-ocean ridge basalt and ocean island basalt magmas, with high concentrations of alkalis, Ti, Ba, Sr and Zr, but varying to very low concentrations of Al and Ca. Such alkali-rich, strongly evolved melts might have resulted from extreme (>95%) fractional crystallization of the parental magmas, assuming their chemical compositions to be similar to those of the respective lavas. However, this estimate is about three times higher than the modal amount of phenocrysts in the lavas that could be reinforced by the presence of excess cognate crystals and/or xenocrysts in the lavas. Strong oxidation from FMQ?+?2 to FMQ?+?4 to +?5•2?atm log units of the evolved mafuritic magmas at ?900-1000°C has occurred during the final stage of magma evolution at very shallow crustal depths or possibly directly in the lava flow.
Abstract: The basins and orogens of the Mediterranean region ultimately result from the opening of oceans during the early break-up of Pangea since the Triassic, and their subsequent destruction by subduction accommodating convergence between the African and Eurasian Plates since the Jurassic. The region has been the cradle for the development of geodynamic concepts that link crustal evolution to continental break-up, oceanic and continental subduction, and mantle dynamics in general. The development of such concepts requires a first-order understanding of the kinematic evolution of the region for which a multitude of reconstructions have previously been proposed. In this paper, we use advances made in kinematic restoration software in the last decade with a systematic reconstruction protocol for developing a more quantitative restoration of the Mediterranean region for the last 240 million years. This restoration is constructed for the first time with the GPlates plate reconstruction software and uses a systematic reconstruction protocol that limits input data to marine magnetic anomaly reconstructions of ocean basins, structural geological constraints quantifying timing, direction, and magnitude of tectonic motion, and tests and iterations against paleomagnetic data. This approach leads to a reconstruction that is reproducible, and updatable with future constraints. We first review constraints on the opening history of the Atlantic (and Red Sea) oceans and the Bay of Biscay. We then provide a comprehensive overview of the architecture of the Mediterranean orogens, from the Pyrenees and Betic-Rif orogen in the west to the Caucasus in the east and identify structural geological constraints on tectonic motions. We subsequently analyze a newly constructed database of some 2300 published paleomagnetic sites from the Mediterranean region and test the reconstruction against these constraints. We provide the reconstruction in the form of 12 maps being snapshots from 240 to 0 Ma, outline the main features in each time-slice, and identify differences from previous reconstructions, which are discussed in the final section.
Galloway, M., Nowicki, T., Van Coller, B., Mukodzani, B., Siemens, K., Hetman, C., Webb, K., Gurney, J.
Constraining kimberlite geology through integration of geophysical, geological and geochemical methods: a case study of the Mothae kimberlite, northern Lesotho.
Abstract: The mantle beneath the Cullinan kimberlite (formerly known as "Premier") is a unique occurrence of diamondiferous cratonic mantle where diamonds were generated contemporaneously and shortly following a mantle upwelling that led to the formation of a Large Igneous Province that produced the world's largest igneous intrusion - the 2056?Ma Bushveld Igneous Complex (BIC). We studied 332 diamond inclusions from 202 Cullinan diamonds to investigate mantle thermal effects imposed by the formation of the BIC. The overwhelming majority of diamonds come from three parageneses: (1) lithospheric eclogitic (69%), (2) lithospheric peridotitic (21%), and (3) sublithospheric mafic (9%). The lithospheric eclogitic paragenesis is represented by clinopyroxene, garnet, coesite and kyanite. Main minerals of the lithospheric peridotitic paragenesis are forsterite, enstatite, Cr-pyrope, Cr-augite and spinel; the sublithospheric mafic association includes majorite, CaSiO3 phases and omphacite. Diamond formation conditions were calculated using an Al-in-olivine thermometer, a garnet-clinopyroxene thermometer, as well as majorite and Raman barometers. The Cullinan diamonds may be unique on the global stage in recording a cold geotherm of 40?mW/m2 in cratonic lithosphere that was in contact with underlying convecting mantle at temperatures of 1450-1550?°C. The studied Cullinan diamonds contain a high proportion of inclusions equilibrated at temperatures exceeding the ambient 1327?°C adiabat, i.e. 54% of eclogitic diamonds and 41% of peridotitic diamonds. By contrast, ? 1% of peridotitic diamond inclusions globally yield equally high temperatures. We propose that the Cullinan diamond inclusions recorded transient, slow-dissipating thermal perturbations associated with the plume-related formation of the ~2?Ga Bushveld igneous province. The presence of inclusions in diamond from the mantle transition zone at 300-650?km supports this view. Cullinan xenoliths indicative of the thermal state of the cratonic lithosphere at ~1.2?Ga are equilibrated at the relatively low temperatures, not exceeding adiabatic. The ability of diamonds to record super-adiabatic temperatures may relate to their entrainment from the deeper, hotter parts of the upper mantle un-sampled by the kimberlite in the form of xenoliths or their equilibration in a younger lithosphere after a decay of the thermal disturbance.
Aspects of the Geochemistry of Kimberlites from the Premier mine and Other Selected South African Occurrences with Particular Reference to the Rare Earth Elements.
Physics and Chemistry of the Earth., Vol. 9, PP. 687-707.
Carbon isotope composition, nitrogen content and inclusion composition Of diamonds from the Roberts Victor kimberlite, South Africa- evidence for C 13depletion in t
Geochem. Cosmochem.Acta, Vol. 51, No. 5, May pp. 1227-1243
Comparison of Siberian and South African Diamondiferous kimberlites
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
Diamond resources on the continental shelf of southern Africa
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) ., Session on Diamonds at The Canadian Institute of Mining, Metallurgy and Petroleum (CIM) Annual Meeting April, Vol. 84, No. 947, March p. 99. Abstract
Age, origin and emplacement of diamonds: a review of the scientific advances in the decade 1981-1990
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) ., Session on Diamonds at The Canadian Institute of Mining, Metallurgy and Petroleum (CIM) Annual Meeting April, Vol. 84, No. 947, March p. 90. Abstract
Variations in trapping temperatures and trace elements in peridotite-suite inclusions African diamonds- evidence 2 inclusion suites implications lithosphere stratigr
Contributions to Mineralogy and Petrology, Vol. 110, No. 1, March pp. 1-15
The track record for diamond indicator minerals: a summary of casehistories
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) , Annual Meeting Abstracts approximately 10 lines, Vol. 86, No. 968, March ABSTRACT p. 70
A subduction wedge origin for Paleoarchean peridotitic diamonds and harzburgites from the PAnd a kimberlite, Slave Craton: evidence from Re Os isotope systematics.
Contributions to Mineralogy and Petrology, Vol. 152, 3, pp. 275-294.
Re-Os isotopic composition of peridotitic sulphide inclusions in diamonds from Ellendale, Australia: age constraints on Kimberley cratonic lithosphere.
Geochimica et Cosmochimica Acta, Vol. 74, 11, pp. 3292-3306.
Proceedings of the 5th. International Kimberlite Conference, Vol. 2, pp. 78-97.
United States, Arkansas
Microdiamonds
Abstract: The first report of diamond in igneous rock in the United States originated at Prairie Creek, Arkansas. We have examined the morphological, carbon isotope, and inclusion characteristics of diamonds from Prairie Creek, and from the Twin Knobs # 1, #2, Black Lick, and American lamproites. White is the most common macrodiamond color at Prairie Creek (62% of total), with 20% brown and 16% yellow. This contrasts with Australian lamproites where brown predominates, and with other North American localities such as the Sloan, Colorado kimberlites where yellow is rare. Lamination lines indicate ductile deformation at mantle conditions. The macrodiamonds are very resorbed; 82% are equiform or distorted tetrahexahedroida and none are octahedra. Low relief surfaces indicate prolonged and/or intense resorption. Microdiamonds differ dramatically, with octahedra and fragn~ents common and tetrahexahedroida abscnL Serrate laminae, knob-like asperities, pointed plates, ribbing, and non-uniform resorption are the most common surface features. Diamonds from the Twin Knobs # 1 lamproite are similar to microdiamonds with respect to size and surface features. Magnetite and olivine (F093) are the only primary inclusions foqnd in the diamonds, although inclusions of peridotitic and eclogitic parageneses have been reported in previous studies. Carbon isotope B13c values for Prairie Creek macrodiamonds peak at-3.0 to -6.2 %o (ave. -4.7 %o for 19 stones) and -10.3 to -10.6 %o (ave. -10.5 %o for 2 stones). The diamonds with magnetite and olivine inclusions have B13c values of -5.1 %o and -4.0 %o respectively. Microdiamonds from Prairie Creek, Twin Knobs #2, American, and Black Lick are similar to Prairie Creek macrodiamonds ( -0.5 to -7 .8; ave. -4.1 %o for 8 stones). A Prairie Creek and a Black Lick microdiamond have B13c values of -26.1 and -25.2%orespectively, and the latter exhibits non-uniform resorption. Lamination lines on macrodiamonds and xenocrystic surface features on microdiamonds imply that both are xenocrysts in lamproite. Carbon isotopes are characteristic of a peridotitic or primordial carbon reservoir. Two 13cdepleted microdiamonds may be from a subducted carbon source. In comparison to macrodiamond populations from most kimberlites, Prairie Creek macrodiamonds are intensely resorbed, and lamproite may be more corrosive than kimberlite \\ ith respect to diamond resorption. Microdiamonds were probably encapsulated in xenolith material ani.! esca•,)ed resorption. The differences in size and color of Prairie Creek macrodiamonds relative to Sloan kimberlitic diamonds are genetic, and may be related to their formation in lithosphere of differing age and tectonic history.
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: In this study, a private collection of diamondiferous eclogite xenoliths has been made available for non-destructive investigation. All samples have at least one diamond visible. The samples are predominantly sourced from the Excelsior and Newlands mines (South Africa), with additional samples from Roberts Victor mine (South Africa) and Orapa (Botswana). 3D volume models of the samples were created using X-ray tomography. The 3D images reveal abundant secondary veining that is clearly younger than the eclogite. Diamonds are located in fluid pathways and occur in both altered garnet and altered clinopyroxene. Most of the veining is unrelated to the spatial positioning of diamond in the samples. In some instances, early veining has annealed or partially annealed, suggesting a range in timing of at least some of the several metasomatic events that have affected the rock. Importantly, in the most graphic examples, a clear distinction can be seen between diamond-bearing and non-diamond-bearing veins, even where sulphide is present in abundance in the non-diamond-bearing veins. The amount of diamond detected in the xenoliths varies from a single crystal to well over 50 diamonds forming more than 9% of the rock. This extreme value contrasts with the diamond recovery from currently viable diamond mines of less than 2ppm or 0.0002%. The morphology of the diamonds includes step-faced flat-faced octahedra, single crystals and aggregates. This is particularly a feature of diamonds in the Excelsior specimens. In the samples from Newlands and Orapa, in contrast, diamond surfaces reflect resorption processes such as rounding and corrosion of the diamonds. The following conclusions can be drawn from this study: Diamonds in this collection, sourced from within the Kalahari craton, appear to have formed by a metasomatic process during which fluids infiltrated pre-existing mantle-derived eclogite; Several metasomatic events have occurred during the residence of the eclogite in the mantle; Some of these metasomatic events have been diamond-friendly, whilst others have been diamond-neutral or diamond-destructive; Diamond can be present at very high concentrations along particular metasomatic fluid pathways in eclogitic mantle rocks; The absolute timing of diamond formation is still to be determined.
Nestola, F., Korolev, N., Kopylova, M., Rotiroti, N., Pearson, D.G., Pamato, M.G., Alvaro, M., Peruzzo, L., Gurney, J.J., Moore, A.E., Davidson, J.
CaSiO3 perovskite in diamond indicates the recycling of oceanic crust into the lower mantle.
Nature, Vol. 555, March 8, pp. 237-241.
Mantle
deposit - Cullinan
Abstract: Laboratory experiments and seismology data have created a clear theoretical picture of the most abundant minerals that comprise the deeper parts of the Earth’s mantle. Discoveries of some of these minerals in ‘super-deep’ diamonds—formed between two hundred and about one thousand kilometres into the lower mantle—have confirmed part of this picture1,2,3,4,5. A notable exception is the high-pressure perovskite-structured polymorph of calcium silicate (CaSiO3). This mineral—expected to be the fourth most abundant in the Earth—has not previously been found in nature. Being the dominant host for calcium and, owing to its accommodating crystal structure, the major sink for heat-producing elements (potassium, uranium and thorium) in the transition zone and lower mantle, it is critical to establish its presence. Here we report the discovery of the perovskite-structured polymorph of CaSiO3 in a diamond from South African Cullinan kimberlite. The mineral is intergrown with about six per cent calcium titanate (CaTiO3). The titanium-rich composition of this inclusion indicates a bulk composition consistent with derivation from basaltic oceanic crust subducted to pressures equivalent to those present at the depths of the uppermost lower mantle. The relatively ‘heavy’ carbon isotopic composition of the surrounding diamond, together with the pristine high-pressure CaSiO3 structure, provides evidence for the recycling of oceanic crust and surficial carbon to lower-mantle depths.https://www.nature.com/articles/nature25972
Mineralogy and Petrology, doi.org/10.1007/s710-018-0601-z 15p.
Africa, South Africa
deposit - Cullinan
Abstract: We studied a suite of Cullinan diamonds (<0.3 ct) with mineral inclusions, which comprised 266 Type I and 75 blank Type II (<20 ppm N) diamonds, as classified by infrared spectroscopy. More than 90% (n?=?68) of Type II diamonds do not luminesce. In contrast, 51.9% (n?=?177) of Type I diamonds luminesce, with blue colors of different intensity. Carbon isotopic compositions of Type I and II diamonds are similar, with ?13CVPDB ranging from ?2.1 to ?7.7‰for Type I diamonds (n?=?25), and from ?1.3 to ?7.8- for Type II diamonds (n?=?20). The Type II diamonds are sourced from three parageneses, lithospheric lherzolitic (45%), lithospheric eclogitic (33%), and sublithospheric mafic (22%). The lherzolitic suite contains Cr-pyrope, forsterite, enstatite, clinopyroxene and Cr-spinel formed at 1090-1530 °C and P?=?4.6-7.0 GPa. Lithospheric eclogitic diamonds containing garnet, omphacite, kyanite and coesite comprise 33% of Type II diamonds. The sublithospheric mafic paragenesis is mainly represented by Cr-free majorite, various CaSiO3 phases and omphacite equilibrated at 11.6-26 GPa, in the transition zone and the lower mantle. The lherzolitic paragenesis predominates in Type II diamonds, whereas 79% Type I diamonds are sourced from eclogites. The higher incidence of sublithospheric inclusions was found in Type II diamonds, 22% against 6% in Type I diamonds. The similarity of the mineral parageneses and C isotopic compositions in the small Cullinan Type II and Type I diamonds indicate the absence of distinct mantle processes and carbon sources for formation of studied Type II diamonds. The parent rocks and the carbon sources generally vary for Type II diamonds within a kimberlite and between kimberlites.
Abstract: Volcanic hotspot tracks featuring linear progressions in the age of volcanism are typical surface expressions of plate tectonic movement on top of narrow plumes of hot material within Earth’s mantle1. Seismic imaging reveals that these plumes can be of deep origin2=probably rooted on thermochemical structures in the lower mantle3, 4, 5, 6. Although palaeomagnetic and radiometric age data suggest that mantle flow can advect plume conduits laterally7, 8, the flow dynamics underlying the formation of the sharp bend occurring only in the Hawaiian-Emperor hotspot track in the Pacific Ocean remains enigmatic. Here we present palaeogeographically constrained numerical models of thermochemical convection and demonstrate that flow in the deep lower mantle under the north Pacific was anomalously vigorous between 100 million years ago and 50 million years ago as a consequence of long-lasting subduction systems, unlike those in the south Pacific. These models show a sharp bend in the Hawaiian-Emperor hotspot track arising from the interplay of plume tilt and the lateral advection of plume sources. The different trajectories of the Hawaiian and Louisville hotspot tracks arise from asymmetric deformation of thermochemical structures under the Pacific between 100 million years ago and 50 million years ago. This asymmetric deformation waned just before the Hawaiian-Emperor bend developed, owing to flow in the deepest lower mantle associated with slab descent in the north and south Pacific.
Abstract: Volcanic hotspot tracks featuring linear progressions in the age of volcanism are typical surface expressions of plate tectonic movement on top of narrow plumes of hot material within Earth’s mantle1. Seismic imaging reveals that these plumes can be of deep origin2—probably rooted on thermochemical structures in the lower mantle3, 4, 5, 6. Although palaeomagnetic and radiometric age data suggest that mantle flow can advect plume conduits laterally7, 8, the flow dynamics underlying the formation of the sharp bend occurring only in the Hawaiian–Emperor hotspot track in the Pacific Ocean remains enigmatic. Here we present palaeogeographically constrained numerical models of thermochemical convection and demonstrate that flow in the deep lower mantle under the north Pacific was anomalously vigorous between 100 million years ago and 50 million years ago as a consequence of long-lasting subduction systems, unlike those in the south Pacific. These models show a sharp bend in the Hawaiian–Emperor hotspot track arising from the interplay of plume tilt and the lateral advection of plume sources. The different trajectories of the Hawaiian and Louisville hotspot tracks arise from asymmetric deformation of thermochemical structures under the Pacific between 100 million years ago and 50 million years ago. This asymmetric deformation waned just before the Hawaiian–Emperor bend developed, owing to flow in the deepest lower mantle associated with slab descent in the north and south Pacific.
Abstract: Global deep?time plate motion models have traditionally followed a classical rigid plate approach, even though plate deformation is known to be significant. Here we present a global Mesozoic?Cenozoic deforming plate motion model that captures the progressive extension of all continental margins since the initiation of rifting within Pangea at ~240 Ma. The model also includes major failed continental rifts and compressional deformation along collision zones. The outlines and timing of regional deformation episodes are reconstructed from a wealth of published regional tectonic models and associated geological and geophysical data. We reconstruct absolute plate motions in a mantle reference frame with a joint global inversion using hotspot tracks for the last 80 million years and minimizing global trench migration velocities and net lithospheric rotation. In our optimized model net rotation is consistently below 0.2°/Myr, and trench migration scatter is substantially reduced. Distributed plate deformation reaches a Mesozoic peak of 30 million km2 in the Late Jurassic (~160?155 Ma), driven by a vast network of rift systems. After a mid?Cretaceous drop in deformation it reaches a high of 48 million km2 in the Late Eocene (~35 Ma), driven by the progressive growth of plate collisions and the formation of new rift systems. About a third of the continental crustal area has been deformed since 240 Ma, partitioned roughly into 65% extension and 35% compression. This community plate model provides a framework for building detailed regional deforming plate networks and form a constraint for models of basin evolution and the plate?mantle system.
Muller, D., Zahirovic, S., Williams, S.E., Cannon, J., Seton, M., Bower, D.J., Tetley, M., Heine, C., Le Breton, E., Liu, S., Russell, S.H.J., Yang, T., Leonard, J., Gurnis, M.
Abstract: Global deep?time plate motion models have traditionally followed a classical rigid plate approach, even though plate deformation is known to be significant. Here we present a global Mesozoic-Cenozoic deforming plate motion model that captures the progressive extension of all continental margins since the initiation of rifting within Pangea at ~240 Ma. The model also includes major failed continental rifts and compressional deformation along collision zones. The outlines and timing of regional deformation episodes are reconstructed from a wealth of published regional tectonic models and associated geological and geophysical data. We reconstruct absolute plate motions in a mantle reference frame with a joint global inversion using hot spot tracks for the last 80 million years and minimizing global trench migration velocities and net lithospheric rotation. In our optimized model, net rotation is consistently below 0.2°/Myr, and trench migration scatter is substantially reduced. Distributed plate deformation reaches a Mesozoic peak of 30 × 106 km2 in the Late Jurassic (~160-155 Ma), driven by a vast network of rift systems. After a mid?Cretaceous drop in deformation, it reaches a high of 48 x 106 km2 in the Late Eocene (~35 Ma), driven by the progressive growth of plate collisions and the formation of new rift systems. About a third of the continental crustal area has been deformed since 240 Ma, partitioned roughly into 65% extension and 35% compression. This community plate model provides a framework for building detailed regional deforming plate networks and form a constraint for models of basin evolution and the plate?mantle system.
Journal of Geophysical Research Solid Earth, 10.1029/2019JB0 17442 28p. Pdf
Global
geodynamics
Abstract: The absolute motion of tectonic plates since Pangea can be derived from observations of hotspot trails, paleomagnetism, or seismic tomography. However, fitting observations is typically carried out in isolation without consideration for the fit to unused data or whether the resulting plate motions are geodynamically plausible. Through the joint evaluation of global hotspot track observations (for times <80 Ma), first?order estimates of net lithospheric rotation (NLR), and parameter estimation for paleo-trench migration (TM), we present a suite of geodynamically consistent, data?optimized global absolute reference frames from 220 Ma to the present. Each absolute plate motion (APM) model was evaluated against six published APM models, together incorporating the full range of primary data constraints. Model performance for published and new models was quantified through a standard statistical analyses using three key diagnostic global metrics: root?mean square plate velocities, NLR characteristics, and TM behavior. Additionally, models were assessed for consistency with published global paleomagnetic data and for ages <80 Ma for predicted relative hotspot motion, track geometry, and time dependence. Optimized APM models demonstrated significantly improved global fit with geological and geophysical observations while performing consistently with geodynamic constraints. Critically, APM models derived by limiting average rates of NLR to ~0.05°/Myr and absolute TM velocities to ~27?mm/year fit geological observations including hotspot tracks. This suggests that this range of NLR and TM estimates may be appropriate for Earth over the last 220 Myr, providing a key step toward the practical integration of numerical geodynamics into plate tectonic reconstructions.
Abstract: In the northern part of the Anabar Shield, orthopyroxene plagiogneisses of the granulite Daldyn Group host lenses of mafic rocks surrounded by melanocratic rims. According to their chemical composition, the mafic rocks correspond to subalkaline gabbro, the plagiogneisses correspond to granodiorites contaminated with mafic material, and the rims are diorites. The orthopyroxene plagiogneisses of granodiorite composition have 147Sm/144Nd = 0.1097, ?Nd(?) = 1.6, TNd(DM) = 3.47 Ga and are metamorphosed anatectic granitoids with an age of 3.34 Ga. The mafic rocks have high Zr, Th, and Pb contents, are enriched in REE (?REE = 636 ppm), with a high degree of fractionation [(La/Yb)N = 17.73] and a well-defined Eu minimum (Eu/Eu* = 0.51), and have 147Sm/144Nd = 0.099, ?Nd(?) = 1.4 and TNd(DM) = 3.65 Ga. It is assumed that these rocks crystallized from melt derived from an enriched mantle (plume) source. Based on U-Pb (SHRIMP-II) dating of 50 zircon grains from the mafic rocks, a group of grains with concordant ages from 3567 to 1939 Ma was distinguished, along with a large number of discordant values. Multiple measurements in zircon grains with discordant age values make it possible to identify seven grains of Eoarchean age, with upper intercepts of the discordia corresponding to 3987 ± 71 to 3599 ± 33 Ma. The Lu-Hf systematics of 14 zircon grains is characterized by ?Hf(T) = +3.7 and by close values of THf(DM) = 3.95 and TCHf = 3.93 Ga (3.99 Ga for the oldest zircon). The Paleoarchean (3.57 Ga) zircons are characterized by negative values of ?Hf(T) = -5.3 and -6.8, THf(DM) = 3.92-3.98 Ga, and TCHf = 4.14-4.24 Ga, which indicate recycling of the preexisting Eoarchean and Hadean continental crust. The younger zircon (3287-2410 Ma) was also formed when the preexisting crust was recycled.