Hello Guest User, You are visiting this website from a computer with an IP address of 172.71.254.107 with the name of '?' since Sat Apr 27, 2024 at 6:24:26 AM PT for approx. 0 minutes now.
SDLRC - Scientific Articles all years by Author - De-Dn
The Sheahan Diamond Literature Reference Compilation
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcementscalled the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Resource Center
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
The SDLRC provides 3 types of references identified in the reference code. DS for scientific article, DM for a media article, and DC for a corporate announcement. Consider DS0512-0001. The DS stands for "diamond scientific". 05 stands for 2005, the year the reference was posted. 12 represents the month the reference was posted. For all years prior to 2015 the default month is 12. -0001 is the reference's identifier and it does not mean anything. The number below the refence code, ie 2015, is the year the article was published. Note that the posted year may sometimes be later than the published year.
Sort Order
References are sorted by the "author" name and when the reference was posted to the compilation.
Most Recent
If the reference code is highlighted yellow, the reference was made available through the most recent monthly compilation of new literature. Use this to check out new references. When new references are posted, we make it our priority to track down an online link and obtain an abstract. With regard to older references, tracking down an abstract and an online link is a work in progress.
Link to external location of article:
If the title has a link, it means we have found a location online where you can either retrieve the full article free, or purchase access to it. The Sheahan Diamond Literature Service is not a technical article procurement service; if you want a restricted article, you must deal directly with the vendor who controls the copyright to the article.
Searching this page for a specific term or author
In your Firefox browser click Edit in the menu bar and then Find. In the Find box that shows up at the bottom of the web page enter your search term. Firefox will highlight all occurrences. This is particularly helpful when the author you are seeking was not the lead author by whom the compilation is sorted.
Sending or sharing a reference
The left column (Posted/Published) has an embedded hyperlink for each reference. In Firefox, if you right click on it, you can obtain the link url for that reference's location within the page, which you can copy and paste into an email or any other document. You can also use the "share this link" option to tweet, facebook etc the link.
Journal of South American Earth Sciences, Vol. 94, 102208 13p. Pdf
South America, Brazil
carbonatite
Abstract: Carbonatites are peculiar igneous rocks, consisting mainly of greater than 50% carbonate minerals, which arouse an economic interest due to the potentiality of high phosphate content and Light Rare Earth Elements (LREE) associated with their occurrence. The Passo Feio Carbonatite (PFC) is located 17?km Southwest of Caçapava do Sul city and constitutes NW dipping body, which is interposed with Passo Feio Formation metamorphic rocks. The PFC varies texturally from massive to foliated, being mainly composed of calcites and dolomites and on a smaller scale by apatites, phlogopites and tremolites. The opaque minerals correspond to hematites, magnetites, pyrites and barites, while the accessory minerals are represented by zircons, monazites- (Ce) and aeschynites- (Ce). Probably those REE mineral phases correspond to a hydrothermal stage, with the REE remobilization from apatites into those latter REE-rich mineral phases - this hypothesis is corroborated by geochemistry, mineral chemistry and microtextures found. Considering the results of mineral chemistry and taking into account the textural criteria, it was possible to classify carbonatite as an alvikite, with geochemical patterns that do not indicate economic potential for REE. However, soil geochemistry showed an important enrichment in REE, reflecting a probable concentration of monazites- (Ce) and aeschynites- (Ce), and because of this, it was possible to establish a zone in which the Passo Feio Carbonatite would probably be extended. In the stable isotope analyzes, the ?13C values varied between ?4.14 and ?3.89‰ while those of ?18O between 10.01 and 11.32‰ which can be attributed to the cooling of the magma itself, without suggesting metamorphic processes or subsequent changes. The deformation found in this carbonatite was probably developed in late-magmatic conditions, guided by tectonics associated with horizontal movements in shear zones. Thus, this work suggests that this carbonatite was the product of the reactivation of mantle sources, within a post-collision magmatic context of the Sul-Riograndense Shield.
Journal of South American Earth Sciences, Vol. 94, 102208 13p. Pdf
South America, Brazil
carbonatite
Abstract: Carbonatites are peculiar igneous rocks, consisting mainly of greater than 50% carbonate minerals, which arouse an economic interest due to the potentiality of high phosphate content and Light Rare Earth Elements (LREE) associated with their occurrence. The Passo Feio Carbonatite (PFC) is located 17?km Southwest of Caçapava do Sul city and constitutes NW dipping body, which is interposed with Passo Feio Formation metamorphic rocks. The PFC varies texturally from massive to foliated, being mainly composed of calcites and dolomites and on a smaller scale by apatites, phlogopites and tremolites. The opaque minerals correspond to hematites, magnetites, pyrites and barites, while the accessory minerals are represented by zircons, monazites- (Ce) and aeschynites- (Ce). Probably those REE mineral phases correspond to a hydrothermal stage, with the REE remobilization from apatites into those latter REE-rich mineral phases - this hypothesis is corroborated by geochemistry, mineral chemistry and microtextures found. Considering the results of mineral chemistry and taking into account the textural criteria, it was possible to classify carbonatite as an alvikite, with geochemical patterns that do not indicate economic potential for REE. However, soil geochemistry showed an important enrichment in REE, reflecting a probable concentration of monazites- (Ce) and aeschynites- (Ce), and because of this, it was possible to establish a zone in which the Passo Feio Carbonatite would probably be extended. In the stable isotope analyzes, the ?13C values varied between ?4.14 and ?3.89‰ while those of ?18O between 10.01 and 11.32‰ which can be attributed to the cooling of the magma itself, without suggesting metamorphic processes or subsequent changes. The deformation found in this carbonatite was probably developed in late-magmatic conditions, guided by tectonics associated with horizontal movements in shear zones. Thus, this work suggests that this carbonatite was the product of the reactivation of mantle sources, within a post-collision magmatic context of the Sul-Riograndense Shield.
Abstract: The breakup of supercontinents is accompanied by the emplacement of continental flood basalts and dike swarms, the origin of which is often attributed to mantle plumes. However, convection modeling has showed that the formation of supercontinents result in the warming of the sub-continental asthenospheric mantle (SCAM), which could also explain syn-breakup volcanism. Temperature variations during the formation then breakup of supercontinents are therefore fundamental to understand volcanism related to supercontinent cycles. Magmatic minerals record the thermal state of their magmatic sources. Here we present a data mining analysis on the first global compilation of chemical information on magmatic rocks and minerals formed over the past 600 million years: a time period spanning the aggregation and breakup of Pangea, the last supercontinent. We show that following a period of increasingly hotter Mg-rich magmatism with dominant tholeiitic affinity during the aggregation of Pangea, lower-temperature minerals crystallized within Mg-poorer magma with a dominant calc-alkaline affinity during Pangea disassembly. These trends reflect temporal changes in global mantle climate and global plate tectonics in response to continental masses assembly and dispersal. We also show that the final amalgamation of Pangea at ~300 Myr led to a long period of lithospheric collapse and cooling until the major step of Pangea disassembly started at ~125 Myr. The geological control on the geosphere magma budget has implications on the oxidation state and temperature of the Earth’s outer envelopes in the Phanerozoic and may have exerted indirect influence on the evolution of climate and life on Earth.
Brazil Journal of Geology ( www.scielo.br) ENG, 15p. Pdf
South America, Brazil
deposit - Parana
Abstract: The Paraná deposit, located at Southwestern Rio Grande do Norte state, in Brazil, is one of the few emerald deposits found at Borborema Province. The mineralization occurs in phlogopite schists and actinolite-phlogopite schists associated with pegmatites and albitites within the Portalegre Shear Zone. Unlike other well-known Brazilian emerald deposits, the mineralogy of Paraná emeralds has remained poorly investigated for the last 40 years. In this study, we conducted mineralogical characterization of theses emeralds through gemological testing, mineral chemistry, absorption and reflectance spectroscopy, and thermal analysis. The Paraná emeralds are bluish-green colored, characterized by high refractive index, several two-phase fluid inclusions and mica is the main mineral inclusion. Electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometry analyses detected the presence of Fe2+ (0.43-1.94 wt.% FeO) and Cr3+ (0.04-0.14 wt.% Cr2O3) as the main chromophores replacing octahedral Al3+ in the crystal structure. In addition, substantial amounts of MgO (0.40-2.72 wt.%), Na2O (0.50-1.81 wt.%), and Cs2O (0.07-0.44 wt.%) were also identified. The main causes for its coloration were attributed to Cr3+ absorption features in visible spectral range, which were corroborated by absorption and reflectance spectra. The presence of types I and II H2O at channel-sites was recorded in Fourier-transform infrared spectra and demonstrated by dehydration processes observed in different thermal and thermogravimetric analyses.
De Assis Janasi, V., Andrade, S., Svisero, D.P.,Vieira de Almeida, V.
Inferencias sobre a evolucao petrologica do manto no sudeste brasileiro a partir de microanalises de elementos traco em piroxenios e olivin a de xenolitos de espinelio peridotitos.
5th Brasilian Symposium on Diamond Geology, Nov. 6-12, abstract p. 85.
Composition and thermal structure of the lithosphere beneath the Ethiopian plateau: evidence from mantle xenoliths in basanites, Injibara Lake Tana Province.
Mineralogy and Petrology, Vol. 93, 1-2, pp. 47-78.
Viola, G., Henderson, I.H.C., Bingen, B., Thomas, R.J., Smethurst, M.A., De Azavedo, S.
Growth and collapse of a deeply eroded orogen: insights from structural, geophysical, and geochronological constraints on Pan-African evolution of NE Mozambique.
Azzone, R.G., Enrich, G.E.R., De Barros Goes, C., Ruberti, E.
Trace element composition of parental magmas from mafic-ultramafic cumulates by in situ mineral analyses: the Juquia mafic-ultramafic alkaline carbonatite massif, SE Brazil.
Journal of South American Earth Sciences, Vol. 41, pp. 5-21.
Azzone, R.G., Enrich, G.E.R., De Barros Gomes, C., Ruberti, E.
Trace element composition of parental magmas from mafic-ultramafic cumulates determined by in situ mineral analyses: the Juquia mafic-ultramafic alkaline-carbonatite massif, SE Brazil.
Journal of South American Earth Sciences, in press available, 17p.
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.
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: 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.
Trumbull, R.B., Reid, D.L., De Beer, C., Van Acken, D., Romer, R.L.
Magmatism and continental breakup at the west margin of southern Africa: a geochemical comparison of dolerite dikes from northwestern Namibia and the Western Cape.
South African Journal of Geology, Vol. 110, 2-3, Sept. pp. 477-502.
Brazil Journal of Geology ( www.scielo.br) ENG, 15p. Pdf
South America, Brazil
deposit - Parana
Abstract: The Paraná deposit, located at Southwestern Rio Grande do Norte state, in Brazil, is one of the few emerald deposits found at Borborema Province. The mineralization occurs in phlogopite schists and actinolite-phlogopite schists associated with pegmatites and albitites within the Portalegre Shear Zone. Unlike other well-known Brazilian emerald deposits, the mineralogy of Paraná emeralds has remained poorly investigated for the last 40 years. In this study, we conducted mineralogical characterization of theses emeralds through gemological testing, mineral chemistry, absorption and reflectance spectroscopy, and thermal analysis. The Paraná emeralds are bluish-green colored, characterized by high refractive index, several two-phase fluid inclusions and mica is the main mineral inclusion. Electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometry analyses detected the presence of Fe2+ (0.43-1.94 wt.% FeO) and Cr3+ (0.04-0.14 wt.% Cr2O3) as the main chromophores replacing octahedral Al3+ in the crystal structure. In addition, substantial amounts of MgO (0.40-2.72 wt.%), Na2O (0.50-1.81 wt.%), and Cs2O (0.07-0.44 wt.%) were also identified. The main causes for its coloration were attributed to Cr3+ absorption features in visible spectral range, which were corroborated by absorption and reflectance spectra. The presence of types I and II H2O at channel-sites was recorded in Fourier-transform infrared spectra and demonstrated by dehydration processes observed in different thermal and thermogravimetric analyses.
Abstract: We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts, ilmenite, phlogopite, spinel, zircon, and uncommon minerals (titanite, calzirtite, anatase, baddeleyite and pyrochlore) of a newly discovered Late Cretaceous kimberlite (U-Pb zircon age 90.0 ± 1.3 Ma; 2?) named Osvaldo França 1, located in the Alto Paranaíba Igneous Province (APIP), southeastern Brazil. Pyrope grains are lherzolitic (Lherz-1, Lherz-2 and Lherz-3), harzburgitic (Harz-3) and wehrlitic (Wehr-2). The pyrope xenocrysts cover a wide mantle column in the subcratonic lithosphere (66-143 km; 20-43 kbar) at relatively low temperatures (811-875 °C). The shallowest part of this mantle is represented by Lherz-1 pyropes (20-32 kbar), which have low-Cr (Cr2O3 = 1.74-6.89 wt.%) and fractionated middle to heavy rare earth elements (MREE-HREE) pattern. The deepest samples are represented by Lherz-2, Lherz-3, Harz-3, and Wehr-2 pyropes (36-43 kbar). They contain high-Cr contents (Cr2O3 = 7.36-11.19 wt.%) and are characterized by sinusoidal (Lherz-2 and Wehr-2) and spoon-like (Lherz-3 and Harz-3) REE patterns. According to their REE and trace elements, pyrope xenocrysts have enriched nature (e.g., Ce and Yb vs. Cr2O3), indicating that the cratonic lithosphere has been affected by a silicate melt with subalkaline/tholeiite composition due to their low Zr, Ti and Y concentrations. Besides minerals with typical kimberlitic signatures, such as ilmenite and zircon, the exotic compositions of phlogopite and ulvöspinel suggest an enriched component in the magma source. The formation of rare mineral phases with strong enrichment of light-REE (LREE) and high field strength elements (HFSE) is attributed to the late-stage kimberlitic melt. We propose a tectonic model where a thermal anomaly, represented by the low-velocity seismic anomaly observed in P-wave seismic tomography images, supplied heat to activate the alkaline magmatism from a metasomatized cratonic mantle source during the late-stages of Gondwana fragmentation and consequent South Atlantic Ocean opening. The metasomatism recorded by mineral phases is a product of long-lived recycling of subducted oceanic plates since the Neoproterozoic (Brasiliano Orogeny) or even older collisional events, contributing to the exotic character of the Osvaldo França 1 kimberlite, as well as to the cratonic lithospheric mantle.
Cyclic development of sedimentary basins at convergent plate margins - 1.structural and tectono-thermal evolution of some Gondwana Basins of easternAustralia
Journal of Geodynamics, Vol. 16, No. 4, pp. 241-282
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.
Earth and Planetary Science Letters, Vol. 456, pp. 134-145.
Mantle
Bridgemanite
Abstract: The lower mantle is estimated to be composed of mostly bridgmanite and a smaller percentage of ferropericlase, yet very little information exists for two-phase deformation of these minerals. To better understand the rheology and active deformation mechanisms of these lower mantle minerals, especially dislocation slip and the development of crystallographic preferred orientation (CPO), we deformed mineral analogs neighborite (NaMgF3, iso-structural with bridgmanite) and halite (NaCl, iso-structural with ferropericlase) together in the deformation-DIA at the Advanced Photon Source up to 51% axial shortening. Development of CPO was recorded in situ with X-ray diffraction, and information on microstructural evolution was collected using X-ray microtomography. Results show that when present in as little as 15% volume, the weak phase (NaCl) controls the deformation. Compared to single phase NaMgF3 samples, samples with just 15% volume NaCl show a reduction of CPO in NaMgF3 and weakening of the aggregate. Microtomography shows both NaMgF3 and NaCl form highly interconnected networks of grains. Polycrystal plasticity simulations were carried out to gain insight into slip activity, CPO evolution, and strain and stress partitioning between phases for different synthetic two-phase microstructures. The results suggest that ferropericlase may control deformation in the lower mantle and reduce CPO in bridgmanite, which implies a less viscous lower mantle and helps to explain why the lower mantle is fairly isotropic.
International Engineering Journal, Ouro Preto *** IN: PORT, Vol. 71, 1, pp. 27-35.
South America, Brazil
deposit - Chapada
Abstract: The Chapada Diamantina, located in the central region of the State of Bahia, is of important historical significance due to its diamond occurrences. Discovered in the nineteenth century, comprehensive research about the regional diamonds and their origins are still limited, demanding more investigation in the matter. Looking for insights about their genesis, mineral inclusions in 23 alluvial diamonds from 4 garimpos located in the Chapada Diamantina were analyzed through the use of Raman micro spectroscopy. Additionally, the characteristics of nitrogen aggregation of the host diamonds were measured using Fourier-transform infrared spectroscopy (FTIR). The diamonds from Chapada Diamantina consist mainly of well-formed crystals, with dominant dodecahedral habits, characterized by faint to very light yellow body colors, typically with green and brown radiation spots on their surface. The main surface textures observed are related to processes that took place in the late stage resorption and during the residence of the diamonds in placer environments. The diamonds are predominantly type IaAB, with a significant occurrence of poorly aggregated nitrogen (Type IaA diamond). The main mineral assemblages of the studied peridotitic inclusions refer to a harzburgitic paragenesis.
Journal of South American Earth Sciences, Vol. 111, 15p. Pdf
South America, Brazil, Minas Gerais
deposit - Coromandel
Abstract: In the midwestern Minas Gerais, Brazil, kimberlite intrusions, particularly kamafugite and alkaline complexes, occur along the NW-SE-oriented Alto Paranaíba structural high. Diamonds in this region were discovered in the Bagagem River and later in the alluvial deposits of the Santo Antônio do Bonito, Santo Inácio and Douradinho rivers. Diamond-bearing kimberlites observed in the region are the primary sources of diamond deposits, as in the case of the Vargem Bonita diggings-in the upper São Francisco River. However, the primary sources of the alluvial diamonds that occur in the Santo Antônio do Bonito, Santo Inácio, and Douradinho rivers have not been clarified. These diamond populations have characteristics common to all three drainage area, where large stones are frequently recovered. Diamond accumulation in the alluvium is due to the erosion and re-concentration of material from basal conglomerate of the Capacete Formation. There is evidence that the sources that fed the conglomerate are local diamond-bearing kimberlites of approximately 90-120 Ma underlying the Capacete Formation, which in an upper unit of the Mata da Corda Group. Recent fieldwork led to the location of a kimberlite intrusion in the Santo Inácio River Basin, southeast of Coromandel. The intrusion fulfills the requirements constituting a primary source of diamonds in the area.
Abstract: We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts, ilmenite, phlogopite, spinel, zircon, and uncommon minerals (titanite, calzirtite, anatase, baddeleyite and pyrochlore) of a newly discovered Late Cretaceous kimberlite (U-Pb zircon age 90.0 ± 1.3 Ma; 2?) named Osvaldo França 1, located in the Alto Paranaíba Igneous Province (APIP), southeastern Brazil. Pyrope grains are lherzolitic (Lherz-1, Lherz-2 and Lherz-3), harzburgitic (Harz-3) and wehrlitic (Wehr-2). The pyrope xenocrysts cover a wide mantle column in the subcratonic lithosphere (66-143 km; 20-43 kbar) at relatively low temperatures (811-875 °C). The shallowest part of this mantle is represented by Lherz-1 pyropes (20-32 kbar), which have low-Cr (Cr2O3 = 1.74-6.89 wt.%) and fractionated middle to heavy rare earth elements (MREE-HREE) pattern. The deepest samples are represented by Lherz-2, Lherz-3, Harz-3, and Wehr-2 pyropes (36-43 kbar). They contain high-Cr contents (Cr2O3 = 7.36-11.19 wt.%) and are characterized by sinusoidal (Lherz-2 and Wehr-2) and spoon-like (Lherz-3 and Harz-3) REE patterns. According to their REE and trace elements, pyrope xenocrysts have enriched nature (e.g., Ce and Yb vs. Cr2O3), indicating that the cratonic lithosphere has been affected by a silicate melt with subalkaline/tholeiite composition due to their low Zr, Ti and Y concentrations. Besides minerals with typical kimberlitic signatures, such as ilmenite and zircon, the exotic compositions of phlogopite and ulvöspinel suggest an enriched component in the magma source. The formation of rare mineral phases with strong enrichment of light-REE (LREE) and high field strength elements (HFSE) is attributed to the late-stage kimberlitic melt. We propose a tectonic model where a thermal anomaly, represented by the low-velocity seismic anomaly observed in P-wave seismic tomography images, supplied heat to activate the alkaline magmatism from a metasomatized cratonic mantle source during the late-stages of Gondwana fragmentation and consequent South Atlantic Ocean opening. The metasomatism recorded by mineral phases is a product of long-lived recycling of subducted oceanic plates since the Neoproterozoic (Brasiliano Orogeny) or even older collisional events, contributing to the exotic character of the Osvaldo França 1 kimberlite, as well as to the cratonic lithospheric mantle.
Brod, J.A., Junqueira-Brod, T.C., Gaspar, J.C., Petrinovic, I.A., De Castro Valente, S., Corval, A.
Decoupling of paired elements, crossover REE patterns and mirrored spider diagrams: fingerprinting liquid immiscibility in the Tapira alkaline carbonatite complex, SE Brazil.
Journal of South American Earth Sciences, Vol. 41, pp. 41-56.
Abstract: The Natal Valley, offshore Mozambique, is a key area for understanding the evolution of East Gondwana. Within the scope of the integrated multidisciplinary PAMELA project, we present new wide?angle seismic data and interpretations, which considerably alter Geoscience paradigms. These data reveal the presence of a 30?km?thick crust that we argue to be of continental nature. This falsifies all the most recent palaeo?reconstructions of the Gondwana. This 30?km?thick continental crust 1,000 m below sea level implies a complex history with probable intrusions of mantle?derived melts in the lower crust, connected to several occurrences of magmatism, which seems to evidence the crucial role of the lower continental crust in passive margin genesis.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 265-6.
Global
Fluoresence
Abstract: The effect of fluorescence on the appearance of diamonds has been a subject of debate for many years (Moses et al., 1997). In the trade, fluorescence is generally perceived as an undesirable characteristic. Nearly 80% of diamonds graded at HRD Antwerp receive a “nil” fluorescence grade, while the remainder are graded as “slight,” “medium,” and “strong,” their value decreasing with level of fluorescence. To understand how fluorescence might change diamond appearance, a selection of 160 round brilliant-cut diamonds were investigated in detail. This study focused on the effect of 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.
Marine diamonds: the geological controls governing the marine and terrestrial diamond deposits occurring along the West Coast of the Republic of South Africa
Indiaqua, Industrial Diamond ANNUAL, 1991 pp. 53-56, 58-59
Abstract: We have investigated a locality very well known to mineral collectors, the Yates U-Th prospect near Otter Lake, Québec. There, dikes of orange to pink calcite enclose euhedral prisms of fluorapatite, locally aligned. Early investigators pointed out the importance of micro-inclusions in the prisms. We describe and image the micro-inclusions in two polished sections of fluorapatite prisms, one of them with a millimetric globule of orange calcite similar to that in the matrix. We interpret the globule to have been an inclusion of melt trapped during growth. Micro-globules disseminated in the fluorapatite are interpreted to have crystallized in situ from aliquots of the boundary-layer melt enriched in constituents rejected by the fluorapatite; the micro-globules contain a complex jigsawed assemblage of carbonate, silicate, and sulfate minerals. Early minerals to crystallize are commonly partly dissolved and partly replaced by lower-temperature phases. Such jigsawed assemblages seem to be absent in the carbonate matrix sampled away from the fluorapatite prisms. The pressure and temperature attained at the Rigolet stage of the Grenville collisional orogeny were conducive to the anatexis of marble in the presence of H2O. The carbonate melt is considered to have become silicocarbonatitic by assimilation of the enclosing gneisses, which were also close to their melting point. Degassing was important, and the melt froze quickly. The evidence points to a magmatic origin for the carbonate dikes and the associated clinopyroxenite, rather than a skarn-related association.
International Journal of Earth Sciences, Vol. 108, pp. 89-113.
Africa, Angola
thermochronology
Abstract: There are two main points of view regarding how continental margins evolve. The first one argues that the present-day margins have been developed by long-term denudation since a major exhumation episode, probably driven by rifting or another relevant tectonic event. The second one argues that continental margins underwent alternating burial and exhumation episodes related to crustal tectonic and surface uplift and subsidence. To demonstrate that the proximal domain of the southwestern Angolan margin has evolved in a polycyclic pattern, we present a review of geological and thermochronological information and integrate it with new combined apatite fission-track and (U-Th)/He data from Early Cretaceous volcanic and Precambrian basement samples. We also provide hypotheses on the possible mechanisms able to support the vertical crustal movements of this margin segment, which are also discussed based on some modern rifting models proposed for Central South Atlantic. The central apatite fission-track ages range from 120.6?±?8.9 to 272.9?±?21.6 Ma, with the mean track lengths of approximately 12 µm. The single-grain apatite (U-Th)/He ages vary between 52.2?±?1 and 177.2?±?2.6 Ma. The integration of the thermochronological data set with published geological constraints supports the following time-temperature evolution: (1) heating since the Carboniferous-Permian, (2) cooling onset in the Early Jurassic, (3) heating onset in the Early Cretaceous, (4) cooling onset in the Mid- to Late Cretaceous, (5) heating onset in the Late Cretaceous, and (6) cooling onset in the Oligocene-Miocene. The thermochronological data and the geological constraints, support that the proximal domain of the southwestern Angolan margin was covered in the past by pre-, syn-, and post-rift sediments, which were eroded during succeeding exhumation events. For this margin segment, we show that a development based on long-term denudation is less realistic than one based on burial and exhumation episodes during the last 130 Myr.
Abstract: All chondrites accreted ?3.5 wt.% C in their matrices, the bulk of which was in a macromolecular solvent and acid insoluble organic material (IOM). Similar material to IOM is found in interplanetary dust particles (IDPs) and comets. The IOM accounts for almost all of the C and N in chondrites, and a significant fraction of the H. Chondrites and, to a lesser extent, comets were probably the major sources of volatiles for the Earth and the other terrestrial planets. Hence, IOM was both the major source of Earth’s volatiles and a potential source of complex prebiotic molecules. Large enrichments in D and 15N, relative to the bulk solar isotopic compositions, suggest that IOM or its precursors formed in very cold, radiation-rich environments. Whether these environments were in the interstellar medium (ISM) or the outer Solar System is unresolved. Nevertheless, the elemental and isotopic compositions and functional group chemistry of IOM provide important clues to the origin(s) of organic matter in protoplanetary disks. IOM is modified relatively easily by thermal and aqueous processes, so that it can also be used to constrain the conditions in the solar nebula prior to chondrite accretion and the conditions in the chondrite parent bodies after accretion. Here we review what is known about the abundances, compositions and physical nature of IOM in the most primitive chondrites. We also discuss how the IOM has been modified by thermal metamorphism and aqueous alteration in the chondrite parent bodies, and how these changes may be used both as petrologic indicators of the intensity of parent body processing and as tools for classification. Finally, we critically assess the various proposed mechanisms for the formation of IOM in the ISM or Solar System.
Contributions to Mineralogy and Petrology, Vol. 174, 28p. Pdf
Africa, Ghana
deposit - Akwatia
Abstract: Trace-element concentrations in olivine and coexisting garnets included in diamonds from the Akwatia Mine (Ghana, West African Craton) were measured to show that olivine can provide similar information about equilibration temperature, diamond paragenesis and mantle processes as garnet. Trace-element systematics can be used to distinguish harzburgitic olivines from lherzolite ones: if Ca/Al ratios of olivine are below the mantle lherzolite trend (Ca/Al??300 µg/g Ca or?>?60 µg/g Na are lherzolitic. Conventional geothermobarometry indicates that Akwatia diamonds formed and resided close to a 39 mW/m2 conductive geotherm. A similar value can be derived from Al in olivine geothermometry, with TAl-ol ranging from 1020 to 1325 °C. Ni in garnet temperatures is on average somewhat higher (TNi-grt?=?1115-1335 °C) and the correlation between the two thermometers is weak, which may be not only due to the large uncertainties in the calibrations, but also due to disequilibrium between inclusions from the same diamond. Calcium in olivine should not be used as a geothermobarometer for harzburgitic olivines, and often gives unrealistic P-T estimates for lherzolitic olivine as well. Diamond-hosted olivine inclusions indicate growth in an extremely depleted (low Ti, Ca, Na, high Cr#) environment with no residual clinopyroxene. They are distinct from olivines from mantle xenoliths which show higher, more variable Ti contents and lower Cr#. Hence, most olivine inclusions in Akwatia diamonds escaped the refertilisation processes that have affected most mantle xenoliths. Lherzolitic inclusions are probably the result of refertilisation after undergoing high-degree melting first. Trivalent cations appear to behave differently in harzburgitic diamond-hosted olivine inclusions than lherzolitic inclusions and olivine from mantle xenoliths. Some divalent chromium is predicted to be present in most olivine inclusions, which may explain high concentrations up to 0.16 wt% Cr2O3 observed in some diamond inclusions. Strong heterogeneity of Cr, V and Al in several inclusions may also result in apparent high Cr contents, and is probably due to late-stage processes during exhumation. However, in general, diamond-hosted olivine inclusions have lower Cr and V than expected compared to mantle xenoliths. Reduced Na activity in depleted harzburgites limits the uptake of Cr, V and Sc via Na-M3+ exchange. In contrast, Al partitioning in harzburgites is not significantly reduced compared to lherzolites, presumably due to uptake of Al in olivine by Al-Al exchange.
Contributions to Mineralogy and Petrology, Vol. 174, (12) doi: 10.1007/s00410-019-1634-y
Africa, Ghana
deposit - Akwatia
Abstract: Trace-element concentrations in olivine and coexisting garnets included in diamonds from the Akwatia Mine (Ghana, West African Craton) were measured to show that olivine can provide similar information about equilibration temperature, diamond paragenesis and mantle processes as garnet. Trace-element systematics can be used to distinguish harzburgitic olivines from lherzolite ones: if Ca/Al ratios of olivine are below the mantle lherzolite trend (Ca/Al??300 µg/g Ca or?>?60 µg/g Na are lherzolitic. Conventional geothermobarometry indicates that Akwatia diamonds formed and resided close to a 39 mW/m2 conductive geotherm. A similar value can be derived from Al in olivine geothermometry, with TAl-ol ranging from 1020 to 1325 °C. Ni in garnet temperatures is on average somewhat higher (TNi-grt?=?1115-1335 °C) and the correlation between the two thermometers is weak, which may be not only due to the large uncertainties in the calibrations, but also due to disequilibrium between inclusions from the same diamond. Calcium in olivine should not be used as a geothermobarometer for harzburgitic olivines, and often gives unrealistic P-T estimates for lherzolitic olivine as well. Diamond-hosted olivine inclusions indicate growth in an extremely depleted (low Ti, Ca, Na, high Cr#) environment with no residual clinopyroxene. They are distinct from olivines from mantle xenoliths which show higher, more variable Ti contents and lower Cr#. Hence, most olivine inclusions in Akwatia diamonds escaped the refertilisation processes that have affected most mantle xenoliths. Lherzolitic inclusions are probably the result of refertilisation after undergoing high-degree melting first. Trivalent cations appear to behave differently in harzburgitic diamond-hosted olivine inclusions than lherzolitic inclusions and olivine from mantle xenoliths. Some divalent chromium is predicted to be present in most olivine inclusions, which may explain high concentrations up to 0.16 wt% Cr2O3 observed in some diamond inclusions. Strong heterogeneity of Cr, V and Al in several inclusions may also result in apparent high Cr contents, and is probably due to late-stage processes during exhumation. However, in general, diamond-hosted olivine inclusions have lower Cr and V than expected compared to mantle xenoliths. Reduced Na activity in depleted harzburgites limits the uptake of Cr, V and Sc via Na-M3+ exchange. In contrast, Al partitioning in harzburgites is not significantly reduced compared to lherzolites, presumably due to uptake of Al in olivine by Al-Al exchange.
Apparent partial loss 40Ar 39 Ar age spectra of hornblende from the Paleoproterozic Lapland Kola orogen ( Arctic European Russia): insights into modelling ....
Geosciences Journal, Vol. 13, 3, Sept. pp. 317-329.
Geochemical and petrographical study of megacrysts and mantle xenoliths from Gemsbok Hollow and Gruendorn kimberlites in the Warmbad kimberlite Province, South Africa
Prencipe, M., Bruno, M., Nestola, F., De La Pierre, M., Nimis, P.
Toward an accurate ab initio estimation of compressibility and thermal expansion of diamond in the (0, 3000K) temperature and (0,30 Gpa) pressure ranges, at the hybrid HF/DFT theoretical level.
Abstract: Pangea results from the progressive amalgamation of continental blocks achieved at 320?Ma. Assuming that the ancient concept of “active” versus “passive” rifting remains pertinent as end-members of more complex processes, we show that the progressive Pangea breakup occurred through a succession of rifting episodes characterized by different tectonic evolutions. A first episode of passive continental rifting during the Upper Carboniferous and Permian led to the formation of the Neo-Tethys Ocean. Then at the beginning of Triassic times, two short episodes of active rifting associated to the Siberian and Emeishan large igneous provinces (LIPs) failed. The true disintegration of Pangea resulted from (1) a Triassic passive rifting leading to the emplacement of the central Atlantic magmatic province (200?Ma) LIP and the subsequent opening of the central Atlantic Ocean during the lowermost Jurassic and from (2) a Lower Jurassic active rifting triggered by the Karoo-Ferrar LIP (183?Ma), which led to the opening of the West Indian Ocean. The same sequence of passive then active rifting is observed during the Lower Cretaceous with, in between, the Parana-Etendeka LIP at 135?Ma. We show that the relationships between the style of rifts and their breakdown or with the type of resulting margins (as magma poor or magma dominated) are not straightforward. Finally, we discuss the respective role of mantle global warming promoted by continental agglomeration and mantle plumes in the weakening of the continental lithosphere and their roles as rifting triggers.
Abstract: Engineering coherent systems is a central goal of quantum science. Color centers in diamond are a promising approach, with the potential to combine the coherence of atoms with the scalability of a solid-state platform. We report a color center that shows insensitivity to environmental decoherence caused by phonons and electric field noise: the neutral charge state of silicon vacancy (SiV0). Through careful materials engineering, we achieved >80% conversion of implanted silicon to SiV0. SiV0 exhibits spin-lattice relaxation times approaching 1 minute and coherence times approaching 1 second. Its optical properties are very favorable, with ~90% of its emission into the zero-phonon line and near -transform-limited optical linewidths. These combined properties make SiV0 a promising defect for quantum network applications.
Brazil Journal of Geology ( www.scielo.br) ENG, 15p. Pdf
South America, Brazil
deposit - Parana
Abstract: The Paraná deposit, located at Southwestern Rio Grande do Norte state, in Brazil, is one of the few emerald deposits found at Borborema Province. The mineralization occurs in phlogopite schists and actinolite-phlogopite schists associated with pegmatites and albitites within the Portalegre Shear Zone. Unlike other well-known Brazilian emerald deposits, the mineralogy of Paraná emeralds has remained poorly investigated for the last 40 years. In this study, we conducted mineralogical characterization of theses emeralds through gemological testing, mineral chemistry, absorption and reflectance spectroscopy, and thermal analysis. The Paraná emeralds are bluish-green colored, characterized by high refractive index, several two-phase fluid inclusions and mica is the main mineral inclusion. Electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometry analyses detected the presence of Fe2+ (0.43-1.94 wt.% FeO) and Cr3+ (0.04-0.14 wt.% Cr2O3) as the main chromophores replacing octahedral Al3+ in the crystal structure. In addition, substantial amounts of MgO (0.40-2.72 wt.%), Na2O (0.50-1.81 wt.%), and Cs2O (0.07-0.44 wt.%) were also identified. The main causes for its coloration were attributed to Cr3+ absorption features in visible spectral range, which were corroborated by absorption and reflectance spectra. The presence of types I and II H2O at channel-sites was recorded in Fourier-transform infrared spectra and demonstrated by dehydration processes observed in different thermal and thermogravimetric analyses.
Brazil Journal of Geology ( www.scielo.br) ENG, 17p. Pdf
South America, Brazil
deposit - Tres Estradas
Abstract: Carbonatites were recently discovered in Southern Brazil, which increased the interest to evaluate the economic potential of these uncommon rocks, especially the Três Estradas Carbonatite. Carbonates are the dominant minerals of fresh rock followed by apatite, but the weathering process makes apatite abundant. We focused on apatite from the carbonatite using conventional petrography and electronic microscopy associated with microprobe, micro-Raman and Fourier-transform infrared spectroscopy. Results demonstrate the existence of four types. The primary type is associated with the rock crystallization and the subsequent three others are associated with weathering processes. The alteration mechanism was favorable for initial carbonate leaching and subsequent increase of phosphate with late precipitation of three new apatite generations. The deduced model involves long exposure during polycyclic climate changes, intercalating periods of warm dry with humid climate. The apatite types differ chemically and morphologically and have distinctive characteristics that are suitable to be used to differentiate them. These properties should be considered in future planes of industrial processes to transform apatite into single superphosphate, a basic input for fertilizer production.
International Engineering Journal, Ouro Preto *** IN: PORT, Vol. 71, 1, pp. 27-35.
South America, Brazil
deposit - Chapada
Abstract: The Chapada Diamantina, located in the central region of the State of Bahia, is of important historical significance due to its diamond occurrences. Discovered in the nineteenth century, comprehensive research about the regional diamonds and their origins are still limited, demanding more investigation in the matter. Looking for insights about their genesis, mineral inclusions in 23 alluvial diamonds from 4 garimpos located in the Chapada Diamantina were analyzed through the use of Raman micro spectroscopy. Additionally, the characteristics of nitrogen aggregation of the host diamonds were measured using Fourier-transform infrared spectroscopy (FTIR). The diamonds from Chapada Diamantina consist mainly of well-formed crystals, with dominant dodecahedral habits, characterized by faint to very light yellow body colors, typically with green and brown radiation spots on their surface. The main surface textures observed are related to processes that took place in the late stage resorption and during the residence of the diamonds in placer environments. The diamonds are predominantly type IaAB, with a significant occurrence of poorly aggregated nitrogen (Type IaA diamond). The main mineral assemblages of the studied peridotitic inclusions refer to a harzburgitic paragenesis.
The Orange and Riet River alluvial diamond deposits in the vicinity of Douglas, Northern Cape Province: geology, evaluation, and exploitation of unique South African large diamond producing deposits.
IGC 35th., Session Mineral Exploration 1p. Abstract
Journal of South American Earth Sciences, Vol. 58, March pp. 72-81.
South America, Paraguay, Brazil
Geochronology
Abstract: The magmatic rocks from Alto Paraguay (High Paraguay River extensional lineament), western Apa craton, mainly consist of several major circular alkaline complexes and some rhyolitic domes and ignimbrites. The former are characterized by intrusive Na-alkaline rock-types (nepheline syenites and syenites and effusive equivalents) topped by lava flows and ignimbrites. Two main evolved suites were defined using petrochemical and Sr- isotope data: an agpaitic suite in the north and a miaskitic suite in the south. The domes of subalkaline rhyolitic lavas and ignimbrites occur to the north of the alkaline complexes, along the Paraguay River, near the town of Fuerte Olimpo. The emplacement ages of the alkaline complexes were constrained using the K-Ar, Ar-Ar, Rb-Sr and Sm-Nd dating methods on whole rocks and/or mineral separates (amphibole, alkali feldspar and biotite). Ages are quite variable (Upper Permian to Middle Triassic), with average K-Ar and Ar-Ar ages of 248.8 ± 4.8 and 241.8 ± 1.1 Ma, respectively, and Rb-Sr and Sm-Nd age data giving best values from 248 ± 4 to 244 ± 27 Ma and from 256 ± 3 to 257 ± 3 Ma, respectively. In contrast, the Fuerte Olimpo volcanics show a Mesoproterozoic age (1.3 Ga, K-Ar and Ar-Ar radiometric methods; and 1.42 ± 0.24 to 1.30 ± 0.03 Ga, Rb-Sr and Sm-Nd methods, respectively). Rb-Sr systematics (87Sr/86Sr initial ratios ? 0.7038) highlight a relatively "primitive" character of the Na-alkaline magmatic source(s), in contrast with the "crustal" values (87Sr/86Sr initial ratio ? 0.7105) of the Fuerte Olimpo rhyolites. Thus, magmatism in the Alto Paraguay area is related to two extensional events: a younger event corresponding to the Permian-Triassic alkaline rocks, and an older event connected to the Precambrian volcanic acidic rocks.
Journal of South American Earth Sciences, Vol. 71, pp. 248-261.
South America, Paraguay
Carbonatite
Abstract: This work describes rare accessory minerals in volcanic and subvolcanic silica-undersaturated peralkaline and agpaitic rocks from the Permo-Triassic Cerro Boggiani complex (Eastern Paraguay) in the Alto Paraguay Alkaline Province. These accessory phases consist of various minerals including Th-U oxides/silicates, Nb-oxide, REE-Sr-Ba bearing carbonates-fluorcarbonates-phosphates-silicates and Zr-Na rich silicates. They form a late-stage magmatic to deuteric/metasomatic assemblage in agpaitic nepheline syenites and phonolite dykes/lava flows made of sodalite, analcime, albite, fluorite, calcite, ilmenite-pyrophanite, titanite and zircon. It is inferred that carbonatitic fluids rich in F, Na and REE percolated into the subvolcanic system and metasomatically interacted with the Cerro Boggiani peralkaline and agpaitic silicate melts at the thermal boundary layers of the magma chamber, during and shortly after their late-stage magmatic crystallization and hydrothermal deuteric alteration.
Journal of South American Earth Sciences, Vol. 72, pp. 302-314.
South America, Paraguay
Magmatism
Abstract: The amalgamation of the Western Gondwana (including the Greater Gondwana supercraton) occurred at 600 Ma during the Brazilian - Pan African orogeny. A plate junction related to this event is marked by the Transbrazilian lineament which separates the South American continent into two sectors: the Eastern Paraguay-Brazilian and Central Andean domains. An overview of the geodynamic data from these two sectors indicates that the two domains were subjected to distinct evolutions from the Proterozoic to the present. The Andean domain is characterized by long-lived subduction processes linked to the convergence and consequent collision of microplates since the Middle Proterozoic (western Amazonian Craton) with a peak at about 600-580 Ma. The Paraguay-Brazilian domain remained relatively stable but was affected by extension episodes that reactivated ancient (Early and Middle Proterozoic) suture zones. These different geodynamic evolutions seem to reflect broadly distinct mantle compositions. In the subduction zones of the Andean domain the mantle was deeply modified by metasomatic processes following the subduction of oceanic plates. Consequently, the Andean type magma sources show a clear HIMU imprint inherited from the MORB, whereas the Paraguay-Brazilian sector shows a prevalent EMI and subordinate EMII character. The petrological data mainly from Mesozoic and Cenozoic magmatic events in the two sectors are reviewed to investigate the current mantle plume and mantle dome models for the uprising of the asthenospheric (or sub-lithospheric) material.
Boletin del Museo Nacional de Historia Narural del Paraguay, Vol. 20, 2, pp. 188-204. pdf available in * Port
South America, Paraguay
Geochemistry - indicator minerals
Abstract: Many diamondiferous kimberlites in the Lac de Gras region of the Northwest Territories are concealed by glacial drift, rendering them challenging to detect by traditional exploration techniques that exploit residual surface chemistry. Much research has been aimed at the development of deep penetrating geochemical exploration technologies to increase the rate of discovery whilst reducing risk and exploration cost. However, results from a detailed study of soil geochemistry above the DO-18 kimberlite (Peregrine Diamonds) demonstrate the potential to apply conventional surface geochemical techniques coupled with surface material mapping and landscape evolution models to the evaluation of discrete targets. 50 soil samples from the oxidized upper B-horizon in a detailed grid crossing the concealed kimberlite were collected. Samples, screened to -180 microns, were analysed by multi-element ICP-MS following 4-acid, aqua-regia and deionized water extractions. Fp-XRF was utilised as an equivalent total method to evaluate its applicability. Sequential leach on selected samples was undertaken to understand the deportment of the elements of interest within the soils. Surficial mapping included soil type, topographic variation, landforms, environment and vegetation. This allows an assessment of surface controls on the geochemistry, in particular the generation of false anomalies from chemical traps such as swamps; and allows the generation of a landscape development model. Hydrocarbons, analysed using the SGH and Gore-sorber techniques, were evaluated to characterize the type and abundance of complex hydrocarbons above the kimberlite relative to above the host granitic gneiss. Geochemical data is subject to landform generation processes. The northern half of the grid comprises till with numerous frost boils. The southern half, at lower topography below a distinct break, is dominated by sand-rich material and fine clay. Results from the 4-acid and aqua regia extraction show a dispersion of Nb, Ni, Mg, Ce, Cr and Cs from directly above the northern part of the kimberlite to the edge of the sampling grid, approximately 500 metres to the northwest, following glacial dispersion. SGH-hydrocarbon results exhibit a similar pattern in light-alkyl benzenes. Fp-XRF data repeats the pattern in all elements except Mg, where the concentrations are too low for reliable detection. In the southern half of the grid, at a lower topographic level, geochemical responses are considerably more subtle. It is hypothesised that anomaly formation in the till followed standard glacial dispersion in the down ice trend. Material was entrained to the surface from deeper in the till, locally above the kimberlite, by frost boil action. The southern part of the area is considered to have been inundated with water, the remains of which comprise the current lake over the DO-27 kimberlite approximately 400m to the south. Sediments in this area are clay rich - comprising material deposited by the lake, or re-worked sandy material along the palaeo-lake margin and subsequent erosional channels. These later processes acted to further disperse, conceal and dilute the signal of the underlying body.
Bol. Mus. Nac. Hist. Parag. *** ENG, Vol. 20, 2, pp. 188-204. pdf
South America, Paraguay
geochemistry
Abstract: After some works of Jaime Baez-Presse that quoted the presence of diamonds in Eastern Paraguay, we have perfprmed a whole sampling a study relative to the indicator mineral for diamonds. Indicator minerals are mineral species that, when appearing as transported grains in clastic sediments, indicate the presence in bedrock of a specific type of mineralization, hydrothermal alteration or lithology. Their physical and chemical characteristics, including a relatively high density (heavy minerals), facilitate their preservation and identification. The heavy minerals represent an important exploration method for detecting a variety of ore deposit types including diamond, gold, Ni-Cu, PGE, and so on.. One of the most significant events in the application of indicator mineral methods in the past was the diamond exploration. This paper provides an overview of indicator mineral methods, i.e. presence of Cr-diopside, Pyrope-rich garnet and Picroilmenite, for diamond exploration along the Eastern Paraguay river. Unfortunately the above heavy mineraks, generally associated to the diamonds, do not appear in Eastern Paraguay, excluding this Country as a potential source for the diamond as economic potential source.
Omarini, R.H., Gasparon, M., De Min, A.M., Comin-Chiaramonti, P.
An overview of the Mesozoic-Cenozoic magmatism and tectonics of Eastern Paraguay and central Andes ( western Gondwana): implications for the composition of mantle sources.
Journal of South American Earth Sciences, In press available, 19p.
Barry, P.H., de Moor, J.M., Giovannelli, D., Schrenk, M., Hummer, D.R., Lopez, T., Pratt, C.A., Alpizar Segua, Y., Battaglia, A., Beaudry, A., Bini, G., Cascante, M., d'Errico, G., di Carlo, M., Fattorini, D., Fullerton, K., H+Gazel, E., Gonzalez, G., Hal
Abstract: Carbon and other volatiles in the form of gases, fluids or mineral phases are transported from Earth’s surface into the mantle at convergent margins, where the oceanic crust subducts beneath the continental crust. The efficiency of this transfer has profound implications for the nature and scale of geochemical heterogeneities in Earth’s deep mantle and shallow crustal reservoirs, as well as Earth’s oxidation state. However, the proportions of volatiles released from the forearc and backarc are not well constrained compared to fluxes from the volcanic arc front. Here we use helium and carbon isotope data from deeply sourced springs along two cross-arc transects to show that about 91 per cent of carbon released from the slab and mantle beneath the Costa Rican forearc is sequestered within the crust by calcite deposition. Around an additional three per cent is incorporated into the biomass through microbial chemolithoautotrophy, whereby microbes assimilate inorganic carbon into biomass. We estimate that between 1.2 × 108 and 1.3 × 1010 moles of carbon dioxide per year are released from the slab beneath the forearc, and thus up to about 19 per cent less carbon is being transferred into Earth’s deep mantle than previously estimated.
Bruno, H., Helibron, M., Strachen, R., Fowler, M., de MorrisonValeriano , C., Bersan, S., Moreira, H., Cutts, K., Dunlop, J., Almeida, R., Almeida, J., Storey, C.
Abstract: A zircon Hf isotope data set from Archean and Paleoproterozoic magmatic and metasedimentary rocks of the southern São Francisco craton (Brazil) is interpreted as evidence of accretionary and collisional plate tectonics since at least the Archean-Proterozoic boundary. During the Phanerozoic, accretionary and collisional orogenies are considered the end members of different plate tectonic settings, both involving preexisting stable continental lithosphere and consumption of oceanic crust. However, mechanisms for the formation of continental crust during the Archean and Paleoproterozoic are still debated, with the addition of magmatic rocks to the crust being explained by different geodynamic models. Hf isotopes can be used to quantify the proportion of magmatic addition into the crust: positive ?Hf values are usually interpreted as indications of magmatic input from the mantle, whereas crust-derived rocks show more negative ?Hf. We show that the crust of the amalgamated Paleoproterozoic tectonostratigraphic terranes that make up the southern São Francisco craton were generated from different proportions of mantle and crustal isotopic reservoirs. Plate tectonic processes are implied by a consistent sequence of events involving (1) the generation of juvenile subduction-related magmatic arc rocks, followed by (2) collisional orogenesis and remelting of older crust, and (3) post-collisional bimodal magmatism.
Journal of South American Earth Sciences, Vol. 111, 10p. Pdf
South America, Brazil
geophysics - seismics
Abstract: The crustal thickness in South America has been mostly determined using seismological techniques. However, because these techniques provide point constraint or profile-specific results, the crustal thickness maps become especially dependent on both the number and spatial distribution of seismological stations. In the Amazonian Craton, the extensive forest cover restricts the number of existing stations, not allowing to elaborate a solely seismological crustal thickness model with homogeneous data coverage. Therefore, to overcome this difficulty, this work proposes a crustal thickness model for the Amazonian Craton developed based on the Parker-Oldenburg method and the Global Geopotential Model called GECO, considering the relationships between wavelengths and depths of the investigation sources. Furthermore, the developed iterative process allowed to determine the average depth of the crust-mantle interface, the density contrast at the interface, and the minimum and maximum frequencies used in the signal filtering process, making the model more robust for defining the used constants. The average crustal thickness of the Amazonian Craton was estimated as 40.25 km, with a standard deviation of the differences of 4.91 km, compared to crustal thickness defined by the seismological data. The estimated model shows great consistency with the data set used while allowing important inferences about craton compartmentation. Also, the geological provinces displayed an N-S connecting trend under the Amazonas, Solimões, and Acre basins, correlating the Guyana Shield with the Central Brazil Shield. Additionally, we observed various tectonic cycles acting on the craton while significantly modifying the structure of the provinces, possibly removing cratonic roots and rejuvenating the crust in older provinces.
Abstract: Despite the advanced stage of diamond thin-film technology, with applications ranging from superconductivity to biosensing, the realization of a stable and atomically thick two-dimensional diamond material, named here as diamondene, is still forthcoming. Adding to the outstanding properties of its bulk and thin-film counterparts, diamondene is predicted to be a ferromagnetic semiconductor with spin polarized bands. Here, we provide spectroscopic evidence for the formation of diamondene by performing Raman spectroscopy of double-layer graphene under high pressure. The results are explained in terms of a breakdown in the Kohn anomaly associated with the finite size of the remaining graphene sites surrounded by the diamondene matrix. Ab initio calculations and molecular dynamics simulations are employed to clarify the mechanism of diamondene formation, which requires two or more layers of graphene subjected to high pressures in the presence of specific chemical groups such as hydroxyl groups or hydrogens.
Abstract: The Late Cretaceous Catalão I carbonatite complex consists of ultramafic silicate rocks, phoscorites, nelsonites and carbonatites. The latest stages of the evolution of the complex are characterized by several nelsonite (magnetite-apatite rock) and carbonatite dykes, plugs and veins crosscutting earlier alkaline rocks. The interaction between the latter and late-stage carbonatites and/or carbo-hydrothermal fluids, converted the original dunites and bebedourites to metasomatic phlogopitites. Late-stage nelsonites (N1), pseudonelsonites (N2) and various types of dolomite carbonatites (DC) including norsethite-, magnesite- and/or monazite-bearing varieties show significant whole-rock Nd and Sr isotopic variations. To elucidate whether magmatic or metasomatic processes, or both, were responsible for these isotope variations we characterized the Nd and Sr isotope compositions of major mineral phases (i.e. apatite, dolomite, norsethite, pyrochlore and tetraferriphlogopite) in these late-stage rocks. Mineral isotope data recorded the same differences observed between N1 and N2 whole-rocks with N2 minerals showing more enriched isotopic signatures than minerals from N1. Sr isotopic disequilibrium among minerals from N2 pseudonelsonites and spatially related dolomite carbonatite pockets implies formation from batches of carbonate melts with distinct isotopic compositions. A detailed investigation of Nd and Sr isotopes from whole-rocks and minerals suggests that the most evolved rocks of the Catalão I complex probably derive from two different evolution paths. We propose that an earlier magmatic trend (path A) could be explained by several batches of immiscible and/or residual melts derived from carbonated-silicate parental magma (e.g. phlogopite picrite) contaminated with continental crust to a variable extent, in an AFCLI-like process. A second trend (path B) comprises highly variable 143Nd/144Ndi at nearly constant 87Sr/86Sri coupled with high ?18O in carbonates. This is interpreted here as the result of the interaction of previously-formed dolomite carbonatites with carbo-hydrothermal fluids.
Petrology, geochemistry and Sm-Nd systematics of the Paleoproterozoic Itagurra retroeclogite from Sao Francisco/Congo craton: one of the oldest records of the modern-style plate tectonics.
De Oliveira Cordeiro, P.F., Brod, J.A., Ventura Santos, R., Dantas, E.L., Gouvieia de Oliveira, C., Soares Rocha, Barbosa, E.
Stable ( C,O) and radiogenic (Sr, Nd) isotopes of carbonates as indicators of magmatic and post magmatic processes of phoscorite series rocks and carbonatites f
Contributions to Mineralogy and Petrology, In press available, 14p.
Soares Rocha Barbosa, E., Brod, J.A., Junqueira-Brod, T.C., Dantas, E.L., De Oliveira Cordeiro, P.F., Siqueira Gomide, C.
Bebdourite from its type area Sailtre 1 complex: a key petrogenetic series in the Late-Cretaceous Alto Paranaiba kamafugite carbonatite phoscorite association, central Brazil.
Abstract: We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts, ilmenite, phlogopite, spinel, zircon, and uncommon minerals (titanite, calzirtite, anatase, baddeleyite and pyrochlore) of a newly discovered Late Cretaceous kimberlite (U-Pb zircon age 90.0 ± 1.3 Ma; 2?) named Osvaldo França 1, located in the Alto Paranaíba Igneous Province (APIP), southeastern Brazil. Pyrope grains are lherzolitic (Lherz-1, Lherz-2 and Lherz-3), harzburgitic (Harz-3) and wehrlitic (Wehr-2). The pyrope xenocrysts cover a wide mantle column in the subcratonic lithosphere (66-143 km; 20-43 kbar) at relatively low temperatures (811-875 °C). The shallowest part of this mantle is represented by Lherz-1 pyropes (20-32 kbar), which have low-Cr (Cr2O3 = 1.74-6.89 wt.%) and fractionated middle to heavy rare earth elements (MREE-HREE) pattern. The deepest samples are represented by Lherz-2, Lherz-3, Harz-3, and Wehr-2 pyropes (36-43 kbar). They contain high-Cr contents (Cr2O3 = 7.36-11.19 wt.%) and are characterized by sinusoidal (Lherz-2 and Wehr-2) and spoon-like (Lherz-3 and Harz-3) REE patterns. According to their REE and trace elements, pyrope xenocrysts have enriched nature (e.g., Ce and Yb vs. Cr2O3), indicating that the cratonic lithosphere has been affected by a silicate melt with subalkaline/tholeiite composition due to their low Zr, Ti and Y concentrations. Besides minerals with typical kimberlitic signatures, such as ilmenite and zircon, the exotic compositions of phlogopite and ulvöspinel suggest an enriched component in the magma source. The formation of rare mineral phases with strong enrichment of light-REE (LREE) and high field strength elements (HFSE) is attributed to the late-stage kimberlitic melt. We propose a tectonic model where a thermal anomaly, represented by the low-velocity seismic anomaly observed in P-wave seismic tomography images, supplied heat to activate the alkaline magmatism from a metasomatized cratonic mantle source during the late-stages of Gondwana fragmentation and consequent South Atlantic Ocean opening. The metasomatism recorded by mineral phases is a product of long-lived recycling of subducted oceanic plates since the Neoproterozoic (Brasiliano Orogeny) or even older collisional events, contributing to the exotic character of the Osvaldo França 1 kimberlite, as well as to the cratonic lithospheric mantle.
Brazil Journal of Geology ( www.scielo.br) ENG, 14p. Pdf
South America, Brazil
deposit - Fazenda Bonfim
Abstract: The Fazenda Bonfim emerald deposit lies within the Seridó Belt. It is a classic example of deposit formed through metasomatic interactions between Be-rich granite intrusions and Cr(± V)-rich mafic-ultramafic rocks. The setting of the emerald mineralization was built under strong strike-slip dynamics, which produced serpentinization and talcification of mafic-ultramafic host-rocks, and was followed by syn-kinematic emplacement of Be-rich albite granite, favoring hydrothermal/metasomatic processes. The structural control and lithological-contrast were fundamental to the fluid flow and the best ore-shoot geometry, developed in the S-foliation intra-plane at the contact zone (phlogopite hornfels) between mafic-ultramafic rocks and the albite granite. Subsequently, an albitization process, linked to the final-stage of magmatic crystallization, led to an overall mineralogical and chemical change of the albite granite. 207U-235Pb data revealed inheritance ages from Archean to Neoproterozoic and a crystallization age of 561 ± 4 Ma for albite granite. However, 40Ar/39Ar data revealed plateau age of 553 ± 4 Ma for phlogopite hornfels, interpreted as the closure time for the metasomatic event responsible for the nucleation and growth of emerald crystals. The short interval of time between U-Pb and Ar-Ar data indicates an intense, but not protracted, metasomatic history, probably due to low volume of intrusive magma.
De Oliveire Cordeiro, P.F., Brod, J.A., Ventura Santos, R., Dantas, E.L., Gouveia de Oliveira, C., Soares Rochas Barbosa, E.
Stable (C,O) and radiogenic (Sr,Nd) isotopes of carbonates as indicators of magmatic and post-magmatic processes of phoscorite series rocks and carbonatites from Catalao 1, central Brazil.
Contributions to Mineralogy and Petrology, Vol. 161, 3, pp. 451-464.
Precambrian Research, in press available, 55p. pdf
South America, Brazil
craton
Abstract: Broadband and long period magnetotelluric (MT) data were collected along an east-west oriented, 580-km-long profile across the northern São Francisco Craton where extensive Proterozoic and Phanerozoic sedimentary cover and lack of deep-probing geophysical surveys have prevented to establish unequivocally the cratonic character of the Archean-Paleoproterozoic lithosphere. Following dimensionality analyses, the MT dataset was interpreted using both 2-D and 3-D inversion procedures. The near-surface structure is better resolved in the 2-D model due to its finer resolution. A huge upper crustal conductor is found all along the shallow early Neoproterozoic Irecê Basin in the central domain of the craton, extending laterally for approximately 150?km and restricting signal propagation below the basin. Its high conductance is explained by a combination of high porosity and high fluid salinity in the sedimentary package. Another upper crustal conductor is observed on the west side of the profile, interpreted as fractured metasedimentary rocks of the Rio Preto belt thrusted on top of the craton basement during Neoproterozoic marginal collision. The 3-D model explains significantly better the measured data related to deep structure. Contrary to what is expected for a stable cratonic block, the geoelectric model shows pronounced electrical complexity and heterogeneity, an indication that the cratonic lithosphere was multiply reworked in the past by tectonothermal events. Different lithospheric resistive blocks bounded by major conductive zones are identified. Constrained by geochemical and isotopic data, these vertical conductive interfaces are interpreted as cryptic suture zones due to large-scale amalgamation of continents and microcontinents leading to the assembly of the São Francisco Craton in the Paleoproterozoic. The conductivity enhancement is more likely explained by emplacement of sulfides along previous suture zones during mafic magmatism. At upper mantle depths, high conductivity observed below most of the profile indicates that metasomatism or refertilization processes with incompatible elements caused by the Paleoproterozoic subducting slabs and Mesoproterozoic to Neoproterozoic upwelling of deep fluids and melts reworked this portion of the craton mantle.
Abstract: The Upper Ruvubu Alkaline Plutonic Complex (URAPC) in Burundi consists of three separate intrusions, each with a specific emplacement age and petrological composition. Three main units are recognized: an outer unit with silica-saturated plutonic rocks (from gabbro to granite), an inner unit with silica-undersaturated plutonic rocks (feldspathoidal syenite with subordinate feldspathoidal monzonite and ijolite) and a carbonatitic body in the subsoil, known by drilling. The URAPC is quite large in size (?24?km long and up to 10?km wide). It is considered to have been intruded syntectonically in an overall extensional context, thanks to the kilometric shear zones that accommodated its emplacement. Radiometric ages from literature range from 748 to 705?Ma and point to structurally-controlled magmatic differentiation followed by long-lived circulations of late-stage fluids postdating the emplacement of a part of the undersaturated rocks and the carbonatites. In the north-western part of the outer unit, gabbro likely has been emplaced at a deeper structural level than the granite, which represents a more apical structural level of emplacement. This petrological, geochemical and isotopic (Sr-Nd-Hf) study concentrates on the processes that generated the URAPC: (i) fractional crystallization, evidenced by the chemical evolution trends of the major and trace elements, and by marked P, Ti and Ba anomalies in the trace element patterns; (ii) crustal assimilation/contamination, as shown by the wide range of Nd isotope compositions and the general increase of the Sr isotope ratios with increasing SiO2 contents, and (iii) late-magmatic/hydrothermal alteration inducing an increase of the Sr isotope composition without changing significantly the Nd isotope composition. The isotopic data are consistent with an asthenospheric mantle source, though less depleted than the Depleted Mantle (DM), contaminated by the Subcontinental Lithospheric Mantle (SCLM). The silicate and carbonate magmatic series are cogenetic. The outer unit is clearly more contaminated than the inner unit, whereas the carbonatitic body could have evolved by liquid immiscibility. The URAPC lies within East Africa’s Western Rift Valley, which is marked by 23 alkaline plutonic complexes. Their emplacement has been ascribed to reactivation of Proterozoic lithospheric weakness zones resulting from the breakup of the Neoproterozoic supercontinent Rodinia supercontinent.
Abstract: Much of the present-day volume of Earth’s continental crust had formed by the end of the Archean Eon, 2.5 billion years ago, through the conversion of basaltic (mafic) crust into sodic granite of tonalite, trondhjemite and granodiorite (TTG) composition. Distinctive chemical signatures in a small proportion of these rocks, the so-called high-pressure TTG, are interpreted to indicate partial melting of hydrated crust at pressures above 1.5?GPa (>50?km depth), pressures typically not reached in post-Archean continental crust. These interpretations significantly influence views on early crustal evolution and the onset of plate tectonics. Here we show that high-pressure TTG did not form through melting of crust, but through fractionation of melts derived from metasomatically enriched lithospheric mantle. Although the remaining, and dominant, group of Archean TTG did form through melting of hydrated mafic crust, there is no evidence that this occurred at depths significantly greater than the ~40?km average thickness of modern continental crust.
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.
Journal of South American Earth Sciences, Vol. 114, 103712, 20p. Pdf
South America, Brazil, Mato Grosso
deposit - Itiquira, alluvials
Abstract: The Itiquira River, Mato Grosso state (western Brazil), hosts several diamond placer deposits, mined intermittently over the last century. It runs over volcanic and sedimentary rocks of the Paraná Basin until it discharges in the northern Pantanal Basin. The bedrocks were deposited in marine, continental desertic, alluvial deltaic, and fluvial environments. The meanders of the Itiquira River are controlled by NE-SW, ENE-WSW, NNW-SSE, N-S, and NW-SE fractures and normal faults, developed in response to the evolution of the Paraná Basin and by neotectonics, linked with the development of the Pantanal Basin since the Paleogene. The Itiquira River middle valley, in which the diamondiferous placers are found, is controlled by NE-SW structures inherited from the Neoproterozoic Transbrasiliano Lineament. The landscape comprises dissected plateaus and structure-controlled valleys formed by Cretaceous, Paleogene, and Neogene planation processes. The diamonds occur in the muddy-sandy matrix of the current stream bed and older terraces gravels. In the Itiquira River, the diamond deposits are related to the following traps: point bars, cut-and-fill channels, pockets, and potholes. Sapphire, garnet (including kimberlitic), ilmenite, zircon, rutile, gold, and iron oxides are documented as heavy minerals in the gravels. Detrital zircon dating of grains extracted from the Itiquira River diamond deposits resulted in the ages of 2057, 1184, 873, 645-508, 307-207, and 144-142 Ma. The potential zircon sources are the Goiás Magmatic Arc granitoids, Paraguay Belt metavolcanics and granites, and Serra Geral Formation volcanics. The ages between 307 and 207 Ma are likely to be from an unknown (possibly kimberlitic) source. The Itiquira River tectonic, geomorphological, and sedimentological evolutions suggest potential sources for the diamond placers and paleoplacers.
Method and trace elements for monitoring the seperation of diamonds And diamond containing particles from gangue particles on the basis of Ramanscattering
Patent: De Beers Ind. Diamond Division, Title cont'd.Laser radiation, British U.K.No. 2210686 A1, June 14, 1989. 9p
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.
Netherlands Journal of Geolsciences, Vol. 95, 4, pp. 491-522.
South America, Suriname
Guiana shield
Abstract: The Proterozoic basement of Suriname consists of a greenstone-tonalite-trondhjemite-granodiorite belt in the northeast of the country, two high-grade belts in the northwest and southwest, respectively, and a large granitoid-felsic volcanic terrain in the central part of the country, punctuated by numerous gabbroic intrusions. The basement is overlain by the subhorizontal Proterozoic Roraima sandstone formation and transected by two Proterozoic and one Jurassic dolerite dyke swarms. Late Proterozoic mylonitisation affected large parts of the basement. Almost 50 new U-Pb and Pb-Pb zircon ages and geochemical data have been obtained in Suriname, and much new data are also available from the neighbouring countries. This has led to a considerable revision of the geological evolution of the basement. The main orogenic event is the Trans-Amazonian Orogeny, resulting from southwards subduction and later collision between the Guiana Shield and the West African Craton. The first phase, between 2.18 and 2.09 Ga, shows ocean floor magmatism, volcanic arc development, sedimentation, metamorphism, anatexis and plutonism in the Marowijne Greenstone Belt and the adjacent older granites and gneisses. The second phase encompasses the evolution of the Bakhuis Granulite Belt and Coeroeni Gneiss Belt through rift-type basin formation, volcanism, sedimentation and, between 2.07 and 2.05 Ga, high-grade metamorphism. The third phase, between 1.99 and 1.95 Ga, is characterised by renewed high-grade metamorphism in the Bakhuis and Coeroeni belts along an anticlockwise cooling path, and ignimbritic volcanism and extensive and varied intrusive magmatism in the western half of the country. An alternative scenario is also discussed, implying an origin of the Coeroeni Gneiss Belt as an active continental margin, recording northwards subduction and finally collision between a magmatic arc in the south and an older northern continent. The Grenvillian collision between Laurentia and Amazonia around 1.2-1.0 Ga caused widespread mylonitisation and mica age resetting in the basement.
Geonomos *** IN POR, Vol. 19, 1, pp. 39-45. *** In Portuguese
South America, Brazil
Deposit - Coromandel area
Abstract: Hundreds of kimberlite intrusions and related rocks are known in the Coromandel region (MG), in the "Alto Paranaiba Diamondiferous Province", although the knowledge of these rocks is still scarce. Among these intrusions, it emphasizes the Grota do Cedro kimberlite, which outcrops in the drainage of same name at south of Coromandel (MG), hosted in micaschists of the Araxá Group (Neoproterozoic). The body has a roughly elliptical surface shape with 350 and 300 m axis; its chemical composition is similar to others of the province, and mineral chemistry of Cr-pyrope shows a strong concentration in the "G9" and "G5" fields. These chemical fields generally characterize diamond-poor or infertile intrusions.
REM: Revista Escola de Minas, Vol. 67, 2, pp. 159-165. *** in Portuguese
South America, Brazil
Alluvials, diamonds
Abstract: A representative set of diamonds from the Borrachudo River (114 stones) was described for the first time as to their physical characteristics, among them weight, morphology, dissolution figures, and abrasion. Most samples are low in weight (~51% less than 0.30 ct) and only ~3.5% are above 3.0 ct. However, considering the total weight, ~47% of it is concentrated in the range above 3 ct. On the other hand, there even occur diamonds with dozen of carats. The most common crystallographic forms are originated from the octahedral form by dissolution, although irregular shapes like chips and flats also occur. The diamonds show differences in form compared to their weight ranges; those up to 0.30 ct have various shapes; in the range of 0.31 to 1 ct the most common are octahedral forms and their descendants; and in stones larger than 1 ct chips and flats predominate. The general habitus of the crystals, identified by their final tetrahexahedroid shapes, chips and flats, corroborated by the dissolution figures, indicate that the diamond were submitted to strong dissolution in a magmatic environment. Residual hillocks and holes represent the final stage of dissolution. The study indicates that the abrasion by the fluvial transport was not expressive enough to cause mechanical wear, thus ca. 97% shows no sign of wear. This fact suggests a proximal source for most of these diamonds.
Volcanoes of the central Andes.Section of the book -large silicic systemsxeroxed. Not well in black and white as the photographs are in colour in thebook
Abstract: Kimberlites often entrain crustal felsic xenoliths, which show alteration and metamorphism as a result of interaction with the host kimberlite. We studied granite and gneiss xenoliths in the Renard 65 kimberlite pipe (Northern Québec, Canada). The study comprised a detailed petrographic examination of 45 thin sections, a scanning electron microscopy and an X-ray powder diffractometry of a sample sub-set. Two major units of the Renard 65 pipe (Unit A and Unit B/D) distinguished by abundance of crustal xenoliths along with the degree of their alteration, were investigated. Unit A is a volcaniclastic kimberlite with 40-90 % xenoliths, whereas Unit B/D is a hypabyssal kimberlite with textures transitional to pyroclastic, containing 15-40 % more intensely altered xenoliths. Both units carry xenoliths of coarse-grained leucogranite (K-feldspar, plagioclase, quartz, biotite with accessory garnet, apatite, and zircon) and medium-grained gneiss (plagioclase, quartz, biotite, orthopyroxene with accessory garnet, apatite and zircon). The Unit A xenoliths are partially replaced by chlorite, sericite, epidote, serpentine, richterite, actinolite and clinochlore vermiculite. In Unit B/D four distinct metamorphic and metasomatic mineral assemblages almost completely replace xenoliths. The assemblages include aegirine, pectolite, garnet, wollastonite, xonotlite, prehnite, calcite, K-feldspar and richterite in various proportions. Secondary K-feldspar and calcite may indicate the granite protolith, whereas wollastonite may be the signature of the gneiss protolith. The presence of secondary garnet and wollastonite, the hallmark skarn minerals, suggests the analogy between the classical skarn geological processes at the contact between felsic rocks and the host hot carbonate-rich melts. The observed mineralogy of the Renard 65 felsic xenoliths will be compared with the theoretically predicted mineralogy modelled using Theriak-Domino or Perplex software for the known bulk hybrid kimberlite compositions. The comparison will enable constraints on temperatures, volatile contents and thermal history of the kimberlite melt during emplacement.
Abstract: Assimilation of country rock xenoliths by the host kimberlite can result in the development of concentric reaction zones within the xenoliths and a reaction halo in the surrounding contaminated kimberlite. Petrographic and geochemical changes across the reaction zones in the xenoliths and the host kimberlite were studied using samples with different kimberlite textures and contrasting xenolith abundances from the Renard 65 kimberlite pipe. The pipe, infilled by Kimberley-type pyroclastic (KPK) and hypabyssal kimberlite (HK) and kimberlite with transitional textures, was emplaced into granitoid and gneisses of the Superior Craton. Using samples of zoned, medium-sized xenoliths of both types, mineralogical and geochemical data were collected across xenolith-to-kimberlite profiles and contrasted with those of fresh unreacted country rock and hypabyssal kimberlite. The original mineralogy of the unreacted xenoliths (potassium feldspar-plagioclase-quartz-biotite in granitoid and plagioclase-quartz-biotite-orthopyroxene in gneiss) is replaced by prehnite, pectolite, and diopside. In the kimberlite halo, olivine is completely serpentinized and diopside and late phlogopite crystallized in the groundmass. The xenoliths show the progressive degrees of reaction, textural modification, and mineral replacement in the sequence of kimberlite units KPK — transitional KPK — transitional HK. The higher degree of reaction observed in the HK-hosted xenoliths as compared to the KPK-hosted xenoliths in this study and elsewhere may partly relate to higher temperatures in xenoliths included in an HK melt. The correlation between the degree of reaction and the kimberlite textures suggests that the reactions are specific to and occur within each emplaced batch of magma and cannot result from external post-emplacement processes that should obliterate the textural differences across the kimberlite units. Xenolith assimilation may have started in the melt, as suggested by the textures in the xenoliths and the surrounding halos and proceeded in the subsolidus. Elevated CaO at the kimberlite-xenolith contact appears to be an important factor in producing the concentric mineralogical zoning in assimilated xenoliths.
Abstract: The sustainability of a mineral resource depends, among other aspects, on what the mineral in question will be used for, price fluctuations, future resource requirements, and downstream manufacturing. A balance must be struck between the long-term commitment of developing a mineral deposit against the short-term threats of a changing commercial and social environment. Long-term resource sustainability is dependent both on increased efficiency, which improves profitability, and on revitalizing marginal mines. This is illustrated through breakthroughs in the processing of low-grade copper and refractory gold ores, as well as nickel laterite ores. Retreatment of mine wastes and tailings can also increase the sustainability of mining activity. Ongoing research and development is also helping to sustain mineral resource exploitation.
Abstract: In the present work, we report the chemical composition of representative emerald crystals from some of the most important worldwide deposits. Major and trace elements were investigated using Electron Microprobe Analysis (EMPA) and Secondary Ion Mass Spectrometry (SIMS) techniques. Binary, ternary and spider diagrams along with statistical analysis, i.e., Principal Component Analysis (PCA), were used to discriminate each deposit with high reliability. PCA of SiO2, Al2O3, V, Sc, B, Li content identified distinct groups. The use of binary and ternary diagrams contributed to discriminate among emerald crystals from various deposits, which are included in the same clusters of the PCA analysis. In addition, the geochemical features of each group were linked to the geological environment and genetic processes which leaded to emerald formation. In particular, the emeralds related to granitic-pegmatitic intrusions (Type-1) or those occurring in environments controlled by tectonic events (Type-2) were distinguished using the concentrations of major and trace elements. The results of this study can contribute to improve the existing genetic models and classification schemes as well as to identify useful geochemical fingerprints for provenance purposes.-
Progress in Earth and Planetary Science, Vol. 3, 7p.
Mantle
Melting
Abstract: Because of the high energies involved, giant impacts that occur during planetary accretion cause large degrees of melting. The depth of melting in the target body after each collision determines the pressure and temperature conditions of metal-silicate equilibration and thus geochemical fractionation that results from core-mantle differentiation. The accretional collisions involved in forming the terrestrial planets of the inner Solar System have been calculated by previous studies using N-body accretion simulations. Here we use the output from such simulations to determine the volumes of melt produced and thus the pressure and temperature conditions of metal-silicate equilibration, after each impact, as Earth-like planets accrete. For these calculations a parameterised melting model is used that takes impact velocity, impact angle and the respective masses of the impacting bodies into account. The evolution of metal-silicate equilibration pressures (as defined by evolving magma ocean depths) during Earth’s accretion depends strongly on the lifetime of impact-generated magma oceans compared to the time interval between large impacts. In addition, such results depend on starting parameters in the N-body simulations, such as the number and initial mass of embryos. Thus, there is the potential for combining the results, such as those presented here, with multistage core formation models to better constrain the accretional history of the Earth.
Abstract: The metallurgical recovery processes in diamond mining may, under certain circumstances, cause an under-recovery of large diamonds. In order to predict high quantiles or tail probabilities we use a Bayesian approach to fit a truncated Generalized Pareto Type distribution to the tail of the data consisting of the weights of individual diamonds. Based on the estimated tail probability, the expected number of diamonds larger than a specified weight can be estimated. The difference between the expected and observed frequencies of diamond weights above an upper threshold provides an estimate of the number of diamonds lost during the recovery process.
Paleomagnetism and geochronology of mafic dykes in south Siberia, Russia: the first precisely dated Permian paleomagnetic pole from the Siberian Craton.
Geophysical Journal International, Vol. 167, 2, pp. 649-658.
Ratre, K., De Waele, B., Kumar, Biswal, T., Sinha, S.
Shrimp geochronology for the 1450 Ma Lakhna dyke swarm: its implication for the presence of Eoarchean crust in the Bastar Craton and the 1450-517 Ma depositional age
Journal of Asian Earth Sciences, Vol. 39, 6, pp. 565-577.
Journal of Asian Earth Sciences, in press available, 47p.
India, Africa, Madagascar
tectonis
Abstract: It has long been recognised that Madagascar was contiguous with India until the Late Cretaceous. However, the timing and nature of the amalgamation of these two regions remain highly contentious as is the location of Madagascar against India in Gondwana. Here we address these issues with new U-Pb and Lu-Hf zircon data from five metasedimentary samples from the Karwar Block of India and new Lu-Hf data from eight previously dated igneous rocks from central Madagascar and the Antongil-Masora domains of eastern Madagascar. New U-Pb data from Karwar-region detrital zircon grains yield two dominant age peaks at c. 3100 Ma and c. 2500 Ma. The c. 3100 Ma population has relatively juvenile ?Hf(t) values that trend toward an evolved signature at c. 2500 Ma. The c. 2500 Ma population shows a wide range of ?Hf(t) values reflecting mixing of an evolved source with a juvenile source at that time. These data, and the new Lu-Hf data from Madagascar, are compared with our new compilation of over 7000 U-Pb and 1000 Lu-Hf analyses from Madagascar and India. We have used multidimensional scaling to assess similarities in these data in a statistically robust way. We propose that the Karwar Block of western peninsular India is an extension of the western Dharwar Craton and not part of the Antananarivo Domain of Madagascar as has been suggested in some models. Based on ?Hf(t) signatures we also suggest that India (and the Antongil-Masora domains of Madagascar) were palaeogeographically isolated from central Madagascar (the Antananarivo Domain) during the Palaeoproterozoic. This supports a model where central Madagascar and India amalgamated during the Neoproterozoic along the Betsimisaraka Suture.
Abstract: Madagascar hosts several Paleoproterozoic sedimentary sequences that are key to unravelling the geodynamic evolution of past supercontinents on Earth. New detrital zircon U-Pb and Hf data, and a substantial new database of ?15,000 analyses are used here to compare and contrast sedimentary sequences in Madagascar, Africa and India. The Itremo Group in central Madagascar, the Sahantaha Group in northern Madagascar, the Maha Group in eastern Madagascar, and the Ambatolampy Group in central Madagascar have indistinguishable age and isotopic characteristics. These samples have maximum depositional ages > 1700 Ma, with major zircon age peaks at c. 2500 Ma, c. 2000 Ma and c. 1850 Ma. We name this the Greater Itremo Basin, which covered a vast area of Madagascar in the late Paleoproterozoic. These samples are also compared with those from the Tanzania and the Congo cratons of Africa, and the Dharwar Craton and Southern Granulite Terrane of India. We show that the Greater Itremo Basin and sedimentary sequences in the Tanzania Craton of Africa are correlatives. These also tentatively correlate with sedimentary protoliths in the Southern Granulite Terrane of India, which together formed a major intra?Nuna/Columbia sedimentary basin that we name the Itremo?Muva?Pandyan Basin. A new Paleoproterozoic plate tectonic configuration is proposed where central Madagascar is contiguous with the Tanzania Craton to the west and the Southern Granulite Terrane to the east. This model strongly supports an ancient Proterozoic origin for central Madagascar and a position adjacent to the Tanzania Craton of East Africa.
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)
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 24p.
Global
Database, ages
Abstract: Of the approximately 6,500 kimberlites known to date, less than 3% are diamondiferous and less than half of those are economically viable. Admittedly, this is a global figure and varies from area to area much depending on geological, logistical, and political circumstances. The number of diarnond~bearing kimberlites is an important geobarometer, and an age-frequency profile has been compiled in order to identify diamond metallogenic epochs and provinces. Four primary diamond clusters have been recognised: Siberia (332-370Ma), Kaapvaal South(114-144Ma), Kaapvaal Southwest(84-95Ma), and Slave (52-55Ma). In addition, three secondary clusters are the Kaapvaal Central (S06-S38Ma), the Man Craton (139-lS3Ma) and the Kasai Craton (120- 130Ma). The Kaapvaal is the only area where kimberlites with diamonds were discovered between 1870 and 1925, except for the Prairie Creek discovery in North America in 1906 and Colossus in Zimbabwe in 1907. Between 1906 and 1940 no major diamondiferous kimbcrlites were discovered, but many of Africa's alluvial deposits were found and exploited in that period. Advances in scientific prospecting in the 1940's rapidly accelerated the discovery rate that peaked in the 1990's; 39 diamondiferous occurrences were found in the 1980's, of which 8 became mines and 5 are in feasibility, and nearly 450ha of kimberlite was added to the global resource base. Despite the accelerated exploration expenditure, the last decade (e.g. 2000-2009) has been the leanest since the 1940's. The most important reason for this decline in exploration success is that any undiscovered deposits are largely buried by younger cover. Understanding complex and geophysical noisy basement geology, as well as decomposition of path-finder minerals are major challenges to further exploration success. Major investments in basic geological expertise and field research will have to be made in order to secure future diamond resources. Integrated geophysical studies, geochemistry diamond forensics, and improved imaging of Earth's upper mantle are perhaps foremost in this requirement, but the costs of these activities are such that government support will be required if countries are to sustain their diamond mining industry.
De Wit, M., Bhebhe, Z., Davidson, J., Haggerty, S.E., Hundt, P., Jacob, J., Lynn, M., Marshall, T.R., Skinner, C., Smithson, K., Stiefenhofer, J., Robert, M., Revitt, A., Spaggiari, R., Ward, J.
Abstract: From the discovery of diamonds in South Africa in 1866 until the end of 2013, Africa is estimated to have produced almost 3.2 Bct out of a total global production of 5.03 Bct, or 63.6% of all diamonds that have ever been mined. In 2013 African countries ranked 2nd (Botswana), 3rd (DRC), 6th (Zimbabwe), 7th (Angola), 8th (South Africa), and 9th (Namibia), in terms of carat production and 1st (Botswana), 4th (Namibia), 5th (Angola), 6th (South Africa), 7th (Zimbabwe), and 9th (DRC), in terms of value of the diamonds produced. In 2013 Africa produced 70.6 Mct out of a global total of 130.5 Mct or 54.1%, which was valued at US$ 8.7 billion representing 61.5% of the global value of US$ 14.1 billion.
Earth and Planetary Science Letters, Vol. 484, pp. 1-14.
Mantle
kimberlite, origin, magmatism
Abstract: Kimberlite magmatism has occurred in cratonic regions on every continent. The global age distribution suggests that this form of mantle melting has been more prominent after 1.2 Ga, and notably between 250-50 Ma, than during early Earth history before 2 Ga (i.e., the Paleoproterozoic and Archean). Although preservation bias has been discussed as a possible reason for the skewed kimberlite age distribution, new treatment of an updated global database suggests that the apparent secular evolution of kimberlite and related CO2-rich ultramafic magmatism is genuine and probably coupled to lowering temperatures of Earth's upper mantle through time. Incipient melting near the CO2- and H2O-bearing peridotite solidus at >200 km depth (1100-1400?°C) is the petrologically most feasible process that can produce high-MgO carbonated silicate melts with enriched trace element concentrations akin to kimberlites. These conditions occur within the convecting asthenospheric mantle directly beneath thick continental lithosphere. In this transient upper mantle source region, variable CHO volatile mixtures control melting of peridotite in the absence of heat anomalies so that low-degree carbonated silicate melts may be permanently present at ambient mantle temperatures below 1400?°C. However, extraction of low-volume melts to Earth's surface requires tectonic triggers. Abrupt changes in the speed and direction of plate motions, such as typified by the dynamics of supercontinent cycles, can be effective in the creation of lithospheric pathways aiding kimberlite magma ascent. Provided that CO2- and H2O-fluxed deep cratonic keels, which formed parts of larger drifting tectonic plates, existed by 3 Ga or even before, kimberlite volcanism could have been frequent during the Archean. However, we argue that frequent kimberlite magmatism had to await establishment of an incipient melting regime beneath the maturing continents, which only became significant after secular mantle cooling to below 1400?°C during post-Archean times, probably sometime shortly after 2 Ga. At around this time kimberlites replace komatiites as the hallmark mantle-derived magmatic feature of continental shields worldwide. The remarkable Mesozoic-Cenozoic ‘kimberlite bloom’ between 250-50 Ma may represent the ideal circumstance under which the relatively cool and volatile-fluxed cratonic roots of the Pangea supercontinent underwent significant tectonic disturbance. This created more than 60% of world's known kimberlites in a combination of redox- and decompression-related low-degree partial melting. Less than 2% of world's known kimberlites formed after 50 Ma, and the tectonic settings of rare ‘young’ kimberlites from eastern Africa and western North America demonstrate that far-field stresses on cratonic lithosphere enforced by either continental rifting or cold subduction play a crucial role in enabling kimberlite magma transfer to Earth's surface.
Mineralogy and Petrology, June 14, DOI:10.1007/ s00710-018- 0593-8, 14p.
Africa, Botswana
deposit - Nxau
Abstract: The Nxau Nxau kimberlites in northwest Botswana belong to the Xaudum kimberlite province that also includes the Sikereti, Kaudom and Gura kimberlite clusters in north-east Namibia. The Nxau Nxau kimberlites lie on the southernmost extension of the Congo Craton, which incorporates part of the Damara Orogenic Belt on its margin. The Xaudum kimberlite province is geographically isolated from other known clusters but occurs within the limits of the NW-SE oriented, Karoo-aged Okavango Dyke Swarm and near NE-SW faults interpreted as the early stages of the East African Rift System. Petrographic, geochronological and isotopic studies were undertaken to characterise the nature of these kimberlites and the timing of their emplacement. The Nxau Nxau kimberlites exhibit groundmass textures, mineral phases and Sr-isotope compositions (87Sr/86Sri of 0.7036?±?0.0002; 2?) that are characteristic of archetypal (Group I) kimberlites. U-Pb perovskite, 40Ar/39Ar phlogopite and Rb-Sr phlogopite ages indicate that the kimberlites were emplaced in the Cretaceous, with perovskite from four samples yielding a preferred weighted average U-Pb age of 84?±?4 Ma (2?). This age is typical of many kimberlites in southern Africa, indicating that the Xaudum occurrences form part of this widespread Late Cretaceous kimberlite magmatic province. This time marks a significant period of tectonic stress reorganisation that could have provided the trigger for kimberlite magmatism. In this regard, the Nxau Nxau kimberlites may form part of a NE-SW oriented trend such as the Lucapa corridor, with implications for further undiscovered kimberlites along this corridor.
Abstract: The structural depression that occupies the Okavango Basin in southern Africa comprises a depo?centre within the intracratonic Kalahari Basin where sediments of the Cenozoic Kalahari Group have accumulated. The Okavango Basin has been formed due to stretching and subsidence at an area of diffused deformation, southwestwards to the main East African Rift System (EARS). Sediments from two full Kalahari Group sequences, located on opposite sides of the Gumare Fault that forms a major fault within the Okavango Basin, were studied to determine their provenance and chronology. Terrestrial Cosmogenic Nuclide (TCN) 26Al/10Be burial dating was used to constrain a chronostratigraphical framework, and Pb, Sr, and Nd isotopic ratios combined with geochemical and sedimentological analyses were applied to track the source areas of the sediments.Results indicate the following sequence of basin filling: (a) Accumulation between ca. 4-3 Ma during which the currently downthrown (southern) block received a mixture of sediments mostly from the Choma?Kalomo, Ghanzi?Chobe, and Damara terranes, and possibly from the Lufilian Belt and/or Karoo basalts during earlier stages of deposition. Simultaneously, the upthrown (northern) block received sediments from more distant Archean sources in the Zimbabwe and/or Kasai cratons, (b) Hiatus in sedimentation occurred at both sites between ca. 3-2 Ma, (c) Sediments on both sides of the Gumare Fault share a similar source (Angolan Shield) with minor distinct contributions to the downthrown block from the Kasai Craton and local sources input to the upthrown block, and (d) Regional distribution of aeolian sand since at least 1 Ma. The change in source areas is attributed to rearrangements of the drainage systems that were probably linked to vertical crustal movements on the margins of the Okavango Basin. The tectonically induced morphodynamics controlled the landscape evolution of the endorheic basin where vast lakes, wetlands and salt pans have developed through time.
South African Journal of Geology, doi:10.25131/sajg.124.0028 10p. Pdf
Africa, South Africa
deposit - Salpeterkop
Abstract: The Salpeterkop volcano forms part of what has been referred to as the Upper Cretaceous Sutherland Suite of alkaline rocks, an igneous province composed of olivine melilitites, carbonatites, trachytes and ultramafic lamprophyres. Salpeterkop is a remnant of the summit tuff ring structure that surrounds a crater which is almost 1 km in diameter and is filled with epiclastic strata. Five pieces of silicified wood were collected from the crater filled sediments, sectioned and identified as a new species of Cupressinoxylon, C. widdringtonioides. This is the first example of the fossil genus in South Africa. Only one member of the Cupressaceae s.l. occurs in southern Africa today. From the wide and indistinct growth rings in the fossil wood it can be deduced that the local climate was warm and humid with little or no seasonality, in support of global records of a warm Late Cretaceous. The preservation of the crater further signifies the low level of erosion the region has experienced since its emplacement.
The geochemistry (platinum group elements (PGE)) in kimberlites and constraints of the nature platinum group elements (PGE) insubcratonic lithospheric mantle.
International Symposium Upper Mantle, Aug. 14-19, 1994, Extended abstracts pp. 73-75.
Mineralogy and Petrology, in press available, 16p.
Africa, Botswana
diamond exploration
Abstract: Bechuanaland/Botswana has a long and colourful history in exploration and mining. Here these activities are subdivided into three phases: pre-historic, historic and modern. Quarrying stone in Botswana was ongoing 500,000 years ago during the Early Stone Age (ESA). Actual mining of stones probably only started during the Middle Stone Age (MSA) i.e. post 250,000 BP, and the first prehistoric hard rock mining of specularite and limonite, likely started during the Late Stone Age (LSA) 20,000 to 2,000 BP. In east Botswana iron and copper were mined from AD 800 onwards; the mining of gold started in the thirteenth century. Historic mining started with the re-discovery of gold close to Francistown in 1865 and lasted until the 1950s. Rumours of diamonds in Bechuanaland had already surfaced in the 1880s, and it was Ngamiland, in the northwest, that was first explored systematically for diamonds and gold between 1896 and 1899. A joint initiative between Anglo American and De Beers started serious prospecting parts of eastern Bechuanaland between 1932 and 1938; and in 1938 the first diamond finds in Bechuanaland were reported. Modern mining and exploration started with the signing of an agreement in 1959, allowing Consolidated African Selection Trust Ltd. (CAST) into the Bamangwato Tribal Reserve. CAST found a few diamonds in the Motloutse River, but concluded that these were reworked and dropped the exploration rights. De Beers believed that these diamonds had come from west of the Motloutse headwaters, across the watershed in the Kalahari. This ultimately led to the discovery of the Orapa kimberlite field in 1967, a year after Botswana became independent. This discovery triggered a major exploration boom across Botswana adding important diamond-bearing kimberlites such as at Letlhakane (1968), Jwaneng (1973), Gope (1981) and Lerala (1991).
Geology and resource potential of the Congo Basin, Springer Regional Geology Reviews, Chapter 17, 9p.
Africa, Democratic Republic of Congo
Overview, history
Abstract: An overview is provided of the exploration history and geological setting of the kimberlites in the Democratic Republic of Congo (DRC). Exploration for diamonds, in what was then known as Congo Belge, started in 1900 and the first diamonds were found in 1903 in Shaba (now Katanga) Province, in 1907 in Kasai Occidental Province near Tshikapa Town and in 1918 in Kasai Oriental Province near Mbuji Mayi Town. While the Kundelungu kimberlites in Katanga Province were discovered in 1908, other kimberlite fields were discovered much later (Mbuji Mayi 1946; Tshibwe 1956; Bas-Congo 1974; Kasendou and Lukashi 2005), during exploration work by Forminière (Société Internationale Forestière et Minière du Congo-Tshikapa), MIBA (Societé Minière de Bakwanga-Mbuji Mayi), the De Beers Group and Bugeco S.A. Published age constraints on the kimberlites show Late Cretaceous ages for the Mbuji Mayi kimberlites (~70 Ma) and Eocene-Oligocene ages for the Kundelungu kimberlites (~32 Ma). Emplacement of the Late Cretaceous kimberlites (Mbuji Mayi, Tshibwe, Kasendou and Lukashi) was concomitant with the deposition of Cretaceous sedimentary sequences. The majority of the pipes show crater-facies preservation and some of the pipes are flared displaying so-called ‘champagne glass-shaped’ morphologies, suggesting emplacement into unconsolidated sediments overlying basement. The age of the Eocene-Oligocene Kundelungu kimberlites corresponds to lithospheric extension associated with the southward propagation of the East African Rift.
Geology and resource potential of the Congo Basin, Springer Regional Geology Reviews, Chapter 16, 20p.
Africa, Democratic Republic of Congo
Overview, history
Abstract: Since the mid-1950s the Kwango River has been a major target for alluvial diamonds which are and continue to be mined from its terraces, younger river flats and present-day river channel. The terraces have maximum ages of Early to Middle Pleistocene. Most of the diamonds have been recovered from large diamond placers in and along the Angolan section of this river—the Cuango River. Smaller deposits have been worked further downstream, where the Kwango River forms the international border between Angola and the Democratic Republic of Congo (DRC), also referred to as the ‘international’ Kwango. The prospecting history of this river goes back to 1906 but a systematic exploration program over the lower Kwango was only initiated in 2005. The application of geophysics to explore the terrace deposits and river flats has been very useful, but a programme of drilling and pitting was required to accurately define gravel and overburden thicknesses, and outline palaeo-channels. A diamond study used to glean information on diamond sizes and characteristics added significant value to the understanding of these alluvial deposits. For most of the international Kwango, terraces and river flats overly aeolian facies of Upper Kwango Group. Basement rocks, providing more favourable sites for diamond concentrations, are only exposed over a relatively short section, just upstream from Tembo. The basal part of the Cretaceous Kwango Group is locally composed of chemically mature gravels with diamonds in economic quantities only proximal to primary sources and no such settings were found in the project area. The size frequency of the diamonds from the international Kwango indicates that these form the distal head of the diamond trail that have been eroded out of the Cretaceous Kwango Group sediments and kimberlites in the Upper Cuango basin in Angola since the Pleistocene. The diamonds below the two major waterfalls along the international Kwango near Tembo, the Guiliame and Francois-Joseph Falls, show a high percentage of breakage and a decline in average diamond size from around 0.25 cts/stn above the falls to between 0.07 and 0.1 cts/stn in the Nzasi Muadi to Kitangu area between 20 km and 130 km below the Falls. However, local variations due to geomorphological influences affect diamond concentrations and sizes. A preliminary assessment of the terrace deposits suggests that these are uneconomic at present using modern mining methods. This is largely due to thick overburden (up to 12 m of sand) combined with thin and hence low-volume, medium-grade basal gravel, and the dominance of small diamonds of lower value.
South African Journal of Geology, Vol. 119, 4, pp. 585-606.
Africa, South Africa
deposit - Lichtenburg Ventersdorp
Abstract: Diamond-bearing gravels of the Lichtenburg-Ventersdorp area of the North West Province are associated with north-south orientated sinuous ‘runs’ that occur almost entirely on a flat erosional surface of the Malmani dolomites (Transvaal Supergroup) at some 1,500 m elevation. East to west, this dolomite plain measures 150 km, and north-south it is on average 40 km wide. This unconformity, which first developed before the Pretoria Group sedimentation over a period of at least 80 Myr, is marked by siliceous breccias (palaeo-karst infill) and conglomerates (reworked breccias). It was exhumed in pre-Karoo and post-Gondwana times. Glacial pavements and remnants of thin Lower Karoo sediments are also found on this polyphase surface. The gravels that make up these ‘runs’ and sinkholes directly or indirectly linked to these runs, are coarse-grained, very poorly-sorted, and are best described as diamictites. The ‘runs’ are narrow, elongated, generally positive ridges that meander across the dolomite surface and are up to 30 km long and between 80 to 300 m wide. They have always been regarded as post-Cretaceous drainage features linked to southward-flowing river systems. Diamonds were discovered in these ‘runs’ and they have produced some 12 million carats. However, no Cainozoic fossils or artefacts have ever been found in almost 90 years of mining. From new field evidence, geomorphological studies, age dating from inclusions in diamond and zircon and clay analyses, it is proposed that these coarse-grained runs represent proximal palaeoeskers of the last deglaciation of the Dwyka continental ice sheet, that are preserved on this ancient ‘palimpsest’ surface. The age of the deposit is constrained by two populations of agate within the diamictites that are linked to two separate volcanic units of the Pretoria Group. In addition, the youngest crustal zircon ages from the gravels are 1 Ba, but mantle zircons from Lichtenburg suggest that these have been derived from Cambrian age kimberlites. Analysis of inclusions in diamond support a Neoproterozoic to Cambrian source for the diamonds, so the absence of diamonds from Mesozoic kimberlites and Cainozoic fossils within the gravels support the conclusion that the runs are of Karoo age.
Mineralogy and Petrology, 10.1007/ s00710-018- 0568-9, 15p.
Africa, South Africa
geochronology
Abstract: The West Coast of Namaqualand in South Africa hosts extensive detrital diamond deposits, but considerable debate exists as to the provenance of these diamonds. Some researchers have suggested derivation of the diamonds from Cretaceous-Jurassic kimberlites (also termed Group I kimberlites) and orangeites (also termed Group II kimberlites) located on the Kaapvaal Craton. However, others favour erosion of diamonds from the ca.300 Ma Dwyka Group sediments, with older, pre-Karoo kimberlites being the original source(s). Previous work has demonstrated that 40Ar/39Ar analyses of clinopyroxene inclusions, extracted from diamonds, yield ages approaching the time(s) of source kimberlite emplacement, which can be used to constrain the provenance of placer diamond deposits. In the current study, 40Ar/39Ar analyses were conducted on clinopyroxene inclusions from two similar batches of Namaqualand detrital diamonds, yielding (maximum) ages ranging from 117.5?±?43.6 Ma to 3684?±?191 Ma (2?) and 120.6?±?15.4 Ma to 688.8?±?4.9 Ma (2?), respectively. The vast majority of inclusions (88%) produced ages younger than 500 Ma, indicating that most Namaqualand diamonds originated from Cretaceous-Jurassic kimberlites/orangeites, with few, if any, derived from the Dwyka tillites. The provenance of the Namaqualand diamonds from ca.115-200 Ma orangeites is consistent with Late Cretaceous paleo-drainage reconstructions, as these localities could have been sampled by the ‘paleo-Karoo’ River and transported to the West Coast via an outlet close to the current Olifants River mouth. At ca.90 Ma, this drainage system appears to have been captured by the ‘paleo-Kalahari’ River, a precursor to the modern Orange River system. This latter drainage is considered to have transported diamonds eroded from both ca.80-90 Ma kimberlites and ca.115-200 Ma orangeites to the West Coast, which were subsequently reworked along the Namibian coast, forming additional placer deposits.
Mineralogy and Petrology, doi:10.1007/ s00710-018-0568-9 15p.
Africa, South Africa
deposit - Group I, orangeites Group II
Abstract: The West Coast of Namaqualand in South Africa hosts extensive detrital diamond deposits, but considerable debate exists as to the provenance of these diamonds. Some researchers have suggested derivation of the diamonds from Cretaceous-Jurassic kimberlites (also termed Group I kimberlites) and orangeites (also termed Group II kimberlites) located on the Kaapvaal Craton. However, others favour erosion of diamonds from the ca.300 Ma Dwyka Group sediments, with older, pre-Karoo kimberlites being the original source(s). Previous work has demonstrated that 40Ar/39Ar analyses of clinopyroxene inclusions, extracted from diamonds, yield ages approaching the time(s) of source kimberlite emplacement, which can be used to constrain the provenance of placer diamond deposits. In the current study, 40Ar/39Ar analyses were conducted on clinopyroxene inclusions from two similar batches of Namaqualand detrital diamonds, yielding (maximum) ages ranging from 117.5?±?43.6 Ma to 3684?±?191 Ma (2s) and 120.6?±?15.4 Ma to 688.8?±?4.9 Ma (2s), respectively. The vast majority of inclusions (88%) produced ages younger than 500 Ma, indicating that most Namaqualand diamonds originated from Cretaceous-Jurassic kimberlites/orangeites, with few, if any, derived from the Dwyka tillites. The provenance of the Namaqualand diamonds from ca.115-200 Ma orangeites is consistent with Late Cretaceous paleo-drainage reconstructions, as these localities could have been sampled by the ‘paleo-Karoo’ River and transported to the West Coast via an outlet close to the current Olifants River mouth. At ca.90 Ma, this drainage system appears to have been captured by the ‘paleo-Kalahari’ River, a precursor to the modern Orange River system. This latter drainage is considered to have transported diamonds eroded from both ca.80-90 Ma kimberlites and ca.115-200 Ma orangeites to the West Coast, which were subsequently reworked along the Namibian coast, forming additional placer deposits.
Journal of African Earth Sciences, Vol. 185, 104414, 19p. Pdf
Africa, South Africa
carbonatite
Abstract: The Salpeterkop volcano is spatially part of the Sutherland Suite of alkaline rocks in the Northern Cape. It is one of the best preserved volcanoes in South Africa with part of the tuff ring still intact, remnants of the ejecta mantle outside the crater still preserved, and is host to epiclastic rocks, including ash and lapillistone and water lain sediments, inside the crater. New dates from apatites and phlogopites from Salpeterkop suggest an age close to 70 Ma. This Upper Cretaceous age is supported by the silicified wood found within the epiclastic sediments. Its relationship to the alkaline rocks in this cluster is not entirely clear but field evidence suggests that initial olivine melilitites and ultramafic bodies were followed by the main eruption that produced Salpeterkop. Clear igneous components in the associated breccias and pyroclastics are rare but indicate that this volcano was linked to an alkaline (trachytic) intrusion driven by phreatic magmatism. This is further highlighted by the presence of (nepheline?) syenite xenoliths in some carbonatite breccias and dykes that are part of the later carbonatites with its associated hydrothermal alteration products. Although the carbonatites are largely late-stage, there is evidence of earlier carbonatite activity from a precursor carbonatite dyke that has off-set an olivine melilitite dyke. Relatively unaltered pyroclastics, associated with two vents, northeast and northwest of the crater respectively, represent the final phase of this volcanic centre. Finally, the preservation of the Salpeterkop crater and its associated volcaniclastics highlights the change of intense landscape denudation, that ensued from Gondwana break-up to the end of the Cretaceous, to a period of drastically reduced erosion rates during the Cenozoic Era.
Paleomagnetism of the NeoArchean Pongola and Ventersdorp Supergroups and an appriasal of the 3.0 - 1.9 Ga apparent polar wander path of Kaapvaal Craton
Science Advance AEON and Earth Stewardship Science Research Institute, Nelson Mandela Metropolitan Univerisity, 13p.
Technology
Glacial remnants, exosphere, silica pipes
Abstract: Estimates of ocean temperatures on Earth 3.5 billion years ago (Ga) range between 26° and 85°C. We present new data from 3.47- to 3.43-Ga volcanic rocks and cherts in South Africa suggesting that these temperatures reflect mixing of hot hydrothermal fluids with cold marine and terrestrial waters. We describe fossil hydrothermal pipes that formed at ~200°C on the sea floor >2 km below sea level. This ocean floor was uplifted tectonically to sea level where a subaerial hydrothermal system was active at 30° to 270°C. We also describe shallow-water glacial diamictites and diagenetic sulfate mineral growth in abyssal muds. These new observations reveal that both hydrothermal systems operated in relatively cold environments and that Earth’s surface temperatures in the early Archean were similar to those in more recent times.
Abstract: Apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated (>1000?m above sea level) but low-relief Tanzanian Craton. Coeval episodes of sedimentation documented within adjacent Paleozoic to Mesozoic basins of southern Tanzania and the Congo basin of the Democratic Republic of Congo indicate that most of the cooling in the basement rocks in Tanzania was linked to erosion. Basement samples were from an exploration borehole located within the craton and up to 2200?m below surface. Surface samples were also analyzed. AFT dates range between 317?±?33?Ma and 188?±?44?Ma. Alpha (Ft)-corrected AHe dates are between 433?±?24?Ma and 154?±?20?Ma. Modeling of the data reveals two important periods of cooling within the craton: one during the Carboniferous-Triassic (340-220?Ma) and a later, less well constrained episode, during the late Cretaceous. The later exhumation is well detected proximal to the East African Rift (70?Ma). Thermal histories combined with the estimated geothermal gradient of 9°C/km constrained by the AFT and AHe data from the craton and a mean surface temperature of 20°C indicate removal of up to 9?±?2?km of overburden since the end of Paleozoic. The correlation of erosion of the craton and sedimentation and subsidence within the Congo basin in the Paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far-field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes.
South African Journal of Geology, Vol. 120, 3, pp. 371-384.
Africa, Zimbabwe, South Africa
diamond morphology
Abstract: The morphological, chemical impurities and carbon isotope properties of diamonds may reveal subtle details of their mantle source and growth characteristics, supporting efforts towards identifying their original place of harvesting. Here we investigate the mantle carbon and nitrogen sources and growth patterns from selected diamonds mined from four kimberlites: macro-sized diamonds from River Ranch kimberlite in Zimbabwe and the Swartruggens and Klipspringer kimberlitic deposits from South Africa, and micro-sized diamonds from the Klipspringer and Premier kimberlite intrusions in South Africa. Type IaAB diamonds are found in all the samples; Type IaB diamonds only occur in samples from the Swartruggens, River Ranch and Premier kimberlites. A single Type II diamond (nitrogen below the detection limit) was also observed in the River Ranch and Premier kimberlites. Both the micro- and macro-sized diamonds from Klipspringer have similar nitrogen contents. Based on the % B-defect, the diamonds from Klipspringer are grouped into low- and high-nitrogen aggregates (i.e. % of B-defect <40% and >56%, respectively) that likely represent two different diamond forming episodes. Time averaged mantle storage temperatures for Type IaAB diamonds are calculated to have been: 1060°C for Swartruggens; 1190°C for River Ranch; 1100°C (low aggregated); and 1170°C (highly aggregated) for Klipspringer, and 1210°C for Premier diamonds. The CL-images of the River Ranch, Klipspringer and Premier diamonds reveal multi-oscillatory growth zones. The carbon isotopic analyses on the diamonds reveal an average ?13CVPDB value of: -4.5‰ for Swartruggens; -4.7‰ for River Ranch; -4.5‰ for Klipspringer; and -3‰ for Premier. With the exception of the diamond from Premier, the average ?13C value of the diamonds are similar to the average ?13C value of the mantle (-5‰), which is similar to the occurrence of diamonds in the other kimberlites. The internal carbon isotopic variation of individual diamonds from Swartruggens, Klipspringer and Premier are less than 4‰, which is similar to the variability of most other diamond occurrences reported from elsewhere in the world. Up to 6.7‰ internal carbon isotopic variation was observed in a single diamond from River Ranch. The internal carbon isotopic studies of the diamonds reveal that the primary carbon in the Swartruggens and Klipspringer was derived from an oxidation of CH4-bearing fluid, whereas in the River Ranch the primary carbon was derived from the reduction of carbonate-or CO2-bearing fluids. The Swartruggens diamonds also reveal a secondary carbon sourced from a reduction of CO2- or carbonate-rich fluid or melt. Diamonds from Klipspringer exhibit a cyclic change in ?13C values that reflects fluctuation in a complex mantle perturbation system or periodic change in fugacity of the mantle. Based on this study, we conclude that, in principle, a selected range of diamond signatures might be used to fingerprint their origins; especially when linked to their other physical properties such as a low temperature magnetic signature.
Earth and Planetary Science Letters, Vol. 429, pp. 101-109.
Mantle
Seismic -anisotropy
Abstract: The seismic structure of Earth’s inner core is highly complex, displaying strong anisotropy and further regional variations. However, few seismic waves are sensitive to the inner core and fundamental questions regarding the origin of the observed seismic features remain unanswered. Thus, newtechniques to observe different types of inner corewaves are imperative to improve data coverage. Here, we detail our method for detecting exotic inner core phases such as PKJKP and PKIIKP, using inner core compressional waves as proof of concept.We use phase weighted stacking on long period data from a global distribution of stations, and employ several synthetic methods, including normal mode summation and SPECFEM, to identify and confirm the inner core phases. We present evidence for two observations of exotic inner core compressional waves, and apply the technique to a previously detected inner core shear wave. A possible new inner core shear wave remains unconfirmed. Additionally, we show how our method is important for rejecting potential observations, and distinguishing between waves with similar traveltime and slowness. The method is most successful for detecting exotic inner core compressional waves, and will provide a new approach for studying the compressional wave structures in the upper inner core.
Abstract: Enrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitized phonolite breccia, focusing on small satellite occurrences 1-2 km from the Songwe Hill carbonatite, Malawi. Within the breccia groundmass, a HREE-bearing mineral assemblage comprises xenotime, zircon, anatase/rutile and minor huttonite/thorite, as well as fluorite and apatite. A genetic link between HREE mineralization and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitization. We propose that the HREE are retained in hydrothermal fluids which are residually derived from a carbonatite after precipitation of LREE minerals. Brecciation provides a focusing conduit for such fluids, enabling HREE transport and xenotime precipitation in the fenite. Continued fluid-rock interaction leads to dissolution of HREE-bearing minerals and further precipitation of xenotime and huttonite/thorite. At a maximum Y content of 3100 µg g?1, HREE concentrations in the presented example are not sufficient to constitute ore, but the similar composition and texture of these rocks to other cases of carbonatite-related HREE enrichment suggests that all form via a common mechanism linked to fenitization. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focusing conduit for fenitizing fluids, reducing fluid - country-rock interaction. Enrichment of HREE and Th in fenite breccia serves as an indicator of fluid expulsion from a carbonatite, and may indicate the presence of LREE mineralization within the source carbonatite body at depth.
Abstract: The rare earth elements (REE) are critical metals that have been the subject of considerable recent research. In the published literature, REE deposits are typically divided into classes, which commonly include ‘alkaline igneous rocks’ and ‘carbonatites’ [1]. However, our recent work, carried out as part of the EURARE and HiTech AlkCarb projects, suggests that many deposits of the REE and other critical metals may be formed where late-stage carbonatites and associated fluids interact with alkaline igneous rocks. A key question is whether these carbonatites are formed by liquid immiscibility from the host alkaline magmas, or whether they are introduced from other sources. A classic example of a mineral deposit formed in this way is at Ivigtut in Greenland, where late-stage F and CO2 rich fluids interacted with alkali granitic melts to form a cryolite (Na3AlF6) deposit, with associated metasomatism and REE mobilisation. Isotopic evidence indicates that these late-stage fluids may have been carbonatite-derived [2]. Our more recent work indicates that REE enrichment in many alkaline igneous complexes may be generated by a similar mechanism. In the alkaline igneous province of NW Scotland, late-stage metasomatism by CO2-rich fluids has generated metasomatised veins with TREO up to 2 wt% [3]. Similar features are observed in the Ditra? Alkaline Igneous complex in Romania, where REE mineralisation is represented by monazite- and carbonate-rich veins cutting syenitic host rocks [4]; and at the Kizilcaören REE deposit in Turkey. This talk will provide an overview of the formation of REE mineralisation in this type of magmatic-hydrothermal system and consider future research questions.
Abstract: Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization. Although many attempts have been made in the literature to categorize and name fenites, it is recommended that the IUGS metamorphic nomenclature be used to describe predominant mineralogy and textures. Complexing anions greatly enhance the solubility of REE and Nb in these fenitizing fluids, mobilizing them into the surrounding country rock, and precipitating REE- and Nb-enriched micro-mineral assemblages. As such, fenites have significant potential to be used as an exploration tool to find mineralized intrusions in a similar way alteration patterns are used in other ore systems, such as porphyry copper deposits. Strong trends have been identified between the presence of more complex veining textures, mineralogy and brecciation in fenites with intermediate stage Nb-enriched and later stage REE-enriched magmas. However, compiling this evidence has also highlighted large gaps in the literature relating to fenitization. These need to be addressed before fenite can be used as a comprehensive and effective exploration tool.
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: Many of the world's largest known REE deposits are associated with post-collisional alkaline-carbonatite magmatic complexes (e.g., the Minanning-Dechang belt, China). These systems are potassic to ultrapotassic in composition and contain LREE-dominated mineralisation associated with F and Ba-rich carbonatite breccias, carbonatite dykes and carbo-hydrothermal veins. They are typically emplaced through major shear zones during a period of 'relaxation' that postdates continental collision by up to 75 Ma. The subduction of sediment during continental collision is potentially a key control on the 'fertility' of the mantle source, and understanding the role of sediment is a crucial step towards better exploration models. However, the identification of sediment source components to alkaline systems has not been straightforward because their petrological complexity precludes traditional methods such as trace-element ratios and major-element modelling of crystal fractionation. We use a global database of Sr, Nd and Hf isotope compositions for alkaline and carbonatite systems, alongside geodynamic reconstructions to identify favourable source components for mineralisation and to provide direct information about the origin of the metals of interest. Subduction of shale and carbonate sequences is likely to introduce REE + HFSE and potentially mineralising ligands (F-, CO3 2-) into the mantle source for post-collisional alkaline systems; clastic sediments are poorer in these vital components. This research provides a framework through which the mineral exploration industry can identify tectonic environments that are predisposed to form REE mineralisation, providing regional-scale (100-1000 km) guidance especially for systems hidden beneath sedimentary cover.
Open pit mining at Argyle diamond mine, western Australia
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) , Annual Meeting Abstracts LESS than approximately 10, Vol. 86, No. 968, March ABSTRACT p. 75.
Abstract: Thermal history models, that have been used to understand the geological history of Earth, are now being coupled to climate models to map conditions that allow planets to maintain surface water over geologic time - a criteria considered crucial for life. However, the lack of intrinsic uncertainty assessment has blurred guidelines for how thermal history models can be used toward this end. A model, as a representation of something real, is not expected to be complete. Unmodeled effects are assumed to be small enough that the model maintains utility for the issue(s) it was designed to address. The degree to which this holds depends on how unmodeled factors affect the certainty of model predictions. We quantify this intrinsic uncertainty for several parameterized thermal history models (a widely used subclass of planetary models). Single perturbation analysis is used to determine the reactance time of different models. This provides a metric for how long it takes low amplitude, unmodeled effects to decay or grow. Reactance time is shown to scale inversely with the strength of the dominant feedback (negative or positive) within a model. A perturbed physics analysis is then used to determine uncertainty shadows for model outputs. This provides probability distributions for model predictions and tests the structural stability of a model. That is, do model predictions remain qualitatively similar, and within assumed model limits, in the face of intrinsic uncertainty. Once intrinsic uncertainty is accounted for, model outputs/predictions and comparisons to observational data should be treated in a probabilistic way.
Abstract: The Magnet Cove Alkaline Intrusive Complex contains several silica-undersaturated igneous rock types (e.g. nepheline syenite, ijolite, carbonatite) that form a concentric ring map pattern approximately 4.6 square miles in area. These rings, which are likely the result of several nearly contemporaneous magma injection events during the mid Cretaceous, become increasingly silica-undersaturated from rim to core, and have been previously mapped as separate geologic units. The outer ring contains nepheline syenite, the intermediate ring contains both garnet ijolite and garnet biotite ijolite, and the core contains carbonatite. Though the detailed modal mineralogy differs somewhat between the silicate (i.e. syenite and ijolite) rock types, they all have in common the presence of mica group minerals. The purpose of this study is to examine and characterize the diversity of mica group minerals found in the silica-undersaturated rocks of Magnet Cove. Syenite and ijolite rock samples were collected from several locations within the complex, and thin sections were prepared for petrographic and electron microscope analysis using facilities and equipment at the UALR Rock Preparation Laboratory. Overall mineralogy from these samples indicates the presence of potassium feldspar, plagioclase feldspar, several feldspathoid minerals (nepheline, sodalite, altered leucite), amphiboles, pyroxenes (primarily aegerine and aegerine-augite), black Ti-bearing garnets (melanite, schorlomite), and various opaque minerals (e.g. magnetite, pyrite). Previously, micas in these rocks have been labeled simply as “biotite”. However, the ranges of color (yellowish-brown to bluish-green), crystal size (millimeter to several centimeters in diameter), and crystal habit (clusters of euhedral grains) in hand sample and variable pleochroism, ranging interference colors, reaction coronas, and zoning in thin section indicate a more interesting and complex chemical history.
Journal of Earth System Science, Vol. 127, 6, pp. 76- doi:10.1007/s12040-018-0988-2
India
cratons
Abstract: The northern part of the Nellore-Khammam schist belt and the Karimnagar granulite belt, which are juxtaposed at high angle to each other have unique U-Pb zircon age records suggesting distinctive tectonothermal histories. Plate accretion and rifting in the eastern part of the Dharwar craton and between the Dharwar and Bastar craton indicate multiple and complex events from 2600 to 500 Ma. The Khammam schist belt, the Dharwar and the Bastar craton were joined together by the end of the Archaean. The Khammam schist belt had experienced additional tectonic events at ?1900 and ?1600 Ma. The Dharwar and Bastar cratons separated during development of the Pranhita-Godavari (P-G) valley basin at ?1600 Ma, potentially linked to the breakup of the Columbia supercontinent and were reassembled during the Mesoproterozoic at about 1000 Ma. This amalgamation process in southern India could be associated with the formation of the Rodinia supercontinent. The Khammam schist belt and the Eastern Ghats mobile belt also show evidence for accretionary processes at around 500 Ma, which is interpreted as a record of Pan-African collisions during the Gondwana assembly. From then on, southern India, as is known today, formed an integral part of the Indian continent.
Springer Nature , book review in Mineralium Deposita diamonds mentioned p. 6-7. of review
India
diamonds
Abstract: Nonrenewable natural resources - metallic and non-metallic minerals, industrial rocks and energy resources (both organic and inorganic), have been treated in a holistic manner in this book, including two important resources (soil and water), not commonly covered in most books on this topic. For the uninitiated reader, an introductory chapter looks into some basic definitions as well as nature and characteristics of mineral deposits followed by a chapter on the different crustal processes that produce the various ore deposits in the endogenous and exogenous environments. The strength of the book lies in its critical treatment of the genetic processes of the mineral deposits, their classification and the geodynamic context of metallogeny, and coverage of sustainable development of mineral deposits with special reference to various socio-economic as well as regulatory and environmental issues that face the Indian mining industry today. The text is punctuated with examples of Indian deposits, balanced with classical deposits around the world, to cater to the interests of Indian students and the international readership. This is a book for advanced undergraduate and post-graduate students of Geology, Environmental Sciences and Natural Resource Management.
Murphy, D.T., Brandon, A.D., Debaille, V., Burgess, R., Ballentine, C.
In search of a hidden long term isolated sub-chondritic 142 Nd 144Nd reservoir in the deep mantle: implications for the Nd isotope systematics of the Earth.
Geochimica et Cosmochimica Acta, Vol. 74, 2, pp. 738-750.
Nd-Hf isotope systematics of megacrysts from the Mbuji-Mayi kimberlites, D.R. Congo: evidence for a metasomatic origin related to kimberlite interaction with the cratonic lithosphere mantle.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 123-136.
Abstract: The Upper Ruvubu Alkaline Plutonic Complex (URAPC) in Burundi consists of three separate intrusions, each with a specific emplacement age and petrological composition. Three main units are recognized: an outer unit with silica-saturated plutonic rocks (from gabbro to granite), an inner unit with silica-undersaturated plutonic rocks (feldspathoidal syenite with subordinate feldspathoidal monzonite and ijolite) and a carbonatitic body in the subsoil, known by drilling. The URAPC is quite large in size (?24?km long and up to 10?km wide). It is considered to have been intruded syntectonically in an overall extensional context, thanks to the kilometric shear zones that accommodated its emplacement. Radiometric ages from literature range from 748 to 705?Ma and point to structurally-controlled magmatic differentiation followed by long-lived circulations of late-stage fluids postdating the emplacement of a part of the undersaturated rocks and the carbonatites. In the north-western part of the outer unit, gabbro likely has been emplaced at a deeper structural level than the granite, which represents a more apical structural level of emplacement. This petrological, geochemical and isotopic (Sr-Nd-Hf) study concentrates on the processes that generated the URAPC: (i) fractional crystallization, evidenced by the chemical evolution trends of the major and trace elements, and by marked P, Ti and Ba anomalies in the trace element patterns; (ii) crustal assimilation/contamination, as shown by the wide range of Nd isotope compositions and the general increase of the Sr isotope ratios with increasing SiO2 contents, and (iii) late-magmatic/hydrothermal alteration inducing an increase of the Sr isotope composition without changing significantly the Nd isotope composition. The isotopic data are consistent with an asthenospheric mantle source, though less depleted than the Depleted Mantle (DM), contaminated by the Subcontinental Lithospheric Mantle (SCLM). The silicate and carbonate magmatic series are cogenetic. The outer unit is clearly more contaminated than the inner unit, whereas the carbonatitic body could have evolved by liquid immiscibility. The URAPC lies within East Africa’s Western Rift Valley, which is marked by 23 alkaline plutonic complexes. Their emplacement has been ascribed to reactivation of Proterozoic lithospheric weakness zones resulting from the breakup of the Neoproterozoic supercontinent Rodinia supercontinent.
Abstract: The Siilinjärvi phosphate deposit (Finland) is hosted by an Archean carbonatite complex. The main body is composed of glimmerite, carbonatite and combinations thereof. It is surrounded by a well-developed fenitization zone. Almost all the rocks pertaining to the glimmerite-carbonatite series are considered for exploitation of phosphate. New petrological and in-situ geochemical as well as spectroscopic data obtained by cathodoluminescence, Raman and laser-induced breakdown spectroscopy make it possible to constrain the genesis and evolution of apatite through time. Apatite in the glimmerite-carbonatite series formed by igneous processes. An increase in rare earth elements (REE) content during apatite deposition can be explained by re-equilibration of early apatite (via sub-solidus diffusion at the magmatic stage) with a fresh carbonatitic magma enriched in these elements. This late carbonatite emplacement has been known as a major contributor to the overall P and REE endowment of the system and is likely connected to fenitization and alkali-rich fluids. These fluids - enriched in REE - would have interacted with apatite in the fenite, resulting in an increase in REE content through coupled dissolution-reprecipitation processes. Finally, a marked decrease in LREE is observed in apatite hosted by fenite. It highlights the alteration of apatite by a REE-poor fluid during a late-magmatic/hydrothermal stage. Regarding the potential for REE exploitation, geochemical data combined with an estimation of the reserves indicate a sub-economic potential of REE to be exploited as by-products of phosphate mining. Spectroscopic analyses further provide helpful data for exploration, by determining the P and REE distribution and the enrichment in carbonatite and within apatite.
Chemical Geology, doi.org/10.1016/ j.chemgeo.2019. 119307
Mantle
craton
Abstract: Establishing the mode and rate of formation of the continental crust is crucial for quantifying mass exchange between Earth’s crust and mantle. The limited crustal rock record, particularly of early Archean rocks, has led to a variety of different models of continental growth. Here, we present an open-system model of silicate Earth evolution incorporating the Sm-Nd and Lu-Hf isotope systematics with the aim to constrain crustal growth during the Archean and its effect on the chemical and isotopic evolution of Earth’s crust-mantle system. Our model comprises four reservoirs: the bulk continental crust (CC), depleted upper mantle (UM), lower mantle (LM), and an isolated reservoir (IR) where recycled crust is stored transiently before being mixed with the LM. The changing abundance of isotope species in each reservoir is quantified using a series of first order linear differential equations that are solved numerically using the fourth order Runge-Kutta method at 1 Myr time steps for 4.56 Gyr (the age of the Earth). The model results show that only continuous and exponential crustal growth reproduces the present-day abundances and isotope ratios in the terrestrial reservoirs. Our preferred crustal growth model suggests that the mass of the CC by the end of Hadean (4.0 Ga) and end of Archean (2.5 Ga) was ?30% and ?75% of the present-day mass of the CC, respectively. Models proposing formation of most (?90%) of the present-day CC during the initial 1 Gyr or nearly 50-60% during the last 1 Gyr are least favorable. Significant mass exchange between crust and mantle, that is, both the formation and recycling of crust, started in the Hadean with Sm-Nd and Lu-Hf isotope evolution typical for mafic rocks. Depletion of the UM (in incompatible elements) during the early Archean is mitigated by the input of recycled crust, so that the UM maintained a near-primitive Hf-Nd isotope composition. The LM also retained a near-primitive Hf-Nd isotope composition during the Archean, but for different reasons. In contrast to the UM, the crustal return flux into the LM is transiently stored (? 1 Gyr) in an isolated reservoir (IR), which limits the mass flux into and out of the LM. The IR in our model is distinct from other mantle reservoirs and possibly related to stable crustal blocks or, alternatively, to recycled crust in the mantle that remains temporarily isolated, perhaps at the core-mantle boundary (LLSVPs).
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.
Abstract: The existence of undulations of the geoid, gravity and bathymetry in ocean basins, as well as anomalies in heat flow, point to the existence of small scale convection beneath tectonic plates. The instabilities that could develop at the base of the lithosphere are sufficiently small scale (< 500 km) that they remain mostly elusive from seismic detection. We take advantage of 3D spherical numerical geodynamic models displaying plate-like behavior to study the interaction between large-scale flow and small-scale convection. We find that finger-shaped instabilities develop at seafloor ages > 60 Ma. They form networks that are shaped by the plate evolution, slabs, plumes and the geometry of continental boundaries. Plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere. They create a wake in which new instabilities form downstream. These wakes form channels that are about 1000 km wide, and thus are possibly detectable by seismic tomography. Beneath fast plates, cold sinking instabilities are tilted in the direction opposite to plate motion, while they sink vertically for slow plates. These instabilities are too small to be detected by usual seismic methods, since they are about 200 km in lateral scale. However, this preferred orientation of instabilities below fast plates could produce a pattern of large-scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models.
Abstract: The existence of undulations of the geoid, gravity and bathymetry in ocean basins, as well as anomalies in heat flow, point to the existence of small scale convection beneath tectonic plates. The instabilities that could develop at the base of the lithosphere are sufficiently small scale (< 500 km) that they remain mostly elusive from seismic detection. We take advantage of 3D spherical numerical geodynamic models displaying plate-like behavior to study the interaction between large-scale flow and small-scale convection. We find that finger-shaped instabilities develop at seafloor ages > 60 Ma. They form networks that are shaped by the plate evolution, slabs, plumes and the geometry of continental boundaries. Plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere. They create a wake in which new instabilities form downstream. These wakes form channels that are about 1000 km wide, and thus are possibly detectable by seismic tomography. Beneath fast plates, cold sinking instabilities are tilted in the direction opposite to plate motion, while they sink vertically for slow plates. These instabilities are too small to be detected by usual seismic methods, since they are about 200 km in lateral scale. However, this preferred orientation of instabilities below fast plates could produce a pattern of large-scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models.
Earth and Planetary Science Letters, Vol. 451, pp. 232-240.
Mantle
Peridotite
Abstract: We present new Zn elemental and isotope data on seventeen fertile and refractory mantle peridotite xenoliths. Eleven fertile peridotites are garnet and spinel lherzolites from Vitim and Tariat (Siberia and Mongolia) and represent some of the most pristine fertile peridotites available. Six refractory peridotites are spinel harzburgites from the Udachnaya kimberlite (Siberian craton) that are nearly pristine residues of high-degree polybaric melting at high pressure (7-4 GPa). Geochemical data suggest that Zn isotopic compositions in the peridotites have not been affected by post-melting processes such as metasomatism, contamination by the host-magmas or alteration. The fertile peridotites have uniform Zn concentrations (59±2 ppm59±2 ppm) and Zn isotopic compositions with ?66Zn (relative to JMC-Lyon-03-0749l)?=?+0.30?±?0.03‰ consistent with the Bulk Silicate Earth estimates of ?66Zn?=?+0.28?±?0.05‰ (Chen et al., 2013). The refractory peridotites have Zn concentrations ranging from 30 to 48 ppm and ?66Zn from +0.10±0.01‰+0.10±0.01‰ to +0.18±0.01‰+0.18±0.01‰ with an average of +0.14±0.03‰+0.14±0.03‰. Our data suggest that the lithospheric mantle has a heterogeneous Zn isotopic composition. Modeling of Zn isotope partitioning during partial melting of fertile mantle suggests that high degrees of melt extraction (>30%) may significantly fractionate Zn isotopes (up to 0.16‰) and that during mantle melting, Zn concentrations and isotopic compositions are mainly controlled by the stability of clinopyroxene and garnet within the melting residue. Because the stability of clinopyroxene and garnet is mainly pressure dependent we suggest that both the depth and the degrees of melt extraction may control Zn isotope fractionation during mantle melting.
Abstract: The study of sediment cores allows for the reconstruction of past climate and environment through physical-chemical analysis. Nevertheless, this interpretation suffers from many drawbacks that can be overcome with the newest technologies. Hyperspectral imaging is one of these and allows a fast, high resolution, and non-destructive analysis of sediment cores. In this study, we use visible and near-infrared hyperspectral imaging to predict particle size fractions and distribution (PSD) at a resolution of 200??m on a previously well-studied sediment core taken from Lake Bourget (Western Alps, France). These predictions agree with previous studies on this core. Then, the PSD was used to estimate sedimentary deposit sources using the PSD unmixing algorithm AnalySize. It permitted estimation of the contribution of five sources (micrite, small and large bio-induced calcite crystals, diatom frustules, detrital particles), which had previously been characterized. The spatial dimension allowed for laminae to be discretized and counted, in agreement with the age-depth model previously established. We then evaluated the particle size and spectral signatures of each of these annual laminae, hence characterizing their physico-chemical composition. These high-resolution data also allowed for estimation of the accumulation rate (cm/year) of each of the main sources in the laminated unit and inferring the trophic status and the presence of instantaneous events of the lake.
Midende, G., Boulais, P., Tack, L., Melcher, F., Gerdes,A., Dewaele, S., Demaiffe, D., Decree, S.
Petrography, geochemistry and U Pb zircon age of the Matongo carbonatite Massif ( Burundi): implication for the Neoproterozoic geodynamic evolution of Central Africa.
Journal of African Earth Sciences, Vol. 100, pp. 656-674.
Midende, G., Boulvais, P., Tack, L., Melcher, F., Gerdes, A., Dewaele, S., Demaiffe, D., Decree, S.
Petrography, geochemistry and U-Pb zircon age of the Matongo carbonatite Massif ( Burundi): implication for the Neoproterozoic geodynamic evolution of Central Africa.
Journal of African Earth Sciences, Vol. 100, pp. 656-674.
Abstract: The Matongo carbonatite intrusive body in the Neoproterozoic Upper Ruvubu alkaline plutonic complex (URAPC) in Burundi is overlain by an economic phosphate ore deposit that is present as breccia lenses. The ore exhibits evidence of supergene enrichment but also preserves textures related to the concentration of fluorapatite in the carbonatitic system. Magmatic fluorapatite is abundant in the ore and commonly occurs as millimeter-sized aggregates. It is enriched in light rare earth elements (LREE), which is especially apparent in the final generation of magmatic fluorapatite (up to 1.32 wt% LREE2O3). After an episode of metasomatism (fenitization), which led to the formation of K-feldspar and albite, the fluorapatite-rich rocks were partly brecciated. Oxygen and carbon isotope compositions obtained on the calcite forming the breccia matrix (?18O?=?22.1?- and ?13C?=??1.5?‰) are consistent with the involvement of a fluid resulting from the mixing of magmatic-derived fluids with a metamorphic fluid originating from the country rocks. In a subsequent postmagmatic event, the carbonates hosting fluorapatite were dissolved, leading to intense brecciation of the fluorapatite-rich rocks. Secondary carbonate-fluorapatite (less enriched in LREE with 0.07-0.24 wt% LREE2O3 but locally associated with monazite) and coeval siderite constitute the matrix of these breccias. Siderite has ?18O values between 25.4 and 27.7?- and very low ?13C values (from ?12.4 to ?9.2?, which are consistent with the contribution of organic-derived low ?13C carbon from groundwater. These signatures emphasize supergene alteration. Finally, the remaining voids were filled with a LREE-poor fibrous fluorapatite (0.01 wt% LREE2O3), forming hardened phosphorite, still under supergene conditions. Pyrochlore and vanadiferous magnetite are other minerals accumulated in the eluvial horizons. As a consequence of the supergene processes and fluorapatite accumulation, the phosphate ore, which contains 0.72 to 38.01 wt% P2O5, is also enriched in LREE (LaN/YbN from 47.1 to 83.5; ?REE between 165 and 5486 ppm), Nb (up to 656 ppm), and V (up to 1232 ppm). In the case of phosphate exploitation at Matongo, REE could prove to have a subeconomic potential to be exploited as by-products of phosphates.
Mineralogy and Petrology, in press available, 17p.
Africa, Burundi
Carbonatite
Abstract: The Matongo carbonatite intrusive body in the Neoproterozoic Upper Ruvubu alkaline plutonic complex (URAPC) in Burundi is overlain by an economic phosphate ore deposit that is present as breccia lenses. The ore exhibits evidence of supergene enrichment but also preserves textures related to the concentration of fluorapatite in the carbonatitic system. Magmatic fluorapatite is abundant in the ore and commonly occurs as millimeter-sized aggregates. It is enriched in light rare earth elements (LREE), which is especially apparent in the final generation of magmatic fluorapatite (up to 1.32 wt% LREE2O3). After an episode of metasomatism (fenitization), which led to the formation of K-feldspar and albite, the fluorapatite-rich rocks were partly brecciated. Oxygen and carbon isotope compositions obtained on the calcite forming the breccia matrix (?18O?=?22.1?‰ and ?13C?=??1.5?‰) are consistent with the involvement of a fluid resulting from the mixing of magmatic-derived fluids with a metamorphic fluid originating from the country rocks. In a subsequent postmagmatic event, the carbonates hosting fluorapatite were dissolved, leading to intense brecciation of the fluorapatite-rich rocks. Secondary carbonate-fluorapatite (less enriched in LREE with 0.07-0.24 wt% LREE2O3 but locally associated with monazite) and coeval siderite constitute the matrix of these breccias. Siderite has ?18O values between 25.4 and 27.7?‰ and very low ?13C values (from ?12.4 to ?9.2?‰), which are consistent with the contribution of organic-derived low ?13C carbon from groundwater. These signatures emphasize supergene alteration. Finally, the remaining voids were filled with a LREE-poor fibrous fluorapatite (0.01 wt% LREE2O3), forming hardened phosphorite, still under supergene conditions. Pyrochlore and vanadiferous magnetite are other minerals accumulated in the eluvial horizons. As a consequence of the supergene processes and fluorapatite accumulation, the phosphate ore, which contains 0.72 to 38.01 wt% P2O5, is also enriched in LREE (LaN/YbN from 47.1 to 83.5; ?REE between 165 and 5486 ppm), Nb (up to 656 ppm), and V (up to 1232 ppm). In the case of phosphate exploitation at Matongo, REE could prove to have a subeconomic potential to be exploited as by-products of phosphates.
Abstract: The Upper Ruvubu Alkaline Plutonic Complex (URAPC) in Burundi consists of three separate intrusions, each with a specific emplacement age and petrological composition. Three main units are recognized: an outer unit with silica-saturated plutonic rocks (from gabbro to granite), an inner unit with silica-undersaturated plutonic rocks (feldspathoidal syenite with subordinate feldspathoidal monzonite and ijolite) and a carbonatitic body in the subsoil, known by drilling. The URAPC is quite large in size (?24?km long and up to 10?km wide). It is considered to have been intruded syntectonically in an overall extensional context, thanks to the kilometric shear zones that accommodated its emplacement. Radiometric ages from literature range from 748 to 705?Ma and point to structurally-controlled magmatic differentiation followed by long-lived circulations of late-stage fluids postdating the emplacement of a part of the undersaturated rocks and the carbonatites. In the north-western part of the outer unit, gabbro likely has been emplaced at a deeper structural level than the granite, which represents a more apical structural level of emplacement. This petrological, geochemical and isotopic (Sr-Nd-Hf) study concentrates on the processes that generated the URAPC: (i) fractional crystallization, evidenced by the chemical evolution trends of the major and trace elements, and by marked P, Ti and Ba anomalies in the trace element patterns; (ii) crustal assimilation/contamination, as shown by the wide range of Nd isotope compositions and the general increase of the Sr isotope ratios with increasing SiO2 contents, and (iii) late-magmatic/hydrothermal alteration inducing an increase of the Sr isotope composition without changing significantly the Nd isotope composition. The isotopic data are consistent with an asthenospheric mantle source, though less depleted than the Depleted Mantle (DM), contaminated by the Subcontinental Lithospheric Mantle (SCLM). The silicate and carbonate magmatic series are cogenetic. The outer unit is clearly more contaminated than the inner unit, whereas the carbonatitic body could have evolved by liquid immiscibility. The URAPC lies within East Africa’s Western Rift Valley, which is marked by 23 alkaline plutonic complexes. Their emplacement has been ascribed to reactivation of Proterozoic lithospheric weakness zones resulting from the breakup of the Neoproterozoic supercontinent Rodinia supercontinent.
Contributions to Mineralogy and Petrology, Vol. 175, 34 31p. Pdf
Africa, South Africa
carbonatite
Abstract: The Phalaborwa world-class phosphate deposit (South Africa) is hosted by a Paleoproterozoic alkaline complex mainly composed of phoscorite, carbonatite, pyroxenitic rocks, and subordinate fenite. In addition, syenite and trachyte occur in numerous satellite bodies. New petrological and in-situ geochemical data along with O and Sr isotope data obtained on apatite demonstrate that apatite is in the principal host rocks (pyroxenitic rocks, phoscorite and carbonatite) formed primarily by igneous processes from mantle-derived carbonatitic magmas. Early-formed magmatic apatite is particularly enriched in light rare earth elements (LREE), with a decrease in the REE content ascribed to magma differentiation and early apatite fractionation in isolated interstitial melt pockets. Rayleigh fractionation favored a slight increase in ?18O (below 1%) at a constant Sr isotopic composition. Intrusion of fresh carbonatitic magma into earlier-formed carbonatite bodies locally induced re-equilibration of early apatite with REE enrichment but at constant O and Sr isotopic compositions. In fenite, syenite and trachyte, apatite displays alteration textures and LREE depletion, reflecting interaction with fluids. A marked decrease in ?18O in apatite from syenite and trachyte indicates a contribution from ?18O-depleted meteoric fluids. This is consistent with the epizonal emplacement of the satellite bodies. The general increase of the Sr isotope ratios in apatite in these rocks reflects progressive interaction with the country rocks over time. This study made it possible to decipher, with unmatched precision, the succession of geological processes that led to one of the most important phosphate deposits worldwide.
Abstract: The rare earth element (REE) mineralization of Gakara (Burundi) has first been discovered in 1936 and has periodically been the subject of geological studies, at times when the exploitation of bastnäsite-(Ce) and monazite-(Ce) was economically interesting. This study focuses on the establishment of a mineral paragenesis for Gakara, with special attention to the REE-bearing phases, to understand the formation history of the deposit. The paragenesis can be subdivided into 3 stages: primary ore deposition, brecciation stage and supergene alteration. Evidence for fenitization processes (i.e. pinkish-red cathodoluminescence of K-feldpar, brecciation stage) and the strong enrichment of light REEs in bastnäsite and monazite substantiate the hypothesis of a structurally controlled hydrothermal mineralization with a strong carbonatitic affinity. This likely confirms the association of the Gakara REE deposit with the Neoproterozoic alignment of alkaline complexes and carbonatites along the present-day Western Rift. It suggests a direct link with a - currently unidentified - carbonatitic body at depth, possibly derived from a predominantly metasomatized lithospheric mantle.
Abstract: The Siilinjärvi phosphate deposit (Finland) is hosted by an Archean carbonatite complex. The main body is composed of glimmerite, carbonatite and combinations thereof. It is surrounded by a well-developed fenitization zone. Almost all the rocks pertaining to the glimmerite-carbonatite series are considered for exploitation of phosphate. New petrological and in-situ geochemical as well as spectroscopic data obtained by cathodoluminescence, Raman and laser-induced breakdown spectroscopy make it possible to constrain the genesis and evolution of apatite through time. Apatite in the glimmerite-carbonatite series formed by igneous processes. An increase in rare earth elements (REE) content during apatite deposition can be explained by re-equilibration of early apatite (via sub-solidus diffusion at the magmatic stage) with a fresh carbonatitic magma enriched in these elements. This late carbonatite emplacement has been known as a major contributor to the overall P and REE endowment of the system and is likely connected to fenitization and alkali-rich fluids. These fluids - enriched in REE - would have interacted with apatite in the fenite, resulting in an increase in REE content through coupled dissolution-reprecipitation processes. Finally, a marked decrease in LREE is observed in apatite hosted by fenite. It highlights the alteration of apatite by a REE-poor fluid during a late-magmatic/hydrothermal stage. Regarding the potential for REE exploitation, geochemical data combined with an estimation of the reserves indicate a sub-economic potential of REE to be exploited as by-products of phosphate mining. Spectroscopic analyses further provide helpful data for exploration, by determining the P and REE distribution and the enrichment in carbonatite and within apatite.
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: Mantle flow can cause the Earth’s surface to uplift and subside, but the rates and durations of these motions are, in general, poorly resolved due to the difficulties in making measurements of relatively small vertical movements (hundreds of metres) over sufficiently large distances (about 1,000?km). Here we examine the effect of mantle upwelling through a study of Quaternary uplift along the coast of Angola. Using both optically stimulated luminescence on sediment grains, and radiocarbon dating of fossil shells, we date a 25?m coastal terrace at about 45 thousand years old, when sea level was about 75?m lower than today, indicating a rapid uplift rate of 1.8-2.6?mm?yr?1 that is an order of magnitude higher than previously obtained rates averaged over longer time periods. Automated extraction and correlation of coastal terrace remnants from digital topography uncovers a symmetrical uplift with diameter of more than 1,000?km. The wavelength and relatively short timescale of the uplift suggest that it is associated with a mantle process, possibly convective upwelling, and that the topography may be modulated by rapid short-lived pulses of mantle-derived uplift. Our study shows that stable continental regions far from the effects of glacial rebound may experience rapid vertical displacements of several millimetres per year.
Abstract: On million-year time scales, Earth’s climate fluctuates between warm and cool baselines. For example, the past 40 m.y. has been relatively cool and characterized by a permanent ice sheet on Antarctica, while the interval between 150 and 50 m.y. ago was characterized by warm temperatures and no permanent ice sheets (Royer et al., 2004; Zachos et al., 2008). What controls these fluctuations is debated, but to first order, the average surface temperature of Earth reflects the balance of incoming solar insolation (energy in) versus planetary albedo and greenhouse gas concentrations (energy out). It is generally thought that over the past billion years, the most important control on long-term climate is variations in greenhouse gases in the atmosphere, namely CO2 (Berner, 1991). What controls long-term CO2 are variations in geologic inputs and the efficiency of CO2 sequestration, the former through volcanic and metamorphic degassing and oxidative weathering of organic carbon, and the latter through silicate weathering (and eventual carbonate precipitation) and organic carbon burial. Importantly, the efficiency of silicate weathering and organic carbon burial is widely thought to scale directly and indirectly with atmospheric pCO2; CO2’s impact on global temperature and the hydrologic cycle serves as a negative feedback, enhancing (mitigating) carbon sequestration mechanisms given increased (decreased) inputs of CO2. As the residence time of CO2 in the exogenic system (ocean-atmosphere-biosphere) is on the order of 10-100 k.y., exogenic carbon contents on million-year time scales are at steady state, where inputs equal outputs (Berner and Caldeira, 1997). Changes in exogenic cabon over greater than million-year time scales thus reflect secular changes in the steady-state baseline, driven by changes in inputs and/or the kinetics of carbon sequestration (Fig. 1).
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.
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.
The Journal of the Southern African Institute of Mining and Metallurgy, Vol. 118, Jan. pp. 1-6.
Africa, South Africa
diamond size frequency
Abstract: Diamond size frequency distribution (SFD) curves, combined with the associated dollar per carat per size class, play an important role in the diamond industry. Value per size class is unique for each deposit and typically varies from less than a dollar per carat to several thousands of dollars per carat for special stones. Recovery of large stone therefore contributes significantly to the bottom line of a large diamond producer. While the design of the process plant should prevent damage and possible breakage of large stones, it should also ensure adequate liberation of the finer diamonds. Innovative solutions are required to protect and recover type I and II diamonds if prominent within the resource. X-ray transmission (XRT) sorting presents the opportunity to develop flow sheet designs that incorporate a balance between exploitation of the resource and process efficiency, as well as practical capital and operating costs. This paper serves to illustrate the role and application of SFD curves in determining optimum cut-off and re-crush sizes within the flow sheet of a large diamond producer. A thorough understanding of the unique technical and economic aspects of a deposit provides the basis from where new and innovative technologies can be proposed, allowing mining companies to maintain and improve profit margins. It highlights the results of various plant trials and newly commissioned XRT sorters for larger size fractions. It also provides recommendations for future applications of XRT machines in the diamond process flow sheet.
Abstract: The objective of our study is to establish an assessment of four volcanic hazards in a country threatened by the eruption of the OlDoinyo Lengai volcano. The last major eruption dates back to 2007-2008 but stronger activity in 2019 has revived the memory of volcanic threats to the Maasai and Bantu communities and human activities (agro-pastoral and tourism). The methods chosen have had to be adapted to the scarce and incomplete data. The volcanic hazards and their probability of occurrence were analysed on the basis of data available in the scientific literature and were supplemented by two field missions combining geography and hydro-geomorphology. Our study enabled us to map the hazards of ash fall, lava flows, lahars and avalanches of debris. Each hazard was spatialised by being ascribed an intensity. They are sometimes synchronous with the eruption sometimes they occur several months or years after a volcanic eruption. The results are the first step towards developing a volcanic risk management strategy, especially for the pastoral communities living around Lengai and for the growing tourist activities in this area.
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.
Russia, South America, Brazil, Africa, Tanzania, Zimbabwe, India, Asia, Sri Lanka
alexandrite
Abstract: The gem and jewelry trade has come to place increasing importance on the geographic origin of alexandrite, as it can have a significant impact on value. Alexandrites from Russia and Brazil are usually more highly valued than those from other countries. In 2016, GIA began researching geographic origin of alexandrite with the intent of offering origin determination as a laboratory service. Unfortunately, collecting reliable samples with known provenance can be very difficult. Alexandrite is often recovered as a byproduct of mining for other gemstones (e.g., emerald and corundum), so it can be difficult to secure reliable parcels of samples because production is typically erratic and unpredictable. The reference materials studied here were examined thoroughly for their trace element chemistry profiles, characteristic color-change ranges under daylight-equivalent and incandescent illumination, and inclusion scenes. The data obtained so far allow us to accurately determine geographic origin for alexandrites from Russia, Brazil, Sri Lanka, Tanzania, and India. Future work may help to differentiate alexandrites from other localities.
Abstract: Magmatic processes on Earth govern the mass, energy and chemical transfer between the mantle, crust and atmosphere. To understand magma storage conditions in the crust that ultimately control volcanic activity and growth of continents, an evaluation of the mass and heat budget of the entire crustal column during magmatic episodes is essential. Here we use a numerical model to constrain the physical conditions under which both lower and upper crustal magma bodies form. We find that over long durations of intrusions (greater than 105 to 106?yr), extensive lower crustal mush zones develop, which modify the thermal budget of the upper crust and reduce the flux of magma required to sustain upper crustal magma reservoirs. Our results reconcile physical models of magma reservoir construction and field-based estimates of intrusion rates in numerous volcanic and plutonic localities. Young igneous provinces (less than a few hundred thousand years old) are unlikely to support large upper crustal reservoirs, whereas longer-lived systems (active for longer than 1 million years) can accumulate magma and build reservoirs capable of producing super-eruptions, even with intrusion rates smaller than 10?3 to 10?2?km3?yr?1. Hence, total duration of magmatism should be combined with the magma intrusion rates to assess the capability of volcanic systems to form the largest explosive eruptions on Earth.
Abstract: Storage pressures of magma chambers influence the style, frequency and magnitude of volcanic eruptions. Neutral buoyancy or rheological transitions are commonly assumed to control where magmas accumulate and form such chambers. However, the density of volatile-rich silicic magmas is typically lower than that of the surrounding crust, and the rheology of the crust alone does not define the depth of the brittle-ductile transition around a magma chamber. Yet, typical storage pressures inferred from geophysical inversions or petrological methods seem to cluster around 2?±?0.5?kbar in all tectonic settings and crustal compositions. Here, we use thermomechanical modelling to show that storage pressure is controlled by volatile exsolution and crustal rheology. At pressures ?1.5?kbar, and for geologically realistic water contents, chamber volumes and recharge rates, the presence of an exsolved magmatic volatile phase hinders chamber growth because eruptive volumes are typically larger than recharges feeding the system during periods of dormancy. At pressures >rsim2.5?kbar, the viscosity of the crust in long-lived magmatic provinces is sufficiently low to inhibit most eruptions. Sustainable eruptible magma reservoirs are able to develop only within a relatively narrow range of pressures around 2?±?0.5?kbar, where the amount of exsolved volatiles fosters growth while the high viscosity of the crust promotes the necessary overpressurization for eruption.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 8p.
Technology
Sorting - X-ray DE-XRT
Abstract: In May 2009 a test work programme was carried out at the sorting laboratory of CommodasUltrasort in Germany which was aimed at establishing the efficiency of Xray transmission sorting for the recovery of unliberated diamonds in a size range of +8mm. A PRO Tertiary XRT belt sorter with a working width of 600mm and a belt speed of - 3mJs was used for the test work. Dual energy X-ray transmission sorting is a type of sensor-based sorting which uses an X-ray scanning system, The attenuation of the transmitting X-rays is measured at two different energy levels, Thereby it is possible to eliminate the effect of the particle thickness on the measurement result. The effective measurement results in a classification by elemental order based on the periodic table. Diamonds are mostly composed of Carbon, which has an atomic number of 6. Compared to Silicon with an atomic number of 14, Diamonds show up much lighter on an XRT image than Silica-based gravels (typical DMS sinks material usually has more dense minerals such as Fe, and Mn which shows an even, darker X-ray image than Si). Tests were run with ele.ven Boarl Diamonds in a size range of -15+8mm mixed into 13kg alluvial DMS sinks gravel in a size range of -25+8mm. All liberated Boart Diamonds were recovered at feed rates of 15t/h at average yields of 7.6g per ejected Diamond. In a second test series the Boarl Diamonds were covered in 20% Gypsum by weight and run again at 15t/h. All unliberated Diamonds were recovered at an average yield of 12.2g per ejected Diamond. The paper outlines the test work results in detail.
Geophysical Journal International, Vol. 214, 2, pp. 1281-1300. doi:1093/gji/ggy193
Canada, Nunavut
Geophysics - gravity
Abstract: Mesozoic to Cenozoic continental rifting, breakup and spreading between North America and Greenland led to the opening, from south to north, of the Labrador Sea and eventually Baffin Bay between Baffin Island, northeast Canada and northwest Greenland. Baffin Bay lies at the northern limit of this extinct rift, transform and spreading system and remains largely underexplored. With the sparsity of existing crustal-scale geophysical investigations of Baffin Bay, regional potential field methods and quantitative deformation assessments based on plate reconstructions provide two means of examining Baffin Bay at the regional scale and drawing conclusions about its crustal structure, its rifting history and the role of pre-existing structures in its evolution. Despite the identification of extinct spreading axes and fracture zones based on gravity data, insights into the nature and structure of the underlying crust have only been gleaned from limited deep seismic experiments, mostly concentrated in the north and east where the continental shelf is shallower and wider. Baffin Bay is partially underlain by oceanic crust with zones of variable width of extended continental crust along its margins. 3-D gravity inversions, constrained by bathymetric and depth to basement constraints, have generated a range of 3-D crustal density models that collectively reveal an asymmetric distribution of extended continental crust, approximately 25-30?km thick, along the margins of Baffin Bay, with a wider zone on the Greenland margin. A zone of 5-13?km thick crust lies at the centre of Baffin Bay, with the thinnest crust (5?km thick) clearly aligning with Eocene spreading centres. The resolved crustal thicknesses are generally in agreement with available seismic constraints, with discrepancies mostly corresponding to zones of higher density lower crust along the Greenland margin and Nares Strait. Deformation modelling from independent plate reconstructions using GPlates of the rifted margins of Baffin Bay was performed to gauge the influence of original crustal thickness and the width of the deformation zone on the crustal thicknesses obtained from the gravity inversions. These results show the best match with the results from the gravity inversions for an original unstretched crustal thickness of 34-36?km, consistent with present-day crustal thicknesses derived from teleseismic studies beyond the likely continentward limits of rifting around the margins of Baffin Bay. The width of the deformation zone has only a minimal influence on the modelled crustal thicknesses if the zone is of sufficient width that edge effects do not interfere with the main modelled domain.
International Journal of Remote Sensing, Vol. 38, 3, pp. 827-849.
Technology
LIDAR
Abstract: In recent years lidar technology has experienced a noticeable increase in its relevance and usage in a number of scientific fields. Therefore, software capable of handling lidar data becomes a key point in those fields. In this article, we present GPU-based viewer lidar (GVLiDAR), a novel web framework for visualization and geospatial measurement of lidar data point sets. The design of the framework is focused on achieving three key objectives: performance in terms of real-time interaction, functionality, and online availability for the lidar datasets. All lidar files are pre-processed and stored in a lossless data structure, which minimizes transfer requirements and offers an on-demand lidar data web framework.
Deijanin, B., Simic, D., Zaitsev, A., Chapman, J., Dobrinets, I., Widemann, A., Del Re, N., Middleton, T., Dijanin, E., Se Stefano, A.
Characterization of pink diamonds of different origin: natural ( Argyle, non-Argyle), irradiated and annealed, treated with multi-process, coated and synthetic.
Diamond and Related Materials, Vol. 17, 7-10, pp. 1169-1178.
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
Abstract: Knowing how much Earth’s rotation axis has moved in the geological past - the so-called "polar wander" - has important implications for understanding geological processes. For example, it exposes significant areas on Earth to rapidly changing climatic conditions. The polar shift is a consequence of the Earth responding to a changed center of gravity, caused by processes such as slab-break-off...(no abstract, good graphics)
Geochemistry, Geophysics, Geosystems, Vol. 20, 9, pp. 4426-4456.
Russia
xenoliths
Abstract: This paper presents the results of a study of rare rock fragments (xenoliths) that were transported from the Earth's deep interior to the surface during an eruption of Kharchinsky volcano, Kamchatka. The chemical compositions, mineralogy, and textures of the samples were studied with the goal of understanding the processes that affected rocks, which may play a role in the formation of magmas in the Kamchatka subduction zone. The key process that affected the xenoliths involved the addition of fluids and dissolved elements to the samples at temperatures of 500-700 °C. These fluids are derived from seawater that was transported to 30? to 50?km depths by subduction of the Pacific Plate beneath Kamchatka. Subsequent to the addition of fluid, there was a shift in the position of the Kamchatka?Pacific Plate boundary that led to an increase in temperature and the formation of small quantities of melt that crystallized to a distinctive group of secondary minerals that are present in the samples and that postdate (overprint) the initial effects of fluid addition. The final step in the evolution of the samples was infiltration by an Fe? and Mg?rich magma that crystallized principally amphibole?group minerals.
Annual Review of Earth and Planetary Sciences, Vol. 48, 21p. pdf
Mantle
mineralogy
Abstract: Recent progress in theoretical mineral physics based on the ab initio quantum mechanical computation method has been dramatic in conjunction with the rapid advancement of computer technologies. It is now possible to predict stability, elasticity, and transport properties of complex minerals quantitatively with uncertainties that are comparable to or even smaller than those attached in experimental data. These calculations under in situ high-pressure (P) and high-temperature conditions are of particular interest because they allow us to construct a priori mineralogical models of the deep Earth. In this article, we briefly review recent progress in studying high-P phase relations, elasticity, thermal conductivity, and rheological properties of lower mantle minerals including silicates, oxides, and some hydrous phases. Our analyses indicate that the pyrolitic composition can describe Earth's properties quite well in terms of density and P- and S-wave velocity. Computations also suggest some new hydrous compounds that could persist up to the deepest mantle and that the postperovskite phase boundary is the boundary of not only the mineralogy but also the thermal conductivity. 1) The ab initio method is a strong tool to investigate physical properties of minerals under high pressure and high temperature.
2) Calculated thermoelasticity indicates that the pyrolytic composition is representative to the chemistry of Earth's lower mantle. 3) Simulations predict new dense hydrous phases stable in the whole lower mantle pressure and temperature condition. 4) Calculated lattice thermal conductivity suggests a heat flow across the core mantle boundary no greater than 10 TW.
Annual Review of Earth and Planetary Sciences, Vol. 48, 1, pp. 99-119.
Mantle
mineralogy
Abstract: Recent progress in theoretical mineral physics based on the ab initio quantummechanical computation method has been dramatic in conjunction with the rapid advancement of computer technologies. It is now possible to predict stability, elasticity, and transport properties of complex minerals quantitatively with uncertainties that are comparable to or even smaller than those attached in experimental data. These calculations under in situ high-pressure (P) and high-temperature conditions are of particular interest because they allow us to construct a priori mineralogical models of the deep Earth. In this article, we briefly review recent progress in studying high-P phase relations, elasticity, thermal conductivity, and rheological properties of lower mantle minerals including silicates, oxides, and some hydrous phases. Our analyses indicate that the pyrolitic composition can describe Earth’s properties quite well in terms of density and P- and S-wave velocity. Computations also suggest some new hydrous compounds that could persist up to the deepestmantle and that the postperovskite phase boundary is the boundary of not only the mineralogy but also the thermal conductivity.
Abstract: Diamond is commonly regarded as an indicator of ultra-high pressure conditions in Earth System Science. This canonical view is challenged by recent data and interpretations that suggest metastable growth of diamond in low pressure environments. One such environment is serpentinisation of oceanic lithosphere, which produces highly reduced CH4-bearing fluids after olivine alteration by reaction with infiltrating fluids. Here we report the first ever observed in situ diamond within olivine-hosted, CH4-rich fluid inclusions from low pressure oceanic gabbro and chromitite samples from the Moa-Baracoa ophiolitic massif, eastern Cuba. Diamond is encapsulated in voids below the polished mineral surface forming a typical serpentinisation array, with methane, serpentine and magnetite, providing definitive evidence for its metastable growth upon low temperature and low pressure alteration of oceanic lithosphere and super-reduction of infiltrated fluids. Thermodynamic modelling of the observed solid and fluid assemblage at a reference P-T point appropriate for serpentinisation (350 °C and 100 MPa) is consistent with extreme reduction of the fluid to logfO2 (MPa) = ?45.3 (?logfO2[Iron-Magnetite] = ?6.5). These findings imply that the formation of metastable diamond at low pressure in serpentinised olivine is a widespread process in modern and ancient oceanic lithosphere, questioning a generalised ultra-high pressure origin for ophiolitic diamond.
Deijanin, B., Simic, D., Zaitsev, A., Chapman, J., Dobrinets, I., Widemann, A., Del Re, N., Middleton, T., Dijanin, E., Se Stefano, A.
Characterization of pink diamonds of different origin: natural ( Argyle, non-Argyle), irradiated and annealed, treated with multi-process, coated and synthetic.
Diamond and Related Materials, Vol. 17, 7-10, pp. 1169-1178.
Abstract: We document an exceptionally large, 25.85 ct diamond that shows a slight colour change but exhibits some atypical properties for chameleon diamonds, including white luminescence to long- and short-wave UV radiation, as well as a network-like pattern seen in most orientations with the DiamondView. In considering whether to call this a chameleon diamond, we undertook a review of available data to compile the properties that are commonly exhibited by these gems. We found that, in addition to their defining photochromic and thermochromic behaviour, nine characteristics all must be present: long-lasting yellow phosphorescence, a zoned DiamondView growth pattern showing yellow-green/blue/inert areas, the presence of dominant A aggregates and also some hydrogen in the infrared spectrum, a continuum of absorption in the visible range related to a very weak type Ib character, a 480 nm absorption band that is possibly related to trace amounts of oxygen, a 425 nm absorption band, a weaker absorption band in the red to near-infrared region consistent with hydrogen-related defects, and traces of nickel detected with photoluminescence spectroscopy.
Philosphical Transactions of the Royal Society, http://dx.doi.org/ 10.1098/rsta .2017.0403 12p. Available
Mantle
plate tectonics
Abstract: Less than 25% of the volume of the juvenile continental crust preserved today is older than 3?Ga, there are no known rocks older than approximately 4?Ga, and yet a number of recent models of continental growth suggest that at least 60-80% of the present volume of the continental crust had been generated by 3?Ga. Such models require that large volumes of pre-3?Ga crust were destroyed and replaced by younger crust since the late Archaean. To address this issue, we evaluate the influence on the rock record of changing the rates of generation and destruction of the continental crust at different times in Earth's history. We adopted a box model approach in a numerical model constrained by the estimated volumes of continental crust at 3?Ga and the present day, and by the distribution of crust formation ages in the present-day crust. The data generated by the model suggest that new continental crust was generated continuously, but with a marked decrease in the net growth rate at approximately 3?Ga resulting in a temporary reduction in the volume of continental crust at that time. Destruction rates increased dramatically around 3 billion years ago, which may be linked to the widespread development of subduction zones. The volume of continental crust may have exceeded its present value by the mid/late Proterozoic. In this model, about 2.6-2.3 times of the present volume of continental crust has been generated since Earth's formation, and approximately 1.6-1.3 times of this volume has been destroyed and recycled back into the mantle.
Geological Society of America Bulletin, Vol. 128, 1/2, pp. 169-186.
Africa, Tanzania
Lineaments
Abstract: The North Tanzanian divergence zone along the East African Rift is characterized by active faults and several large volcanoes such as Meru, Ol Doinyo Lengai, and Kilimanjaro. Based on systematic morphostructural analysis of the Shuttle Radar Topographic Mission digital elevation model and targeted field work, 14 debris avalanche deposits were identified and characterized, some of them being - to our knowledge - previously unknown. Our field survey around Mount Meru allowed previous "lahar" deposits to be reinterpreted as debris avalanche deposits and three major collapse events to be distinguished, with the two older ones being associated with eruptions. We used topographic lineaments and faults across the North Tanzanian divergence zone to derive the main tectonic trends and their spatial variations and highlight their control on volcano collapse orientation. Based on previous analogue models, the tectonic regime is inferred from the orientation of the collapse scars and/or debris avalanche deposits. We infer two types of regime: extensional and transtensional/strike-slip. The strike-slip regime dominates along the rift escarpment, but an extensional regime is inferred to have operated for the recent sector collapses. The proposed interpretation of sector collapse scars and debris avalanche deposits therefore provides constraints on the tectonic regime in the region. It is possible that, in some cases, movement on regional faults triggered sector collapse.
Abstract: Hydrogen (H) and carbon (C) have probably been delivered to the Earth mainly during accretion processes at High Temperature (HT) and High Pressure (HP) and at variable redox conditions. We performed HP (1-15?GPa) and HT (1600-2300°C) experiments, combined with state-of-the-art analytical techniques to better understand the behavior of H and C during planetary differentiation processes. We show that increasing pressure makes H slightly siderophile and slightly decreases the highly siderophile nature of C. This implies that the capacity of a growing core to retain significant amounts of H or C is mainly controlled by the size of the planet: small planetary bodies may retain C in their cores while H may have rather been lost in space; larger bodies may store both H and C in their cores. During the Earth's differentiation, both C and H might be sequestrated in the core. However, the H content of the core would remain one or two orders of magnitude lower than that of C since the (H/C)core ratio might range between 0.04 and 0.27.
Abstract: The first small manufactured industrial diamonds were produced in 1953 by the Swedish company ASEA but their accomplishment went unannounced. In 1970, General Electric (GE) produced synthetic diamond crystals using the HPHT method with a belt type of press and created a 0.78ct polished RBC colorless diamond. In the 1980’s and 1990’s Russians used their own technology (“BARS” and “TOROID” high pressure apparatus (HPA) with high pressure presses of up to 25 MN load) to grow industrial and crystals up to 2.00ct in polished size, mostly orange to yellow in colour. In the last 15 years, companies including Lucent, Chatham, AOTC, Gemesis (now IIa Technologies) and many other producers in China, Germany, India, Russia, Ukraine, USA and Taiwan have improved the technology yet again and used their expertise to successfully grow diamond crystals that cut to 1.00ct up to 2.00ct in size. This “next generation” of diamonds exhibited high clarities (VS and VVS) and colours (D-H), as well as new blue and pink colours (after irradiation). Other companies (ref. 2-3) including Scio Diamonds, Washington Diamonds, Taidiam, PDC diamonds and Pure Grown Diamonds (selling arm of IIa technologies) are also using a very different technology/process of Chemical Vapour Deposition (CVD) to produce laboratory-grown diamonds up to 3.00 ct in size (table 1).
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 305-6.
Global
synthetics
Abstract: Laboratory-grown diamonds are created using either high-pressure, high-temperature (HPHT) or chemical vapor deposition (CVD). With the influx of manmade diamonds on the market over the past few years, instrument producers and labs have launched screening and detection instruments to help dealers and jewelers spot HPHTor CVD-grown specimens. Most standard instruments are inaccurate testers or just type I and type II screening devices that do not give a definite answer about diamond genesis. Over the last four annual Mediterranean Gemmological and Jewellery Conferences and more than 30 workshops given in 17 countries, we have assembled a portable new Synthetic Diamond Identification Kit. The kit comprises two portable instruments and two booklets: 1) A PL inspector (mini UV lamp with magnifier) to inspect laboratory-grown, treated, and natural diamonds using long- and short-wave fluorescence and phosphorescence 2) A 2017 handbook with images and explanation of longand short-wave reactions of diamonds of all types 3) A mini foldable polariscope with portable light to separate natural diamonds using characteristic birefringence patterns from HPHT and CVD diamonds 4) A 2010 handbook with images and explanations of crosspolarized filter reactions of diamonds of all types The combination of this kit with professional training could identify all HPHT-grown diamonds and most CVD-grown diamonds on the market, loose or mounted. Also available are melee and jewelry inspectors consisting of larger UV lamps with magnifiers designed for identification of small loose or mounted diamonds. Different diamond types and subtypes can exhibit different birefringence under cross-polarized filters. A clear majority of natural diamonds exhibit some degree of internal strain, with type II natural diamonds showing a weak “tatami” pattern. HPHTgrown diamonds are free of such strain, and CVD-grown diamonds show mostly coarse columnar patterns. Most natural diamonds have a strong reaction to long-wave UV; this reaction is usually weaker (mostly blue) at shorter wavelengths. Laboratory-grown diamonds generally exhibit more intense fluorescence with short-wave UV compared to long-wave UV, with a chalky coloring tinged with green or yellow. Most HPHT-grown diamonds also phosphoresce. If a diamond is free of inclusions, fluorescence is a reliable screening test to flag suspicious stones that should be further checked under cross-polarized filters (figure 1). In the case of some rare near-colorless clean CVD-grown diamonds that do not show fluorescence or have a birefringence pattern that is coarse but resembling tatami in type IIa and weak patterns in natural Ia diamonds, additional tests using advanced spectroscopy and strong short-wave UV light to observe growth patterns are needed to confirm diamond genesis.
Gemmological Research Industries Inc. Vancouver B.C., isbn 978-1777369231 184p.
Global
Book - notice
Abstract: For those who have some portable and advanced instruments, this book will serve as a handbook with many useful spectra, cross polarised filters and fluorescence reactions to compare, plus an Appendix with results of tests conducted using 11 portable instruments on 64 samples, and suggestions as to what instruments to use depending on budget and needs. Even if you are not a diamond specialist but are merely interested in the science of diamond, or you trade in diamonds, the information in this book will make you more knowledgeable and confident to talk about this beautiful gem with friends and clients.
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.
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.
Geophysical Research Abstracts EGRU2019-3091, Vol. 21, 3091, 1p.
Europe, Alps
garnet
Abstract: Garnet is the paradigmatic cubic mineral of metamorphic and igneous rocks, and is generally regarded as optically isotropic. Nonetheless, evident birefringence is observed, particularly in the rare CaFe 3+ hydrogarnets, which is attributed to the coexistence of two or more cubic phases. A weak birefringence, with rare examples of optical sector zoning, has also been documented in much more common Fe 2+-Mg-Mn garnets, but an adequate explanation for its cause is, so far, lacking. Here we show that optically anisotropic garnets are much more widespread than previously thought, both in blueschists and blueschist-facies rocks, as well as in lower greenschist-facies phyllites, but they are frequently overlooked when working with conventional, 30-µm-thick thin sections. Utilizing a multi-technique approach including optical microstructural analysis, BSEM, EMPA, EBSD, FTIR, TEM, EDT and single-crystal XRD, we demonstrate here that the birefringence in these garnets is related to their tetragonal symmetry, that it is not due to strain, and that crystals are twinned according to a merohedral law. We also show that the birefringent garnets from blueschists and phyllites are anhydrous, lacking any hydrogarnet component, and have compositions dominated by almandine (58-79%) and grossular (19-30%) with variable spessartine (0-21%) and very low pyrope (1-7%). Considering the widespread occurrence of optically anisotropic OH-free garnets in blueschists and phyllites, their common low-grade metamorphic origin, and the occurrence of optically isotropic garnets with similar Ca-rich almandine composition in higher-grade rocks, we conclude that garnet does not grow with cubic symmetry in low-temperature rocks (< 400 • C). The tetragonal structure appears to be typical of Fe-Ca-rich compositions, with very low Mg contents. Cubic but optically sector-zoned garnet in a lower amphibolite-facies metapelite from the eastern Alps suggests that preservation of tetragonal garnet is favored in rocks which did not progress to T> ?500 • C, where transition to the cubic form, accompanied by change of stable chemical composition, would take place. Our data show that the crystal-chemistry of garnet, its thermodynamics and, in turn, its use in unravelling petrogenetic processes in cold metamorphic environments need to be reassessed.
Abstract: Garnet is the archetypal cubic mineral, occurring in a wide variety of rock types in Earth’s crust and upper mantle. Owing to its prevalence, durability and compositional diversity, garnet is used to investigate a broad range of geological processes. Although birefringence is a characteristic feature of rare Ca-Fe3+ garnet and Ca-rich hydrous garnet, the optical anisotropy that has occasionally been documented in common (that is, anhydrous Ca-Fe2+-Mg-Mn) garnet is generally attributed to internal strain of the cubic structure. Here we show that common garnet with a non-cubic (tetragonal) crystal structure is much more widespread than previously thought, occurring in low-temperature, high-pressure metamorphosed basalts (blueschists) from subduction zones and in low-grade metamorphosed mudstones (phyllites and schists) from orogenic belts. Indeed, a non-cubic symmetry appears to be typical of common garnet that forms at low temperatures (<450?°C), where it has a characteristic Fe-Ca-rich composition with very low Mg contents. We propose that, in most cases, garnet does not initially grow cubic. Our discovery indicates that the crystal chemistry and thermodynamic properties of garnet at low-temperature need to be re-assessed, with potential consequences for the application of garnet as an investigative tool in a broad range of geological environments.
Agrosi, G., Tempesta, G., Mele, D., Caggiani, MC., Mangone, A., Della Ventura, G., Cestelli-Guidi, M., Allegretta, I., Hutchison, M.T., Nimis, P., Nestola, F.
Abstract: Super-deep diamonds and their mineral inclusions preserve very precious information about Earth’s deep mantle. In this study, we examined multiphase inclusions entrapped within a diamond from the Rio Vinte e um de Abril, São Luiz area (Juina, Brazil), using a combination of non-destructive methods. Micro-Computed X-ray Tomography (?-CXRT) was used to investigate the size, shape, distribution and X-Ray absorption of inclusions and mapping by micro X-ray Fluorescence (?-XRF), ?-Raman Spectroscopy and micro-Fourier Transform Infrared Spectroscopy (?-FTIR) were used to determine the chemical and mineralogical composition of the inclusions. Four large inclusions enclosed in the N-rich diamond core consist of dominant ferropericlase-magnesiowüstite and locally exsolved magnesioferrite. FTIR maps, obtained integrating the band at 1430 cm?1, show also the presence of carbonates. A fifth large inclusion (ca 100 ?m) was remarkable because it showed a very unusual flask shape, resembling a fluid/melt inclusion. Based on ?CXRT tomography and ?-Raman mapping, the flask-shaped inclusion is polyphase and consists of magnetite and hematite partly replacing a magnesiowüstite core and small-volume of gas/vacuum. ?-Raman spectra on the same inclusion revealed local features that are ascribed to post-spinel polymorphs, such as maohokite or xieite, which are stable at P ? 18 GPa, and to huntite, a carbonate with formula CaMg3(CO3)4. This represents the first finding of maohokite and huntite in diamond. We interpret the composition of the inclusions as evidence of formation of ferropericlase-magnesiowüstite and diamond in a carbonate-rich environment at depths corresponding at least to the Transition Zone, followed by oxidation of ferropericlase-magnesiowüstite by reaction with relatively large-volume entrapped melt during diamond ascent.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 10p.
Technology
Sorting
Abstract: The introduction of new diamond sorting technologies as well as additional manufacturers I vendors, has increased the equipment selection and combinations available for recovery flowsheet design. This paper describes the methodology used and the benefits realized in the design of recovery plants by ensuring a fundamental understanding of the advantages and limitations of the technology embodied in the equipment, a comprehensive knowledge of the ore body characteristics, and the effective matching of this information in combination with operational commercial requirements.
Abstract: The paper brings together the language of diamond numbers and the underlying principles for calculation of diamond liberation, followed by estimation of process efficiency at circuit and complete plant levels. In this way it provides a reference point, albeit a mixture of the theoretical and empirical, to assess the effectiveness of diamond plant accounting systems in the field. Having established today's baseline, the wider aim is ongoing education, peer technical debate, and progression to a more exact science.
Zircon geochronology and Sm Nd isotopic study: further constraints for the Archean and Paleoproterozoic geodynamical evolution of southe eastern Guiana Shield.
Abstract: Observing and controlling macroscopic quantum systems has long been a driving force in quantum physics research. In particular, strong coupling between individual quantum systems and mechanical oscillators is being actively studied. Whereas both read-out of mechanical motion using coherent control of spin systems and single-spin read-out using pristine oscillators have been demonstrated, temperature control of the motion of a macroscopic object using long-lived electronic spins has not been reported. Here we observe a spin-dependent torque and spin-cooling of the motion of a trapped microdiamond. Using a combination of microwave and laser excitation enables the spins of nitrogen-vacancy centres to act on the diamond orientation and to cool the diamond libration via a dynamical back-action. Furthermore, by driving the system in the nonlinear regime, we demonstrate bistability and self-sustained coherent oscillations stimulated by spin-mechanical coupling, which offers the prospect of spin-driven generation of non-classical states of motion. Such a levitating diamond-held in position by electric field gradients under vacuum-can operate as a ‘compass’ with controlled dissipation and has potential use in high-precision torque sensing, emulation of the spin-boson problem15 and probing of quantum phase transitions. In the single-spin limit and using ultrapure nanoscale diamonds, it could allow quantum non-demolition read-out of the spin of nitrogen-vacancy centres at ambient conditions, deterministic entanglement between distant individual spins and matter-wave interferometry.
Mollex, G., France, L., Furi, E., Bonnet, R., Botcharnikov, R.E., Zimmermann, L., Wilke, S., Deloule, E., Chazot, G., Kazimoto. E.O., Marty, B., Burnard, P.
Abstract: Cognate xenoliths that have been emitted during the last sub-plinian eruption in 2007-08 at Oldoinyo Lengai (OL) represent a unique opportunity to document the igneous processes occuring within the active magma chamber. Detailed petrographic descriptions coupled to a thermobarometric approach, and to the determination of volatile solubility models, allow us to identify the melt evolution at magma chamber conditions, and the storage parameters (P, T). Results indicate that a fresh phonolite melt (~1060°C) was injected into a crustal magma chamber at 11.5 ±3.5 km depth, in agreement with geophysical surveys performed during the eruption. The phonolite contains high volatile contents: 3.2 wt.% H2O and 1.4 wt.% CO2. The liquid line of descent highlights an evolution to nephelinite compositions by cooling down to 880°C. Our results support previous results related to this eruption, and are similar to the historical products emitted during the whole volcano history, allowing us to suggest that no major modification in the plumbing system has occured during the OL evolution. New noble gas results show that: i. fumaroles display constant He isotopic signature since 1988; ii. Cognate xenoliths documenting the active magma chamber and fumaroles display similar He isotopic values (6.58±0.46RA, and 7.31±0.40RA, respectively); iii. OL He isotopic composition is similar to that of other silicate volcanoes of the Arusha region, and comparable to the typical subcontinental lithospheric mantle (SCLM) range (5.2 to 7.0 RA); iv. Ne isotopic ratio of OL is following the MORB signature. Those results are interpreted as showing that 1/ no major modification in the hydrothermal system architecture has occured since 1988 despite major modification of the summit crater morphology, 2/ no contamination by either the atmospheric gases, or crustal material assimilation has occured between the magma chamber and the surface, and 3/ the source of OL and of the other silicate volcanoes in the Arusha region is a SCLM metasomatized by asthenospheric fluids.
Abstract: Eclogite xenoliths brought to the surface by kimberlites are high pressure mafic rocks whose origin (magmatic vs crustal) remains debated. In addition to disagreement on how to interpret eclogite compositions, mantle metasomatism overprints the mineralogy and geochemistry of some of these rocks, making the question of their protolith undoubtedly more complex. In this contribution we aim to test the robustness of multiple S-isotope signatures in highly metasomatized eclogitic sulfides. We selected 12 interstitial sulfides from Mir (n=4) and Udachnaya (n=8) eclogites, intergrown with garnet and omphacite. We analysed their lead (including Pb204) and S-isotope (32S, 33S, 34S and 36S) compositions, insitu, using a Cameca ims 1280. The samples consist of complex assemblages of pyrrhotite pentlandite intergrowth with K- and Cl-rich sulfides (djerfisherite) invaded by veinlets of alteration minerals (mainly chlorite). All our samples display internal zoning in Pb concentration (118 ppm to 4.2 wt%) but are homogeneous in isotopic compositions (e.g. 208Pb/204Pb = 38.09 ± 0.35‰). Pb-Pb ages of eclogitic sulfides are modern and undoubtedly reflect the metasomatic overprint by a Cl- and K-rich kimberlitic melt (consistent with the presence of djerfisherite). Sulfur isotope signatures of these sulfide (G34S = -1.3‰ ±2‰) fall within the canonical mantle range and cannot be distinguished from the composition of sulfides in the kimberlite (-1.4 ±2.2‰, Kitayama et al., 2016). Furthermore, Mir and Udachanaya eclogitic sulfides carry the largest mass independant fractionation (MIF) ever reported in mantle rocks. The overall trend reveals negative ?33S (down to - 1.1‰) associated to positive ?36S (up to 3‰). This observed correlation between ?33S and ?36S is consistent with the composition of sulfate aerosols formed in the Archean by photolysis reactions and likely dissolved in the ocean [4]. Our results indicate that multiple sulfur isotopes survive intense metasomatism (because isotope fractionation does not create S-MIF), and provide further evidence that the protoliths of Siberian eclogites were mafic rocks altered by seawater in the Archean.
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.
Contributions to Mineralogy and Petrology, Vol. 175, 34 31p. Pdf
Africa, South Africa
carbonatite
Abstract: The Phalaborwa world-class phosphate deposit (South Africa) is hosted by a Paleoproterozoic alkaline complex mainly composed of phoscorite, carbonatite, pyroxenitic rocks, and subordinate fenite. In addition, syenite and trachyte occur in numerous satellite bodies. New petrological and in-situ geochemical data along with O and Sr isotope data obtained on apatite demonstrate that apatite is in the principal host rocks (pyroxenitic rocks, phoscorite and carbonatite) formed primarily by igneous processes from mantle-derived carbonatitic magmas. Early-formed magmatic apatite is particularly enriched in light rare earth elements (LREE), with a decrease in the REE content ascribed to magma differentiation and early apatite fractionation in isolated interstitial melt pockets. Rayleigh fractionation favored a slight increase in ?18O (below 1%) at a constant Sr isotopic composition. Intrusion of fresh carbonatitic magma into earlier-formed carbonatite bodies locally induced re-equilibration of early apatite with REE enrichment but at constant O and Sr isotopic compositions. In fenite, syenite and trachyte, apatite displays alteration textures and LREE depletion, reflecting interaction with fluids. A marked decrease in ?18O in apatite from syenite and trachyte indicates a contribution from ?18O-depleted meteoric fluids. This is consistent with the epizonal emplacement of the satellite bodies. The general increase of the Sr isotope ratios in apatite in these rocks reflects progressive interaction with the country rocks over time. This study made it possible to decipher, with unmatched precision, the succession of geological processes that led to one of the most important phosphate deposits worldwide.
Nature Communications, doi:.org/10.1038/ s41467-020-17442 -8 11p. Pdf
Africa, South Africa, Russia, Siberia
water
Abstract: Trace amounts of water dissolved in minerals affect density, viscosity and melting behaviour of the Earth’s mantle and play an important role in global tectonics, magmatism and volatile cycle. Water concentrations and the ratios of hydrogen isotopes in the mantle give insight into these processes, as well as into the origin of terrestrial water. Here we show the presence of molecular H2 in minerals (omphacites) from eclogites from the Kaapvaal and Siberian cratons. These omphacites contain both high amounts of H2 (70 to 460 wt. ppm) and OH. Furthermore, their ?D values increase with dehydration, suggesting a positive H isotope fractionation factor between minerals and H2-bearing fluid, contrary to what is expected in case of isotopic exchange between minerals and H2O-fluids. The possibility of incorporation of large quantities of H as H2 in nominally anhydrous minerals implies that the storage capacity of H in the mantle may have been underestimated, and sheds new light on H isotope variations in mantle magmas and minerals.
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: Investing in a mining venture can be risky and stakeholders need transparent, unbiased reports to understand the Mineral Resources and Mineral Reserves a mining company holds. Readability and textual choice can be used consciously to manipulate perceptions, or it can be done unconsciously. This exploratory study investigates the readability and textual choice of supplementary Mineral Resources and Mineral Reserves Reports of companies listed on the Johannesburg Stock Exchange. The results indicate that narrative manipulation occurs in these reports through word choices that make the reports difficult to read, as well as specific narrative selections. This reduces the informational value of the reports. The results of the study will be useful to various stakeholders, such as mining company management, investors, investment specialists, financial analysts, and even employees and the general community, who all use these reports to make important decisions. It is also useful for the preparers of the Mineral Resources and Mineral Reserves Reports, Competent Persons, and other technical specialists to be aware of readability and that certain textual choices can affect the interpretation of these reports. It is recommended that bodies such as the JSE and the SAMREC and SAMVAL Code committees consider adding a plain language requirement to regulations, guidelines, and codes to ensure transparent, unbiased, and objective reports.
Geological Society of America Bulletin, Vol. 128, 1/2, pp. 169-186.
Africa, Tanzania
Lineaments
Abstract: The North Tanzanian divergence zone along the East African Rift is characterized by active faults and several large volcanoes such as Meru, Ol Doinyo Lengai, and Kilimanjaro. Based on systematic morphostructural analysis of the Shuttle Radar Topographic Mission digital elevation model and targeted field work, 14 debris avalanche deposits were identified and characterized, some of them being - to our knowledge - previously unknown. Our field survey around Mount Meru allowed previous "lahar" deposits to be reinterpreted as debris avalanche deposits and three major collapse events to be distinguished, with the two older ones being associated with eruptions. We used topographic lineaments and faults across the North Tanzanian divergence zone to derive the main tectonic trends and their spatial variations and highlight their control on volcano collapse orientation. Based on previous analogue models, the tectonic regime is inferred from the orientation of the collapse scars and/or debris avalanche deposits. We infer two types of regime: extensional and transtensional/strike-slip. The strike-slip regime dominates along the rift escarpment, but an extensional regime is inferred to have operated for the recent sector collapses. The proposed interpretation of sector collapse scars and debris avalanche deposits therefore provides constraints on the tectonic regime in the region. It is possible that, in some cases, movement on regional faults triggered sector collapse.
Heterogeneity of the subcontinental mantle: uranium-lead (U-Pb) (U-Pb) and Lu-Hf in megacrysts of baddeleyite and zircon from the Mbuji-Mayi kimberlite
Eos Transactions, Vol. 73, No. 14, April 7, supplement abstracts p.339
Magmatic evolution of the differentiated ultramafic, alkaline and carbonatite intrusion of Vuoriyarvi, Kola Peninsula, Russia, A LA ICP MS study of apatite.
Magmatic evolution of the differentiated ultramafic, alkaline and carbonatite intrusion of Vuoriyarvi ( Kola Peninsula) Russia, A LA-ICP-MS study of apatite.
Pivin, M., Valley, J.W., Spicuzza, M.J., Demaiffe, D.
Oxygen isotopic composition of garnet clinopyroxene and zircon megacrysts from kimberlites in Democratic Republic of Congo: insights into their petrogenesis.
International Mineralogical Association meeting August Budapest, abstract p. 560.
Midende, G., Boulais, P., Tack, L., Melcher, F., Gerdes,A., Dewaele, S., Demaiffe, D., Decree, S.
Petrography, geochemistry and U Pb zircon age of the Matongo carbonatite Massif ( Burundi): implication for the Neoproterozoic geodynamic evolution of Central Africa.
Journal of African Earth Sciences, Vol. 100, pp. 656-674.
Midende, G., Boulvais, P., Tack, L., Melcher, F., Gerdes, A., Dewaele, S., Demaiffe, D., Decree, S.
Petrography, geochemistry and U-Pb zircon age of the Matongo carbonatite Massif ( Burundi): implication for the Neoproterozoic geodynamic evolution of Central Africa.
Journal of African Earth Sciences, Vol. 100, pp. 656-674.
Giuliani, G.,Pivin, M., Fallick, A.E., Ohnenstetter, D., Song, Y., Demaiffe, D.
Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite, Democratic Republic of Congo and the Changle alkali basalt, China.
Comptes Rendus Geoscience, Vol. 347, 1, pp. 24-34.
Feneyrol, J., Giuliani, G., Demaiffe, D., Ohenstetter, D., Fallick, A.E., Dubessy, J., Martelet, J-E., Rakotondrazafy, A.F.M., Omito, E., Ichangi, D., Nyamai, C., Wamunyu, W.
Abstract: The genetic model previously proposed for tsavorite- (and tanzanite-) bearing mineralization hosted in the Neoproterozoic Metamorphic Mozambique Belt (stretching from Kenya through Tanzania to Madagascar) is refined on the basis of new Sm-Nd age determinations and detailed Sr-O-S isotope and fluid-inclusion studies. The deposits are hosted within meta-sedimentary series composed of quartzites, graphitic gneisses, calc-silicate rocks intercalated with meta-evaporites, and marbles. Tsavorite occurs either in nodules (also called “boudins”) oriented parallel to the metamorphic foliation in all of the deposits in the metamorphic belt or in quartz veins and lenses located at the hinges of anticlinal folds (Lelatema fold belt and Ruangwa deposits, Tanzania). Gem tanzanite occurs in pockets and lenses in the Lelatema fold belt of northern Tanzania. The Sm-Nd isotopic data for tsavorites and tanzanites hosted in quartz veins and lenses from Merelani demonstrate that they formed at 600 Ma, during the retrograde metamorphic episode associated with the East African Orogeny. The tsavorites hosted in nodules do not provide reliable ages: their sedimentary protoliths had heterogeneous compositions and their Sm-Nd system was not completely rehomogenized, even at the local scale, by the fluid-absent metamorphic recrystallization. The initial 87Sr/86Sr isotopic ratios of calcite from marble and tanzanites from Merelani fit with the strontium isotopic composition of Neoproterozoic marine carbonates. Seawater sediment deposition in the Mozambique Ocean took place around 720 Ma. The quartz-zoisite O-isotopic thermometer indicates a temperature of formation for zoisite between 385 and 448 °C. The sulfur isotopic composition of pyrite (between –7.8 and –1.3‰ V-CDT) associated with tsavorite in the Lelatema fold belt deposits suggests the contribution of reduced marine sulfate. The sulfur in pyrite in the marbles was likely derived from bacterial sulfate reduction which produced H2S. Fluid inclusion data from tsavorite and tanzanite samples from the Merelani mine indicate the presence of a dominant H2S-S8±(CH4)±(N2)±(H2O)-bearing fluid. In the deposits in Kenya and Madagascar, the replacement of sulfate by tsavorite in the nodules and the boron isotopic composition of tourmaline associated with tsavorite are strong arguments in favor of the participation of evaporites in garnet formation.
Journal of African Earth Sciences, Vol. 145, pp. 274-283.
Africa, Chad
lineaments
Abstract: This work reports an analysis of the relationships existing between the structural lineaments and the Cenozoic volcanism of the Tibesti area (northern Chad). Shield volcanoes, cinder cones, structural lineaments, intersection points of lineaments and faults are mapped using the combination of Shuttle Radar Topography Mission (SRTM), Digital Elevation Models (DEMs) and Landsat satellite images of the Tibesti Volcanic Province. The interpretation of the distribution of these structural and morphological features allows constraining the structural/tectonic setting of the Tibesti. We show that the relationships between the lineaments and the volcanic centres of the Tibesti province can locally be explained as the result of the combination of two Riedel dextral tectonic systems, respectively oriented at N120°E and N30-35°E. Taking into account the geological features of the area, a geodynamical model is proposed: the emplacement of the Tibesti Volcanic Province results from the reactivation of inherited structures of the Saharan metacraton, characterized by relict rigid cratonic nuclei and metacratonic areas reworked during the Pan-African orogeny, among which is located the Tibesti. The contrasted behaviour of these rheologically different zones can explain the location and the evolution of the Tibesti swell and volcanism. The new data presented in this paper and their interpretation in terms of the emplacement of the Tibesti volcanic province in the Saharan metacraton bring a new and major information about the behaviour of the African plate within its collisional context with Europe.
Abstract: The Upper Ruvubu Alkaline Plutonic Complex (URAPC) in Burundi consists of three separate intrusions, each with a specific emplacement age and petrological composition. Three main units are recognized: an outer unit with silica-saturated plutonic rocks (from gabbro to granite), an inner unit with silica-undersaturated plutonic rocks (feldspathoidal syenite with subordinate feldspathoidal monzonite and ijolite) and a carbonatitic body in the subsoil, known by drilling. The URAPC is quite large in size (?24?km long and up to 10?km wide). It is considered to have been intruded syntectonically in an overall extensional context, thanks to the kilometric shear zones that accommodated its emplacement. Radiometric ages from literature range from 748 to 705?Ma and point to structurally-controlled magmatic differentiation followed by long-lived circulations of late-stage fluids postdating the emplacement of a part of the undersaturated rocks and the carbonatites. In the north-western part of the outer unit, gabbro likely has been emplaced at a deeper structural level than the granite, which represents a more apical structural level of emplacement. This petrological, geochemical and isotopic (Sr-Nd-Hf) study concentrates on the processes that generated the URAPC: (i) fractional crystallization, evidenced by the chemical evolution trends of the major and trace elements, and by marked P, Ti and Ba anomalies in the trace element patterns; (ii) crustal assimilation/contamination, as shown by the wide range of Nd isotope compositions and the general increase of the Sr isotope ratios with increasing SiO2 contents, and (iii) late-magmatic/hydrothermal alteration inducing an increase of the Sr isotope composition without changing significantly the Nd isotope composition. The isotopic data are consistent with an asthenospheric mantle source, though less depleted than the Depleted Mantle (DM), contaminated by the Subcontinental Lithospheric Mantle (SCLM). The silicate and carbonate magmatic series are cogenetic. The outer unit is clearly more contaminated than the inner unit, whereas the carbonatitic body could have evolved by liquid immiscibility. The URAPC lies within East Africa’s Western Rift Valley, which is marked by 23 alkaline plutonic complexes. Their emplacement has been ascribed to reactivation of Proterozoic lithospheric weakness zones resulting from the breakup of the Neoproterozoic supercontinent Rodinia supercontinent.
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: The subparallel maflc dykes of the Aorida-Durazno-S.José region (SW Uruguay) trend N60-80W and vary in thickness from 0.6 to 50 m. They are part of the mafic dyke swarms intrudlng granitic-gnelssic basement that were mappecl by BOSSI et ai. (1989), In an ares approximately 200 km In length and 100 km in bresdth. Plagioclass, augite, subcalclc augite (plgeonite) and opaques are the maln components of the dykes. Orthopyroxene and oIlvine are very rare. Blotite and homblende are secondary minerais. Quartz-feldspar Intergrowths occur In the coarser gralnecl dykes. The characterlstlc textures are subophitic and intersertal.
Abstract: We conducted a magnetotelluric (MT) study from Paleoproterozoic Rio de la Plata Craton, in Uruguay, toward Paleozoic?Mesozoic Paraná Basin, in Brazil. The 850?km?long MT transect comprises 35 evenly spaced broadband electromagnetic soundings sites. In the Paraná Basin, 11 additional long?period measurements were acquired to extend the maximum depth of investigation. All data were inverted using two? and three?dimensional approaches obtaining the electrical resistivity structure from the surface down to 200 km. The Rio de la Plata Craton is >200?km thick and resistive (~2,000 ?m). Its northern limit is electrically defined by a lithosphere scale lateral transition and lower crust conductive anomalies (1-10 ?m) interpreted as a Paleoproterozoic suture at the southern edge of Rivera?Taquarembó Block. The latter is characterized by an approximately 100?km thick and moderate resistive (>500 ?m) upper mantle. The Ibaré shear zone is another suture where an ocean?ocean subduction generated the 120?km thick and resistive (>1,000 ?m) São Gabriel juvenile arc. Proceeding northward, a 70? to 80?km thick, 150?km wide, and inclined resistive zone is imaged. This zone could be remnant of an oceanic lithosphere or island arcs accreted at the southern border of Paraná Basin. The MT transect terminates within the southern Paraná Basin where a 150? to 200?km?thick less resistive lithosphere (<1,000 ?m) may indicate refertilization processes during plate subduction and ocean closure in Neoproterozoic?Cambrian time. Our MT data support a tectonic model of NNE-SSW convergence for this segment of SW Gondwanaland.
Abstract: New high-resolution airborne magnetic data of Uruguay allowed constructing new maps concerning the spatial distribution of dyke swarms, main faults and other magnetic bodies, which compose the Uruguayan Shield. We combined geophysical analyses (vertical derivatives, upward continuation, Euler deconvolution), structural analyses of the magnetic maps and previous geological data in order to discriminate the main structural features of the Uruguayan Shield and contribute to a better understanding of its tectonic evolution. The magnetic maps revealed several outstanding features in the Uruguayan Shield. The Paleoproterozoic dyke swarm is larger, denser, more widespread and complex than originally thought, suggesting a possible plume origin. In addition, a new Mesozoic dyke swarm, as complex as the previous one, was identified crosscutting the Paleoproterozoic dyke swarm and the Neoproterozoic orogenic structures. Moreover, this swarm is connected to volcanic calderas in the Merín basin, and shows displacements along Neoproterozoic shear zones, in the magnetic maps, revealing its brittle reactivation during Mesozoic times. The new observations clarify how Proterozoic basement structures controlled the development of the Mesozoic rift. Paleoproterozoic dyke swarms were reactivated as normal faults and Neoproterozoic structures hindered the rift growth, deflecting the deformation in transcurrent movements. Meanwhile, the Mesozoic dyke swarm was developed in a perpendicular direction to the Neoproterozoic structures. Moreover, these findings contradict the current rift model for Uruguay and rise a new model in which the Mesozoic rift developed as two rift basins connected by a central transfer zone, generated by the reactivation of Dom Feliciano Belt structures, between the Sierra Ballena and Sarandí del Yí Shear Zones.
Journal of South American Earth Sciences, Vol. 108, 103206, 17p. Pdf
South America, Uruguay
geophysics
Abstract: The integrated observation of geology, gammaspectrometry and magnetometry of southern Uruguay was made possible by the high-resolution aerogeophysical survey carried out during 2014 and 2015. This survey covers nearly the outcropping area of the Uruguayan basement. Previous studies focused on the Proterozoic and Mesozoic dike swarms present in the Uruguayan basement. In this work we address features previously undescribed and unstudied in the Uruguayan basement. Structures previously grouped with the dike swarm are separated (subcircular structures and a lava river) and the basement general patterns were studied. Distinctive magnetic and radiometric features characterize each structural block in the Uruguayan basement, supporting the main tectonic units previously described, but improving the location of their limits.
Chakhmouradian, A.R., Demeny, A., Reguir, E.P., Hegner, E., Halden, N.M., Yang, P.
'Kimberlite' from Wekusko Lake, Manitoba: re-assessment and implications for further exploration. Beforsite ( primary dolomite carbonatite)... 'notion' could be
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.
Casillas, R., Demeny, A., Nagy, G., Ahijado, A., Fernandez, C.
Metacarbonatites in the Basal Complex of Fuerteventura ( Canary Islands). The role of fluid/rock interactions during contact metamorphism and anatexis.
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.
Soderlund, U., Bleeker, W., Demirer, K., Srivastava, R.K., Hamilton, M., Nilsson, M., Personen, L.J., Samal, A.K., Jayananda, M., Ernst, R.E., Srinivas, M.
Abstract: Large igneous provinces (LIPs) and especially their dyke swarms are pivotal to reconstruction of ancient supercontinents. The Dharwar craton of southern Peninsular India represents a substantial portion of Archean crust and has been considered to be a principal constituent of Superia, Sclavia, Nuna/Columbia and Rodinia supercontinents. The craton is intruded by numerous regional-scale mafic dyke swarms of which only a few have robustly constrained emplacement ages. Through this study, the LIP record of the Dharwar craton has been improved by U-Pb geochronology of 18 dykes, which together comprise seven generations of Paleoproterozoic dyke swarms with emplacement ages within the 2.37-1.79 Ga age interval. From oldest to youngest, the new ages (integrated with U-Pb ages previously reported for the Hampi swarm) define the following eight swarms with their currently recommended names: NE-SW to ESE-WNW trending ca. 2.37 Ga Bangalore-Karimnagar swarm. N-S to NNE-SSW trending ca. 2.25 Ga Ippaguda-Dhiburahalli swarm. N-S to NNW-SSE trending ca. 2.22 Ga Kandlamadugu swarm. NW-SE to WNW-ESE trending ca. 2.21 Ga Anantapur-Kunigal swarm. NW-SE to WNW-ESE trending ca. 2.18 Ga Mahbubnagar-Dandeli swarm. N-S, NW-SE, and ENE-WSW trending ca. 2.08 Ga Devarabanda swarm. E-W trending 1.88-1.89 Ga Hampi swarm. NW-SE ca. 1.79 Ga Pebbair swarm. Comparison of the arcuate trends of some swarms along with an apparent oroclinal bend of ancient geological features, such as regional Dharwar greenstone belts and the late Archean (ca. 2.5 Ga) Closepet Granite batholith, have led to the hypothesis that the northern Dharwar block has rotated relative to the southern block. By restoring a 30° counter clockwise rotation of the northern Dharwar block relative to the southern block, we show that pre-2.08 Ga arcuate and fanning dyke swarms consistently become approximately linear. Two possible tectonic models for this apparent bending, and concomitant dyke rotations, are discussed. Regardless of which deformation mechanisms applies, these findings reinforce previous suggestions that the radial patterns of the giant ca. 2.37 Ga Bangalore-Karimnagar dyke swarm, and probably also the ca. 2.21 Ga Anantapur-Kunigal swarm, may not be primary features.
Abstract: A dike -vein complex of potassic type of alkalinity recently discovered in the Baikal ledge, western Baikal area, southern Siberian craton, includes calcite and dolomite -ankerite carbonatites, silicate-bearing carbonatite, phlogopite metapicrite, and phoscorite. The most reliable 40Ar -39Ar dating of the rocks on magnesioriebeckite from alkaline metasomatite at contact with carbonatite yields a statistically significant plateau age of 1017.4 ± 3.2 Ma. The carbonatite is characterized by elevated SiO2 concentrations and is rich in K2O (K2O/Na2O ratio is 21 on average for the calcite carbonatite and 2.5 for the dolomite -ankerite carbonatite), TiO2, P2O5 (up to 9 wt %), REE (up to 3300 ppm), Nb (up to 400 ppm), Zr (up to 800 ppm), Fe, Cr, V, Ni, and Co at relatively low Sr concentrations. Both the metapicrite and the carbonatite are hundreds of times or even more enriched in Ta, Nb, K, and LREE relative to the mantle and are tens of times richer in Rb, Ba, Zr, Hf, and Ti. The high (Gd/Yb)CN ratios of the metapicrite (4.5 -11) and carbonatite (4.5 -17) testify that their source contained residual garnet, and the high K2O/Na2O ratios of the metapicrite (9 -15) and carbonatite suggest that the source also contained phlogopite. The Nd isotopic ratios of the carbonatite suggest that the mantle source of the carbonatite was mildly depleted and similar to an average OIB source. The carbonatites of various mineral composition are believed to be formed via the crystallization differentiation of ferrocarbonatite melt, which segregated from ultramafic alkaline melt.
Doklady Earth Sciences, Vol. 471, 1, pp. 1104-1207.
Russia
Deposit - Udachnaya
Abstract: New results of Rb-Sr and Sm-Nd isotope analyses have been obtained on samples of deformed peridotite xenoliths collected from the Udachnaya kimberlite pipe (Yakutia). The data obtained imply two main stages of metasomatic alteration of the lithospheric mantle base matter in the central part of the Siberian Craton. Elevated ratios of Sr isotopes may be considered as evidence of an ancient stage of metasomatic enrichment by a carbonatite melt. The acquired Nd isotope composition together with the geochemistry of the deformed peridotite xenoliths suggests that the second stage of metasomatic alteration took place shortly before formation of the kimberlite melt. The metasomatic agent of this stage had a silicate character and arrived from an asthenosphere source, common for the normal OIB type (PREMA) and the Group-I kimberlite.
Abstract: Two typical lamproitic dykes were found in Noril'sk region of the north-western Siberian Craton, which according to mineralogical, geochemical and isotopic criteria belong to anorogenic, non-diamondiferous type of lamproites. According to the geologic relationships, they cut through the Noril'sk-1 intrusion of the Siberian flood basalt province and thus are younger than ~251 Ma. 40Ar/39Ar dating of the two dykes yielded ages of 235.24 ± 0.19 Ma and 233.96 ± 0.19 Ma, showing that they were emplaced in Carnian of the Late Triassic, about 16 Ma after the flood basalt event. There are some indications that there were multiple lamproitic dyke emplacements, including probably emplacement of diamondiferous lamproites, which produced Carnian-age diamond-rich placer deposits in other parts of the Siberian Craton and in adjacent regions. Lead isotope modelling shows that the source of the studied lamproites was formed with participation of recycled crust, which underwent modification of its U/Pb ratio as early as 2.5 Ga. However, the exactmechanismof the recycling cannot be deciphered now. It could be either through delamination of the cratonic crust or subduction of amix of ancient terrigenous sediments into the mantle transition zone.
Abstract: A classification suggested for alkaline ultramafic rocks of the Ary-Mastakh and Staraya Rechka fields, Northern Anabar Shield, is based on the modal mineralogical composition of the rocks and the chemical compositions of their rock-forming and accessory minerals. Within the framework of this classification, the rocks are indentified as orangeite and alkaline ultramafic lamprophyres: aillikite and damtjernite. To estimate how much contamination with the host rocks has modified their composition when the diatremes were formed, the pyroclastic rocks were studied that abound in xenogenic material (which is rich in SiO2, Al2O3, K2O, Rb, Pb, and occasionally also Ba) at relatively low (La/Yb)PM, (La/Sm)PM, and not as much also (Sm/Zr)PM and (La/Nb)PM ratios. The isotopic composition of the rocks suggests that the very first melt portions were of asthenospheric nature. The distribution of trace elements and REE indicates that one of the leading factors that controlled the diversity of the mineralogical composition of the rocks and the broad variations in their isotopic-geochemical and geochemical characteristics was asthenosphere-lithosphere interaction when the melts of the alkaline ultramafic rocks were derived. The melting processes involved metasomatic vein-hosted assemblages of carbonate and potassic hydrous composition (of the MARID type). The alkaline ultramafic rocks whose geochemistry reflects the contributions of enriched vein assemblages to the lithospheric source material, occur in the northern Anabar Shield closer to the boundary between the Khapchan and Daldyn terranes. The evolution of the aillikite melts during their ascent through the lithospheric mantle could give rise to damtjernite generation and was associated with the separation of a C-H-O fluid phase. Our data allowed us to distinguish the evolutionary episodes of the magma-generating zone during the origin of the Triassic alkaline ultramafic rocks in the northern Anabar Shield.
Abstract: The Siberian Craton was assembled in a Paleoproterozoic episode at about 1.88?Ga by the collision of older blocks, followed at about 1.86?Ga by post-collisional felsic magmatism. We have found a set of extremely fresh mica-bearing lamprophyre-looking rocks within the Sharyzhalgay metamorphic complex of the south-western Siberian Craton. Zircon from these rocks yields a UPb TIMS age of 1864.7?±?1.8?Ma, which coincides perfectly with the peak of the post-collisional granite ages and postdates by ~15?Ma the peak of ages obtained for metamorphism. The same ages were reported earlier for a mafic dyke with ocean island basalt (OIB) geochemical signatures and a Pt-bearing mafic-ultramafic intrusion found in the same region. Mineralogy, major and trace element geochemistry and Sr-Nd-Pb isotopes show that the studied rocks (1) have shoshonitic affinity, (2) are hybrid rocks with mineral assemblages which could not be in equilibrium, (3) where derived by recycling of an Archean crustal source and (4) resemble post-collision Tibetan shoshonitic series. The genesis of these rocks is considered to be due to melting of crustal lithologies and metasomatized lithospheric mantle within a subducted slab. Some of the resulting melts ascended through the lithospheric column and fractionated to low-Mg absarokites, whereas other melts were contaminated by orthopyroxenitic mantle material and attained unusual high-Mg mafic compositions. According to our model, the post-collisional magmatism (shoshonite- and OIB-type) occurred due to upwelling of hot asthenosphere through a slab window, when the active collision ceased as a result of the slab break off and loss of the slab pull force. Overall, our study shows that in the Paleoproterozoic shoshonitic melts were emplaced within a similar tectonic setting as seen today in modern orogenic systems.
Russian Geology and Geophysics, Vol. 62, pp. 605-618.
Russia
deposit - Aikhal
Abstract: A dike of rock similar in composition to carbonatites has been found in the Aikhal diamondiferous pipe of the Alakit-Markha field of the Yakutian kimberlite province (YaKP). The fine-grained rock of essentially carbonate composition (dolomite and calcite) rich in thin-platy phlogopite contains minerals typical of carbonatites: monazite, baddeleyite, and pyrochlore. In the high contents and distribution of incompatible elements the rock differs significantly from kimberlites and is transitional from kimberlites to carbonatites. The content of incompatible elements in this rock is 3-5 times lower than that in carbonatite breccias of the pipes in the Staraya Rechka kimberlite field of the YaKP (Nomokhtookh site). The compositions of accessory trace element minerals from the Aikhal dike rock and the Nomokhtookh carbonatite breccias are compared. An assumption is made that the high contents of incompatible elements in the carbonatite-like rock, which caused the crystallization of accessory minerals, are due to the differentiation of kimberlite melt/fluid. The high Sr isotope ratios indicate that the rock altered during hydrothermal and metasomatic processes. The obtained data on the composition of the carbonatite-like rock cannot serve as an argument for the genetic relationship between the Aikhal kimberlites and typical carbonatites. The genetic relationship between kimberlites and carbonatites in the northern fields of the YaKP remains an open issue.
Abstract: To test if hydrogen incorporation by ionic diffusion can occur between a volatile-rich kimberlitic liquid and forsterite, results of high-pressure and high-temperature experiments using a piston-cylinder apparatus at 1200–1300 °C and 1 GPa for durations of 1 min, 5 h, and 23 h, are reported here. Kim-berlitic liquid in the system CaO-MgO-Al 2 O 3-SiO 2-CO 2-H 2 O and synthetic forsterite single crystals were chosen as a first simplification of the complex natural kimberlite composition. Unpolarized Fourier transform infrared spectroscopy was used to quantify the concentrations of OH in the crystallographically oriented forsterite. Scanning electron microscopy, electron backscattered diffraction, electron microprobe analyses, and transmission electron microscopy were performed to identify the run products. After 5 and 23 h, a forsterite overgrowth crystallized with the same orientation as the initial forsterite single crystal. The kimberlitic liquid has crystallized as micrometer-scale euhedral forsterite neocrystals with random crystallographic orientations, as well as a nanoscale aluminous phase and a calcic phase. Despite theoretical water-saturation of the system and long duration, none of the initial forsterite single crystals display signs of hydration such as hydrogen diffusion profile from the border toward the center of the crystal. Most likely, the presence of CO 2 in the system has lowered the H 2 O fugacity to such an extent that there is no significant hydration of the starting forsterite single crystal or its overgrowth. Also, the presence of CO 2 enhances rapid forsterite crystal growth. Forsterite growth rate is around 2 × 10 8 mm 3 /h at 1250 °C. These experimental results suggest a deep mantle origin of the high OH content found in natural mantle-derived xenoliths transported in kimberlites, as reported from the Kaapvaal craton. In agreement with previous studies, it also points out to the fact that significant hydration must take place in a CO 2-poor environment.
Abstract: Hydrogen as an atomic impurity in mantle minerals is recurrently proposed as a key element impacting significantly on many mantle properties and processes such as melting temperature and mechanical strength. Nevertheless, interpretation based on the natural samples remains weak as we do not have yet a robust world-wild database for hydrogen concentrations in mantle minerals and rocks. Here, we report the first hydrogen concentrations in nominally anhydrous minerals from a rare selection of ultramafic rocks and minerals embedded in Mesoproterozoic Wajrakarur kimberlites (Eastern Dharwar craton, India). Based on key chemical elements, we demonstrate that olivine, pyroxenes and garnet from the Dharwar craton are of mantle origin. We quantify the hydrogen concentrations using Fourier transform infrared spectroscopy (FTIR) and mineral-specific FTIR calibrations. Calculated hydrogen concentrations are, in average, 18 ppm wt H2O in olivine, 70 ppm wt H2O in orthopyroxene and 207 ppm wt H2O in clinopyroxene. Garnet has highly variable hydrogen concentration ranging from 0 to 258 ppm wt H2O, probably influenced by nano-scale inclusions. The average of clean garnet spectra yields 14.5 ppm wt H2O. The reconstructed hydrogen bulk concentrations of Dharwar peridotites yields ppm wt H2O. This value is two to five times lower than the estimated hydrogen concentration in the lithospheric mantle, and agree well with the lower range of hydrogen bulk concentration from the current data base for the upper mantle minerals transported by kimberlites from other cratons (e.g., South Africa, Siberia). The low hydrogen concentration in mantle minerals, together with petrological and geochemical evidence of carbonated silicate melt metasomatism in Dharwar cratonic lithospheric mantle, suggest that these xenoliths are possibly related to proto-kimberlite melts with low water activity prior to being transported to the surface by the Mesoproterozoic Wajrakarur kimberlites. These observations, valid to a depth of ~165-km, suggest that cratonic lithosphere beneath the Dharwar craton may not be particularly indicative of an abnormal hydrogen-rich southern Indian lithosphere in the late Archean and that hydroxylic weakening in olivine would induced a negligible effect on the mantle viscosity of Indian subcontinent.
Precambrian Research, Vol. 352, doi.org/1016 /j.precamres .2020.105982 15p. Pdf
India
deposit - Wajrakarur
Abstract: Hydrogen as an atomic impurity in mantle minerals is recurrently proposed as a key element impacting significantly on many mantle properties and processes such as melting temperature and mechanical strength. Nevertheless, interpretation based on the natural samples remains weak as we do not have yet a robust world-wild database for hydrogen concentrations in mantle minerals and rocks. Here, we report the first hydrogen concentrations in nominally anhydrous minerals from a rare selection of ultramafic rocks and minerals embedded in Mesoproterozoic Wajrakarur kimberlites (Eastern Dharwar craton, India). Based on key chemical elements, we demonstrate that olivine, pyroxenes and garnet from the Dharwar craton are of mantle origin. We quantify the hydrogen concentrations using Fourier transform infrared spectroscopy (FTIR) and mineral-specific FTIR calibrations. Calculated hydrogen concentrations are, in average, 18 ppm wt H2O in olivine, 70 ppm wt H2O in orthopyroxene and 207 ppm wt H2O in clinopyroxene. Garnet has highly variable hydrogen concentration ranging from 0 to 258 ppm wt H2O, probably influenced by nano-scale inclusions. The average of clean garnet spectra yields 14.5 ppm wt H2O. The reconstructed hydrogen bulk concentrations of Dharwar peridotites yields ppm wt H2O. This value is two to five times lower than the estimated hydrogen concentration in the lithospheric mantle, and agree well with the lower range of hydrogen bulk concentration from the current data base for the upper mantle minerals transported by kimberlites from other cratons (e.g., South Africa, Siberia). The low hydrogen concentration in mantle minerals, together with petrological and geochemical evidence of carbonated silicate melt metasomatism in Dharwar cratonic lithospheric mantle, suggest that these xenoliths are possibly related to proto-kimberlite melts with low water activity prior to being transported to the surface by the Mesoproterozoic Wajrakarur kimberlites. These observations, valid to a depth of ~165-km, suggest that cratonic lithosphere beneath the Dharwar craton may not be particularly indicative of an abnormal hydrogen-rich southern Indian lithosphere in the late Archean and that hydroxylic weakening in olivine would induced a negligible effect on the mantle viscosity of Indian subcontinent.
Abstract: The search for deposits of lead, zinc, copper, and nickel might soon become much less of a hit-and-miss activity. Instead of trying their luck over wide areas, mining companies should focus their efforts—and billions of dollars in exploration expenses—on the contours of thick, old pieces of lithosphere strewn across Earth’s continents: cratons. Lithospheric thickness can serve as a treasure map, according to Mark Hoggard, an Earth scientist at Harvard University and Columbia University, and his colleagues from the United Kingdom and Australia. They reported their findings in Nature Geoscience.
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.
Zhao, D., Smith, D.G.W. Smith, Zhou, M., Jang, J., Deng, C., Huang, Y.
Yinniugou lamproites in Datong, northern Shanxi Province, Chin a: first occurrence in the North Chin a craton.
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 133-140.
Mo, X., Zhao, Z., Deng, J., Flower, M., Yu, X., Luo, Z., Li, Y., Zhou, S., Deng, G., Zhu, D.
Petrology and geochemistry of post collisional volcanic rocks from the Tibetan plateau: implications for lithosphere heterogeneity and collision induced mantle
Geological Society of America, Special Paper, No. 409, pp. 507-530.
Mo, X., Zhao, Z., Deng, J., Flower, M., Yu, X., Luo, Z., Li, Y., Zhou, S., Deng, G., Zhu, D.
Petrology and geochemistry of post collisional volcanic rocks from the Tibetan plateau: implications for lithosphere heterogeneity and collision induced mantle
Geological Society of America, Special Paper, No. 409, pp. 507-530.
Journal of Geophysical Research: Solid Earth, Vol. 124, pp. 4566-4575.
Mantle
boundary
Abstract: Ultralow?velocity zones (ULVZs) are 5-40?km?thick patches lying above Earth's core-mantle boundary. They are characterized with anomalously low seismic velocities compared with the ambient mantle and may contain important clues on the thermochemical evolution of the Earth. A recent experimental study argued that ULVZs may be caused by the accumulation of pyrite?type FeO2Hx (P phase) at the bottom of the mantle. Here for the first time, we systematically study the thermoelastic properties of both FeO2Hx solid and liquid phases. We find that P phase is likely melted near the core-mantle boundary and thus cannot be the source of ULVZs. Furthermore, in order for the molten product of P phase to cause ULVZs, the dense and nearly inviscid melts must be dynamically stable and confined within the ULVZs, which requires that the mantle is highly viscous and/or convects vigorously.
Geophysical Research Letters, Vol. 46, doi.org/ 10.1029/ 2019GL083633
Mantle
melting
Abstract: Planetary?scale melting is ubiquitous after energetic impacts early in Earth's history. Therefore, determining key melt properties, such as density, is of great significance to better understand Earth's formation and subsequent evolution. In this study, we performed state?of?art first?principles molecular dynamics simulations to examine the density of deep mantle melts, namely, hydrous Fe?rich silicate melts. We find that such hydrous melts can be gravitationally stable near Earth's core?mantle boundary given their likely high iron content. This has great implications for Earth's thermochemical evolution, as well as Earth's volatile cycle.
Geophysical Research Letters, Vol. 46, 16, pp. 9466-9473.
Mantle
water
Abstract: Planetary-scale melting is ubiquitous after energetic impacts early in Earth's history. Therefore, determining key melt properties, such as density, is of great significance to better understand Earth's formation and subsequent evolution. In this study, we performed state-of-art first-principles molecular dynamics simulations to examine the density of deep mantle melts, namely, hydrous Fe-rich silicate melts. We find that such hydrous melts can be gravitationally stable near Earth's core-mantle boundary given their likely high iron content. This has great implications for Earth's thermochemical evolution, as well as Earth's volatile cycle.
Physics of the Earth and Planetary Interiors, in press available, 51p. Pdf
United States, Canada
geophysics - seismics
Abstract: We examined SKS-SKKS splitting intensity discrepancies for phases that sample the lowermost mantle beneath North America, which has previously been shown to exhibit seismic anisotropy using other analysis techniques. We examined data from 25 long-running seismic stations, along with 244 stations of the temporary USArray Transportable Array, located in the eastern, southeastern and western U.S. We identified 279 high-quality SKS-SKKS wave pairs that yielded well-constrained splitting intensity measurements for both phases. Of the 279 pairs, a relatively small number (15) exhibited discrepancies in splitting intensity of 0.4 s or greater, suggesting a contribution to the splitting of one or both phases from anisotropy in the lowermost mantle. Because only a small minority of SK(K)S phases examined in this study show evidence of being affected by lowermost mantle anisotropy, the traditional interpretation that splitting of these phases primarily reflects anisotropy in the upper mantle directly beneath the stations is appropriate. The discrepant pairs exhibited a striking geographic trend, sampling the lowermost mantle beneath the southern U.S. and northern Mexico, while other regions were dominated by non-discrepant pairs. We carried out ray theoretical modeling of simple anisotropy scenarios that have previously been suggested for the lowermost mantle beneath North America, invoking the alignment of post-perovskite due to flow induced by the impingement of the remnant Farallon slab on the core-mantle boundary. We found that our measurements are generally consistent with this model and with the idea of slab-driven flow, but relatively small-scale lateral variations in the strength and/or geometry of lowermost mantle anisotropy beneath North America are also likely present.
Physics of the Earth and Planetary Interiors, Vol. 305, 106504, 15p. Pdf
United States
geophysics - seismics
Abstract: We examined SKS-SKKS splitting intensity discrepancies for phases that sample the lowermost mantle beneath North America, which has previously been shown to exhibit seismic anisotropy using other analysis techniques. We examined data from 25 long-running seismic stations, along with 244 stations of the temporary USArray Transportable Array, located in the eastern, southeastern and western U.S. We identified 279 high-quality SKS-SKKS wave pairs that yielded well-constrained splitting intensity measurements for both phases. Of the 279 pairs, a relatively small number (15) exhibited discrepancies in splitting intensity of 0.4 s or greater, suggesting a contribution to the splitting of one or both phases from anisotropy in the lowermost mantle. Because only a small minority of SK(K)S phases examined in this study show evidence of being affected by lowermost mantle anisotropy, the traditional interpretation that splitting of these phases primarily reflects anisotropy in the upper mantle directly beneath the stations is appropriate. The discrepant pairs exhibited a striking geographic trend, sampling the lowermost mantle beneath the southern U.S. and northern Mexico, while other regions were dominated by non-discrepant pairs. We carried out ray theoretical modeling of simple anisotropy scenarios that have previously been suggested for the lowermost mantle beneath North America, invoking the alignment of post-perovskite due to flow induced by the impingement of the remnant Farallon slab on the core-mantle boundary. We found that our measurements are generally consistent with this model and with the idea of slab-driven flow, but relatively small-scale lateral variations in the strength and/or geometry of lowermost mantle anisotropy beneath North America are also likely present.
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.
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.
Abstract: Preliminary mineralogical and geochemical studies have been carried out on dolomite marble drill cores from the Bayan Obo REE deposit in China. Three types of apatites and four types of monazites have been identified based on textural features: Type 1 apatite occurs as grains with minor monazite (Type 1 monazite) on its border; Type 2 apatite veinlet shows clusters of assemblages with abundant bastnäsite and parisite at the rim; Type 3 apatite has a linear array associated with fluorite and bastnäsite veinlets. Type 2 monazite occurs as clusters intergrowing with parisite and fluorite. Type 3 and 4 monazites occur as polymineralic (fluorite and bastnäsite) and monomineralic veinlets, respectively. These four types of monazites have similar LREE composition but variable Y content (Y2O3 ranging from below determination limits to 0.7?wt%). The three types of apatites also show different REE content and distribution patterns, ranging from high REE abundance (?REE?+?Y: 27243-251789?ppm) and strong LREE enrichment [(La/Yb)CN ?101] in Type 1, less LREE enrichment [(La/Yb)CN ?8] in Type 2 to relatively low REE abundance (?REE?+?Y: 4323-11175?ppm) but high REE fractionation [(La/Yb)CN ?58] in Type 3. The primary apatite has high Sr (5461-6892?ppm) and REE content, implying a carbonatite origin. The late-stage apatites (Types 2 and 3) show different Sr and REE abundances. Significant differences in their Sr composition (6189?±?573, 6041?±?549 and 3492?±?802 for Types 1-3 samples, respectively) and Y/Ho ratio (20.9?±?0.11, 19.5?±?0.17 and 17.4?±?0.37, respectively) indicate that the three types of apatites may have crystallized from different metasomatic fluids. Multi-stage metasomatism resulted in remobilization and redeposition of primary REE minerals to form the Bayan Obo REE deposit.
Contributions to Mineralogy and Petrology, Vol. 171, 7, 20p.
Africa, Tanzania
Tectonics
Abstract: North Tanzanian Divergence is the first stage of continental break-up of East African Rift (<6 Ma) and is one of the most concentrated areas of carbonatite magmatism on Earth, with singular Oldoinyo Lengai and Kerimasi volcanoes. Hanang volcano is the southernmost volcano in the North Tanzanian Divergence and the earliest stage of rift initiation. Hanang volcano erupted silica-undersaturated alkaline lavas with zoned clinopyroxene, nepheline, andradite-schorlomite, titanite, apatite, and pyrrhotite. Lavas are low MgO-nephelinite with low Mg# and high silica content (Mg# = 22.4–35.2, SiO2 = 44.2–46.7 wt%, respectively), high incompatible element concentrations (e.g. REE, Ba, Sr) and display Nb–Ta fractionation (Nb/Ta = 36–61). Major elements of whole rock are consistent with magmatic differentiation by fractional crystallization from a parental melt with melilititic composition. Although fractional crystallization occurred at 9–12 km and can be considered as an important process leading to nephelinite magma, the complex zonation of cpx (e.g. abrupt change of Mg#, Nb/Ta, and H2O) and trace element patterns of nephelinites recorded magmatic differentiation involving open system with carbonate–silicate immiscibility and primary melilititic melt replenishment. The low water content of clinopyroxene (3–25 ppm wt. H2O) indicates that at least 0.3 wt% H2O was present at depth during carbonate-rich nephelinite crystallization at 340–640 MPa and 1050–1100 °C. Mg-poor nephelinites from Hanang represent an early stage of the evolution path towards carbonatitic magmatism as observed in Oldoinyo Lengai. Paragenesis and geochemistry of Hanang nephelinites require the presence of CO2-rich melilititic liquid in the southern part of North Tanzanian Divergence and carbonate-rich melt percolations after deep partial melting of CO2-rich oxidized mantle source.
Doklady Earth Sciences, Vol. 493, 1, pp. 513-516. pdf
Russia, Yakutia
subduction
Abstract: The data available indicate the complex evolution of deformed peridotites of mantle xenoliths, the P-T parameters of which indicate that they are fragments of the metasomatized lower part of the cratonic lithosphere. The zoning established in garnets from xenoliths in kimberlite pipes is interpreted as a result of metasomatism that occurred shortly before xenoliths reached the surface. Metasomatic alterations in xenoliths of deformed harzburgites were manifested not only in the development of zoning of minerals. The study results show that there is a discrepancy between the data calculated based on the contents of incompatible elements in minerals of xenoliths and those obtained due to direct measurements of the bulk composition of xenoliths. To determine the balance of incompatible elements, we have carried out experiments on leaching xenoliths of deformed lherzolites from the Udachnaya kimberlite pipe. It was established that a significant part of LREEs in the studied xenoliths occurs in the intergranular space. The distribution pattern of incompatible elements and, in particular, the presence of a positive Eu anomaly indicate that the appearance of the intergranular component is not associated with contamination of xenoliths by the kimberlite melt. Quite a few xenoliths demonstrate a positive Eu anomaly, which indicates the influence of the subducted crustal component at one of the modification stages of xenoliths.
Geochemistry International, Vol. 59, 8, pp. 743-756. pdf
Russia, Siberia
deposit - Nyurba, Botuoba
Abstract: The paper presents data on garnets from serpentinized peridotite xenoliths in the Nyurba and Botuoba kimberlite pipes of the Nakyn kimberlite field. The major and trace-element compositions of the garnets were analyzed to determine their compositional specifics and genesis. Based on the REE content and chondrite-normalized distribution patterns, the garnets are divided into two types with sinusoidal ((Sm/Er)n > 1) and normal ((Sm/Er)n < 1) REE distribution patterns. In terms of the Y, Zr, Ti, and Eu relations, and the shape of REE distribution pattern, all the garnets correspond to garnets of metasomatized peridotites, except for one sample falling into the field of depleted garnets of harzburgite-dunite paragenesis. The geochemical characteristics of the garnets record two types of metasomatic agents: carbonatite/fluid for type 1 garnets and silicate/melt for type 2 garnets. The carbonatite metasomatic agent produced harzburgitic garnet and its further transformation into lherzolitic garnet. Silicate metasomatism, which led to the formation of the REE pattern of type 2 garnets, likely overprinted two different types of garnets and, respectively, gave two evolutionary trends. These are depleted residual garnets and type 1 garnets previously subjected to carbonatite metasomatism. The low Y and Th contents in combination with the low Ti/Eu ratios in garnets suggest a moderate reworking of lithospheric peridotites by silicate melts, which is consistent with the high diamond grade of the Nakyn kimberlite field.
Proceedings of National Academy of Science USA, Vol. 113, no. 40, pp. 11127-11130.
Mantle
UHP
Abstract: We performed laser-heated diamond anvil cell experiments combined with state-of-the-art electron microanalysis (focused ion beam and aberration-corrected transmission electron microscopy) to study the distribution and valence of iron in Earth's lower mantle as a function of depth and composition. Our data reconcile the apparently discrepant existing dataset, by clarifying the effects of spin (high/low) and valence (ferrous/ferric) states on iron partitioning in the deep mantle. In aluminum-bearing compositions relevant to Earth's mantle, iron concentration in silicates drops above 70 GPa before increasing up to 110 GPa with a minimum at 85 GPa; it then dramatically drops in the postperovskite stability field above 116 GPa. This compositional variation should strengthen the lowermost mantle between 1,800 km depth and 2,000 km depth, and weaken it between 2,000 km depth and the D" layer. The succession of layers could dynamically decouple the mantle above 2,000 km from the lowermost mantle, and provide a rheological basis for the stabilization and nonentrainment of large low-shear-velocity provinces below that depth.
Nature Communications, doe:10.1038/ s41467-018- 030808-6 6p. Pdf
Technology
ureilite
Abstract: Planetary formation models show that terrestrial planets are formed by the accretion of tens of Moon- to Mars-sized planetary embryos through energetic giant impacts. However, relics of these large proto-planets are yet to be found. Ureilites are one of the main families of achondritic meteorites and their parent body is believed to have been catastrophically disrupted by an impact during the first 10 million years of the solar system. Here we studied a section of the Almahata Sitta ureilite using transmission electron microscopy, where large diamonds were formed at high pressure inside the parent body. We discovered chromite, phosphate, and (Fe,Ni)-sulfide inclusions embedded in diamond. The composition and morphology of the inclusions can only be explained if the formation pressure was higher than 20?GPa. Such pressures suggest that the ureilite parent body was a Mercury- to Mars-sized planetary embryo.
Journal of Volcanology and Geothermal Research, Vol. 354, pp. 87-101.
Mantle
magmatism
Abstract: Volcanic eruptions are fed by plumbing systems that transport magma from its source to the surface, mostly fed by dykes. Here we present laboratory experiments that model dyke ascent to eruption using a tank filled with a crust analogue (gelatine, which is transparent and elastic) that is injected from below by a magma analogue (dyed water). This novel experimental setup allows, for the first time, the simultaneous measurement of fluid flow, sub-surface and surface deformation during dyke ascent. During injection, a penny-shaped fluid-filled crack is formed, intrudes, and traverses the gelatine slab vertically to then erupt at the surface. Polarised light shows the internal stress evolution as the dyke ascends, and an overhead laser scanner measures the surface elevation change in the lead-up to dyke eruption. Fluorescent passive-tracer particles that are illuminated by a laser sheet are monitored, and the intruding fluid's flow dynamics and gelatine's sub-surface strain evolution is measured using particle image velocimetry and digital image correlation, respectively. We identify 4 previously undescribed stages of dyke ascent. Stage 1, early dyke growth: the initial dyke grows from the source, and two fluid jets circulate as the penny-shaped crack is formed. Stage 2, pseudo-steady dyke growth: characterised by the development of a rapidly uprising, central, single pseudo-steady fluid jet, as the dyke grows equally in length and width, and the fluid down-wells at the dyke margin. Sub-surface host strain is localised at the head region and the tail of the dyke is largely static. Stage 3, pre-eruption unsteady dyke growth: an instability in the fluid flow appears as the central fluid jet meanders, the dyke tip accelerates towards the surface and the tail thins. Surface deformation is only detected in the immediate lead-up to eruption and is characterised by an overall topographic increase, with axis-symmetric topographic highs developed above the dyke tip. Stage 4 is the onset of eruption, when fluid flow is projected outwards and focused towards the erupting fissure as the dyke closes. A simultaneous and abrupt decrease in sub-surface strain occurs as the fluid pressure is released. Our results provide a comprehensive physical framework upon which to interpret evidence of dyke ascent in nature, and suggest dyke ascent models need to be re-evaluated to account for coupled intrusive and extrusive processes and improve the recognition of monitoring signals that lead to volcanic eruptions in nature.
Abstract: Permian dikes, sills, and diatremes in southern Illinois and northwestern Kentucky (the Omaha, Wildcat Hills, Cottage Grove, Will Scarlet, Williams, Grant, and Clay Lick intrusions) share similar geochemistry and are classified as ultramafic lamprophyres. Major element compositions are 30-35 wt% SiO2, 6-7% Al2O3, 12-14% FeOt, 16-19% MgO, 3-5% TiO2, 11-16% CaO, 0.1-0.7% Na2O, 1.2-2.7% K2O, and 0.4-1.3% P2O5. The Grant Intrusive Breccia is an exception, with lower SiO2, Al2O3, FeOt, MgO, TiO2, and higher CaO. Typically, these rocks are fine grained, with phlogopite, serpentinized olivine ( Fo88), diopside, perovskite, Fe-Ti-spinel, apatite, and calcite. Blocky and lath-shaped pseudomorphs in some samples probably represent melilite, which would make the rocks alnöites. The Grant and Williams diatremes contain sedimentary and igneous clasts (including amphibole megacrysts) within a carbonate-rich matrix. The Grant exhibits pelletal lapilli and is characterized as a lamprophyre?carbonatite tuffisite. Trace element patterns exhibit enrichment of LREE, strong REE fractionation, and relative depletions of K, Sr, Zr, and Hf, closely matching those of the mela-aillikites of Aillik Bay, Labrador. The Grant Intrusive exhibits even greater REE enrichment and notable peaks at Nb, La, and Ce. Geochemical characteristics, including distributions of 143Nd/144Nd and 87Sr/86Sr, are consistent with near-primary melts from a metasomatized peridotite source containing phlogopite-rich veins. Derivation of the lamprophyres from carbonate-rich parental melts similar to the Grant Intrusive could be achieved by separation of carbonatite. A narrow range of initial 87Sr/86Sr (0.70301-0.70449), and initial ?Nd (3.7-5.1), suggests a uniform mantle source close to Bulk Earth. T-depleted mantle model ages range from 540 to 625 Ma, and might correlate with timing of enrichment of a lithospheric mantle source during the breakup of Rodinia.
Abstract: The concept of a mineral deposit forming via a mineral system that operates across areas of perhaps 1000s of squares kilometres and to mantle depths has important implications for greenfields mineral exploration. Geographically widespread datasets and deep penetrating geophysical methods are required to map key mineral system elements such as fluid/metal source zones and migration paths. Developed primarily for academic studies of the deep crust, there are several established geophysical techniques that can potentially be used to identify elements of mineral systems in the deep crust and upper mantle. Although the seismic reflection method produces the highest quality images, it is prohibitively expensive and the recommended approach is a combination of MT surveys and receiver function recordings with CCP stacking. Mineral system elements that can be detected in this fashion include major structures and geological boundaries which are potential controls on fluid flow and also areas of crust and mantle that have been altered by one or both of fluid creation and migration.
Abstract: As mineral exploration seeks deeper targets there will be a greater reliance on geophysical data and a better understanding of the geological meaning of the responses will be required, and this must be achieved with less geological control from drilling. Also, exploring based on the mineral system concept requires particular understanding of geophysical responses associated with altered rocks. Where petrophysical datasets of adequate sample size and measurement quality are available, physical properties show complex variations, reflecting the combined effects of various geological processes. Large datasets, analysed as populations, are required to understand the variations. We recommend the display of petrophysical data as frequency histograms as the nature of the data distribution is easily seen with this form of display. A petrophysical data set commonly contains a combination of overlapping sub?populations, influenced by different geological factors. To understand the geological controls on physical properties in hard rock environments it is necessary to analyse the petrophysical data not only in terms of the properties of different rock types. It is also necessary to consider the effects of processes such as alteration, weathering, metamorphism, and strain, and variables such as porosity and stratigraphy. To address this complexity requires that much more supporting geological information be acquired than is current practice. The widespread availability of field portable instruments means quantitative geochemical and mineralogical data can now be readily acquired, making it unnecessary to rely primarily on categorical rock classification schemes. The petrophysical data can be combined with geochemical, petrological and mineralogical data to derive explanations for observed physical property variations based not only on rigorous rock classification methods, but also in combination with quantitative estimates of alteration and weathering. To understand how geological processes will affect different physical properties it is useful to define three end?member forms of behaviour. Bulk behaviour depends on the physical properties of the dominant mineral components. Density and, to a lesser extent, seismic velocity show such behaviour. Grain and texture behaviour occur when minor components of the rock are the dominate controls on its physical properties. Grain size and shape control grain properties, and for texture properties the relative positions of these grains are also important. Magnetic and electrical properties behave in this fashion. Thinking in terms of how geological processes change the key characteristics of the major and minor mineralogical components allows the resulting changes in physical properties to be understood and anticipated.
Abstract: The southeast Mojave Desert hosts one of the world’s largest rare earth element (REE) deposits at Mountain Pass, California. Although surface geology has been studied, a full understanding of the carbonatite and associated intrusive suite complex requires subsurface geophysical characterization. In this study, a combination of geophysical methods, including magnetotelluric (MT), magnetics, and gravity are used to create a two-dimensional (2D) geophysical model to a depth of about 10 km. An electrically conductive body is found 2-3 km below and west of the deposit that is associated with a magnetic high that could be connected to a deeper (10 km) conductive body related to possible intrusions or hydrothermal systems. The carbonatite body coincides with a steep magnetic gradient and a bench or terrace in the gravity data that may reflect relative lower-density intrusive rocks. Although carbonatite rocks are typically magnetic, the carbonatite rocks, associated intrusive suite, and host rocks in this area are essentially non-magnetic. Combined geophysical data indicate that the enriched REE deposit may be related to a regional extensive hydrothermal alteration event.
Paleomagnetic geochemical and U Pb geochronological study of Proterozoic dykes in Greenland and Arctic Canada and their role in plate tectonic reconstruction.
GAC Annual Meeting Halifax May 15-19, Abstract 1p.
Abstract: The Archean Yilgarn Craton in Western Australia is intruded by numerous mafic dykes of varying orientations, which are poorly exposed but discernible in aeromagnetic maps. Previous studies have identified two craton-wide dyke swarms, the 2408?Ma Widgiemooltha and the 1210?Ma Marnda Moorn Large Igneous Provinces (LIP), as well as limited occurrences of the 1075?Ma Warakurna LIP in the northern part of the craton. We report here a newly identified NW-trending mafic dyke swarm in southwestern Yilgarn Craton dated at 1888?±?9?Ma with ID-TIMS U-Pb method on baddeleyite from a single dyke and at 1858?±?54?Ma, 1881?±?37 and 1911?±?42?Ma with in situ SHRIMP U-Pb on baddeleyite from three dykes. Preliminary interpretation of aeromagnetic data indicates that the dykes form a linear swarm several hundred kilometers long, truncated by the Darling Fault in the west. This newly named Boonadgin dyke swarm is synchronous with post-orogenic extension and deposition of granular iron formations in the Earaheedy basin in the Capricorn Orogen and its emplacement may be associated with far field stresses. Emplacement of the dykes may also be related to initial stages of rifting and formation of the intracratonic Barren Basin in the Albany-Fraser Orogen, where the regional extensional setting prevailed for the following 300?million years. Recent studies and new paleomagnetic evidence raise the possibility that the dykes could be part of the coeval 1890?Ma Bastar-Cuddapah LIP in India. Globally, the Boonadgin dyke swarm is synchronous with a major orogenic episode and records of intracratonic mafic magmatism on many other Precambrian cratons.
Abstract: The Indian Shield is cross-cut by a number of distinct Paleoproterozoic mafic dyke swarms. The density of dykes in the Dharwar and Bastar Cratons is amongst the highest on Earth. Globally, boninitic dyke swarms are rare compared to tholeiitic dyke swarms and yet they are common within the Southern Indian Shield. Geochronology and geochemistry are used to constrain the petrogenesis and relationship of the boninitic dykes (SiO2?=?51.5 to 55.7?wt%, MgO?=?5.8 to 18.7?wt%, and TiO2?=?0.30?wt% to 0.77?wt%) from the central Bastar Craton (Bhanupratappur) and the NE Dharwar Craton (Karimnagar). A single U-Pb baddeleyite age from a boninitic dyke near Bhanupratappur yielded a weighted-mean 207Pb/206Pb age of 2365.6?±?0.9?Ma that is within error of boninitic dykes from the Dharwar Craton near Karimnagar (2368.5?±?2.6?Ma) and farther south near Bangalore (2365.4?±?1.0?Ma to 2368.6?±?1.3?Ma). Rhyolite-MELTS modeling indicates that fractional crystallization is the likely cause of major element variability of the boninitic dykes from Bhanupratappur whereas trace element modeling indicates that the primary melt may be derived from a pyroxenite mantle source near the spinel-garnet transition zone. The Nd isotopes (?Nd(t)?=??6.4 to +4.5) of the Bhanupratappur dykes are more variable than the Karimnagar dykes (?Nd(t)?=??0.7 to +0.6) but they overlap. The variability of Sr-Nd isotopes may be related to crustal contamination during emplacement or is indicative of an isotopically heterogeneous mantle source. The chemical and temporal similarities of the Bhanupratappur dykes with the dykes of the Dharwar Craton (Karimnagar, Penukonda, Chennekottapalle) indicate they are members of the same giant radiating dyke swarm. Moreover, our results suggest that the Bastar and Dharwar Cratons were adjacent but likely had a different configuration at 2.37?Ga than the present day. It is possible that the 2.37Ga dyke swarm was related to a mantle plume that assisted in the break-up of an unknown or poorly constrained supercontinent.
Abstract: The NNW trending tholeiitic Sonakhan mafic dyke swarm of the Northern Bastar Craton is comprised of basalt to basaltic andesite (SiO2?=?46.3?wt% to 55.3?wt%; Mg#?=?37 to 70) dykes. A single basaltic dyke yielded a weighted-mean 207Pb/206Pb baddeleyite age of 1851.1?±?2.6?Ma. The Sr and Nd isotopes (87Sr/86Sri?=?0.70396 to 0.70855; ?Nd(t)?=??5.7 to +2.0) are variable which is a consequence of crustal contamination. Trace element modeling suggests the dykes were likely derived by partial melting of a spinel-bearing mantle source. The Sonakhan dykes are 30 million years younger than the 1.88?Ga Bastar-Cuddapah dykes (Bastanar-Hampi swarm) of the southern and central Bastar Craton indicating they represent a distinct period of magmatism. However, much like the 1.88?Ga dykes, the Sonakhan dykes appear to be correlative with dykes from the Yilgarn Craton (Yalgoo dyke?=?1854?±?5?Ma) of Western Australia. The temporal and compositional similarity of the Sonakhan dykes with the Yalgoo dyke is evidence that they are petrologically related and may represent different branches of the same dyke swarm. The existence of two distinct Paleoproterozoic dyke swarms in the Bastar Craton that each have a correlative unit in the Yilgarn Craton is supportive of a link between India and Australia before 1.9?Ga. Moreover, it suggests that the break-up of India and Western Australia was protracted and lasted for at least 30 million years.
The continental crustal age distribution: methods of determining mantle seperation ages from samarium-neodymium (Sm-Nd) isotopic dat a and application to the southwest UnitedStates
Journal of Geophysical Research, Pt. B., Vol. 96, No. B2, February 10, pp. 2071-2088
neodymium and Strontium isotope chronostratigraphy of Colorado Plateau lithosphere: implications for magmatic and tectonic underplating of the continental crust.
Earth and Planetary Science Letters, Vol. 116, No. 1/4, April pp. 23-44.
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.
Abstract: Both mantle-derived clinopyroxene and orthopyroxene are generally homogeneous in water concentration, while water content in the coexisting olivine is affected by partial or complete loss during the ascent of the hosting magma. Here, we report the first record of water content profiles (higher water in the cores than in the rims) in natural orthopyroxene grains in peridotite xenoliths hosted by Cenozoic alkali basalts in Tianchang volcano, eastern China. The water contents of the coexisting clinopyroxene grains are homogeneous and are twice that measured in the cores of orthopyroxene grains, confirming previous chemical equilibrium between the two pyroxenes. The olivines (ol) are nearly dry (?0 ppm). These observations demonstrate that H diffusion in mantle orthopyroxene (opx) is faster than in clinopyroxene (cpx), and the relative mobility of H in each mineral phase could be quantified as: Graphic (where is the chemical diffusion coefficient of hydrogen). Combining this with experimental diffusion coefficients from the literature, we infer that (1) the xenoliths remained in contact with the magma below 900 °C for several months, and (2) clinopyroxene remains the more reliable recorder of water from depth, and orthopyroxene should be used more cautiously but can be considered with olivine for tracing slow transport and cooling of magma.
Geochemistry, Geophysics, Geosystems, Vol. 19, 5, pp. 1690-1712.
Africa
geomorphology
Abstract: West African drainage reorganization during Cretaceous opening of the Atlantic Ocean is deciphered here from geochemical provenance studies of Central Atlantic sediments. Changes in the geochemical signature of marine sediments are reflected in major and trace element concentrations and strontium?neodymium radiogenic isotopic compositions of Cretaceous sedimentary rocks from eight Deep Sea Drilling Project (DSDP) sites and one exploration well. Homogeneous major and trace element compositions over time indicate sources with average upper (continental) crust signatures. However, detailed information on the ages of these sources is revealed by neodymium isotopes (expressed as ?Nd). The ?Nd(0) values from the DSDP sites show a three?step decrease during the Late Cretaceous: (1) the Albian?Middle Cenomanian ?Nd(0) values are heterogeneous (-5.5 to ?14.9) reflecting the existence of at least three subdrainage basins with distinct sedimentary sources (Hercynian/Paleozoic, Precambrian, and mixed Precambrian/Paleozoic); (2) during the Late Cenomanian?Turonian interval, ?Nd(0) values become homogeneous in the deepwater basin (-10.3 to ?12.4), showing a negative shift of 2 epsilon units interpreted as an increasing contribution of Precambrian inputs; (3) this negative shift continues in the Campanian?Maastrichtian (?Nd(0)?=??15), indicating that Precambrian sources became dominant. These provenance changes are hypothesized to be related to the opening of the South and Equatorial Atlantic Ocean, coincident with tectonic uplift of the continental margin triggered by Africa?Europe convergence. Finally, the difference between ?Nd(0)values of Cretaceous sediments from the Senegal continental shelf and from the deepwater basins suggests that ocean currents prevented detrital material from the Mauritanides reaching deepwater areas.
Journal of Geophysical Research,, Vol. 120, 12, pp. 8259-8280.
Mantle
Ringwoodite
Abstract: Review of recent mineral physics literature shows consistent trends for the influence of Fe and H2O on the bulk modulus (K0) of wadsleyite and ringwoodite, the major phases of Earth's mantle transition zone (410-660?km). However, there is little consensus on the first pressure derivative, K0??=?(dK/dP)P=0, which ranges from about 4 to >5 across experimental studies and compositions. Here we demonstrate the importance of K0? in evaluating the bulk sound velocity of the transition zone in terms of water content and provide new constraints on the effect of H2O on K0? for wadsleyite and ringwoodite by conducting a comparative compressibility study. In the experiment, multiple crystals of hydrous Fo90 wadsleyite containing 2.0 and 0.25?wt?% H2O were loaded into the same diamond anvil cell, along with hydrous ringwoodite containing 1.4?wt?% H2O. By measuring their pressure-volume evolution simultaneously up to 32?GPa, we constrain the difference in K0? independent of the pressure scale, finding that H2O has no effect on K0?, whereas the effect of H2O on K0 is significant. The fitted K0? values of hydrous wadsleyite (0.25 and 2.0?wt?% H2O) and hydrous ringwoodite (1.4?wt?% H2O) examined in this study were found to be identical within uncertainty, with K0? ~3.7(2). New secondary-ion mass spectrometry measurements of the H2O content of these and previously investigated wadsleyite samples shows the bulk modulus of wadsleyite is reduced by 7.0(5)?GPa/wt?% H2O, independent of Fe content for upper mantle compositions. Because K0? is unaffected by H2O, the reduction of bulk sound velocity in very hydrous regions of transition zone is expected to be on the order of 1.6%, which is potentially detectible in high-resolution, regional seismology studies.
Abstract: Carbon has been suggested as one of the light elements existing in the Earth's core. Under core conditions, iron carbide Fe7C3 is likely the first phase to solidify from a Fe-C melt and has thus been considered a potential component of the inner core. The crystal structure of Fe7C3, however, is still under debate, and its thermoelastic properties are not well constrained at high pressures. In this study, we performed synchrotron-based single-crystal X-ray diffraction experiment using an externally heated diamond-anvil cell to determine the crystal structure and thermoelastic properties of Fe7C3 up to 80 GPa and 800 K. Our diffraction data indicate that Fe7C3 adopts an orthorhombic structure under experimentally investigated conditions. The pressure-volume-temperature data for Fe7C3 were fitted by the high-temperature Birch-Murnaghan equation of state, yielding ambient-pressure unit-cell volume V0 = 745.2(2) Å3, bulk modulus K0 = 167(4) GPa, its first pressure derivative K0? = 5.0(2), dK/dT = -0.02(1) GPa/K, and thermal expansion relation ?T = 4.7(9) × 10-5 + 3(5) × 10-8 × (T - 300) K-1. We also observed anisotropic elastic responses to changes in pressure and temperature along the different crystallographic directions. Fe7C3 has strong anisotropic compressibilities with the linear moduli Ma > Mc > Mb from zero pressure to core pressures at 300 K, rendering the b axis the most compressible upon compression. The thermal expansion of c3 is approximately four times larger than that of a3 and b3 at 600 and 700 K, implying that the high temperature may significantly influence the elastic anisotropy of Fe7C3. Therefore, the effect of high temperature needs to be considered when using Fe7C3 to explain the anisotropy of the Earth's inner core.
Earth and planetary Science Letters, Vol. 579, 117359, 11p. Pdf
Mantle
ringwoodite
Abstract: The 520 km discontinuity (the 520) and the 660 km discontinuity (the 660) are primarily caused by the wadsleyite to ringwoodite and ringwoodite to bridgmanite + ferropericlase phase transitions, respectively. Global seismic studies show significant regional variations of the 520, which are likely due to chemical and thermal heterogeneities in the Mantle Transition Zone (MTZ). However, the effects of chemical composition and temperature on the detectability of the 520 are unclear. Additionally, it remains unknown whether the possibly existing metastable ringwoodite in the core of the cold and fast subducting slabs could create a detectable seismic signature near the top of the lower mantle. Our understanding of both issues is hindered by the lack of single-crystal elasticity measurements of ringwoodite at simultaneous high pressure-temperature (P-T) conditions. In this study, we measured the single-crystal elasticity of an anhydrous Fe-bearing ringwoodite up to 32 GPa and 700 K by Brillouin spectroscopy, and then modeled the composition-dependent elastic properties of ringwoodite to calculate the compositional effects on the velocity jumps at the 520. We found that opposite to the effect of Fe, water enhances the Vp (P-wave velocity) jump, yet decreases the Vs (S-wave velocity) jump of the 520 across the wadsleyite to ringwoodite transition. Higher temperature increases both Vp and Vs contrasts across the 520. At depths between 660-700 km in the lower mantle, the existence of metastable ringwoodite may only result in ?1-2% low velocity anomaly, which is seismically difficult to resolve. The low velocity anomaly caused by metastable ringwoodite increases to 5-7% at 750 km depth due to the weak pressure dependence of Vs in ringwoodite at lower mantle conditions, but whether it is seismically detectable depends on the extension of the regions in subducted slabs that are sufficiently cold to host metastable ringwoodite.
Geophysical Research Letters, Vol. 46, 1, pp. 179-188.
Mantle
subduction
Abstract: Eclogite mainly consists of pyrope?almandine?grossular garnet and sodium?rich pyroxene (omphacite) and is a key component of the Earth's upper mantle and oceanic crust. It plays an important role in the mantle convection. The lack of thermoelastic parameters of eclogitic garnets and omphacites hampers accurate modeling of eclogite density at deep?Earth pressure?temperature conditions. In this study, we obtained the thermoelastic parameters of natural eclogitic garnets and omphacites and then modeled the densities of high?Fe and low?Fe eclogites in the subducted oceanic crust and the normal upper mantle. In the upper mantle, eclogite enhances the slab subduction into the transition zone; however, the presence of the metastable low?Fe eclogite would promote the slab stagnation within the upper range of the transition zone. Additionally, eclogite can explain positive density anomalies at depths of 100-200 km of the upper mantle of Asia identified by seismic observations.
Journal of Synchrotron Radiation, Vol. 26, doi.org/10.1107 /S1600577519006854 6p. Pdf
Mantle
diamond inclusions
Abstract: Mineral inclusions in natural diamond are widely studied for the insight that they provide into the geochemistry and dynamics of the Earth's interior. A major challenge in achieving thorough yet high rates of analysis of mineral inclusions in diamond derives from the micrometre-scale of most inclusions, often requiring synchrotron radiation sources for diffraction. Centering microinclusions for diffraction with a highly focused synchrotron beam cannot be achieved optically because of the very high index of refraction of diamond. A fast, high-throughput method for identification of micromineral inclusions in diamond has been developed at the GeoSoilEnviro Center for Advanced Radiation Sources (GSECARS), Advanced Photon Source, Argonne National Laboratory, USA. Diamonds and their inclusions are imaged using synchrotron 3D computed X-ray microtomography on beamline 13-BM-D of GSECARS. The location of every inclusion is then pinpointed onto the coordinate system of the six-circle goniometer of the single-crystal diffractometer on beamline 13-BM-C. Because the bending magnet branch 13-BM is divided and delivered into 13-BM-C and 13-BM-D stations simultaneously, numerous diamonds can be examined during coordinated runs. The fast, high-throughput capability of the methodology is demonstrated by collecting 3D diffraction data on 53 diamond inclusions from Juína, Brazil, within a total of about 72 h of beam time.
Composition and thermal structure of the lithosphere beneath the Ethiopian plateau: evidence from mantle xenoliths in basanites, Injibara Lake Tana Province.
Mineralogy and Petrology, Vol. 93, 1-2, pp. 47-78.
Pure and Applied Physics, Vol. 175, 6, pp. 2141-2151.
Canada, Northwest Territories
mining
Abstract: Kimberlite is an igneous rock which sometimes bears diamonds. Most of the diamonds mined in the world today are found in kimberlite ores. Burst potential in kimberlite has not been investigated, because kimberlite is mostly mined using open-pit mining, which poses very little threat of rock bursting. However, as the mining depth keeps increasing, the mines convert to underground mining methods, which can pose a threat of rock bursting in kimberlite. This paper focuses on the burst potential of kimberlite at a diamond mine in northern Canada. A combined model with the methods of principal component analysis (PCA) and fuzzy comprehensive evaluation (FCE) is developed to process data from 12 different locations in kimberlite pipes. Based on calculated 12 fuzzy evaluation vectors, 8 locations show a moderate burst potential, 2 locations show no burst potential, and 2 locations show strong and violent burst potential, respectively. Using statistical principles, a Mahalanobis distance is adopted to build a comprehensive fuzzy evaluation vector for the whole mine and the final evaluation for burst potential is moderate, which is verified by a practical rockbursting situation at mine site.
From subcalcic pyropes to uvarovites: experimental crystalllization of Cr-rich garnets in ultramafic systems with presence of Ca bearing hydrous fluid.
European Journal of Mineralogy, Vol. 28, pp. 329-336.
Russia
Deposit - Udachnaya
Abstract: An experimental simulation of serpentine and chromite interaction was conducted at the pressure (P) and temperature (T) conditions of garnet-peridotite stability in order to clarify the potential role of serpentinite as a source for the crystallization of subcalcic garnet in the depleted subcratonic mantle. The experiments were performed at 4 GPa and 1100 C and 5.5 GPa and 1200 C using the high-pressure apparatus BARS. Natural antigorite from ophiolites of the Eastern Sayan (Russia) was used as a starting material. Two groups of chromite grains with different Cr# ¼ 100Cr/(Cr þ Al) ratios (from spinel peridotite xenoliths from the Udachnaya kimberlite pipe, Yakutia) were added to the antigorite. Newly formed garnet, spinel, olivine and orthopyroxene were observed as the products in the experiments. Garnet formed only around chromite grains with the lower Cr# value (46.4). Garnet has low CaO contents (,0.05 -1.10 wt.%) with chromium contents showing wide intra- and inter-grain variations (Cr# ¼ 0.7 -33.5). The Cr content increases from core to rim with the outer zones corresponding most closely to the equilibrium composition of the relevant bulk composition. The garnet total FeO content is in the range 3.4 -5.8 wt.%. The experiments demonstrate that serpentinite decomposed at a temperature of 700 C to olivine þ orthopyroxene þ water. If mingled mechanically with spinel-bearing mantlewedge peridotite upon subduction, it could react to form the range of subcalcic garnet compositions found as inclusions in diamonds.
Moscow University Bulletin, Vol. 76, 5, pp. 471-481.
South America, Brazil, Russia, India, China
legal
Abstract: The mineral resource sectors of BRICS countries complement each other perfectly; one of the possible areas for their cooperation in this field is the expansion of mutual trade in mineral commodities and metals in order to provide continuous supplies and price stability. In 2006-2018, the principal beneficiaries of such cooperation were Republic of South Africa and Brazil, which managed to sharply increase their exports of mineral commodities. At the same time, close cooperation with these countries allowed China to become the largest purchaser of mineral commodities and metals in the global market, to ensure continuous supplies and price stability, and to obtain access to mineral resources of the other countries from the organization. However, the expectations of future cooperation among BRICS countries relating to regulation of the global market of mineral resources were to be too high for a number of reasons.
Abstract: Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry, and electromagnetic data can identify contemporary melt zones, magma reservoirs, and, or, crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs), and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community.
The Proterozoic sulphide-alteration pipe of Sidi Flah and its host series.New dat a for the geotectonic evolution of the Pan-African belt in the eastern Anti-Atlas (
Ore Geology Reviews, Vol. 6, No. 6, December pp. 501-536
Abstract: Knowledge of the Guiana Shield evolution during the Gondwana break-up is key to a better understanding of craton dynamics and margin response to transtensional opening. To improve this knowledge, we investigated the dynamics and thermal evolution of French Guiana, using several low-temperature thermochronology methods applied to basement rocks, including apatite and zircon (U-Th)/He and apatite fission tracks. Inverse modelling of results allows us to reconstruct the Phanerozoic thermal history of French Guiana margin and to give a preview of the Guiana Shield evolution. Three main events are inferred: firstly, a long-term period of relative stability since ~1.2 Ga, with no strong evidence for any erosional or burial event (>5-7 km); secondly, a heating phase between ~210 and ~140 Ma consistent with the Central Atlantic Magmatic Province-related event. Finally, an exhumation phase between ~140 and ~90 Ma, triggered by the Equatorial Atlantic opening, brought samples close to the surface (<40°C).
Abstract: "Super-deep" diamonds are thought to crystallize between 300 and 800 km depth because some of the inclusions trapped within them are considered to be the products of retrograde transformation from lower mantle or transition zone precursors. In particular, single inclusion CaSiO3-walstromite is believed to derive from CaSiO3-perovskite, although its real depth of origin has never been proven. Our aim is therefore to determine for the first time the pressure of formation of the diamond-CaSiO3-walstromite pair by “single-inclusion elastic barometry” and to determine whether CaSiO3-walstromite derives from CaSiO3-perovskite or not. We investigated several single phases and assemblages of Ca-silicate inclusions still trapped in a diamond coming from Juina (Brazil) by in-situ analyses (single-crystal X-ray diffraction and micro-Raman spectroscopy) and we obtained a minimum entrapment pressure of ~ 5.7 GPa (? 180 km) at 1500 K. However, the observed coexistence of CaSiO3-walstromite, larnite (?-Ca2SiO4) and CaSi2O5-titanite in one multiphase inclusion within the same diamond indicates that the sample investigated is sub-lithospheric with entrapment pressure between ~ 9.5 and ~ 11.5 GPa at 1500 K, based on experimentally-determined phase equilibria. In addition, thermodynamic calculations suggested that, within a diamond, single inclusions of CaSiO3-walstromite cannot derive from CaSiO3-perovskite, unless the diamond around the inclusion expands by ~ 30% in volume.
International Journal of Luminescence and Applications, Vol. 5, 3, pp. 293-297.
Technology
Luminescence
Abstract: Some of the minerals like Corundum, chryso beryl, beryllium alumino silicate (emerald) and also Diamond exhibit exceptional optical properties[1] and in some cases attractive colours; in India these were recognized quite early since the days of Indus valley civilization. In more recent times there has been a lot of scientific interest in colours and colour modifications in Gem minerals and in Diamonds. Science of gem stones deals with their identification by non destructive means and understanding of origin of colour and excellent optical properties[1]. Optical methods have long been used to obtain properties like ‘Refractive Index’ which still remains an important parameter as a preliminary test to identify the gemstone/mineral. The spectroscopic studies of gem grade minerals are essentially directed towards some of these features in identifying and understanding the spectral properties of chromophores, either chemical impurities and/or radiation induced point defects, in solids. In this context a variety of spectroscopic methods are used to address the problems of the Gem stone identification and identification of origin of colours and colour modification treatments. The methods frequently used in Gem testing labs are the following: (i)Electronic absorption in UV-Visible-NIR range.(ii)UV-Vis excited luminescence, (iii) Vibrational spectra – Absorption in the Infra red range (iv) Vibrational spectra using Light Scattering (Raman spectroscopy) (v)Surface Fluorescence mapping Under deep UV excitation. The present paper deals with the luminescence studies in rubies, sapphires, emeralds and diamonds. Special mention may be made of fluorescence mapping using deep UV excitation (around 205 nm) corresponding to the band gap of diamond. Under such an excitation inter band excitation takes place creating a e-h pair and most of the absorption and subsequent emission being restricted to the surface. This makes surface mapping possible and thereby elucidating the growth patterns. This is invaluable in the diagnostics for the detection of synthetic diamonds. In this introductory presentation on the Luminescence methods in Gemmology, we give a brief account of optical spectroscopic methods which mainly deal with identification of corundum based gem stones (rubies, sapphire) and diamonds including the electronic absorption and luminescence of chromophore centres. In gem testing infrared absorption and Raman scattering methods are main work horses and they will be brought in as and when necessary to give a complete picture.
Abstract: Potential environmental issues associated with the mining of carbonatites are receiving increased attention due to the importance of critical metals for green technologies. This study investigates the chemistry of tailings seepage at the former Saint Lawrence Columbium mine near Oka, Québec, Canada, which produced pyrochlore concentrate and ferroniobium from a carbonatite-hosted Nb-REE deposit. Detailed field sampling and laboratory methods were used to characterize the hydraulic properties of the tailings, their bulk chemistry, mineralogy, pore water and effluent chemistries. The tailings are composed of REE-enriched calcite (64-89 wt %) and fluorapatite (2-22 wt %), as well as biotite (6-17 wt %) and chlorite (0-7 wt %). Minor minerals include ankerite, pyrite, sphalerite, molybdenite, magnetite and unrecovered pyrochlore. Secondary minerals include gypsum, barite, strontianite and rhodochrosite. Geochemical mass balance modeling, constrained by speciation modeling, was used to identify dissolution, precipitation and exchange reactions controlling the chemical evolution of pore water along its flow path through the tailings impoundment. In the unsaturated zone, these reactions include sulfide oxidation and calcite dissolution with acid neutralization. Below the water table, gypsum dissolution is followed by sulfate reduction and FeS precipitation driven by the oxidation of organic carbon in the tailings. Incongruent dissolution of biotite and chlorite releases K, Mg, Fe, Mn, Ba and F and forms kaolinite and Ca-smectite. Cation exchange reactions further remove Ca from solution, increasing concentrations of Na and K. Fluoride concentrations reach 23 mg/L and 8 mg/L in tailings pore water and effluent, respectively. These values exceed Canadian guidelines for the protection of aquatic life. In the mildly alkaline (pH 8.3) pore waters, Mo is highly mobile and reaches an average concentration of 83 ?g/L in tailings effluent, which slightly exceeds environmental guidelines. Concentrations (unfiltered) of Zn reach 1702 ?g/L in tailings pore water although values in effluent are usually less than 20 ?g/L. At the ambient pH, Zn is strongly adsorbed by Fe-Mn oxyhydroxides. Although U forms mobile complexes in tailings pore water, concentrations do not exceed 16 ?g/L due to the low solubility of its pyrochlore host. Adsorption and the low solubility of pyrochlore limit concentrations of Nb to less than 49 ?g/L. Cerium, from calcite dissolution, is strongly adsorbed although it reaches concentrations (unfiltered) in excess of 1 mg/L and 100 ?g/L in pore water and effluent, respectively. Results of this study show that mine tailings from carbonatite deposits are enriched in a wide variety of incompatible elements with multiple mineral hosts of varying solubility. Some of these elements, such as F and Mo, may represent contaminants of concern because of their mobility in alkaline tailings waters.
Journal of Geophysical Research: Planets, doei:10.1029/ 2018JE005698
Mantle
water
Abstract: People have long had curiosity in the origin of Earth's water (equivalently hydrogen). Solar nebula has been given the least attention among existing theories, although it was the predominating reservoir of hydrogen in our early solar system. Here we present a first model for Earth's water origin that quantifies contribution from the solar nebula in addition to that from chondrites, the primary building blocks of Earth. The model considers dissolution of nebular hydrogen into the early Earth's magma oceans and reaction between hydrogen and iron droplets within the magma ocean. Such processes not only delivered countless hydrogen atoms from the mantle to the core but also generated an appreciable difference in hydrogen isotopic composition (2H/1H ratio) between the mantle and core. Fitting the model to current knowledge about Earth's hydrogen produces best combinations of nebular and chondritic contributions to Earth's water. We find that nearly one out of every 100 water molecules on Earth came from the solar nebula. Our planet hides majority of its water inside, with roughly two oceans in the mantle and four to five oceans in the core. These results suggest inevitable formation of water on sufficiently large rocky planets in extrasolar systems.
Earth and Planetary Science Letters, Vol. 554, doi:10.1016/j.epsl.2020.116685
Mantle
geophysics - seismic
Abstract: Here, we provide a reappraisal of potential LLSVPs compositions based on an improved mineralogical model including, for instance, the effects of alumina. We also systematically investigate the effects of six parameters: FeO and Al2O3 content, proportion of CaSiO3 and bridgmanite (so that the proportion of ferropericlase is implicitly investigated), Fe3+/?Fe and temperature contrast between far-field mantle and LLSVPs. From the 81 millions cases studied, only 79000 cases explain the seismic observations. Nevertheless, these successful cases involve a large range of parameters with, for instance, FeO content between 12--25~wt\% and Al2O3 content between 3--17~wt\%. We then apply a principal component analysis (PCA) to these cases and find two robust results: (i) the proportion of ferropericlase should be low (<6vol\%); (ii) the formation of Fe3+-bearing bridgmanite is much more favored than other iron-bearing phases. Following these results, we identify two end-member compositions, Bm-rich and CaPv-rich, and discuss their characteristics. Finally, we discuss different scenarios for the formation of LLSVPs and propose that investigating the mineral proportion produced by each scenario is the best way to evaluate their relevance. For instance, the solidification of a primitive magma ocean may produce FeO and Al2O3 content similar to those suggested by our analysis. However, the mineral proportion of such reservoirs is not well-constrained and may contain a larger proportion of ferropericlase than what is allowed by our results.
Abstract: Hints from seismic tomography and geochemistry indicate that Earth's mantle is heterogeneous at large scale. Numerical simulations of mantle convection show that, if it started enriched in silicates, the lower mantle may remain unmixed today.
Harvey, S., Read, G., DesGagnes, B., Shimell, M., Danoczi, J., Van Breugel, B., Fourie, L., Stilling, A.
Utilization of olivine macrocryst grain size and abundance dat a as a proxy for diamond size and grade in pyroclastic deposits of the Orion South kimberlite Fort a la Corne, Sasakatchewan, Canada.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
Utilization of olivine macrocryst grain size and abundance dat a as a proxy for diamond size and grade in pyroclastic deposits of the Orion South kimberlite, Fort a la Corne, Saskatchewan, Canada.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 79-95.
Harvey, S., Read, G., DesGagnes, B., Shimell, M., van Breugel, B., Fourie, L.
Utilization of olivine macrocryst grain size and abundance dat a as a proxy for diamond size and grade in pyroclastic deposits of the Orion South kimberlite, Fort a la Corne, Saskatchewan, Canada.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, pp. 79-96.
Abstract: The NNW trending tholeiitic Sonakhan mafic dyke swarm of the Northern Bastar Craton is comprised of basalt to basaltic andesite (SiO2?=?46.3?wt% to 55.3?wt%; Mg#?=?37 to 70) dykes. A single basaltic dyke yielded a weighted-mean 207Pb/206Pb baddeleyite age of 1851.1?±?2.6?Ma. The Sr and Nd isotopes (87Sr/86Sri?=?0.70396 to 0.70855; ?Nd(t)?=??5.7 to +2.0) are variable which is a consequence of crustal contamination. Trace element modeling suggests the dykes were likely derived by partial melting of a spinel-bearing mantle source. The Sonakhan dykes are 30 million years younger than the 1.88?Ga Bastar-Cuddapah dykes (Bastanar-Hampi swarm) of the southern and central Bastar Craton indicating they represent a distinct period of magmatism. However, much like the 1.88?Ga dykes, the Sonakhan dykes appear to be correlative with dykes from the Yilgarn Craton (Yalgoo dyke?=?1854?±?5?Ma) of Western Australia. The temporal and compositional similarity of the Sonakhan dykes with the Yalgoo dyke is evidence that they are petrologically related and may represent different branches of the same dyke swarm. The existence of two distinct Paleoproterozoic dyke swarms in the Bastar Craton that each have a correlative unit in the Yilgarn Craton is supportive of a link between India and Australia before 1.9?Ga. Moreover, it suggests that the break-up of India and Western Australia was protracted and lasted for at least 30 million years.
Report on the Preliminary Investigation for Diamonds in The banganapalle Conglomerate and Tungabhadra Krishna River Gravels in Parts of Kurnool and Mahaboolonagar Districts.
India Geological Survey Program Report, FOR 1967-1968
A Report on the Diamond Investigation, Krishna Gravel Areas and Reconnaissance Work in Srisailam and Penner River Areas, Krishna, Kurnool and Cuddapah Districts
India Geological Survey Program Report, FOR 1969-1970
A Report on the Assessment of Diamond Resources, Krishna River Gravels and Reconnaissance Work of Ustapalle and Mellwalli Areas, Krishna District, A.p.
India Geological Survey Program Report, FOR 1970-1971
Abstract: The Saint-Honoré alkaline complex located near the Saguenay River (Grenville Province, Québec) has a syenite outer rim and concentric units of calcio-, magnesio- to ferro-carbonatite moving towards the centre. The Mg-carbonatite hosts a niobium deposit, and the Fe-carbonatite hosts a rare earth-rich zone at its centre. The Nb mineralization has a close spatial relationship to the syenite enclaves suggesting that the syenites may have played a critical role in concentrating the pyrochlore (Pcl). There are two forms of Nb mineralization: high- and low-grade. Low-grade mineralization is characterized by highly variable Pcl chemistry with higher U concentrations and a low abundance of fluoroapatite (Ap), whereas high-grade mineralization has a consistent Pcl chemistry (low-U), abundant Ap (with many acicular crystals) and more abundant phlogopite and magnetite. Some of the Pcl crystals have been altered to columbite by hydrothermal processes. It is interpreted that the metamict Pcl (rich in radioactive elements) was altered more readily than the Pcl having undamaged crystal structure. The high-grade mineralization is generally located near the syenite enclaves. Syenite enclaves (from a centimetre scale to several tens of metres in size) reacted with the carbonatite magma to produce a phlogopite rim. Ap is also abundant along the immediate contact between the enclaves and Mg-carbonatite. Large enclaves show hydro-fracturing by the carbonatite suggesting they were crystalline enough to be brittle. There are smaller textures (3-6 mm in diameter) that share many similarities with the syenite enclaves; however, these textures are rounded and could be interpreted as being related to liquid immiscibility. The interaction of carbonatite magma with syenite enclaves is interpreted to have started with abundant crystallization of acicular Ap which depleted the magma in F and lowered the magma's Nb-solubility. Pcl then crystallized in abundance in the vicinity of the syenite enclaves to create the economic Nb-rich zone.
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.
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.
Watremez, L., Leroy, S., d'Acremont, E., Roche, V., Evain, M., Lepretre, A., Verrier, F., Aslanian, D., Dias, N., Afilhado, A., Schnurle, P., Castilla, R., Despinois, F., Moulin, M.
Abstract: A variety of structures results from the interplay of evolving far-field forces, plate kinematics, and magmatic activity during continental break-up. The east Limpopo transform margin, offshore northern Mozambique, formed as Africa and Antarctica separated during the mid-Jurassic period break-up of the Gondwana supercontinent. The nature of the crust onshore has been discussed for decades in an effort to resolve issues with plate kinematic models. Two seismic refraction profiles with coincident multichannel seismic reflection profiles allow us to interpret the seismic velocity structures across the margin, both onshore and offshore. These seismic profiles allow us to (a) delineate the major regional crustal domains; (b) identify widespread indications of magmatic activity; and (c) map crustal structure and geometry of this magma-rich transform margin. Careful examination of the profiles allows us to make the following observations and interpretations: (a) on land, continental crust is overlain by a >10-km thick volcano-sedimentary wedge related to an early rifting stage, (b) offshore, thick oceanic crust formed due to intense magmatic activity, and between the two (c) a 50-60-km wide transform zone where the crustal structures are affected by intense magmatic activity and faulting. The prominent presence of intrusive and extrusive igneous units may be attributed to the combination of a deep-seated melting anomaly and a trans-tensional fault zone running through thinned lithosphere that allowed melt to reach the surface. A comparison of the crustal thinning along other transform margins shows a probable dependence with the thermal and/or tectonic history of the lithosphere.
Geochemical Perspectives Letters, Vol. 3, pp. 230-237.
Canada, Quebec, Anticosti Island
carbon cycle
Abstract: Chemical weathering of silicate rocks is a primary drawdown mechanism of atmospheric carbon dioxide. The processes that affect weathering are therefore central in controlling global climate. A temperature-controlled “weathering thermostat” has long been proposed in stabilising long-term climate, but without definitive evidence from the geologic record. Here we use lithium isotopes (?7Li) to assess the impact of silicate weathering across a significant climate-cooling period, the end-Ordovician Hirnantian glaciation (~445 Ma). We find a positive ?7Li excursion, suggestive of a silicate weathering decline. Using a coupled lithium-carbon model, we show that initiation of the glaciation was likely caused by declining CO2 degassing, which triggered abrupt global cooling, and much lower weathering rates. This lower CO2 drawdown during the glaciation allowed climatic recovery and deglaciation. Combined, the data and model provide support from the geological record for the operation of the weathering thermostat.
Yellowknife Forum NWTgeoscience.ca, abstract volume poster p.105-106.
Canada, Northwest Territories
drift prospecting
Abstract: In the glaciated terrain of the Northwest Territories, successful diamond exploration projects depend on the implementation of drift prospecting. Drift prospecting combines surficial sediment sampling with an understanding of glacial sediment transport history so that geochemical anomalies can be properly interpreted. However, deglacial meltwater processes that may rework, erode, transport, and deposit previously emplaced till are commonly overlooked or misidentified in sample collection and data interpretation. Exactly how deglacial meltwater processes affect the concentration of kimberlite indicator minerals in glacial sediments is poorly understood. The aim of this study is to determine if syn- and post-depositional meltwater processes affect kimberlite indicator mineral concentrations and distributions. The study area is approximately 225 km2, located in the Winter Lake area, in the southern Slave region. This area was chosen for its multiple subglacial meltwater corridors with numerous meltwater related landforms adjacent to relatively unmodified till. It is a prospective area for kimberlites based on the kimberlite indicator minerals identified during previous till sampling programs. The project incorporates terrain mapping, fieldwork and geochemical analysis. Progress so far includes a desktop study using existing air photos and surficial maps of the region and fieldwork. Stereo image visualization and mapping software (Summit EvolutionTM) combined with digital air photos of the area were utilized to complete a preliminary 1:10 000 scale digital terrain map. Fieldwork was completed in the summer of 2019: the nature and distribution of surficial materials were described, ice flow indicators identified and recorded, and surficial material samples collected. Sampling targeted sediments that experienced varying degrees of meltwater modification; materials collected cover the spectrum from unmodified till to washed till to sorted glaciofluvial sand and gravel. Analysis and interpretation are ongoing. Clast shape and lithology analysis has been completed. Grain size analysis will be completed for the presentation. Samples have been sent to commercial labs for geochemical analysis of the silt and clay fraction as well as heavy mineral separation followed by picking of kimberlite, base metal and gold indicator minerals. Potential kimberlite indicator minerals will be analyzed by electron microprobe to verify the mineralogy; their chemistry will be related to diamond potential. The results of these analysis will not be available in time for the presentation. Field descriptions and photogrammetry indicate that many meltwater corridors contain hummocks and elongate ridges composed of diamicton that is sandier and contains less silt than an unmodified till. The morphology and directionality of these identified landforms suggest they are not esker segments. Comparison of grain size, clast shape and lithology data between till and modified sediments will be related to landform genesis. The observations of surficial materials, landforms and ice flow indicators are being used to update the preliminary 1:10 000 scale terrain map of the area, as well as to interpret the local glacial history of the study area. The results of this project will have significant implications in the planning and execution of diamond exploration programs in the Northwest Territories as well as in effectively interpreting the results of drift prospecting campaigns.
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.
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.
REM: Revista Escola de Minas, Vol. 67, 2, pp. 159-165. *** in Portuguese
South America, Brazil
Alluvials, diamonds
Abstract: A representative set of diamonds from the Borrachudo River (114 stones) was described for the first time as to their physical characteristics, among them weight, morphology, dissolution figures, and abrasion. Most samples are low in weight (~51% less than 0.30 ct) and only ~3.5% are above 3.0 ct. However, considering the total weight, ~47% of it is concentrated in the range above 3 ct. On the other hand, there even occur diamonds with dozen of carats. The most common crystallographic forms are originated from the octahedral form by dissolution, although irregular shapes like chips and flats also occur. The diamonds show differences in form compared to their weight ranges; those up to 0.30 ct have various shapes; in the range of 0.31 to 1 ct the most common are octahedral forms and their descendants; and in stones larger than 1 ct chips and flats predominate. The general habitus of the crystals, identified by their final tetrahexahedroid shapes, chips and flats, corroborated by the dissolution figures, indicate that the diamond were submitted to strong dissolution in a magmatic environment. Residual hillocks and holes represent the final stage of dissolution. The study indicates that the abrasion by the fluvial transport was not expressive enough to cause mechanical wear, thus ca. 97% shows no sign of wear. This fact suggests a proximal source for most of these diamonds.
Earth and Planetary Science Letters, Vol. 432, pp. 159-168.
Europe, Iceland
Mantle plume
Abstract: The presence of a mantle plume beneath Iceland has long been hypothesised to explain its high volumes of crustal volcanism. Practical constraints in seismic tomography mean that thin, slow velocity anomalies representative of a mantle plume signature are difficult to image. However it is possible to infer the presence of temperature anomalies at depth from the effect they have on phase transitions in surrounding mantle material. Phase changes in the olivine component of mantle rocks are thought to be responsible for global mantle seismic discontinuities at 410 and 660 km depth, though exact depths are dependent on surrounding temperature conditions. This study uses P to S seismic wave conversions at mantle discontinuities to investigate variation in topography allowing inference of temperature anomalies within the transition zone. We employ a large data set from a wide range of seismic stations across the North Atlantic region and a dense network in Iceland, including over 100 stations run by the University of Cambridge. Data are used to create over 6000 receiver functions. These are converted from time to depth including 3D corrections for variations in crustal thickness and upper mantle velocity heterogeneities, and then stacked based on common conversion points. We find that both the 410 and 660 km discontinuities are depressed under Iceland compared to normal depths in the surrounding region. The depression of 30 km observed on the 410 km discontinuity could be artificially deepened by un-modelled slow anomalies in the correcting velocity model. Adding a slow velocity conduit of ?1.44% reduces the depression to 18 km; in this scenario both the velocity reduction and discontinuity topography reflect a temperature anomaly of 210 K. We find that much larger velocity reductions would be required to remove all depression on the 660 km discontinuity, and therefore correlated discontinuity depressions appear to be a robust feature of the data. While it is not possible to definitively rule out the possibility of uncorrected velocity anomalies causing the observed correlated topography we show that this is unlikely. Instead our preferred interpretation is that the 660 km discontinuity is controlled by a garnet phase transition described by a positive Clapeyron slope, such that depression of the 660 is representative of a hot anomaly at depth.
Earth and Planetary Science Letters, Vol. 433, pp. 159-168.
Europe, Iceland
Mantle - 660 km
Abstract: The presence of a mantle plume beneath Iceland has long been hypothesised to explain its high volumes of crustal volcanism. Practical constraints in seismic tomography mean that thin, slow velocity anomalies representative of a mantle plume signature are difficult to image. However it is possible to infer the presence of temperature anomalies at depth from the effect they have on phase transitions in surrounding mantle material. Phase changes in the olivine component of mantle rocks are thought to be responsible for global mantle seismic discontinuities at 410 and 660 km depth, though exact depths are dependent on surrounding temperature conditions. This study uses P to S seismic wave conversions at mantle discontinuities to investigate variation in topography allowing inference of temperature anomalies within the transition zone. We employ a large data set from a wide range of seismic stations across the North Atlantic region and a dense network in Iceland, including over 100 stations run by the University of Cambridge. Data are used to create over 6000 receiver functions. These are converted from time to depth including 3D corrections for variations in crustal thickness and upper mantle velocity heterogeneities, and then stacked based on common conversion points. We find that both the 410 and 660 km discontinuities are depressed under Iceland compared to normal depths in the surrounding region. The depression of 30 km observed on the 410 km discontinuity could be artificially deepened by un-modelled slow anomalies in the correcting velocity model. Adding a slow velocity conduit of ?1.44% reduces the depression to 18 km; in this scenario both the velocity reduction and discontinuity topography reflect a temperature anomaly of 210 K. We find that much larger velocity reductions would be required to remove all depression on the 660 km discontinuity, and therefore correlated discontinuity depressions appear to be a robust feature of the data. While it is not possible to definitively rule out the possibility of uncorrected velocity anomalies causing the observed correlated topography we show that this is unlikely. Instead our preferred interpretation is that the 660 km discontinuity is controlled by a garnet phase transition described by a positive Clapeyron slope, such that depression of the 660 is representative of a hot anomaly at depth.
Earth and Planetary Science Letters, Vol. 494, pp. 226-238.
Mantle
perovskite
Abstract: Lower mantle tomography models consistently feature an increase in the ratio of shear-wave velocity () to compressional-wave velocity () variations and a negative correlation between shear-wave and bulk-sound velocity () variations. These seismic characteristics, also observed in the recent SP12RTS model, have been interpreted to be indicative of large-scale chemical variations. Other explanations, such as the lower mantle post-perovskite (pPv) phase, which would not require chemical heterogeneity, have been explored less. Constraining the origin of these seismic features is important, as geodynamic simulations predict a fundamentally different style of mantle convection under both scenarios. Here, we investigate to what extent the presence of pPv explains the observed high ratios and negative - correlation globally. We compare the statistical properties of SP12RTS with the statistics of synthetic tomography models, derived from both thermal and thermochemical models of 3-D global mantle convection. We convert the temperature fields of these models into seismic velocity structures using mineral physics lookup tables with and without pPv. We account for the limited tomographic resolution of SP12RTS using its resolution operator for both and structures. This allows for direct comparisons of the resulting velocity ratios and correlations. Although the tomographic filtering significantly affects the synthetic tomography images, we demonstrate that the effect of pPv remains evident in the ratios and correlations of seismic velocities. We find that lateral variations in the presence of pPv have a dominant influence on the / ratio and - correlation, which are thus unsuitable measures to constrain the presence of large-scale chemical variations in the lowermost mantle. To explain the decrease in the / ratio of SP12RTS close to the CMB, our results favour a pPv-bearing CMB region, which has implications for the stability field of pPv in the Earth's mantle.
Canadian Institute of Mining and Metallurgy, Vol. 6, 3, 14p.
Africa, Botswana
Deposit - Orapa
Abstract: The degree of uncertainty associated with a natural diamond resource is important to quantify from the time of discovery through the production lifetime. Data collection occurs during the discovery, exploration, delineation, and production or recovery phases. Quantifying the relationship between data and uncertainty is an important component of project valuation. The value of data is measured as their potential to reduce uncertainty if they are collected. A method is developed using Monte Carlo simulation for predicting resource uncertainty and valuing data during critical phases of development, particularly discovery and exploration. The technique is applied to diamond pipe deposits.
Geochimica et Cosmochinica Acta, Vol. 213, pp. 346-374.
United States, Colorado
volatiles
Abstract: The Colorado Plateau hosts several large accumulations of naturally occurring, non-hydrocarbon gases, including CO2, N2, and the noble gases, making it a good field location to study the fluxes of these gases within the crust and to the atmosphere. In this study, we present a compilation of 1252 published gas-composition measurements. The data reveal at least three natural gas associations in the field area, which are dominated by hydrocarbons, CO2, and N2 + He + Ar, respectively. Most gas accumulations of the region exhibit compositions that are intermediate between the three end members. The first non-hydrocarbon gas association is characterized by very high-purity CO2, in excess of 75 mol% (hereafter, %). Many of these high-purity CO2 fields have recently been well described and interpreted as magmatic in origin. The second non-hydrocarbon gas association is less well described on the Colorado Plateau. It exhibits He concentrations on the order of 1-10%, and centered log ratio biplots show that He occurs proportionally to both N2 and Ar. Overall ratios of N2 to He to Ar are ?100:10:1 and correlation in concentrations of these gases suggests that they have been sourced from the same reservoir and/or by a common process. To complement the analysis of the gas-composition data, stable isotope and noble-gas isotope measurements are compiled or newly reported from 11 representative fields (previously published data from 4 fields and new data from 7 fields). Gas sampled from the Harley Dome gas field in Utah contains nearly pure N2 + He + Ar. The various compositional and stable and noble gas isotopic data for this gas indicate that noble gas molecule/isotope ratios are near crustal radiogenic production values and also suggest a crustal N2 source. Across the field area, most of the high-purity N2 + He + Ar gas accumulations are associated with the mapped surface trace of structures or sutures in the Precambrian basement and are often accumulated in lower parts of the overlying Phanerozoic sedimentary cover. The high-purity gas association mostly occurs in areas interior to the plateau that are characterized by a narrow range of elevated, moderate heat flow values (53-74 mW/m2) in the ancient (1.8-1.6 Ga) basement terranes of the region. Collectively, the geochemical and geological data suggest that (1) the N2 + He + Ar gas association is sourced from a crustal reservoir, (2) the gas association migrates preferentially along structures in the Precambrian basement, and (3) the sourcing process relates to heating of the crust. Prospecting for noble-gas accumulations may target areas with elevated Cenozoic heat flow, ancient crust, and deep crustal structures that focus gas migration. High-purity CO2 gas may also migrate through regional basement structures, however, there is not always a clear spatial association. Rather, CO2 accumulations are more clearly associated with zones of high heat flow (>63 mW/m2) that sit above hot upper mantle and are proximal to Cenozoic volcanic rocks near the plateau margins. These observations are consistent with previous interpretations of a magmatic gas source, which were based on geochemical measurements.
Relationships between crustal contamination and crystallization in continental flood basalt magmas with special reference to the Deccan Traps Of the Westrn Ghats, In
Earth and Planetary Science Letters, Vol. 84, No. 1, June pp. 59-68
Abstract: The extent to which water and halogens in Earth’s mantle have primordial origins, or are dominated by seawater-derived components introduced by subduction is debated. About 90% of non-radiogenic xenon in the Earth’s mantle has a subducted atmospheric origin, but the degree to which atmospheric gases and other seawater components are coupled during subduction is unclear. Here we present the concentrations of water and halogens in samples of magmatic glasses collected from mid-ocean ridges and ocean islands globally. We show that water and halogen enrichment is unexpectedly associated with trace element signatures characteristic of dehydrated oceanic crust, and that the most incompatible halogens have relatively uniform abundance ratios that are different from primitive mantle values. Taken together, these results imply that Earth’s mantle is highly processed and that most of its water and halogens were introduced by the subduction of serpentinized lithospheric mantle associated with dehydrated oceanic crust.
Earth and Planetary Science Letters, Vol. 413, March 1, pp. 158-166.
Mantle
Geochronology
Abstract: This study investigates the impact of Earth's core formation on the metal-silicate partitioning of Sm and Nd, two rare-earth elements assumed to be strictly lithophile although they are widely carried by the sulphide phases in reducing material (e.g. enstatite chondrites). The partition coefficients of Sm and Nd (DSmDSm and DNdDNd) between molten CI and EH chondrites model compositions and various Fe-rich alloys (in the Fe-Ni-C-Si-S system) have been determined in a multi-anvil between 3 and 26 GPa at various temperatures between 2073 and 2440 K, and at an oxygen fugacity ranging from 1 to 5 log units below the iron-wüstite (IW) buffer. The chemical compositions of the run products and trace concentrations in Sm and Nd elements were determined using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry. Our results demonstrate the non-fractionation of Sm and Nd during the segregation of the metallic phases: the initial Sm/Nd ratio of about 1 in the starting materials yields precisely the same ratio in the recovered silicate phases after the equilibration with the metal phases at all conditions investigated in this study. In addition, DSmDSm and DNdDNd values range between 10?310?3 and 10?510?5 representing a low solubility in the metal. An increase of the partition coefficients is observed with decreasing the oxygen fugacity, or with an increase of S content of the metallic phase at constant oxygen fugacity. Thus, based on the actual Sm and Nd concentrations in the bulk Earth, the core should contain less than 0.4 ppb for Sm and less than 1 ppb for Nd. These estimates are three orders of magnitude lower than what would be required to explain the reported 142Nd excess in terrestrial samples relative to the mean chondritic value, using the core as a Sm-Nd complementary reservoir. In other words, the core formation processes cannot be responsible for the increase of the Sm/Nd ratio in the mantle early in Earth history.
Contributions to Mineralogy and Petrology, Vol. 171, pp. 95-121.
Technology
Melting - peridotite
Abstract: Melting experiments have been performed at 3 GPa, between 1150 and 1450 °C, on a phlogopite-peridotite source in the garnet stability field. We succeeded to extract and determine the melt compositions of both phlogopite-bearing lherzolite and harzburgite from low to high degrees of melting (? = 0.008-0.256). Accounting for the presence of small amounts of F in the mantle, we determined that phlogopite coexists with melt >150 °C above the solidus position (1150-1200 °C). Fluorine content of phlogopite continuously increases during partial melting from 0.2 to 0.9 wt% between 1000 and 1150 °C and 0.5 to 0.6 wt% between 1150 and 1300 °C at 1 and 3 GPa, respectively. The phlogopite continuous breakdown in the lherzolite follows the reaction: 0.59 phlogopite + 0.52 clinopyroxene + 0.18 garnet = 0.06 olivine + 0.23 orthopyroxene + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.93 phlogopite + 0.46 garnet = 0.25 olivine + 0.14 orthopyroxene + 1.00 melt. Melts from phlogopite-peridotite sources at 3 GPa are silica-undersaturated and are foiditic to trachybasaltic in composition from very low (0.8 wt%) to high (25.6 wt%) degrees of melting. As observed at 1 GPa, the potassium content of primary mantle melts is buffered by the presence of phlogopite, but the buffering values are higher, from 6.0 to 8.0 wt% depending on the source fertility. We finally show that phlogopite garnet-peridotite melts are very close to the composition of the most primitive post-collisional lavas described worldwide.
Contributions to Mineralogy and Petrology, Vol. 171, pp. 95-106.
Mantle
Melting
Abstract: Melting experiments have been performed at 3 GPa, between 1150 and 1450 °C, on a phlogopite-peridotite source in the garnet stability field. We succeeded to extract and determine the melt compositions of both phlogopite-bearing lherzolite and harzburgite from low to high degrees of melting (? = 0.008-0.256). Accounting for the presence of small amounts of F in the mantle, we determined that phlogopite coexists with melt >150 °C above the solidus position (1150-1200 °C). Fluorine content of phlogopite continuously increases during partial melting from 0.2 to 0.9 wt% between 1000 and 1150 °C and 0.5 to 0.6 wt% between 1150 and 1300 °C at 1 and 3 GPa, respectively. The phlogopite continuous breakdown in the lherzolite follows the reaction: 0.59 phlogopite + 0.52 clinopyroxene + 0.18 garnet = 0.06 olivine + 0.23 orthopyroxene + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.93 phlogopite + 0.46 garnet = 0.25 olivine + 0.14 orthopyroxene + 1.00 melt. Melts from phlogopite-peridotite sources at 3 GPa are silica-undersaturated and are foiditic to trachybasaltic in composition from very low (0.8 wt%) to high (25.6 wt%) degrees of melting. As observed at 1 GPa, the potassium content of primary mantle melts is buffered by the presence of phlogopite, but the buffering values are higher, from 6.0 to 8.0 wt% depending on the source fertility. We finally show that phlogopite garnet-peridotite melts are very close to the composition of the most primitive post-collisional lavas described worldwide.
Abstract: Carbonatites are rare magmas containing almost no silica; their igneous counterparts represent the main rare earth element deposits inoperation. No consensus exists on their origin, genesis and evolution. Oldoinyo Lengai (Tanzania) is the only active carbonatite volcano, but the alkali-rich natrocarbonatites it erupts are unique among the >500 reported fossil carbonatite occurrences. Here, we use threephase melt inclusions hosted in minerals from cognate cumulates (clinopyroxene, nepheline, Ti-garnet, interstitial melt)— which sampled the active Oldoinyo Lengai magma chamber during the 2007-08 sub-Plinian explosive eruption—to track the carbonatite presence within the plumbing system, and to eventually quantify its composition at depth. We show that although natrocarbonatites are emitted at the Oldoinyo Lengai summit, more classical calciocarbonatites are present at magma chamber depth (~3.5 kbar, 1050 to 900°C), which is consistent with the model of rare natrocarbonatites deriving from calciocarbonatites by further magma differentiation. We also show that those calciocarbonatites are not isolated but rather conjugated with alkali-rich silica melt suggesting a joint evolution. We eventually present the first direct measurements of major and trace element partition coefficients between natural coexisting carbonate and silicate melts. Partitioning behaviour and recent experiments support our conclusion that natrocarbonatites derive from calciocarbonatites by fractionating Ca-rich, Na-poor phases. As natrocarbonatites are highly unstable at surface conditions, they were likely erupted but not preserved in association with fossil calciocarbonatites worldwide. Oldoinyo Lengai can then be considered as representative of other carbonatite systems, and provide us with the unique opportunity to observe the plumbing system architecture, and to constrain the protracted differentiation path of a carbonatite system.
Contributions to Mineralogy and Petrology, Vol. 175, 15p. Pdf
Africa, Tanzania
deposit - Oldoinyo Lengai
Abstract: Trace element partitioning between minerals and liquids provides crucial constraints on igneous processes. We quantified trace element concentrations in clinopyroxene (Cpx) phenocrysts and their phonolite melt inclusions from the 2007-08 eruption of Oldoinyo Lengai (Tanzania), and report Cpx-melt partition coefficients (D) and corresponding partitioning equations for rare earth elements (REE) and high field strength elements (HFSE) in alkaline magmas. Heavy REE (HREE: Er, Tm, Yb, Lu) are enriched relative to middle REE in alkaline Cpx and display a specific partitioning behavior that is characteristic of alkaline systems. HFSE (Ti, Zr, Hf) and HREE have similar D values (DHf?=?0.25; DLu?=?0.4) that are significantly higher than MREE (DSm?=?0.06). High DHREE/DMREE are strongly correlated with the high values of DZr and DHf relative to the low DMREE values. In this study, REE partitioning between phonolite melt and Cpx is not consistent with standard models assuming incorporation of all REE in the Cpx M2 site, but rather highlights HREE substitution in both the M1 and M2 sites. Here we highlight the preferential incorporation of HREE in the VI-coordinated M1 site, whereas light REE and MREE remain mostly distributed in the VIII-coordinated M2 site. REE partitioning is strongly dependent on Cpx chemistry: the ideal ionic radius and HREE incorporation in the M1 site increase with increasing Fe3+ content and decrease with increasing Mg2+ and AlVI content. In our study, we focus on alkaline evolved magmas, and update existing models to obtain adequate DHREE for alkaline evolved melts. We provide equations to quantify REE and HFSE partitioning, and HREE enrichment in Cpx that are based on Cpx major element composition and temperature. We propose a new model based on the lattice strain approach that predicts HREE partitioning between Cpx and alkaline magmas. The knowledge of the melt composition or of the trace element contents is not required to obtain DREE from the new model. An improved parameterization of HFSE partitioning between Cpx and phonolite and trachy-phonolite melts is also provided herein. We discuss the potential implications of the new data on our understanding of REE deposits that are commonly associated with igneous alkaline complexes.
Abstract: Carbonatites are rare magmas containing almost no silica; the corresponding crystallized rocks represent the main rare earth elements (REE) deposits in production. Oldoinyo Lengai (Tanzania) is the only active carbonatite volcano on Earth, and may be used as a natural laboratory to identify the parameters responsible for the genesis of the >500 reported fossil occurrences of carbonatite magmas. Nevertheless the carbonatites emitted at Oldoinyo Lengai are unique as alkalirich (natrocarbonatites), and their origin may not be representative of the fossil carbonatites (calcio-, ferro-, magnesio-carbonatites). Here we use three-phases melt inclusions trapped in clinopyroxenes and nephelines from cognate cumulates – that sample the active magma chamber of Oldoinyo Lengai – emitted during the 2007-08 sub-plinian explosive eruption to track the carbonatite presence within the plumbing system, and to eventually quantify its composition at depth. We show that although natrocarbonatites are emitted at Oldoinyo Lengai summit, more classical calciocarbonatites are present at magma chamber depth, consistent with rare natrocarbonatites being derived from more classical calciocarbonatites by further magma differentiation. Those unique cognate samples allows us to provide the first direct measurements of partition coefficients for major and trace elements of natural coexisting in equilibrium carbonatite and silicate melts. Partition coefficients suggests that natrocarbonatites derive from calciocarbonatites by fractionating Ca-rich, and Na-poor phases. The Oldoinyo Lengai can therefore be used as a perfect analogue of fossil igneous systems that are now exhumed, commonly lacking any relation with the initial geodynamic setting, and form REE mineral deposits.
The Canadian Mineralogist, Vol. 55, 4, pp. 521-533.
Technology
Deposit - Renard 2
Abstract: The purpose of this work is to investigate the crystal growth parameters necessary or sufficient to obtain a crystal specifically of gem quality. We assume adequate chemistry is available. First, nucleation must occur with only a limited number of nuclei, otherwise too many crystals will be produced, and they will be too small to be faceted into a gem. Two growth mechanisms are readily documented for gems: Most commonly there is slow growth, driven by a spiral growth mechanism, leading to large single individuals. There are only a few examples of fast growth leading to gem-quality edifices: examples include “gota de aceite” Colombian emerald or the dendritic “pseudo cube” for gem diamonds. We have not documented the intermediate conditions between these two extremes in the Sunagawa diagram, which would correspond to 2D nucleation growth. The presence of inclusions is to be limited to desirable ones. Thus, in general, a good stability of the growth interface is the best guarantee of good clarity in the final gem. As for the interface, in general, growth conditions must be relatively stable over the period necessary to achieve growth. Perhaps surprisingly, it has become well documented that gem-quality near-colorless diamonds may have experienced quite a complex growth history. Therefore, the term stability has to be re-defined for each system producing a given gem. The length of time it takes to achieve crystallization of the gem has rarely been studied or estimated. Scientific evidence from experimental petrology and the growth of synthetic gems indicates that it does not take millions of years to grow a gem, but that this exercise may be achieved in a week to, arguably, a few years at the most. Available free space to grow does not appear always necessary, but it helps. Otherwise deformation, inclusions, and other negative effects may occur. Finally, no dramatic post-growth events, such as fracturing or etching, should affect the gem crystal.
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Magnetic inversion of three airborne dat a sets over the Tli Kwi Cho kimberlite complex.
SEG Annual Meeting Denver, pp. 1790-1794 extended abstract
Fournier, D., Heagy, L., Corcoran, N., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Multi-EM systems inversion - towards a common conductivity model for Tli Kwi Cho complex.
SEG Annual Meeting Denver, pp. 1795-1798. Extended abstract
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO-18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO- 18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Marchant, M., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
Society of Exploration Geophysicists, Interpretation, August T 299, 13p.
Canada, Northwest Territories
deposit - Tli Kwi Cho
Abstract: The Tli Kwi Cho (TKC) kimberlite complex contains two pipes, called DO-27 and DO-18, which were discovered during the Canadian diamond exploration rush in the 1990s. The complex has been used as a testbed for ground and airborne geophysics, and an abundance of data currently exist over the area. We have evaluated the historical and geologic background of the complex, the physical properties of interest for kimberlite exploration, and the geophysical surveys. We have carried out 3D inversion and joint interpretation of the potential field data. The magnetic data indicate high susceptibility at DO-18, and the magnetic inversion maps the horizontal extent of the pipe. DO-27 is more complicated. The northern part is highly magnetic and is contaminated with remanent magnetization; other parts of DO-27 have a low susceptibility. Low densities, obtained from the gravity and gravity gradiometry data, map the horizontal extents of DO-27 and DO-18. We combine the 3D density contrast and susceptibility models into a single geologic model that identifies three distinct kimberlite rock units that agree with drilling data. In further research, our density and magnetic susceptibility models are combined with information from electromagnetic data to provide a multigeophysical interpretation of the TKC kimberlite complex.
Midende, G., Boulais, P., Tack, L., Melcher, F., Gerdes,A., Dewaele, S., Demaiffe, D., Decree, S.
Petrography, geochemistry and U Pb zircon age of the Matongo carbonatite Massif ( Burundi): implication for the Neoproterozoic geodynamic evolution of Central Africa.
Journal of African Earth Sciences, Vol. 100, pp. 656-674.
Midende, G., Boulvais, P., Tack, L., Melcher, F., Gerdes, A., Dewaele, S., Demaiffe, D., Decree, S.
Petrography, geochemistry and U-Pb zircon age of the Matongo carbonatite Massif ( Burundi): implication for the Neoproterozoic geodynamic evolution of Central Africa.
Journal of African Earth Sciences, Vol. 100, pp. 656-674.
Abstract: The rare earth element (REE) mineralization of Gakara (Burundi) has first been discovered in 1936 and has periodically been the subject of geological studies, at times when the exploitation of bastnäsite-(Ce) and monazite-(Ce) was economically interesting. This study focuses on the establishment of a mineral paragenesis for Gakara, with special attention to the REE-bearing phases, to understand the formation history of the deposit. The paragenesis can be subdivided into 3 stages: primary ore deposition, brecciation stage and supergene alteration. Evidence for fenitization processes (i.e. pinkish-red cathodoluminescence of K-feldpar, brecciation stage) and the strong enrichment of light REEs in bastnäsite and monazite substantiate the hypothesis of a structurally controlled hydrothermal mineralization with a strong carbonatitic affinity. This likely confirms the association of the Gakara REE deposit with the Neoproterozoic alignment of alkaline complexes and carbonatites along the present-day Western Rift. It suggests a direct link with a - currently unidentified - carbonatitic body at depth, possibly derived from a predominantly metasomatized lithospheric mantle.
Andersen, T.B., Jamveit, B., Dewey, J.F., Swensson, E.
Subduction and education of continental crust: major mechanisms during continent-continent collision and orogenic extensional collapse, a model Based on Norweg
IN: Cycle Concepts in Plate Tectonics, editors Wilson and Houseman , Geological Society of London special publication 470, pp. 18-38.
Mantle
plate tectonics
Abstract: In the first application of the developing plate tectonic theory to the pre-Pangaea world 50 years ago, attempting to explain the origin of the Paleozoic Appalachian-Caledonian orogen, J. Tuzo Wilson asked the question: ‘Did the Atlantic close and then reopen?’. This question formed the basis of the concept of the Wilson cycle: ocean basins opening and closing to form a collisional mountain chain. The accordion-like motion of the continents bordering the Atlantic envisioned by Wilson in the 1960s, with proto-Appalachian Laurentia separating from Europe and Africa during the early Paleozoic in almost exactly the same position that it subsequently returned during the late Paleozoic amalgamation of Pangaea, now seems an unlikely scenario. We integrate the Paleozoic history of the continents bordering the present day basin of the North Atlantic Ocean with that of the southern continents to develop a radically revised picture of the classic Wilson cycle The concept of ocean basins opening and closing is retained, but the process we envisage also involves thousands of kilometres of mainly dextral motion parallel with the margins of the opposing Laurentia and Gondwanaland continents, as well as complex and prolonged tectonic interaction across an often narrow ocean basin, rather than the single collision suggested by Wilson.
Canadian Journal of Earth Sciences, Vol. 56, pp. 1077-1094.
Mantle
Tectonic history
Abstract: I outline and discuss my career in the context of the history of structural geology and tectonics, the progressive developments that led to plate tectonics, the people who have encouraged and influenced me, the events that changed my life, my fifty six doctoral students who have taught me so much, and my principal interests in tectonics. I discuss, in particular, nine topics of special current interest: the evolution of Tibet, the geomorphology of the British Isles, transtension, the Precambrian, the complexities of plate boundary evolution, Appalachian-Caledonian evolution, ophiolites, the structure and strength of the lithosphere, and the subducting slab.
South African Journal of Geology, Vol. 124, 1, pp. 141-162. pdf
Mantle
tectonics
Abstract:
Space probes in our solar system have examined all bodies larger than about 400 km in diameter and shown that Earth is the only silicate planet with extant plate tectonics sensu stricto. Venus and Earth are about the same size at 12 000 km diameter, and close in density at 5 200 and 5 500 kg.m-3 respectively. Venus and Mars are stagnant lid planets; Mars may have had plate tectonics and Venus may have had alternating ca. 0.5 Ga periods of stagnant lid punctuated by short periods of plate turnover. In this paper, we contend that Earth has seen five, distinct, tectonic periods characterized by mainly different rock associations and patterns with rapid transitions between them; the Hadean to ca. 4.0 Ga, the Eo- and Palaeoarchaean to ca. 3.1 Ga, the Neoarchaean to ca. 2.5 Ga, the Proterozoic to ca. 0.8 Ga, and the Neoproterozoic and Phanerozoic. Plate tectonics sensu stricto, as we know it for present-day Earth, was operating during the Neoproterozoic and Phanerozoic, as witnessed by features such as obducted supra-subduction zone ophiolites, blueschists, jadeite, ruby, continental thin sediment sheets, continental shelf, edge, and rise assemblages, collisional sutures, and long strike-slip faults with large displacements. From rock associations and structures, nothing resembling plate tectonics operated prior to ca. 2.5 Ga. Archaean geology is almost wholly dissimilar from Proterozoic-Phanerozoic geology. Most of the Proterozoic operated in a plate tectonic milieu but, during the Archaean, Earth behaved in a non-plate tectonic way and was probably characterised by a stagnant lid with heat-loss by pluming and volcanism, together with diapiric inversion of tonalite-trondjemite-granodiorite (TTG) basement diapirs through sinking keels of greenstone supracrustals, and very minor mobilism. The Palaeoarchaean differed from the Neoarchaean in having a more blobby appearance whereas a crude linearity is typical of the Neoarchaean. The Hadean was probably a dry stagnant lid Earth with the bulk of its water delivered during the late heavy bombardment, when that thin mafic lithosphere was fragmented to sink into the asthenosphere and generate the copious TTG Ancient Grey Gneisses (AGG). During the Archaean, a stagnant unsegmented, lithospheric lid characterised Earth, although a case can be made for some form of mobilism with “block jostling”, rifting, compression and strike-slip faulting on a small scale. We conclude, following Burke and Dewey (1973), that there is no evidence for subduction on a global scale before about 2.5 Ga, although there is geochemical evidence for some form of local recycling of crustal material into the mantle during that period. After 2.5 Ga, linear/curvilinear deformation belts were developed, which “weld” cratons together and palaeomagnetism indicates that large, lateral, relative motions among continents had begun by at least 1.88 Ga. The “boring billion”, from about 1.8 to 0.8 Ga, was a period of two super-continents (Nuna, also known as Columbia, and Rodinia) characterised by substantial magmatism of intraplate type leading to the hypothesis that Earth had reverted to a single plate planet over this period; however, orogens with marginal accretionary tectonics and related magmatism and ore genesis indicate that plate tectonics was still taking place at and beyond the bounds of these supercontinents. The break-up of Rodinia heralded modern plate tectonics from about 0.8 Ga. Our conclusions are based, almost wholly, upon geological data sets, including petrology, ore geology and geochemistry, with minor input from modelling and theory.
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.
Road log from Las Vegas Nevada to Mountain Pass, California.Rare earth mineral deposits ,San Bernardino County-geochemistry of shonkinites, syenite sand granites wit
Seg Guidebook Series, Proterozoic Ore Deposits Of The Southwestern U.s., No. 1, pp. 1-56
Abstract: Pyrochlore-group minerals are common in the Neoproterozoic Sevattur carbonatite complex. This complex is composed of dolomite-, calcite-, banded- and blue carbonatite together with pyroxenite, albitite and diverse syenites. This work reports the paragenetic-textural types and compositional variation of pyrochlore hosted by dolomite carbonatite, banded carbonatite and albitite together with that of alteration assemblages containing belkovite and baotite. On the basis of composition, five different types of pyrochlore are recognised and termed Pcl-I through to Pcl-V. The Pb-rich Pcl-I are present exclusively as inclusions in U-rich Pcl-IIa in dolomite carbonatite. The alteration assemblages of Pb-poor Pcl-IIb + Ba-rich or Ba-Si- rich Pcl-IV + belkovite (dolomite carbonatite) and Si-rich Pcl-V + baotite (banded carbonatite) formed after Pcl-IIa differ in these carbonatites. The albitite hosts extremely U-Ti-rich Pcl-III, mantled by Ba-rich potassium feldspar. In common with the banded carbonatite, Pcl-V is formed by alteration of Pcl-III where this mantle is partially, or completely broken. The Ba-Si-enrichment of Pcl-IV and Pcl-V together with the ubiquitous presence of baryte in all Sevattur lithologies suggests late-stage interaction with a Ba-Si-rich acidic hydrothermal fluid. This fluid was responsible for leaching silica from the associated silicates and produced Pcl-V in the silicate-rich lithologies of the banded carbonatite and albitite. The absence of Pcl-V in dolomite carbonatite is a consequence of the low modal abundance of silicates. The complex compositional diversity and lithology specific pyrochlore alteration assemblages suggest that all pyrochlore (Pcl-I to Pcl-IV) were formed initially in an unknown source and transported subsequently in their respective hosts as altered antecrysts.
Abstract: The Neoproterozoic Sevattur complex is composed essentially of calcite and dolomite carbonatites together with pyroxenites and diverse syenites. This work reports the compositions and paragenesis of different pyrochlore generations hosted by albitite veins in this complex. The pyrochlore are distinctive, being exceptionally rich in uranium (26 to 36 wt.% UO2). Five types of pyrochlore (Pcl-I to Pcl-V) are recognised on the basis of composition and texture. With the exception of Pcl-V, the majority of the pyrochlore (Pcl-II to Pcl-IV) are surrounded by a thick orbicular mantle of Ba-rich potassium feldspar. This mantle around Pcl-V is partially-broken. Pcl-I is restricted to the cores of crystals, and associated with Pcl-II and -III and is relatively rich in Nb (0.53-0.62 apfu) together with more A-site vacancies (0.37-0.71 apfu) compared to Pcl-II to Pcl-IV. Other pyrochlore (Pcl-II to Pcl-IV) are characterised by elevated Ca and Ti compared to Pcl-I, which are related to the (3Nb5+ + Na+ ? 3Ti4+ + U4+) and (2Nb5+ ? 2Ti4+ + Ca2+) substitutions, respectively. These substitutions represent replacement of Pcl-II to Pcl-IV. Alteration and Ba-enrichment in all the pyrochlore are marked by interaction with an externally-derived Ba-rich hydrothermal fluid following the (2Nb5+ ? 2Ti4+ + Ba2+) substitution. This substitution, coupled with extensive metamictisation leads to the formation of Ba-rich (15.9-16.3 wt.% BaO) patchy-zoned Pcl-V. The orbicular mantles around Pcl-I to Pcl-IV have prevented extensive metamictisation and extensive secondary alteration compared to Pcl-V, where mantling is partially disrupted. The compositional and textural variation suggests that Pcl-II to Pcl-IV form by nucleation on Pcl-I, and are transported subsequently as antecrysts in the host albitite.
Geological Society of London Special Publication: Continent formation through time., No. 389, pp. 135-163.
India
Geotectonics
Abstract: Field and geochemical studies combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating set important constraints on the timing and petrogenesis of volcanic rocks of the Neoarchaean Kadiri greenstone belt and the mechanism of crust formation in the eastern Dharwar craton (EDC). The volcanic rocks are divided into three suites: tholeiitic basalts, calc-alkaline high-Mg# andesites and dominant dacites-rhyolites. The basalts (pillowed in places) show flat rare earth element (REE) and primordial mantle-normalized trace element patterns, but have minor negative Nb and Ta anomalies. They are interpreted as mantle plume-related oceanic plateau basalts whose source contained minor continental crustal input. The andesites are characterized by high Mg# (0.66-0.52), Cr and Ni, with depletion of high-field strength elements (HFSE) and enrichment of light REE (LREE) and large-ion lithophile elements (LILE). They were probably derived from a metasomatized mantle wedge overlying a subducted slab in a continental margin subduction zone. The dacites-rhyolites are silicic rocks (SiO2 = 61-72 wt%) with low Cr and Ni, K2O/Na2O mostly 0.5-1.1, highly fractionated REE patterns, enrichments of LILE and distinctly negative HFSE anomalies. One rhyolite sample yielded a zircon U-Pb age of 2353 ± 32 Ma. This suite is similar to potassic adakites and is explained as the product of deep melting of thickened crust in the arc with a significant older crustal component. Collision between a continental margin arc with an oceanic plateau followed by slab break-off, upwelling of hot asthenosphere and extensive crustal reworking in a sustained compressional regime is proposed for the geodynamic evolution of the area. This is in corroboration with the scenario of EDC as a Neoarchaean hot orogen as suggested recently by some workers.
Abstract: The Neoarchaean era is characterized by rapid crustal growth corresponding to some fundamental global changes in geodynamic processes. However, the nature of crustal growth including the mechanism and tectonic setting of the Neoarchaean are controversial issues. The eastern Dharwar Craton (EDC) exposes widespread Neoarchaean granite?greenstone belts, which provide an opportunity to evaluate the various models proposed for Neoarchaean crustal growth. In this study, we present field, petrographic, and geochemical data and discuss the petrogenesis and significance for crustal evolution for a suite of previously undescribed banded gneisses, TTG (tonalite-trondhjemite-granodiorite), biotite granites, alkali feldspar granite and gabbro. These rocks are associated with Neoarchaean metavolcanic and metapelites rocks of the Tsundupalle greenstone belt, in the eastern fringe of the EDC. Whole?rock major and trace element geochemical data are consistent with diverse sources, including both crust and enriched mantle in an evolving subduction zone. A convergent orogenic setting is proposed for interpreting the association of various granitoids in the Tsundupalle area. Finally, intrusion of crustally derived, highly silicic, alkali?rich granite, and mantle?derived gabbro emplaced in a post?subduction regime is proposed. Lithospheric delamination and attendant mantle melting are suggested as possible mechanisms for generation of these rocks. The understanding of generation of the different granitoid types along with gabbro provides significant insights into the mechanism of Neoarchaean crustal growth.
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.
Geology, petrology, geochemistry and mineral chemistry of new kimberlite fields in the Wajrakarur kimberlite field, Anantapur district, Andhra Pradesh.
National Seminar on Exploration Survey, Geological Society of India Special Publication, No. 58, pp. 593-602.
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.
South African Journal of Geology, Vol. 124, pp. 519-536.
Africa, South Africa
geodynamics
Abstract: The delicate interplay of various Earth’s systems processes in the Critical Zone is vital in ensuring an equilibrium across the different spheres of life. The upper crust forms a thin veneer on the Earth’s surface that is defined by an interconnected network of brittle structures. These brittle structures enable various Earth System processes. Increased anthropogenic interactions within the very upper crust have seemingly resulted in a growing number of negative natural effects, including induced seismicity, mine water drainage and land degradation. Brittle structures across South Africa are investigated. These structures include various fractures and dykes of different ages and geodynamic evolutions. The orientation of these structures is compared to the underlying tectonic domains and their bounding suture zones. The orientations corroborate an apparent link between the formation of the brittle structures and the tectonic evolution of the southern African crust. Reactivation and the creation of new structures are also apparent. These are linked to the variability of the surrounding stress field and are shown to have promoted magmatism, e.g., Large Igneous Provinces, and the movement of hydrothermal fluids. These fluids were commonly responsible for the formation of important mineral deposits. The preferred structural orientations and their relationship to underlying tectonic zones are also linked to fractured groundwater aquifers. Subsurface groundwater displays a link to structural orientations. This comparison is extended to surficial water movement. Surface water movement also highlights an apparent link to brittle structures. The apparent correlation between these Earth’s systems processes and the interconnectivity developed by brittle structures are clear. This highlights the importance of high-resolution geological and structural mapping and linking this to further development of the Earth’s Critical Zone.
Abstract: Although a mantle origin of carbonatites has long been advocated, a few carbonatite bodies with crustal fingerprints have been identified. The Eppawala carbonatites in Sri Lanka are more similar to orogenic carbonatites than those formed in stable cratons and within plate rifts. They occur within the Pan-African orogenic belt and have a formation age of ca. 475 Ma newly obtained in this study with no contemporary mantle-related magmatism. These carbonatites have higher (87Sr/86Sr)i ratios (0•70479-0•70524) and more enriched Nd and Hf isotopic compositions than carbonatites reported in other parts of the world. Model ages (1•3-2•0 Ga) of both Nd and Hf isotopes [apatite ?Nd(t)?=??9•2 to ?4•7; rutile ?Hf(t)?=??22•0 to ?8•02] are in the age range of metamorphic basement in Sri Lanka, and the carbon and oxygen isotopic compositions (?13CPDB?=??2•36 to ?1•71; ?18OSMOW?=?13•91-15•13) lie between those of mantle-derived carbonatites and marble. These crustal signatures are compatible with the chemistry of accessory minerals in the carbonatites, such as Ni-free olivine and Al- and Cr-poor rutile. Modeling results demonstrate that the Eppawala carbonatite magmas originated from a mixture of basement gneisses and marbles, probably during regional metamorphism. This interpretation is supported by the occurrence of the carbonatites along, or near, the axes of synforms and antiforms where granitic gneiss and marble are exposed. Therefore, we propose that the Eppawala carbonatites constitute another rare example of a carbonatitic magma that was derived from melting of a sedimentary carbonate protolith. Our findings suggest that other orogenic carbonatites with similar features should be re-examined to re-evaluate their origin.
Abstract: We present petrology, geochemistry and radiogenic isotope (Sr and Nd) data of thirteen post-Deccan lamprophyre dykes in the Narmada rift zone from the Chhotaudepur alkaline province of the Deccan Large Igneous Province (DLIP). Mineralogically, these dykes show affinity towards alkaline (sannaite and camptonite) as well as ultramafic (damtjernite) varieties of lamprophyres. Their major oxides and certain trace element ratios increase with increasing silica content highlighting the strong influence of fractionation processes. Their Nb/U and Ce/Pb ratios are similar to the mantle array defined by MORBs and OIBs and suggests an uncontaminated nature. Major oxide (K2O, Na2O, SiO2 and TiO2) contents show geochemical similarity towards shoshonitic volcanic series, whereas elevated Zr/Hf and Nb/La coupled with suppressed Rb/Nb and Zr/b display their affinity towards HIMU-type intraplate basalts. Their radiogenic initial 87Sr/86Sr (0.706034-0.710582) and sub-chondritic initial ?Nd (?8.6 to 2.1) are akin to those of the (i) ca. 65?Ma Ambadongar carbonatite, NW India, and (ii) ca. 65?Ma orangeites from Bastar Craton, central India, highlighting an enriched lithospheric mantle source. REE inversion modeling suggests ~3% enrichment of an undepleted mantle followed by small degrees of melting of this enriched mantle source are sufficient- as in the case of ocean island basalts (OIB)- to reproduce their observed REE concentrations. Their TDM Nd model ages (564-961?Ma) are consistent with widespread convergent margin-related magmatism during the amalgamation of the Rodinia supercontinent. We propose that enriched lithospheric mantle developed during the Neoproterozoic was metasomatized by small-volume CO2-rich melts imparting a HIMU-type geochemical character during Late Cretaceous, when the mantle plume (viz., Réunion) responsible for the flood basalt eruption, impinged at the base of the NW Indian lithosphere. From the presence of F-rich apatite and high K/Rb in mica, we infer the (i) presence of F-phlogopite in their source regions, and (ii) that the depth of post-Deccan lithosphere-asthenosphere boundary (LAB) beneath NW India was at least ~100?km at ca. 65?Ma.
Abstract: Carbonatites and associated alkaline rocks are the primary sources for REE mineralization. The Ambadongar Carbonatite Complex (ADCC) from NW Deccan Volcanic Province (DVP) constitutes the largest Carbonatite Associated REE Deposits (CARD) in India. ADCC belongs to the final stages of the Late Cretaceous alkaline-carbonatite magmatism associated with main Deccan basalt volcanic activity. The ADCC is an envisioned diatreme structure in which four carbonatitic phases are recognized, mainly calcio-carbonatites and ferro-carbonatites. Each successive carbonatite phase shows higher REE enrichment. The primary REE mineralization with bastnäsite as the dominant REE phase is hosted by pervasive hydrothermally altered ferro-carbonatite plugs. The secondary mineralogy formed with barites in the main orebody during late- to post-magmatic hydrothermal fluid alteration is fluorite, quartz, ankerite, and other REE-bearing minerals like bastnäsite, parisite, synchysite, strontianite, florencite, monazite and columbite. Carbonatite samples contain 18.61% to 52.42% of CaO, and the LOI varies from 5.28% to 38.79%. Most can be classified as calcio-carbonatites. Since all the samples also contain an appreciable amount of Fe2O3 (4.13% to 20.20%) and MnO (0.07% to 5.46%), some may be classified as ferro-carbonatites. Total REE content varies from 0.6 to 4%, with a high Ce concentration and LREE/HREE ratio. The highest values for La, Ce, Pr, and Nd are 1.95%, 1.56%, 0.16%, and 0.45%, respectively. Metasomatism of SCLM from asthenospheric melts followed by the low degree partial melting of the SCLM region is responsible for fertile carbonatite generation in ADCC. The multiphase liquid immiscibility of carbonatite melts from carbonate-silicate magma followed by immiscibility of REE rich carbonatite melt and REE deficient fluoride-rich aqueous fluids explain the higher level of REE enrichment in each successive phases of carbonatites in ADCC. The mineralizing fluids were probably the result of residual magmatic volatiles that brought mainly REE and later SiO2 into the overprinted rocks. Ambadongar carbonatites' stable isotopic compositions agree with a magmatic origin (?13C = ?4.1 ± 1.9‰ [PDB] and ?l8O = 10.3 ± 1.7‰ [SMOW]). The C-O stable isotopic modeling indicates re-equilibration under hydrothermal conditions between 180 °C and 70 °C. Significant amounts of REE fluorocarbonate minerals, relatively Sr- and Th-rich, were deposited during re-equilibration. The REE fluorocarbonate bastnäsite-(Ce) occurs as late individual crystals, overgrown on the synchysite and parisite polycrystals. Textural and chemical reactions between the REE fluorocarbonates provide insights into rare-earth elements' mobility during fluid-rock interaction. Early crystallization of synchysite/parisite indicates the high activity of Ca2+, OH?, (SO4)2?, Al and Si in the fluid. Later, the fluid was characterized by increased activity of F?, (SO4)2?, REE and Si, and decreased activity of Ca2+ as reflected in the association of barite, fluorite, quartz, and bastnäsite typical of strongly overprinted ferro-carbonatites. Re-equilibration and recrystallization of the primary minerals in the presence of OH?, (SO4)2?, F?, REE, Al, and Si carried in solution by the hydrothermal fluid is the leading cause behind the refixing of REE in the form of REE fluorocarbonate in REE rich ferro-carbonatites.
Geochemistry and origin of the Proterozoic kimberlites, ultramafic and ultrapotassic magmatic rocks from Indravati Basin in Bastar Craton, central India.
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 94-97.
Abstract: The continental crust is the archive of Earth’s history. Its rock units record events that are heterogeneous in time with distinctive peaks and troughs of ages for igneous crystallization, metamorphism, continental margins, and mineralization. This temporal distribution is argued largely to reflect the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. Isotopic and elemental data from zircons and whole rock crustal compositions suggest that the overall growth of continental crust (crustal addition from the mantle minus recycling of material to the mantle) has been continuous throughout Earth’s history. A decrease in the rate of crustal growth ca. 3.0 Ga is related to increased recycling associated with the onset of plate tectonics. We recognize five stages of Earth’s evolution: (1) initial accretion and differentiation of the core/mantle system within the first few tens of millions of years; (2) generation of crust in a pre-plate tectonic regime in the period prior to 3.0 Ga; (3) early plate tectonics involving hot subduction with shallow slab breakoff over the period from 3.0 to 1.7 Ga; (4) Earth’s middle age from 1.7 to 0.75 Ga, characterized by environmental, evolutionary, and lithospheric stability; (5) modern cold subduction, which has existed for the past 0.75 b.y. Cycles of supercontinent formation and breakup have operated during the last three stages. This evolving tectonic character has likely been controlled by secular changes in mantle temperature and how that impacts on lithospheric behavior. Crustal volumes, reflecting the interplay of crust generation and recycling, increased until Earth’s middle age, and they may have decreased in the past ~1 b.y.
Annual Review of Earth and Planetary Sciences, Vol. 45, pp. 169-198.
Mantle
tectonics
Abstract: The record of the continental lithosphere is patchy and incomplete; no known rock is older than 4.02 Ga, and less than 5% of the rocks preserved are older than 3 Ga. In addition, there is no recognizable mantle lithosphere from before 3 Ga. We infer that there was lithosphere before 3 Ga and that ?3 Ga marks the stabilization of blocks of continental lithosphere that have since survived. This was linked to plate tectonics emerging as the dominant tectonic regime in response to thermal cooling, the development of a more rigid lithosphere, and the recycling of water, which may in turn have facilitated plate tectonics. A number of models, using different approaches, suggest that at 3 Ga the volume of continental crust was ?70% of its present day volume and that this may be a minimum value. The continental crust before 3 Ga was on average more mafic than that generated subsequently, and this pre-3 Ga mafic new crust had fractionated Lu/Hf and Sm/Nd ratios as inferred for the sources of tonalite-trondhjemite-granodiorite and later granites. The more intermediate composition of new crust generated since 3 Ga is indicated by its higher Rb/Sr ratios. This change in composition was associated with an increase in crustal thickness, which resulted in more emergent crust available for weathering and erosion. This in turn led to an increase in the Sr isotope ratios of seawater and in the drawdown of CO2. Since 3 Ga, the preserved record of the continental crust is marked by global cycles of peaks and troughs of U-Pb crystallization ages, with the peaks of ages appearing to match periods of supercontinent assembly. There is increasing evidence that the peaks of ages represent enhanced preservation of magmatic rocks in periods leading up to and including continental collision in the assembly of supercontinents. These are times of increased crustal growth because more of the crust that is generated is retained within the crust. The rates of generation of continental crust and mantle lithosphere may have remained relatively constant at least since 3 Ga, yet the rates of destruction of continental crust have changed with time. Only relatively small volumes of rock are preserved from before 3 Ga, and so it remains difficult to establish which of these are representative of global processes and the extent to which the rock record before 3 Ga is distorted by particular biases.
Abstract: There is much discussion of the timing of the onset of plate tectonics, yet there is increasing evidence that magma types similar to those from recent within plate and subduction related settings were generated in different areas at broadly similar times in the early Archaean. It may therefore be helpful to consider when plate tectonics became the dominant mechanism associated with the generation of continental crust, rather than just when it started. To do this we discuss the geochemical and mechanical characteristics of the lithosphere in the search to discern differences between Early Archaean and younger tectonic environments. Seismic tomography allows increasingly detailed mapping of the lithosphere, and it provides some evidence that the degree of anisotropy is different in different Archaean terrains. Structural styles also appear to vary from basin and swell, or vertical tectonics, as in the Australia Pilbara and southern Africa, to those with more strongly developed regional fabrics and greater seismic anisotropy, as in North America. These terrains tend to be characterized by inferred within-plate and subduction-related magmatism respectively, and we consider possible links between the degree of crustal and mantle anisotropy and the nature of the magmatic record. At least in some areas, terrains with stronger regional fabrics may be younger than those in which such fabrics are less well developed. A model is developed for the generation and stabilization of continental lithosphere in the Archaean. It seeks to reconcile evidence for hot shallow melting with melt fractions up to 40% to generate residual peridotites now preserved as mantle xenoliths, and the lower degrees of melting required to generate the mafic sources of TTGs (fractionated Lu/Hf and Sm/Nd, and perhaps not Rb/Sr).
Annual Review of Earth and Planetary Sciences, Vol. 45, pp. 169-198.
Mantle
Geochronology
Abstract: The record of the continental lithosphere is patchy and incomplete; no known rock is older than 4.02 Ga, and less than 5% of the rocks preserved are older than 3 Ga. In addition, there is no recognizable mantle lithosphere from before 3 Ga. We infer that there was lithosphere before 3 Ga and that ?3 Ga marks the stabilization of blocks of continental lithosphere that have since survived. This was linked to plate tectonics emerging as the dominant tectonic regime in response to thermal cooling, the development of a more rigid lithosphere, and the recycling of water, which may in turn have facilitated plate tectonics. A number of models, using different approaches, suggest that at 3 Ga the volume of continental crust was ?70% of its present-day volume and that this may be a minimum value. The continental crust before 3 Ga was on average more mafic than that generated subsequently, and this pre-3 Ga mafic new crust had fractionated Lu/Hf and Sm/Nd ratios as inferred for the sources of tonalite-trondhjemite-granodiorite and later granites. The more intermediate composition of new crust generated since 3 Ga is indicated by its higher Rb/Sr ratios. This change in composition was associated with an increase in crustal thickness, which resulted in more emergent crust available for weathering and erosion. This in turn led to an increase in the Sr isotope ratios of seawater and in the drawdown of CO2. Since 3 Ga, the preserved record of the continental crust is marked by global cycles of peaks and troughs of U-Pb crystallization ages, with the peaks of ages appearing to match periods of supercontinent assembly. There is increasing evidence that the peaks of ages represent enhanced preservation of magmatic rocks in periods leading up to and including continental collision in the assembly of supercontinents. These are times of increased crustal growth because more of the crust that is generated is retained within the crust. The rates of generation of continental crust and mantle lithosphere may have remained relatively constant at least since 3 Ga, yet the rates of destruction of continental crust have changed with time. Only relatively small volumes of rock are preserved from before 3 Ga, and so it remains difficult to establish which of these are representative of global processes and the extent to which the rock record before 3 Ga is distorted by particular biases.
Philosphical Transactions of the Royal Society, rsta.royalsociety publishing.org 30p. Available
Mantle
tectonics, geochemistry
Abstract: Plate tectonics, involving a globally linked system of lateral motion of rigid surface plates, is a characteristic feature of our planet, but estimates of how long it has been the modus operandi of lithospheric formation and interactions range from the Hadean to the Neoproterozoic. In this paper, we review sedimentary, igneous and metamorphic proxies along with palaeomagnetic data to infer both the development of rigid lithospheric plates and their independent relative motion, and conclude that significant changes in Earth behaviour occurred in the mid- to late Archaean, between 3.2?Ga and 2.5?Ga. These data include: sedimentary rock associations inferred to have accumulated in passive continental margin settings, marking the onset of sea-floor spreading; the oldest foreland basin deposits associated with lithospheric convergence; a change from thin, new continental crust of mafic composition to thicker crust of intermediate composition, increased crustal reworking and the emplacement of potassic and peraluminous granites, indicating stabilization of the lithosphere; replacement of dome and keel structures in granite-greenstone terranes, which relate to vertical tectonics, by linear thrust imbricated belts; the commencement of temporally paired systems of intermediate and high dT/dP gradients, with the former interpreted to represent subduction to collisional settings and the latter representing possible hinterland back-arc settings or ocean plateau environments. Palaeomagnetic data from the Kaapvaal and Pilbara cratons for the interval 2780-2710?Ma and from the Superior, Kaapvaal and Kola-Karelia cratons for 2700-2440?Ma suggest significant relative movements. We consider these changes in the behaviour and character of the lithosphere to be consistent with a gestational transition from a non-plate tectonic mode, arguably with localized subduction, to the onset of sustained plate tectonics.
Philosphical Transactions of the Royal Society, http://dx.doi.org/ 10.1098/rsta .2017.0403 12p. Available
Mantle
plate tectonics
Abstract: Less than 25% of the volume of the juvenile continental crust preserved today is older than 3?Ga, there are no known rocks older than approximately 4?Ga, and yet a number of recent models of continental growth suggest that at least 60-80% of the present volume of the continental crust had been generated by 3?Ga. Such models require that large volumes of pre-3?Ga crust were destroyed and replaced by younger crust since the late Archaean. To address this issue, we evaluate the influence on the rock record of changing the rates of generation and destruction of the continental crust at different times in Earth's history. We adopted a box model approach in a numerical model constrained by the estimated volumes of continental crust at 3?Ga and the present day, and by the distribution of crust formation ages in the present-day crust. The data generated by the model suggest that new continental crust was generated continuously, but with a marked decrease in the net growth rate at approximately 3?Ga resulting in a temporary reduction in the volume of continental crust at that time. Destruction rates increased dramatically around 3 billion years ago, which may be linked to the widespread development of subduction zones. The volume of continental crust may have exceeded its present value by the mid/late Proterozoic. In this model, about 2.6-2.3 times of the present volume of continental crust has been generated since Earth's formation, and approximately 1.6-1.3 times of this volume has been destroyed and recycled back into the mantle.
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: 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: Carbonatite and alkaline magma constitute one of the principal resources of rare metals (REE, Nb, Ti, Zr). Carbonatite rare metals enrichment is mainly considered as the result of hydrothermal or supergen processes. However, the magmatic processes linked to carbonatites genesis and differentiation are still debated and whether these processes can significantly impact on the rare metal concentrations remains unclear. Experimental studies have shown that immiscibility processes between carbonate and silicate melts can lead to both REE enrichments and depletions in carbonatites. Anionic species (F, Cl, P or S) and water may impact both melt compositions and expand the immiscibility gap. Morever, anionic species are assumed to play an important role in REE behaviour in carbonate melts [1]. Indeed, halogens may occur in carbonatites as immiscible salt melts in melt inclusions [2] and primary REE- fluoride minerals have been identified as magmatic phases in carbonatites. Such occurrences thus question on the role of salt (carbonate, phosphate, fluoride and chloride) melts in REE and other rare metals partitioning. F, Cl, P and also H2O may all significantly increase the window of primary REE enrichment in carbonatites. Here we present high pressure and high temperature experiments made in piston-cylinder (850 to 1050°C, 8kb) simulating the immiscibility between carbonate and differentiated alkaline melts. We added F, Cl, P and H2O in order to assess the effect of salts and water on the immiscibility gap and on the rare metals partitoning between carbonatite and evolved silicate melts. The partitioning data are analysed using LA-ICP-MS, nano-SIMS, FTIR and RAMAN. The characterization of rare metal partition coefficients allow to determine the relative importance of F, Cl, P and H2O on carbonatites rare metal enrichments at evolved magmatic stage.
Abstract: uncommon type of magmatic rocks dominates by carbonate, are broadly enriched in rare earth elements (REE) relative to the majority of igneous silicate rocks. While more than 500 carbonatites are referenced worldwide [1], only a few contain economic REE concentrations that are widely considered as resulting from late magmatic-hydrothermal or supergene processes. Magmatic pre-enrichment, linked to the igneous processes at the origin of carbonatites, are, however, likely to contribute to the REE fertilisation. Field observations [1] and experimental surveys [2, 3] suggest that a large part of the carbonatite melts can be produced as immiscible liquids with silicate magmas. Experimental constraints reveals that such immiscibility processes can lead to both REE enrichments and depletions in carbonatites [2, 3], making the magmatic processes controlling REE enrichments unclear. Here we present results of high-pressure and hightemperature experiments, simultaneously addressing crystal fractionation of alkaline magmas and immiscibility between carbonate and silicate melts. The experimental data reveal that the degree of differentiation, controlling the chemical composition of alkaline melts is a key factor ruling the REE concentration of the coexisting immiscible carbonatites. The parameterization of the experimental data together with the compilation of geochemical data from various alkaline provinces show that REE concentrations similar to those of highly REE enriched carbonatites (?REE > 30000 ppm) can be produced by immiscibility with phono-trachytic melt compositions, while more primitive alkaline magma can only be immiscible with carbonatites that are not significantly enriched in REE.
Geochimica et Cosmochimica Acta, in press available 57p. Pdf
Mantle
carbonatite
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.
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.
Mineralogy and Petrology, Vol. 114, pp. 363-373. pdf
Mantle
melting
Abstract: Sulfur is an important element present in natural kimberlites and along with CO2, S can play a role in the kimberlite degassing. We have investigated experimentally the change in S content and CO2 solubility in synthetic kimberlitic melts in response to a range of pressure (0.5 to 2.0 GPa) and temperature (1500 to 1525 °C). Several initial S concentrations were investigated ranging from 0 to 24000 ppm. The dissolved CO2 and S were determined by Raman spectroscopy and Electron Probe Micro-Analyses. Under the investigated oxidizing conditions (?FMQ?+?1), S is dissolved in the glass only as S6+ forming sulfate molecular groups (SO42?). The measured S concentration in the glasses increases from 2900 to 22000 ppm. These results suggest that the experimental conditions were below saturation with respect to S and that the S solubility is higher than 22000 ppm for kimberlitic melts; regardless of the experimental conditions considered here. CO2 is dissolved as CO32? molecular groups. The CO2 solubility ranges from 3.0 to 11.3 wt% between 0.5 and 2.0 GPa. CO2 solubility is not affected by the presence of S; which suggests that SO42? and CO32? clusters have two distinct molecular environments not interacting together. This result implies that both CO2 and S are efficiently transported by kimberlitic melt from the upper mantle towards the atmosphere.
Comptes Rendus Geoscience, Vol. 353, no S2, pp. 217-231.
Global
carbonatite
Abstract: Carbonatites host Earth’s main REE deposits, with bastnaesite (LREE)CO
F being the main economic REE-bearing mineral. However, bastnaesite mineralisation processes are debated between hydrothermal or magmatic origin. This study aims to assess if bastnaesite can be magmatic, and to characterise the REE behaviour during carbonatite crystallisation. Crystallisation experiments have been performed from 900 to 600 °C at 1 kbar, on a REE-rich calciocarbonatitic composition. REE-bearing calcite is the dominant crystallising mineral, driving the residual melt towards natrocarbonatitic compositions. Both halogens (i.e., Cl and F) and water decrease the temperature of calcite saturation. REE are slightly incompatible with calcite: for all REE, partition coefficients between carbonate melt and calcite are comprised between 1 and 11, and increase with temperature decrease. Britholite (REE, Ca) (Si,P)O) (F,OH) crystallises at high temperatures (700-900 °C), while pyrochlore (Ca,Na,REE) NbO (OH,F) crystallises at low temperatures (600-700 °C), as well as REE-rich apatite (600-650 °C). No bastnaesite is found in crystallisation experiments. We thus performed a bastnaesite saturation experiment at 600 °C. The bastnaesite-saturated melt contains 20 wt% of REE: such magmatic saturation is unlikely to happen in nature. Textural evidences imply a Na, Cl, REE-rich fluid at high temperatures and hydrous conditions. We propose that fluids are the main mineralising agent for bastnaesite at hydrothermal stage (600 °C).
Comptes Rendus Geoscience, Vol. 353, no S2, pp. 233-272. pdf
Global
carbonatites
Abstract: This study presents new insights into the effects of halogens (F and Cl) and phosphorous (P) on rare earth element (REE) partitioning between carbonatite and alkaline silicate melts. F, Cl and P are elements that are abundant in carbonatites and alkaline magmatic systems and they are considered to play an important role on the REE behaviour. Nonetheless, their effect on REE partitioning between carbonate and alkaline silicate melts has not yet been constrained. Here we present new experimental data on REE partitioning between carbonate and alkaline silicate melts doped in F, Cl and P, in order to (1) test the Nabyl et al. [2020] REE partitioning model in F-, Cl- and P-rich systems, and (2) identify the possible role of F, Cl and P in carbonate melt REE enrichments during alkaline–carbonatite magma differentiation. The experiments were performed at 850–1050 °C and 0.8 GPa using piston-cylinder devices. Starting materials consisted of carbonatite and phonolite compositions doped in F, Cl and P. The experimental results show that REE partitioning is similar in F-Cl-P-rich and -poor systems. The silicate melt composition and its molecular structure (i.e. SiO contents, the alumina saturation index and the alkali/alkaline-earth element ratio), which have already been identified as controlling REE partitioning in F-, Cl- and P-poor systems, still operate in doped systems. No direct effect of the F, Cl or P melt concentrations on REE partitioning has been identified. We also propose an application to natural systems.
Barry, P.H., de Moor, J.M., Giovannelli, D., Schrenk, M., Hummer, D.R., Lopez, T., Pratt, C.A., Alpizar Segua, Y., Battaglia, A., Beaudry, A., Bini, G., Cascante, M., d'Errico, G., di Carlo, M., Fattorini, D., Fullerton, K., H+Gazel, E., Gonzalez, G., Hal
Abstract: Carbon and other volatiles in the form of gases, fluids or mineral phases are transported from Earth’s surface into the mantle at convergent margins, where the oceanic crust subducts beneath the continental crust. The efficiency of this transfer has profound implications for the nature and scale of geochemical heterogeneities in Earth’s deep mantle and shallow crustal reservoirs, as well as Earth’s oxidation state. However, the proportions of volatiles released from the forearc and backarc are not well constrained compared to fluxes from the volcanic arc front. Here we use helium and carbon isotope data from deeply sourced springs along two cross-arc transects to show that about 91 per cent of carbon released from the slab and mantle beneath the Costa Rican forearc is sequestered within the crust by calcite deposition. Around an additional three per cent is incorporated into the biomass through microbial chemolithoautotrophy, whereby microbes assimilate inorganic carbon into biomass. We estimate that between 1.2 × 108 and 1.3 × 1010 moles of carbon dioxide per year are released from the slab beneath the forearc, and thus up to about 19 per cent less carbon is being transferred into Earth’s deep mantle than previously estimated.
Abstract: A high-temperature rheometer equipped with a graphite furnace, characterized by an air-bearing-supported synchronous motor, has been enhanced by a custom-made Pt-Au concentric cylinder assembly. With this adaptation, viscosity measurements of highly fluid melts can be achieved at high temperatures, up to 1273 K. Due to the air-bearing-supported motor, this apparatus can perform measurements of extremely low torque ranging between 0.01 ?Nm and 230 mNm (resolution of 0.1 nNm), extending the typical range of viscosity measurements accessible in the present configuration to 10?3.5-103.5 Pa•s and shear rates up to 102 of s?1. We calibrated the system with distilled water, silicone oils, and the DGG-1 standard glass. We further present new data for the viscosity of Na2CO3, K2CO3, and Li2CO3 liquids. Finally, a comparison between our results and literature data is provided, to illustrate the effect of chemical composition and oxygen fugacity on the viscosity of alkali carbonate melts, which serve as analogs for both carbonatitic melts and molten carbonates of industrial relevance. This study substantially improves the database of alkali carbonate melts and dramatically increases the accuracy with respect to previous measurement attempts. The very low viscosity range data and their temperature dependence also helps to constrain very well the activation energy of these highly fluid systems and confirms the estimate of a universal pre-exponential factor for non-Arrhenian viscosity-temperature relationships.
Abstract: A VTEM survey was flown over the Drybones kimberlite in 2005, followed by a ZTEM survey in 2009. These data sets were inverted on multiple previous occasions using various 1D, 2D, 3D and plate modelling algorithms. VTEM data showed AIP effects, manifested as negative voltages and otherwise skewed transients. This created artefacts in conventional inversions of VTEM data, which showed some inconsistencies with ZTEM inversions, as well as with the known geology. In 2015 the VTEM data were transferred to Aarhus Geophysics, reprocessed and reinverted using the modified "AarhusINV" code with Cole-Cole modelling. The results are presented in current abstract, they appear to be more interpretable and provide better data fit, than previous inversion attempts.
Abstract: We present the joint interpretation of airborne electromagnetic and aeromagnetic data, acquired to study kimberlite pipes. We analyse the data surveyed in 2005 over Drybones Bay, Archean Slave Province of the Northwest Territories, northern Canada. This area hosts a recently discovered kimberlite province with >150 kimberlite pipes. Magnetic and electromagnetic data were each one modelled by 1D inversion. For magnetic data we inverted vertical soundings built through upward continuations of the measured data at various altitudes. The validity of the method was prior verified by tests on synthetic data. Electromagnetic data were processed and inverted using the modified AarhusINV code, with Cole-Cole modelling, in order to take into account induced polarization effects, consisting in negative voltages and otherwise skewed transients. The integrated study of the two kinds of data has led to a better understanding of the structures at depth, even though the comparison between the magnetic and the electromagnetic models shows the different sensitivity of the two methods with respect to the geological structure at Drybones Bay.
Abstract: The origin of the diamond-forming fluids are routinely addressed with the stable isotopes of carbon and nitrogen, where average ?13C and ?15N values are -5 ± 3‰ and -5 ± 4‰, respectively. Because these values differ from crustal sources the application of C-N stable isotopes are applied as tracers of recycled crustal volatiles into the mantle. Additionally, fluid inclusions in fast-growing diamonds provides a unique opportunity to further examine the origin of diamond-forming fluids using noble gas geochemistry. Here we combine C-N isotopes, N concentrations from the diamond with He isotopes released from trapped fluids by in vacuo crushing of mg-sized polycrystalline diamonds. The samples examined are dominantly eclogitic to websteritic abd originate from Southern Africa. ?13C values range from -4.3 to -22.2 ‰ and ?15N values from -4.9 to +23.2 ‰. These data require a significant contribution of material that is 13Cdepleted and 15N-enriched relative to mantle, akin to altered oceanic crust or deep ocean sediments. 3He/4He ratios range from typical mantle values (8.5 Ra) to those dominated by radiogenic He (< 0.1 Ra). These new data show 3He/4He correlates with 3He concentration, suggesting that the low 3He/4He are, at least in part, the result of ingrowth of radiogenic 4He in He-poor diamonds after their formation. 13C-depleted and 15N-enriched diamonds dominate the population studied here. This indicates that subducted altered oceanic crust is essential for diamondite-formation within the SCLM beneath southern Africa. However, the fluids trapped in the low ?13C diamondites (< -15 ‰) have 3He/4He ratios that indicate an origin in the convective upper asthenospheric mantle. Ergo, helium reveals what carbon and nitrogen cannot. When the carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamondformation, helium isotopes reveal an unambiguos mantle component hidden within strongly 13C-depleted diamond.
Abstract: Hundred-micrometer-sized calcium-aluminum-silicates (CAS) inclusions occur in moissanite-4H, moissanite-15R, and moissanite-6H from Turkey. These inclusions commonly consist of tabular exsolution lamellae of two different minerals. By combined electron microprobe and Raman spectroscopy analysis, at least eight different, essentially Mg- and Fe-free Ca-Al-silicate or Al-silicate phases have been discerned. The most common phase is dmisteinbergite, a hexagonal modification of CaAl2Si2O8, occurring in association with lamellae of Cax(Al,Si)1?xO3 or Ca1?x(Al,Si)2+xO5 compositions. All three phases contain significant amounts of BaO (up to 2 mol% of celsiane component in dmisteinbergite), SrO, SO3, and light rare earth elements (LREE). In particular, Ca1?x(Al,Si)2+xO5 contains up to 2.1 wt% of LREE, 3.9 wt% of F, and significant traces of Cl, while it is also associated to osbornite (TiN). Pure ghelenite, Ca2Al2SiO7, and three additional compositions, namely CaAl4-xSixO7, Ca1-x(Al,Si)3+xO6, and Ca3-x(Al,Si)6+xO14 have been found, either occurring as single grains or forming exsolution lamellae. They also contain significant amounts of BaO, SrO, SO3, and LREE. One last intriguing phase is composed in average of 65.9 wt% SiO2, 17.4% Al2O3, 3.0% alkalis, 6.0% BaO, 2.0% CaO+MgO, 0.9% ZrO2, and up to 0.5% LREE. Dmisteinbergite and ghelenite show Raman peaks in very good agreement with literature data, Cax(Al,Si)1-xO3 shows main Raman modes at 416 and 1009 cm?1, Ca1-x(Al,Si)3+xO6 at 531 and 1579 cm?1 while Ca3-x(Al,Si)6+xO14 has a strong peak at 553 cm?1. CaAl4-xSixO7 shows a weak Raman pattern, while Ca1-x(Al,Si)2+xO5 has no detectable Raman modes. Since the association moissanite-CAS is thermodynamically not stable at ambient pressure and moissanite crystals hosting the CAS phases have ?13C values typical of deep-mantle origin, we interpret the CAS inclusions as partially retrogressed HP minerals. Striking analogies exist between observed CAS compositions and experimentally obtained HP-HT mineralogy. For instance, Cax(Al,Si)1-xO3 contains up to 25 mol% of Al2O3, which is considered as the upper limit of alumina solubility in Ca-perovskite. The study confirms that CAS phases are an important mantle depository for large ion lithophile elements (LILE) and LREE.
Arabian Journal of Geosciences, Vol. 13, , 209 orchid.org/ 0000-002-3287-9537
Africa, Mauritania
craton
Abstract: We used remote sensing, geographical information systems, Google Earth™ images, and regional geology in order to (i) improve the mapping of linear structures and understand the chronology of different mafic dyke swarms in the Ahmeyim area that belongs to the Archean Tasiast-Tijirit Terrane of the Reguibat Shield, West African craton, NW Mauritania. The spatial and temporal distributions with the trends of the dyke swarms provide important information about geodynamics. The analysis of the mafic dyke swarms map and statistical data allow us to distinguish four mafic dyke swarm sets: a major swarm trending NE-SW to NNE-SSW (80%) and three minor swarms trending EW to ENE-WSW (9.33%), NW-SE to WNW-ESE (9.06%), and NS (1.3%). The major swarms extend over 35 km while the minor swarms do not exceed 13 km. The Google Earth™ images reveal relative ages through crossover relationships. The major NE-SW to NNE-SSW and the minor NS swarms are the oldest generations emplaced in the Ahemyim area. The NW-SE-oriented swarm dykes which are cutting the two former swarms are emplaced later. The minor E-W to WSW-ENE swarms are probably the youngest. A precise U-Pb baddeleyite age of 2733?±?2 Ma has been obtained for the NNE-SSW Ahmeyim Great Dyke. This dyke is approximately 1500 m wide in some zone and extends for more than 150 km. The distinct mafic dyke swarms being identified in this study can potentially be linked with coeval magmatic events on other cratons around the globe to identify reconstructed LIPs and constrain continental reconstructions.
Abstract: A lack of liquid water limits life on glaciers worldwide but specialized microbes still colonize these environments. These microbes reduce surface albedo, which, in turn, could lead to warming and enhanced glacier melt. Here we present results from a replicated, controlled field experiment to quantify the impact of microbes on snowmelt in red-snow communities. Addition of nitrogen-phosphorous-potassium fertilizer increased alga cell counts nearly fourfold, to levels similar to nitrogen-phosphorus-enriched lakes; water alone increased counts by half. The manipulated alga abundance explained a third of the observed variability in snowmelt. Using a normalized-difference spectral index we estimated alga abundance from satellite imagery and calculated microbial contribution to snowmelt on an icefield of 1,900?km2. The red-snow area extended over about 700?km2, and in this area we determined that microbial communities were responsible for 17% of the total snowmelt there. Our results support hypotheses that snow-dwelling microbes increase glacier melt directly in a bio-geophysical feedback by lowering albedo and indirectly by exposing low-albedo glacier ice. Radiative forcing due to perennial populations of microbes may match that of non-living particulates at high latitudes. Their contribution to climate warming is likely to grow with increased melt and nutrient input.
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.
The Extractive Industries and Society, Vol. 4, pp. 489-496.
Africa, Guinea
artisanal mining
Abstract: The period of protracted conflict in Sierra Leone and Liberia brought the politics of alluvial diamond mining in West Africa to the forefront of academic and policy-oriented discussions. Using social contract theory, this paper moves away from discussions on how minerals have perpetuated conflict in the region, and interrogates how the governance of diamond mining in Guinea impacts regime stability and social insecurity. More importantly, it attempts to illustrate how artisanal diamond mining contributes to stability. The paper situates this discussion within the broad spectrum of the social contract between state and citizens and an analysis of how these are at play in diamond mining areas. It illustrates how artisanal diamond mining enables specific social contracts to emerge and how this in turn contributes to stability in the regions where they are extracted.
Abstract: In May of 2000, a meeting was convened in Kimberley, South Africa, by representatives of the diamond industry and leaders of African governments to develop a certification process intended to assure that export shipments of rough diamonds were free of conflict concerns. Outcomes of the meeting were formally supported later in December of 2000 by the United Nations in a resolution adopted by the General Assembly. By 2002, the Kimberley Process Certification Scheme (KPCS) was ratified and signed by diamond-producing and diamond-importing countries. The goal of this study was to estimate the alluvial diamond resource endowment and the current production capacity of the alluvial diamond mining sector of Guinea. A modified volume and grade methodology was used to estimate the remaining diamond reserves within Guinea’s diamondiferous regions, while the diamond-production capacity of these zones was estimated by inputting the number of artisanal miners, the number of days artisans work per year, and the average grade of the deposits into a formulaic expression. Guinea’s resource potential was estimated to be approximately 40 million carats, while the production capacity was estimated to lie within a range of 480,000 to 720,000 carats per year. While preliminary results have been produced by integrating historical documents, five fieldwork campaigns, and remote sensing and GIS analysis, significant data gaps remain. The artisanal mining sector is dynamic and is affected by a variety of internal and external factors. Estimates of the number of artisans and deposit variables, such as grade, vary from site to site and from zone to zone. This report has been developed on the basis of the most detailed information available at this time. However, continued fieldwork and evaluation of artisanally mined deposits would increase the accuracy of the results.
Hetenyl, G., Cattin, R., Brunet, F., Bollinger, L., Vergne, J., Nabalek, J.L., Diament, M.
Density distribution of the India plate beneath the Tibetan plateau: geophysical and petrological constraints on kinetics of lower crustal eclogitization
Earth and Planetary Science Letters, Vol. 264, 1-2, pp. 226-244.
Abstract: discovered a unique type of diamond in the Tolbachik volcano frozen lava- in the Kamchatka Peninsula of far East Russia. Scientists from all over the world are quite impressed with these findings and they now classify the gemstones as completely new & unique types of diamonds. The new types of diamonds are named after the place they were found- Tolbachik Diamonds. According to geologists, the Tolbachik diamonds were born under a very peculiar natural formation where the gemstones crystalized under the direct influence of the electric discharge of lightning and the pressure from the volcanic gases. Geologists also agree that several decades ago experts wanted to create a synthetic type of diamond where the result would have been exactly the one now created naturally in the Tolbachik diamonds. In the early 60s the French wanted to create a synthetic diamond where they would use gas as primary substance and then using electric discharge for the crystallization of the gemstone. At a closer look, the Tolbachik diamonds are similar to the synthetic diamonds found on today’s market. However, the main mineralogical and geological structure of the diamond is unlike any other type of precious stone discovered until now. These diamonds are particularly large in size (200 and up to 700 microns!). According to scientists, this is actually the first time when they discovered lava rocks that show such very dense diamond content. This discovery actually gave way to very intense disputes among specialists regarding if the Lab Grown diamonds should be regarded as ‘synthetic’ any longer. The Tolbachik diamonds resemble almost in every aspect the structure of the lab grown diamonds and they are naturally formed in lava. Therefore, the Lab Grown diamonds should be more highly regarded just as the naturally mined diamonds are. Another important fact to keep in mind is that only a true expert is actually able to tell the difference between the Tolbachik diamonds created naturally and the lab grown diamonds. Someone who is not an expert would simply regard the Tolbachik diamonds as synthetic diamonds, given their different structure from the regular diamonds formed in magmatic melt. Experts found an extremely high quantity of diamonds (several hundreds of pieces) in only a small sample of frozen lava in the Kamchatka Lava in Russia. When or how these gemstones will enter the fascinating diamond industry, and if the price of jewelry featuring Tolbachik diamonds will be more budget friendly is yet to be settled…
Abstract: The Natal Valley, offshore Mozambique, is a key area for understanding the evolution of East Gondwana. Within the scope of the integrated multidisciplinary PAMELA project, we present new wide?angle seismic data and interpretations, which considerably alter Geoscience paradigms. These data reveal the presence of a 30?km?thick crust that we argue to be of continental nature. This falsifies all the most recent palaeo?reconstructions of the Gondwana. This 30?km?thick continental crust 1,000 m below sea level implies a complex history with probable intrusions of mantle?derived melts in the lower crust, connected to several occurrences of magmatism, which seems to evidence the crucial role of the lower continental crust in passive margin genesis.
Watremez, L., Leroy, S., d'Acremont, E., Roche, V., Evain, M., Lepretre, A., Verrier, F., Aslanian, D., Dias, N., Afilhado, A., Schnurle, P., Castilla, R., Despinois, F., Moulin, M.
Abstract: A variety of structures results from the interplay of evolving far-field forces, plate kinematics, and magmatic activity during continental break-up. The east Limpopo transform margin, offshore northern Mozambique, formed as Africa and Antarctica separated during the mid-Jurassic period break-up of the Gondwana supercontinent. The nature of the crust onshore has been discussed for decades in an effort to resolve issues with plate kinematic models. Two seismic refraction profiles with coincident multichannel seismic reflection profiles allow us to interpret the seismic velocity structures across the margin, both onshore and offshore. These seismic profiles allow us to (a) delineate the major regional crustal domains; (b) identify widespread indications of magmatic activity; and (c) map crustal structure and geometry of this magma-rich transform margin. Careful examination of the profiles allows us to make the following observations and interpretations: (a) on land, continental crust is overlain by a >10-km thick volcano-sedimentary wedge related to an early rifting stage, (b) offshore, thick oceanic crust formed due to intense magmatic activity, and between the two (c) a 50-60-km wide transform zone where the crustal structures are affected by intense magmatic activity and faulting. The prominent presence of intrusive and extrusive igneous units may be attributed to the combination of a deep-seated melting anomaly and a trans-tensional fault zone running through thinned lithosphere that allowed melt to reach the surface. A comparison of the crustal thinning along other transform margins shows a probable dependence with the thermal and/or tectonic history of the lithosphere.
Review of African Political Economy, Vol. 45, no. 158, pp. 522-540.
Africa, Sierra Leone
history
Abstract: This article explores the relationship between the 2014-2016 Ebola outbreak and the political economy of diamond mining in Kono District, Sierra Leone. The authors argue that foreign companies have recycled colonial strategies of indirect rule to facilitate the illicit flow of resources out of Sierra Leone. Drawing on field research conducted during the outbreak and in its aftermath, they show how this ‘indirect rule redux’ undermines democratic governance and the development of revenue-generation institutions. Finally, they consider the linkages between indirect rule and the Ebola outbreak, vis-à-vis the consequences of the region’s intentionally underdeveloped health care infrastructure and the scaffolding of outbreak containment onto the paramount chieftaincy system.
Abstract: The Mesoproterozoic Midcontinent Rift System (MRS) of North America hosts a diverse suite of magmatic and hydrothermal mineral deposits in the Lake Superior region where rift rocks are exposed at or near the surface. Historically, hydrothermal deposits, such as Michigan’s native copper deposits and the White Pine sediment-hosted stratiform copper deposit, were major MRS metal producers. On-going exploration for and potential development of copper-nickel sulfide deposits hosted by the Duluth Complex of Minnesota and the opening of the Eagle nickel mine in Michigan indicate an expanding interest in MRS magmatic deposits. MRS hydrothermal and magmatic mineral deposits, many of which are significant past, present, and likely future providers of critical minerals, here are placed into a space and time metallogenic framework. To construct this framework, regional MRS mineral deposits extracted from the U.S. Geological Survey Mineral Resources Data System (MRDS) and the Ontario Ministry of Energy, Northern Development and Mines Mineral Deposit Inventory (MDI) were supplemented by other known and recently recognized mineral deposits described in the literature. All mineral deposits were classified by deposit type, host rock age and type, and estimated timing of mineralization. Deposits were then put into a tectonic evolutionary framework (stages) for the MRS, which shows that deposits formed within discrete spatial and temporal stages of rift evolution. Each stage of rift evolution is characterized by specific mineral deposit types that are largely confined both by their physical location in rift rocks and type and timing of mineralization. Examples include MRS nickel-rich conduit-type magmatic sulfide deposits, which are restricted to an early magmatic stage MRS history when magma compositions were characterized by Ni-rich picrites and high Mg basalts. In contrast, contact-type magmatic sulfide deposits with Cu > Ni were derived from more evolved Al-rich tholeiitic magmas that dominated a later time of voluminous magmatic activity. Hydrothermal sediment-hosted stratiform chalcocite mineralization along the margins of western Lake Superior in Michigan is economic only where fluid flow through red beds was concentrated by structures along the margins of a post-volcanic MRS sedimentary basin. Widespread native Cu and native Ag mineralization occurred about 40 million years after formation of host basalt lava flows and interflow sedimentary rocks. These descriptions of the diverse mineral deposits in the Lake Superior region cataloged within the space of the local and regional geology and over the more than 60-million-year mineralizing history of the MRS are indicative of the complex and at times overlapping magmatic and hydrothermal mineral systems that operated within this major large igneous province.
Geophysical Journal International, Vol. 214, 2, pp. 1281-1300. doi:1093/gji/ggy193
Canada, Nunavut
Geophysics - gravity
Abstract: Mesozoic to Cenozoic continental rifting, breakup and spreading between North America and Greenland led to the opening, from south to north, of the Labrador Sea and eventually Baffin Bay between Baffin Island, northeast Canada and northwest Greenland. Baffin Bay lies at the northern limit of this extinct rift, transform and spreading system and remains largely underexplored. With the sparsity of existing crustal-scale geophysical investigations of Baffin Bay, regional potential field methods and quantitative deformation assessments based on plate reconstructions provide two means of examining Baffin Bay at the regional scale and drawing conclusions about its crustal structure, its rifting history and the role of pre-existing structures in its evolution. Despite the identification of extinct spreading axes and fracture zones based on gravity data, insights into the nature and structure of the underlying crust have only been gleaned from limited deep seismic experiments, mostly concentrated in the north and east where the continental shelf is shallower and wider. Baffin Bay is partially underlain by oceanic crust with zones of variable width of extended continental crust along its margins. 3-D gravity inversions, constrained by bathymetric and depth to basement constraints, have generated a range of 3-D crustal density models that collectively reveal an asymmetric distribution of extended continental crust, approximately 25-30?km thick, along the margins of Baffin Bay, with a wider zone on the Greenland margin. A zone of 5-13?km thick crust lies at the centre of Baffin Bay, with the thinnest crust (5?km thick) clearly aligning with Eocene spreading centres. The resolved crustal thicknesses are generally in agreement with available seismic constraints, with discrepancies mostly corresponding to zones of higher density lower crust along the Greenland margin and Nares Strait. Deformation modelling from independent plate reconstructions using GPlates of the rifted margins of Baffin Bay was performed to gauge the influence of original crustal thickness and the width of the deformation zone on the crustal thicknesses obtained from the gravity inversions. These results show the best match with the results from the gravity inversions for an original unstretched crustal thickness of 34-36?km, consistent with present-day crustal thicknesses derived from teleseismic studies beyond the likely continentward limits of rifting around the margins of Baffin Bay. The width of the deformation zone has only a minimal influence on the modelled crustal thicknesses if the zone is of sufficient width that edge effects do not interfere with the main modelled domain.
Alkaline hybrid mafic magmas of the Yampa area, northwest Colorado, and their relationship to the Yellowstone mantle plume and lithospheric mantle domains
Contributions to Mineralogy and Petrology, Vol. 107, No. 3, pp. 310-327
Symposium on critical and strategic materials, British Columbia Geological Survey Paper 2015-3, held Nov 13-14 2015, pp. 5-12.
Global
Rare earths
Abstract: Rare earth elements (REE), as defi ned by the International Union of Pure and Applied Chemistry (IUPAC), include yttrium (Y), scandium (Sc), and the lanthanides, comprising lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). In the scientifi c community, subdivisions into light (LREE) and heavy (HREE) categories are based on electron confi guration. In this context, LREE include La, Ce, Pr, Nd, Pm, Sm, Eu, and Gd and HREE include Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu (Connelly et al., 2005). Industry commonly refers to LREE as lanthanides from La to Sm, and HREE as lanthanides from Eu to Lu, plus Y (Simandl, 2014). World mine production of rare earth oxides (REO) for 2014 is estimated at approximately 117,000 tonnes, including Y2O3, which accounts for 7000 tonnes of the total (Gambogi, 2015a, b). In 2014, the main producing countries for REE less Y were China, with 86% of worldwide production (Gambogi, 2015a), the United States, India, Australia, Russia, and Thailand (Fig. 1).
Abstract: Precambrian supercontinents Nuna-Columbia (1.7 to 1.3 billion years ago) and Rodinia (1.1 to 0.7 billion years ago) have been proposed. However, the arrangements of crustal blocks within these supercontinents are poorly known. Huge, dominantly basaltic magmatic outpourings and intrusions, covering up to millions of square kilometres, termed Large Igneous Provinces, typically accompany (super) continent breakup, or attempted breakup and offer an important tool for reconstructing supercontinents. Here we focus on the Large Igneous Province record for Siberia and Laurentia, whose relative position in Nuna-Columbia and Rodinia reconstructions is highly controversial. We present precise geochronology—nine U -Pb and six Ar -Ar ages—on dolerite dykes and sills, along with existing dates from the literature, that constrain the timing of emplacement of Large Igneous Province magmatism in southern Siberia and northern Laurentia between 1,900 and 720 million years ago. We identify four robust age matches between the continents 1,870, 1,750, 1,350 and 720 million years ago, as well as several additional approximate age correlations that indicate southern Siberia and northern Laurentia were probably near neighbours for this 1.2-billion-year interval. Our reconstructions provide a framework for evaluating the shared geological, tectonic and metallogenic histories of these continental blocks.
Physicsa Status Solidi , doi:10.1002/pssa.201900888
Global
HPHT
Abstract: Various samples of multisectoral high?pressure high?temperature (HPHT) single?crystal diamond plate (IIa type) (4?×?4?×?0.53?mm) are tested for particle detection applications. The samples are investigated by X?ray diffractometry, photoluminescence spectroscopy, Raman spectroscopy, Fourier?transform infrared, and visible/ultraviolet (UV) absorption spectroscopy. High crystalline perfection and low impurity concentration (in the {100} growth sector) are observed. To investigate detector parameters, circular 1.0 and 1.5?mm diameter Pt Schottky barrier contacts are created on {111} and {100} growth sectors. On the backside, a Pt contact (3.5?×?3.5?mm) is produced. The {100} growth sector is proved to be a high?quality detector: the full width at half maximum energy resolution is 0.94% for the 5.489?MeV 226Ra ??line at an operational bias of +500?V. Therefore, it is concluded that the HPHT material {100} growth sector is used for radiation detector production, whose quality is not worse than the chemical vapor deposition method or specially selected natural diamond detectors.
Geochimica et Cosmochimica Acta, Vol. 173, pp. 304-318.
Mantle
Perovskite
Abstract: Partitioning of hydrogen (often referred to as H2O) between periclase (pe) and perovskite (pvk) at lower mantle conditions (24-80 GPa) was investigated using quantum mechanics, equilibrium reaction thermodynamics and by monitoring two H-incorporation models. One of these (MSWV) was based on replacements provided by Mg2+ ? 2H+ and Si4+ ? 4H+; while the other (MSWA) relied upon substitutions in 2Mg2+ ? Al3+ + H+ and Si4+ ? Al3+ + H+. H2O partitioning in these phases was considered in the light of homogeneous (Bulk Silicate Earth; pvk: 75%-pe:16% model contents) and heterogeneous (Layered Mantle; pvk:78%-pe:14% modal contents) mantle geochemical models, which were configured for lower and upper bulk water contents (BWC) at 800 and 1500 ppm, respectively. The equilibrium constant, BWCK(P,T), for the reactions controlling the H-exchange between pe and pvk exhibited an almost negligible dependence on P, whereas it was remarkably sensitive to T, BWC and the hydrogen incorporation scheme. Both MSWV and MSWA lead to BWCK(P,T) ? 1, which suggests a ubiquitous shift in the exchange reaction towards an H2O-hosting perovskite. This took place more markedly in the latter incorporation mechanism, indicating that H2O-partitioning is affected by the uptake mechanism. In general, the larger the BWC, the smaller the BWCK(P,T). Over the BWC reference range, MSWV led to BWCK(P,T)-grand average (?BWCK?) calculated along lower mantle P-T-paths of ?0.875. With regard to the MSWA mechanism, ?BWCK? was more sensitive to BWC (and LM over BSE), but its values remained within the rather narrow 0.61-0.78 range. The periclase-perovskite H2O concentration-based partition coefficient, View the MathML sourceKdH2Ope/pvk, was inferred using ?BWCK ?, assuming both hydrous and anhydrous-dominated systems. MSWV revealed a View the MathML sourceKdH2Ope/pvk-BWC linear interpolation slope which was close to 0 and View the MathML sourceKdH2Ope/pvk values of 0.36 and 0.56 (for anhydrous and hydrous system, respectively). MSWA, in turn, yielded a View the MathML sourceKdH2Ope/pvk trend with a slightly steeper negative BWC -slope, while it may also be considered nearly invariant with View the MathML sourceKdH2Ope/pvk values of 0.31-0.47 in the 800-1500 ppm interval. Combining the MSWV and MSWA results led to the supposition that View the MathML sourceKdH2Ope/pvk lies in the narrow 0.31-0.56 interval, as far as the P-T-BWC values of interest are concerned. This implies that water always prefers pvk to pe. Furthermore, it also suggests that even in lower mantle with low or very low bulk water content, periclase rarely becomes a pure anhydrous phase.
High pressure intermediate temperature metamorphism in the southern Barbarton granitoid greenstone terrain, South Africa: a consequence of subduction driven ...
Benn, K., Mareschal, J-C., Condie, K.C. Archean Geodynamics and Environments, AGU Geophysical Monograph, No. 164, pp. 239-254.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0631-6 13p.
Canada, Northwest Territories
deposit - Kelvin
Abstract: The early Cambrian to late Neoproterozoic Kelvin kimberlite pipe is located in the southeast of the Archean Slave Craton in northern Canada, eight km northeast of the Gahcho Kué diamond mine. Kelvin was first discovered in 2000 by De Beers Canada. Subsequent exploration undertaken by Kennady Diamonds Inc. between 2012 and 2016 resulted in the discovery of significant thicknesses of volcaniclastic kimberlite that had not previously been observed. Through extensive delineation drilling Kelvin has been shown to present an atypical, steep-sided inclined L-shaped pipe-like morphology with an overall dip of 15 to 20°. With a surface expression of only 0.08 ha Kelvin dips towards the northwest before turning north. The body (which remains open at depth) has been constrained to a current overall strike length of 700 m with varying vertical thickness (70 to 200 m) and width (30 to 70 m). Detailed core logging, petrography and microdiamond analysis have shown that the pipe infill comprises several phases of sub-horizontally oriented kimberlite (KIMB1, KIMB2, KIMB3, KIMB4, KIMB7 and KIMB8) resulting from multiple emplacement events. The pipe infill is dominated by Kimberley-type pyroclastic kimberlite or “KPK”, historically referred to as tuffisitic kimberlite breccia or “TKB”, with less common hypabyssal kimberlite (HK) and minor units with textures transitional between these end-members. An extensive HK sheet complex surrounds the pipe. The emplacement of Kelvin is believed to have been initiated by intrusion of this early sheet system. The main pipe-forming event and formation of the dominant KPK pipe infill, KIMB3, was followed by late stage emplacement of additional minor KPK and a hypabyssal to transitional-textured phase along the upper contact of the pipe, cross-cutting the underlying KIMB3. Rb-Sr age dating of phlogopite from a late stage phase has established model ages of 531 ± 8 Ma and 546 ± 8 Ma. Texturally and mineralogically, the Kelvin kimberlite is similar to other KPK systems such as the Gahcho Kué kimberlites and many southern African kimberlites; however, the external morphology, specifically the sub-horizontal inclination of the pipe, is unique. The morphology of Kelvin and the other kimberlites in the Kelvin-Faraday cluster defines a new type of exploration target, one that is likely not unique to the Kennady North Project area. Extensive evaluation work by Kennady Diamonds Inc. has resulted in definition of a maiden Indicated Mineral Resource for Kelvin of 8.5 million tonnes (Mt) of kimberlite at an average grade of 1.6 carats per tonne (cpt) with an average diamond value of US$ 63 per carat (ct).
Abstract: In the last two decades, airborne laser scanning (ALS) has found widespread application and driven fundamental advances in the Earth sciences. With increasing availability and accessibility, multi-temporal ALS data have been used to advance key research topics related to dynamic Earth surface processes. This review presents a comprehensive compilation of existing applications of ALS change detection to the Earth sciences. We cover a wide scope of material pertinent to the broad field of Earth sciences to encourage the cross-pollination between sub-disciplines and discuss the outlook of ALS change detection for advancing scientific discovery. While significant progress has been made in applying repeat ALS data to change detection, numerous approaches make fundamental assumptions that limit the full potential of repeat ALS data. The use of such data for 3D change detection is, therefore, in need of novel, scalable, and computationally efficient processing algorithms that transcend the ever-increasing data density and spatial coverage. Quantification of uncertainty in change detection results also requires further attention, as it is vitally important to understand what 3D differences detected between epochs represent actual change as opposed to limitations in data or methodology. Although ALS has become increasingly integral to change detection across the Earth sciences, the existence of pre- and post-event ALS data is still uncommon for many isolated hazard events, such as earthquakes, volcanic eruptions, wildfires, and landslides. Consequently, data availability is still a major limitation for many ALS change detection applications.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 265.
Global
DiamondView
Abstract: Diamond, known for its splendor in exquisite jewelry, has been synthesized since the 1950s. In the last six decades, the perfection of laboratory-grown single-crystal diamond has vastly improved through the research and development of two main synthesis techniques. One replicates Earth’s natural process, where the diamond is grown in the laboratory under conditions of diamond stability at high temperature and high pressure (HPHT). The other technique relies on the dissociation of methane (or other carbon-containing source gas) and hydrogen and the subsequent deposition of diamond at low pressures from the gaseous phase in a process known as chemical vapor deposition (CVD). In the latter case, diamond is not the stable form of carbon, but the kinetics in the CVD process are such that diamond wins out. Large gem-quality synthetic diamonds are now possible, and a 6 ct CVD (2018) and a 15.32 ct HPHT (2018) have been reported. It is of course possible to differentiate laboratory-grown from natural diamond based on how extended and point defects are incorporated into the crystal. Furthermore, treated diamond can be identified utilizing knowledge of how defects are produced and how they migrate and aggregate in both natural and synthetic diamond samples. Room-temperature confocal photoluminescence microscopy can be used to image the emission of light from defects in diamond with a spatial resolution limited only by the diffraction limit; a lateral spatial resolution approaching 300 nm is routinely achieved (figure 1). It is possible with this tool to identify point defects with concentrations less than 1 part per trillion (1011 cm–3). This talk will outline the experimental setup, how this tool has been used to identify the decoration of dislocations with point defects in CVD lab-grown diamond, and how different mechanisms for defect incorporation operate at growth sector boundaries in HPHT synthetic diamond.
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.
Diamond and Related Materials, in press available 33p.
Global
DiaMap
Abstract: Type IIb diamonds are those that contain more boron than nitrogen. The presence of this uncompensated boron gives rise to absorption in the infrared part of the electromagnetic spectrum, extending into the visible region and often resulting in blue colouration. Here we report on the expansion of the DiaMap freeware (for the automated spectral deconvolution of Type I [nitrogen containing] diamonds) to work on Type IIb diamonds, returning concentrations from three boron-related absorption bands, and determining which band provides the most reliable value. The program uses the calibration coefficients of Collins (2010), which show good relative agreement between the three bands, but might require some further study to confirm their absolute accuracy to the uncompensated boron concentration. The methodology of DiaMap_IIb is applicable to all Type IIb diamonds, both natural and synthetic. Analysis of high-resolution Fourier-transform infrared (FTIR) maps of two high-pressure high-temperature (HPHT) synthetic diamonds using DiaMap_IIb, confirm the growth sector dependence of the boron incorporation. Partitioning of boron strongly favours the octahedral {111} sectors.
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.
Diamonds & Related Materials, In press available, 30p. Pdf
Global
synthetics
Abstract: Type IIb diamonds are those that contain more boron than nitrogen. The presence of this uncompensated boron gives rise to absorption in the infrared part of the electromagnetic spectrum, extending into the visible region and often resulting in blue colouration. Here we report on the expansion of the DiaMap freeware (for the automated spectral deconvolution of Type I [nitrogen containing] diamonds) to work on Type IIb diamonds, returning concentrations from three boron-related absorption bands, and determining which band provides the most reliable value. The program uses the calibration coefficients of Collins (2010), which show good relative agreement between the three bands, but might require some further study to confirm their absolute accuracy to the uncompensated boron concentration. The methodology of DiaMap_IIb is applicable to all Type IIb diamonds, both natural and synthetic. Analysis of high-resolution Fourier-transform infrared (FTIR) maps of two high-pressure high-temperature (HPHT) synthetic diamonds using DiaMap_IIb, confirm the growth sector dependence of the boron incorporation. Partitioning of boron strongly favours the octahedral {111} sectors.
Deijanin, B., Simic, D., Zaitsev, A., Chapman, J., Dobrinets, I., Widemann, A., Del Re, N., Middleton, T., Dijanin, E., Se Stefano, A.
Characterization of pink diamonds of different origin: natural ( Argyle, non-Argyle), irradiated and annealed, treated with multi-process, coated and synthetic.
Diamond and Related Materials, Vol. 17, 7-10, pp. 1169-1178.
Abstract: Lamprophyres represent hydrous alkaline mantle melts that are a unique source of information about the composition of continental lithosphere. Throughout southwest Britain, post-Variscan lamprophyres are (ultra)potassic with strong incompatible element enrichments. Here we show that they form two distinct groups in terms of their Sr and Nd isotopic compositions, occurring on either side of a postulated, hitherto unrecognized terrane boundary. Lamprophyres emplaced north of the boundary fall on the mantle array with ?Nd ?1 to +1.6. Those south of the boundary are enriched in radiogenic Sr, have initial ?Nd values of ?0.3 to ?3.5, and are isotopically indistinguishable from similar-aged lamprophyres in Armorican massifs in Europe. We conclude that an Armorican terrane was juxtaposed against Avalonia well before the closure of the Variscan oceans and the formation of Pangea. The giant Cornubian Tin-Tungsten Ore Province and associated batholith can be accounted for by the fertility of Armorican lower crust and mantle lithosphere.
Abstract: In recent years we have confirmed the existence of ophiolite-hosted diamonds on Earth, which occur in mantle peridotites and podiform chromitites of many ophiolites. These diamonds differ significantly from most kimberlite varieties, particularly in their inclusions. The typical inclusions in the diamonds are Mn-rich phases, i.e., NiMnCo alloy, native Mn, MnO, galaxite, Mn olivine and Mn garnet. Ca-silicate perovskite, a typical lower mantle mineral, was identified as mineral inclusions in diamond. One occurs as a 60-nanometer, euhedral grain associated with NiMnCo alloy and graphite, while another one occurs as a 50-nanometer grain within a large inclusion containing both NiMnCo alloy and Nd-Se-Cu-S phase. By EDS the perovskite has Ca 48.3%, Si 37.7% and Mn 14.1% with oxygen. TEM diffraction data show that the inclusion has d-spacings and angles between adjacent lattice planes are consistent to the Ca-silicate perovskite with an orthorhombic structure. The only known source of such light carbon is organic material in surface sediments and the best known sources of abundant manganese are Fe-Mn-rich sediments and Mn nodules, both of which are common on the seafloor. Many parts of the modern seafloor are also covered by sediments with a continental provenance. Phases such as SiO2 and Al2O3 are not expected in mantle peridotites and must have been introduced from shallow levels. We propose that subduction of oceanic lithosphere carries C, Mn, Si, Al and REE to the transition zone or lower mantle where the material is mixed with highly reduced material, perhaps derived from greater depths. Crystallization of diamond from a C-rich fluid encapsulates the observed inclusions. The diamonds and associated minerals are incorporated into chromite grains during chromite crystallization at depth of mantle transition zone, and are carried to shallower levels by mantle convection. Accumulation of chromite grains produces podiform chromitites containing a range of exotic minerals. However, the presence of diamonds and other UHP minerals in ophiolitic peridotites indicates that such phases can persist far outside their normal stability fields.
Geological Society of America Annual Meeting, Vol. 47, 7, p. 163. abstract
Europe, Turkey
Moissanite
Abstract: The Aladag ophiolite in the eastern Tauride belt, southern Turkey, is a well-preserved remnant of oceanic lithosphere. It consists of, in ascending order, harzburgitic to dunitic tectonites, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes and basaltic pillow lavas. Podiform chromitites are common in the mantle peridotites. Thus far, more than 200 grains of microdiamond and more than 100 grains of moissanite (SiC) have been separated from one sample of podiform chromitite. The microdiamonds occur mostly as subhedral to euhedral, colorless to pale yellow grains, about 50-300 ?m in size. Moissanite grains are generally subhedral, light blue to deep blue in color and variable in size. These grains of diamond and moissanite are very similar to in-situ grains in podiform chromitites of Tibet and the Polar Urals of Russia (Yang et al., 2014; 2015), indicating that they are natural minerals, not the result of natural or anthropogenic contamination. As reported elsewhere, the diamonds and moissanite are accompanied by a range of other minerals, including rutile, zircon, quartz and sulfides. The discovery of diamond, moissanite and other unusual minerals in the podiform chromitites of the Aladag massif provide additional evidence for the widespread occurrence of these minerals in ophiolites, indicating that they are related to global mantle processes.
Haissen, F., Cambeses, A., Montero, P., Bea, F., Dilek, Y., Mouttaqi, A.
The Archean kaisilite nepheline syenites of the Awsard intrusive massif ( Reguibat Shield, West African craton, Morocco) and its relationship to alkaline magmatism of Africa.
Journal of African Earth Sciences, Vol. 127, pp. 16-50.
Abstract: Various combinations of diamond, moissanite, zircon, corundum, rutile and titanitehave been recovered from the Bulqiza chromitites. More than 10 grains of diamond have been recovered, most of which are pale yellow to reddish–orange to colorless. The grains are all 100–300 ?m in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm?1 and 1333 cm?1, mostly at 1331.51 cm?1 or 1326.96 cm?1. This investigation extends the occurrence of diamond and moissanite to the Bulqiza chromitites in the Eastern Mirdita Ophiolite. Integration of the mineralogical, petrological and geochemical data of the Bulqiza chromitites suggests their multi–stage formation. Magnesiochromite grains and perhaps small bodies of chromitite formed at various depths in the upper mantle, and encapsulated the ultra–high pressure, highly reduced and crustal minerals. Some oceanic crustal slabs containing the magnesiochromite and their inclusion were later trapped in suprasubduction zones, where they were modified by tholeiitic and boninitic arc magmas, thus changing the magnesiochromite compositions and depositing chromitite ores in melt channels.
Abstract: The Pozanti-Karsanti ophiolite situated in the eastern Tauride belt, southern Turkey, is a well-preserved oceanic lithosphere remnant comprising, in ascending order, mantle peridotite, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes, and basaltic pillow lavas. Two types of chromitites are observed in the Pozanti-Karsanti ophiolite. One type of chromitites occurs in the cumulate dunites around the Moho, and the other type of chromitites is hosted by the mantle harzburgites below the Moho. The second type of chromitites has massive, nodular, and disseminated textures. We have conducted the mineral separation work on the podiform chromitites hosted by harzburgites. So far, more than 100 grains of microdiamond and moissanite (SiC) have been recovered from the podiform chromitite. The diamonds and moissanite are accompanied by large amounts of rutile. Besides zircon, monazite and sulfide are also very common phases within the separated minerals. The discovery of diamond, moissanite, and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new evidences for the common occurrences of these unusual minerals in ophiolitic peridotites and chromitites. This discovery also suggests that deep mantle processes and materials have been involved in the formation of podiform chromitite.
Abstract: Ophiolitic peridotites represent variously depleted residues of the primitive mantle after multiple episodes of partial melting, melt extraction, and melt-rock interactions. They display a wide range of compositional and geochemical heterogeneities at different scales, and their incompatible bulk-rock compositions and mineral chemistries are commonly inconsistent with their evolution through simple partial melting processes at shallow mantle depths. Approaching these issues from different perspectives, the papers in this volume concentrate on (1) melt evolution and magmatic construction of ophiolites in various tectonic settings, and (2) the occurrence of microdiamonds, ultrahigh-pressure (UHP) minerals, and crustal material as inclusions in ophiolitic chromitites and peridotites. Crustal and mantle rock units exposed in different ophiolites show that the mantle melt sources of ophiolitic magmas undergo progressive melting, depletion, and enrichment events, constantly modifying the melt compositions and the mineralogical and chemical makeup of residual peridotites. Formation and incorporation of microdiamonds and UHP minerals into chromite grains occurs at depths of 350-660 km in highly reducing conditions of the mantle transition zone. Carbon for microdiamonds and crustal minerals are derived from subduction-driven recycling of surface material. Host peridotites with their UHP mineral and diamond inclusions are transported into shallow mantle depths by asthenospheric upwelling, associated with either slab rollback-induced channel flow or superplumes. Decompression melting of transported mantle rocks beneath oceanic spreading centers and their subsequent flux melting in mantle wedges result in late-stage formation of podiform chromitites during the upper mantle petrogenesis of ophiolites. Future studies should demonstrate whether diamonds and UHP minerals also occur in peridotites and chromitites of nonsubduction-related ophiolites.
Abstract: We document in this study the geological occurrence of diamonds and other ultrahigh-pressure (UHP) minerals in ophiolitic mantle peridotites and podiform chromitites from different orogenic belts. These minerals exist in both high-Cr and high-Al chromitites. Most ophiolite-hosted diamonds are small (? 200-500 ?m across), and some contain distinctive inclusions (i.e., coesite, Ni-Mn-Co alloys, spessartite, tephroite). All of the analyzed diamonds have extremely light carbon isotope compositions (?13C = -28.7 to -18.3‰) and variable trace element contents that distinguish them from most kimberlitic and UHP metamorphic varieties. A wide range of highly reduced minerals, such as native elements, Ni-Mn-Co alloys, Fe-Si and Fe-C phases and moissanite (SiC) also occuras accompanying mineral separates confirming the super-reducing conditions of their environment of formation. The presence of exsolution lamellae of diopside and coesite in some chromite grains suggests chromite crystallization depths around >380 km, near the mantle transition zone. Carbon and other recycled crustal materials at these depths are likely to have been derived from previously subducted material. The peridotites encapsulating the podiform chromitites and diamonds were transported to shallow mantle by convection cells beneath oceanic spreading centers. The chromitites may have formed in the deep mantle or in shallow suprasubduction zone environments. Our observations suggest that diamonds, UHP minerals and recycled crustal material are likely to be ubiquitous in the oceanic mantle.
Contributions to Mineralogy and Petrology, doi.org:10.1007/ s00410-018-1499-5 19p.
Europe, Turkey
diamond inclusions
Abstract: The Pozanti-Karsanti ophiolite (PKO) is one of the largest oceanic remnants in the Tauride belt, Turkey. Micro-diamonds were recovered from the podiform chromitites, and these diamonds were investigated based on morphology, color, cathodoluminescence, nitrogen content, carbon and nitrogen isotopes, internal structure and inclusions. The diamonds recovered from the PKO are mainly mixed-habit diamonds with sectors of different brightness under the cathodoluminescence images. The total ?13C range of the PKO diamonds varies between ? 18.8 and ? 28.4‰, with a principle ?13C mode at ? 25‰. Nitrogen contents of the diamonds range from 7 to 541 ppm with a mean value of 171 ppm, and the ?15N values range from ? 19.1 to 16.6‰, with a ?15N mode of ? 9‰. Stacking faults and partial dislocations are commonly observed in the Transmission Electron Microscopy foils whereas inclusions are rather rare. Combinations of ( Ca0.81Mn0.19)SiO3, NiMnCo-alloy and nanosized, quenched fluid phases were observed as inclusions in the PKO diamonds. We believe that the 13C-depleted carbon signature of the PKO diamonds derived from previously subducted crustal matter. These diamonds may have crystallized from C-saturated fluids in the asthenospheric mantle at depth below 250 km which were subsequently carried rapidly upward by asthenospheric melts.
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:
We report new U-Pb zircon age data, zircon in situ oxygen isotope, mineral chemistry, whole-rock geochemistry and Sr-Nd isotopic compositions from the Early Devonian ultrapotassic Gucheng pluton in the North China Craton, and discuss its petrogenesis. The Gucheng pluton is exposed in the northern part of the North China Craton and forms a composite intrusion, consisting of K-feldspar-bearing clinopyroxenite, clinopyroxene-bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene (Wo43-48En19-35Fs18-38), sanidine (An0Ab3-11Or89-97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in situ oxygen isotopes (?18O = 5.2-6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the North China Craton. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the North China Craton facing the Palaeo-Asian Ocean.
Lithos, doi. 10.1016/j. lithos.2020.105949 1p. Pdf
Mantle
subduction
Abstract: Crystal morphologies are essential for deciphering the reaction history of igneous and metamorphic rocks because they often record the interplay between nucleation and growth rates controlled by the departure from equilibrium. Here, we report an exceptional record of the morphological transition of olivine formed during subduction metamorphism and high-pressure dehydration of antigorite-serpentinite to prograde chlorite-harzburgite in the Almirez ultramafic massif (Nevado-Filábride Complex, Betic Cordillera, SE Spain). In this massif, rare varied-textured chlorite-harzburgite (olivine+enstantite+chlorite+oxides) —formed after high-P dehydration of antigorite-serpentinite— exhibits large olivine porphyroblasts made up of rounded cores mantled by coronas of tabular olivine grains, similar to single tabular olivines occurring in the matrix. The correlative X-ray ?-CT and EBSD study of two varied-textured chlorite-harzburgite samples show that tabular olivine in coronas is tabular on (100)Ol with c > b >> a, and grew in nearly the same crystallographic orientation as the rounded olivine cores of the porphyroblast. Quantitative textural analysis and mass balance indicate that varied-textured chlorite-harzburgite is the result of a two-stage nucleation and growth of olivine during the progress of the high-P dehydration of antigorite-serpentinite to chlorite-harzburgite reaction. The first stage occurred under a low affinity (?Gr) and affinity rate (?Gr/dt) of the antigorite dehydration reaction that resulted in a low time-integrated nucleation rate and isotropic growth of olivine, forming rounded olivine porphyroblasts. With further progress of the dehydration reaction, a second stage of relatively higher affinity and affinity rate resulted in a higher time-integrated nucleation rate of olivine coeval with a shift from isotropic to anisotropic olivine growth, leading to tabular olivines. The two-stage evolution resulted in olivine porphyroblasts made up of rounded cores mantled by coronas of tabular olivine grains characteristic of varied-texture chlorite-harzburgite. Although a switch to anisotropic tabular olivine in the second stage is consistent with the relative increase in the affinity and affinity rate, these changes cannot solely account for the growth of Almirez olivine tabular on (100). Tabular olivines in komatiites and other igneous rocks are tabular on (010)Ol with either a > c >> b, or a ? c > > b, in agreement with experimentally determined growth rates of olivine phenocrysts under moderate to high undercooling and cooling rates. On the other hand, olivine tabular on (100) is expected in the presence of highly polymerized fluids where inhibited growth of the olivine (100) and (010) interfaces occurs, respectively, due to dissociative and molecular adsorption of water monolayers. Rounded and tabular olivines in Almirez varied-textured chlorite-harzburgite show differing trace element compositions that we interpret as due to the infiltration of external fluids during antigorite dehydration. Isothermal infiltration of highly polymerized fluids would explain the shift in the affinity and affinity rate of the antigorite dehydration reaction, as well as the olivine morphology tabular on (100) due to the inhibited growth on the (100) and, to a lesser extent, (010). Our study shows that surface-active molecules may play an essential role in shaping the morphology of growing crystals during fluid-present metamorphic crystallization.
Stratigraphy and sedimentology of an early Archean felsic volcanicsequence, eastern Pilbara block, Western Australia, with special reference to Duffer F.
Precambrian Research, Vol. 44, No. 2, August pp. 147-
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 18p.
Africa, Botswana
Deposit - Jwaneng
Abstract: Slope stability is a fundamental part of a successful mining operation. It impacts directly on the safety of personnel and the economics of the mine. The Jwaneng Mine in Botswana is planning a push back of 644m depth in the south east wall. Mining will commence in 2010 and be completed in 2024. This will put the mine in the deep pit category. The mine has invested in extensive geotechnical data gathering and design programs that started as far back as 2003 for the south east wall. The south east wail of Jwaneng Mine is characterized by foliation that dips adversely into the mining faces. The orienta1ion of the foliation is variable due to intense tectonic movements that have also caused the occurrence of faults that are sub-vertical. A considerable amount of information on the characteristics of structural patterns and the rock mass has been collected. This paper presents an overview of the feasibility level geotechnical design that is about to be concluded.
Journal of South African Institute of Mining and Metallurgy, Vol. 116, Feb. pp. 189-201.
Canada, Northwest Territories
Deposit - Fox, Ekati
Abstract: Multiple-point simulation (MPS) methods have been developed over the last decade as a means of reproducing complex geological patterns while generating stochastic simulations. Some geological spatial configurations are complex, such as the spatial geometries and patterns of diamond-bearing kimberlite pipes and their internal facies controlling diamond quality and distribution. Two MPS methods were tested for modelling the geology of a diamond pipe located at the Ekati mine, NT, Canada. These are the single normal equation simulation algorithm SNESIM, which captures different patterns from a training image (TI), and the filter simulation algorithm FILTERSIM, which classifies the patterns founded on the TI. Both methods were tested in the stochastic simulation of a four-category geology model: crater, diatreme, xenoliths, and host rock. Soft information about the location of host rock was also used. The validation of the simulation results shows a reasonable reproduction of the geometry and data proportions for all geological units considered; the validation of spatial statistics, however, shows that although simulated realizations from both methods reasonably reproduce the fourth-order spatial statistics of the TI, they do not reproduce well the same spatial statistics of the available data (when this differs from the TI). An interesting observation is that SNESIM better imitated the shape of the pipe, while FILTERSIM yielded a better reproduction of the xenolith bodies.
Precamrbian terranes in the southwestern framing of the Siberian craton: isotopic provinces, stages of crustal evolution and accretion collision events.
Russian Geology and Geophysics, Vol. 48, pp. 61-70.
Abstract: In the Rodinia supercontinent, Laurentia is placed at the center because it was flanked by late Neoproterozoic rifted margins; however, the conjugate margin for western Laurentia is still enigmatic. In this study, new paleomagnetic results have been obtained from 15 ca. 775 Ma mafic dikes in eastern Hebei Province, North China craton (NCC). Stepwise thermal demagnetization revealed a high-temperature component, directed northeast or southwest with shallow inclinations, with unblocking temperatures of as high as 580 °C. Rock magnetism suggests the component is carried by single-domain and pseudo-single-domain magnetite grains. Its primary origin is supported by a positive reversal test and regional remanence direction correlation test, and the paleomagnetic pole (29.0°S, 64.7°E, A95 = 5.4°) is not similar to any published younger poles of the NCC. Matching the late Mesoproterozoic to early Neoproterozoic (ca. 1110-775 Ma) apparent polar wander paths of the NCC and Laurentia suggests that the NCC could have been the conjugate margin for northwestern Laurentia in Rodinia, rather than sitting off the northeast coast of the main Rodinian landmass. Geological data indicate that breakup of the NCC and Laurentia occurred between ca. 775 and 720 Ma.
Abstract: The location of the West African craton (WAC) has been poorly constrained in the Paleoproterozoic-Mesoproterozoic supercontinent Nuna (also known as Columbia). Previous Nuna reconstruction models suggested that the WAC was connected to Amazonia in a way similar to their relative position in Gondwana. By an integrated paleomagnetic and geochronological study of the Proterozoic mafic dikes in the Anti-Atlas Belt, Morocco, we provide two reliable paleomagnetic poles to test this connection. Incorporating our new poles with quality-filtered poles from the neighboring cratons of the WAC, we propose an inverted WAC-Amazonia connection, with the northern WAC attached to northeastern Amazonia, as well as a refined configuration of Nuna. Global large igneous province records also conform to our new reconstruction. The inverted WAC-Amazonia connection suggests a substantial change in their relative orientation from Nuna to Gondwana, providing an additional example of large-magnitude cumulative azimuthal rotations between adjacent continental blocks over supercontinental cycles.
Abstract: This paper presents the first major and trace element compositions of mantle-derived garnet xenocrysts from the diamondiferous No. 30 kimberlite pipe in the Wafangdian region, and these are used to constrain the nature and evolution of mantle metasomatism beneath the North China Craton (NCC). The major element data were acquired using an electron probe micro-analyzer and the trace element data were obtained using laser ablation inductively coupled plasma-mass spectrometry. Based on Ni-in-garnet thermometry, equilibrium temperatures of 1107-1365 °C were estimated for peridotitic garnets xenocrysts from the No. 30 kimberlite, with an average temperature of 1258 °C, and pressures calculated to be between 5.0 and 7.4 GPa. In a CaO versus Cr2O3 diagram, 52% of the garnets fall in the lherzolite field and 28% in the harzburgite field; a few of the garnets are eclogitic. Based on rare earth element patterns, the lherzolitic garnets are further divided into three groups. The compositional variations in garnet xenocrysts reflect two stages of metasomatism: early carbonatite melt/fluid metasomatism and late kimberlite metasomatism. The carbonatite melt/fluids are effective at introducing Sr and the light rare earth elements, but ineffective at transporting much Zr, Ti, Y, or heavy rare earth elements. The kimberlite metasomatic agent is highly effective at element transport, introducing, e.g., Ti, Zr, Y, and the rare earth elements. Combined with compositional data for garnet inclusions in diamonds and megacrysts from the Mengyin and Wafangdian kimberlites, we suggest that these signatures reflect a two-stage evolution of the sub-continental lithospheric mantle (SCLM) beneath the NCC: (1) early-stage carbonatite melt/fluid metasomatism resulting in metasomatic modification of the SCLM and likely associated with diamond crystallization; (2) late-stage kimberlite metasomatism related to the eruption of the 465 Ma kimberlite.
Abstract: The grade and morphological character of kimberlite-hosted diamonds were compared to crystallization temperature (T) and oxygen fugacity ( f O 2 ) estimated from groundmass spinels in six kimberlite pipes in the North China Craton (NCC). Crystallization temperatures calculated at an assumed pressure of 1 GPa are in the range of 1037-1395 °C, with a mean of 1182 °C. At these temperatures, the estimated f O 2 varies from 1.2 to 3.1 log units below the nickel-nickel oxide (NNO) buffer. Generally, individual kimberlite pipe shows a small variation of the T (50-100 °C) and f O 2 (0.4-0.6 log units), whereas different kimberlite pipes present great changes of T and f O 2 which can be up to 300 °C and 2 units respectively. The f O 2 of kimberlite magma shows a strong negative correlation with the diamond grade of kimberlite, suggesting that the f O 2 plays an important role in diamond resorption, whereas the T shows no relationship with the diamond grade, indicating the T plays no role in diamond resorption. The conditions of kimberlite crystallization ( f O 2 ) can be a useful parameter in evaluating diamond survival in diamond exploration.
Abstract: Conventional diamond exploration seldom searches directly for diamonds in rock and soil samples. Instead, it focuses on the search for indicator minerals like chrome spinel, which can be used to evaluate diamond potential. Chrome spinels are preserved as pristine minerals in the early Paleozoic (?465 Ma), hydrothermally altered, Group I No. 30 pipe kimberlite that intruded the Neoproterozoic Qingbaikou strata in Wafangdian, North China Craton (NCC). The characteristics of the chrome spinels were investigated by petrographic observation (BSE imaging), quantitative chemical analysis (EPMA), and Raman spectral analysis. The results show that the chrome spinels are mostly sub-rounded with extremely few grains being subhedral, and these spinels are macrocrystic, more than 500 µm in size. The chrome spinels also have compositional zones: the cores are classified as magnesiochromite as they have distinctly chromium-rich (Cr2O3 up to 66.56 wt%) and titanium-poor (TiO2 < 1 wt%) compositions; and the rims are classified as magnetite as they have chromium-poor and iron-rich composition. In the cores of chrome spinels, compositional variations are controlled by Al3+-Cr3+ isomorphism, which results in a strong Raman spectra peak (A1g mode) varying from 690 cm?1 to 702.9 cm?1. In the rims of chrome spinel, compositional variations result in the A1g peak varying from 660 cm?1 to 672 cm?1. The morphology and chemical compositions indicate that the chrome spinels are mantle xenocrysts. The cores of the spinel are remnants of primary mantle xenocrysts that have been resorbed, and the rims were formed during kimberlite magmatism. The compositions of the cores are used to evaluate the diamond potential of this kimberlite through comparison with the compositions of chrome spinels from the Changmazhuang and No. 50 pipe kimberlites in the NCC. In MgO, Al2O3 and TiO2 versus Cr2O3 plots, the chrome spinels from the Changmazhuang and No. 50 pipe kimberlites are mostly located in the diamond stability field. However, only a small proportion of chrome spinels from No. 30 pipe kimberlite have same behavior, which indicates that the diamond potential of the former two kimberlites is greater than that of the No. 30 pipe kimberlite. This is also supported by compositional zones in the spinel grains: there is with an increase in Fe3+ in the rims, which suggests that the chrome spinels experienced highly oxidizing conditions. Oxidizing conditions may have been imparted by fluids/melts that have a great influence on diamond destruction. Here, we suggest that chrome spinel compositions can be a useful tool for identifying the target for diamond potential in the North China Craton.
Abstract: The grade and morphological character of kimberlite-hosted diamonds were compared to crystallization temperature (T) and oxygen fugacity (fO2) estimated from groundmass spinels in six kimberlite pipes in the North China Craton (NCC). Crystallization temperatures calculated at an assumed pressure of 1 GPa are in the range of 1037-1395 °C, with a mean of 1182 °C. At these temperatures, the estimated fO2 varies from 1.2 to 3.1 log units below the nickel-nickel oxide (NNO) buffer. Generally, individual kimberlite pipe shows a small variation of the T (50-100 °C) and fO2 (0.4-0.6 log units), whereas different kimberlite pipes present great changes of T and fO2 which can be up to 300 °C and 2 units respectively. The fO2 of kimberlite magma shows a strong negative correlation with the diamond grade of kimberlite, suggesting that the fO2 plays an important role in diamond resorption, whereas the T shows no relationship with the diamond grade, indicating the T plays no role in diamond resorption. The conditions of kimberlite crystallization (fO2) can be a useful parameter in evaluating diamond survival in diamond exploration.
Abstract: The Bayan Obo REE-Nb-Fe deposit, which reserves the current largest REE resources globally, also hosts over 70% of China’s Nb resources. Unlike many world-class carbonatite-related Nb deposits (e.g. Morro dos Seis Lagos and Araxá, Brazil) with igneous or secondary origin, Nb was mainly stored in Nb-bearing minerals (aeschynite, ilmenorutile, baotite, fergusonite etc.) of hydrothermal origin at Bayan Obo, supported by evidence from petrography, element and isotopic geochemistry. Although igneous fersmite and columbite were occasionally discovered in local carbonatite dykes, the Mesoproterozoic and Paleozoic hydrothermal metasomatism occurred in the ore-hosting dolomite, related to carbonatite intrusion and the closure of Paleo-Asian Ocean respectively, has played a more significant role during the ultimate Nb enrichment. REE, however, was significantly enriched during both the carbonatite-related magmatic and hydrothermal processes. Consequently, there was differentiated mineralization between REE and Nb in the carbonatite dykes and the ores. Niobium mineralization at Bayan Obo is rather limited in Mesoproterozoic carbonatite, whereas more extensive in the metasomatized ore-hosting dolomite, and generally postdating the REE mineralization at the same stage. According to mineral geochemistry, Bayan Obo aeschynite was classified into 3 groups: aeschynite-(Nd) with convex REE patterns (Group 1); aeschynite-(Ce) (Group 2) and nioboaeschynite (Group 3) with nearly flat REE patterns. Aeschynite (Group 1), ilmenorutile and fergusonite precipitated from Paleozoic hydrothermal fluids with advanced fractionation of Ce-rich REE minerals. The Mesoproterozoic hydrothermal Nb mineralization, represented by aeschynite (Group 3) and baotite, occurred postdating REE mineralization at same stage. Besides, fersmite and aeschynite (Group 2) precipitated from the Mesoproterozoic REE-unfractionated melt and hydrothermal fluids, respectively. All above Nb-bearing minerals exhibit extreme Nb-Ta fractionation as a primary geochemical characteristic of mantle-derived carbonatite. The forming age of the aeschynite megacrysts (Group 1) has not been accurately determined. However, the potential age was constrained to ~430 Ma or alternatively ~270-280 Ma subjected to subduction and granite activity, respectively. These aeschynite crystals inherited REEs from multiphase former REE mineralization, with an intermediate apparent Sm-Nd isochron age between the Mesoproterozoic and the Paleozoic REE mineralization events.
Abstract: The finite element method is used to simulate the steady-state temperature field in diamond synthesis cell. The 2D and 3D models of the China-type cubic press with large deformation of the synthesis cell was established successfully, which has been verified by situ measurements of synthesis cell. The assembly design, component design and process design for the HPHT synthesis of diamond based on the finite element simulation were presented one by one. The temperature field in a high-pressure synthetic cavity for diamond production is optimized by adjusting the cavity assembly. A series of analysis about the influence of the pressure media parameters on the temperature field are examined through adjusting the model parameters. Furthermore, the formation mechanism of wasteland was studied in detail. It indicates that the wasteland is inevitably exists in the synthesis sample, the distribution of growth region of the diamond with hex-octahedral is move to the center of the synthesis sample from near the heater as the power increasing, and the growth conditions of high quality diamond is locating at the center of the synthesis sample. These works can offer suggestion and advice to the development and optimization of a diamond production process.
Earth and Planetary Science Letters, Vol. 459, pp. 183-195.
Mantle
Peridotite
Abstract: Magmatism at mid ocean ridges is one of the main pathways of S outflux from deep Earth to the surface reservoirs and is a critical step in the global sulfur cycle, yet our understanding of the behavior of sulfide during decompression melting of the upper mantle is incomplete. In order to constrain the sulfur budget of the mantle and reconcile the sulfur and chalcophile element budget of mantle partial melts parental to primitive mid-ocean ridge basalts (MORBs), here we developed a model to describe the behavior of sulfide and Cu during decompression melting by combining the pMELTS thermodynamic model and empirical sulfur contents at sulfide concentration (SCSS) models, taking into account the effect of the presence of Ni and Cu in sulfides on SCSS of mantle-derived melts. Calculation of SCSS along melting adiabat at mantle potential temperature of 1380?°C with variable initial S content in the mantle indicates that the complete consumption or partial survival of sulfide in the melting residue depends on initial S content and degree of melting. Primitive MORBs (Mg# > 60) with S and Cu mostly concentrated in 800-1000 ppm and 80-120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100-200 ppm S. Model calculations to capture the effects of variable mantle potential temperatures (1280-1420?°C) indicate that for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle owing to the effect of temperature in enhancing sulfide solubility in silicate melt, and higher mantle temperature stabilizing partial melt with higher FeO?FeO? and lower SiO2 and Al2O3, all of which generally enhance sulfide solubility. However, sulfide can still be exhausted by ?10-15%?10-15% melting with bulk S of 100-150 ppm in the mantle when TPTP is as low as 1300?°C. We also show that although variation of View the MathML sourceDCuperidotite/melt and initial Cu in the mantle can all affect the Cu concentration of primitive MORBs, 100-200 ppm S in the MORB source mantle can satisfy both S and Cu geochemistry of partial melts parental to ocean floor basalts.
Abstract: The oxygen fugacity of the upper mantle is 3-4 orders of magnitude higher than that of the lower mantle and this has been attributed to Fe2 + disproportionating into Fe3 + plus Fe0 at pressures > 24 GPa. The upper mantle might therefore have been expected to have evolved to more oxidizing compositions through geological time, but it appears that the oxygen fugacity of the upper mantle has remained constant for the last 3.5 billion years. Thus, it indicates that the mantle has been actively buffered from the accumulation of Fe3 +, and that this is linked to oxidation of diamond to carbonate coupled with reduction of Fe3 + to Fe2 +. When subducted plates penetrate into the lower mantle, compensational upwelling transports bridgmanite into the transition zone, where it breaks down to ringwoodite and majorite, releasing the ferric iron. The system returns to equilibrium through oxidation of diamond. Early in Earth history, diamond may have been enriched at the base of the transition zone in the Magma Ocean, because it is denser than peridotite melts at depths shallower than 660 km, and it is more buoyant below. Ongoing oxidation of diamond forms carbonate, leading to relatively high carbonate concentrations in the source of ocean island basalts.
Geological Society of America Annual Meeting, Vol. 47, 7, p. 163. abstract
Europe, Turkey
Moissanite
Abstract: The Aladag ophiolite in the eastern Tauride belt, southern Turkey, is a well-preserved remnant of oceanic lithosphere. It consists of, in ascending order, harzburgitic to dunitic tectonites, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes and basaltic pillow lavas. Podiform chromitites are common in the mantle peridotites. Thus far, more than 200 grains of microdiamond and more than 100 grains of moissanite (SiC) have been separated from one sample of podiform chromitite. The microdiamonds occur mostly as subhedral to euhedral, colorless to pale yellow grains, about 50-300 ?m in size. Moissanite grains are generally subhedral, light blue to deep blue in color and variable in size. These grains of diamond and moissanite are very similar to in-situ grains in podiform chromitites of Tibet and the Polar Urals of Russia (Yang et al., 2014; 2015), indicating that they are natural minerals, not the result of natural or anthropogenic contamination. As reported elsewhere, the diamonds and moissanite are accompanied by a range of other minerals, including rutile, zircon, quartz and sulfides. The discovery of diamond, moissanite and other unusual minerals in the podiform chromitites of the Aladag massif provide additional evidence for the widespread occurrence of these minerals in ophiolites, indicating that they are related to global mantle processes.
Abstract: A high-temperature rheometer equipped with a graphite furnace, characterized by an air-bearing-supported synchronous motor, has been enhanced by a custom-made Pt-Au concentric cylinder assembly. With this adaptation, viscosity measurements of highly fluid melts can be achieved at high temperatures, up to 1273 K. Due to the air-bearing-supported motor, this apparatus can perform measurements of extremely low torque ranging between 0.01 ?Nm and 230 mNm (resolution of 0.1 nNm), extending the typical range of viscosity measurements accessible in the present configuration to 10?3.5-103.5 Pa•s and shear rates up to 102 of s?1. We calibrated the system with distilled water, silicone oils, and the DGG-1 standard glass. We further present new data for the viscosity of Na2CO3, K2CO3, and Li2CO3 liquids. Finally, a comparison between our results and literature data is provided, to illustrate the effect of chemical composition and oxygen fugacity on the viscosity of alkali carbonate melts, which serve as analogs for both carbonatitic melts and molten carbonates of industrial relevance. This study substantially improves the database of alkali carbonate melts and dramatically increases the accuracy with respect to previous measurement attempts. The very low viscosity range data and their temperature dependence also helps to constrain very well the activation energy of these highly fluid systems and confirms the estimate of a universal pre-exponential factor for non-Arrhenian viscosity-temperature relationships.
Abstract: The metamorphic core of the Betic-Rif orogenic chain (Alboran Domain) is made up of lower crustal rocks forming the envelope of the Ronda (Spain) and Beni Bousera (Morocco) peridotites. The deepest sections of the crustal envelopes are made of migmatitic granulites associated with diffuse acidic magmatic products, making these exposure and ideal site to investigate the textural and petrological connection between crustal anatexis and granite magmatism in the contintental crust. However, still debated is the timing of intracrustal emplacement of the peridotite bodies, with models proposing either Alpine (early Miocene) or Hercynian ages, and still uncertain is the linkage between peridotite emplacement and crustal anatexis. In this study, by combining rock textures with whole-rock geochemistry, metamorphic thermobarometry, the U-Pb zircon geochronology and the analysis of the garnet and zircon REE chemistry, we document the P-T-t evolution of the granulite facies migmatites that form the immediate envelope of the Beni Bousera peridotites of the Rif belt. A main episode of Permo-Carboniferous (ca. 300-290?Ma) deep crustal anatexis, melt extraction and migration is documented that we link to the crustal emplacement of the Beni Bousera peridotites during collapse of the Hercynian orogen. Correlation at a regional scale suggests that the Beni-Bousera section can be tentatively correlated with the pre-Alpine (Permo-Carboniferous) basement units tectonically interleaved within the orogenic structure of the Alpine chain. The results of this study provide ultimate constraints to reconstruct the tectono-metamorphic evolution of the Alboran Domain in the Western Mediterranean and impose re-assessment of the modes and rates through which Alpine orogenic construction and collapse occurred and operated in the region.
Geochemical Perspectives Letters, Vol. 11, pp. 39-43. pdf
Africa, Botswana, South America, French Guiana
deposit - Dachine, Orapa
Abstract: Fluid inclusions trapped in fast-growing diamonds provide a unique opportunity to examine the origin of diamonds, and the conditions under which they formed. Eclogitic to websteritic diamondites from southern Africa show 13C-depletion and 15N-enrichment relative to mantle values (?13C = -4.3 to -22.2 ‰ and ?15N = -4.9 to +23.2 ‰). In contrast the 3He/4He of the trapped fluids have a strong mantle signature, one sample has the highest value so far recorded for African diamonds (8.5 ± 0.4 Ra). We find no evidence for deep mantle He in these diamondites, or indeed in any diamonds from southern Africa. A correlation between 3He/4He ratios and 3He concentration suggests that the low 3He/4He are largely the result of ingrowth of radiogenic 4He in the trapped fluids since diamond formation. The He-C-N isotope systematics can be best described by mixing between fluid released from subducted altered oceanic crust and mantle volatiles. The high 3He/4He of low ?13C diamondites reflects the high 3He concentration in the mantle fluids relative to the slab-derived fluids. The presence of post-crystallisation 4He in the fluids means that all 3He/4He are minima, which in turn implies that the slab-derived carbon has a sedimentary organic origin. In short, although carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamond-formation, helium isotopes reveal an unambiguous mantle component hidden within a strongly 13C-depleted system.
Abstract: This study focuses on the causes, modalities and obstacles of sediment transfer in the longest cell of littoral sand drift documented on Earth so far. Sand derived from the Orange River is dragged by swell waves and persistent southerly winds to accumulate in four successive dunefields in coastal Namibia to Angola. All four dunefields are terminated by river valleys, where aeolian sand is flushed back to the ocean; and yet sediment transport continues at sea, tracing an 1800 km long submarine sand highway. Sand drift would extend northward to beyond the Congo if the shelf did not become progressively narrower in southern Angola, where drifting sand is funnelled towards oceanic depths via canyon heads connected to river mouths. Garnet-magnetite placers are widespread along this coastal stretch, indicating systematic loss of the low-density feldspatho-quartzose fraction to the deep ocean. More than half of Moçamedes Desert sand is derived from the Orange River, and the rest in similar proportions from the Cunene River and from the Swakop and other rivers draining the Damara Orogen in Namibia. The Orange fingerprint, characterized by basaltic rock fragments, clinopyroxene grains and bimodal zircon-age spectra with peaks at ca 0•5 Ga and ca 1•0 Ga, is lost abruptly at Namibe, and beach sands further north have abundant feldspar, amphibole-epidote suites and unimodal zircon-age spectra with a peak at ca 2•0 Ga, documenting local provenance from Palaeoproterozoic basement. Along with this oblique-rifted continental margin, beach placers are dominated by Fe-Ti-Cr oxides with more monazite than garnet and thus have a geochemical signature sharply different from beach placers found all the way along the Orange littoral cell. High-resolution mineralogical studies allow us to trace sediment dispersal over distances of thousands of kilometres, providing essential information for the correct reconstruction of ‘source to sink’ relationships in hydrocarbon exploration and to predict the long-term impact of man-made infrastructures on coastal sediment budgets.
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.
Geochemistry of boninite type volcanic rocks in the Frotet Evans greenstone belt, Opawica subprovince Quebec: implications for the evolution of Archean belts
Precambrian Research, Vol. 115, No.1-4, pp. 349-71.
Abstract: Permian dikes, sills, and diatremes in southern Illinois and northwestern Kentucky (the Omaha, Wildcat Hills, Cottage Grove, Will Scarlet, Williams, Grant, and Clay Lick intrusions) share similar geochemistry and are classified as ultramafic lamprophyres. Major element compositions are 30-35 wt% SiO2, 6-7% Al2O3, 12-14% FeOt, 16-19% MgO, 3-5% TiO2, 11-16% CaO, 0.1-0.7% Na2O, 1.2-2.7% K2O, and 0.4-1.3% P2O5. The Grant Intrusive Breccia is an exception, with lower SiO2, Al2O3, FeOt, MgO, TiO2, and higher CaO. Typically, these rocks are fine grained, with phlogopite, serpentinized olivine ( Fo88), diopside, perovskite, Fe-Ti-spinel, apatite, and calcite. Blocky and lath-shaped pseudomorphs in some samples probably represent melilite, which would make the rocks alnöites. The Grant and Williams diatremes contain sedimentary and igneous clasts (including amphibole megacrysts) within a carbonate-rich matrix. The Grant exhibits pelletal lapilli and is characterized as a lamprophyre?carbonatite tuffisite. Trace element patterns exhibit enrichment of LREE, strong REE fractionation, and relative depletions of K, Sr, Zr, and Hf, closely matching those of the mela-aillikites of Aillik Bay, Labrador. The Grant Intrusive exhibits even greater REE enrichment and notable peaks at Nb, La, and Ce. Geochemical characteristics, including distributions of 143Nd/144Nd and 87Sr/86Sr, are consistent with near-primary melts from a metasomatized peridotite source containing phlogopite-rich veins. Derivation of the lamprophyres from carbonate-rich parental melts similar to the Grant Intrusive could be achieved by separation of carbonatite. A narrow range of initial 87Sr/86Sr (0.70301-0.70449), and initial ?Nd (3.7-5.1), suggests a uniform mantle source close to Bulk Earth. T-depleted mantle model ages range from 540 to 625 Ma, and might correlate with timing of enrichment of a lithospheric mantle source during the breakup of Rodinia.
Optimizing carbon capture and storage in kimberlite tailings for environmental benefit and operational efficiency.
Vancouver Kimberlite Cluster, Nov. 6, 1p. Abstract
Global
carbon
Abstract: Ultramafic mine tailings, including those from kimberlite-hosted diamond mines, offer potential operational and environmental benefit through reaction with carbon dioxide from air and power plant flue gas. The carbon dioxide is sequestered from the environment through the precipitation of carbonate minerals, thus reducing or offsetting the greenhouse gas emissions associated with mining. Additional benefits can include tailings stabilization, dust reduction, acid mine drainage prevention, and toxic metal encapsulation. In this talk I will present an overview of the processes and controls on carbonation reactions within tailings at active mines with a focus on acceleration of carbon sequestration within kimberlite tailings. Carbonation reactions can be limited by transport (rate of CO2 supply) and by reaction kinetics (mineral dissolution or mineral precipitation). Field studies of accidental passive carbonation within tailings at operating mines supplemented with laboratory experiment and reactive transport modelling has been key to identifying the rate limits to carbon sequestration at each mine site. With these limits identified, acceleration approaches can be tailored to the local climate, gangue mineralogy, and mine design, all of which can exert a primary control on carbon sequestration rates. The result is a methodology for evaluating the carbon sequestration potential of a mine site and a toolbox of acceleration strategies which together allow for site selection and project design. In the coming years, these systems will be deployed on site at active mines to further test and advance the technology. I will end with a perspective on the role that mining of ultramafic-hosted deposits can play in achieving net negative CO2 emissions as is projected to be required by the end of this century if we are to avoid net global warming in excess of two degrees centigrade.
Quantifying carbon fixation in trace minerals from processed kimberlite: a comparative study of quantitative methods using X-ray powder diffraction data
Mineralogy and Petrology, 10.1007/ s00710-018- 0589-4, 14p.
Africa, South Africa, Canada, Northwest Territories, Ontario
deposit - Venetia, Voorspoed, Gahcho Kue, Victor, Snap Lake
Abstract: De Beers kimberlite mine operations in South Africa (Venetia and Voorspoed) and Canada (Gahcho Kué, Victor, and Snap Lake) have the potential to sequester carbon dioxide (CO2) through weathering of kimberlite mine tailings, which can store carbon in secondary carbonate minerals (mineral carbonation). Carbonation of ca. 4.7 to 24.0 wt% (average?=?13.8 wt%) of annual processed kimberlite production could offset 100% of each mine site’s carbon dioxide equivalent (CO2e) emissions. Minerals of particular interest for reactivity with atmospheric or waste CO2 from energy production include serpentine minerals, olivine (forsterite), brucite, and smectite. The most abundant minerals, such as serpentine polymorphs, provide the bulk of the carbonation potential. However, the detection of minor amounts of highly reactive brucite in tailings from Victor, as well as the likely presence of brucite at Venetia, Gahcho Kué, and Snap Lake, is also important for the mineral carbonation potential of the mine sites.
Contrasting termite transported indicator mineral concentrations in the Kgalafadi of central district Botswana: Macrotermes micaelseni vs Hodotermes mossambicus.
Vancouver Kimberlite Cluster, March 9, 1p. Abstract
Mineralogy and Petrology, 10.1007/s00710-018-0608-5 8p.
Africa, Botswana
indicator minerals
Abstract: The majority of the diamond mines in Botswana were discovered as a direct consequence of soil sampling for indicator minerals such as garnet and picroilmenite. Over the past 60 years the application of soil sampling for indicator minerals as a primary exploration tool has declined while aeromagnetic surveys have increased in popularity. The rate of kimberlite discovery in Botswana has declined significantly. The obvious magnetic kimberlites have been discovered. The future of new kimberlite discoveries is once again dependent on soil sampling for kimberlite indicator minerals. It is essential to have an in depth understanding of the transport mechanism of kimberlite indicator minerals from the kimberlite to the modern day surface of the Kalahari Formation, which is solely via termite bioturbation. Field observations indicate that the concentration of indicator minerals at surface is directly dependent on the physical characteristics and capabilities as well as behavioural patterns of the particular termite species dominant in the exploration area. The discovery of future diamond mines in Botswana will be closely associated with an in depth understanding of the relationship between size and concentration of kimberlite indicator minerals in surface soils and the seasonal behaviour, depth penetration capabilities, earthmoving efficiencies and mandible size of the dominant termite species within the exploration area. Large areas in Botswana, where kimberlite indicator minerals recovered from soil samples have been described as distal from source or background, will require re-evaluation. Without detailed termite studies the rate of discovery will continue to decline.
Abstract: Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite (more than 8 wt.% MgO). This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios (Zr/Hf = 32-35, Ti/Zr = 107-147), producing flat REE and HFSE profiles [(La/Yb)pm = 0.9-1.3], with abundances of 3-10 times primitive mantle and with minor negative anomalies of Nb [(Nb/La)pm = 0.6-0.8] and Th [(Th/La)pm = 0.6-0.9]. Initial isotope compositions (?Nd) range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping ?50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between ?2830 and ?2820 Ma; are compositionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition.
Abstract: The detrital zircon population in quartzitic conglomerates from the northern Tanzania Craton yield ages between 2640 Ma and 2790 Ma which includes most of the igneous history from this part of the craton. The igneous evolution is characterised by mafic volcanism with an oceanic plateau?like geochemical signature at ~2800 Ma followed by diorite and tonalite-trondhjemite-granodiorite dominated magmatism between 2790 and 2700 Ma, which transitioned into more evolved high?K magmatism between 2700 and 2620 Ma. The ?Hf values of the detrital zircons range from +2.4 to ?1.4 and change with time from radiogenic Hf pre?2700 Ma (98% positive ?Hf) to unradiogenic Hf post?2700 Ma (41% positive ?Hf). The petrological progression from mafic to felsic crust is reflected in the detrital age distribution and Hf isotopes and is consistent with juvenile mafic crust slowly maturing into more evolved felsic crust through a series of successive partial melting events in an oceanic?plateau?like environment.
GSA Annual Meeting, Paper 300-2, 1p. Abstract only Booth
Technology
GIA library
Abstract: Through its digitization project, the Gemological Institute of America’s (GIA) Richard T. Liddicoat Library is making available digital copies of historic and unique books to geoscientists worldwide. By the end of 2015, more than 100 volumes from the library’s rare book collection, including the gemology and mineralogy library collected by John and Marjorie Sinkankas, will be posted online in a readily accessible, searchable format.
In December 2014, the library purchased the BC100 Book Capture system by Digital Transitions. This equipment includes two Phase One digital camera backs with Schneider Kreuznach lenses, which photograph the two pages of an open book simultaneously. Images are captured at a minimum resolution of 300 pixels per inch, with higher resolutions used for books that are small, have highly detailed color images, or have small or faded text.
Capture One imaging software is used to generate a TIFF image for each page. These TIFF files will be saved indefinitely as preservation masters, from which derivative files can be created and modified for future use. Then docWorks post-processing software by Content Conversion Specialists is used to perform optical character recognition (OCR) and generate a searchable PDF and ePub output for each book. The OCR supports a variety of languages including those using European and Cyrillic alphabets, as well as Chinese, Japanese, and Korean characters.
A number of GIA’s digital books are now available online to view and download for free through Internet Archive (https://archive.org/details/@gia_library) with new books being uploaded every few days. Available books include out-of-copyright titles dating from 1496 to the 1920s. One of the oldest digitized books, Libellus de Lapidibus Preciosis (Book of Precious Stones) by Marbode, Bishop of Rennes, printed in 1511, was originally written in the 11th century and discusses properties of 60 gems. Another title, René Just Haüy’s influential Traité de Minéralogie (Treatise of Mineralogy) published in 1801, describes the laws governing crystal structure and was the first rational system for identifying and classifying minerals. The digitization project is ongoing with plans to ultimately post hundreds of volumes related to gems and minerals.
Abstract: The Taj Mahal evokes an image of a monumental building and reflecting pool—its classic view. But the Taj Mahal complex is much more than that. It is actually a series of beautiful buildings and gardens in Agra, India, built in the seventeenth century in loving memory of Mumtaz Mahal. This name, given by the Mughal emperor Shah Jahan to one of his brides, means “Chosen One of the Palace.” Famed for its architectural magnificence, the landmark holds additional significance for the gemologist. Upon closer investigation, one is impressed with the intricacies of the inlay of numerous gems to create thousands of designs throughout the buildings on the grounds. This article sheds light on the gems used in decorating the Taj Mahal and in the extraordinary jewelry collected by Shah Jahan and other Mughals. These gems often took intricate trade routes to Agra, which are also discussed, along with the craft used to create the inlays and the efforts undertaken to preserve this Wonder of the World.
Abstract: Rare earth elements (REEs) including fifteen lanthanides, yttrium and scandium are found in more than 250 minerals, worldwide. REEs are used in various high-tech applications across various industries, such as electrical and electronics, automotive, renewable energy, medical and defence. Therefore, the demand for REEs in the global market is increasing day by day due to the surging demand from various sectors, such as emerging economies, green technology and R&D sectors. Rare earth (RE) deposits are classified on the basis of their genetic associations, mineralogy and form of occurrences. The Bayan Obo, Mountain Pass, Mount Weld and China’s ion adsorption clays are the major RE deposits/mines in the world to date and their genesis, chronology and mineralogy are discussed in this review. In addition, there are other RE deposits, which are currently being mined or in the feasibility or exploration stages. Most of the RE resources, production, processing and supply are concentrated in the Asia-Pacific region. In this regard, China holds the dominancy in the RE industry by producing more than 90% of the current rare earth requirements. Thus, REEs are used as a powerful tool by China in trade wars against other countries, especially against USA in 2019. However, overwhelming challenges in conventional RE explorations and mining make secondary RE resources, such as electric and electronic waste (e-waste) and mine tailings as promising resources in the future. Due to the supply risk of REEs and the monopoly of the REEs market, REEs recycling is currently considered as an effective method to alleviate market fluctuations. However, economical and sustainable processing techniques are yet to be established to exploit REEs via recycling. Moreover, there are growing ecological concerns along with social resistance towards the RE industry. To overcome these issues, the RE industry needs to be assessed to maintain long-term social sustainability by fostering the United Nations sustainable development goals (SDGs).
Petrochemical interpretation of carbonatite-like rocks from the Chagatai Complex of the Tien Shan with appllication to the problem of diamond potential.
Journal of Geophysical Research: Solid Earth, Vol. 123, 10, pp. 8459-8473.
Mantle
olivine
Abstract: Olivine is the most abundant and among the weakest phases in Earth's upper mantle, and thus, its rheological properties play a critical role in governing thermal structure and convective flow in the upper mantle. A persistent obstacle to constraining the in situ flow properties of olivine by laboratory experiment has been the difficulty in resolving the effect of pressure, which is weak within the 0? to ~2?GPa pressure range of conventional laboratory deformation instruments but potentially strong over the 1? to ~14?GPa range of the upper mantle. Using a deformation?DIA, one of a new generation of bonafide deformation devices designed for operation to ?10 GPa, we have deformed dry, polycrystalline San Carlos olivine in high?temperature creep with the singular intent of providing the best achievable measurement of activation volume V* and a comprehensive statement of uncertainty. Under strictly dry conditions, at constant temperature (1,373 K) and strain rate (1 × 10?5 s?1), varying only pressure (1.8 to 8.8 GPa), we measure V* = 15 ± 5 cm3/mol. We have reproduced the well?known mechanism change from [100]?slip to [001]?slip near 5 GPa and determined that, whatever the change in V* associated with the change in slip system, the effective value of 15 ± 5 cm3/mol is still accurate for modeling purposes in the 2? to 9?GPa pressure range. This is a substantial pressure effect, which in the absence of a temperature gradient would represent a viscosity increase from the top to bottom of the upper mantle of 5 ± 2 orders of magnitude.
Abstract: Ihouhaouene area in In Ouzzal terrane (Hoggar, South Algeria) is exceptional by numerous carbonatite complexes systematically associated to syenites. They constitute one of the oldest carbonatite emplaced at 2 Ga. Various types of carbonatites are distinguished by their successive placement and pegmatitic to brecciated appearance. The first-generation of carbonatites are always brecciated with elements of syenite and carbonate cement with calcite, apatite, alkali feldspar, wollastonite, clinopyroxene +/- sphene, allanite, quartz and garnet. Late carbonatite intrusions appear in small pegmatitic veins rich in apatite (3-50 mm). All carbonatites are calciocarbonatites (38-50 wt% CaO) with silica content ranging from 5 to 21 wt% SiO2. The high silica content is interpreted as assimilation of syenite material during emplacement. Carbonatites have high Rare Earth Element (REE) concentrations with high Ligh REE/Heavy REE fractionation (e.g. 1088 ppm La, La/Yb= 144-198) and variable concentrations in Th (26.5-197 ppm). The REE concentrations are mainly controlled by apatite phenocrysts (30-40 vol.%) with 4-9 wt% REE. In late pegmatitic carbonatite, REE-rich apatites are green-yellow phenocrysts with britholite exsolution (up to 40 vol.%, Ca4(REE)6 (SiO4,PO4)6 (OH,F,Cl)2). Britholites are hexagonal and occur as fine lamellar exsolutions (<10 um) in the same crystallographic axis (001) than apatites or as irregularshaped grains (10-200 um). All britholites contain 8-16 wt% La, 21-43 wt% Ce and 7-12 wt% Nd. The apatite-britholite exsolutions correspond to a substitution of the trivalent rareearth elements (REE3+) and Si4+ for Ca2+ and P5+. The REE substitution is accompanied by a change in volatile composition with F-rich apatite and Cl-rich britholite indicating that Si and Cl-rich hydrothermal fluids are present at the late stage of carbonatite evolution leading to REEenrichment and the crystallization of REE minerals.
Abstract: Carbonatites and syenites from Ihouhaouene (2 Ga; In Ouzzal terrane, Hoggar, South of Algeria) have close spatial relationships. Their analogous mineral assemblages with diopside/hedenbergite (cpx), apatite, wollastonite +/- calcite and alkali-feldspar suggest that they were emplaced from a common igneous parental event. Carbonatites from In Ouzzal terrane are calciocarbonatites and form a continuous range of whole-rock major and trace element composition from Sipoor carbonatite (<20 wt.% SiO2; 24-36 wt.% CO2) to Si-rich carbonatite (20-35 wt.% SiO2; 11-24 wt.% CO2) then white syenite (52-58 wt.% SiO2; 0.1-6.5 wt.% CO2) and red syenite (57-65 wt.% SiO2; 0.1-0.4 wt.% CO2). Equilibrium calculations reveal that apatite (Ce/Lu= 1690-6182; Nb/Ta >50) and cpx (Ce/Lu= 49-234; Nb/Ta<10) from Si-rich carbonatites and white syenites crystallized from a REEenriched carbonate melt and an evolved silicate melt, respectively. Likewise, Si-poor carbonatites have a higher REE contents in calculated apatite equilibrium melts than in their cpx and a wide range of Nb/Ta ratios with a majority of subchondritic value (<10) that reflects the segregation of the carbonate fraction from an evolved parental melt. Otherwise, red syenites have similar REE contents in apatite and clinopyroxene equilibrium melts (Nb/Ta>10) suggesting an origin from homogeneous evolved melt batches. Both mineralogical and geochemical features reveal the intimate link between carbonatites and syenites and their cogenetic signature. Immiscibility and fractional crystallization processes modelling explain the trace element contents and low Nb/Ta ratio in minerals. These processes were partly counterbalanced by intermingling of partially crystallized melt fractions and hybridization of segregated minerals during the progressive cooling of a silico-carbonated mantle melt.
Journal of African Earth Sciences, Vol. 179, 104206, 21p. pdf
Africa, Cameroon, Central African Republic
geophysics
Abstract: The Bangui Magnetic Anomaly (BMA) is one of the largest magnetic anomalies in the world whose origin is still not known. This research investigated the crustal thickness, Curie depths and thermal structures in the Central African sub-regions - Cameroon, Central African Republic and adjacent countries - which are largely characterized by the Bangui Magnetic Anomaly. To achieve a better understanding and clearer idea of the location of the possible sources of the BMA, analyses of geothermal structures were conducted. Two potential methods were used: gravity to evaluate the crustal thickness and magnetics for geothermal analysis. Spectral analysis of gravity data shows that crustal thickness range between 14 and 55 km. The highest depths were found in Central African Republic. The lower values of crustal thickness were obtained in South-Chad basin with a minimum of roughly 14 km. Geothermal analysis is carried out using the Curie point depth, thermal gradient and heat-flow evaluations. The results show that the BMA is related to a thick crust of roughly 40 km. Depth to the bottom of possible sources does not exceed the lower crust. The mean Curie point depth estimated is 38 km with an error of ±2 km. Geothermal results also show the difference in the thermal behaviour between the crust in the Pan African and Precambrian domain. The mobile zone which constitutes the Pan African domain is associated with a thin crust of high heat-flow values of 65 mW/m2. However, the Precambrian domain beneath the BMA is associated with a thick crust with lower heat-flow values (roughly 45 mW/m2). The difference between crustal thickness and Curie point depths shows that all the sources of the BMA are crustal. The present results are in favour of a geological origin for the Bangui Magnetic anomaly.
Abstract: The Small and Junior diamond mining industry, which is dominated by alluvial diamond miners, and a few remaining small kimberlite operations, produced a high proportion of diamonds in South Africa in the late 1950s and early 1960s, prior to the discovery and development of major kimberlite mines such as Finsch and Venetia, in the 1970’s and 80’s. Subsequent to these discoveries the Small and Junior sector remained an active and important participant in the local diamond industry, particularly in respect of the highly sought after top-quality gemstone diamonds produced from the extensive alluvial deposits of South Africa. Since 2004 the sector has shown a strong decline. This report highlights the challenges faced by the Small and Junior diamond miners and makes recommendations for the revival of this sector.
Journal of Physics Condensed Matter, Vol. 541, 1, pp. 12090-12094.
Technology
Diamond crystallography
Abstract: The optical and ODMR spectra of nitrogen vacancy (NV) centers in diamond were investigated as applied to the development of micro-scale diamond magnetometer sensor. It is shown that the NV0 center (unlike the NV? center) has an absorption line in the UV range. The inversion of the zero phonon line of the NV? center was observed and explained. ODMR signals in high magnetic fields (as compared with inner fields in the crystal) as well as in weak fields also were investigated. We propose the solution to the problem of the magnetometer sensitivity reduction at low magnetic field, that is of interest for quantum magnetometry applications. Peculiarities of optical and ODMR Spectra of Nitrogen-Vacancy Color Centers in Diamond Crystals.
Oxygen, Carbon and Strontium isotopic composition of calcites in garnet megacrysts and carbonatized granulitic xenoliths from the Udachnaya kimberlite pipe, Yakutia
Proceedings of Fifth International Kimberlite Conference held Araxa June, pp. 558-559
