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Sisir K Mondal - One of the best experts on this subject based on the ideXlab platform.
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Trace-element fingerprints of Chromite, magnetite and sulfides from the 3.1 Ga ultramafic–mafic rocks of the Nuggihalli greenstone belt, Western Dharwar craton (India)
Contributions to Mineralogy and Petrology, 2015Co-Authors: Ria Mukherjee, José M. González-jiménez, Sisir K Mondal, William L. Griffin, Norman J Pearson, Suzanne Y. O’reillyAbstract:The 3.1 Ga Nuggihalli greenstone belt in the Western Dharwar craton is comprised of chromitite-bearing sill-like ultramafic–mafic rocks that are surrounded by metavolcanic schists (compositionally komatiitic to komatiitic basalts) and a suite of tonalite–trondhjemite–granodiorite gneissic rocks. The sill-like plutonic unit consists of a succession of serpentinite (after dunite)–peridotite–pyroxenite and gabbro with bands of titaniferous magnetite ore. The chromitite ore-bodies (length ≈30–500 m; width ≈2–15 m) are hosted by the serpentinite–peridotite unit. Unaltered Chromites from massive chromitites (>80 % modal Chromite) of the Byrapur and Bhaktarhalli Chromite mines in the greenstone belt are characterized by high Cr# (100Cr/(Cr + Al)) of 78–86 and moderate Mg# (100 Mg/(Mg + Fe^2+)) of 45–55. In situ trace-element analysis (LA-ICPMS) of unaltered Chromites indicates that the parental magma of the chromitite ore-bodies was a komatiite lacking nickel-sulfide mineralization. In the Ga/Fe^3+# versus Ti/Fe^3+# diagram, the Byrapur Chromites plot in the field of suprasubduction zone (SSZ) Chromites while those from Bhaktarhalli lie in the MOR field. The above results corroborate our previous results based on major-element characteristics of the Chromites, where the calculated parental melt of the Byrapur Chromites was komatiitic to komatiitic basalt, and the Bhaktarhalli Chromite was derived from Archean high-Mg basalt. The major-element Chromite data hinted at the possibility of a SSZ environment existing in the Archean. Altered and compositionally zoned Chromite grains in our study show a decrease in Ga, V, Co, Zn, Mn and enrichments of Ni and Ti in the ferritchromit rims. Trace-element heterogeneity in the altered Chromites is attributed to serpentinization. The trace-element patterns of magnetite from the massive magnetite bands in the greenstone belt are similar to those from magmatic Fe–Ti–V-rich magnetite bands in layered intrusions, and magnetites from andesitic melts, suggesting that magnetite crystallized from an evolved gabbroic melt. Enrichments of Ni, Co, Te, As and Bi in disseminated millerite and niccolite occurring within chromitites, and in disseminated bravoite within magnetites, reflect element mobility during serpentinization. Monosulfide solid solution inclusions within pyroxenes (altered to actinolite) in pyroxenite, and interstitial pyrites and chalcopyrites in magnetite, retain primary characteristics except for Fe-enrichment in chalcopyrite, probably due to sub-solidus re-equilibration with magnetite. Disseminated sulfides are depleted in platinum-group elements (PGE) due to late sulfide saturation and the PGE-depleted nature of the mantle source of the sill-like ultramafic–mafic plutonic rocks in the Nuggihalli greenstone belt.
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compositional variations in the mesoarchean Chromites of the nuggihalli schist belt western dharwar craton india potential parental melts and implications for tectonic setting
Contributions to Mineralogy and Petrology, 2010Co-Authors: Ria Mukherjee, Sisir K Mondal, Minik T Rosing, Robert FreiAbstract:The Chromite deposits in the Archean Nuggihalli schist belt are part of a layered ultramafic–mafic sequence within the Western Dharwar Craton of the Indian shield. The 3.1-Ga ultramafic–mafic units occur as sill-like intrusions within the volcano-sedimentary sequences of the Nuggihalli greenstone belt that are surrounded by the tonalite–trondhjemite–granodiorite (TTG) suite of rocks. The entire succession is exposed in the Tagdur mining district. The succession has been divided into the lower and the upper ultramafic units, separated by a middle gabbro unit. The ultramafic units comprise of deformed massive chromitite bodies that are hosted within Chromite-bearing serpentinites. The chromitite bodies occur in the form of pods and elongated lenses (~60–500 m by ~15 m). Detailed electron microprobe studies reveal intense compositional variability of the Chromite grains in silicate-rich chromitite (~50% modal Chromite) and serpentinite (~2% modal Chromite) throughout the entire ultramafic sequence. However, the primary composition of Chromite is preserved in the massive chromitites (~60–75% modal Chromite) from the Byrapur and the Bhaktarhalli mining district of the Nuggihalli schist belt. These are characterized by high Cr-ratios (Cr/(Cr + Al) = 0.78–0.86) and moderate Mg-ratios (Mg/(Mg + Fe2+) = 0.38–0.58). The compositional variability occurs due to sub-solidus re-equilibration in the accessory Chromite in the serpentinite (Mg-ratio = 0.01–0.38; Cr-ratio = 0.02–0.99) and in silicate-rich chromitite (Mg-ratio = 0.06–0.48; Cr-ratio = 0.60–0.99). In the massive chromitites, the sub-solidus re-equilibration for Chromite is less or absent. However, the re-equilibration is prominent in the co-existing interstitial and included olivine (Fo96–98) and pyroxene grains (Mg-numbers = 97–99). Compositional variability on the scale of a single Chromite grain occurs in the form of zoning, and it is common in the accessory Chromite grains in serpentinite and in the altered grains in chromitite. In the zoned grains, the composition of the core is modified and the rim is ferritchromit. In general, ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains. In this case, ferritchromit formation is not very extensive. This indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization. In some occurrences, the ferritchromit rim is very well developed, and only a small relict core appears to remain in the Chromite grain. However, complete alteration of the Chromite grains to ferritchromit without any remnant core is also present. The regular, well-developed and continuous occurrence of ferritchromit rims around the Chromite grain boundaries, the complete alteration of the Chromite grains and the modification of the core composition indicate the alteration in the Nuggihalli schist belt to be intense, pervasive and affected by later low-grade metamorphism. The primary composition of Chromite has been used to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton. Tectonic discrimination diagrams using the primary composition of Chromites indicate a supra-subduction zone setting (SSZ) for the Archean chromitites of Nuggihalli and derivation from a boninitic magma. The composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back-arc rift settings. Formation of the massive chromitites in Nuggihalli may be due to magma mixing process involving hydrous high-Mg magmas or may be related to intrusions of Chromite crystal laden magma; however, there is little scope to test these models because the host rocks are highly altered, serpentinized and deformed. The present configurations of the chromitite bodies are related to the multistage deformation processes that are common in Archean greenstone belts.
Norman J Pearson - One of the best experts on this subject based on the ideXlab platform.
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Trace-element fingerprints of Chromite, magnetite and sulfides from the 3.1 Ga ultramafic–mafic rocks of the Nuggihalli greenstone belt, Western Dharwar craton (India)
Contributions to Mineralogy and Petrology, 2015Co-Authors: Ria Mukherjee, José M. González-jiménez, Sisir K Mondal, William L. Griffin, Norman J Pearson, Suzanne Y. O’reillyAbstract:The 3.1 Ga Nuggihalli greenstone belt in the Western Dharwar craton is comprised of chromitite-bearing sill-like ultramafic–mafic rocks that are surrounded by metavolcanic schists (compositionally komatiitic to komatiitic basalts) and a suite of tonalite–trondhjemite–granodiorite gneissic rocks. The sill-like plutonic unit consists of a succession of serpentinite (after dunite)–peridotite–pyroxenite and gabbro with bands of titaniferous magnetite ore. The chromitite ore-bodies (length ≈30–500 m; width ≈2–15 m) are hosted by the serpentinite–peridotite unit. Unaltered Chromites from massive chromitites (>80 % modal Chromite) of the Byrapur and Bhaktarhalli Chromite mines in the greenstone belt are characterized by high Cr# (100Cr/(Cr + Al)) of 78–86 and moderate Mg# (100 Mg/(Mg + Fe^2+)) of 45–55. In situ trace-element analysis (LA-ICPMS) of unaltered Chromites indicates that the parental magma of the chromitite ore-bodies was a komatiite lacking nickel-sulfide mineralization. In the Ga/Fe^3+# versus Ti/Fe^3+# diagram, the Byrapur Chromites plot in the field of suprasubduction zone (SSZ) Chromites while those from Bhaktarhalli lie in the MOR field. The above results corroborate our previous results based on major-element characteristics of the Chromites, where the calculated parental melt of the Byrapur Chromites was komatiitic to komatiitic basalt, and the Bhaktarhalli Chromite was derived from Archean high-Mg basalt. The major-element Chromite data hinted at the possibility of a SSZ environment existing in the Archean. Altered and compositionally zoned Chromite grains in our study show a decrease in Ga, V, Co, Zn, Mn and enrichments of Ni and Ti in the ferritchromit rims. Trace-element heterogeneity in the altered Chromites is attributed to serpentinization. The trace-element patterns of magnetite from the massive magnetite bands in the greenstone belt are similar to those from magmatic Fe–Ti–V-rich magnetite bands in layered intrusions, and magnetites from andesitic melts, suggesting that magnetite crystallized from an evolved gabbroic melt. Enrichments of Ni, Co, Te, As and Bi in disseminated millerite and niccolite occurring within chromitites, and in disseminated bravoite within magnetites, reflect element mobility during serpentinization. Monosulfide solid solution inclusions within pyroxenes (altered to actinolite) in pyroxenite, and interstitial pyrites and chalcopyrites in magnetite, retain primary characteristics except for Fe-enrichment in chalcopyrite, probably due to sub-solidus re-equilibration with magnetite. Disseminated sulfides are depleted in platinum-group elements (PGE) due to late sulfide saturation and the PGE-depleted nature of the mantle source of the sill-like ultramafic–mafic plutonic rocks in the Nuggihalli greenstone belt.
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fingerprints of metamorphism in Chromite new insights from minor and trace elements
Chemical Geology, 2014Co-Authors: Norman J Pearson, Vanessa Colás, Isabel Fanlo, Fernando Gervilla, Jose Maria Gonzalezjimenez, W L Griffin, Suzanne Y Oreilly, Thomas KerestedjianAbstract:Abstract A suite of minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in Chromite grains from ophiolitic chromitites subjected to high-pressure metamorphism defines a metamorphic signature. A two-stage process associated with the infiltration of fluids during retrograde metamorphism from eclogite- to amphibolite-facies has produced four types of Chromites: (1) porous Chromite strongly enriched in Cr and Fe 2 + but depleted in Al and Mg, with abundant chlorite filling the pores; (2) non-porous Chromite strongly enriched in Fe 3+ (i.e., ferrian Chromite); (3) partly altered Chromite with primary cores surrounded by chlorite-bearing porous Chromite; and (4) zoned Chromite made up of primary cores surrounded by non-porous rims of ferrian Chromite. Compared to spinels from unmetamorphosed chromitites the cores of partly altered Chromites after primary high-Cr Chromite are enriched in Zn, Co and Mn but strongly depleted in Ga, Ni and Sc. This distribution of minor- and trace-elements is related to a decrease in Mg# [Mg/(Mg + Fe 2+ )] and Al, produced by the crystallization of chlorite in the pores of porous Chromite. Non-porous Chromite is enriched in Ti, Ni, Zn, Co, Mn and Sc but depleted in Ga, suggesting that fluid-assisted processes have obliterated the primary magmatic signature. Zoned Chromites have cores depleted in Ga, Ni and Sc but are progressively enriched in Zn, Co and Mn as Mg# and Al decrease toward the rims; they have overall lower concentrations in Ga, Ni and Sc and higher Zn and Co than the non-porous rims of ferrian Chromite. The complex variation of the minor- and trace-elements vs Fe 3+ /(Fe 3+ + Fe 2+ ) in the different types of Chromite suggests a complex interplay of substitutions, linked with the ability of fluids to infiltrate the Chromite and the extent of the re-equilibration between pre-existing cores and newly-formed rims. The results demonstrate that metamorphism can seriously disturb the original magmatic distribution of minor and trace elements in Chromite. The abundances of these elements, and by inference the major elements, can be strongly modified even in the cores of grains that appear “unaltered” in terms of major elements. The use of the major elements as indicators of magmatic processes therefore must be linked to careful evaluation of metamorphic effects, using LA-ICP-MS analysis of minor and trace elements.
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Taking fingerprints of metamorphism in Chromite using minor and trace elements
2014Co-Authors: Vanessa Colás, José M. González-jiménez, William L. Griffin, Norman J Pearson, Isabel Fanlo, Fernando Gervilla, Thomas KerestedjianAbstract:A two-stage mechanism associated with the infiltration of fluids during retrograde metsamorphism from eclogite to amphibolite facies in the ultramafic massif of Golyamo Kamenyane (Bulgaria) has produced four types of microstructures in Chromites: i) porous Chromite, with chlorite in the pores, ii) non-porous Chromite, iii) partly altered Chromite, with primary cores surrounded by porous Chromite, and iv) zoned Chromite, with primary cores surrounded by nonporous rims. LA-ICP-MS analysis shows that partly altered Chromite cores and their surrounding rims of Fe 2+ -rich porous Chromite show a distribution of minor and trace elements similar to Chromite from MORB, except for a strong depletion in Ga and Sc. However, single grains of porous Chromite produced after the massive infiltration of fluids show a significant enrichment in Zn, Co and Mn but depletion in Ga, Ni and Sc. Non-porous Chromite (i.e., ferrian Chromite), forming single grains and rims on zoned Chromites, are also enriched in Zn, Co, Mn and depleted in Ga but are distictively enriched in Ti, Ni and Sc. This suggests that oxidising fluids have substantially obliterated the geochemical fingerprint of the magmatic Chromite. The cores of zoned Chromite show higher contents of Zn, Co and Mn as well as lower Ga, Ti and Sc than the cores of partly altered Chromite. The complex changes in these elements with respect to the Fe
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Ruthenium in komatiitic Chromite
Geochimica et Cosmochimica Acta, 2011Co-Authors: Marek Locmelis, Norman J Pearson, Stephen Barnes, Marco L. FiorentiniAbstract:Abstract The distinction between Ru in solid solution and Ru-bearing inclusions is essential for the predictive modeling of platinum-group element (PGE) geochemistry in applications such as the lithogeochemical exploration for magmatic sulfide deposits in komatiites. This study investigates the role of Chromite in the fractionation of Ru in ultramafic melts by analyzing Chromite grains from sulfide-undersaturated komatiites and a komatiitic basalt from the Yilgarn Craton in Western Australia. In situ analysis using laser ablation ICP-MS yields uniform Ru concentrations in Chromites both within-grain and on a sample scale, with concentrations between 220 and 540 ppb. All other platinum-group elements are below the detection limit of the laser ablation ICP-MS analysis. Carius tube digestion isotope dilution ICP-MS analysis of Chromite concentrates confirms the accuracy of the in-situ method. Time resolved laser ablation ICP-MS analyses have identified the presence of sub-micron Ir-bearing inclusions in some Chromite grains from the komatiitic basalt. However, Ru-bearing inclusions have not been recognized in the analyzed Chromites and this combined with the in situ data suggests that Ru exists in solid solution in the crystal lattice of Chromite. These results show that Chromite can control the fractionation and concentration of Ru in ultramafic systems.
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garnet geotherms pressure temperature data from cr pyrope garnet xenocrysts in volcanic rocks
Journal of Geophysical Research, 1996Co-Authors: Chris Ryan, W L Griffin, Norman J PearsonAbstract:The temperatures and pressures of equilibration of single peridotitic garnet xenocrysts are estimated using a combination of major- and trace-element data, determined using electron microprobe (EMP) and proton-induced X ray emission (PIXE). This new method enables the use of xenocrysts found in kimberlites and other volcanic rocks to determine the local paleogeotherm at the time of eruption of the magma which sampled and transported the xenocrysts. The “Ni thermometer” of Griffin et al. [1989], based on the strong temperature dependence of the partitioning of Ni between garnet and olivine, is refined using an expanded database. Pressure is calculated from garnet composition using an algorithm that combines a modification of the geobarometer of Nickel [1989], based on Cr solubility in coexisting garnet and orthopyroxene, with the composition of a hypothetical coexisting orthopyroxene. The orthopyroxene composition is estimated by inverting the geothermometry equations of Gasparik [1987], Brey and Kohler [1990], and Harley [1984], and combining these with empirical relationships describing Cr in orthopyroxene in Cr-saturated peridotite (Chromite present). The derived pressure (PCr) gives the equilibration pressure of peridotic garnets provided they were in equilibrium with Chromite; garnets from Cr-undersaturated rocks will produce underestimates of pressure. Therefore, the locus of maximum PCr at a given TNi defines the “garnet geotherm”, and provides a method for the determination of paleogeotherms based solely on PIXE and EMP analyses of garnet grains in concentrates. The assumption of coexisting Chromite is tested by comparing the temperature distributions of garnets and Chromites from the same concentrate. Chromite equilibration temperature is estimated using the “Zn thermometer”, based on the strong temperature dependence of the partitioning of Zn between Chromite and olivine. This thermometer is calibrated against the new Ni thermometer using a suite of garnet-Chromite intergrowths. The garnet geotherm technique provides an estimate of the geotherm with an accuracy comparable to xenolith-derived geotherms and provides a means of mapping the thermal state of the lithosphere where xenoliths are rare or absent.
Ria Mukherjee - One of the best experts on this subject based on the ideXlab platform.
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Trace-element fingerprints of Chromite, magnetite and sulfides from the 3.1 Ga ultramafic–mafic rocks of the Nuggihalli greenstone belt, Western Dharwar craton (India)
Contributions to Mineralogy and Petrology, 2015Co-Authors: Ria Mukherjee, José M. González-jiménez, Sisir K Mondal, William L. Griffin, Norman J Pearson, Suzanne Y. O’reillyAbstract:The 3.1 Ga Nuggihalli greenstone belt in the Western Dharwar craton is comprised of chromitite-bearing sill-like ultramafic–mafic rocks that are surrounded by metavolcanic schists (compositionally komatiitic to komatiitic basalts) and a suite of tonalite–trondhjemite–granodiorite gneissic rocks. The sill-like plutonic unit consists of a succession of serpentinite (after dunite)–peridotite–pyroxenite and gabbro with bands of titaniferous magnetite ore. The chromitite ore-bodies (length ≈30–500 m; width ≈2–15 m) are hosted by the serpentinite–peridotite unit. Unaltered Chromites from massive chromitites (>80 % modal Chromite) of the Byrapur and Bhaktarhalli Chromite mines in the greenstone belt are characterized by high Cr# (100Cr/(Cr + Al)) of 78–86 and moderate Mg# (100 Mg/(Mg + Fe^2+)) of 45–55. In situ trace-element analysis (LA-ICPMS) of unaltered Chromites indicates that the parental magma of the chromitite ore-bodies was a komatiite lacking nickel-sulfide mineralization. In the Ga/Fe^3+# versus Ti/Fe^3+# diagram, the Byrapur Chromites plot in the field of suprasubduction zone (SSZ) Chromites while those from Bhaktarhalli lie in the MOR field. The above results corroborate our previous results based on major-element characteristics of the Chromites, where the calculated parental melt of the Byrapur Chromites was komatiitic to komatiitic basalt, and the Bhaktarhalli Chromite was derived from Archean high-Mg basalt. The major-element Chromite data hinted at the possibility of a SSZ environment existing in the Archean. Altered and compositionally zoned Chromite grains in our study show a decrease in Ga, V, Co, Zn, Mn and enrichments of Ni and Ti in the ferritchromit rims. Trace-element heterogeneity in the altered Chromites is attributed to serpentinization. The trace-element patterns of magnetite from the massive magnetite bands in the greenstone belt are similar to those from magmatic Fe–Ti–V-rich magnetite bands in layered intrusions, and magnetites from andesitic melts, suggesting that magnetite crystallized from an evolved gabbroic melt. Enrichments of Ni, Co, Te, As and Bi in disseminated millerite and niccolite occurring within chromitites, and in disseminated bravoite within magnetites, reflect element mobility during serpentinization. Monosulfide solid solution inclusions within pyroxenes (altered to actinolite) in pyroxenite, and interstitial pyrites and chalcopyrites in magnetite, retain primary characteristics except for Fe-enrichment in chalcopyrite, probably due to sub-solidus re-equilibration with magnetite. Disseminated sulfides are depleted in platinum-group elements (PGE) due to late sulfide saturation and the PGE-depleted nature of the mantle source of the sill-like ultramafic–mafic plutonic rocks in the Nuggihalli greenstone belt.
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compositional variations in the mesoarchean Chromites of the nuggihalli schist belt western dharwar craton india potential parental melts and implications for tectonic setting
Contributions to Mineralogy and Petrology, 2010Co-Authors: Ria Mukherjee, Sisir K Mondal, Minik T Rosing, Robert FreiAbstract:The Chromite deposits in the Archean Nuggihalli schist belt are part of a layered ultramafic–mafic sequence within the Western Dharwar Craton of the Indian shield. The 3.1-Ga ultramafic–mafic units occur as sill-like intrusions within the volcano-sedimentary sequences of the Nuggihalli greenstone belt that are surrounded by the tonalite–trondhjemite–granodiorite (TTG) suite of rocks. The entire succession is exposed in the Tagdur mining district. The succession has been divided into the lower and the upper ultramafic units, separated by a middle gabbro unit. The ultramafic units comprise of deformed massive chromitite bodies that are hosted within Chromite-bearing serpentinites. The chromitite bodies occur in the form of pods and elongated lenses (~60–500 m by ~15 m). Detailed electron microprobe studies reveal intense compositional variability of the Chromite grains in silicate-rich chromitite (~50% modal Chromite) and serpentinite (~2% modal Chromite) throughout the entire ultramafic sequence. However, the primary composition of Chromite is preserved in the massive chromitites (~60–75% modal Chromite) from the Byrapur and the Bhaktarhalli mining district of the Nuggihalli schist belt. These are characterized by high Cr-ratios (Cr/(Cr + Al) = 0.78–0.86) and moderate Mg-ratios (Mg/(Mg + Fe2+) = 0.38–0.58). The compositional variability occurs due to sub-solidus re-equilibration in the accessory Chromite in the serpentinite (Mg-ratio = 0.01–0.38; Cr-ratio = 0.02–0.99) and in silicate-rich chromitite (Mg-ratio = 0.06–0.48; Cr-ratio = 0.60–0.99). In the massive chromitites, the sub-solidus re-equilibration for Chromite is less or absent. However, the re-equilibration is prominent in the co-existing interstitial and included olivine (Fo96–98) and pyroxene grains (Mg-numbers = 97–99). Compositional variability on the scale of a single Chromite grain occurs in the form of zoning, and it is common in the accessory Chromite grains in serpentinite and in the altered grains in chromitite. In the zoned grains, the composition of the core is modified and the rim is ferritchromit. In general, ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains. In this case, ferritchromit formation is not very extensive. This indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization. In some occurrences, the ferritchromit rim is very well developed, and only a small relict core appears to remain in the Chromite grain. However, complete alteration of the Chromite grains to ferritchromit without any remnant core is also present. The regular, well-developed and continuous occurrence of ferritchromit rims around the Chromite grain boundaries, the complete alteration of the Chromite grains and the modification of the core composition indicate the alteration in the Nuggihalli schist belt to be intense, pervasive and affected by later low-grade metamorphism. The primary composition of Chromite has been used to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton. Tectonic discrimination diagrams using the primary composition of Chromites indicate a supra-subduction zone setting (SSZ) for the Archean chromitites of Nuggihalli and derivation from a boninitic magma. The composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back-arc rift settings. Formation of the massive chromitites in Nuggihalli may be due to magma mixing process involving hydrous high-Mg magmas or may be related to intrusions of Chromite crystal laden magma; however, there is little scope to test these models because the host rocks are highly altered, serpentinized and deformed. The present configurations of the chromitite bodies are related to the multistage deformation processes that are common in Archean greenstone belts.
Robert Frei - One of the best experts on this subject based on the ideXlab platform.
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compositional variations in the mesoarchean Chromites of the nuggihalli schist belt western dharwar craton india potential parental melts and implications for tectonic setting
Contributions to Mineralogy and Petrology, 2010Co-Authors: Ria Mukherjee, Sisir K Mondal, Minik T Rosing, Robert FreiAbstract:The Chromite deposits in the Archean Nuggihalli schist belt are part of a layered ultramafic–mafic sequence within the Western Dharwar Craton of the Indian shield. The 3.1-Ga ultramafic–mafic units occur as sill-like intrusions within the volcano-sedimentary sequences of the Nuggihalli greenstone belt that are surrounded by the tonalite–trondhjemite–granodiorite (TTG) suite of rocks. The entire succession is exposed in the Tagdur mining district. The succession has been divided into the lower and the upper ultramafic units, separated by a middle gabbro unit. The ultramafic units comprise of deformed massive chromitite bodies that are hosted within Chromite-bearing serpentinites. The chromitite bodies occur in the form of pods and elongated lenses (~60–500 m by ~15 m). Detailed electron microprobe studies reveal intense compositional variability of the Chromite grains in silicate-rich chromitite (~50% modal Chromite) and serpentinite (~2% modal Chromite) throughout the entire ultramafic sequence. However, the primary composition of Chromite is preserved in the massive chromitites (~60–75% modal Chromite) from the Byrapur and the Bhaktarhalli mining district of the Nuggihalli schist belt. These are characterized by high Cr-ratios (Cr/(Cr + Al) = 0.78–0.86) and moderate Mg-ratios (Mg/(Mg + Fe2+) = 0.38–0.58). The compositional variability occurs due to sub-solidus re-equilibration in the accessory Chromite in the serpentinite (Mg-ratio = 0.01–0.38; Cr-ratio = 0.02–0.99) and in silicate-rich chromitite (Mg-ratio = 0.06–0.48; Cr-ratio = 0.60–0.99). In the massive chromitites, the sub-solidus re-equilibration for Chromite is less or absent. However, the re-equilibration is prominent in the co-existing interstitial and included olivine (Fo96–98) and pyroxene grains (Mg-numbers = 97–99). Compositional variability on the scale of a single Chromite grain occurs in the form of zoning, and it is common in the accessory Chromite grains in serpentinite and in the altered grains in chromitite. In the zoned grains, the composition of the core is modified and the rim is ferritchromit. In general, ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains. In this case, ferritchromit formation is not very extensive. This indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization. In some occurrences, the ferritchromit rim is very well developed, and only a small relict core appears to remain in the Chromite grain. However, complete alteration of the Chromite grains to ferritchromit without any remnant core is also present. The regular, well-developed and continuous occurrence of ferritchromit rims around the Chromite grain boundaries, the complete alteration of the Chromite grains and the modification of the core composition indicate the alteration in the Nuggihalli schist belt to be intense, pervasive and affected by later low-grade metamorphism. The primary composition of Chromite has been used to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton. Tectonic discrimination diagrams using the primary composition of Chromites indicate a supra-subduction zone setting (SSZ) for the Archean chromitites of Nuggihalli and derivation from a boninitic magma. The composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back-arc rift settings. Formation of the massive chromitites in Nuggihalli may be due to magma mixing process involving hydrous high-Mg magmas or may be related to intrusions of Chromite crystal laden magma; however, there is little scope to test these models because the host rocks are highly altered, serpentinized and deformed. The present configurations of the chromitite bodies are related to the multistage deformation processes that are common in Archean greenstone belts.
Vanessa Colás - One of the best experts on this subject based on the ideXlab platform.
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The role of silica in the hydrous metamorphism of Chromite
Ore Geology Reviews, 2017Co-Authors: Vanessa Colás, José M. González-jiménez, Isabel Fanlo, José Alberto Padrón-navarta, Vicente López Sánchez-vizcaíno, Fernado Gervilla, Ricardo CastroviejoAbstract:Retrograde hydrous metamorphism has produced three types of microstructures in Chromite grains from chromitites and enclosing rocks of the Tapo Ultramafic Massif (Central Peruvian Andes). In semi-massive chromitites (60–80 vol% Chromite), (i) partly altered Chromite with homogeneous cores surrounded by lower Al2O3 and MgO but higher Cr2O3 and FeO porous Chromite with chlorite filling the pores. In serpentinites (ii) zoned Chromite with homogeneous cores surrounded by extremely higher Fe2O3 non-porous Chromite and magnetite rims, and (iii) non-porous Chromite grains. The different patterns of zoning in Chromite grains are the consequences of the infiltration of reducing and SiO2-rich fluids and the subsequent heterogeneous interaction with more oxidizing and Fe-bearing fluids. During the first stage of alteration under reduced conditions magmatic Chromite is dissolved meanwhile new metamorphogenic porous Chromite crystallizes in equilibrium with chlorite. This reaction that involves dissolution and precipitation of minerals is here modeled thermodynamically for the first time. µSiO2-µMgO pseudosection calculated for unaltered semi-massive chromitites at 2 kbar and 300 °C, the lowest P-T conditions inferred from the Tapo Ultramafic Massif and Maranon Complex, predicts that Chromite + chlorite (i.e., partly altered Chromite) is stable instead of Chromite + chlorite + brucite at progressive higher µSiO2 but lower µMgO. Our observation is twofold as it reveals that the important role of SiO2 and MgO and the open-nature of this process. P-T-X diagrams computed using the different P-T pathways estimated for the enclosing Tapo Ultramafic Massif reproduce well the partial equilibrium sequence of mineral assemblages preserved in the chromitites. Nevertheless, it is restricted only to the P-T conditions of the metamorphic peak and that of the latest overprint. Our estimations reveal that a high fluid/rock ratio (1:40 ratio) is required to produce the microstructures and compositional changes observed in the chromitites from the Tapo Ultramafic Massif. The circulation of SiO2-rich fluids and the mobilization of MgO from the chromitite bodies are linked with the formation of garnet amphibolites and carbonate-silica hydrothermalites (i.e., listwaenites and birbirites) in the ultramafic massif. The origin of these fluids is interpreted as a result of the dissolution of orthopyroxene and/or olivine from the metaharzburgites and metagabbros enclosed in the Tapo Ultramafic Massif.
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Fluid-present deformation AIDS chemical modification of Chromite: Insights from Chromites from Golyamo Kamenyane, SE Bulgaria
Lithos, 2015Co-Authors: Takako Satsukawa, José M. González-jiménez, William L. Griffin, Vanessa Colás, Isabel Fanlo, Fernando Gervilla, Sandra Piazolo, Suzanne Y. O'reilly, Thomas KerestedjianAbstract:Abstract Chemical signatures of chromitites are commonly used to track the evolution of the Earth's mantle. However, chemical modification during deformation may have important implications for the interpretation of Chromites' signatures. Here, we describe the details of how deformation promotes chemical modification in Chromite. Physicochemical characteristics of the Chromites were quantified by measuring crystallographic orientation relationships using Electron Back-Scattered Diffraction (EBSD) and electron microprobe analysis (EMP). Chromites show porphyroclastic textures with coarse-grained porphyroclasts (ca. 0.2–5 mm) and fine-grained neoblasts ( 3 + and Cr and lower Mg# values than the cores of large grains. We interpret F1 and F2 to represent Chromite recrystallized by heterogeneous nucleation and subgrain rotation recrystallization, respectively. Crystallographic preferred orientation (CPO) and misorientation data on the well-developed low-angle (subgrain) boundaries in coarse grains and F2 grains indicate that deformation in Chromite was accommodated mainly by dislocation creep with the dominant activation of the {111} slip system. The retrograde P–T exhumation path predicted by thermodynamic and chemical modeling suggests that these fine-grained Chromites were produced when the initial chromitites reacted with oxidizing fluids during retrograde metamorphism (~ 1.0 GPa and 500–700 °C). Our results show that deformation in the dislocation-creep regime in a chemically open system has induced chemical modification and homogenization within Chromite aggregates as well as strain localization. This close physicochemical link offers new avenues of interpreting the chemical signatures of Chromites, utilizing their microstructurally controlled variation or lack thereof.
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Fluid-present deformation aids chemical modification of Chromite: Insights from Chromites from Golyamo Kamenyane, SE Bulgaria
Lithos, 2015Co-Authors: Takako Satsukawa, José M. González-jiménez, William L. Griffin, Vanessa Colás, Isabel Fanlo, Fernando Gervilla, Sandra Piazolo, Suzanne Y. O'reilly, Thomas N. KerestedjianAbstract:Artículo de publicación ISIChemical signatures of chromitites are commonly used to track the evolution of the Earth's mantle. However, chemical modification during deformation may have important implications for the interpretation of Chromites' signatures. Here, we describe the details of how deformation promotes chemical modification in Chromite. Physicochemical characteristics of the Chromites were quantified by measuring crystallographic orientation relationships using Electron Back-Scattered Diffraction (EBSD) and electron microprobe analysis (EMP). Chromites show porphyroclastic textures with coarse-grained porphyroclasts (ca. 0.2-5 mm) and fine-grained neoblasts (
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fingerprints of metamorphism in Chromite new insights from minor and trace elements
Chemical Geology, 2014Co-Authors: Norman J Pearson, Vanessa Colás, Isabel Fanlo, Fernando Gervilla, Jose Maria Gonzalezjimenez, W L Griffin, Suzanne Y Oreilly, Thomas KerestedjianAbstract:Abstract A suite of minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in Chromite grains from ophiolitic chromitites subjected to high-pressure metamorphism defines a metamorphic signature. A two-stage process associated with the infiltration of fluids during retrograde metamorphism from eclogite- to amphibolite-facies has produced four types of Chromites: (1) porous Chromite strongly enriched in Cr and Fe 2 + but depleted in Al and Mg, with abundant chlorite filling the pores; (2) non-porous Chromite strongly enriched in Fe 3+ (i.e., ferrian Chromite); (3) partly altered Chromite with primary cores surrounded by chlorite-bearing porous Chromite; and (4) zoned Chromite made up of primary cores surrounded by non-porous rims of ferrian Chromite. Compared to spinels from unmetamorphosed chromitites the cores of partly altered Chromites after primary high-Cr Chromite are enriched in Zn, Co and Mn but strongly depleted in Ga, Ni and Sc. This distribution of minor- and trace-elements is related to a decrease in Mg# [Mg/(Mg + Fe 2+ )] and Al, produced by the crystallization of chlorite in the pores of porous Chromite. Non-porous Chromite is enriched in Ti, Ni, Zn, Co, Mn and Sc but depleted in Ga, suggesting that fluid-assisted processes have obliterated the primary magmatic signature. Zoned Chromites have cores depleted in Ga, Ni and Sc but are progressively enriched in Zn, Co and Mn as Mg# and Al decrease toward the rims; they have overall lower concentrations in Ga, Ni and Sc and higher Zn and Co than the non-porous rims of ferrian Chromite. The complex variation of the minor- and trace-elements vs Fe 3+ /(Fe 3+ + Fe 2+ ) in the different types of Chromite suggests a complex interplay of substitutions, linked with the ability of fluids to infiltrate the Chromite and the extent of the re-equilibration between pre-existing cores and newly-formed rims. The results demonstrate that metamorphism can seriously disturb the original magmatic distribution of minor and trace elements in Chromite. The abundances of these elements, and by inference the major elements, can be strongly modified even in the cores of grains that appear “unaltered” in terms of major elements. The use of the major elements as indicators of magmatic processes therefore must be linked to careful evaluation of metamorphic effects, using LA-ICP-MS analysis of minor and trace elements.
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Taking fingerprints of metamorphism in Chromite using minor and trace elements
2014Co-Authors: Vanessa Colás, José M. González-jiménez, William L. Griffin, Norman J Pearson, Isabel Fanlo, Fernando Gervilla, Thomas KerestedjianAbstract:A two-stage mechanism associated with the infiltration of fluids during retrograde metsamorphism from eclogite to amphibolite facies in the ultramafic massif of Golyamo Kamenyane (Bulgaria) has produced four types of microstructures in Chromites: i) porous Chromite, with chlorite in the pores, ii) non-porous Chromite, iii) partly altered Chromite, with primary cores surrounded by porous Chromite, and iv) zoned Chromite, with primary cores surrounded by nonporous rims. LA-ICP-MS analysis shows that partly altered Chromite cores and their surrounding rims of Fe 2+ -rich porous Chromite show a distribution of minor and trace elements similar to Chromite from MORB, except for a strong depletion in Ga and Sc. However, single grains of porous Chromite produced after the massive infiltration of fluids show a significant enrichment in Zn, Co and Mn but depletion in Ga, Ni and Sc. Non-porous Chromite (i.e., ferrian Chromite), forming single grains and rims on zoned Chromites, are also enriched in Zn, Co, Mn and depleted in Ga but are distictively enriched in Ti, Ni and Sc. This suggests that oxidising fluids have substantially obliterated the geochemical fingerprint of the magmatic Chromite. The cores of zoned Chromite show higher contents of Zn, Co and Mn as well as lower Ga, Ti and Sc than the cores of partly altered Chromite. The complex changes in these elements with respect to the Fe