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Max W. Schmidt - One of the best experts on this subject based on the ideXlab platform.
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The Melting of Carbonated Pelites from 70 to 700 km Depth
Journal of Petrology, 2011Co-Authors: Daniele Grassi, Max W. SchmidtAbstract:Phase assemblages, melting relations and melt compositions of a dry carbonated Pelite (DG2) and a carbonated Pelite with 1·1wt % H2O (AM) have been experimentally investigated at 5·5^ 23·5 GPa and 1070^15508C.The subsolidus mineralogies to 16 GPa contain garnet, clinopyroxene, coesite or stishovite, kyanite or corundum, phengite or potassium feldspar ( 8 GPa with and without H2O, respectively), and then K-hollandite, a Ti phase and ferroan dolomite/Mg-calcite or aragonite þ ferroan magnesite at higher pressures. The breakdown of clinopyroxene at 416 GPa causes Na-rich Ca-carbonate containing up to 11wt % Na2O to replace aragonite and leads to the formation of an Na-rich CO2 fluid. Further pressure increase leads to typical Transition Zone minerals such as the CAS phase and one or two perovskites, which completely substitute garnet at the highest investigated pressure (23·5 GPa). Melting at 5·5^23·5 GPa yields alkali-rich magnesio-dolomitic (DG2) to ferro-dolomitic (AM) carbonate melts at temperatures 200^3508C below the mantle geotherm, lower than for any other studied natural composition. Melting reactions are controlled by carbonates and alkali-hosting phases: to 16 GPa clinopyroxene remains residual, Na is compatible and the magnesioto ferro-dolomitic carbonate melts have extremely high K2O/Na2O ratios. K2O/Na2O weight ratios decrease from 26^41 at 8 GPa to 1·2 at 16 GPa when K-hollandite expands its stability field with increasing pressure. At 416 GPa, Na is repartitioned between several phases, and again becomes incompatible as at53 GPa, leading to Na-rich carbonate melts with K2O/Na2O ratios 1.This leaves the pressure interval of c. 4^15 GPa for ultrapotassic metasomatism. Comparison of the solidus with typical subducting slab-surface temperatures yields two distinct depths of probable carbonated Pelite melting: at 6^9 GPa where the solidus has a negative Clapeyron slope between the intersection of the silicate and carbonate melting reactions at 5 GPa, and the phengite or potassium feldspar stability limit at 9 GPa. The second opportunity is related to possible slab deflection along the 660 km discontinuity, leading to thermal relaxation and partial melting of the fertile carbonated Pelites, thus recycling sedimentary CO2, alkalis and other lithophile and strongly incompatible elements back into the mantle.
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Melting of carbonated Pelites at 8–13 GPa: generating K-rich carbonatites for mantle metasomatism
Contributions to Mineralogy and Petrology, 2010Co-Authors: Daniele Grassi, Max W. SchmidtAbstract:The melting behaviour of three carbonated Pelites containing 0–1 wt% water was studied at 8 and 13 GPa, 900–1,850°C to define conditions of melting, melt compositions and melting reactions. At 8 GPa, the fluid-absent and dry carbonated Pelite solidi locate at 950 and 1,075°C, respectively; >100°C lower than in carbonated basalts and 150–300°C lower than the mantle adiabat. From 8 to 13 GPa, the fluid-present and dry solidi temperatures then increase to 1,150 and 1,325°C for the 1.1 wt% H2O and the dry composition, respectively. The melting behaviour in the 1.1 wt% H2O composition changes from fluid-absent at 8 GPa to fluid-present at 13 GPa with the pressure breakdown of phengite and the absence of other hydrous minerals. Melting reactions are controlled by carbonates, and the potassium and hydrous phases present in the subsolidus. The first melts, which composition has been determined by reverse sandwich experiments, are potassium-rich Ca–Fe–Mg-carbonatites, with extreme K2O/Na2O wt ratios of up to 42 at 8 GPa. Na is compatible in clinopyroxene with \( D_{\text{Na}}^{{{\text{cpx}}/{\text{carbonatite}}}} = 10{-}18 \) at the solidus at 8 GPa. The melt K2O/Na2O slightly decreases with increasing temperature and degree of melting but strongly decreases from 8 to 13 GPa when K-hollandite extends its stability field to 200°C above the solidus. The compositional array of the sediment-derived carbonatites is congruent with alkali- and CO2-rich melt or fluid inclusions found in diamonds. The fluid-absent melting of carbonated Pelites at 8 GPa contrasts that at ≤5 GPa where silicate melts form at lower temperatures than carbonatites. Comparison of our melting temperatures with typical subduction and mantle geotherms shows that melting of carbonated Pelites to 400-km depth is only feasible for extremely hot subduction. Nevertheless, melting may occur when subduction slows down or stops and thermal relaxation sets in. Our experiments show that CO2-metasomatism originating from subducted crust is intimately linked with K-metasomatism at depth of >200 km. As long as the mantle remains adiabatic, low-viscosity carbonatites will rise into the mantle and percolate upwards. In cold subcontinental lithospheric mantle keels, the potassic Ca–Fe–Mg-carbonatites may freeze when reacting with the surrounding mantle leading to potassium-, carbonate/diamond- and incompatible element enriched metasomatized zones, which are most likely at the origin of ultrapotassic magmas such as group II kimberlites.
Jay J Ague - One of the best experts on this subject based on the ideXlab platform.
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evidence for major mass transfer and volume strain during regional metamorphism of Pelites
Geology, 1991Co-Authors: Jay J AgueAbstract:Systematic examination of published sedimentary and metamorphosed Pelite analyses has revealed evidence of significant mass transfer and volume strain during regional metamorphism. Statistical analysis of the data shows that Barrovian zone metamorphism of pelitic schist generally causes increases in the whole-rock concentrations of the low-solubility elements Ti and All The observed increases in Ti and Al contents as functions of metamorphic grade are almost certainly due to residual enrichment caused by the removal of other more soluble species. Application of mass-balance principles to the petite compositional trends indicates that the average Pelite may lose as much as 30% of its mass and volume during progressive metamorphism from subgreenschist to amphibolite facies conditions. The bulk of the lost mass is silica, not volatiles. In addition, other elements, particularly Ca, Na, and K, appear to be highly mobile in deep-crustal metamorphism. Contrary to conventional interpretations, it is concluded that the regional metamorphism of Pelites is not an isochemical process.
Toshiro Morikiyo - One of the best experts on this subject based on the ideXlab platform.
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Graphitization of carbonaceous matter during metamorphism with references to carbonate and pelitic rocks of contact and regional metamorphisms, Japan
Contributions to Mineralogy and Petrology, 1994Co-Authors: Hideki Wada, Toshihiro Tomita, Kazuhiro Matsuura, Keisuke Tuchi, Toshiro MorikiyoAbstract:This study is an attempt to correlate the graphitization process of carbonaceous matter during metamorphism with metamorphic grade. Graphitization can be parameterized using crystal structure and chemical and isotopic compositions. The extent of graphitization could be characterized mainly by temperature, duration of metamorphism and rock composition. We compared the graphitization trends for two metamorphic terrains, a contact aureole of the Kasuga area and a regional metamorphic terrain of high-temperature/low pressure type of the Ryoke metamorphic terrain in Northern Kiso area, Central Japan, and for two different lithologies (carbonate and Pelite), using X-ray diffractogram, DTA-TG analysis, and chemical and stable isotope analyses. During contact metamorphism, graphitization and carbon isotopic exchange reactions proceeded simultaneously in pelitic and carbonate rocks. The decreases in basal spacing d(002) of the carbonaceous matter in carbonate rocks is greatly accelerated at temperatures higher than about 400° C. Furthermore, carbon isotopic ratios of graphite in carbonate rocks also change to ^13C-enriched values implying exchange with carbonates. The beginning of this enrichment of ^13C in the carbonaceous matter coincides with an abrupt increase of the graphitization processes. Carbon isotopic shifting up to 5‰ in Pelites could be observed as metamorphic temperature increased probably by about 400° C. Carbonaceous matter in pelitic rocks is sometimes a mixture of poorly crystallized organic matter and well-crystallized graphite detritus. DTA-TG analysis is an effective tool for the distinction of detrital graphitic material. Two sources for the original carbon isotopic composition of carbonaceous matter in Pelites in the Kasuga contact aureole can be distinguished, about-28‰ and-24‰ regardless of the presence of detrital graphite, and were mainly controlled by depositional environment of the sediments. Graphitization in limestones and pelitic rocks in regional metamorphism proceeds further than in a contact aureole. In the low-temperature range, the differences in extent of graphitization between the two metamorphic regions is large. However, at temperatures higher than 600° C, the extent of graphitization in both regions is indistinguishable. The degree of graphitization is different in limestones and pelitic rocks from the Ryoke metamorphic terrain. We demonstrate that the graphitization involves a progressive re-construction process of the crystal structure. The sequence of the first appearance of crystal inter planar spacing correlates with the metamorphic grade and indicates the crystal growth of three-dimensional structured graphite.
Daniele Grassi - One of the best experts on this subject based on the ideXlab platform.
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The Melting of Carbonated Pelites from 70 to 700 km Depth
Journal of Petrology, 2011Co-Authors: Daniele Grassi, Max W. SchmidtAbstract:Phase assemblages, melting relations and melt compositions of a dry carbonated Pelite (DG2) and a carbonated Pelite with 1·1wt % H2O (AM) have been experimentally investigated at 5·5^ 23·5 GPa and 1070^15508C.The subsolidus mineralogies to 16 GPa contain garnet, clinopyroxene, coesite or stishovite, kyanite or corundum, phengite or potassium feldspar ( 8 GPa with and without H2O, respectively), and then K-hollandite, a Ti phase and ferroan dolomite/Mg-calcite or aragonite þ ferroan magnesite at higher pressures. The breakdown of clinopyroxene at 416 GPa causes Na-rich Ca-carbonate containing up to 11wt % Na2O to replace aragonite and leads to the formation of an Na-rich CO2 fluid. Further pressure increase leads to typical Transition Zone minerals such as the CAS phase and one or two perovskites, which completely substitute garnet at the highest investigated pressure (23·5 GPa). Melting at 5·5^23·5 GPa yields alkali-rich magnesio-dolomitic (DG2) to ferro-dolomitic (AM) carbonate melts at temperatures 200^3508C below the mantle geotherm, lower than for any other studied natural composition. Melting reactions are controlled by carbonates and alkali-hosting phases: to 16 GPa clinopyroxene remains residual, Na is compatible and the magnesioto ferro-dolomitic carbonate melts have extremely high K2O/Na2O ratios. K2O/Na2O weight ratios decrease from 26^41 at 8 GPa to 1·2 at 16 GPa when K-hollandite expands its stability field with increasing pressure. At 416 GPa, Na is repartitioned between several phases, and again becomes incompatible as at53 GPa, leading to Na-rich carbonate melts with K2O/Na2O ratios 1.This leaves the pressure interval of c. 4^15 GPa for ultrapotassic metasomatism. Comparison of the solidus with typical subducting slab-surface temperatures yields two distinct depths of probable carbonated Pelite melting: at 6^9 GPa where the solidus has a negative Clapeyron slope between the intersection of the silicate and carbonate melting reactions at 5 GPa, and the phengite or potassium feldspar stability limit at 9 GPa. The second opportunity is related to possible slab deflection along the 660 km discontinuity, leading to thermal relaxation and partial melting of the fertile carbonated Pelites, thus recycling sedimentary CO2, alkalis and other lithophile and strongly incompatible elements back into the mantle.
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Melting of carbonated Pelites at 8–13 GPa: generating K-rich carbonatites for mantle metasomatism
Contributions to Mineralogy and Petrology, 2010Co-Authors: Daniele Grassi, Max W. SchmidtAbstract:The melting behaviour of three carbonated Pelites containing 0–1 wt% water was studied at 8 and 13 GPa, 900–1,850°C to define conditions of melting, melt compositions and melting reactions. At 8 GPa, the fluid-absent and dry carbonated Pelite solidi locate at 950 and 1,075°C, respectively; >100°C lower than in carbonated basalts and 150–300°C lower than the mantle adiabat. From 8 to 13 GPa, the fluid-present and dry solidi temperatures then increase to 1,150 and 1,325°C for the 1.1 wt% H2O and the dry composition, respectively. The melting behaviour in the 1.1 wt% H2O composition changes from fluid-absent at 8 GPa to fluid-present at 13 GPa with the pressure breakdown of phengite and the absence of other hydrous minerals. Melting reactions are controlled by carbonates, and the potassium and hydrous phases present in the subsolidus. The first melts, which composition has been determined by reverse sandwich experiments, are potassium-rich Ca–Fe–Mg-carbonatites, with extreme K2O/Na2O wt ratios of up to 42 at 8 GPa. Na is compatible in clinopyroxene with \( D_{\text{Na}}^{{{\text{cpx}}/{\text{carbonatite}}}} = 10{-}18 \) at the solidus at 8 GPa. The melt K2O/Na2O slightly decreases with increasing temperature and degree of melting but strongly decreases from 8 to 13 GPa when K-hollandite extends its stability field to 200°C above the solidus. The compositional array of the sediment-derived carbonatites is congruent with alkali- and CO2-rich melt or fluid inclusions found in diamonds. The fluid-absent melting of carbonated Pelites at 8 GPa contrasts that at ≤5 GPa where silicate melts form at lower temperatures than carbonatites. Comparison of our melting temperatures with typical subduction and mantle geotherms shows that melting of carbonated Pelites to 400-km depth is only feasible for extremely hot subduction. Nevertheless, melting may occur when subduction slows down or stops and thermal relaxation sets in. Our experiments show that CO2-metasomatism originating from subducted crust is intimately linked with K-metasomatism at depth of >200 km. As long as the mantle remains adiabatic, low-viscosity carbonatites will rise into the mantle and percolate upwards. In cold subcontinental lithospheric mantle keels, the potassic Ca–Fe–Mg-carbonatites may freeze when reacting with the surrounding mantle leading to potassium-, carbonate/diamond- and incompatible element enriched metasomatized zones, which are most likely at the origin of ultrapotassic magmas such as group II kimberlites.
Tochukwu A. S. Ugwoke - One of the best experts on this subject based on the ideXlab platform.
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Influence of Petrogenesis on Suitability of Some Pelitic Rocks as Construction Aggregates in South-Eastern Nigeria
Geotechnical and Geological Engineering, 2015Co-Authors: C. O. Okogbue, Tochukwu A. S. UgwokeAbstract:Ten rock samples, collected from five studied Albian rock quarry units, were subjected to hand specimen and X-ray diffraction studies to determine their petrography and also to two degradability test versions of soaking and drying to simulate their resistance to intra-seasonal and inter-seasonal weathering that occurs in south-eastern Nigeria. Nine of the samples, which are pelitic in composition, were further subjected to Los Angeles abrasion (LAAV) and aggregate impact value (AIV) tests to determine their mechanical properties. Field and petrographic studies revealed that the quarry units are made up of rocks of varying petrogenetic origins namely: hydrothermal alteration of Pelites and volcanic bomb, contact metamorphism of Pelites, low-grade regional metamorphism of Pelites and volcanic/igneous activity. Results of the degradability tests revealed that the rocks that formed through regional metamorphism of Pelites showed significant deterioration (≥1 %) in both short soaking/partial drying and long soaking/complete drying test while three of the hydrothermally altered Pelites that contain sizeable amount of soluble minerals, showed significant deterioration (≥1 %) in only long soaking/complete drying test. Results of the LAAV and AIV tests showed that all the rocks satisfy the mechanical property criteria for construction of all sections of pavements. However, a combination of the petrographic, degradability, LAAV and AIV test results reveal that the regional metamorphosed Pelites are not suitable for construction of base and wearing surface of pavements and structural concretes. The results also reveal that only the contact metamorphosed Pelites and hydrothermally altered volcanic bomb are suitable for construction of all types of civil engineering structures.