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Xiaoying Gao - One of the best experts on this subject based on the ideXlab platform.
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Multiphase solid inclusions in ultrahigh-pressure metamorphic rocks: A snapshot of anatectic melts during continental collision
Journal of Asian Earth Sciences, 2017Co-Authors: Xiaoying Gao, Yixiang Chen, Qiang-qiang ZhangAbstract:Abstract Partial Melting of crustal rocks may take place during continental collision, giving rise to anatectic melts. Their composition and evolution are major concerns in the chemical geodynamics of continental subduction zones. They are often recorded by multiphase solid (MS) inclusions enclosed by peritectic minerals in ultrahigh-pressure (UHP) metamorphic terranes inside collisional orogens. These MS inclusions generally show negative crystal shapes with the wedge-shaped off-shoot structure in garnet, omphacite and kyanite. They contain a number of silicate, carbonate and sulfate minerals such as K-feldspar, plagioclase, quartz, epidote, calcite and barite, with occasional occurrences of magnetite, zircon and pyrite. An integrated study of petrology, mineralogy and geochemistry indicates that the MS inclusions are the primary crystallization product of former silicate and carbonate melts. The silicate melts were derived from Dehydration Melting of hydrous minerals such as phengite and paragonite in UHP metamorphic rocks, and the carbonate melts were produced by partial Melting of subducted carbonate minerals. Some MS inclusions show remarkably high Na contents, suggesting their derivation from Dehydration Melting of paragonite. In contrast, K-bearing MS inclusions are produced by Dehydration Melting of K-bearing hydrous minerals like phengite. Many studies have been devoted to the mineralogical and geochemical compositions of MS inclusions in UHP metamorphic rocks, with the aim to determine the time and mechanism of crustal anataxis during collisional orogeny. Various analytical methods were used to characterize the morphology, texture, mineral chemistry and trace element composition of MS inclusions. The results provide insights into the physicochemical properties of anatectic melts in continental subduction zones. The partial Melting of deeply subducted crustal rocks would lead to their significant differentiation in lithochemistry and geochemistry. This process has great bearing on the tectonothermal evolution of continental subduction zones and the exhumation mechanism of deeply subducted crustal slices.
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Dehydration Melting of ultrahigh pressure eclogite in the dabie orogen evidence from multiphase solid inclusions in garnet
Journal of Metamorphic Geology, 2012Co-Authors: Xiaoying Gao, Yongfei Zheng, Yixiang ChenAbstract:Several types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh-pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K-feldspar to pure quartz, with predominance of intermediate types consisting of K-feldspar + quartz ± silicate (plagioclase or epidote) ± barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 57-73% and highly variable SiO2 contents of 0.32-25.85% that are positively correlated with BaO and SO3 contents. The occurrence of MS inclusions provides petrographic evidence for partial Melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO2 contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local Dehydration Melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial Melting during the continental subduction-zone metamorphism.
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Dehydration Melting of ultrahigh‐pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet
Journal of Metamorphic Geology, 2011Co-Authors: Xiaoying Gao, Yongfei Zheng, Yixiang ChenAbstract:Several types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh-pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K-feldspar to pure quartz, with predominance of intermediate types consisting of K-feldspar + quartz ± silicate (plagioclase or epidote) ± barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 57-73% and highly variable SiO2 contents of 0.32-25.85% that are positively correlated with BaO and SO3 contents. The occurrence of MS inclusions provides petrographic evidence for partial Melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO2 contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local Dehydration Melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial Melting during the continental subduction-zone metamorphism.
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partial Melting fluid supercriticality and element mobility in ultrahigh pressure metamorphic rocks during continental collision
Earth-Science Reviews, 2011Co-Authors: Yongfei Zheng, Qiongxia Xia, Renxu Chen, Xiaoying GaoAbstract:Abstract Partial Melting at continental lithosphere depths plays an important role in generating geochemical variations in igneous rocks. In particular, Dehydration Melting of ultrahigh-pressure (UHP) metamorphic rocks during continental collision provides a petrological link to intracrustal differentiation with respect to the compositional evolution of continental crust. While island arc magmatism represents one end-member of fluid-induced large-scale Melting in the mantle wedge during subduction of the oceanic crust, the partial Melting of UHP rocks can be viewed as the other end-member of fluid-induced small-scale anatexis during exhumation of the deeply subducted continental crust. This latter type of Melting is also triggered by metamorphic Dehydration in response to P–T changes during the continental collision. It results in local occurrences of hydrous melts (even supercritical fluids) as felsic veinlets between boundaries of and multiphase solid inclusions in UHP metamorphic minerals as well as local accumulation of veinlet-like felsic leucosomes in foliated UHP metamorphic rocks and metamorphically grown zircons in orogenic peridotites. Thus, very low-degree melts of UHP rocks provide a window into magmatic processes that operated in continental subduction zones. This article presents a review on available results from experimental petrology concerning the possibility of partial Melting under conditions of continental subduction-zone metamorphism, and petrological evidence for the occurrence of Dehydration-driven in-situ partial Melting in natural UHP rocks during the continental collision. Although the deeply subducted continental crust is characterized by a relative lack of aqueous fluids, the partial Melting in UHP rocks commonly takes place during decompression exhumation to result in local in-situ occurrences of felsic melts at small scales. This is caused by the local accumulation of aqueous fluids due to the breakdown of hydrous minerals and the exsolution of structural hydroxyl and molecular water from nominally anhydrous minerals in UHP rocks during the exhumation. The Dehydration Melting of UHP rocks would not only have bearing on the formation of supercritical fluids during subduction-zone metamorphism, but also contribute to element mobility and ultrapotassic magmatism in continental collision orogens. Therefore, the study of Dehydration Melting and its effects on element transport in UHP slabs, rocks and minerals is a key to chemical geodynamics of continental subduction zones.
Yixiang Chen - One of the best experts on this subject based on the ideXlab platform.
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Multiphase solid inclusions in ultrahigh-pressure metamorphic rocks: A snapshot of anatectic melts during continental collision
Journal of Asian Earth Sciences, 2017Co-Authors: Xiaoying Gao, Yixiang Chen, Qiang-qiang ZhangAbstract:Abstract Partial Melting of crustal rocks may take place during continental collision, giving rise to anatectic melts. Their composition and evolution are major concerns in the chemical geodynamics of continental subduction zones. They are often recorded by multiphase solid (MS) inclusions enclosed by peritectic minerals in ultrahigh-pressure (UHP) metamorphic terranes inside collisional orogens. These MS inclusions generally show negative crystal shapes with the wedge-shaped off-shoot structure in garnet, omphacite and kyanite. They contain a number of silicate, carbonate and sulfate minerals such as K-feldspar, plagioclase, quartz, epidote, calcite and barite, with occasional occurrences of magnetite, zircon and pyrite. An integrated study of petrology, mineralogy and geochemistry indicates that the MS inclusions are the primary crystallization product of former silicate and carbonate melts. The silicate melts were derived from Dehydration Melting of hydrous minerals such as phengite and paragonite in UHP metamorphic rocks, and the carbonate melts were produced by partial Melting of subducted carbonate minerals. Some MS inclusions show remarkably high Na contents, suggesting their derivation from Dehydration Melting of paragonite. In contrast, K-bearing MS inclusions are produced by Dehydration Melting of K-bearing hydrous minerals like phengite. Many studies have been devoted to the mineralogical and geochemical compositions of MS inclusions in UHP metamorphic rocks, with the aim to determine the time and mechanism of crustal anataxis during collisional orogeny. Various analytical methods were used to characterize the morphology, texture, mineral chemistry and trace element composition of MS inclusions. The results provide insights into the physicochemical properties of anatectic melts in continental subduction zones. The partial Melting of deeply subducted crustal rocks would lead to their significant differentiation in lithochemistry and geochemistry. This process has great bearing on the tectonothermal evolution of continental subduction zones and the exhumation mechanism of deeply subducted crustal slices.
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multiphase solid inclusions in zoisite bearing eclogite evidence for partial Melting of ultrahigh pressure metamorphic rocks during continental collision
Lithos, 2014Co-Authors: Yixiang Chen, Yongfei Zheng, Zhaochu HuAbstract:Abstract Multiphase solid (MS) inclusions in both garnet and omphacite were investigated for zoisite-bearing eclogite from the ultrahigh-pressure (UHP) metamorphic zone in the Sulu orogen. The results provide petrological evidence for local anatexis during exhumation of deeply subducted continental crust. There are three types of MS inclusions: (1) plagioclase + quartz, (2) plagioclase + quartz + K-feldspar, and (3) barite + plagioclase + K-feldspar ± zoisite/epidote. The host minerals mostly exhibit radial fractures surrounding the MS inclusions. The first and second types of MS inclusions were analyzed for their bulk compositions, yielding high SiO 2 and Na 2 O but very low FeO + MgO + TiO 2 with variable K 2 O for the second type. Trace element analyses of representative MS inclusions yield generally very low concentrations except such large ion lithophile elements as Sr, Ba and Rb. These features suggest different origins for the three types of MS inclusions. The first type of MS inclusions would be primarily derived from Dehydration Melting of paragonite, whereas the second type of MS inclusions would be derived from Dehydration Melting of both paragonite and phengite. The third type of MS inclusion may result from the interaction between metamorphic fluid and host mineral in view of the occurrence of barite as a filling phase in the fractures of host minerals. Zoisite breakdown is also indicated by its highly cuspate shape in coexistence with quartz, providing components for growth of garnet during the exhumation. Therefore the anatexis of zoisite-bearing UHP eclogite is primarily driven by the breakdown of hydrous minerals such as phengite and paragonite. The major and trace element compositions of MS inclusions in the eclogites mainly depend on the species of hydrous minerals involved in the mineralogical reactions of Dehydration Melting in subduction channel.
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Dehydration Melting of ultrahigh pressure eclogite in the dabie orogen evidence from multiphase solid inclusions in garnet
Journal of Metamorphic Geology, 2012Co-Authors: Xiaoying Gao, Yongfei Zheng, Yixiang ChenAbstract:Several types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh-pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K-feldspar to pure quartz, with predominance of intermediate types consisting of K-feldspar + quartz ± silicate (plagioclase or epidote) ± barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 57-73% and highly variable SiO2 contents of 0.32-25.85% that are positively correlated with BaO and SO3 contents. The occurrence of MS inclusions provides petrographic evidence for partial Melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO2 contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local Dehydration Melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial Melting during the continental subduction-zone metamorphism.
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Dehydration Melting of ultrahigh‐pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet
Journal of Metamorphic Geology, 2011Co-Authors: Xiaoying Gao, Yongfei Zheng, Yixiang ChenAbstract:Several types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh-pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K-feldspar to pure quartz, with predominance of intermediate types consisting of K-feldspar + quartz ± silicate (plagioclase or epidote) ± barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 57-73% and highly variable SiO2 contents of 0.32-25.85% that are positively correlated with BaO and SO3 contents. The occurrence of MS inclusions provides petrographic evidence for partial Melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO2 contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local Dehydration Melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial Melting during the continental subduction-zone metamorphism.
Yongfei Zheng - One of the best experts on this subject based on the ideXlab platform.
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Experimental melts from crustal rocks: A lithochemical constraint on granite petrogenesis
Lithos, 2016Co-Authors: Peng Gao, Yongfei Zheng, Zi–fu ZhaoAbstract:Abstract Many studies of experimental petrology have devoted to partial Melting of crustal rocks. In order to provide lithochemical constraints on granite petrogenesis, this paper presents a compilation and synthesis of available experimental data for the major element compositions of felsic melts derived from partial Melting of natural or synthetic materials in the compositional range of crustal rocks. The experimental melts are categorized into four types according to the species of hydrous minerals in starting materials: (I) amphibole-bearing; (II) amphibole- and biotite-bearing; (III) biotite-bearing; and (IV) biotite- and muscovite-bearing. If Dehydration Melting takes place at normal crustal conditions (P = 5–10 kbar, T ≤ 1000 °C), experimental melts are rich in SiO 2 but poor in MgO + FeO T except those from amphibole-bearing sources. A comprehensive comparison of compositions between experimental melts and starting materials indicates that geochemical fractionation is variable for different major elements and their ratios. Source composition and Melting temperature exert stronger controls on the compositional variations of experimental melts than pressure and fluid. By comparing the experimental melts with natural granites, the following insights into granite petrogenesis can be got: (1) while peritectic assemblage entrainment may be the dominant mechanism for the compositional variations of garnet/cordierite-rich S-type granites, fractional crystallization of diverse melts from heterogeneous metasedimentary precursors probably governs the compositional variations of garnet/cordierite-poor S-type granites; (2) relatively K 2 O-rich mafic to intermediate rocks are appropriate sources for calc-alkaline I-type granites. The compositional variations of calc-alkaline granites are jointly controlled by peritectic assemblage entrainment and subsequent fractional crystallization; (3) while Dehydration Melting at T > 950 °C is appropriate for the production of ferroan and alkali-rich granitic melts from intermediate magnesian tonalite or granodiorite, it is also possible for ferroan, alkali–rich and fluorine-rich granitic melts to be produced by Dehydration Melting of moderately magnesian mica–bearing materials at T ≤ 900 °C. Nevertheless, the low-T melts are more peraluminous than the high-T ones. Therefore, the composition of source rocks exerts the first-order control on the composition of granitic melts in closed systems. In addition, the Dehydration Melting of crustal rocks under different conditions is also responsible for variations in the composition of granites.
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multiphase solid inclusions in zoisite bearing eclogite evidence for partial Melting of ultrahigh pressure metamorphic rocks during continental collision
Lithos, 2014Co-Authors: Yixiang Chen, Yongfei Zheng, Zhaochu HuAbstract:Abstract Multiphase solid (MS) inclusions in both garnet and omphacite were investigated for zoisite-bearing eclogite from the ultrahigh-pressure (UHP) metamorphic zone in the Sulu orogen. The results provide petrological evidence for local anatexis during exhumation of deeply subducted continental crust. There are three types of MS inclusions: (1) plagioclase + quartz, (2) plagioclase + quartz + K-feldspar, and (3) barite + plagioclase + K-feldspar ± zoisite/epidote. The host minerals mostly exhibit radial fractures surrounding the MS inclusions. The first and second types of MS inclusions were analyzed for their bulk compositions, yielding high SiO 2 and Na 2 O but very low FeO + MgO + TiO 2 with variable K 2 O for the second type. Trace element analyses of representative MS inclusions yield generally very low concentrations except such large ion lithophile elements as Sr, Ba and Rb. These features suggest different origins for the three types of MS inclusions. The first type of MS inclusions would be primarily derived from Dehydration Melting of paragonite, whereas the second type of MS inclusions would be derived from Dehydration Melting of both paragonite and phengite. The third type of MS inclusion may result from the interaction between metamorphic fluid and host mineral in view of the occurrence of barite as a filling phase in the fractures of host minerals. Zoisite breakdown is also indicated by its highly cuspate shape in coexistence with quartz, providing components for growth of garnet during the exhumation. Therefore the anatexis of zoisite-bearing UHP eclogite is primarily driven by the breakdown of hydrous minerals such as phengite and paragonite. The major and trace element compositions of MS inclusions in the eclogites mainly depend on the species of hydrous minerals involved in the mineralogical reactions of Dehydration Melting in subduction channel.
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Dehydration Melting of ultrahigh pressure eclogite in the dabie orogen evidence from multiphase solid inclusions in garnet
Journal of Metamorphic Geology, 2012Co-Authors: Xiaoying Gao, Yongfei Zheng, Yixiang ChenAbstract:Several types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh-pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K-feldspar to pure quartz, with predominance of intermediate types consisting of K-feldspar + quartz ± silicate (plagioclase or epidote) ± barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 57-73% and highly variable SiO2 contents of 0.32-25.85% that are positively correlated with BaO and SO3 contents. The occurrence of MS inclusions provides petrographic evidence for partial Melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO2 contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local Dehydration Melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial Melting during the continental subduction-zone metamorphism.
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Dehydration Melting of ultrahigh‐pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet
Journal of Metamorphic Geology, 2011Co-Authors: Xiaoying Gao, Yongfei Zheng, Yixiang ChenAbstract:Several types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh-pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K-feldspar to pure quartz, with predominance of intermediate types consisting of K-feldspar + quartz ± silicate (plagioclase or epidote) ± barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 57-73% and highly variable SiO2 contents of 0.32-25.85% that are positively correlated with BaO and SO3 contents. The occurrence of MS inclusions provides petrographic evidence for partial Melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO2 contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local Dehydration Melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial Melting during the continental subduction-zone metamorphism.
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partial Melting fluid supercriticality and element mobility in ultrahigh pressure metamorphic rocks during continental collision
Earth-Science Reviews, 2011Co-Authors: Yongfei Zheng, Qiongxia Xia, Renxu Chen, Xiaoying GaoAbstract:Abstract Partial Melting at continental lithosphere depths plays an important role in generating geochemical variations in igneous rocks. In particular, Dehydration Melting of ultrahigh-pressure (UHP) metamorphic rocks during continental collision provides a petrological link to intracrustal differentiation with respect to the compositional evolution of continental crust. While island arc magmatism represents one end-member of fluid-induced large-scale Melting in the mantle wedge during subduction of the oceanic crust, the partial Melting of UHP rocks can be viewed as the other end-member of fluid-induced small-scale anatexis during exhumation of the deeply subducted continental crust. This latter type of Melting is also triggered by metamorphic Dehydration in response to P–T changes during the continental collision. It results in local occurrences of hydrous melts (even supercritical fluids) as felsic veinlets between boundaries of and multiphase solid inclusions in UHP metamorphic minerals as well as local accumulation of veinlet-like felsic leucosomes in foliated UHP metamorphic rocks and metamorphically grown zircons in orogenic peridotites. Thus, very low-degree melts of UHP rocks provide a window into magmatic processes that operated in continental subduction zones. This article presents a review on available results from experimental petrology concerning the possibility of partial Melting under conditions of continental subduction-zone metamorphism, and petrological evidence for the occurrence of Dehydration-driven in-situ partial Melting in natural UHP rocks during the continental collision. Although the deeply subducted continental crust is characterized by a relative lack of aqueous fluids, the partial Melting in UHP rocks commonly takes place during decompression exhumation to result in local in-situ occurrences of felsic melts at small scales. This is caused by the local accumulation of aqueous fluids due to the breakdown of hydrous minerals and the exsolution of structural hydroxyl and molecular water from nominally anhydrous minerals in UHP rocks during the exhumation. The Dehydration Melting of UHP rocks would not only have bearing on the formation of supercritical fluids during subduction-zone metamorphism, but also contribute to element mobility and ultrapotassic magmatism in continental collision orogens. Therefore, the study of Dehydration Melting and its effects on element transport in UHP slabs, rocks and minerals is a key to chemical geodynamics of continental subduction zones.
H. Bureau - One of the best experts on this subject based on the ideXlab platform.
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Dehydration Melting below the undersaturated transition zone
Geochemistry Geophysics Geosystems, 2020Co-Authors: W Panero, C Thomas, R. Myhill, C. Raepsaet, Jeffrey S Pigott, H. BureauAbstract:A reflector 70-130 km below the base of the transition zone beneath Tibet is observed in receiver functions and underside seismic reflections, at depths consistent with the transition of garnet to bridgmanite. Contrast in water storage capacity between the minerals of the Earth's transition zone and lower mantle suggests the possibility for Dehydration Melting at the top of the lower mantle. First-principles calculations combined with laboratory synthesis experiments constrain the mantle water capacity across the base of the transition zone and into the top of the lower mantle. We interpret the observed seismic signal as consistent with 3-4 vol % hydrous melt resulting from Dehydration Melting in the garnet to bridgmanite transition. Should seismic signals evident in downwelling region result from water contents representative of upper mantle water globally, this constrains the water stored in nominally anhydrous minerals in the mantle to <30% the mass of the surface oceans. Plain Language Summary The dynamic mantle circulates material between the upper and lower mantle. At 70-130 km below the upper-to-lower mantle boundary, seismic waves indicate an abrupt change in the material properties of the mantle at that depth. We offer a new interpretation of these seismic signals through the calculation of the effects of water on the materials moving from the upper into the lower mantle, which suggests that the decrease in water storage upon pressure-induced breakdown of garnet can explain the observations through the generation of deep melt. Together with the synthesis of water-bearing garnet at the relevant pressures, we demonstrate that the synthesis of the observations suggest that the mantle may be significantly limited in water storage, with as little as 30% of the water that is found at the surface of the Earth.
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Dehydration Melting Below the Undersaturated Transition Zone
Geochemistry Geophysics Geosystems, 2020Co-Authors: W Panero, C Thomas, R. Myhill, J Pigott, C. Raepsaet, H. BureauAbstract:A reflector 70-130 km below the base of the transition zone beneath Tibet is observed in receiver functions and underside seismic reflections, at depths consistent with the transition of garnet to bridgmanite. Contrast in water storage capacity between the minerals of the Earth's transition zone and lower mantle suggests the possibility for Dehydration Melting at the top of the lower mantle. First-principles calculations combined with laboratory synthesis experiments constrain the mantle water capacity across the base of the transition zone and into the top of the lower mantle. We interpret the observed seismic signal as consistent with 3-4 vol % hydrous melt resulting from Dehydration Melting in the garnet to bridgmanite transition. Should seismic signals evident in downwelling region result from water contents representative of upper mantle water globally, this constrains the water stored in nominally anhydrous minerals in the mantle to
O’brien - One of the best experts on this subject based on the ideXlab platform.
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Dehydration Melting and devolatilization during exhumation of high-grade metapelites; the Tatra Mountains, Western Carpathians
Journal of Metamorphic Geology, 1999Co-Authors: Janák, Hurai, Ludhová, O’brienAbstract:Partial Melting and retrogression related to Variscan tectonic exhumation have been recognized in the high-grade metapelites of the Tatra Mountains, Western Carpathians. Staurolite and kyanite relics document an early stage of the prograde metamorphism at c. 600 °C and 9–10 kbar. An increase in temperature to >730 °C at 11–12 kbar resulted in partial Melting and incipient migmatization in the stability field of kyanite. Further heating at decreasing pressure during the earliest stage of exhumation led to the Dehydration-Melting of muscovite and biotite at >750–800 °C and 6–10 kbar, producing garnet-bearing granite as leucosomes in migmatite. Subsequent cooling is documented by garnet resorption by biotite and sillimanite (a reversal of the prograde biotite Dehydration-Melting reaction). This was followed by nearly isothermal decompression to c. 4–5 kbar producing cordierite and some melt due to biotite decomposition. Later nearly isobaric cooling led to cordierite pinitization and formation of orthoamphibole, chlorite and carbonates. Densities of primary, monophase CO2–N2 inclusions (0.69–1.06 g cm−3) from the migmatite leucosome are consistent with the near-peak and retrograde conditions. Highly varying N2 contents (5–30 mol%) are thought to result from the nitrogen uptake in retrograde K-bearing minerals, or dilution by CO2 liberated during interaction of melt-derived water with metapelite graphite. The relatively high nitrogen content, not observed until now in migmatites, could have been inherited from the high-pressure metamorphism stage. It is assumed that the water-absent composition of fluid inclusions is not representative of the bulk water content (XH2O≤0.7), which was masked by mechanical separation of the CO2- and H2O-dominated immiscible phases, and/or by post-entrapment modifications of the fluid inclusions. Decompression and the final stage of exhumation were accomplished by top-to-the-south thrusting as well as west–east (orogen-parallel) extension. They were most probably related to regional uplift and gravitational collapse of thermally weakened Variscan crust.