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Xun Wei - One of the best experts on this subject based on the ideXlab platform.
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origin of high an plagioclase in the early permian 280 ma xiaohaizi wehrlite northwest china insights from melt inclusions in Clinopyroxene macrocrysts and zircon oxygen isotopes
International Geology Review, 2016Co-Authors: Xun Wei, Zhong-yuan Ren, Zhen-yu LuoAbstract:The Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in Clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5-48.7wt.%) and Na2O+K2O (2.4-3.8wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the Clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host Clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9-4.7) of melt inclusions are the upper limit of the parental magma of the Clinopyroxenes due to high CaO (21.5-23.0wt.%) and very low Na2O (0.22-0.34wt.%) contents in the host Clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7kbar) and T (1000 degrees C), and assumed H2O (~1.5wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the wehrlite. The results confirm that H2O exerts a major control. Zircon delta O-18 (VSMOW) values (2.99-3.71 parts per thousand) are significantly lower than that of mantle-derived zircon (5.3 +/- 0.6 parts per thousand). Such low zircon delta O-18 values may be due to incorporation of large amounts of low-delta O-18, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.
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Origin of high-An plagioclase in the early Permian (~280 Ma) Xiaohaizi wehrlite, Northwest China: insights from melt inclusions in Clinopyroxene macrocrysts and zircon oxygen isotopes
2016Co-Authors: Xun Wei, Zhong-yuan Ren, Zhen-yu LuoAbstract:The Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in Clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5–48.7 wt.%) and Na2O + K2O (2.4–3.8 wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the Clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host Clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9–4.7) of melt inclusions are the upper limit of the parental magma of the Clinopyroxenes due to high CaO (21.5–23.0 wt.%) and very low Na2O (0.22–0.34 wt.%) contents in the host Clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000°C), and assumed H2O (~1.5 wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the wehrlite. The results confirm that H2O exerts a major control. Zircon δ18O (VSMOW) values (2.99–3.71‰) are significantly lower than that of mantle-derived zircon (5.3 ± 0.6‰). Such low zircon δ18O values may be due to incorporation of large amounts of low-δ18O, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.
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Composition of the Tarim mantle plume: Constraints from Clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China
Lithos, 2015Co-Authors: Xun Wei, Jianxin Zhao, Zhen-yu Luo, Yuexing FengAbstract:Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant Clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg# = 80-89) and the host dyke (Mg# = 39, corresponding to Mg# = 73 of Clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The Clinopyroxene macrocrysts are characterized by low TiO2 (026-1.09 wt.%), Al2O3 (1.15-3.10 wt.%) and Na2O (0.16-0.37 wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The Clinopyroxene macrocrysts and the Clinopyroxenes from the Xiaohaizi cumulate wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenoaysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The Sr-87/Sr-86(i) (0.7035-0.7037) and epsilon Nd-i (4.5-4.8) of the Clinopyroxene macrocrysts with high Mg# (80-89) are apparently lower and higher than their respective ratios of the Clinopyroxenes in the wehrlites (Mg# = 75-84, Sr-87/Sr-86(i) = 0.7038-0.7041, epsilon Nd-i = 1.0-1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the Clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts. (C) 2015 Elsevier B.V. All rights reserved.
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Petrology and Sr-Nd Isotopic Disequilibrium of the Xiaohaizi Intrusion, NW China: Genesis of Layered Intrusions in the Tarim Large Igneous Province
Journal of Petrology, 2014Co-Authors: Xun Wei, Chuanlin Zhang, Jianxin Zhao, Yuexing FengAbstract:Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi wehrlite intrusion is composed mainly of olivine (Fo69-75), Clinopyroxene (Mg# = 75-84), intercumulus plagioclase (An53-86) and Fe-Ti oxides. Both petrography and mineral compositions suggest that olivine and Clinopyroxene crystallized earlier than plagioclase and Fe-Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35-39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive epsilon Nd-i values (4.3-4.8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these Clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi wehrlite intrusion and dykes. The Xiaohaizi wehrlite intrusion is characterized by Sr-Nd isotopic disequilibrium between Clinopyroxene and plagioclase separates: Sr-87/(86)Sri (0.7038-0.7041) and epsilon Nd-i (1.0-1.9) of Clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (Sr-87/Sr-86(i) = 0.7042-0.7043, epsilon Nd-i = 0.4-1.0). The Sr-87/(86)Sri and epsilon Nd-i of Clinopyroxene separates correlate positively and negatively with Zr/ Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi wehrlite intrusion. Sr-87/(86)Sri increases and epsilon Nd-i decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and epsilon Nd-i of Clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean-Neoproterozoic basement and carbonates as contaminants.
Yuexing Feng - One of the best experts on this subject based on the ideXlab platform.
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Composition of the Tarim mantle plume: Constraints from Clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China
Lithos, 2015Co-Authors: Xun Wei, Jianxin Zhao, Zhen-yu Luo, Yuexing FengAbstract:Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant Clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg# = 80-89) and the host dyke (Mg# = 39, corresponding to Mg# = 73 of Clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The Clinopyroxene macrocrysts are characterized by low TiO2 (026-1.09 wt.%), Al2O3 (1.15-3.10 wt.%) and Na2O (0.16-0.37 wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The Clinopyroxene macrocrysts and the Clinopyroxenes from the Xiaohaizi cumulate wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenoaysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The Sr-87/Sr-86(i) (0.7035-0.7037) and epsilon Nd-i (4.5-4.8) of the Clinopyroxene macrocrysts with high Mg# (80-89) are apparently lower and higher than their respective ratios of the Clinopyroxenes in the wehrlites (Mg# = 75-84, Sr-87/Sr-86(i) = 0.7038-0.7041, epsilon Nd-i = 1.0-1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the Clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts. (C) 2015 Elsevier B.V. All rights reserved.
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Petrology and Sr-Nd Isotopic Disequilibrium of the Xiaohaizi Intrusion, NW China: Genesis of Layered Intrusions in the Tarim Large Igneous Province
Journal of Petrology, 2014Co-Authors: Xun Wei, Chuanlin Zhang, Jianxin Zhao, Yuexing FengAbstract:Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi wehrlite intrusion is composed mainly of olivine (Fo69-75), Clinopyroxene (Mg# = 75-84), intercumulus plagioclase (An53-86) and Fe-Ti oxides. Both petrography and mineral compositions suggest that olivine and Clinopyroxene crystallized earlier than plagioclase and Fe-Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35-39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive epsilon Nd-i values (4.3-4.8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these Clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi wehrlite intrusion and dykes. The Xiaohaizi wehrlite intrusion is characterized by Sr-Nd isotopic disequilibrium between Clinopyroxene and plagioclase separates: Sr-87/(86)Sri (0.7038-0.7041) and epsilon Nd-i (1.0-1.9) of Clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (Sr-87/Sr-86(i) = 0.7042-0.7043, epsilon Nd-i = 0.4-1.0). The Sr-87/(86)Sri and epsilon Nd-i of Clinopyroxene separates correlate positively and negatively with Zr/ Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi wehrlite intrusion. Sr-87/(86)Sri increases and epsilon Nd-i decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and epsilon Nd-i of Clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean-Neoproterozoic basement and carbonates as contaminants.
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petrology and sr nd isotopic disequilibrium of the xiaohaizi intrusion nw china genesis of layered intrusions in the tarim large igneous province
Journal of Petrology, 2014Co-Authors: Yigang Xu, Chuanlin Zhang, Jianxin Zhao, Yuexing FengAbstract:Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi wehrlite intrusion is composed mainly of olivine (Fo69-75), Clinopyroxene (Mg# = 75-84), intercumulus plagioclase (An53-86) and Fe-Ti oxides. Both petrography and mineral compositions suggest that olivine and Clinopyroxene crystallized earlier than plagioclase and Fe-Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35-39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive epsilon Nd-i values (4.3-4.8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these Clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi wehrlite intrusion and dykes. The Xiaohaizi wehrlite intrusion is characterized by Sr-Nd isotopic disequilibrium between Clinopyroxene and plagioclase separates: Sr-87/(86)Sri (0.7038-0.7041) and epsilon Nd-i (1.0-1.9) of Clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (Sr-87/Sr-86(i) = 0.7042-0.7043, epsilon Nd-i = 0.4-1.0). The Sr-87/(86)Sri and epsilon Nd-i of Clinopyroxene separates correlate positively and negatively with Zr/ Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi wehrlite intrusion. Sr-87/(86)Sri increases and epsilon Nd-i decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and epsilon Nd-i of Clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean-Neoproterozoic basement and carbonates as contaminants.
Zhen-yu Luo - One of the best experts on this subject based on the ideXlab platform.
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origin of high an plagioclase in the early permian 280 ma xiaohaizi wehrlite northwest china insights from melt inclusions in Clinopyroxene macrocrysts and zircon oxygen isotopes
International Geology Review, 2016Co-Authors: Xun Wei, Zhong-yuan Ren, Zhen-yu LuoAbstract:The Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in Clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5-48.7wt.%) and Na2O+K2O (2.4-3.8wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the Clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host Clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9-4.7) of melt inclusions are the upper limit of the parental magma of the Clinopyroxenes due to high CaO (21.5-23.0wt.%) and very low Na2O (0.22-0.34wt.%) contents in the host Clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7kbar) and T (1000 degrees C), and assumed H2O (~1.5wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the wehrlite. The results confirm that H2O exerts a major control. Zircon delta O-18 (VSMOW) values (2.99-3.71 parts per thousand) are significantly lower than that of mantle-derived zircon (5.3 +/- 0.6 parts per thousand). Such low zircon delta O-18 values may be due to incorporation of large amounts of low-delta O-18, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.
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Origin of high-An plagioclase in the early Permian (~280 Ma) Xiaohaizi wehrlite, Northwest China: insights from melt inclusions in Clinopyroxene macrocrysts and zircon oxygen isotopes
2016Co-Authors: Xun Wei, Zhong-yuan Ren, Zhen-yu LuoAbstract:The Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in Clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5–48.7 wt.%) and Na2O + K2O (2.4–3.8 wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the Clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host Clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9–4.7) of melt inclusions are the upper limit of the parental magma of the Clinopyroxenes due to high CaO (21.5–23.0 wt.%) and very low Na2O (0.22–0.34 wt.%) contents in the host Clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000°C), and assumed H2O (~1.5 wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the wehrlite. The results confirm that H2O exerts a major control. Zircon δ18O (VSMOW) values (2.99–3.71‰) are significantly lower than that of mantle-derived zircon (5.3 ± 0.6‰). Such low zircon δ18O values may be due to incorporation of large amounts of low-δ18O, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.
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Composition of the Tarim mantle plume: Constraints from Clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China
Lithos, 2015Co-Authors: Xun Wei, Jianxin Zhao, Zhen-yu Luo, Yuexing FengAbstract:Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant Clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg# = 80-89) and the host dyke (Mg# = 39, corresponding to Mg# = 73 of Clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The Clinopyroxene macrocrysts are characterized by low TiO2 (026-1.09 wt.%), Al2O3 (1.15-3.10 wt.%) and Na2O (0.16-0.37 wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The Clinopyroxene macrocrysts and the Clinopyroxenes from the Xiaohaizi cumulate wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenoaysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The Sr-87/Sr-86(i) (0.7035-0.7037) and epsilon Nd-i (4.5-4.8) of the Clinopyroxene macrocrysts with high Mg# (80-89) are apparently lower and higher than their respective ratios of the Clinopyroxenes in the wehrlites (Mg# = 75-84, Sr-87/Sr-86(i) = 0.7038-0.7041, epsilon Nd-i = 1.0-1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the Clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts. (C) 2015 Elsevier B.V. All rights reserved.
Jianxin Zhao - One of the best experts on this subject based on the ideXlab platform.
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Composition of the Tarim mantle plume: Constraints from Clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China
Lithos, 2015Co-Authors: Xun Wei, Jianxin Zhao, Zhen-yu Luo, Yuexing FengAbstract:Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant Clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg# = 80-89) and the host dyke (Mg# = 39, corresponding to Mg# = 73 of Clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The Clinopyroxene macrocrysts are characterized by low TiO2 (026-1.09 wt.%), Al2O3 (1.15-3.10 wt.%) and Na2O (0.16-0.37 wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The Clinopyroxene macrocrysts and the Clinopyroxenes from the Xiaohaizi cumulate wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenoaysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The Sr-87/Sr-86(i) (0.7035-0.7037) and epsilon Nd-i (4.5-4.8) of the Clinopyroxene macrocrysts with high Mg# (80-89) are apparently lower and higher than their respective ratios of the Clinopyroxenes in the wehrlites (Mg# = 75-84, Sr-87/Sr-86(i) = 0.7038-0.7041, epsilon Nd-i = 1.0-1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the Clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts. (C) 2015 Elsevier B.V. All rights reserved.
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Petrology and Sr-Nd Isotopic Disequilibrium of the Xiaohaizi Intrusion, NW China: Genesis of Layered Intrusions in the Tarim Large Igneous Province
Journal of Petrology, 2014Co-Authors: Xun Wei, Chuanlin Zhang, Jianxin Zhao, Yuexing FengAbstract:Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi wehrlite intrusion is composed mainly of olivine (Fo69-75), Clinopyroxene (Mg# = 75-84), intercumulus plagioclase (An53-86) and Fe-Ti oxides. Both petrography and mineral compositions suggest that olivine and Clinopyroxene crystallized earlier than plagioclase and Fe-Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35-39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive epsilon Nd-i values (4.3-4.8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these Clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi wehrlite intrusion and dykes. The Xiaohaizi wehrlite intrusion is characterized by Sr-Nd isotopic disequilibrium between Clinopyroxene and plagioclase separates: Sr-87/(86)Sri (0.7038-0.7041) and epsilon Nd-i (1.0-1.9) of Clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (Sr-87/Sr-86(i) = 0.7042-0.7043, epsilon Nd-i = 0.4-1.0). The Sr-87/(86)Sri and epsilon Nd-i of Clinopyroxene separates correlate positively and negatively with Zr/ Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi wehrlite intrusion. Sr-87/(86)Sri increases and epsilon Nd-i decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and epsilon Nd-i of Clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean-Neoproterozoic basement and carbonates as contaminants.
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petrology and sr nd isotopic disequilibrium of the xiaohaizi intrusion nw china genesis of layered intrusions in the tarim large igneous province
Journal of Petrology, 2014Co-Authors: Yigang Xu, Chuanlin Zhang, Jianxin Zhao, Yuexing FengAbstract:Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi wehrlite intrusion is composed mainly of olivine (Fo69-75), Clinopyroxene (Mg# = 75-84), intercumulus plagioclase (An53-86) and Fe-Ti oxides. Both petrography and mineral compositions suggest that olivine and Clinopyroxene crystallized earlier than plagioclase and Fe-Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35-39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive epsilon Nd-i values (4.3-4.8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these Clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi wehrlite intrusion and dykes. The Xiaohaizi wehrlite intrusion is characterized by Sr-Nd isotopic disequilibrium between Clinopyroxene and plagioclase separates: Sr-87/(86)Sri (0.7038-0.7041) and epsilon Nd-i (1.0-1.9) of Clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (Sr-87/Sr-86(i) = 0.7042-0.7043, epsilon Nd-i = 0.4-1.0). The Sr-87/(86)Sri and epsilon Nd-i of Clinopyroxene separates correlate positively and negatively with Zr/ Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi wehrlite intrusion. Sr-87/(86)Sri increases and epsilon Nd-i decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and epsilon Nd-i of Clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean-Neoproterozoic basement and carbonates as contaminants.
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a review of the lattice strain and electrostatic effects on trace element partitioning between Clinopyroxene and melt applications to magmatic systems saturated with tschermak rich Clinopyroxenes
Earth-Science Reviews, 2020Co-Authors: Silvio Mollo, Jonathan D Blundy, Piergiorgio Scarlato, Francesco Vetere, Francois Holtz, Olivier Bachmann, Mario GaetaAbstract:Abstract The purpose of this review study is to reappraise in a more comprehensive form the thermodynamic principles behind the partitioning of trace elements between Clinopyroxene and melt. The original corollary is that the partitioning energetics controlling the crystal-melt exchange are described by two distinct but complementary contributions: ΔGpartitioning = ΔGstrain + ΔGelectrostatic. ΔGstrain is the excess of strain energy quantifying the elastic response of the crystallographic site to insertion of trace cations with radius different from that of the major cation at the site. ΔGelectrostatic is the excess of electrostatic energy requiring that an electrostatic energy penalty is paid when a trace cation entering the lattice site without strain has charge different from that of the resident cation. Lattice strain and electrostatic parameters for different isovalent groups of cations hosting the same lattice site from literature have been discussed in comparison with new partitioning data measured between Tschermak-rich Clinopyroxenes and a primitive phonotephritic melt assimilating variable amounts of carbonate material. Through such a comparatively approach, we illustrate that the type and number of trace cation substitutions are controlled by both charge-balanced and -imbalanced configurations taking place in the structural sites of Tschermak-rich Clinopyroxenes. A virtue of this complementary relationship is that the control of melt composition on the partitioning of highly charged cations is almost entirely embodied in the crystal chemistry and structure, as long as these crystallochemical aspects are the direct expression of both ΔGstrain and ΔGelectrostatic. A size mismatch caused by cation substitution is accommodated by elastic strain in the surrounding lattice of Clinopyroxene, whereas the charge mismatch is enabled via increasing amounts of charge-balancing Tschermak components, as well as the electrostatic work done on transferring the trace cations from melt to crystallographic sites, and vice versa. The influence of the melt chemistry on highly-charged (3+ and 4+) cation partitioning is greatly subordinate to the lattice strain and electrostatic energies of substitutions, in agreement with the thermodynamic premise that both these energetic quantities represent simple-activity composition models for the crystal phase. The various charge-balanced and -imbalanced configurations change principally with aluminium in tetrahedral coordination and the Clinopyroxene volume change produced by heterovalent cation substitutions. In contrast, for low-charged (1+ and 2+) cations, the role of melt chemistry cannot be properly deconvoluted from the structural changes of the crystal lattice. The incorporation of these cations into the Clinopyroxene lattice depends on the number of structural sites critically important to accommodating network-modifying cations in the melt structure, implying that the partitioning energetics of monovalent and divalent cations are strictly controlled by both crystal and melt properties. We conclude that the competition between charge-balanced and charge-imbalanced substitutions may selectively change the ability of trace elements to be compatible or incompatible in the Clinopyroxene structure, with important ramifications for the modeling of natural igneous processes in crustal magma reservoirs which differentiate under closed- and open-system conditions.
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an integrated p t h2o lattice strain model to quantify the role of Clinopyroxene fractionation on ree y and hfse patterns of mafic alkaline magmas application to eruptions at mt etna
Earth-Science Reviews, 2018Co-Authors: Jonathan D Blundy, Piergiorgio Scarlato, S Mollo, Serena Pia De Cristofaro, Vanni Tecchiato, Flavio Di Stefano, Francesco Vetere, Francois Holtz, Olivier BachmannAbstract:Abstract A correct description and quantification of the geochemical behaviour of REE+Y (rare earth elements and Y) and HFSE (high field strength elements) is a key requirement for modeling petrological and volcanological aspects of magma dynamics. In this context, mafic alkaline magmas (MAM) are characterized by the ubiquitous stability of Clinopyroxene from mantle depths to shallow crustal levels. On one hand, Clinopyroxene incorporates REE+Y and HFSE at concentration levels that are much higher than those measured for olivine, plagioclase, and magnetite. On the other hand, the composition of Clinopyroxene is highly sensitive to variations in pressure, temperature, and melt-water content, according to exchange-equilibria between jadeite and melt, and between jadeite/Ca-Tschermak and diopside-hedenbergite. As a consequence, the dependence of the partition coefficient on the physicochemical state of the system results in a variety of DREE+Y and DHFSE values that are sensitive to the magmatic conditions at which Clinopyroxenes nucleate and grow. In order to better explore magma dynamics using Clinopyroxene chemical changes, an integrated P-T-H2O-lattice strain model specific to MAM compositions has been developed. The model combines a set of refined Clinopyroxene-based barometric, thermometric and hygrometric equations with thermodynamically-derived expressions for the lattice strain parameters, i.e., the strain-free partition coefficient (D0), the site radius (r0), and the effective elastic modulus (E). Through this approach, it is found that the incorporation of REE+Y and HFSE into M2 and M1 octahedral sites of Clinopyroxene is determined by a variety of physicochemical variables that may or may not change simultaneously during magma differentiation. The applicability of the P-T-H2O-lattice strain model to natural environments has been verified using Clinopyroxene-melt pairs from a great number of volcanic eruptions at Mt. Etna volcano (Sicily, Italy). DREE+Y and DHFSE values recovered by the model have been used as input data to quantify fractional crystallization processes in natural MAM compositions. Results from calculation illustrate that the concentration of REE+Y and HFSE in the magma is primary controlled by the geochemical evolution of Clinopyroxene in terms of major cation exchange-equilibria and trace cation lattice strain properties.
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the partitioning of trace elements between Clinopyroxene and trachybasaltic melt during rapid cooling and crystal growth
Contributions to Mineralogy and Petrology, 2013Co-Authors: Silvio Mollo, Piergiorgio Scarlato, Gianluca Iezzi, Jon D Blundy, Antonio LangoneAbstract:We present the variation in trace element partition coefficients measured at the interface between rapidly cooled Clinopyroxene crystals and co-existing melts. Results indicate that, as the cooling rate is increased, Clinopyroxene crystals are progressively depleted in Si, Ca and Mg counterbalanced by enrichments in Al (mainly tetrahedral Aliv), Na and Ti. Partition coefficients (Ds) for rare earth elements (REE), high field strength elements (HFSE) and transition elements (TE) increase with increasing cooling rate, in response to Clinopyroxene compositional variations. The entry of REE into the M2 site is facilitated by a coupled substitution where either Na substitutes for Ca on the M2 site or Aliv substitutes for Si in the tetrahedral site. The latter substitution reflects an increased ease of locally balancing the excess charge at M2 as the number of surrounding Aliv atoms increases. Due to the lower concentration of Ca in rapidly cooled Clinopyroxenes, divalent large ion lithophile elements (LILE) on M2 decrease at the expense of monovalent cations. Conversely, higher concentrations of HFSE and TE on the M1 site are facilitated as the average charge on this site increases with the replacement of divalent-charged cations by Alvi. Although crystallization kinetics modify Clinopyroxene composition, deviations from equilibrium partitioning are insufficient to change the tendency of a trace element to be compatible or incompatible. Consequently, there are regular relationships between ionic radius, valence of the trace element and D. At both equilibrium and cooling rate conditions, Ds for isovalent cations define parabola-like curves when plotted against ionic radius, consistent with the lattice strain model, demonstrating that the partitioning of trace elements is driven by charge balance mechanisms; cation substitution reactions can be treated in terms of the energetics of the various charge-imbalanced configurations.
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a new test for equilibrium based on Clinopyroxene melt pairs clues on the solidification temperatures of etnean alkaline melts at post eruptive conditions
Chemical Geology, 2013Co-Authors: Silvio Mollo, Keith Putirka, Valeria Misiti, Michele Soligo, Piergiorgio ScarlatoAbstract:Abstract We have performed new global regression analyses to calibrate a model of equilibrium between Clinopyroxene and co-existing melt. Then we have applied this model to a restricted but important range of Clinopyroxene and melt compositions from Mt. Etna volcano. The degree of disequilibrium is determined through the comparison between components “predicted” for Clinopyroxene via regression analyses of Clinopyroxene–liquid pairs in equilibrium conditions, with those “measured” in the analyzed crystals. The model is tested using compositions not included into the calibration dataset, i.e., Clinopyroxene–melt pairs obtained from equilibrium and cooling rate experiments conducted at ambient pressure on an Etnean trachybasalt. The experiments were duplicated at the NNO + 1.5 and QFM oxygen buffering conditions estimated for magmas at Mt. Etna. Both equilibrium and disequilibrium Clinopyroxene–melt pairs from the experiments were also used as input data for one of the most recent thermometers based on the Jd–DiHd exchange reaction. Results from calculations indicate that, under rapid cooling rate conditions, Clinopyroxenes do not equilibrate with the melt. Consequently, the thermometers predict higher crystallization temperatures compared to the final experimental temperature, prior to rapid quenching of the experiment. The systematic difference between expected and measured compositions and temperatures allows us to calibrate a model that describes undercooling based on disequilibrium exchange reactions. We use this new tool to estimate the thermal history of naturally cooled lava flows and dikes at Mt. Etna volcano.
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dependence of Clinopyroxene composition on cooling rate in basaltic magmas implications for thermobarometry
Lithos, 2010Co-Authors: Silvio Mollo, Piero Del Gaudio, Guido Ventura, Gianluca Iezzi, Piergiorgio ScarlatoAbstract:Abstract The compositional variation of Clinopyroxene and the partitioning of major elements between Clinopyroxene and melt are estimated as a function of the cooling rate. Clinopyroxenes were crystallized under variable cooling regimes (15, 9.4, 3, 2.1, and 0.5 °C/min from 1250 down to 1000 °C) and at isothermal conditions of 1000 °C from a basaltic composition at a pressure of 500 MPa under anhydrous and hydrous (H 2 O = 1.3 wt.%) conditions. The Clinopyroxene chemistry shows that, as the cooling rate increases, crystals are progressively depleted in Ca, Mg, Fe 2+ and Si and enriched in Na, Fe 3+ , Al (mainly Al IV ), and Ti. Di and Hd versus CaTs and CaFeTs form a continuous binary solid solution characterized by higher amounts of tschermakitic components with increasing cooling rate. Two parameters (DH = Di + Hd and TE = CaTs + CaFeTs + En) are calculated to describe the effect of cooling rate on the Clinopyroxene composition. The variation of DH/TE with increasing cooling rate evidences the kinetic process induced by rapid cooling in basic rocks under hydrous and anhydrous conditions. Dynamic crystallization conditions affect the partitioning of major elements between Clinopyroxene and melt; with increasing cooling rate, the value of crystal–melt partition coefficient departs from that obtained at the isothermal condition. However, in spite of these variations, the values of cpx–melt Kd Fe–Mg remain almost constant. Therefore, the Fe 2 –Mg exchange between Clinopyroxene and melt is not suitable to prove the (dis)equilibrium conditions in basaltic cooling magmas, giving rise to possible mismatches in the application of thermobarometers. The results of our study are consistent with that observed at the margin of dikes or in the exterior portions of lavas, where the cooling rate is maximized and disequilibrium compositions of Clinopyroxene have been found.