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Othmar Muntener - One of the best experts on this subject based on the ideXlab platform.
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Fractional Crystallization of primitive, hydrous arc magmas: an experimental study at 0.7 GPa
Contributions to Mineralogy and Petrology, 2014Co-Authors: Rohit H. Nandedkar, Peter Ulmer, Othmar MuntenerAbstract:Differentiation of mantle-derived, hydrous, basaltic magmas is a fundamental process to produce evolved intermediate to SiO_2-rich magmas that form the bulk of the middle to shallow continental and island arc crust. This study reports the results of Fractional Crystallization experiments conducted in a piston cylinder apparatus at 0.7 GPa for hydrous, calc-alkaline to arc tholeiitic magmas. Fractional Crystallization was approached by synthesis of starting materials representing the liquid composition of the previous, higher temperature experiment. Temperatures ranged from near-liquidus at 1,170 °C to near-solidus conditions at 700 °C. H_2O contents varied from 3.0 to more than 10 wt%. The liquid line of descent covers the entire compositional range from olivine–tholeiite (1,170 °C) to high-silica rhyolite (700 °C) and evolves from metaluminous to peraluminous compositions. The following Crystallization sequence has been established: olivine → clinopyroxene → plagioclase, spinel → orthopyroxene, amphibole, titanomagnetite → apatite → quartz, biotite. Anorthite-rich plagioclase and spinel are responsible for a marked increase in SiO_2-content (from 51 to 53 wt%) at 1,040 °C. At lower temperatures, fractionation of amphibole, plagioclase and Fe–Ti oxide over a temperature interval of 280 °C drives the SiO_2 content continuously from 53 to 78 wt%. Largest Crystallization steps were recorded around 1,040 °C and at 700 °C. About 40 % of ultramafic plutonic rocks have to crystallize to generate basaltic–andesitic liquids, and an additional 40 % of amphibole–gabbroic cumulate to produce granitic melts. Andesitic liquids with a liquidus temperature of 1,010 °C only crystallize 50 % over an 280 °C wide range to 730 °C implying that such liquids form mobile crystal mushes (
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Fractional Crystallization of primitive hydrous arc magmas an experimental study at 0 7 gpa
Contributions to Mineralogy and Petrology, 2014Co-Authors: Rohit H. Nandedkar, Peter Ulmer, Othmar MuntenerAbstract:Differentiation of mantle-derived, hydrous, basaltic magmas is a fundamental process to produce evolved intermediate to SiO2-rich magmas that form the bulk of the middle to shallow continental and island arc crust. This study reports the results of Fractional Crystallization experiments conducted in a piston cylinder apparatus at 0.7 GPa for hydrous, calc-alkaline to arc tholeiitic magmas. Fractional Crystallization was approached by synthesis of starting materials representing the liquid composition of the previous, higher temperature experiment. Temperatures ranged from near-liquidus at 1,170 °C to near-solidus conditions at 700 °C. H2O contents varied from 3.0 to more than 10 wt%. The liquid line of descent covers the entire compositional range from olivine–tholeiite (1,170 °C) to high-silica rhyolite (700 °C) and evolves from metaluminous to peraluminous compositions. The following Crystallization sequence has been established: olivine → clinopyroxene → plagioclase, spinel → orthopyroxene, amphibole, titanomagnetite → apatite → quartz, biotite. Anorthite-rich plagioclase and spinel are responsible for a marked increase in SiO2-content (from 51 to 53 wt%) at 1,040 °C. At lower temperatures, fractionation of amphibole, plagioclase and Fe–Ti oxide over a temperature interval of 280 °C drives the SiO2 content continuously from 53 to 78 wt%. Largest Crystallization steps were recorded around 1,040 °C and at 700 °C. About 40 % of ultramafic plutonic rocks have to crystallize to generate basaltic–andesitic liquids, and an additional 40 % of amphibole–gabbroic cumulate to produce granitic melts. Andesitic liquids with a liquidus temperature of 1,010 °C only crystallize 50 % over an 280 °C wide range to 730 °C implying that such liquids form mobile crystal mushes (<50 % crystals) in long-lived magmatic systems in the middle crust, allowing for extensive fractionation, assimilation and hybridization with periodic replenishment of more mafic magmas from deeper magma reservoirs.
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Crystallization pressures of mid ocean ridge basalts derived from major element variations of glasses from equilibrium and Fractional Crystallization experiments
Journal of Geophysical Research, 2007Co-Authors: Samuel Villiger, Othmar Muntener, Peter UlmerAbstract:[1] A new method for calculating fractionation pressures of mid-ocean ridge basalts (MORB) that are saturated in clinopyroxene and plagioclase is presented. This mineral assemblage is the major control on the CaO versus Mg # (molar Mg/(Mg + Fetot), all Fe as Fe2+) chemical variations of basaltic liquids. By combining new equilibrium and Fractional Crystallization experiments on primitive mid-ocean ridge basalts at high pressure with previously published experiments on natural basaltic systems, we derive an expression for fractionation pressures that depends only on the CaO content and the Mg # of the liquid. P (kbar) = [CaO (wt %) − 3.98(±0.17) − 14.96(±0.34) × Mg # (molar)]/[−0.260(±0.008)], r2 = 0.92. We compare our formulation of a liquid barometer with those of Grove et al. (1992), Yang et al. (1996), and Herzberg (2004). The equation can be used to predict Crystallization pressures of dry tholeiitic liquids with Mg # < 0.6. Low H2O contents in the liquid (<1 wt %) do not significantly affect the calculated pressures compared to anhydrous liquids. Pressure calculations have been performed on MORB glasses for three mid-ocean ridge systems with different spreading rates (East Pacific Rise, Mid-Atlantic Ridge, Southwest Indian Ridge). Average pressure estimates correlate negatively with spreading rate when the MORB data are filtered for hot spot-affected compositions. Major element variations indicate that MORB crystallizing at elevated pressures also require lower degrees of mantle partial melting. Locations and compositions of MORB glasses crystallized at elevated pressures can be explained with models where conductive cooling is enhanced, either at segmented ridges with slow spreading rates or along ridge segment terminations at fast spreading ridges.
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magnesian andesite and dacite lavas from mt shasta northern california products of Fractional Crystallization of h2o rich mantle melts
Contributions to Mineralogy and Petrology, 2005Co-Authors: T L Grove, Stephen W. Parman, Nilanjan Chatterjee, Linda T Elkinstanton, Michael B Baker, Richard C Price, Othmar MuntenerAbstract:Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through Fractional Crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of Fractional Crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive Crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption.
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Magnesian andesite and dacite lavas from Mt. Shasta, northern California: products of Fractional Crystallization of H_2O-rich mantle melts
Contributions to Mineralogy and Petrology, 2005Co-Authors: Timothy L. Grove, Stephen W. Parman, Nilanjan Chatterjee, Linda T. Elkins-tanton, Michael B Baker, Richard C Price, Othmar MuntenerAbstract:Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO_2. The range of major and trace element compositions of the Shasta lavas can be explained through Fractional Crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H_2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of Fractional Crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive Crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H_2O) to lower crustal depths with magmatic H_2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H_2O-rich magmas containing 10–15 wt.% H_2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H_2O-rich melts could also play an important role in mixing and eruption.
Junfeng Zhang - One of the best experts on this subject based on the ideXlab platform.
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effects of melt Fractional Crystallization on sr nd and lu hf isotope systems a case study of triassic migmatite in the sulu uhp terrane
International Geology Review, 2014Co-Authors: Yanru Song, Junfeng Zhang, Deyuan Wang, Endong LiuAbstract:The Weihai migmatite in the Sulu ultra-high-pressure (UHP) metamorphic terrane, eastern China, underwent partial melting in the Late Triassic during its exhumation. The primary partial melts experienced a decompressional Fractional Crystallization (DFC) process to produce plagioclase (Pl)-rich leucosome crystallized under eclogite to granulite facies conditions and K-feldspar (Kfs)-rich pegmatitic veins crystallized under amphibolite-facies conditions. In this study, our results demonstrate that the DFC process can cause decoupling between whole-rock Sr and Nd isotopes. The Pl-rich leucosome has eNd(t) values (–10.4 to −15.0) and initial (87Sr/86Sr) ratios (0.708173–0.712476) very similar to those of the melanosome, but the Kfs-rich pegmatitic veins have homogeneous eNd(t) values (−14.8 to −15.2) and significantly high initial (87Sr/86Sr) ratios (0.713882–0.716284). Our results also suggest that the DFC process can change zircon 176Yb/177Hf and 176Lu/177Hf isotopic ratios, with no effect on 176Hf/177Hf ra...
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Prograde metamorphism, decompressional partial melting and subsequent melt Fractional Crystallization in the Weihai migmatitic gneisses, Sulu UHP terrane, eastern China
Chemical Geology, 2013Co-Authors: Yanru Song, Junfeng Zhang, Yi Chen, Qiang Liu, Shun GuoAbstract:Abstract In order to decipher fluid/melt activity during subduction and subsequent exhumation of felsic continental crust, we carried out a combined study of zircon internal structure, zircon U–Pb age, zircon trace element composition and whole-rock geochemistry on migmatitic gneisses from the Weihai region in the Sulu ultra-high pressure (UHP) metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase (Pl)-rich leucosome and K-feldspar (Kfs)-rich pegmatite veins. Whole-rock geochemistry suggests that MgO, Fe2O3T, CaO, TiO2, P2O5, rare earth elements (REE), and large ion lithophile elements (HFSE) are nearly completely retained in the melanosome, whereas significant amounts of LILE (e.g., Rb, Ba, K, Sr, Pb) and U are partitioned into the Pl-rich leucosome and the Kfs-rich pegmatite veins. The Kfs-rich pegmatite vein has much higher K2O, U, Pb and Rb, but lower CaO and Na2O contents than those of the Pl-rich leucosome layer. Protolith magmatic zircon domains, with an upper intercept age of about 780 Ma, are common in the cores of zircon grains from the melanosome and the Pl-rich leucosome layers, indicating that the protolith of the migmatitic gneiss is Mid-Neoproterozoic magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as overgrowth mantles on the protolith magmatic zircon cores. They are characterized by remarkably flat heavy rare earth element (HREE) patterns without obviously negative Eu anomalies, and have low Y and Th contents, low Th/U and high Hf/Y ratios, indicating a prograde metamorphism in the HP eclogite-facies during subduction. Recrystallized rims of zircons (228 ± 2 Ma) in the Pl-rich leucosome layer are similar to magmatic zircons in terms of their HREE enriched patterns with negative Eu anomalies, high Y and low Th contents, and low Hf/Y and Th/U ratios. This indicates that the gneiss has experienced decompressional partial melting and the Pl-rich leucosome layers were formed by Fractional Crystallization of the primary melt during the early stage hot exhumation. Zircon grains (219 ± 2 Ma) in the Kfs-rich veins are characterized by aqueous fluid/melt-related signatures with strongly HREE enriched patterns, high U, Pb and low Th contents, and low Th/U and high Lu/Hf ratios, indicating that the pegmatite vein was formed by Crystallization of residual aqueous melt after the Crystallization of Pl-rich leucosome layers during later stage cooling during exhumation. Our results suggest that the migmatitic gneiss successively recorded information concerning the protolith, prograde HP metamorphism during subduction, Fractional Crystallization of primary melt and Crystallization of the residual melt during exhumation.
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Temperature of prograde metamorphism, decompressional partial melting and subsequent melt Fractional Crystallization in the Weihai migmatitic gneisses, Sulu UHP terrane: Constraints from Ti-in-zircon thermometer
Journal of Earth Science, 2012Co-Authors: Junfeng ZhangAbstract:In order to constrain temperature during subduction and subsequent exhumation of felsic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai (威海) region in the Sulu (苏鲁) ultra-high pressure (UHP) metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase (Pl)-rich leucosome and K-feldspar (Kfs)-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompressional partial melting during early stage exhumation and subsequent Fractional Crystallization of primary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the protolith of the migmatitic gneiss is Mid-Neoproterozoic (∼780 Ma) magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as overgrowth mantles on the protolith magmatic zircon cores. The estimated growth temperatures (625–717 °C) of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eclogite-facies condition during subduction. Zircon recrystallized rims (228±2 Ma) in the Pl-rich leucosome layers provide the lower limit of the decompresssional partial melting time during exhumation. The ages from 228±2 to 219±2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the Fractional Crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 °C, and the temperatures of new growth zircon grains (219±2 Ma) in Kfs-rich vein are between 769 and 529 °C. The estimated temper atures have a positive correlation with ages from the Pl-rich leucosome to the Kfs-rich pegmatite vein, strongly indicating that a process of Fractional Crystallization of the partial melt during exhumation.
Shun Guo - One of the best experts on this subject based on the ideXlab platform.
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Prograde metamorphism, decompressional partial melting and subsequent melt Fractional Crystallization in the Weihai migmatitic gneisses, Sulu UHP terrane, eastern China
Chemical Geology, 2013Co-Authors: Yanru Song, Junfeng Zhang, Yi Chen, Qiang Liu, Shun GuoAbstract:Abstract In order to decipher fluid/melt activity during subduction and subsequent exhumation of felsic continental crust, we carried out a combined study of zircon internal structure, zircon U–Pb age, zircon trace element composition and whole-rock geochemistry on migmatitic gneisses from the Weihai region in the Sulu ultra-high pressure (UHP) metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase (Pl)-rich leucosome and K-feldspar (Kfs)-rich pegmatite veins. Whole-rock geochemistry suggests that MgO, Fe2O3T, CaO, TiO2, P2O5, rare earth elements (REE), and large ion lithophile elements (HFSE) are nearly completely retained in the melanosome, whereas significant amounts of LILE (e.g., Rb, Ba, K, Sr, Pb) and U are partitioned into the Pl-rich leucosome and the Kfs-rich pegmatite veins. The Kfs-rich pegmatite vein has much higher K2O, U, Pb and Rb, but lower CaO and Na2O contents than those of the Pl-rich leucosome layer. Protolith magmatic zircon domains, with an upper intercept age of about 780 Ma, are common in the cores of zircon grains from the melanosome and the Pl-rich leucosome layers, indicating that the protolith of the migmatitic gneiss is Mid-Neoproterozoic magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as overgrowth mantles on the protolith magmatic zircon cores. They are characterized by remarkably flat heavy rare earth element (HREE) patterns without obviously negative Eu anomalies, and have low Y and Th contents, low Th/U and high Hf/Y ratios, indicating a prograde metamorphism in the HP eclogite-facies during subduction. Recrystallized rims of zircons (228 ± 2 Ma) in the Pl-rich leucosome layer are similar to magmatic zircons in terms of their HREE enriched patterns with negative Eu anomalies, high Y and low Th contents, and low Hf/Y and Th/U ratios. This indicates that the gneiss has experienced decompressional partial melting and the Pl-rich leucosome layers were formed by Fractional Crystallization of the primary melt during the early stage hot exhumation. Zircon grains (219 ± 2 Ma) in the Kfs-rich veins are characterized by aqueous fluid/melt-related signatures with strongly HREE enriched patterns, high U, Pb and low Th contents, and low Th/U and high Lu/Hf ratios, indicating that the pegmatite vein was formed by Crystallization of residual aqueous melt after the Crystallization of Pl-rich leucosome layers during later stage cooling during exhumation. Our results suggest that the migmatitic gneiss successively recorded information concerning the protolith, prograde HP metamorphism during subduction, Fractional Crystallization of primary melt and Crystallization of the residual melt during exhumation.
Zhaobin Qiu - One of the best experts on this subject based on the ideXlab platform.
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unusual Fractional Crystallization behavior of novel crystalline crystalline polymer blends of poly ethylene suberate and poly ethylene oxide with similar melting points
Macromolecules, 2014Co-Authors: Mengting Weng, Zhaobin QiuAbstract:Novel crystalline/crystalline polymer blends of biodegradable poly(ethylene suberate) (PESub) and biocompatible poly(ethylene oxide) (PEO) were prepared through a solution and casting method. The basic thermal properties, including both glass transition temperature and melting point, of both of the components are very close to each other. The two components are miscible as no obvious phase separation can be detected, forming novel miscible polymer blends of two crystalline polymers. PESub/PEO blends show two or three Crystallization exotherms at different supercoolings. The complex Crystallization behaviors are attributed to the occurrence of the Fractional Crystallization of the minor component of the blends. Depending on the blend composition, the major component of the blend crystallizes first and the minor component crystallizes later. During the Crystallization of the major component, the amorphous minor component is completely included the interlamellar region of the major component or most of the a...
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Unusual Fractional Crystallization Behavior of Novel Crystalline/Crystalline Polymer Blends of Poly(ethylene suberate) and Poly(ethylene oxide) with Similar Melting Points
2014Co-Authors: Mengting Weng, Zhaobin QiuAbstract:Novel crystalline/crystalline polymer blends of biodegradable poly(ethylene suberate) (PESub) and biocompatible poly(ethylene oxide) (PEO) were prepared through a solution and casting method. The basic thermal properties, including both glass transition temperature and melting point, of both of the components are very close to each other. The two components are miscible as no obvious phase separation can be detected, forming novel miscible polymer blends of two crystalline polymers. PESub/PEO blends show two or three Crystallization exotherms at different supercoolings. The complex Crystallization behaviors are attributed to the occurrence of the Fractional Crystallization of the minor component of the blends. Depending on the blend composition, the major component of the blend crystallizes first and the minor component crystallizes later. During the Crystallization of the major component, the amorphous minor component is completely included the interlamellar region of the major component or most of the amorphous minor component is expelled out of the interlamellar region while only a few is incorporated between the lamellae of the major component. In both cases, the Fractional Crystallization of the minor component occurs at large supercooling because of the confinement effect of the lamellae of the major component. In the present work, the Fractional Crystallization of PESub or PEO may occur at large supercooling when its content is low. PESub/PEO blends may be the first model that the Fractional Crystallization of each component occurs at large supercooling when its content is low; thus, they provide a rare system to study the unique crystalline morphology and Crystallization behavior of miscible crystalline/crystalline polymer blends. Such research is very interesting and challenging for a better understanding of the crystalline morphology and Crystallization behavior of crystalline polymer blends from both academic and practical viewpoints
Nilanjan Chatterjee - One of the best experts on this subject based on the ideXlab platform.
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trace element variations in deccan basalts roles of mantle melting Fractional Crystallization and crustal assimilation
Journal of Geological Society of India, 2008Co-Authors: Nilanjan Chatterjee, Somdev BhattacharjiAbstract:The variation in incompatible trace element contents of lower basaltic dykes and flows from different parts of the Deccan province was assessed through modelling of partial melting of primitive mantle and Fractional Crystallization of the parental melts coupled with crustal assimilation (AFC). Variations in Ba/Zr, Rb/Y and Nb/Y can be attributed to different degrees of partial melting of mantle. Variations in Zr, rare earth elements and Rb/Nb indicate Fractional Crystallization and significant coupled crustal assimilation away from the Deccan plume center. The Zr variation near the plume center can be explained through only Fractional Crystallization without the involvement of crust. AFC modelling of available Sr-Nd isotope data requires very high and unreasonable amounts of crustal assimilant implying that the parental magmas may have acquired their isotopic characteristics before AFC occurred in crustal magma chambers. The occurrence of AFC away from the plume center in the Narmada-Tapti rift region may be related to longer and greater magma-Wall rock interaction in shallow crustal magma chambers due to crustal extension-Related enlargement of the magma chambers, recharge with fresh, hot magma and convective mixing.
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magnesian andesite and dacite lavas from mt shasta northern california products of Fractional Crystallization of h2o rich mantle melts
Contributions to Mineralogy and Petrology, 2005Co-Authors: T L Grove, Stephen W. Parman, Nilanjan Chatterjee, Linda T Elkinstanton, Michael B Baker, Richard C Price, Othmar MuntenerAbstract:Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through Fractional Crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of Fractional Crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive Crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption.
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Magnesian andesite and dacite lavas from Mt. Shasta, northern California: products of Fractional Crystallization of H_2O-rich mantle melts
Contributions to Mineralogy and Petrology, 2005Co-Authors: Timothy L. Grove, Stephen W. Parman, Nilanjan Chatterjee, Linda T. Elkins-tanton, Michael B Baker, Richard C Price, Othmar MuntenerAbstract:Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO_2. The range of major and trace element compositions of the Shasta lavas can be explained through Fractional Crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H_2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of Fractional Crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive Crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H_2O) to lower crustal depths with magmatic H_2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H_2O-rich magmas containing 10–15 wt.% H_2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H_2O-rich melts could also play an important role in mixing and eruption.
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Fractional Crystallization and mantle-melting controls on calc-alkaline differentiation trends
Contributions to Mineralogy and Petrology, 2003Co-Authors: Timothy L. Grove, Stephen W. Parman, Othmar Muntener, Nilanjan Chatterjee, Linda T. Elkins-tanton, Glenn A GaetaniAbstract:The phase relations of primitive magnesian andesites and basaltic andesites from the Mt. Shasta region, N California have been determined over a range of pressure and temperature conditions and H_2O contents. The experimental results are used to explore the influence of H_2O and pressure on Fractional Crystallization and mantle melting behavior in subduction zone environments. At 200-MPa H_2O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt. Shasta region lavas. This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO_2 contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO_2. We propose a two-stage process for the origin of these lavas. (1) Extensive hydrous mantle melting produces H_2O-rich (>4.5--6 wt% H_2O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue. Trace elements and H_2O are contributed from a slab-derived fluid and/or melt. (2) This mantle melt ascends into the overlying crust and undergoes Fractional Crystallization. Crustal-level differentiation occurs under near-H_2O saturated conditions producing the distinctive high SiO_2 and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas. In a subset of Mt. Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H_2O contents (>10 wt% H_2O) and lower crustal Crystallization pressures (800 MPa). Igneous rocks that possess major and trace element characteristics similar to those of the Mt. Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite--tonalite--granodiorite suites (TTG suites). We propose that these magmas also form by hydrous mantle melting.
