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Experimental melting of phlogopite-bearing mantle at 1 GPa: Implications for potassic magmatism.

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Title: Experimental melting of phlogopite-bearing mantle at 1 GPa: Implications for potassic magmatism.
Authors: Condamine, Pierre1,2,3 p.condamine@opgc.univ-bpclermont.fr, Médard, Etienne1,2,3
Source: Earth & Planetary Science Letters. Jul2014, Vol. 397, p80-92. 13p.
Subjects: Phlogopite, Magmatism, Fluid dynamics, Extraction (Chemistry), Orthopyroxene, Viscosity
Abstract: We have experimentally investigated the fluid-absent melting of a phlogopite peridotite at 1.0 GPa (1000–1300 °C) to understand the source of K2O- and SiO2-rich magmas that occur in continental, post-collisional and island arc settings. Using a new extraction technique specially developed for hydrous conditions combined with iterative sandwich experiments, we have determined the composition of low- to high-degree melts ( to 24.2 wt.%) of metasomatized lherzolite and harzburgite sources. Due to small amounts of adsorbed water in the starting material, amphibole crystallized at the lowest investigated temperatures. Amphibole breaks down at 1050–1075 °C, while phlogopite-breakdown occurs at 1150–1200 °C. This last temperature is higher than the previously determined in a mantle assemblage, due to the presence of stabilizing F and Ti. Phlogopite–lherzolite melts incongruently according to the continuous reaction: 0.49 phlogopite + 0.56 orthopyroxene + 0.47 clinopyroxene + 0.05 spinel = 0.58 olivine + 1.00 melt. In the phlogopite–harzburgite, the reaction is: 0.70 phlogopite + 1.24 orthopyroxene + 0.05 spinel = 0.99 olivine + 1.00 melt. The K2O content of water-undersaturated melts in equilibrium with residual phlogopite is buffered, depending on the source fertility: from in lherzolite to in harzburgite. Primary melts are silica-saturated and evolve from trachyte to basaltic andesite (63.5–52.1 wt.% SiO2) with increasing temperature. Calculations indicate that such silica-rich melts can readily be extracted from their mantle source, due to their low viscosity. Our results confirm that potassic, silica-rich magmas described worldwide in post-collisional settings are generated by melting of a metasomatized phlogopite-bearing mantle in the spinel stability field. [ABSTRACT FROM AUTHOR]
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Abstract:We have experimentally investigated the fluid-absent melting of a phlogopite peridotite at 1.0 GPa (1000–1300 °C) to understand the source of K2O- and SiO2-rich magmas that occur in continental, post-collisional and island arc settings. Using a new extraction technique specially developed for hydrous conditions combined with iterative sandwich experiments, we have determined the composition of low- to high-degree melts ( to 24.2 wt.%) of metasomatized lherzolite and harzburgite sources. Due to small amounts of adsorbed water in the starting material, amphibole crystallized at the lowest investigated temperatures. Amphibole breaks down at 1050–1075 °C, while phlogopite-breakdown occurs at 1150–1200 °C. This last temperature is higher than the previously determined in a mantle assemblage, due to the presence of stabilizing F and Ti. Phlogopite–lherzolite melts incongruently according to the continuous reaction: 0.49 phlogopite + 0.56 orthopyroxene + 0.47 clinopyroxene + 0.05 spinel = 0.58 olivine + 1.00 melt. In the phlogopite–harzburgite, the reaction is: 0.70 phlogopite + 1.24 orthopyroxene + 0.05 spinel = 0.99 olivine + 1.00 melt. The K2O content of water-undersaturated melts in equilibrium with residual phlogopite is buffered, depending on the source fertility: from in lherzolite to in harzburgite. Primary melts are silica-saturated and evolve from trachyte to basaltic andesite (63.5–52.1 wt.% SiO2) with increasing temperature. Calculations indicate that such silica-rich melts can readily be extracted from their mantle source, due to their low viscosity. Our results confirm that potassic, silica-rich magmas described worldwide in post-collisional settings are generated by melting of a metasomatized phlogopite-bearing mantle in the spinel stability field. [ABSTRACT FROM AUTHOR]
ISSN:0012821X
DOI:10.1016/j.epsl.2014.04.027