Solubility and partitioning of water in synthetic forsterite and enstatite in the system MgO–SiO2–H2O±Al2O3 |
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| Authors: | Kevin J Grant Simon C Kohn Richard A Brooker |
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| Institution: | (1) Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, BS8 1RJ Bristol, UK;(2) Present address: Department of Earth and Planetary Sciences, Macquarie University, 2109 Sydney, NSW, Australia;(3) Present address: Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK |
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| Abstract: | The solubility of water in coexisting enstatite and forsterite was investigated by simultaneously synthesizing the two phases in a series of high pressure and temperature piston cylinder experiments. Experiments were performed at 1.0 and 2.0 GPa at temperatures between 1,100 and 1,420°C. Integrated OH absorbances were determined using polarized infrared spectroscopy on orientated single crystals of each phase. Phase water contents were estimated using the calibration of Libowitzky and Rossman (Am Mineral 82:1111–1115, 1997). Enstatite crystals, synthesized in equilibrium with forsterite and an aqueous phase at 1,350°C and 2.0 GPa, contain 114 ppm H2O. This is reduced to 59 ppm at 1,100°C, under otherwise identical conditions, suggesting a strong temperature dependence. At 1,350°C and 1.0 GPa water solubility in enstatite is 89 ppm, significantly lower than that at 2.0 GPa. In contrast water solubility in forsterite is essentially constant, being in the range 36–41 ppm for all conditions studied. These data give partition coefficients
 in the range 2.28–3.31 for all experiments at 1,350°C and 1.34 for one experiment at 1,100°C. The incorporation of Al2O3 in enstatite modifies the OH stretching spectrum in a systematic way, and slightly increases the water solubility. |
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