HYDROGEN INCORPORATION INTO FORSTERITE IN THE SYSTEMS Mg2SiO4-K2Mg(CO3)2-H2O AND Mg2SiO4-H2O-C at 7.5-14.0 GPa
K.D. Litasova,b, A.F. Shatskiyb, Yu.N. Pal'yanova, A.G. Sokola, T. Katsurac, E. Ohtanib
a Sobolev Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
b Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
c Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori 682-0193, Japan
Keywords: Forsterite, water, upper mantle, carbonate, carbon, fluid
Pages: 1129-1138
Abstract
Experiments on water solubility in forsterite in the systems Mg2SiO4 - K2Mg (CO3)2 - H2O and Mg2SiO4-H2O-C were conducted at 7.5-14.0 GPa and 1200-1600°C. The resulting crystals contain 448 to 1480 ppm water, which is 40-70% less than in the forsterite-water system under the same conditions. This can be attributed to lower water activity in the carbonate-bearing melt. The water content of forsterite was found to vary systematically with temperature and pressure. For instance, at 14 GPa, the H2O content of forsterite in the system forsterite-carbonate-H2O drops from 1140 ppm at 1200°C to 450 ppm at 1600°C, and at 8 GPa it remains constant or increases from 550 to 870 ppm at 1300-1600°C. Preliminary data for D-H-bearing forsterite are reported. Considerable differences were found between IR spectra of D-H- and H-bearing forsterite. The results suggest that CO2 can significantly affect the width of the olivine-wadsleyite transition, i.e., the 410 km seismic discontinuity, which is a function of the water content of olivine and wadsleyite.
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