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 in the system forsterite-carbonate-H2O the H2O content of forsterite 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.
- upper mantle
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