WATER PARTITIONING BETWEEN NOMINALLY ANHYDROUS MINERALS IN THE MGO-SIO2-H2O SYSTEM UP TO 24 GPA: IMPLICATIONS FOR THE DISTRIBUTION OF WATER IN THE EARTH'S MANTLE

Abstract

Multi-anvil experiments have been conducted in the MgO-SiO2-H2O system at pressures of 15-24 GPa and temperatures of 1200-1600°C to investigate the partitioning of water between mantle phases. The water contents were obtained from Fourier transform infrared spectroscopy on doubly polished and clear crystals. Most of the mineral phases coexisted with a hydrous melt. The results show that among all magnesium silicate phases (1) MgSiO3-perovskite is the only phase which does not dissolve a detectable amount of water and (2) the high-pressure polymorphs of Mg2SiO4, wadsleyite and ringwoodite, dissolve about one order of magnitude more water than the polymorphs of MgSiO3 (clinoenstatite, majorite and akimotoite). The following partition coefficients of water were measured: Dwadsleyite/clin°enstatite=3.8 at 15 GPa and 1300°C, Dringw°°dite/akim°t°ite=21 at 19 GPa and 1300°C, Dakim°t°ite/per°vskite#1 at 24 GPa and 1600°C, and Dpericlase/per°vskite>1 at 24 GPa and 1500°C. These results suggest that most of the water in the Earth's mantle is partitioned in the transition zone.