Water storage capacity in olivine and pyroxene to 14 GPa: Implications for the water content of the Earth's upper mantle and nature of seismic discontinuities

Abstract

Experiments were performed under water-saturated conditions in the MFSH (MgO–FeO–SiO2–H2O) and MFASH (MgO–FeO–Al2O3–SiO2–H2O) systems at 2.5, 5, 7.5 and 9GPa, at temperatures from 1175 to 1400°C and H2O initial abundance of 0.5–5wt%. One experiment was performed at 13.5GPa at a temperature of 1400°C in the MFSH system. Water contents were analyzed by Fourier transform infrared spectroscopy. Results show that Al contents in olivine and pyroxene in equilibrium with an aluminous phase decrease significantly with increasing pressure and decreasing temperature. The incorporation of Al enhances water incorporation in olivine and pyroxene, but only at pressures of 2.5 and 5GPa. At 7.5GPa (i.e. 225km depth) the pyroxene is monoclinic, indicating that in a hydrous mantle the orthoenstatite to clinoenstatite phase transition occurs at shallower depths than previously thought, which is more consistent with the Lehmann discontinuity than with the X discontinuity. The partitioning of water between pyroxene and olivine in the MFASH system decreases from a value of 2 at 2.5GPa (80km depth) to 0.9 at 9GPa (270km depth). At 13.5GPa and 1400°C, the water content of olivine is 1700±300ppmwt H2O. The water partition coefficient between coexisting wadsleyite and olivine equals 4.7±0.7. We conclude that the water storage capacity of the upper mantle just above the 410km discontinuity is of 1500±300ppmwt H2O. If we assume that the Low Velocity Layer observed near 350km is due to mantle melting, we can constrain the water content of the mantle at that depth to be ∼850±150ppmwt H2O. This new value is four times higher than previous estimates for the mantle source of Mid Oceanic Ridge Basalts.Finally, comparison of the depth ranges of the L and X seismic discontinuities and the water storage capacity of the upper mantle suggests that the L-discontinuity (180–240km) is concomitant with a kink in the water storage due to the orthorhombic to monoclinic phase transition in enstatite, while the X-discontinuity (240–340km) coincides with a kink in the water storage capacity due to dehydration of garnet.