We present H2O analyses of MgSiO3 pyroxene crystals quenched from hydrous conditions in the presence of olivine or wadsleyite at 8-13.4 GPa and 1,100-1,400°C. Raman spectroscopy shows that all pyroxenes have low clinoenstatite structure, which we infer to indicate that the crystals were high clinoenstatite (C2/c) during conditions of synthesis. H2O analyses were performed by secondary ion mass spectrometry and confirmed by unpolarized Fourier transform infrared spectroscopy on randomly oriented crystals. Measured H2O concentrations increase with pressure and range from 0.08 wt.% H2O at 8 GPa and 1,300°C up to 0.67 wt.% at 13.4 GPa and 1,300°C. At fixed pressure, H2O storage capacity diminishes with increasing temperature and the magnitude of this effect increases with pressure. This trend, which we attribute to diminishing activity of H2O in coexisting fluids as the proportion of dissolved silicate increases, is opposite to that observed previously at low pressure. We observe clinoenstatite 1.4 GPa below the pressure stability of clinoenstatite under nominally dry conditions. This stabilization of clinoenstatite relative to orthoenstatite under hydrous conditions is likely owing to preferential substitution of H2 O into the high clinoenstatite polymorph. At 8-11 GPa and 1,200-1,400°C, observed H2O partitioning between olivine and clinoenstatite gives values of Dol/CEn between 0.65 and 0.87. At 13 GPa and 1,300°C, partitioning between wadsleyite and clinoenstatite, Dwd/CEn, gives a value of 2.8 ± 0.4.
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Acknowledgments We gratefully acknowledge the assistance of Cyril Aubaud, and Yunbin Guan during SIMS analyses at ASU and of Jinping Dong for help with the Raman spectrometer and Ellery Frahm for help with the electron microprobe. We thank Steve Jacobsen for illuminating discussions, and two anonymous reviewers for their thoughtful comments and suggestions. Parts of this work were carried out in the Minnesota Characterization Facility, which receives partial support from NSF through the NNIN program. This work supported by NSF EAR0456405.