H₂O storage capacity of MgSiO₃ clinoenstatite at 8-13 GPa, 1,100-1,400°C

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 D^ol/CEn between 0.65 and 0.87. At 13 GPa and 1,300°C, partitioning between wadsleyite and clinoenstatite, D^wd/CEn, gives a value of 2.8 ± 0.4.