H ₂O storage capacity of olivine at 5-8GPa and consequences for dehydration partial melting of the upper mantle

The H ₂O storage capacities of peridotitic minerals place crucial constraints on the onset of hydrous partial melting in the mantle. The storage capacities of minerals in equilibrium with a peridotite mineral assemblage ("peridotite-saturated" minerals) are lower than when the minerals coexist only with fluid because hydrous partial melt is stabilized at a lower activity of H ₂O. Here, we determine peridotite-saturated olivine H ₂O storage capacities from 5 to 8GPa and 1400-1500°C in layered experiments designed to grow large (~100-150μm) olivine crystals in equilibrium with the full hydrous peridotite assemblage (melt+ol+opx+gar+cpx). The peridotite-saturated H ₂O storage capacity of olivine at 1450°C rises from 57±26ppm (by wt.) at 5GPa to 254±60ppm at 8GPa. Combining these with results of a parallel study at 10-13GPa (Tenner et al., 2011, CMP) yields a linear relation applicable from 5 to 13GPa for peridotite-saturated H ₂O storage capacity of olivine at 1450°C, CH2Oolivine(ppm)=57.6(±16)×P(GPa)-169(±18). Storage capacity diminishes with increasing temperature, but is unaffected by variable total H ₂O concentration between 0.47 and 1.0wt%. Both of these are as predicted for the condition in which the water activity in the melt is governed principally by the cryoscopic requirement of melt stability for a given temperature below the dry solidus. Measured olivine storage capacities are in agreement or slightly greater than those predicted by a model that combines data from experimental freezing point depression and olivine/melt partition coefficients of H ₂O (Hirschmann et al., 2009). Considering the temperature along the mantle geotherm, as well as available constraints on garnet/olivine and pyroxene/olivine partitioning of H ₂O (DH2Ogar/ol,DH2Opx/ol), we estimate the peridotite H ₂O storage capacity in the low velocity zone. The CH2O required to initiate melting between 150 and 250km depth is between 270 and 855ppm. We conclude that hydrous partial melting does not occur at these depths for H ₂O concentrations (50-200ppm) typical of the convecting upper mantle sampled by mid-ocean ridge basalts.