The distribution of H2O between silicate melt and nominally anhydrous peridotite and the onset of hydrous melting in the deep upper mantle

Abstract The partitioning of H 2 O between a mantle peridotite assemblage and low degree hydrous melt has been investigated at 6 GPa (corresponding to ∼180 km depth) at a temperature of 1400 °C. Peridotite mineral phases were analysed from 6 melting experiments performed in a natural chemical system. The experiments contained ∼80 wt% of a low degree hydrous melt that was obtained through a series of experiments where the melt composition was iteratively adjusted until saturation with the appropriate peridotite assemblage was achieved. The melt is fluid-undersaturated at the conditions of the experiment. Ion microprobe measurements of the mineral phases indicate olivine H 2 O concentrations of 434 ± 61 ppm wt and average clinopyroxene (cpx) concentrations of 1268 ± 173 ppm wt H 2 O. Orthopyroxene (opx) and garnet contain 700 ± 46 ppm wt and 347 ± 83 ppm wt H 2 O, respectively. The H 2 O content of the hydrous melts was determined by mass balance to be 11 ± 0.5 wt% H 2 O. H 2 O partition coefficients between minerals and melt ( D H 2 O min / melt = X H 2 O min / X H 2 O melt ) are 0.0040 ± 0.0006 for olivine, 0.0064 ± 0.0004 for opx, 0.0115 ± 0.0016 for cpx and 0.0032 ± 0.0008 for garnet. Using the determined H 2 O partition coefficients the onset and extent of melting at conditions equivalent to 180 km below mid-ocean ridges was determined as a function of mantle H 2 O content. Current estimates for the H 2 O content of the depleted mantle (50–200 ppm wt H 2 O) are insufficient to induce mantle melting at this depth, which requires ∼700 ppm wt H 2 O to produce 0.1% melting and 1600 ppm wt H 2 O for 1% melting, along an adiabat with a potential temperature of 1327 °C. Melting can occur at these conditions within the mantle source of ocean island basalts, which are estimated to contain up to 900 ppm wt H 2 O. If adiabatic temperatures are 200 °C higher within such plume related sources, then melt fractions of over 1% can be reached at 180 km depth. In addition, a model for the distribution of H 2 O between peridotite mineral phases as a function of depth and at H 2 O-undersaturated conditions is constructed. The model indicates that for a fixed mantle composition containing 150 ppm wt H 2 O, the olivine H 2 O content will increase with depth solely due to changes in inter-phase partitioning and modal proportions of minerals. The change in the olivine H 2 O concentration with depth corresponds to proposed changes in the dominant olivine slip system for deformation by dislocation creep, that might provide an explanation for the reduction in seismic anisotropy observed at depths >200 km.