The effect of H₂O on partial melting of garnet peridotite at 3.5 GPa

We present experimental determinations of the influence of H₂O on partial melting of garnet peridotite (+1.5, 2.5, and 5 wt. % added H ₂O) at 3.5 GPa and 1200-1450C. Experiments produced complex polyphase regions of quenched melt and equilibrium partial melt compositions were reconstructed by combined EMP and LA-ICP-MS analyses. Mass balance-derived melt fractions (F) range from 0.18 to 0.33 and dissolved water contents range from 4.5 to 23.5 wt. %. One exceptional experiment quenched glass, allowing independent verification of H₂O concentration by FTIR. The influence of H₂O on melt production is quantified by the temperature difference required to achieve a given F under dry and wet conditions, T, which is controlled by the H₂O concentration in partial melts. Melts with 1.5, 5, 10, and 15 wt. % H₂O yield T values of 50, 150, 250, and 320C, respectively, consistent with a cryoscopic parameterization that assumes 3 oxygens per mole of silicate melt. Based on this parameterization, we calculate that beneath oceanic ridges, peridotite H₂O storage capacity increases from 0 to 240 ppm from 66 to 110 km depth. For H₂O to be solely responsible for melting in the oceanic low velocity zone (LVZ) at least 5.7 wt. % H₂O must be dissolved in the melt at 110 km, and considerably more (e.g., 15 wt.% at 220 km) is required for melting throughout the entire observed interval. The addition of H₂O results in 3.5 GPa partial melts of garnet peridotite (normalized anhydrous) that are SiO ₂ and Al₂O₃ poor (43-50 and 9-11.5 wt. %, respectively), and MgO and CaO rich (18-27 and 7-12 wt. %, respectively) when compared to anhydrous analogues. These effects become highly pronounced deep in the upper mantle, and are opposite to the effect of H₂O on melt compositions in the spinel stability field, potentially owing in part to OH ^- association with network modifying cations in high pressure, depolymerized melts and in part to low-temperature stabilization of garnet, which enhances CaO/Al₂O₃.