THE INFLUENCE OF WATER ON MELTING OF MANTLE PERIDOTITE

This experimental study examines the effects of variable concentration s of dissolved H2O on the compositions of silicate melts and their coe xisting mineral assemblage of olivine + orthopyroxene +/- clinopyroxen e +/- spinel +/- garnet. Experiments were performed at pressures of 1. 2 to 2.0 GPa and temperatures of 1100 to 1345 degrees C, with up to si milar to 12 wt% H2O dissolved in the liquid. The effects of increasing the concentration of dissolved H2O on the major element compositions of melts in equilibrium with a spinel Iherzolite mineral assemblage ar e to decrease the concentrations of SiO2, FeO, MgO, and CaO. The conce ntration of A1(2)O(3) is unaffected. The lower SiO2 contents of the hy drous melts result from an increase in the activity coefficient for Si O2 with increasing dissolved H2O. The lower concentrations of FeO and MgO result from the lower temperatures at which H2O-bearing melts coex ist with mantle minerals as compared to anhydrous melts. These composi tional changes produce an elevated SiO2/ (MgO + FeO) ratio in hydrous peridotite partial melts, making them relatively SiO2 rich when compar ed to anhydrous melts on a volatile-free basis. Hydrous peridotite mel ting reactions are affected primarily by the lowered mantle solidus. T emperature-induced compositional variations in coexisting pyroxenes lo wer the proportion of clinopyroxene entering the melt relative to orth opyroxene. Isobaric batch melting calculations indicate that fluid-und ersaturated peridotite melting is characterized by significantly lower melt productivity than anhydrous peridotite melting, and that the per idotite melting process in subduction zones is strongly influenced by the composition of the H2O-rich component introduced into the mantle w edge from the subducted slab.