The Composition of Hydrous Partial Melts of Garnet Peridotite at 6 GPa: Implications for the Origin of Group II Kimberlites

Chemical compositions of hydrous melts, compatible with those that would form by incipient melting of upper mantle peridotite at 180 km depth, have been determined using a series of iterative crystallization experiments. Experiments were performed in a multi-anvil apparatus at 6 GPa and 14008C and a melt was ultimately produced that was saturated in a residual peridotite assemblage (olivineþ clinopyroxeneþgarnet orthopyroxene). The multiply saturated hydrous melts have higher (MgþFe)/Si and Al/Ca com-pared with hydrous melts produced at lower pressures.The melt com-positions are similar to those determined near the dry peridotite solidus at 17008C, when compared on an H2O-free basis. Melt H2O contents were determined to be 11wt % using mass balance, and these estimates were made more accurate by maintaining a large proportion of melt (470 wt %) in each experiment. If the geophysi-cally inferred seismic low-velocity zone is caused by the presence of