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 multianvil apparatus at 6 GPa and 1400A degrees C and a melt was ultimately produced that was saturated in a residual peridotite assemblage (olivine + clinopyroxene + garnet +/- orthopyroxene). The multiply saturated hydrous melts have higher (Mg + Fe)/Si and Al/Ca compared with hydrous melts produced at lower pressures. The melt compositions are similar to those determined near the dry peridotite solidus at similar to 1700A degrees C, when compared on an H2O-free basis. Melt H2O contents were determined to be similar to 11 wt % using mass balance, and these estimates were made more accurate by maintaining a large proportion of melt (> 70 wt %) in each experiment. If the geophysically inferred seismic low-velocity zone is caused by the presence of H2O-rich melt then at the base of this zone, at similar to 220 km, these results imply that the melt must contain 15-16 wt % H2O. The hydrous melt compositions, when compared on a volatile-free basis, are found to be similar to those of group II kimberlites (orangeites). The low FeO and Na2O but enriched K2O concentrations in group II magmas imply their derivation from melt-depleted cratonic lithosphere enriched by the metasomatic addition principally of K2O and H2O. A simple model is proposed in which this enrichment occurs by the addition of phlogopite to the source peridotite. Using determined K2O and H2O partition coefficients and assuming that the ratio of both components in the source is controlled by their ratio in phlogopite, group II kimberlite magmas can be constrained as being the product of similar to 0 center dot 2 wt % melting of a garnet peridotite source rock enriched with 1 center dot 7 wt % phlogopite, undergoing melting at near-adiabatic temperatures.