Melting Processes at the Base of the Mantle Wedge: Melt Compositions and Melting Reactions for the First Melts of Vapor-Saturated Lherzolite

Abstract:

Vapor-saturated melting experiments have been performed at pressures near the base of the mantle wedge (3.2 GPa). The starting composition is a metasomatized lherzolite containing 3 wt. % H₂O. Near-solidus melts and coexisting mineral phases have been characterized in experiments that span 925 to 1100 ^oC with melt % varying from 6 to 9 wt. %. Olivine, orthopyroxene, clinopyroxene and garnet coexist with melt over the entire interval and rutile is also present at < 1000 ^oC. Melt is andesitic in composition and varies from 60 wt. % SiO₂ at 950 ^oC to 52 wt. % at 1075 ^oC. The Al₂O₃ contents of the melt are 13 to 14 wt. %, and CaO contents range from 1 and 4 wt. %. Melting is peritectic with orthopyroxene + liquid produced by melting of garnet + olivine + high-Ca pyroxene. In addition to quenched melt, we observe a quenched silicate component that is rhyolitic (>72 % SiO₂) that we interpret as a precipitate from the coexisting supercritical H₂O-rich vapor. Extrapolation of the measured compositional variation toward the solidus suggests that the first melt may be very SiO₂ rich (i.e., granitic). We suggest that these granitic melts are the first melts of the mantle near the slab-wedge interface. As these SiO₂-rich melts ascend into shallower, hotter overlying mantle, they continue to interact with the surrounding mantle and evolve in composition. These first melts may elucidate the geochemical and physical processes that accompany the beginnings of H₂O flux melting.