Melt-mantle interactions beneath the Kamchatka arc: Evidence from ultramafic xenoliths from Shiveluch volcano

[ 1] Ultramafic xenoliths of spinel dunite, harzburgite, lherzolite, amphibole/phlogopite- bearing pyroxenite, and clinopyroxenite occur in andesitic pyroclastic debris from the 1964 eruption of Shiveluch volcano, Kamchatka. Peridotites have coarse/protogranular, porphyroclastic, and granuloblastic textures, abundant kink-banded olivine, and refractory mineral compositions with forsteritic olivine (Fo(88-94)) and Cr-rich spinel ( 100* Cr/Cr + Al = 47 - 83). Orthopyroxene (opx) is also Mg-rich but occurs only as a fibrous mineral present along olivine grain boundaries, in monomineralic veins that crosscut coarse olivine, and in veins with amphibole and phlogopite that crosscut coarse-grained peridotites. Textural evidence and mineral compositions indicate that the peridotites and hydrous pyroxenites were replacive dunites that formed by melt-rock reactions involving the dissolution of pyroxene and precipitation of olivine. The fibrous opx and millimeter-scale veins of phlogopite, amphibole, and opx are interpreted as the autometasomatic products of hydrous magmas that were trapped in the uppermost mantle (< 45 km). In this interpretation, opx was produced by reaction between late-stage, silica-rich, hydrous fluids/melts and olivine in the dunite protolith, and the millimeter-scale veins of phlogopite, amphibole, and opx are the volatile-enriched, deuteric products that were liberated during the final stages of magma crystallization. The absence of textural equilibrium suggests that the late-stage replacement process which produced the fibrous opx occurred shortly prior to the eruption that carried the xenoliths to the surface. On the basis of two-pyroxene thermometry and Ca-in-olivine barometry, the xenoliths equilibrated between 800 and 1000 degrees C and 1.03 and 2.21 GPa. This implies that the xenoliths were carried from sub-Moho depths, a result consistent with geophysical estimates of crustal thickness. Olivine-opx-spinel equilibria indicate that the xenoliths are strongly oxidized with fO(2) from + 1.4 - 2.6 log units above the fayalite-magnetite-quartz (Delta FMQ) buffer in peridotites, + 1.7 - 2.3 Delta FMQ in hydrous pyroxenites, and + 2.4 - 3.3 Delta FMQ in cumulate clinopyroxenites. High fO(2) in the peridotites is attributed to the melt-rock reactions that formed the dunite protolith. These results therefore suggest that interaction between oxidized melts and peridotite wall rock at shallow mantle depths plays a significant role in creating and modifying the uppermost mantle and deepest crust in some subduction settings.