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

Abstract

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 (Fo88–94) 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°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 fO2 from +1.4–2.6 log units above the fayalite‐magnetite‐quartz (ΔFMQ) buffer in peridotites, +1.7–2.3 ΔFMQ in hydrous pyroxenites, and +2.4–3.3 ΔFMQ in cumulate clinopyroxenites. High fO2 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.