Hornblende peridotite xenoliths from central Mexico reveal the highly oxidized nature of subarc upper mantle

Abstract

Quaternary hornblende andesites have erupted along a fault zone near El Penon, central Mexico. One of these flows contains 1‐2 cm xenoliths of amphibole-rich spinel lherzolite and chromite websterite. These xenoliths are rare samples of subarc upper mantle from a region of continuing subduction, and they are the most oxidized mantle peridotites yet described (fayalitemagnetite-quartz +1.5 to 2.4). The abundant amphibole and high oxygen fugacities that characterize these xenoliths are significant because they provide direct evidence for metasomatism of the mantle wedge by slab-derived fluids. Phenocrysts of hornblende and lack of plagioclase phenocrysts in the host andesite indicate that it equilibrated with high water content (>8 wt%), and the presence of xenoliths implies rapid (26 km/day) ascent.