An arsenide (low-Ni, high-Co löllingite) was found in a peridotite xenolith, which is strongly metasomatized by slab-derived melt or fluid from Avacha volcano, located in the Kamchatka arc. This is the first finding of a mantle arsenide within a fresh peridotite xenolith that may have been precipitated from metasomatic fluid/melt ultimately derived from the subducting slab. The löllingite is very low in Ni, suggesting low Ni-Fe partitioning between arsenide and olivine at mantle conditions. This is in contrast to sulfide, which favors Ni over Fe. An As-bearing fluid/melt thus plays some role in the metasomatic Fe enrichment in the mantle wedge. Supply of As is one of the characteristics of the upper mantle beneath the volcanic front.

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