Fragments of Metasomatized Forearc: Origin and Implications of Mafic and Ultramafic Xenoliths From Kharchinsky Volcano, Kamchatka

Dunite, harzburgite, and clinopyroxenite xenoliths from Kharchinsky volcano, Kamchatka, have abundances and ratios of incompatible trace elements similar to those in arc volcanic rocks (elevated Ba/Th, La/Yb, Nd/Hf, and Sr/Y). All orthopyroxenes and some clinopyroxenes in the peridotites have U‐shaped rare‐earth element patterns. Negative Ce anomalies are present in orthopyroxenes with Ce/Ce* as low as 0.01 and down to 0.22 in whole‐rock peridotite data. Ce anomaly growth is linked to increasing La/Sm and enrichments in Rb, U, Pb, and Ba over La and Ce. Isotopes (Pb, Sr, Nd, and Hf) indicate pelagic sediment, and hydrothermal crusts play no role in Ce anomaly development. Instead, Ce anomalies appear to be products of fluid transport and elemental scavenging under oxidizing conditions beneath the deep forearc. Textures and compositions of aluminous green spinels indicate most of the peridotites were partially melted and recrystallized at depth. Veins and pockets of amphibole reflect impregnation late in the petrogenesis of the rocks by melts similar to Kamchatka basalts. Orthopyroxenite xenoliths are fine‐grained with fibrous orthopyroxene that has high‐Mg/Mg + Fe (up to 0.96) and generally lower CaO and Al2O3 compared to peridotite orthopyroxenes and perhaps formed by reaction of siliceous fluids with olivine. Kharchinsky xenoliths have Pb, Sr, and Nd isotopes similar to Kamchatka volcanic rocks, but Hf isotopes in clinopyroxenites and gabbros are more radiogenic by 1–3 epsilon units. Patterns in isotopic data indicate a compositional change in the source of Kamchatka volcanism within the past 20 million years. Plain Language Summary This paper presents the results of a study of rare rock fragments (xenoliths) that were transported from the Earth's deep interior to the surface during an eruption of Kharchinsky volcano, Kamchatka. The chemical compositions, mineralogy, and textures of the samples were studied with the goal of understanding the processes that affected rocks, which may play a role in the formation of magmas in the Kamchatka subduction zone. The key process that affected the xenoliths involved the addition of fluids and dissolved elements to the samples at temperatures of 500–700 °C. These fluids are derived from seawater that was transported to 30‐ to 50‐km depths by subduction of the Pacific Plate beneath Kamchatka. Subsequent to the addition of fluid, there was a shift in the position of the Kamchatka‐Pacific Plate boundary that led to an