Melting and Multi-stage Metasomatism in the Mantle Wedge beneath a Frontal Arc Inferred from Highly Depleted Peridotite Xenoliths from the Avacha Volcano, Southern Kamchatka

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Peridotite xenoliths from the Avacha volcano, Kamchatka, derived from the mantle beneath the volcanic front of the Kamchatka arc, are mainly highly depleted clinopyroxene-poor harzburgites with highly forsteritic olivine (Fo 90–92) and high Cr-number spinel (0·5–0·7). The Avacha peridotites have experienced metasomatism to various extents, with the formation of metasomatic orthopyroxene replacing primary olivine, by infiltration of SiO2-rich fluids. The metasomatic orthopyroxenes can be subdivided into two textural types; (1) radially aggregated prismatic grains (opx II-1); (2) stout grains associated with interstitial glass and metasomatic minerals (opx II-2). The Avacha peridotites exhibit light rare earth element (LREE) enrichment relative to heavy REE (HREE), even in primary lithologies with <2 vol.% of metasomatic orthopyroxene. However, the metasomatism has not altered the chemical characteristics of the Avacha peridotites significantly, and the Fo contents of olivine do not increase with an increase in the Cr-number of spinel. The clinopyroxene in the most depleted primary peridotite, with the highest spinel Cr-number, has the highest CeN (where subscript N indicates normalized to primitive mantle) and relatively low YbN contents. We conclude that the Avacha peridotites are residues of high-degree melting induced by infiltration of an aqueous fluid with relatively high Fe/Mg and LREE/HREE ratios. In situ analysis of olivine grain boundaries by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) showed significant concentrations of some trace elements (Ce and Ba), even on glass-free boundaries; such grain boundaries can be a repository for large-ion lithophile elements (LILE). Orthopyroxene and Ca-amphibole contribute to the HREE and middle REE (MREE) budget, respectively, of the bulk peridotites. Three types of mantle metasomatism are recorded in the Avacha peridotites. The agent that formed HREE-poor opx II-1 was a slab-derived aqueous fluid rich in SiO2. It was a successor of the fluid influx involved in the initial partial melting event. On the other hand, the metasomatic minerals in highly metasomatized peridotites have high LILE (Th, U and Sr) and LREE contents relative to HREE, indicating involvement of a hydrous melt of adakitic affinity derived from the subducted slab. Glasses associated with opx II-2 are similar in their trace-element characteristics to the host Avacha volcanic rocks, indicating the likelihood that the metasomatic agent was a forerunner of the recent Avacha arc magmatism.

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