Melt evolution in subarc mantle: evidence from heating experiments on spinel-hosted melt inclusions in peridotite xenoliths from the andesitic Avacha volcano (Kamchatka, Russia)

Glass-bearing inclusions hosted by Cr-spinel in harzburgite xenoliths from Avacha are grouped based on homogenization temperatures and daughter minerals into high-T (1,200A degrees C; opx + cpx), intermediate (900-1,100A degrees C; cpx +/- A amph), and low-T (900A degrees C; amph) and are commonly accompanied by larger "melt pockets". Unlike previous work on unheated inclusions and interstitial glass in xenoliths from Kamchatka, the homogenized glass compositions in this study are not affected by low-pressure melt fractionation during transport and cooling or by interaction with host magma. Primary melt compositions constrained for each inclusion type differ in major and trace element abundances and were formed by different events, but all are silica saturated, Ca-rich, and K-poor, with enrichments in LREE, Sr, Rb, and Ba and negative Nb anomalies. These melts are inferred to have been formed with participation of fluids produced by dehydration of slab materials. The high-T inclusions trapped liquids produced by ancient high-degree, fluid-induced melting in the mantle wedge. The low-T inclusions are related to percolation of low-T melts or hydrous fluids in arc mantle lithosphere. Melt pockets arise from localized heating and fluid-assisted melting induced by rising magmas shortly before the entrapment of the xenoliths. The "high-T" melt inclusions in Avacha xenoliths are unique in preserving evidence of ancient, high-T melting events in arc mantle, whereas the published data appear to characterize pre-eruption enrichment events.