Petrology of Avachites, High-Magnesian Basalts of Avachinsky Volcano, Kamchatka: II. Melt Inclusions in Olivine

The composition and crystallization conditions of the parental melts of avachites were elucidated \nby studying melt inclusions in olivine (Fo85.8–90.7) phenocrysts. The melt inclusions captured \nduring the crys- tallization of primitive magmas subsequently reequilibrated with their host \nminerals and became partly recrys- tallized and decrepitated. The diffusion-controlled \nreequilibration of the melt inclusions with the olivine occurred at temperatures close to ~1100°ë \nand was associated with the crystallization of daughter phases: oli- vine, high-Ca pyroxene, and \nspinel. The composition of the pyroxene and spinel in the inclusions evolved toward extremely high \nAl contents, which is atypical of pyroxene in the rocks and was controlled by plagioclase \nabsence from the daughter phase assemblage of the inclusions. Magma decompression induced the \npartial decrepitation of the melt inclusions, a process that was associated with the escape of \nfluid components (ëé2 and ç2é) and variable amounts of the residual silicate material from the \ninclusions. The initial compositions of the melt inclusions, which were reconstructed using \ntechniques of experimental homogenization and mod- eling, show broad ranges in the contents of \nmajor and trace elements. Compared with the composition of the rocks, the compositions of \ninclusions in the olivine Fo > 90% are higher in CaO, Al2O3, and Na2O at lower concentrations of \nSiO2. Their geochemical characteristics are identical to those of low-Si ankaramite melts occurring \nin many island arcs. The carbonatite metasomatism of the arc mantle, the derivation of nepheline- \nnormative ankaramite magmas, and the significant crustal contamination of these magmas during their \nfraction- ation can be spread more widely than is currently assumed in models for island-arc \npetrogenesis. The evolution of the avachite primitive magmas was controlled by the crystallization \nof early olivine, high-Ca pyroxene, spinel, and, perhaps, the assimilation of crustal rocks in the \nmagmatic chambers at different depths (from 5 to \n30 km). During two (or more) crystallization stages, olivine–pyroxene cumulates were produced, \nremobilized, and transported to the surface by the differentiated hypersthene-normative magmas. \nAvachites are hybrid cumu- \nlative rocks, which were produced in a long-lived open magmatic system. \n