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 by studying melt inclusions in olivine (Fo85.8–90.7) phenocrysts. The melt inclusions captured during the crys- tallization of primitive magmas subsequently reequilibrated with their host minerals and became partly recrys- tallized and decrepitated. The diffusion-controlled reequilibration of the melt inclusions with the olivine occurred at temperatures close to ~1100°e and was associated with the crystallization of daughter phases: oli- vine, high-Ca pyroxene, and spinel. The composition of the pyroxene and spinel in the inclusions evolved toward extremely high Al contents, which is atypical of pyroxene in the rocks and was controlled by plagioclase absence from the daughter phase assemblage of the inclusions. Magma decompression induced the partial decrepitation of the melt inclusions, a process that was associated with the escape of fluid components (ee2 and c2e) and variable amounts of the residual silicate material from the inclusions. The initial compositions of the melt inclusions, which were reconstructed using techniques of experimental homogenization and mod- eling, show broad ranges in the contents of major and trace elements. Compared with the composition of the rocks, the compositions of inclusions in the olivine Fo > 90% are higher in CaO, Al2O3, and Na2O at lower concentrations of SiO2. Their geochemical characteristics are identical to those of low-Si ankaramite melts occurring in many island arcs. The carbonatite metasomatism of the arc mantle, the derivation of nepheline- normative ankaramite magmas, and the significant crustal contamination of these magmas during their fraction- ation can be spread more widely than is currently assumed in models for island-arc petrogenesis. The evolution of the avachite primitive magmas was controlled by the crystallization of early olivine, high-Ca pyroxene, spinel, and, perhaps, the assimilation of crustal rocks in the magmatic chambers at different depths (from 5 to 30 km). During two (or more) crystallization stages, olivine–pyroxene cumulates were produced, remobilized, and transported to the surface by the differentiated hypersthene-normative magmas. Avachites are hybrid cumu- lative rocks, which were produced in a long-lived open magmatic system.

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