Magnesian Basalts of the Medvezhia Caldera: Dominant Magmas and Their Sources, as Exemplified by Menshiy Brat Volcano, Iturup Island, Kuriles

The paper presents new data on the formation conditions of basalts from Menshiy Brat postcaldera volcano in the Medvezhia caldera, Iturup Island. The liquidus mineral assemblage consists of olivine (Fo 85.3–90.1 mol %) and chromium spinel (Cr# = 0.46–0.6), which crystallized at 1090–1170°C and oxygen fugacity at NNO + 0.6 (σ = 0.2) to NNO + 0.2 (σ = 0.14) in the course of the eruption. Data on melt inclusions in the liquidus olivine demonstrate that its parental melts were low-Al₂O₃ and low-K₂O, with up to 15.5 wt % MgO, and with an average H₂O content of 5.5 wt %. The newly obtained data on volatile contents in the olivine-hosted melt inclusions suggest that the mafic melts were derived by the partial melting of a peridotitic-rich source with a small admixture of an olivine-free component at 1225°C, under active influence of the slab-derived fluids. These fluids were separated from the subducting slab at 670–705°C and depths of 95–105 km beneath Iturup Island. Our results enhance our understanding of the evolution of basic magmas that serve as a heat and volatile sources during the formation of large calderas.