Compositions and Formation Conditions of Primitive Magmas of the Karymsky Volcanic Center, Kamchatka: Evidence from Melt Inclusions and Trace-Element Thermobarometry

Abstract

This paper reports the results of a study of naturally and experimentally quenched melt inclusions in magnesian olivine (Fo_77–89) from a basalt sample from the Karymsky volcanic center, which is located in the middle segment of the Eastern Volcanic Front of Kamchatka. The conditions of parental magma formation were estimated using modern methods of trace-element thermometry. Based on direct H₂O measurements in inclusions and thermometry of coexisting olivine and spinel, it was shown that the parent melts contained at least 4.5 wt % H₂O and crystallized at a temperature of 1114 ± 27°C and an oxygen fugacity of ΔQFM = 1.5 ± 0.4. The obtained estimates of H₂O content and crystallization temperature are among the first and currently most reliable data for the Eastern Volcanic Front of Kamchatka. The primary melt of the Karymsky volcanic center was derived from peridotitic material and could be produced by ~12–17% melting of an enriched MORB source (E-DMM) at ~1230–1250°C and ~1.5 GPa. Our estimates of mantle melting temperature beneath Kamchatka are slightly lower than values reported previously and up to 50°C lower than the dry peridotite solidus, which indicates the influence of a slab-derived hydrous melt. The combined approach to the estimation of the initial H₂O content of melt employed in this study can provide a more reliable data in future investigations, and its application will probably decrease the existing temperature estimates for the mantle wedge above subduction zones.