Olivine–Anorthite Spherulites of Kudryavy Volcano: Crystallization Conditions in a Shallow Chamber

The article focuses on the nature of olivine–plagioclase (allivalite) spherulites from a young basaltic andesite lava flow of Kudryavy volcano. The spherulites consist mainly of olivine (Fo81–77) and plagioclase (An95–92) and have a radial structure, which is not typical of allivalites of the Kuril–Kamchatka region. The study of the mineral composition and primary melt inclusions in olivine showed that they formed from a low-alumina, low-potassium melt at temperatures of 970–1075°C and oxygen fugacity corresponding to the NNO + 1.6 – NNO + 2 buffer. A characteristic feature of the initial melts was their elevated water content (4.2 wt % on average). The structural features of the spherulites suggest that the formation of dendritic zones could have occurred during decompression of the shallow chamber under Kudryavy volcano at the beginning of the eruption, with a sharp decrease in temperature by 50–70°C. The coarsely crystalline central zones of spherulites confirm the cumulative nature of their formation. The formation of central zones could have occurred during the destruction of layers and crusts of cumulates on their entry into melts heading to the surface.