Lower crustal magmatic processes and andesite genesis at Shiveluch Volcano

Andrea Elizabeth Goltz, Michael Krawczynski

The silicic melts that eventually erupt at arc volcanoes are produced in the lower crust, yet, the storage conditions of magma in the lower crust have not been the topic of extensive study. In this study, we conduct and analyze hydrous piston cylinder experiments to determine the mid-to-lower magma storage conditions of primitive melt at Shiveluch, an arc volcano located in northern Kamchatka. The near-liquidus assemblage of amphibole and olivine, observed in mafic enclaves, constrains storage of primitive magmas to pressures between 500 MPa and 1 GPa, temperatures from 1000-1100 °C, and high (≥7.2 wt%) H2O contents. Compositions of glasses in our experiments also show that lower crustal differentiation cannot be the only process involved in the production of andesites at Shiveluch. By comparing synthetic and natural compositions of amphibole we determine that anatexis of the lower crust is also likely to significantly contribute to produce andesitic melts at Shiveluch.