MAGMA STORAGE DEPTHS PRIOR TO THE 2018 MOUNT VENIAMINOF ERUPTION, ALASKA

Mount Veniaminof is an open vent, 35 km wide arc tholeiitic volcano in the western Alaska Peninsula with multiple caldera-forming eruptions in the Pleistocene and many Strombolian eruptions of low Volcanic Explosivity Index (VEI) in the Holocene. In 2018, Veniaminof erupted from September to December starting with small lava flows and culminating in a larger explosive event in November that caused a 450 km long ash plume. While this plume was short-lived, it marked the first large, explosive eruption in recent history. Prior to the eruption, seismic unrest was detected at ~20 km depth.

Here, we present new volatile data from olivine-hosted melt inclusions (MIs) to assess the roles of magmatic volatiles for the 2018 eruption. Bulk tephras are basaltic andesites (58 wt. % SiO2) with olivine ranging from Fo_64-79. H₂O concentrations within MIs suggest moderately wet magmas (<2.5 wt.% H₂O). CO₂concentrations in the melt inclusions range from 39-290 ppm. Combined with Raman spectroscopy of shrinkage bubbles within the MIs, we estimate total CO₂ concentrations in the MIs between 250-1690 ppm. This corresponds to entrapment pressures of at least 300 MPa and entrapment depth up to <15 km similar to seismic precursors.