Pre‐Eruptive Volatiles and Magma Storage Depth for the 2018 Eruption at Mount Veniaminof, Alaska

Abstract Mount Veniaminof is a frequently active, open‐system arc volcano on the Alaska Peninsula, part of the Alaska‐Aleutian arc. Mount Veniaminof's September–December 2018 eruption began with small lava flows and culminated in a larger explosive event in November that produced a 450 km long continuous ash plume—one of the volcano's most significant ash‐producing events in recently observed eruptions. A temporally constrained tephra record was sampled after the eruption. We present volatile element concentrations (H, C, S, Cl, F) from a total of 65 olivine‐hosted melt inclusions (MIs) across six tephra layers to assess potential variations in magma chemistry throughout the course of the eruption. No variation is observed in volatile concentrations across the tephra layers, and there is no correlation between volatile concentrations and the explosivity of eruptive pulses leading to the November explosive event. H 2 O concentrations within MI suggest moderately hydrous magmas up to 2.4 wt% H 2 O and a large range in CO 2 concentrations in the MI (14–2,180 ppm) from Secondary Ion Mass Spectrometry and vapor bubble reconstructions. H 2 O and CO 2 measurements correspond to entrapment pressures up to 400 MPa (entrapment depths ∼15 km), but with modal storage around 100–200 MPa. H 2 O contents can be reconciled with previous microlite number density estimates using a polybaric storage history.