Investigating syn- vs. post-eruption hydration mechanisms of the 2012 Havre submarine explosive eruption: Water speciation analysis of pumiceous rhyolitic glass

Volatiles preserved in volcanic glass can record the quenching, fragmentation and solubility conditions during an explosive eruption. The VEI-5 2012 eruption of Havre volcano, which produced >1.5 km3 of rhyolite, provides exciting new insight into deep-submarine explosive eruptions. With no direct observations of the eruption at the 900 mbsl vent, the analysis and interpretation of volatile concentrations and speciation within pyroclasts is essential to constraining the eruption style and quenching mechanisms in this understudied environment. We present here the first detailed water speciation data for a large submarine explosive eruption. Water concentrations were measured in pyroclasts from known deposit localities across the Havre stratigraphic succession after ROV collection in 2015. Variations in total water concentration (H2OT) within pyroclasts were determined using high spatial resolution (1 - 2 µm) micro-Raman spectroscopy and water speciation (molecular water (H2Om) and OH) concentrations were measured using Fourier-transform infrared spectroscopy. H2OT concentrations are consistent between Raman and FTIR analysis, ranging from 0.1 - 1.5 wt % H2OT over different stratigraphic units. Comparison of water speciation data with speciation models suggests the Havre pyroclasts experienced secondary, non-magmatic hydration. Since OH is unaltered by secondary hydration, OH concentrations aid in the interpretation of quench depths and inferring of eruption mechanisms. The variability of excess H2Om across units suggests a more complex glass-hydration mechanism during the eruption instead of exclusively post-eruption, low-temperature secondary rehydration. The young sample ages are inconsistent with our current understanding of low-temperature H2O-diffusivity timescales, implying faster secondary rehydration in a higher-temperature submarine setting. We here explore potentially novel syn-eruptive, higher-temperature hydration mechanisms for deep-submarine pumice.