MAGMA CHAMBER TO THE OCEAN SURFACE: WHAT WATER TELLS US ABOUT PRE- AND SYN-ERUPTIVE SUBMARINE VOLCANIC PROCESSES

Water is the most abundant dissolved volatile in silicic submarine volcanism and a major control on the eruptive processes within this environment. By quantifying water within volcanic glass, both in phenocryst-hosted melt inclusions and bubble-walls, we can piece together a story from deep magmatic storage to shallow conduit ascent to syn-eruptive processes. The VEI-5 submarine rhyolitic eruption of Havre in 2012 was the largest historical eruption in the deep-submarine realm. Total water concentrations (H₂O_T) within pumiceous glass from Havre were determined using micro-Raman spectroscopy; and concentrations of water species (molecular water (H₂O_m) and OH) using FTIR spectroscopic imaging. Secondary ion mass spectrometry quantified H₂O_T and CO₂ concentrations in quartz-hosted melt inclusions in Havre pumice.

Melt inclusions contain 5 – 7 wt % H₂O_T and < 300 ppm CO₂ suggesting deep storage conditions at 150-250 MPa of a very hydrous silicic melt which subsequently underwent closed-system degassing during ascent. Undersaturated residual OH contents in matrix glass, relative to the 650 – 900 m vent depth, imply either: 1) much shallower quenching pressures in the water column from slower cooling rates, or 2) the presence of coupled-volatile solubility conditions during magma ascent. Presence of excess H₂O_m in the matrix glass show that Havre products have been rehydrated post-degassing, but on rapid timescales inconsistent with our current understanding of water diffusion in the marine setting. Micro-Raman measurements spatially resolve this rehydration as mostly limited to bubble rims rather than throughout the glass. This suggests a novel syn-eruptive rehydration mechanism that may be unique to the deep-submarine volcanic setting, which we here explore.