Reconstructing Final H2O Contents of Hydrated Rhyolitic Glasses: Insights into H2O Degassing and Eruptive Style of Silicic Submarine Volcanoes

Abstract:

H₂O degassing influences the evolution of magma viscosity and vesicularity during ascent through the crust, and ultimately the eruptive style. Investigating H₂O degassing requires data on both initial and final H₂O contents. Initial H₂O contents are revealed by melt inclusion data, while final H₂O contents are found from dissolved H₂O contents of volcanic glass. However volcanic glasses, particularly of silicic composition, are susceptible to secondary hydration i.e. the addition of H₂O from the surrounding environment at ambient temperature during the time following pyroclast deposition. Obtaining meaningful final H₂O data therefore requires distinguishing between the original final dissolved H₂O content and the H₂O added subsequently during hydration. Since H₂O added during hydration is added as molecular H₂O (H₂O_m), and the species interconversion between H₂O_m and hydroxyl (OH) species is negligible at ambient temperature, the final OH content of the glass remains unaltered during hydration. By using H₂O speciation models to find the original H₂O_m content that would correspond to the measured OH content of the glass, the original total H₂O (H₂O_t) content of the glass prior to hydration can be reconstructed. These H₂O speciation data are obtained using FTIR spectroscopy. In many cases, particularly where vesicular glasses necessitate thin wafers, OH cannot be measured directly and instead is calculated indirectly as OH = H₂O_t - H₂O_m. Here we demonstrate the importance of using a speciation-dependent H₂O_t molar absorptivity coefficient to obtain accurate H₂O_t and H₂O speciation data and outline a methodology for calculating such a coefficient for rhyolite glasses, with application to hydrated silicic pumice from submarine volcanoes in the Japanese Izu-Bonin Arc. Although hydrated pumice from Kurose Nishi and Oomurodashi now contain ~1.0 - 2.5 wt% H₂O_t, their pre-hydration final H₂O contents were typically ~0.3 – 0.4 wt% H₂O_t. Furthermore, we show that pre-hydration final H₂O contents vary with pumice texture, with Kurose Nishi tube pumice containing ~0.3 wt% H₂O_t while denser pumice with spherical vesicles contains ~1.0 wt% H₂O_t. By combining these new reconstructed final H₂O data with textural characteristics we investigate H₂O degassing and eruption mechanisms of silicic submarine volcanoes.