[S-GC35] Volatiles in the Earth - from Surface to Deep Mantle

[SGC35-09] Investigating shallow submarine eruption processes recorded in dissolved H2O contents of marine tephra: example of Oomurodashi volcano tephra in drill core C9010E

*Iona McIntosh¹, Kaori Aoki² (1.Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology, 2.Research Center for Volcanic Hazards and their Mitigation, Tokyo Metropolitan University)

Keywords:FTIR spectroscopy, H2O speciation, Submarine eruptions, Tephra

Magmatic volatiles (such as H2O, CO2, Cl, S) drive volcanic eruptions through bubble nucleation and growth as magma ascends. Their solubilities vary in response to changes in pressure, temperature and co-existing volatile concentrations. Accordingly, volcanic processes can be reconstructed based on dissolved volatile concentrations recorded in volcanic glasses. H2O is often the most volumetrically abundant volatile in silicic magmas such as rhyolite, and therefore is a key volatile record of eruption processes. However the strong susceptibility of high silica glasses to secondary hydration (i.e. slow diffusive addition of non-magmatic water in the time after eruption and deposition that alters eruptive H2O concentrations) has limited the use of dissolved H2O concentrations in high surface area material such as tephra and pumice, particularly in submarine deposits.

H2O is dissolved in melts and glasses as two species, molecular H2O and OH, which can be measured using Fourier Transform Infra Red spectroscopy (FTIR). Low temperature secondary hydration increases H2Om but does not alter OH. Using imaging FTIR analyses and a species-dependent H2Ot molar absorptivity coefficient (McIntosh et al 2017 Am. Mineral.) to overcome analytical issues relating to thin glasses, it is now possible to accurately measure OH concentrations and thus to investigate volcanic processes even in glasses affected by secondary hydration.

We present here an example of silicic tephra contained in the marine sedimentary core C9010E drilled by the D/V Chikyu in 2009 at a site ~40 km south of the Boso Peninsula. Geochemical analysis has identified the shallow submarine Oomurodashi volcano as the source of this tephra. Here we present FTIR H2O species concentrations for different size fractions from this tephra and use them to investigate eruption processes of this explosive shallow marine eruption. We highlight that similar FTIR volatile analysis has the potential to yield new insights into tephra deposits contained in marine sedimentary archives.