Abstract
The 2012 rhyolitic submarine eruption of Havre volcano occurred in water depths of 700-1300 m. The eruption produced a sea surface pumice raft in addition to effusive and clastic sea floor deposits. Post-eruption AUV mapping along with ROV observations and sampling make Havre an ideal laboratory for studying water's effect on subaqueous eruption processes. Here we present high spatial resolution synchrotron-FTIR measurements of water speciation in glassy ash (125-500 mu m) from subunits 1, 2 and 3 of the 2012 eruption's widespread Ash with Lapilli deposit. Measurements record depletion profiles of OH around vesicles caused by degassing with half distances of 1 to 6 mu m. Enrichment profiles in H2Omol are often found around the same vesicles with similar half distances. In several thick vesicle walls in ash grains far-field regions unaffected by processes at the bubble margins have also been identified. These features are common across ash of different morphologies and in each subunit. Knowing water's pressure dependent solubility in magma, that OH is a non-diffusing species below the glass transition, and the high cooling rates of ash in water, we can derive a precise record of syn-eruptive exsolution conditions by measuring water species. Far field OH concentrations of 0.86 to 0.94 wt% equate to quench pressures of between 7.8 to 9.7 MPa, appropriate for the inferred hydrostatic vent pressure of approximately 9 MPa, suggesting ash grains equilibrated at vent depth. The OH depletion profiles around vesicles indicate a subsequent phase of volatile exsolution prior to quenching driven by decompression of at least 2-4 MPa. Diffusion modelling suggests that at eruption temperature the observed profiles formed through exsolution in <2s. We will discuss the implications our findings have for using dissolved volatile contents to interpret submarine volcanism more broadly, including the formation of low-pressure signature during deep sea volcanism.