High SO2 flux, sulfur accumulation, and gas fractionation at an erupting submarine volcano.

Strombolian-style volcanic activity has persisted for six years at the NW Rota-1 submarine volcano in the southern Mariana Arc, allowing direct observation and sampling of gas-rich fluids produced by actively degassing lavas, and permitting study of the magma-hydrothermal transition zone. Fluids sampled centimeters above erupting lava and percolating through volcaniclastic sediments around an active vent have dissolved sulfite >100 mmol/kg, total dissolved sulfide <30 μmol/kg, pH as low as 1.05, and dissolved Al and Fe >1 mmol/kg. If NW Rota is representative of submarine arc eruptions, then volcanic vent fluids from seawater-lava interaction on submarine arcs have a significant impact on the global hydrothermal flux of sulfur and Al to the oceans, but a minimal impact on Mg removal. Gas ratios (SO₂, CO₂, H₂, and He) are variable on small spatial and temporal scales, indicative of solubility fractionation and gas scrubbing. Elemental sulfur (S_e) is abundant in solid and molten form, produced primarily by disproportionation of magmatic SO₂ injected into seawater. S_e accumulates within the porous rock surrounding the lava conduit connecting the magma source to the seafloor. Accumulated S_e can be heated, melted, and pushed upward by rising magma to produce molten S_e flows and lavas saturated with S_e. Molten S_e near the top of the lava conduit may be ejected up into the water column by escaping gases or boiling water. This mechanism of S_e accumulation and refluxing may underlie the relatively widespread occurrence of S_e deposits of many sizes found on submarine arc volcanoes.