Intermediate products of sulfur disproportional reaction and their physical role in effusive to explosive submarine volcanic activity

Recent direct observations of submarine volcanic activity in the Mariana Arc are giving us a chance to examine the role of volcanic gas in submarine volcanic conduits. Unlike subaerial volcanoes, where hydrogeologic conditions have different character from place to place, the overlying water mass above submarine volcanoes gives a uniform hydrographic setting. Currently, the places where we can directly observe submarine volcanic activity are located deeper than 400 m, which raises the boiling point of seawater to over 240 deg C. This situation allows us to examine the interaction of volcanic gases with ambient seawater at a shorter distance from the magma source than at subaerial volcanic settings. Arc volcano settings give us longer and more frequent opportunities to make observations and provide a more diverse range of submarine volcanism than ridge settings. Among the three major components of volcanic gases (i.e., H2O, CO2 and SO2), water follows a two phase boundary below the critical temperature after volatile components leave from the magmatic source. Milky sulfur sol bearing hydrothermal fluid is commonly observed throughout Mariana active sites. Most of the sulfur sol (colloidal elemental sulfur and polysulfides) might be formed by disproportional reaction of sulfur dioxide with seawater when water vapor shrinks to liquid water. The reaction creates not only sulfur sol but also various types of sulfite, which affects the pH of seawater. We detected short-lived sulfite species in the water column above several active Mariana volcanoes such as NW Rota-1, Daikoku and Nikko by on-board HPLC. Because most observations are made on the liquid phase side of H2O boundary, it is very hard to get data to investigate the physical and chemical sulfur sol forming process occurring on the vapor phase side or at the critical state (i.e., near the magma source process). Carbon dioxide behaves as a gas at a wide range of pressures and temperatures and carries heat and sulfur dioxide effectively and quickly to the seafloor. At Nikko Seamount carbon dioxide bubbles penetrated the intentionally excavated liquid sulfur pond beneath a hydrothermal vent. The bubbles have misty surfaces, which indicated progressive disproportional reaction of carried sulfur dioxide with ambient seawater. Although the temperature along most of the conduit up to the seafloor is above the freezing point of elemental sulfur, coalescence of sulfur sol, which creates masses of liquid sulfur, mostly occur when the mixture of hydrothermal fluid and volcanic gas leaves the two phase boundary of water near the seafloor. The polymerization state of liquid sulfur governs the resistance against volcanic gas flow near the surface of volcanic conduits. Several types of liquid sulfur spherules were sampled at NW Rota-1 eruption site.