CHEMISTRY OF SHALLOW SUBMARINE WARM SPRINGS IN AN ARC-VOLCANIC SETTING - VULCANO ISLAND, AEOLIAN ARCHIPELAGO, ITALY

Results are presented from a geochemical survey of active submarine wa rm springs off the island of Vulcano, an active island-are volcanic ce ntre in the Aeolian Archipelago, Italy. Water-samples were collected f rom submarine 'fumaroles' which discharge hot (similar to 50-100 degre es C), acidic, gas-rich, low-chlorinity fluids into the shallow embaym ent of Porto di Levante, adding dissolved Si, K, Li, Rb, Fe, Mn, NH, a nd H2S into surrounding seawater. These fluids are interpreted as a mi xture of seawater and low-salinity groundwater which has undergone hig h-temperature (> 100 degrees C) hydrothermal alteration, followed by m ixing with cool seawater in the sub-seafloor and during venting. The f luid compositions also suggest the chemical 'overprint' of reactions r esulting from the input of significant amounts of the volcanic gases C O2, SO2 and H2S at this site, specifically, the attack of igneous sili cate phases by dissolved CO,, and the hydrolysis and oxidation of SO, and H2S to SO42-, H+ and elemental sulphur. These overprinting reactio ns have been proposed for other gas-rich submarine hydrothermal fluids collected from shallow island-are and hotspot volcanoes, and may be t ypical of such settings. Several water samples were also collected fro m a site where warm (< 30 degrees C) fluids seep from volcanic sand, a nd are enriched in dissolved Si, K, Li, Rb, Mg, Ca, and particularly F e and Mn relative to ambient seawater. These solutions are interpreted as the result of low-temperature (<100 degrees C) hydrothermal altera tion of seawater, again overprinted by the addition of acidic volcanic gases; the warm, acidic fluids then leach Si and metal cations, inclu ding Mg2+, from the volcanic sands. The elevated H2S concentrations an d low pH of 'ambient' embayment water relative to typical surface ocea n waters suggest that this type of hydrothermal activity significantly alters the redox and pH conditions of local seawater.