The Chemistry of Hydrothermal venting at a Volcano "O", a large Submarine Volcano in the NE Lau Basin

Volcano O is located in the NE Lau basin in a highly extensional region between the Tonga arc and back arc. The NE Lau basin has the highest subduction rates on Earth [Bevis et al., 1995] and is Earth's fastest-opening back-arc basin [Zellmer and Taylor, 2001]. The NE Lau Basin contains abundant recent submarine volcanism with magma production likely driven by water released from the subducting slab and decompression associated with crustal extension. One manifestation of this abundant volcanism is Volcano O, one of the largest discrete active-submarine volcanoes on Earth. It is hard to classify this volcano is either arc or back arc because its location behind the magmatic arc is inconsistent with the eruption of dacitic lavas and highly acidic hydrothermal activity reported here. Volcano O has exhibited ongoing hydrothermal activity as documented in 2004 (Lupton pers. Comm.), 2006 [Kim et al., 2009], and in both 2008 and 2010 as presented here. In 2008 and 2010, we documented intense hydrothermal plumes in several locations in the caldera. The most intense hydrothermal plume was observed in 2010 directly above a small cone on the eastern side of the caldera. A camera tow across the cone revealed fresh blocky lavas with a hint of sulfur-rich waters surrounding the cone. Here we present data on the chemistry of hydrothermal plumes at Volcano O. Of particular interest is a hydrocast conducted directly above the cone. During this hydrocast we collected fluids rich in Fe (≤13,000nM), Mn (≤665nM), particulate Al (≤1500nM), particulate sulfur (≤18000nM), and with large decreases in pH (≤0.9 pH units). These anomalies are extraordinarily large and are much greater than the Fe and Mn levels observed by Kim et al. These chemical enrichments and depletions strongly suggest that Volcano O is actively degassing SO2 which, in turn, suggests that the magma chamber must be fairly shallow. This may indicate an active state of cone building at Volcano O. In addition to the above data, we will also present data on CO2 and 3He in the plumes to better constrain the source of the magmatic volatiles feeding the volcano