Hydrothermal Activity and its Chemical Characteristics in the NE Lau Basin

The NE Lau Basin is a magmatically robust area with an abundance of hydrothermal activity. We conducted exploratory research to the NE Lau Basin during three cruises to the area in November 2008, May 2009, and May 2010. We have found an unusual density of hydrothermal activity in the area bounded by the NE Lau Spreading Center (NELSC) and the Tongan Magmatic arc. Aside from the magmatic activity at the NELSC and the Tonga Arc, this area includes a area of crustal extension, where nine elongate volcanoes (The Matas) lie in a 25km arc extending into the Tonga trench with the summits from ∼1200m to ∼ 2700m depth and a large volcanic feature (Volcano “O”) which is characterized by a caldera >10km diameter with an emergent dome in its SE quadrant. Finally, the basin bounded by the Matas, the NELSC, and Volcano “O” contain many large lava flows with elevated acoustic backscatter suggesting a relatively young age. During the cruise in 2008, two ongoing eruptions were encountered in the region, one at the NELSC and another at W Mata volcano. These eruptions were later confirmed using the Jason II submersible in 2009. W Mata has been continuously erupting over the course of our observations. In addition to the eruption on the NELSC, hydrothermal activity was also observed at Maka and Tafu volcanoes, which are south and north of the eruptive area respectively. Observations of hydrothermal activity at two sites were made within the caldera at Volcano “O” and on the volcanic arc at two depths on Niua Volcano. The cruise in May 2010, revealed dense hydrothermal activity along the Mata chain where eight of the nine Mata volcanoes were hydrothermally active, including W Mata. Hydrothermal activity in this region is very sulfur rich as documented by large amounts of elemental sulfur at “O”, Niua, and seven of the Matas (elemental sulfur is inferred from light scatter and particulate matter color; analyses are pending), by acidic magmatic volatiles at”O” and the erupting W Mata, and non-volatile (sulfide and sulfate) S on the NELSC. Except for the plume above W Mata, the hydrothermal plumes above the other Matas are rich in CH4 which generally ranges from 40-50nM in the plume maxima to > 250nM in the maxima at Matas Ua and Fitu. In 2008, elevated CH4 was also found above the eruptive site along the NELSC. CH4 was low at W Mata, O, Niua. Fe and Mn were≥1000 nM and Mn ≥ 350 nM respectively at Niua, O, W Mata, and above the NELSC during the eruption. (Fe and Mn from the N Matas will be analyzed and presented). Magmatic gasses were abundant in the plumes at Niua, E Mata, the NELSC, and O (2008), however they were highly enriched at O in 2010 and at W. Mata in 2008 and 2010. By comparison, the N Matas were fairly gas poor (indicated from pH data). Chemistry (Fe, Mn, Al, S, CO2,pH,CH4, H2 , and 3He) at each site will be examined in detail and related to temporal and spatial setting of each volcano.