Chemical Characteristics of Plumes above and Around NW-Rota Submarine Volcano: Before and During the March 2004 Eruption

During the March 2004 Ocean Exploration cruise to the Mariana Arc, a small-scale volcanic eruption was observed at the NW Rota-1 submarine volcano by the Canadian ROV ROPOS. CTD tows and casts were conducted above and around the seamount to sample the water for dissolved and particulate chemical species during the eruption. The water column around NW Rota-1 was also sampled in March 2003, providing a baseline for evaluating eruption-induced changes in the chemical composition of the seawater. These investigations mark the first time that water around an erupting underwater volcano has been sampled before and during an eruption. During the 2003 cruise, only an above-summit plume between 400-600 m was observed originating from NW Rota-1. This plume was distinctly hydrothermal, with high values of iron oxides and its associated oxyhdroxides, elemental sulfur, and aluminum in the form of natroalunite. In addition, the natroalunite co-occurs with a silica phase that suggests these Al-rich particles must have formed within the hydrothermal system where magmatic SO2 attacked the host rocks. During the 2004 cruise, prominent plumes were detected both around the flanks and above the summit. The flank plume completely surrounded the volcano from about 700 m to the seafloor (at least 2000 m). Bulk XRF analysis and SEM particle imagery show this plume to consist primarily of glass fragments. This plume may be caused by earthquake tremor associated with the eruption, causing resuspension of eruption debris. The summit plume consisted of a mixture of minerals similar to those sampled during the 2003 cruise, including highly elevated levels of Fe consistent with fresh lava/water interactions and/or release of subsurface rock that has previously interacted with SO2.