Volatile Contents of NW Rota Melt Inclusions: Insight to Explosive Submarine Arc Volcanism

NW Rota Seamount, located in the Southern Seamount Province of the Mariana arc system, provides a unique opportunity to examine an actively erupting submarine arc volcano. Vigorous degassing of SO2 and explosive eruptions of lava fragments at NW Rota's main vent, Brimstone Pit, emphasize the importance of volatiles in driving arc volcanism. In October 2005, we collected fresh volcaniclastic material and lavas from the main vent (550 m water depth), as well as from the surrounding parasitic cones using JAMSTEC's remotely operated vehicle, Hyper-Dolphin. Through melt inclusion studies, we can directly evaluate the role of volatiles in generating NW Rota melts and test whether they correlate with trace element proxies for slab fluids. We have analysed volatiles (H2O, CO2, F, S and Cl), major and trace elements for olivine hosted melt inclusions using SIMS and EMP techniques at the Carnegie Institution of Washington. S contents of melt inclusions are similar to values found at other sites along the arc (up to 1940 ppm) and thus the observed SO2 emissions simply reflect the solubility of S which is effectively exsolved at NW Rota's depth of eruption (rather than a particularly S-rich system). H2O and CO2 contents of melts at the main vent site range from 1.1-3.5 wt % and 5-460 ppm, respectively. Melts preserved in olivines from volcaniclastics and lavas can be explained by closed system degassing from a common source. However, melts from the parasitic cone to the east of the main vent have significantly higher water contents (3.5-5.8 wt %) as high as melts from Agrigan volcano (5.8 wt %), which is the highest among the Mariana arc volcanoes and thought to represent the sediment endmember of the arc. Ba/La (~20) and La/Yb (~4-6) ratios of the parasitic cone and the main vent are indistinguishable, suggesting similar slab fluid contributions and degrees of partial melting. On the basis of higher Mg values and higher volatile contents, we consider the parasitic cone melts to be the least influenced by fractional crystallization and degassing thus, most representative of the underlying source. The water content of the mantle source feeding the NW Rota eruption is therefore among the highest observed along the entire arc system. Intriguingly, Ba/La ratios are among the lowest, suggesting a need to re-evaluate the use of common trace element proxies for slab fluids.