Hydrothermal Helium Plumes over Submarine Volcanoes of the Marianas Arc

During February-March, 2003, as part of the Submarine Ring of Fire project funded by NOAA's Ocean Exploration Program, the R/V T.G. Thompson conducted a comprehensive survey of hydrothermal activity along 1200 km of the Mariana Arc from 13.5° N to 22.5° N [see Embley et al., EOS Trans. AGU, 2003]. Plume surveys were conducted in the water-column above ~50 submarine volcanoes using a CTD/rosette system. A total of 70 CTD casts were completed, and discrete water samples were collected for analysis of a variety of hydrothermal tracers, including ³He, CH₄, CO₂, H₂S, Fe, Mn, pH, and suspended particles. Although shorebased analysis of the samples is still underway, preliminary results indicate that about 11 of the 50 submarine volcanoes surveyed are hydrothermally active. Because many of the Marianas Arc volcanoes rise to within 500 m of the sea surface, hydrothermal plume signals such as light attenuation (suspended particles) and temperature anomaly have limited utility due to masking by near surface effects. For this reason ³He, an unambiguous hydrothermal tracer, has been particularly useful for identifying which of the shallow arc volcanoes are hydrothermally active. Our expectation was that the water-column helium signal might be reduced at shallow depths due to ventilation into the atmosphere. However, we observed very high ³He enrichments at shallow depths both at Maug Islands and at NW Rota #1 (14° 36'N; 144° 46.5'E). The ³He enrichments were strongly correlated with changes in pH, Mn, and other hydrothermal tracers. The three Maug Islands mark the perimeter of a caldera formed by an explosive eruption, and a single hydrocast in the center of the caldera detected a robust helium plume at 120-200 m depth with δ ³He reaching a maximum of 250% at 150m depth. Analysis of the co-variation of [³He] vs. [⁴He] at Maug gave R/R_a = 6.6 for an estimate of the end-member helium isotope ratio (R = ³He/⁴He and R_a = R_air). This value falls well within the range of R/R_a = 5-7 generally observed for helium at subduction zone volcanic systems. At NW Rota #1, we found a strong helium plume reaching a maximum δ ³He = 320% at 460 m depth. Surprisingly, the estimate for the end-member ³He /⁴He at NW Rota gave R/R_a = 8.4, outside the range normally found at subduction zones and similar to that observed along MOR spreading centers. NW Rota #1 is located about 15 km west of the main arc and is part of a cross-arc volcanic chain. Craig et al. [EOS 68, No. 44, p.1531, 1987] found a similar value of R/R_a = 8.6 in vent fluids in the Mariana Trough back-arc system at 18° 13'N. Thus the higher ³He/⁴He ratio at NW Rota may reflect an affinity to the back-arc spreading center as opposed to a pure arc component.