Sulfur Isotope Study of Seafloor Hydrothermal Vent Precipitates from Southernmost Tonga Arc Volcanoes, Southwest Pacific

Abstract

Multiple sulfur isotopes (32S, 33S, 34S) in two submarine volcanoes at southernmost Tonga arc, two hydrothermal vent sites from volcano 19 and a hydrothermal field from volcano 18, were measured by continuous flow-isotope ratio mass spectrometer. In volcano 19, the δ34S values for sulfide are between -10.5 and -3.0 ‰ with Δ33S values from +0.14 to +0.51 ‰ and sulfates display δ34S values from +19.5 to +21.3 ‰ and Δ33S values between +0.45 and +1.00 ‰. The negative δ34S values of sulfide cannot be explained by simple mixing of seawater sulfate and host rock sulfide, suggesting that entire summit area of volcano 19 is pervaded by boiling. Sulfides from sediment covered site have more depleted δ34S values with higher Δ33S than another site where there is no or little sediment, reflecting biogenic contribution of sulfur. Volcano 18 has sulfides ranging δ34S values between +2.0 and +2.6 ‰ with Δ33S values from +0.18 to -0.09 ‰ and sulfates are δ34S values ranging from +21.7 to +23.2 ‰ and Δ33S values from +0.45 to +1.09 ‰. It is because extremely crater forming eruption may supply magmatic sulfur into hydrothermal system. However, the magmatic sulfur signature in hydrothermal fluid might be masked by dilution with large amount of seawater.