S, SR, AND PB ISOTOPIC SYSTEMATICS OF HYDROTHERMAL CHIMNEY FROM THE EASTERN MANUS BACK-ARC BASIN, WESTERN PACIFIC: EVALUATION OF MAGMATIC CONTRIBUTION TO HYDROTHERMAL SYSTEM

Sulfur, strontium, and lead isotopic analyses were performed for chimney samples from the hydrothermal vent fields in the eastern Manus back-arc basin. Chimneys can be classified into sphalerite dominant Zn-rich type from the PACMANUS and chalcopyrite dominant Cu-rich type from the Susu knolls based on their dominant mineral assemblage. Lead isotope composition varies in a narrow range (²⁰⁶Pb/²⁰⁴Pb=18.75 to 18.78, ²⁰⁷Pb/²⁰⁴Pb=15.51 to 15.54, and ²⁰⁸Pb/²⁰⁴Pb=38.31 to 38.40), in which the Susu knolls samples show slightly more radiogenic ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values than the PACMANUS samples. Strontium isotopic ratios (⁸⁷Sr/⁸⁶Sr) of sulfate minerals vary from 0.7051 to 0.7077, lying between those of seawater (0.7090) and the basement rock (0.7036). d³⁴S values of sulfide and sulfate samples vary from -8.0 to +4.3 per mil and from +14.8 to +20.4 per mil, respectively. The d³⁴S values of sulfides (-8.0 to -3.9 per mil) of Cu-rich type chimney are the lowest values so far reported for volcanic-hosted massive sulfides from modern seafloor. The low d³⁴S values of sulfates and sulfides cannot be explained by simple seawater circulation model without additional source for light sulfur. It is also supported by the presence of sulfate in hydrothermal end-member fluid indicated by the mixing relationship between sulfur and strontium isotopic ratios in chimney sulfates. Biogenic sulfur and/or oxidation of H₂S after boiling are excluded from the possible sources of light sulfur because of lacking of sediment cover and discordance between calculated and observed d³⁴S values, respectively. Disproportionation of magmatic SO₂ introduced into hydrothermal system through preeruptive degassing is the most plausible mechanism for the observed light sulfur isotope composition of the studied chimney samples. The lighter d³⁴S values and more radiogenic lead isotope compositions of the Susu knolls samples indicate that the magmatic degassing was more vigorously in the Susu knolls than in the PACMANUS. More systematic studies, however, are required to resolve this question.