SULFUR ISOTOPE GEOCHEMISTRY OF HYDROTHERMAL PRECIPITATES FROM SUBMARINE HYDROTHERMAL SYSTEM IN THE NORTHEASTERN LAU BASIN: THE FONUALEI RIFT AND SPREADING CENTER, NORTHEASTERN LAU SPREADING CENTER, AND AN OFF-AXIS CALDERA

Sulfur isotope composition was measured for hydrothermal chimneys and native sulfur precipitates collected from three active hydrothermal sites in the northeastern Lau Basin: the Fonualei Rift and Spreading Center (FRSC), Northeast Lau Spreading Center (NELSC), and an off-axis caldera (MTJ-1). Sulfide-rich and sulfate-rich chimneys were collected from the FRSC hydrothermal site. Hydrothermal chimneys sampled from the NELSC site mainly composed of chalcopyrite and well-crystallized pyrite that is commonly precipitated under high temperature condition. No chimney was sampled at the central volcanic cone of the MTJ-1 caldera. Instead, pieces of newly formed native sulfur precipitates and hydrothermally altered rocks with fine-grained pyrite were collected from the central volcanic cone. The δ³⁴S values of the hydrothermal chimneys are -0.5 to +2.0 per mil for the FRSC sulfides, +20.3 to 22.4 per mil for the FRSC sulfates, and -1.0 to +0.3 per mil for the NELSC sulfides. Compared with hydrothermal chimneys of other arc and back-arc environments, the hydrothermal chimneys from the FRSC and NELSC have slightly low sulfur isotope composition. But the observed δ³⁴S values of the FRSC and NELSC chimneys are not out of the range of sulfur isotope composition by mixing of seawater and surrounding rock. On the other hand, at the MTJ-1 caldera, sulfur compounds within lava and hydrothermally altered rocks have remarkably low δ³⁴S values (-8.1 to -1.8 per mil for native sulfur, -3.1 to +0.8 for stockwork sulfides, and +13.2 to 19.1 for natroalunite). This result suggests that disproportionation of magmatic SO₂ was introduced into hydrothermal system at the MTJ-1 caldera. The magmatic contribution to hydrothermal system has been widely proposed for arc and back-arc systems, and thus may be typical of subduction-related environments.