Hydrothermal precipitates collected from active hydrothermal sites in the northeastern Lau Basin: the Fonualei Spreading and Rift Center, Northeast Lau Speading Center, and an off-axis caldera

Abstract

Hydrothermal precipitates, including hydrothermal chimneys, sulfide-bearing altered rock and metal-bearing native sulfur, were 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) during the KODOS H-06 hydrothermal expedition. 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. At the central volcanic cone of the MTJ-1 caldera, unusual metal-bearing molten sulfur material as well as pieces of hydrothermally altered rocks with fine-grained pyrite was sampled. The molten sulfur material, composed of yellow and gray sulfur mixture, enveloped the frame and chain bag of the dredge sampler and showed plasticity after recovery of the dredge from the depth of ~1700 m. Fresh dacitic rock and volcanic breccia composed of native sulfur, black lapilli and ash, and alunite and silica were collected from same dredge. 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, hydrothermal precipitates showed remarkably low δ34S values (-8.2 to -1.8 per mil for native sulfur, -3.1 to +0.8 for stockwork sulfides, and +13.2 to 19.1 for natroalunite). The low sulfur isotope composition suggests that disproportionation of magmatic SO2 was introd