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Variations in the stable isotopic composition of seawater Sr ( δ  88 Sr) is a new tool for estimating the rates of global carbonate sedimentation over geologic time, yet the isotope compositions of the major sources and sinks of Sr to the world oceans are still in need of further constraint. We report  δ  88 Sr values of vent fluids from arc/back-arc seafloor hydrothermal systems in the western Pacific. In the sediment-starved hydrothermal fields of the Manus Basin, Izu-Bonin Arc, and Mariana Trough, the  δ  88 Sr values of end-member fluids for each site showed little variation (0.29–0.30‰) and were close to the average value of oceanic volcanic rocks, reflecting dissolved Sr sourced from host rocks. Chlorine-depleted fluids from phase-separated hydrothermal systems in the North Fiji Basin had the end-member  δ  88 Sr values of 0.26, 0.28, and 0.29‰. Thus, both sediment-starved and phase-separated vent fluids had the end-member  δ  88 Sr values indistinguishable from or very close to the range of oceanic volcanic rocks. Therefore, the  δ  88 Sr compositions in these hydrothermal sites are controlled predominantly by Sr sourced from host rock with a small influence from secondary mineral precipitation/re-dissolution. Fluids from the sediment-hosted hydrothermal fields of the Okinawa Trough, however, were characterized by low  δ  88 Sr values of approximately 0.22‰ and high  87 Sr/ 86 Sr ratios, indicating interactions with sedimentary carbonates. As for the modern oceanic  δ  88 Sr budget, the sediment-hosted sites lower the global hydrothermal  δ  88 Sr. Since both sediment-starved and -hosted hydrothermal systems provide a long-term control on the global Sr cycle, the end-member  δ  88 Sr value is an important constraint on the evolution of Sr cycling in past oceans.

frontiers in earth science

A Systematic Assessment of Stable Sr Isotopic Compositions of Vent Fluids in Arc/Back-Arc Hydrothermal Systems: Effects of Host Rock Type, Phase Separation, and Overlying Sediment