table1_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.xlsx

Variations in the stable isotopic composition of seawater Sr (δ88Sr) 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 δ88Sr 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 δ88Sr 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 δ88Sr values of 0.26, 0.28, and 0.29‰. Thus, both sediment-starved and phase-separated vent fluids had the end-member δ88Sr values indistinguishable from or very close to the range of oceanic volcanic rocks. Therefore, the δ88Sr 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 δ88Sr values of approximately 0.22‰ and high 87Sr/86Sr ratios, indicating interactions with sedimentary carbonates. As for the modern oceanic δ88Sr budget, the sediment-hosted sites lower the global hydrothermal δ88Sr. Since both sediment-starved and -hosted hydrothermal systems provide a long-term control on the global Sr cycle, the end-member δ88Sr value is an important constraint on the evolution of Sr cycling in past oceans.

海水锶的稳定同位素组成（δ88Sr）的变化是估算地质时间尺度上全球碳酸盐沉积速率的新兴工具，然而目前全球海洋锶的主要源与汇的同位素组成仍需进一步约束。本研究报道了西太平洋弧/弧后海底热液系统喷口流体的δ88Sr值。在马努斯盆地、伊豆-小笠原弧及马里亚纳海槽的沉积物匮乏热液区中，各站位的端元流体δ88Sr值变化极小（0.29–0.30‰），且接近大洋火山岩的平均数值，反映出溶解锶源自宿主岩石。北斐济盆地相分离热液系统产出的贫氯流体，其端元δ88Sr值分别为0.26‰、0.28‰与0.29‰。综上，无论是沉积物匮乏还是相分离的喷口流体，其端元δ88Sr值均与大洋火山岩的数值范围无法区分或极为接近。因此，这些热液区的δ88Sr组成主要受控于宿主岩石来源的锶，仅受次生矿物沉淀/再溶解作用的微弱影响。然而，冲绳海槽赋存沉积物的热液区流体具有低δ88Sr值（约0.22‰）与高87Sr/86Sr比值的特征，表明其与沉积碳酸盐发生了相互作用。就现代海洋δ88Sr收支而言，赋存沉积物的热液位点会降低全球热液流体的δ88Sr值。由于沉积物匮乏与赋存沉积物的热液系统均对全球锶循环具有长期调控作用，端元δ88Sr值是约束过去海洋锶循环演化的重要参数。