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
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
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DOI：
10.3389/feart.2020.591711
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发表时间：
2020-12
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影响因子：
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通讯作者：
T. Yoshimura;S. Wakaki;T. Ishikawa;T. Gamo;D. Araoka;N. Ohkouchi;H. Kawahata
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文献类型：
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作者：
T. Yoshimura;S. Wakaki;T. Ishikawa;T. Gamo;D. Araoka;N. Ohkouchi;H. Kawahata
海水Sr（δ 88 Sr）稳定同位素组成的变化是估算全球碳酸盐沉积速率的新工具，但世界海洋Sr主要来源和汇的同位素组成仍需进一步约束。本文报道了西太平洋弧/弧后海底热液系统喷口流体的δ 88 Sr值。在马努斯盆地、伊豆-小笠原弧和马里亚纳海槽的贫沉积热液场中，各测点的端员流体δ 88 Sr值变化不大（0.29-0.30‰），接近大洋火山岩的平均值，反映了溶解Sr来源于容矿岩石。来自北斐济盆地相分离热液系统的贫氯流体的端元δ 88 Sr值分别为0.26、0.28和0.29‰。因此，无论是贫沉积物还是相分离的喷口流体，其端元δ 88 Sr值与大洋火山岩的范围没有区别或非常接近。因此，这些热液场所的δ 88 Sr组成主要受来源于寄主岩石的Sr控制，次生矿物沉淀/再溶解的影响较小。然而，来自冲绳海槽沉积物热液区的流体具有低的δ 88 Sr值（约0.22‰）和高的87 Sr/86 Sr比值，表明与沉积碳酸盐的相互作用。对于现代海洋δ 88 Sr收支，沉积物容矿地点降低了全球热液δ 88 Sr。由于缺乏沉积物的热液系统和富沉积物的热液系统都对全球Sr循环具有长期的控制作用，因此端元δ 88 Sr值是过去海洋Sr循环演化的重要制约因素。
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.