Abstract Mid-ocean ridge (MOR) hydrothermal vent fluids are enriched with dissolved barium, but due to barite (BaSO4) precipitation during mixing between Ba-bearing vent fluids and SO4-bearing seawater, the magnitude of hydrothermal Ba input to the ocean remains uncertain. Deep-ocean Ba isotopes show evidence for non-conservative behavior, which might be explained by input of isotopically heavy hydrothermal Ba. In this study we present the first Ba isotope data in mid-ocean ridge hydrothermal vent fluids and particles from systems on the Mid-Atlantic Ridge (Rainbow 36°N and TAG 26°N), the East Pacific Rise (EPR9–10°N and 13°N) and the Juan de Fuca Ridge (MEF and ASHES). The vent fluids display a wide range of dissolved Ba concentrations from 0.43 to 97.9 μmol/kg and δ138/134Ba values from −0.26 to +0.91‰, but are modified relative to initial composition due to precipitation of barite. Calculated endmember vent fluid δ138/134Ba values, prior to barite precipitation, are between −0.17 and +0.09‰, consistent with the values observed in oceanic basalts and pelagic sediments. Water-rock interaction at depth in the oceanic crust appears to occur without Ba isotope fractionation. During subsequent venting and mixing with seawater, barite precipitation preferentially removes isotopically light Ba from vent fluids with a fractionation factor of Δ138/134Bahyd-barite-fluid = −0.35 ± 0.10‰ (2SE, n = 2). Based on knowledge of barite saturation and isotope fractionation during precipitation, the effective hydrothermal Ba component that mixes with seawater after barite precipitation has completed can be calculated: δ138/134Bahyd = +1.7 ± 0.7‰ (2SD). This value is isotopically heavier than deep ocean waters and may explain the observed non-conservative of Ba isotopes in deep waters. These new constraints on hydrothermal Ba compositions enable the hydrothermal input of Ba to Atlantic deep waters to be assessed at ≈3–9% of the observed Ba. Barium isotopes might be used as a tracer to reconstruct the history of hydrothermal Ba inputs and seawater SO4 concentrations in the past.

中文翻译：

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大洋中脊热液喷口流体中的钡同位素：同位素重钡到海洋的来源

摘要 洋中脊（MOR）热液喷口流体富含溶解的钡，但由于含钡喷口流体与含二氧化硫海水混合过程中重晶石（BaSO4）沉淀，热液Ba输入海洋的量级仍不确定. 深海 Ba 同位素显示出非保守行为的证据，这可能是通过输入同位素重热液 Ba 来解释的。在这项研究中，我们展示了大西洋中脊（彩虹 36°N 和 TAG 26°N）、东太平洋海隆（EPR9-10°N）系统中洋中脊热液喷口流体和颗粒的第一个 Ba 同位素数据和 13°N) 和 Juan de Fuca Ridge（MEF 和 ASHES）。排放流体显示的溶解 Ba 浓度范围很广，从 0.43 到 97.9 μmol/kg，δ138/134Ba 值从 -0.26 到 +0.91‰，但由于重晶石的沉淀而相对于初始成分发生了变化。在重晶石沉淀之前，计算的端元喷口流体 δ138/134Ba 值介于 -0.17 到 +0.09‰ 之间，与在大洋玄武岩和远洋沉积物中观察到的值一致。大洋地壳深处的水-岩相互作用似乎在没有 Ba 同位素分馏的情况下发生。在随后的排放和与海水混合期间，重晶石沉淀优先从排放流体中去除同位素轻 Ba，分馏因子为 Δ138/134Bahyd-barite-fluid = -0.35 ± 0.10‰ (2SE, n = 2)。根据沉淀过程中重晶石饱和度和同位素分馏的知识，可以计算出重晶石沉淀完成后与海水混合的有效热液Ba组分：δ138/134Bahyd = +1.7±0.7‰（2SD）。该值在同位素上比深海水域重，可以解释在深水中观察到的 Ba 同位素的非保守性。这些对热液 Ba 成分的新限制使得 Ba 到大西洋深水的热液输入被评估为观察到的 Ba 的 ≈3-9%。钡同位素可用作示踪剂，以重建过去热液 Ba 输入和海水 SO4 浓度的历史。