Calcium isotope systematics at hydrothermal conditions: Mid-ocean ridge vent fluids and experiments in the CaSO4-NaCl-H2O system
Calcium isotope systematics at hydrothermal conditions: Mid-ocean ridge vent fluids and experiments in the CaSO4-NaCl-H2O system
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DOI：
10.1016/j.gca.2018.01.028
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发表时间：
2018-04
影响因子：
5
通讯作者：
Peter P. Scheuermann;D. Syverson;J. Higgins;N. Pester;W. Seyfried
中科院分区：
文献类型：
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作者：
Peter P. Scheuermann;D. Syverson;J. Higgins;N. Pester;W. Seyfried
进行了两组水热实验，以探索水热条件（410–450°C，31.0–50.0MPa）下的Ca同位素分馏和交换率。第一组实验确定了 CaSO4-NaCl-H2O 体系中汽液 Ca 同位素分馏的程度和硬石膏溶解度。数据表明共存蒸气和液体的 Ca 同位素组成之间没有统计差异。第二组实验利用异常 43Ca 尖峰来确定流体和硬石膏之间的 Ca 交换速率，作为总溶解 Ca 浓度的函数。结果表明，交换率随溶解 Ca 浓度 (12–23 mM/kg) 的增加而增加，但当 Ca 浓度从 23 mM/kg Ca 增加到 44 mM/kg Ca 时，没有观察到交换率的变化。在所研究的条件下，需要74-142天才能实现90%的硬石膏-流体Ca同位素交换，而气-液同位素平衡只需要几个小时。我们的实验中缺乏汽液 Ca 同位素分馏，这与洋中脊热液喷口流体的 δ44Ca 保持恒定（无论氯度如何）一致。此外，端元流体δ44Ca的狭窄范围，-0.98至-1.13‰（SW），在很大程度上与MORB δ44Ca无法区分，这表明在现代高温洋中脊热液系统中，深部的相分离和流体-岩石相互作用都不会显着分馏Ca同位素。
Two sets of hydrothermal experiments were performed to explore Ca isotope fractionation and exchange rates at hydrothermal conditions (410–450 °C, 31.0–50.0 MPa). The first set of experiments determined the magnitude of vapor-liquid Ca isotope fractionation and anhydrite solubility in the CaSO4-NaCl-H2O system. The data indicate no statistical difference between the Ca isotopic composition of coexisting vapor and liquid. The second set of experiments utilized an anomalous43Ca spike to determine the rate of Ca exchange between fluid and anhydrite as a function of total dissolved Ca concentration. Results show that the rate of exchange increases with dissolved Ca concentrations (12–23 mM/kg), but no change in exchange rate is observed when the Ca concentration increases from 23 to 44 mM/kg Ca. 74–142 days are required to achieve 90% anhydrite-fluid Ca isotope exchange at the conditions investigated, while only several hours are necessary for vapor-liquid isotopic equilibrium. The lack of vapor-liquid Ca isotope fractionation in our experiments is consistent with δ44Ca of mid-ocean ridge hydrothermal vent fluids that remain constant, regardless of chlorinity. Moreover, the narrow range of end member fluid δ44Ca, −0.98 to −1.13‰ (SW), is largely indistinguishable from MORB δ44Ca, suggesting that neither phase separation nor fluid-rock interactions at depth significantly fractionate Ca isotopes in modern high-temperature mid-ocean ridge hydrothermal systems.