Experimental partitioning of Ca isotopes and Sr into anhydrite: Consequences for the cycling of Ca and Sr in subseafloor mid-ocean ridge hydrothermal systems
Experimental partitioning of Ca isotopes and Sr into anhydrite: Consequences for the cycling of Ca and Sr in subseafloor mid-ocean ridge hydrothermal systems
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DOI：
10.1016/j.gca.2018.03.018
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发表时间：
2018-09
影响因子：
5
通讯作者：
D. Syverson;D. Syverson;D. Syverson;Peter P. Scheuermann;J. Higgins;N. Pester;W. Seyfried
中科院分区：
文献类型：
--
作者：
D. Syverson;D. Syverson;D. Syverson;Peter P. Scheuermann;J. Higgins;N. Pester;W. Seyfried
大洋中脊（MOR）热液系统海水与洋壳之间的 Ca 和 Sr 元素和同位素质量平衡整合了海底的各种物理化学过程，例如原生硅酸盐矿物的溶解、次生矿物的形成以及海底的相分离。特别是，硬石膏的沉淀和重结晶被认为是控制高温喷口流体和共存洋壳的 Ca 和 Sr 元素和同位素组成的重要过程，然而，很少有实验数据来约束这些关键地球化学效应的机制和程度。因此，本研究通过实验检验了 Sr/Ca 分配、Ca 同位素分馏以及硬石膏和溶解成分之间的交换率。然后将这些实验限制与从 TAG 热液系统取样的硬石膏和喷口流体的 Sr/Ca 和 Ca 同位素组成进行比较。因此，硬石膏沉淀和重结晶实验在 175、250 和 350°C、500bar 和活性 MOR 热液系统特有的化学条件下进行。实验数据表明，在将海水夹带并补给至 MOR 热液系统后，硬石膏将迅速沉淀，其中的 Ca 同位素成分与热液相比，重同位素有所减少。 Ca 同位素分馏的大小 Δ44/40Ca(Anh-Fluid) 与温度相关，对于 175、250 和 350°C 分别为 -0.45、-0.22 和 -0.02‰，但可能表明动力学效应。利用溶液中的 a43Ca 加标来量化化学平衡下硬石膏重结晶过程中同位素交换的时间依赖性程度。这些数据表明交换率是温度的函数，其中在 175°C、250°C 和 350°C 下分别在 1322、867、366 小时内发生 12%、46% 和 45% 的交换。沉淀过程中硬石膏和成分溶解物质之间 Sr/Ca 的分配很大程度上取决于每个实验温度下热液相对于硬石膏的饱和状态，175-350°C 下 KD(Anh-Fluid)=1.24-0.55，与 Shikazono 和 Holland (1983) 早期实验观察的结果大致相似。通过明确考虑时间相关的交换幅度来估计平衡 KD（Anh-Fluid）值，在 175、250 和 350°C 时分别产生 0.43、0.36、0.29 的值。将这些实验限制与高温 MOR 热液系统推断的温度梯度结合起来表明，海底附近形成的硬石膏的 Ca 同位素和 Sr 元素组成将保留在初始形成条件下得出的组成，这表明不平衡。相反，在更大的深度和更高的温度下，硬石膏将反映接近平衡的 Sr/Ca 分配和 Ca 同位素分馏条件。本研究提供的实验和自然数据可用于进一步了解洋壳热液循环过程中硬石膏沉淀的影响以及地质时间尺度上海水化学和同位素组成的影响。
The elemental and isotopic mass balance of Ca and Sr between seawater and the oceanic crust at mid-ocean ridge (MOR) hydrothermal systems integrates various physiochemical processes in the subseafloor, such as dissolution of primary silicate minerals, formation of secondary minerals, and phase separation in the subseafloor. In particular, the precipitation and recrystallization of anhydrite are recognized as important processes controlling the Ca and Sr elemental and isotope composition of high temperature vent fluids and coexisting ocean crust, and yet, little experimental data exist to constrain the mechanism and magnitude of these critical geochemical effects. Thus, this study experimentally examines Sr/Ca partitioning, Ca isotope fractionation, and the rate of exchange between anhydrite and dissolved constituents. These experimental constraints are then compared with Sr/Ca and Ca isotope compositions of anhydrite and vent fluids sampled from the TAG hydrothermal system. Accordingly, anhydrite precipitation and recrystallization experiments were performed at 175, 250, and 350 °C and 500 bar at chemical conditions characteristic of active MOR hydrothermal systems. Experimental data suggest that upon entrainment and recharge of seawater into MOR hydrothermal systems anhydrite will rapidly precipitate with a Ca isotopic composition that is depleted in the heavy isotope compared to the hydrothermal fluid. The magnitude of the Ca isotope fractionation, Δ44/40Ca(Anh-Fluid), is temperature dependent, −0.45, −0.22, and −0.02‰, for 175, 250, and 350 °C, respectively, but likely indicative of kinetic effects. Utilization of a43Ca spike in solution was implemented to quantify the time-dependent extent of isotope exchange during anhydrite recrystallization at chemical equilibrium. These data indicate that the rate of exchange is a function of temperature, where 12, 46, and 45% exchange occurred within 1322, 867, 366 h at 175, 250, and 350 °C, respectively. The partitioning of Sr/Ca between anhydrite and constituent dissolved species during precipitation depends greatly on the saturation state of the hydrothermal fluid with respect to anhydrite at each experimental temperature, KD(Anh-Fluid)= 1.24–0.55 at 175–350 °C, broadly similar to results of earlier experimental observations by Shikazono and Holland (1983). Equilibrium KD(Anh-Fluid)values were estimated by taking explicit account of time dependent magnitude of exchange, yielding values of 0.43, 0.36, 0.29 at 175, 250, and 350 °C, respectively. Coupling these experimental constraints with the temperature gradient inferred for high temperature MOR hydrothermal systems suggests that the Ca isotope and Sr elemental composition of anhydrite formed near the seafloor will retain the composition derived upon initial formation conditions, which is indicative of disequilibrium. In contrast, at greater depths and at higher temperatures, anhydrite will reflect close to equilibrium Sr/Ca partitioning and Ca isotope fractionation conditions. The experimental and natural data presented in this study can be used to further understand the effect of anhydrite precipitation during hydrothermal circulation in the oceanic crust and on the chemical and isotopic composition of seawater on geologic timescales.