Experimental study on Fe solubility in vapor-rich hydrothermal fluids at 400–500 °C, 215–510 bar: Implication for Fe mobility in seafloor vent systems
Experimental study on Fe solubility in vapor-rich hydrothermal fluids at 400–500 °C, 215–510 bar: Implication for Fe mobility in seafloor vent systems
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400～500℃、215～510bar 富蒸气热液中铁溶解度的实验研究：对海底喷口系统中铁迁移率的影响
DOI：
10.1016/j.gca.2021.09.027
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发表时间：
2021
影响因子：
5
通讯作者：
Seyfried, William E.
中科院分区：
文献类型：
--
作者：
Xing, Yanlu;Scheuermann, Peter;Seyfried, William E.
在海底热液系统中，喷口流体通常含有较高的溶解铁（Fe），相对于深海海水而言，其含量显著富集。通常认为，由于形成含水Fe-Cl络合物，Fe优先在致密富Cl流体中迁移。然而，富蒸汽低密度流体中的铁富集与低氯浓度（<550 mmol/kg）强调了低氯富蒸汽相在陆上和海底热液系统中传输铁的有效性。目前，由于缺乏高温高压（T-P）溶解度实验和必要的热力学数据，铁在低密度富蒸汽流体中的传输知之甚少。在这里，我们报告了在400-500 °C，215-510 bar下进行的实验中Fe溶解度的新数据，目标是低密度流体（0.1-0.35 g/cm 3）。实验在KCl-H2O体系中进行，以赤铁矿-磁铁矿和钾长石-白云母-石英为矿物缓冲组合。结果表明，铁溶解度与密度和流体氯度呈正相关，而密度和流体氯度受温度和压力的影响。测定了Fe_3 O_4（s）+2 HCl（aq）= Fe_2 O_3（s）+FeCl_2（aq）+H_2 O的平衡常数（logK_m）。将新数据与Helgeson-Kirkham-Flowers（HKF）状态方程计算的数据拟合到密度模型中，外推了宽温压范围内赤铁矿-磁铁矿Fe缓冲反应的logKhm。在考虑氧化还原约束的基础上，还对磁铁矿溶解反应和黄铁矿-磁黄铁矿平衡的密度模型进行了拟合。我们发现，计算的Fe溶解度与测量值在富蒸汽流体中形成的矿物缓冲和玄武岩蚀变实验中的相分离在升高的T-P。密度模型进一步应用于模拟铁在流体中的传输在布兰登热液田在东太平洋隆起（EPR）21°S，与T-P的Si-Cl地质温压计约束。计算表明，所报告的布兰登喷口流体的铁浓度反映了海底深处发生的相分离，温度-压力高达450 °C，400巴，氧化还原条件受到黄铁矿-磁黄铁矿-磁铁矿平衡的缓冲。
In seafloor hydrothermal systems, vent fluids usually contain elevated dissolved iron (Fe) that is significantly enriched relative to deep ocean seawater. It is commonly thought that Fe is preferentially transported in dense Cl-rich fluids due to the formation of aqueous Fe-Cl complexes. However, Fe enrichment in vapor-rich low-density fluids with low Cl concentrations (<550 mmol/kg) underscores the efficacy of the low-Cl vapor-rich phase to transport Fe in both subaerial and submarine hydrothermal systems. Currently, transport of Fe in low-density vapor-rich fluids is poorly understood due to the lack of high temperature–pressure (T-P) solubility experiments and requisite thermodynamic data. Here, we report new data of Fe solubility from experiments conducted at 400–500 °C, 215–510 bar, targeting fluids with low-density (∼0.1–0.35 g/cm3). The experiments were performed in the KCl-H2O system with hematite-magnetite and K-feldspar-muscovite-quartz as mineral buffering assemblages. Our results show that Fe solubility positively correlates with density and fluid chlorinity, which are affected by temperature and pressure. The equilibrium constants (logKhm) for Fe-buffering reaction Fe3O4(s) + 2HCl(aq) = Fe2O3(s) + FeCl2(aq) + H2O were determined. The new data and the data calculated using Helgeson-Kirkham-Flowers (HKF) equation of state were fit into a density model to extrapolate logKhmfor hematite-magnetite Fe buffering reaction over a wide T-P range. The density models for magnetite dissolution reaction and pyrite-pyrrhotite equilibrium were also fit based on HKF to allow redox constraints. We show that calculated Fe solubility are in good agreement with measured values in vapor-rich fluids formed via phase separation in mineral buffered and basalt alteration experiments at elevated T-P. The density model was further applied to model Fe transport in fluids at the Brandon hydrothermal field at East Pacific Rise (EPR) 21°S, with T-P constrained by Si-Cl geothermobarometer. The calculations suggest that the reported Fe concentrations of vent fluids at Brandon reflect phase separation occurring at depth in the seafloor, with T-P up to 450 °C, 400 bar, and redox conditions buffered by pyrite-pyrrhotite-magnetite equilibrium.
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