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 (∼0.1-0.35g/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 (log K_hm) 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 log K_hm for 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.

在海底热液系统中，喷出液通常含有升高的溶解铁 (Fe)，相对于深海海水，这种铁含量显着增加。通常认为，由于形成了含水的 Fe-Cl 络合物，Fe 优先在富含 Cl 的稠密流体中传输。然而，在具有低 Cl 浓度（<550 mmol/kg）的富含蒸汽的低密度流体中富集 Fe 强调了低 Cl 富蒸汽相在海底和海底热液系统中传输 Fe 的功效。目前，由于缺乏高温压力 (TP) 溶解度实验和必要的热力学数据，人们对低密度富含蒸汽的流体中 Fe 的传输知之甚少。在这里，我们报告了在 400-500 下进行的实验中铁溶解度的新数据 °C，215-510 bar，针对低密度流体（~0.1-0.35g/cm ³）。实验在 KCl-H ₂ O 系统中进行，赤铁矿-磁铁矿和钾长石-白云母-石英作为矿物缓冲组合。我们的结果表明，Fe 溶解度与密度和流体氯度呈正相关，后者受温度和压力的影响。Fe 缓冲反应的平衡常数 (log K _hm ) Fe ₃ O ₄ (s) + 2HCl(aq) = Fe ₂ O ₃ (s) + FeCl ₂ (aq) + H ₂O 被确定。将新数据和使用 Helgeson-Kirkham-Flowers (HKF) 状态方程计算的数据拟合到密度模型中以推断 log K _hm用于在宽 TP 范围内进行赤铁矿-磁铁矿 Fe 缓冲反应。磁铁矿溶解反应和黄铁矿-磁黄铁矿平衡的密度模型也基于 HKF 进行拟合，以允许氧化还原约束。我们表明，计算的 Fe 溶解度与在高 TP 下矿物缓冲和玄武岩蚀变实验中通过相分离形成的富含蒸汽的流体中的测量值非常一致。密度模型进一步应用于模拟东太平洋海隆 (EPR) 21°S 的布兰登热液场流体中的 Fe 输运，TP 受 Si-Cl 地温气压计约束。计算表明，所报告的 Brandon 排放流体的 Fe 浓度反映了海底深处发生的相分离，TP 高达 450 °C，400 bar，氧化还原条件由黄铁矿-磁黄铁矿-磁铁矿平衡缓冲。