Fe isotope fractionation during phase separation in the NaCl-H₂O system: An experimental study with implications for seafloor hydrothermal vents

Phase separation has been proposed as a possible mechanism contributing to the Fe isotope composition of hydrothermal fluids at mid-ocean ridges. The uncertainty results largely from the emphasis on field data that can involve competing processes that obscure cause and effect of any one process. To better understand the potential significance of phase separation in the NaCl-Fe-H₂O system on Fe isotope fractionation, temperature and pressure of a Fe-bearing NaCl fluid in a titanium flow reactor were carefully adjusted to produce vapor ± liquid ± halite, while the Fe isotope composition between coexisting phases was monitored. Two different P-T regions were emphasized: (1) 424-420 °C, 35.2-31.5 MPa; and (2) 464-466 °C, 29.8-24.7 MPa. Both regions were chosen to simulate the range of physical conditions that are experienced by hydrothermal fluids at mid-ocean ridges (MORs). Decompression induced phase separation in both P-T regions results in the vapor phase becoming enriched in the heavier isotopes of Fe, as the Fe/Cl ratio decreases. The coexisting NaCl-rich liquid phase remains essentially constant with respect to Fe/Cl ratio and Fe isotope composition. Coinciding with the lowest vapor chlorinity in the vapor-liquid stability field, the Fe/Cl ratio of the vapor abruptly increases, while the Fe isotope fractionation between the vapor and liquid (103lnαV/L56/54) reached a maximum value of +0.145±0.048‰. Subsequently, Fe isotope fractionation decreased upon transition into the vapor-halite stability field (P-T region 2). We infer that the observed Fe isotope fractionation between vapor ± liquid ± halite is caused by differences in Fe speciation among coexisting chloride-bearing phases. The experimental study confirms for the first time that measurable Fe isotope variability can result from phase separation in high temperature hydrothermal systems. The species-dependent Fe isotope fractionation reported here is small relative to predicted mineral-mineral and mineral-fluid fractionations, especially if redox effects are involved as might occur during vent fluid-seawater mixing reactions and/or magmatic activity associated with seafloor eruptive episodes.