Calibration of Br/Cl fractionation during subcritical phase separation of seawater: Possible halite at 9 to 10 degrees N East Pacific Rise

Experiments were conducted to assess Br/Cl fractionation during phase separation of seawater at 400 degrees C and 250 to 275 bars. These conditions are applicable to the 9-10 degrees N EPR system where low CI concentrations and low Br/Cl ratios in vent fluids have been attributed to phase separation of seawater at conditions below the critical point of seawater (408 degrees C, 300 bars). The level of Br/Cl fractionation observed in experiments is well below that needed to account for Br/Cl systematics at EPR 9-10 degrees N. Based on our experimental results, we propose an alternative model involving dissolution and precipitation of halite to account for the anomalous Br/Cl data at 9-10 degrees N. Halite can be predicted to form at pressure and temperature conditions prevailing during eruption of magma at mid-ocean ridges. Subsequent changes in physical conditions, however, would induce halite to dissolve, and thus, decrease Br/Cl ratios of vent fluids. Copyright (C) 1997 Elsevier Science Ltd.