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 con ditions are applicable to the 9-10 degrees N EPR system where low CI c oncentrations and low Br/Cl ratios in vent fluids have been attributed to phase separation of seawater at conditions below the critical poin t of seawater (408 degrees C, 300 bars). The level of Br/Cl fractionat ion observed in experiments is well below that needed to account for B r/Cl systematics at EPR 9-10 degrees N. Based on our experimental resu lts, we propose an alternative model involving dissolution and precipi tation of halite to account for the anomalous Br/Cl data at 9-10 degre es N. Halite can be predicted to form at pressure and temperature cond itions prevailing during eruption of magma at mid-ocean ridges. Subseq uent changes in physical conditions, however, would induce halite to d issolve, and thus, decrease Br/Cl ratios of vent fluids. Copyright (C) 1997 Elsevier Science Ltd.