Hydrogen isotope systematics of phase separation in submarine hydrothermal systems: Experimental calibration and theoretical models

Hydrogen isotope fractionation factors were measured for coexisting brines and vapors formed by phase separation of NaCl/H₂O fluids at temperatures ranging from 399-450°C and pressures from 277-397 bars. It was found that brines are depleted in D compared to coexisting vapors at all conditions studied. The magnitude of hydrogen isotope fractionation is dependent on the relative amounts of Cl in the two phases and can be empirically correlated to pressure using the following relationship: 1000 ln α_(vap-brine) = 2.54(±0.83) + 2.87(±0.69) x log (ΔP), where α_(vap-brine) is the fractionation factor and ΔP is a pressure term representing distance from the critical curve in the NaCl/H₂O system. The effect of phase separation on hydrogen isotope distribution in subseafloor hydrothermal systems depends on a number of factors, including whether phase separation is induced by heating at depth or by decompression of hydrothermal fluids ascending to the seafloor. Phase separation in most subseafloor systems appears to be a simple process driven by heating of seawater to conditions within the two-phase region, followed by segregation and entrainment of brine or vapor into a seawater dominated system. Resulting vent fluids exhibit large ranges in Cl concentration with no measurable effect on δD. Possible exceptions to this include hydrothermal fluids venting at Axial and 9°N on the East Pacific Rise. High δD values of low Cl fluids venting at Axial are consistent with phase separation taking place at relatively shallow levels in the oceanic crust while negative δD values in some low Cl fluids venting at 9°N suggest involvement of a magmatic fluid component or phase separation of D-depleted brines derived during previous hydrothermal activity.