Measurement of stable isotope activities in saline aqueous solutions using optical spectroscopy methods.

The requirement to measure the stable isotopic compositions of saline pore fluids by optical methods has prompted a re-evaluation of the isotopic salt effect for common salts. Hydrogen and oxygen isotopic salt effects were measured at room temperature (21°C) by optical methods. For hydrogen isotopes, our results agree well with those of previous studies and better define these effects at low temperatures. In contrast, measured oxygen isotope salt effects disagree within error for NaCl and CaCl2 solutions from those reported previously. Subtle differences between measurement methods may account for the discrepancy. In studies that involve highly saline fluids, the isotopic salt effect must be taken into account because modern methods that measure stable isotopic compositions as activities or concentrations may be not directly comparable to historical data sets.