A model of isotope fractionation in reacting geochemical systems

We present a numerical technique that predicts how the stable isotopes ²H, ¹³C, ¹⁸O, and ³⁴S fractionate among solvent, aqueous species, minerals, and gases over the course of a geochemical reaction process. Our model is based on mass balance techniques similar to those already presented in the literature but differs from previous techniques in that it allows minerals to be segregated from isotopic exchange instead of remaining in isotopic equilibrium. Such an approach allows us to simulate the fractionation of isotopes between rock and fluid resulting solely from mineral dissolution and precipitation. We test our technique by modeling isotopic fractionation during several reaction processes, including (1) dolomitization of limestone by a migrating pore fluid, (2) diagenetic alteration of the Permian Lyons sandstone in the Denver basin, and (3) hydrothermal alteration of the Okanagan Batholith in southern British Columbia. The results of calculations in which we segregate minerals from isotopic exchange compare well to isotopic trends observed in nature but differ markedly from calculations that assume isotopic equilibrium.