A MODEL OF ISOTOPE FRACTIONATION IN REACTING GEOCHEMICAL SYSTEMS

We present a numerical technique that predicts how the stable isotopes H-2, C-13, O-18, and S-34 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 alrea dy presented in the literature but differs from previous techniques in that it allows minerals to be segregated from isotopic exchange inste ad of remaining in isotopic equilibrium, Such an approach allows us to simulate the fractionation of isotopes between rock and fluid resulti ng solely from mineral dissolution and precipitation, We test our tech nique by modeling isotopic fractionation during several reaction proce sses, including (1) dolomitization of limestone by a migrating pore fl uid, (2) diagenetic alteration of the Permian Lyons sandstone in the D enver 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 tr ends observed in nature but differ markedly from calculations that ass ume isotopic equilibrium.