Implications of kinetically controlled mineral-fluid exchange on the geometry of stable-isotope fronts

Stable isotope fronts at lithologic contacts are robust manifestations of f luid-rock interaction. The geometry of stable isotope fronts contains infor mation on the transport and exchange processes involved. If, during fluid-r ock interaction, stable isotope equilibrium is maintained on a grain scale, front geometries are quantitative and unambiguous documentations of materi al transport. Deviations from a local equilibrium situation may, however, o ccur due to kinetically controlled mineral-fluid isotope exchange. If an is otope front is documented only in one isotope system and if only one consti tuent mineral of a rock is considered as a monitoring phase, the interpreta tion of its geometry may be ambiguous. It is generally not possible to dist inguish between local equilibrium and kinetically controlled isotope exchan ge from a single-tracer single-monitor front. Both, consideration of more t han one monitoring phase and of more than one tracer isotope helps to disce rn between local equilibrium and kinetic scenarios. If more than one minera l is considered as a monitoring phase, the systematics of the inter-mineral fractionations may provide information on the grain-scale equilibration du ring fluid-rock interaction. If more than one tracer isotope is analyzed, t he relative retardation of fronts from different tracers provides additiona l information on the degree of equilibration during fluid-rock interaction. We present theoretical considerations regarding the effect of kinetically controlled mineral fluid exchange on the geometry of stable isotope fronts. We also discuss natural examples to illustrate the most easily observed fe atures of stable isotope Fronts that allow to constrain the nature of fluid -rock interaction.