Stable isotope fronts at lithologic contacts are robust manifestations of fluid-rock interaction. The geometry of stable isotope fronts contains information on the transport and exchange processes involved. If, during fluid-rock interaction, stable isotope equilibrium is maintained on a grain scale, front geometries are quantitative and unambiguous documentations of material transport. Deviations from a local equilibrium situation may, however, occur due to kinetically controlled mineral-fluid isotope exchange. If an isotope front is documented only in one isotope system and if only one constituent mineral of a rock is considered as a monitoring phase, the interpretation of its geometry may be ambiguous. It is generally not possible to distinguish between local equilibrium and kinetically controlled isotope exchange from a single-tracer single-monitor front. Both, consideration of more than one monitoring phase and of more than one tracer isotope helps to discern between local equilibrium and kinetic scenarios. If more than one mineral is considered as a monitoring phase, the systematics of the inter-mineral fractionations may provide information on the grain-scale equilibration during fluid-rock interaction. If more than one tracer isotope is analyzed, the relative retardation of fronts from different tracers provides additional 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 features of stable isotope fronts that allow to constrain the nature of fluid-rock interaction.

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