Reactive dispersion of stable isotopes by mineral reaction during metamorphism

Abstract

The process of reactive dispersion is introduced whereby reaction in one chemical system imparts hydrodynamic dispersion to another apparently unrelated reactive transport system. Reactive dispersion is illustrated with a one-dimensional finite difference model for coupled heat flow, fluid flow, mineral reaction, and oxygen isotopic exchange during contact metamorphism of siliceous dolomite. Infiltration-driven decarbonation reactions impart isotopic heterogeneity to an initially homogeneous system. The effect on fluid composition is explored using a breakthrough curve that tracks fluid composition through time. The corresponding effects on rock composition are illustrated with plots of isotopic alteration vs. distance. Reactive dispersion is especially efficient during infiltration of H 2O into carbonate rocks because 18O is strongly fractionated between CO 2 and H 2O and because mineral reactions may buffer the CO 2-H 2O content of fluid. The spatially heterogeneous isotopic alteration predicted to develop during decarbonation reactions in a homogeneous protolith is similar in magnitude and distribution to that observed downstream of the isotopic depletion front in contact metamorphic aureoles.