Migration of reaction and isotopic fronts in infiltration zones: assessments of fluid flux in metamorphic terrains

Abstract

Criteria are presented for recognising fluid infiltration-driven reactions in metamorphic rocks and quantifying the associated fractionations of oxygen and carbon isotopic compositions. In medium or high grade metamorphic rocks, with local equilibrium between fluid and solid phases, reactions or isotopic changes driven by infiltrating fluid are expected to migrate as relatively sharp fronts with limited diffusive broadening. Fluid leaving such reaction fronts is compositionally buffered by the reacting phases. In suitably reactive rocks, a series of reaction fronts will develop, each driven by infiltration of fluid buffered by the upstream reaction. Knowledge of the spatial arrangement of such fronts allows calculation of the time integrated fluid fluxes (total fluid volume per unit area). Distinctive stable-isotope fractionations are predicted adjacent to infiltration-driven reactions with the distribution and magnitude of the fractionation effects dependent upon the relative velocities of the reaction and isotopic fronts. The various possibilities are illustrated by reference to a hypothetical infiltration-driven decarbonation reaction. The calculated isotopic fractionations do not, in general, resemble those predicted by the open- or closed-system water-rock ratio calculations.