The effects of solid solution and diffusion on infiltration-driven metamorphism with application to metaperidotite, Val d’Efra, Central Swiss Alps

Abstract

The complication introduced by solid solutions in the analysis of infiltration-driven mineral reactions is that the mole fraction of tracer component i in fluid (Xi) changes with reaction progress (ξ). The effect was incorporated into transport models for coupled fluid flow and mineral reaction by parameterizing the relation between Xi and ξ. With specific reference to carbonation and hydration during regional metamorphism of the peridotite body in Val d’Efra, whose constituent minerals are all solid solutions, infiltration of a disequilibrium fluid produces a single sharp reaction front if rock is assumed uniform in composition. The reaction front separates completely unreacted rock downstream from rock upstream with ξ at a steady-state limit (ξss ≤ ξmax) that depends on input fluid composition (ξmax is the maximum possible value). Novel phenomena develop, however, if the flow medium, like the metaperidotite body, is composed of many small domains that differ in initial mineral modes and compositions but with Xi homogenized at a spatial scale larger than the size of the domains (e.g., by diffusion). In this case, infiltration of a disequilibrium fluid produces up to as many different reaction fronts along the flow path as there are domains with 0 ≤ ξ < ξss in all domains except upstream from the slowest moving front where ξ = ξss in all domains. Measured values of ξ in the metaperidotite, (all 0 < ξ < ξmax) are best reproduced by down-temperature infiltration of a disequilibrium fluid with \( X_{{{\text{CO}}_{2} }} = \, 0. 1 9 6 \) into a multi-domain medium with uniform \( X_{{{\text{CO}}_{2} }} \) at each spatial point along the flow path (homogenized across the domains at the m-scale by diffusion), and time-integrated fluid flux ≥1,836 mol fluid/cm2 rock. Results resolve the paradox of the widespread spatial distribution of reactants and products of infiltration-driven decarbonation/dehydration reactions in regional metamorphic terrains (which in the absence of solid solution and compositional domains indicate up-temperature flow) and the prediction of hydrodynamic models that regional metamorphic fluid flow normally is directed vertically upward and down temperature.