Simple models of coupled fluid infiltration and redox reactions in the crust

Abstract

Simple one-dimensional numerical models are presented for coupled advection-hydrodynamic dispersion and kinetically controlled oxidatioin-reduction reactions in graphite-free porous media containing magnetite coexisting with silicate assemblages. Fluid-solid interactions involving either OH (O₂-H₂O-H₂) or COH (O₂-H₂O-H₂-CO₂-CO-CH₄) fluids are considered at ∼500 ^∘C and 5 kbar. The major implications of the modeling are as follows: (1) Regional (km scale) reduction of typical magnetite-bearing rocks originally at f _O2 near NNO may be possible during long-term metamorphic fluid flow if the infiltrating fluids have sufficiently low f _O2 and sufficiently large concentrations of CH₄ and/or H₂. Regional oxidation of such rocks by highly oxidized OH or COH fluids appears to be difficult to achieve. (2) Nearly identical mineral assemblages and modes may be produced by very different kinetic reaction pathways. The model implies that “equilibrium” assemblages preserved in rocks may not always reflect the true kinetic reaction path that evolved during fluid flow, and highlights the need for quantitative measurements of metamorphic reaction rates. (3) Preservation of sharp lithologic contacts between rocks of very different redox states containing accessory amounts of oxides may be unlikely if fluid-rock interaction times exceed 10³10⁴ years. Substantial contact disruption over these times scales is predicted even for oxide-rich rocks if redox contrasts between layers are large. Flow across lithologic contacts may produce asymmetric patterns of metasomatic mineral zonation that may prove useful for mapping flow directions in metamorphic sequences. (4) For fluid flow in typical T gradients through originally homogeneous rock, significant major element metasomatism (e.g., K, Na, Ca) may be possible without producing large changes in oxide abundances.