Thermodynamic Modeling of Fluid-Rock Interaction at Mid-Crustal to Upper-Mantle Conditions

Abstract

Fluid-rock interaction is important in a wide range of settings in the middle crust to the upper mantle. Fluids are liberated in the crust during prograde metamorphism in convergent-margin settings. At greater depth, devolatilization of subducting lithosphere plays a major role in global element cycling, metasomatic alteration of the mantle, and the genesis of arc magmas. Mafic magmas produced in these settings may stall in the lower crust and liberate volatiles that metasomatize surrounding rocks and trigger production of silicic magmas of crustal derivation. Mounting evidence points to an important role for metasomatic alteration by saline brines in the genesis of some lower-crustal granulite-facies metamorphic terranes. Unlike shallow geothermal systems or low-grade metamorphic environments, there has been only limited progress in modeling the fluid-rock interaction that characterizes the deeper geologic systems enumerated above. Such phase-equilibrium models require as input the chemical potential, μ i μ i ≡ ( ∂ G i ∂ n i ) P , T (1) of the participating phases or species i , where G i is the Gibbs free energy of i , n i is the number of moles of i , P is pressure, and T is temperature. At the P and T of interest, μ i = μ i ∘ + R T l n a i (2) Where μ° is standard state chemical potential, a is activity, R is the gas constant, and T is absolute temperature. It is challenging to quantify the right-hand terms in Equation (2) at P and T ranging from the middle crust to the upper mantle. This is chiefly because of (1) limitations in our quantitative knowledge of key properties of relevant solutes and H2O, and (2) uncertainty in how properly to model solute activities at the requisite conditions. As a consequence, aqueous geochemists have been discouraged from plying their trade in the numerous deep geologic systems in which fluid-rock interaction may play a major role in the Earth’s chemical and physical evolution. …