Untangling Element Fluxes From The Subducting Slabs: Aqueous Solutions Through The Electrostatic Lense

Abstract:

Understanding the deep cycling of elements hinges on characterizing the chemistry of fluids in subduction zones. Aqueous solutions dielectric properties determine their characteristics as solvents for rocks. Predicting the composition and speciation of fluid solutions at equilibrium with complex mineral assemblages has been a long lasting challenge. We have developed a model to explore the composition and speciation of metamorphic aqueous solutions to upper mantle pressure and temperature. Our model combines Gibbs free energy minimization of rock and molecular fluids with an electrostatic approach to describe solute-solute and solute-solvent interactions in the fluid phase. Using an extension of the Debye-Onsager-Kirkwood model to characterize the dielectric properties of COH solutions, we derive aqueous fluid speciation by solving the mass action and charge balance of the system. This framework is applied to various slab lithologies. We find, e.g., that the pH of carbonated basalts and pelites is alkaline, i.e. ~2-2.5 pH units above neutrality, in C and Cl-free fluids and shows an isothermal decrease above the albite-jadeite-quartz equilibrium, at P ~ 2 GPa. C and Chlorine (1M) decrease the alkalinity by a combined ~ 1.5 pH unit, with variations tied to the thermal structure and mineral assemblages along typical P–T paths. The results produced are compared with experimental solubility measurements on identical systems. Significant discrepancies reveal that the process of solute polymerization at elevated T and P is more general than previously recognized. Unaccounted species contain, in addition to Na, Si and Al, an important fraction of the total dissolved load of the elements K, Ca and Mg (>50% at T= 600 °C and P = 2 Ga). Finally, we show that knowledge of the acid-base properties of metamorphic fluid solutions affect the (de)coupling between C and alkali-earth mobility, and magnitude of low-T C transfers associated to carbonate dissolution through time. This shows that a better understanding of the mineral and fluid cycling of alkali and halogens in particular, are key to quantifying the deep cycling of elements such as C.