Mineral–water reactions in Earth’s mantle: Predictions from Born theory and ab initio molecular dynamics

Abstract

Recent studies present compelling evidence that a free aqueous fluid phase exists within the upper mantle. Fluid may be present at depths as great as the transition zone (410–660 km) and possibly beyond. The chemical reactivity of such deep fluids can be predicted from the Born model of solvation. To use the Born model, we need to know the dielectric constant of water under mantle conditions. We have used ab initio molecular dynamics simulations to determine the dielectric constant of water up to a pressure of 30 GPa and a temperature of 3000 K. Increased temperature lowers the dielectric constant and decreases ion solvation, but pressure overcomes this effect. The resulting high dielectric constant suggests that aqueous mantle fluids are highly reactive for ion solvation and mineral dissolution. We tested this by using the Helgeson–Kirkham–Flowers equation of state to estimate free energies of several mineral-solution and ion solvation reactions under mantle conditions. The results support previous estimates of carbonate solubility in the mantle. We also find that mantle fluids may play a key role in transporting ore metals: we evaluated the solubility of chalcopyrite and the complexation of Cu and Fe by Cl under mantle conditions and find that metal complexation is as significant as in ore-forming fluids in the crust. At reasonable conditions of pH and fH2, chalcopyrite is highly soluble. We tentatively hypothesize that exsolved fluids from subducted slabs may extract and mobilize primary sulfides in the mantle, implying potentially deep sources for porphyry copper deposits.