Electrolytic fluid speciation by Gibbs energy minimization and implications for subduction zone mass transfer

The number of solute species required to describe the thermodynamic behavior of electrolytic fluids is a hindrance to the incorporation of aqueous geochemistry in petrological Gibbs energy minimization procedures. An algorithm is developed to overcome this problem. Beginning from the solute-free limit, chemical potentials, and phase stability are determined by minimization, the solute speciation and bulk fluid properties consistent with these chemical potentials are then computed and the procedure repeated until the chemical potentials converge. Application of the algorithm to a model for metamorphism of subducted sediment shows that accounting for solute chemistry does not change the conclusion based on molecular fluid models that a pervasive water flux from the subjacent mantle is required to explain island-arc carbon emissions by fluid-mediated slab decarbonation. This putative flux would deplete the sediment in potassium, limiting the capacity of the slab to transport water to greater depth and rendering it refractory to melting. (C) 2018 Elsevier B.V. All rights reserved.