HighPGibbs, a Practical Tool for Fluid-Rock Thermodynamic Simulation in Deep Earth and Its Application on Calculating Nitrogen Speciation in Subduction Zone Fluids

The HighPGibbs program is designed to calculate thermodynamic equilibrium of fluid-rock minerals and solid solutions up to depths of lithospheric mantle. It uses the Gibbs free energy minimization function of the HCh package to calculate mineral-fluid equilibrium assemblages. Chemical potentials of minerals are calculated using the equations of states included in HCh; free energy of aqueous species are calculated using the Deep Earth Water model; and activity coefficients of charged species are estimated using the Davies variant of the Debye-Huckel equation. HighPGibbs was applied to calculate nitrogen speciation in eclogite-buffered fluids from 400 to 790 °C and 30 to 54 kbar, to evaluate the mobility of nitrogen in subducting oceanic crust. Regardless of whether the protolith was altered (and oxidized) or not, N(aq) or NH(aq) are the predominant form of nitrogen in the slab-fluids at sub-arc temperatures, especially in cases of moderate or hot geotherms. Given that molecular nitrogen is highly ...