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-Hückel
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 incompatible in
silicate minerals, the simulation indicates that nitrogen (as NH) in
silicate minerals can be liberated during metamorphic devolatilization.
The majority of nitrogen in subducting crusts can be unlocked during
slab devolatilization and eventually expelled to the atmosphere via
degassing of arc magmas. Therefore, oceanic crusts recycled to deep
earth will be depleted in nitrogen compared to the newly formed crust at
spreading centers. As a result of the long-term mantle convection, large
proportions of the bulk silicate earth may have suffered nitrogen
extraction via subduction, and this may account for the nitrogen
enrichment in the Earth’s atmosphere.