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The solubility of zinc minerals in hydrothermal fluids is enhanced by chloride complexation of Zn2+. Thermodynamic models of these complexation reactions are central to models of Zn transport and ore formation. However, existing thermodynamic models, derived from solubility measurements, are inconsistent with spectroscopic measurements of Zn speciation.Here, we used ab initio molecular dynamics simulations (with the PBE exchange-correlation functional) to predict the speciation of Zn-Cl complexes from 25 to 600°C. We also obtained in situ XAS measurements of Zn-Cl solutions at 30-600°C. Qualitatively, the simulations reproduced the main features derived from in situ XANES and EXAFS measurements: octahedral to tetrahedral transition with increasing temperature and salinity, stability of ZnCl4 2- at high chloride concentration up to ≥500°C, and increasing stability of the trigonal planar [ZnCl3]- complex at high temperature. Having confirmed the dominant species, we directly determined the stability constants for the Zn-Cl complexes using thermodynamic integration along constrained Zn-Cl distances in a series of MD simulations. We corrected our stability constants to infinite dilution using the b-dot model for the activity coefficients of the solute species. In order to compare the ab initio results with experiments, we need to re-model the existing solubility data using the species we identified in our MD simulations. The stability constants derived from refitting published experimental data are in reasonable agreement with those we obtained using ab initio MD simulations. Our new thermodynamic model accurately predicts the experimentally observed changes in ZnO(s) and ZnCO3(s) solubility as a function of chloride concentration from 200 (Psat) to 600°C (2000bar). This study demonstrates that metal speciation and geologically useful stability constants can be derived for species in hydrothermal fluids from ab initio MD simulations even at the generalized gradient approximation for exchange-correlation. We caution, however, that simulations are mostly reliable at high T where ligand exchange is fast enough to yield thermodynamic averages over the timescales of the simulations.
|Number of pages||20|
|Journal||Geochimica et Cosmochimica Acta|
|Publication status||Published - 1 Feb 2015|
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- 1 Finished
TRANSPORT OF LITHOPHILE ELEMENTS IN MAGMATIC-HYDROTHERMAL FLUIDS
Sherman, D. M.
1/09/11 → 1/09/14