Abstract

—The GEOCHEQ_Isotope software package, elaborated previously for modeling chemical and carbon isotope equilibria in hydrothermal and hydrogeochemical systems by minimizing the Gibbs energy, is extended to the simultaneous calculation of carbon and oxygen isotopic effects. Similar to what was done for carbon, the β-factor formalism was used to develop algorithms and a database for calculating the isotopic effects of oxygen. According to the developed algorithm, the Gibbs energy of formation of a rare isotopologue, G*(P, T), is calculated through the Gibbs energy of formation of the main isotopologue, the value of the β18O factor of this substance, and the mass ratio of the rare (18O) and main (16O) isotopes. The isotope mixture is assumed to be ideal. The temperature dependence of the β-factor is unified as a polynomial in reciprocal absolute temperature. Necessary information on oxygen isotope equilibria involving important geochemical compounds was critically analyzed, and the available data were reconciled and modified. The temperature dependences of the β18O-factors were correspondingly optimized. The thermodynamic database was updated by adding information on the temperature dependence of β18O-factors specified by polynomial coefficients for each substance. The usage of the GEOCHEQ_Isotope software package and the corresponding database is demonstrated by modeling the dependence of oxygen and carbon isotope fractionation factors on the acidity of the solution (pH) in a carbonate hydrothermal system. The simulation results are in a good agreement with experimental data available from the literature. The enrichment of dissolved carbonates in the 18O heavy oxygen isotope relative to water decreases with increasing pH of the system. At the same time, a pH increase results in a decrease in the negative carbon isotope shift between calcite and dissolved carbonates. At high pH values (~11), the isotope shift inversion and the enrichment of the dissolved carbonate in the heavy carbon isotope relative to calcite are predicted.

Highlights

  • Studies of the isotopic effects of oxygen is of paramount importance in isotope geochemistry

  • This paper is devoted to the simultaneous computation of chemical and oxygen isotope equilibria with the GEOCHEQ_Isotope software package

  • Equilibrium fractionation in the system calcite–CO2 that was calculated using the values of the β18O-factor for calcite from the GEOCHEQ_Isotope database is generally in a good agreement with the experimentally measured equilibrium oxygen isotope fractionation factor for this system (Fig. 2)

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Summary

Introduction

Studies of the isotopic effects of oxygen is of paramount importance in isotope geochemistry. Experimental data on solute carbonate species (Beck et al, 2005) are in a good agreement with equilibrium isotope fractionation factors between these solute species and water that were calculated based on data from GEOCHEQ_Isotope database (Fig. 1).

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