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Modelling high-pressure aqueous fluids in the system CaO-SiO (sub 2) -H (sub 2) O; a comprehensive semi-empirical thermodynamic formalism

Michael Burchard, Walter V. Maresch, Thomas Fockenberg, Nikos L. Doltsinis and Waheed A. Adeagbo
Modelling high-pressure aqueous fluids in the system CaO-SiO (sub 2) -H (sub 2) O; a comprehensive semi-empirical thermodynamic formalism
European Journal of Mineralogy (June 2011) 23 (3): 409-424

Abstract

There is a distinct need for predictive equations of state for supercritical aqueous solutions, both for understanding fluids in deeper levels of Earth's crust or in subduction zones, as well as in experimental work on mineral solubility. Here we develop a semi-empirical approach introduced by Gerya & Perchuk (1997) based on the P-T partition function of statistical thermodynamics, using experimental data on quartz and wollastonite solubility, as well as data on speciation of dissolved solution components derived from first principles molecular dynamics simulations. Two approaches are possible, differing in the degree of explicit information provided on the nature of the solution modelled and also in the amount of basic data needed to implement them. Both have their potential fields of application. A "simple" model using only the semi-empirical formulation for the H (sub 2) O solvent is useful if independent data on speciation are lacking, overall neutrality of the dissolving species can be assumed, and the number of components is relatively low. Computation is fairly straightforward, because the system can be treated as a simple "mixing" problem, and adopted effective dissolved species are characterized by conventional thermodynamic properties that allow interpolation and extrapolation of fitted experimental solubility data. Application to the system CaO-SiO (sub 2) -H (sub 2) O shows that experimental data on fluids coexisting with wollastonite+quartz/coesite can be successfully modelled up to 900 degrees C and 4 GPa. This simple semi-empirical formulation can lay the groundwork for an "internally consistent data set" allowing descriptions of fluids in relatively simple fluid-rock systems at high pressures. With the addition of independent data on actual speciation, and an optimized model for the dissociation of H (sub 2) O from available literature data, the semi-empirical approach can be extended to a comprehensive description of aqueous solutions in the CaO-SiO (sub 2) -H (sub 2) O system. Both models have one positive feature in common. Because standard thermodynamic properties for dissolved species, oxides or fictive aggregates/clusters can be derived, solutions of arbitrary compositions can be modelled from data obtained from experiments in which fluids are saturated with a given solid phase or phases.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 23
Serial Issue: 3
Title: Modelling high-pressure aqueous fluids in the system CaO-SiO (sub 2) -H (sub 2) O; a comprehensive semi-empirical thermodynamic formalism
Affiliation: Universitaet Heidelberg, Mineralogisches Institut, Heidelberg, Germany
Pages: 409-424
Published: 201106
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 49
Accession Number: 2011-064955
Categories: General geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Secondary Affiliation: Ruhr-Universitaet Bochum, DEU, Germany
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201136
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