Skip to Main Content
GEOREF RECORD

Garnet-spinel-olivine-orthopyroxene equilibria in the FeO-MgO-Al (sub 2) O (sub 3) -SiO (sub 2) -Cr (sub 2) O (sub 3) systems; II, Thermodynamic analysis

Andrei V. Girnis and Gerhard P. Brey
Garnet-spinel-olivine-orthopyroxene equilibria in the FeO-MgO-Al (sub 2) O (sub 3) -SiO (sub 2) -Cr (sub 2) O (sub 3) systems; II, Thermodynamic analysis
European Journal of Mineralogy (August 1999) 11 (4): 619-636

Abstract

Experimental results for the four-phase assemblage in the MASCr (Doroshev et al., 1997) and FMASCr (Brey et al., 1999) systems are evaluated with respect to exchange (Fe-Mg and Cr-Al) and displaced equilibrium reactions (Al solubility in orthopyroxene and spinel-orthopyroxene to garnet-olivine transition). These equilibria are considered on the basis of the thermodynamic data of Berman (1988). The effects of non-ideal mixing in solid solutions is approximated by a subregular Margules formulation, independent for each site of spinel, garnet, and orthopyroxene solid solutions. Mixing parameters were calculated by linear programming considering each experimental point as an independent half-bracket for the concentration term in the respective equilibrium. Cr-Al mixing in all phases is not ideal, but the magnitudes of excess mixing energies are not large. Spinel solid solution shows asymmetric Cr-Al and Mg-Fe excess mixing energy. Cr-Al mixing in garnet is close to an ideal model with a symmetric Margules parameter 0 to 6 kJ/mol. Standard state properties of knorringite, Mg (sub 3) Cr (sub 2) Si (sub 3) O (sub 12) and orthopyroxene end-member MgCrAlSiO (sub 6) are estimated. The major factor determining concentration dependencies of equilibrium constants is the Gibbs free energy changes of reciprocal reactions FeCr+MgAl = FeAl+MgCr in spinel and garnet. The derived thermodynamic parameters are used to calculate phase relationships in the four-mineral region of the FMASCr system, which models depleted mantle harzburgite. The Cr/(Cr+Al) ratio of garnet increases with both pressure and temperature, while that of spinel depends mainly on pressure. The distribution of Cr and Al between spinel and garnet in equilibrium with orthopyroxene and olivine may be used as a thermobarometer for deep mantle xenoliths and inclusions in diamonds.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 11
Serial Issue: 4
Title: Garnet-spinel-olivine-orthopyroxene equilibria in the FeO-MgO-Al (sub 2) O (sub 3) -SiO (sub 2) -Cr (sub 2) O (sub 3) systems; II, Thermodynamic analysis
Affiliation: J. W. Goethe-Universitaet, Institut fuer Mineralogie, Frankfurt am Main, Federal Republic of Germany
Pages: 619-636
Published: 199908
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 64
Accession Number: 1999-066198
Categories: General geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Secondary Affiliation: Institute for Geology of Ore Deposits, RUS, Russian Federation
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 199922
Close Modal

or Create an Account

Close Modal
Close Modal