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The effect of water activity on calculated phase equilibria and garnet isopleth thermobarometry of granulites, with particular reference to Tongbai (east-central China)

Thomas Bader, Leander Franz, Christian de Capitani and Zhang Lifei
The effect of water activity on calculated phase equilibria and garnet isopleth thermobarometry of granulites, with particular reference to Tongbai (east-central China)
European Journal of Mineralogy (March 2014) 26 (1): 5-23

Abstract

Granulites form at low water activities whose exact value is often unknown. This complicates the calculation of equilibrium assemblage phase diagrams and metamorphic pressure (P) and temperature (T) conditions by isopleth thermobarometry, because the fluid characteristics distinctly affect the results. To evaluate the significance of this influence, granulites from Tongbai, east-central China are investigated in this study; equilibrium-assemblage calculations and multiequilibrium calculations are combined with fluid-inclusion studies to determine peak metamorphic P-T conditions together with water activities. Calculations using the Theriak/Domino and the TWQ programs on a felsic and two mafic granulites yield P-T conditions of 750-805 degrees C at 0.75-0.93 GPa and water activities of 0.10-0.65 for the equilibration of the mineral assemblages. Armored relics of Zn-rich Fe-Mg-spinel included in garnet of the felsic granulite point to an earlier metamorphic stage at lower pressures on the same P-T loop. Garnet porphyroblasts from two samples contain primary fluid inclusions comprising vapor and daughter crystals. Using a Raman probe, CO (sub 2) and magnesite were identified in fluid inclusions of the felsic granulite while one mafic granulite yielded fluid inclusions with CO (sub 2) dominating besides CH (sub 4) , H (sub 2) O, graphite and siderite. The carbonic fluid is consistent with the implied low water activities. Possible CO (sub 2) -sources are fluids from marbles associated with the granulites and fluids or magmas from the mantle. Equilibrium assemblage calculations reveal a distinct influence of the water activity on both the stability of considered phases and the location of compositional isopleths of the rock-forming minerals. We demonstrate that an estimate of the water activity using the Theriak/Domino and the TWQ programs in combination with Raman spectroscopy is essential to perform accurate thermobarometry in granulites.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 26
Serial Issue: 1
Title: The effect of water activity on calculated phase equilibria and garnet isopleth thermobarometry of granulites, with particular reference to Tongbai (east-central China)
Affiliation: Peking University, School of Earth and Space Sciences, Beijing, China
Pages: 5-23
Published: 201403
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 107
Accession Number: 2014-028683
Categories: Igneous and metamorphic petrologyMineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes appendices
Illustration Description: illus. incl. geol. sketch map
N32°27'17" - N32°27'17", E113°24'26" - E113°24'26"
N32°30'01" - N32°30'01", E113°17'32" - E113°17'32"
N32°27'38" - N32°27'38", E113°24'33" - E113°24'33"
Secondary Affiliation: Universitaet Basel, CHE, Switzerland
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: 201418
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