Thermodynamic modelling of sodic pyroxene solid-solution and its application in a garnet-omphacite-kyanite-coesite geothermobarometer for UHP metamorphic rocks

Daisuke Nakamura, Shohei Banno

Research output: Contribution to journalArticle

49 Citations (Scopus)

Abstract

A garnet-omphacite-kyanite-coesite geothermobarometer is constructed to estimate the peak metamorphic pressure of ultrahigh-pressure (UHP) eclogites. For this we use a quaternary regular solution model for garnet, and propose a new model for sodic pyroxene that assumes double site solid-solution (Na,Ca)M2(Al,Fe3+,Fe2+,Mg)M1Si2O6, where cations in the M2- and the M1-sites mix randomly but with intersite interactions, not only between nearest neighboring cations but also bewteen next nearest neighboring cations. In the new model, overall excess interaction parameters are quantitatively deduced from the previous experimental data and critical temperature, and the activity of the jadeite component agrees reasonbly with that based on the Landau theory. Furthermore, the relations of the calculated order parameter to temperature and composition are consistent with other experimental works. These facts support the usefulness of the new model. Application of our geothermobarometer to kyanite eclogite gave metamorphic conditions around 3.4 GPa and 800°C for both samples from the Dora Maira Massif, Western Alps, and from Rongcheng County, in the Su-Lu region of Eastern China.

Original languageEnglish
Pages (from-to)93-102
Number of pages10
JournalContributions to Mineralogy and Petrology
Volume130
Issue number1
DOIs
Publication statusPublished - 1997
Externally publishedYes

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Metamorphic rocks
coesite
omphacite
metamorphic rocks
kyanite
Garnets
solid solution
garnets
pyroxene
metamorphic rock
Solid solutions
garnet
solid solutions
thermodynamics
Thermodynamics
Cations
cation
cations
modeling
jadeite

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

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abstract = "A garnet-omphacite-kyanite-coesite geothermobarometer is constructed to estimate the peak metamorphic pressure of ultrahigh-pressure (UHP) eclogites. For this we use a quaternary regular solution model for garnet, and propose a new model for sodic pyroxene that assumes double site solid-solution (Na,Ca)M2(Al,Fe3+,Fe2+,Mg)M1Si2O6, where cations in the M2- and the M1-sites mix randomly but with intersite interactions, not only between nearest neighboring cations but also bewteen next nearest neighboring cations. In the new model, overall excess interaction parameters are quantitatively deduced from the previous experimental data and critical temperature, and the activity of the jadeite component agrees reasonbly with that based on the Landau theory. Furthermore, the relations of the calculated order parameter to temperature and composition are consistent with other experimental works. These facts support the usefulness of the new model. Application of our geothermobarometer to kyanite eclogite gave metamorphic conditions around 3.4 GPa and 800°C for both samples from the Dora Maira Massif, Western Alps, and from Rongcheng County, in the Su-Lu region of Eastern China.",
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AB - A garnet-omphacite-kyanite-coesite geothermobarometer is constructed to estimate the peak metamorphic pressure of ultrahigh-pressure (UHP) eclogites. For this we use a quaternary regular solution model for garnet, and propose a new model for sodic pyroxene that assumes double site solid-solution (Na,Ca)M2(Al,Fe3+,Fe2+,Mg)M1Si2O6, where cations in the M2- and the M1-sites mix randomly but with intersite interactions, not only between nearest neighboring cations but also bewteen next nearest neighboring cations. In the new model, overall excess interaction parameters are quantitatively deduced from the previous experimental data and critical temperature, and the activity of the jadeite component agrees reasonbly with that based on the Landau theory. Furthermore, the relations of the calculated order parameter to temperature and composition are consistent with other experimental works. These facts support the usefulness of the new model. Application of our geothermobarometer to kyanite eclogite gave metamorphic conditions around 3.4 GPa and 800°C for both samples from the Dora Maira Massif, Western Alps, and from Rongcheng County, in the Su-Lu region of Eastern China.

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