Two mineralogic geobarometers based on the assemblage garnet-plagioclase-pyroxeneqvartz, common in granulite-grade quartzofeldspathic and basic lithologies, have been calibrated almost entirely from measured thermodynamic quantities, especially enthalpy of solution and heat capacity measurements For the continuous reaction: CaAl2Si2O6 + Mg2Si2O6: l/3Ca3Al2Si3O12 * 2/3Mg3Al2Si3Or2 + SiO2 (A) (plagioclase) (opx) the resulting geobarometric expression is: (quartz)

http://minsocam.org/ammin/am67/am67_203.pdf

Thermodynamic calibration of geobarometers based on the assemblages garnet-plagioclase-orthopyroxene (clinopyroxene)-quartz

P>We complete the development and description of a thermodynamic method for the computation of phase equilibria and physical properties of multiphase mantle assemblages. Our previous paper focused on the computation of physical properties. In this paper, our focus shifts to the phase equilibria. We further develop our theory to specify the ideal and excess contributions to solution properties and derive properties of multiphase assemblages. We discuss our global inversion strategy for determining the values of the free parameters in our theory and compare inverted parameter values with expectations based on scaling arguments. Comparisons between our method and experimental phase equilibria data encompass the pressure-temperature regime of Earth's mantle. Finally, we present applications of our method that illustrate how it may be used to explore the origins of mantle structure and mantle dynamics. Continuing rapid advances in experimental and theoretical petrology and mineral physics have motivated an expansion of the scope of our model via the addition of several new phases, and of the soda component: an appendix lists all parameters in our model and references to the experimental and theoretical studies that constrain them. Our algorithm for global minimization of the Gibbs free energy is embodied in a code called HeFESTo, and is detailed in a second appendix.

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Thermodynamics of mantle minerals - II. Phase equilibria

Thermochemistry of the high structural state plagioclases

Until recently, the development of mineralogic geobarometry of crustal metamorphic assemblages has lagged far behind mineralogic geothermometry. Temperatures of crystallization of metamorphic rocks can be deduced from a number of relatively reliable thermometers. Pressures of metamorphism, on the other hand, have been estimated mainly semiquantitatively with reference to a few simple univariant equilibria such as the Al2SiO5 polymorphic relations. Various geobarometers based on continuous variation of mineral compositions have been proposed, but most of these are still in the conceptual stage, either lacking an underpinning of experimental or thermodynamic calibration or beset with as yet unresolved experimental difficulties, such as the H2O content of cordierite and its debated effects on the cordierite-garnet (Fe, Mg) partitioning geobarometer (Newton and Wood, 1979).

L.S. Darken

Papers

Statistical thermodynamic models for ideal oxide and silicate solid solutions, with application to plagioclase

Water and magmas: application of the Gibbs-Duhem equation: a response