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Some constraints on the thermodynamic mixing properties of Fe-Mg orthopyroxenes and olivines

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Abstract

Existing data on the temperature and composition dependence of the Fe2+-Mg2+ distribution between Fe-Mg olivine and orthopyroxene, the intra-crystalline distribution of Fe2+ and Mg2+ between M1 and M2 sites in orthopyroxene, and macroscopic activity-composition relations in olivine and orthopyroxene are shown to be inconsistent with generally accepted thermodynamic formulations which assume that the non-configurational Gibbs energy of orthopyroxene is independent of the degree of long-range ordering of Fe2+ and Mg+ between M1 and M2 sites. These data are interpreted in terms of the constraints they provide on the size of Bragg-Williams type energy, entropy, and volume terms for olivine and orthopyroxene. The apparent equilibrium constant for Fe-Mg exchange between olivine and orthopyroxene is shown to be a potentially useful ‘geothermometer’ for olivine-orthopyroxene assemblages with olivines with mole fraction of Fe2SiO4 component less than 0.2 or greater than 0.6. A provisional calibration of this ‘geothermometer’ is presented.

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  1. Department of Geological Sciences, Harvard University, 02138, Cambridge, Massachusetts, USA

    Richard O. Sack

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Sack, R.O. Some constraints on the thermodynamic mixing properties of Fe-Mg orthopyroxenes and olivines. Contr. Mineral. and Petrol. 71, 257–269 (1980). https://doi.org/10.1007/BF00371667

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