Some constraints on the thermodynamic mixing properties of Fe-Mg orthopyroxenes and olivines |
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| Authors: | Richard O Sack |
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| Institution: | (1) Department of Geological Sciences, Harvard University, 02138 Cambridge, Massachusetts, USA |
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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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