Compositional variation of coexisting olivine,orthopyroxene and Fe/Mg-ferrite as a function of T and f_{O_2 }: a geothermometer and oxygen-barometer |
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| Authors: | A Dana Johnston John R Beckett |
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| Institution: | (1) Division of Geological and Planetary Sciences, California Institute of Technology, 91125 Pasadena, CA, USA;(2) Present address: Department of Geology, University of Oregon, 97403-1272 Eugene, OR, USA |
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| Abstract: | A thermodynamic model is developed that describes the compositional variation of coexisting olivine, orthopyroxene, and ferrite (Fe3O4-MgFe2O4) as a function of
and T. The ferrite phase has a cation distribution which varies from nearly inverse to nearly random with increasing T and is described with a model in which the number of sites per formula unit on which mixing occurs varies from 1.67 to 2.0. Given this model and the equilibrium phase composition data for coexisting olivine and ferrite at 1,300° C of Jamieson and Roeder (1984), the ferrite solution is described to an excellent approximation by a symmetric regular solution model with W
ft
=+14.0 ±0.3 kJ/mole. Orthopyroxene and olivine non-ideality are also considered. The T-dependence of the equilibrium constant for the oxidation reaction 6Fs+2Mt=6Fa+O2 and the two Fe/Mg exchange reactions between olivine-ferrite and olivine-orthopyroxene, are used to calculate the compositional variation of coexisting phases as a function of
and T. The results are summarized on an isobaric (1 bar)
–1/Tplot with the compositional variation of olivine, ferrite, and orthopyroxene shown by sets of isopleths. The ferrite isopleths intersect those of olivine and orthopyroxene at sufficiently high angles for this assemblage to serve as a sensitive geothermometer and oxygen-barometer. The model is applied to orthopyroxene-ferrite symplectite in coronas around olivine in a metamorphosed gabbro, to olivine-hosted orthopyroxene-ferrite symplectite in unmetamorphosed gabbros and norites and to olivine-hosted orthopyroxene-ferrite symplectites developed within the rims of lherzolite xenoliths. |
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