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Compositional variation of coexisting olivine, orthopyroxene and Fe/Mg-ferrite as a function of T and f_O_2: a geothermometer and oxygen-barometer

Johnston, A. Dana and Beckett, John R. (1986) Compositional variation of coexisting olivine, orthopyroxene and Fe/Mg-ferrite as a function of T and f_O_2: a geothermometer and oxygen-barometer. Contributions to Mineralogy and Petrology, 94 (3). pp. 323-332. ISSN 0010-7999. doi:10.1007/BF00371441.

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A thermodynamic model is developed that describes the compositional variation of coexisting olivine, orthopyroxene, and ferrite (Fe_3O_4-MgFe_2O_4) as a function of f_O_2 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+O_2 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 f_O_2 and T. The results are summarized on an isobaric (1 bar) f_O_2−1/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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Additional Information:© 1986 Springer-Verlag. Received June 6, 1986; Accepted August 4, 1986. This paper represents an outgrowth of work originally begun by the senior author as part of his Ph.D. thesis at the University of Minnesota. The guidance of Dr. James Stout during this stage of the project is gratefully acknowledged. Thorough reviews by Drs. Martin Engi and an "anonymous" reviewer were very helpful and are appreciated. The manuscript was prepared while the authors held research fellowships at Caltech supported by grants from NASA (NAG9-105) and the Earth Sciences Section of the National Science Foundation (EAR 84-16583).
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NSFEAR 84-16583
Issue or Number:3
Record Number:CaltechAUTHORS:20131016-104614822
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41944
Deposited By: Tony Diaz
Deposited On:16 Oct 2013 18:00
Last Modified:10 Nov 2021 04:36

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