Fe-Mn partitioning between garnet and ilmenite: experimental calibration and applications |
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| Authors: | Mark I. Pownceby Victor J. Wall Hugh St. C. O'Neill |
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| Affiliation: | (1) Earth Sciences Department, Monash University, 3168 Clayton, Australia;(2) R.S.E.S. Australian National University, Canberra, A.C.T., Australia;(3) Present address: C.R.A. Research A.T.D., P.O. Box 42, 2284 Boolaroo, N.S.W., Australia |
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| Abstract: | A new mineralogic geothermometer based on the partitioning of Fe and Mn between garnet and ilmenite has been calibrated by reversal experiments in the P-T range 600–900° C, 2 and 5 kbars and for fO2=QFM. The results constitute a sensitive geothermometer applicable over a broad range of composition and conditions. Garnetilmenite thermometry has advantages relative to existing geothermometers because of its accurate calibration, marked temperature sensitivity and the chemical and structural simplicity of the crystalline solutions involved. Application to natural assemblages reveals that the garnet-ilmenite geothermometer yields temperatures that agree well with other estimates. The reactivity of, and relatively rapid Fe-Mn diffusion in ilmenite may lead to retrograde resetting of high temperature partition values, but these factors may be useful for estimating rock cooling rates. Analysis of the experimental data indicates minor positive deviations from ideality for Fe-Mn garnets and ilmenites. Absolute magnitudes of interaction parameters (WAB) derived from a regression analysis are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the difference between solution parameters. These differences are well constrained with a magnitude of WFeMnilm–WFeMngar 300 cal mol–1. The accuracy and applicability of garnet-ilmenite thermometry will improve with the availability of better thermodynamic data for garnet crystalline solutions.Abbreviations and symbols used in text R universal gas constant (cal/mol/°K) - T absolute temperature (°K or °C) - P pressure (kbars) -  V0 volume change of reaction (1) - H1, T0 standard state enthalpy change of reaction (1) at 1 bar and the T of interest, in cal/mole - ST0 entropy change of reaction (1) at T of interest, in cal/mole/°K - GP,T0 standard free energy change of reaction (1) at the T and P of interest, in cal/mole -  distribution coefficient for Fe-Mn partitioning between garnet and ilmenite - K apparent equilibrium coefficient for reaction (1) -  ij activity of component i in phase j - WA-B binary A-B interaction (Margules) parameter - gar garnet - ilm ilmenite - biot biotite - ol olivine - opx orthopyroxene |
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