A NEW CALIBRATION OF THE FE-TI, TWO-OXIDE GEOTHERMOMETER AND OXYGEN BAROMETER

The thermodynamic formulation of the Fe-Ti oxide geothermometer developed by Ghiorso & Sack (1991, CMP 108, 485-510) is extended and recalibrated in light of recently published experimental measurements of cation ordering in ilmenite-hematite solid solutions (Harrison et al., 2000 AM 85, 194-205) and new two-oxide phase equilibrium experiments conducted over the temperature range 800°-1300°C and oxidation state conditions characterized by NNO-3 to NNO+3. Previous incarnations of the Fe-Ti oxide geothermometer (op. cit.; Andersen & Lindsley, 1988, AM 73, 714-26) were calibrated from experimental data collected largely at oxidation states below NNO, and consequently these calibrations extrapolate poorly to highly oxidized conditions. The new calibration models the experimental constraints equally well over the entire experimental range of temperature and oxidation state conditions. Two-oxide Fe-Ti partitioning data collected at 1200° and 1300°C confirm the necessity for including cation ordering in the expression for the configurational entropy of the rhombohedral phase. Within the framework of the adopted thermodynamic models for these oxides, the newer experimental data are internally consistent with previously published phase equilibrium measurements. Our thermodynamic models for the cubic and rhombohedral oxides permit evaluation of activity-composition relations in the systems (Fe²⁺,Mg)₂TiO₄-(Mg,Fe²⁺)(Al,Cr,Fe³⁺)₂O₄ and (Fe³⁺,Al)₂O₃-(Fe²⁺,Mg,Mn)TiO₃, respectively. This allows us to evaluate chemical potentials of endmember components of the Fe-Ti exchange [FeTiO₃(rhm) + Fe₃O₄(spn)=Fe₂O₃(rhm) + Fe₂TiO₄(spn)] and Fe-oxidation [6Fe₂O₃(rhm)=4Fe₃O₄(spn) + O₂(gas)] reactions without the necessity of projecting compositions into the Fe-Ti subsystem. Phase equilibrium experiments at 800° and 900°C (collected over a broad range of oxidation state conditions) have been designed to systematically evaluate the effects of Al, Mg and Mn on the thermodynamics of the Fe-Ti exchange reaction. These data are utilized in the calibration and are internally consistent with Fe-Ti subsystem experiments.

Presentation Time: 8:00 AM-8:20 AM
A NEW CALIBRATION OF THE FE-TI, TWO-OXIDE GEOTHERMOMETER AND OXYGEN BAROMETER
GHIORSO, Mark S.¹, EVANS, Bernard W.², SAUERZAPF, Ursula³, LATTARD, Dominique³, and SCAILLET, Bruno⁴, (1) Geophysical Sciences, The Univ of Chicago, 5734 S. Ellis Ave, Chicago, IL 60637, ghiorso@u.washington.edu, (2) Earth and Space Sciences, Univ of Washington, Box 351310, Seattle, WA 98195, (3) Mineralogisches Institut, Universität Heidelberg, INF 236, Heidelberg, D-69120, Germany, (4) CNRS-ISTO, 1A rue de la Ferollerie, Orleans Cedex 2, F-45071, France The thermodynamic formulation of the Fe-Ti oxide geothermometer developed by Ghiorso & Sack (1991, CMP 108, 485-510) is extended and recalibrated in light of recently published experimental measurements of cation ordering in ilmenite-hematite solid solutions (Harrison et al., 2000 AM 85, 194-205) and new two-oxide phase equilibrium experiments conducted over the temperature range 800°-1300°C and oxidation state conditions characterized by NNO-3 to NNO+3. Previous incarnations of the Fe-Ti oxide geothermometer (op. cit.; Andersen & Lindsley, 1988, AM 73, 714-26) were calibrated from experimental data collected largely at oxidation states below NNO, and consequently these calibrations extrapolate poorly to highly oxidized conditions. The new calibration models the experimental constraints equally well over the entire experimental range of temperature and oxidation state conditions. Two-oxide Fe-Ti partitioning data collected at 1200° and 1300°C confirm the necessity for including cation ordering in the expression for the configurational entropy of the rhombohedral phase. Within the framework of the adopted thermodynamic models for these oxides, the newer experimental data are internally consistent with previously published phase equilibrium measurements. Our thermodynamic models for the cubic and rhombohedral oxides permit evaluation of activity-composition relations in the systems (Fe²⁺,Mg)₂TiO₄-(Mg,Fe²⁺)(Al,Cr,Fe³⁺)₂O₄ and (Fe³⁺,Al)₂O₃-(Fe²⁺,Mg,Mn)TiO₃, respectively. This allows us to evaluate chemical potentials of endmember components of the Fe-Ti exchange [FeTiO₃(rhm) + Fe₃O₄(spn)=Fe₂O₃(rhm) + Fe₂TiO₄(spn)] and Fe-oxidation [6Fe₂O₃(rhm)=4Fe₃O₄(spn) + O₂(gas)] reactions without the necessity of projecting compositions into the Fe-Ti subsystem. Phase equilibrium experiments at 800° and 900°C (collected over a broad range of oxidation state conditions) have been designed to systematically evaluate the effects of Al, Mg and Mn on the thermodynamics of the Fe-Ti exchange reaction. These data are utilized in the calibration and are internally consistent with Fe-Ti subsystem experiments.
2003 Seattle Annual Meeting (November 2–5, 2003)

Session No. 149 From Oxides to Anorthosites: A Tribute to D.H. Lindsley Washington State Convention and Trade Center: 611/612 8:00 AM-12:00 PM, Tuesday, November 4, 2003 Geological Society of America Abstracts with Programs, Vol. 35, No. 6, September 2003, p. 393
© Copyright 2003 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.

2003 Seattle Annual Meeting (November 2–5, 2003)
Paper No. 149-1