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* OFM-Research—West, 7336 24th Avenue NE, Seattle, Washington 98115
** Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195-1310
Corresponding author: ghiorso{at}ofm-research.org
A model for the thermodynamic properties of rhombohedral oxide solid solutions in the system Fe2O3-FeTiO3-MgTiO3-MnTiO3 (containing minor amounts of Al2O3) is presented. The model accounts for temperature and compositionally dependent long-range cation-order and the related high to low symmetry structural phase transition. The model is calibrated from the cation-ordering data of Harrison and others (2000; Harrison and Redfern, 2001) and experimental data on Fe+2Ti
The new model forms the basis of a revised FeTi-oxide geothermometer/oxygen barometer, which is applied to a newly compiled dataset of natural two oxide pairs from silicic volcanic rocks. Results are compared to previous formulations with the general conclusion that the new model gives a better estimate of oxidation state for magmas that equilibrated under conditions more oxidizing than the nickel-nickel oxide buffer. Estimates of oxygen fugacity are fairly insensitive to analytical uncertainties in oxide compositions. By contrast, temperature estimates are especially sensitive to analytical error and to the abundances of "minor" constituents. Application of the geothermometer to oxide pairs that grew under conditions where the rhombohedral phase was cation disordered (that is high temperature or at oxygen fugacities greater by about one log10 unit than the nickel-nickel oxide buffer) results in an uncertainty due solely to analytical error of at least 50°C and sometimes as high as 100 °C. Temperature estimates from the new geothermometer can be made using either the Fe+2Ti
Software that implements the new thermodynamic model and the two-oxide geothermometer/oxygen barometer is available from http://www.ofm-research.org/.
(Fe+3)2 exchange between rhombohedral oxide and spinel from Lattard and others (2005) and Evans and others (2006). Successful calibration require introduction of an energetic contribution attributed to short-range cation-order, which reduces the configurational entropy of the solid solution. The resultant thermodynamic model for the rhombohedral oxides is internally consistent with the model for spinel solid solutions of Sack and Ghiorso (1991a, 1991b) and with the endmember thermodynamic properties database of Berman (1988); a new model equation for the isobaric heat capacity of ulvöspinel (cubic Fe2TiO4) is proposed and values of the enthalpy of formation, –1490.417 kJ/mol, and third law entropy, 184.199 J/K-mol, at 298.15 K and 105 Pa are recommended. (Fe+3)2 exchange or Fe+2 Mg exchange between the two oxides. Comparison of the two temperature estimates provides a means of evaluating the internal consistency of coexisting oxide compositions and assessing the extent of disequilibrium. Temperatures calculated from the new model are found to be consistent with experimental phase relations for the stability of cummingtonite in silicic volcanics. Other petrologic constraints on derived temperatures are examined including limits on the width of the miscibility gap and the development of self-reversed remanent magnetization in the rhombohedral series.
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  S. A. McEnroe, K. Fabian, P. Robinson, C. Gaina, and L. L. Brown Crustal Magnetism, Lamellar Magnetism and Rocks That Remember Elements, August 1, 2009; 5(4): 241 - 246. [Abstract] [Full Text] [PDF]
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