Stability relations of ilmenite and ulvöspinel in the Fe-Ti-O system and application of these data to lunar mineral assemblages |
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Authors: | Lawrence A. Taylor |
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Affiliation: | Department of Geosciences, Purdue University, Lafayette, Indiana 47907, USA; National Aeronautics and Space Administration Manned Spacecraft Center, Houston, Texas 77058, USA; Department of Geosciences, Purdue University, Lafayette, Indiana 47907, USA |
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Abstract: | The fO2 stability relations of ilmenite and ulvöspinel were determined using C-O H-N gas-flow apparatus with fO2 measured by a solid ceramic (calcia-zirconia) oxygen electrolyte cell. For Fe+TiO2 + 1/2 O2 =FeTiO3 (from 850°–1050°C), 1/2 log fO2=(−11,250/T) + 0.98 and for Fe+FeTiO3 + 1/2 O2 =Fe2TiO4 (from 850°–1210°C), 1/2 logfO2 = (−12,170/T) + 1.93. These curves lie at significantly higher values of ?O2 than determined by previous investigators (i.e., 3/4 and1/4 order of magnitude for ilmenite and ulvöspinel, respectively). In addition, for Fe+ 2TiO2 + 1/2 O2 =FeTi2O5 (1210°C), ΔGr0=−45.8 ± 0.6 kcal. The QFI curve crosses the ulvöspinel reduction curve at ∼950°C and is at lower values of fO2 below this temperature. The occurrences of fayalite reduction to SiO2 + Fe in lunar rock 14053, as well as a new finding of this assemblage in 14072, are evidence for extreme sub-solidus reduction, whereas ulvöspinel breakdown alone occurs under less reducing conditions. The ‘complete’ reduction of ulvöspinel to TiO2 + Fe occurs in 2 steps: first, to ilmenite + Fe and then, however more slowly, to rutile + Fe. Thus, the presence of ulvöspinel but lack of ilmenite reduction in lunar rocks cannot be used as evidence that the fO2 was between the associated curves — only upper limits of fO2 can be inferred. |
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