Stability relations of ilmenite and ulvöspinel in the Fe-Ti-O system and application of these data to lunar mineral assemblages

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), ΔG_r⁰=−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.