Iron-Titanium Oxide Thermometry: Is Chromium a Perturbing Element?
Because Cr is an important supplementary element in Fe-Ti oxide pairs from a variety of terrestrial and lunar rocks equilibrated at high temperatures and reducing conditions (e.g. xenoliths in kimberlites, lunar basalts) relevant phase equilibrium experiments appear necessary to test the new formulations of the Fe-Ti oxide thermometer.
We have performed synthesis and re-equilibration experiments in the Fe-Ti-Cr-O and Fe-Ti-Cr-Mg-O systems at 1 bar in the sub-solidus temperature range 900-1300°C at low to moderate oxygen fugacity values (ΔNNO between -4 and -1). The run products are polycrystalline assemblages of titanomagnetite and ilmenite solid solutions with Cr2O3 contents between 12 and 18 wt.% for the spinels and between 0.7 and 4 wt.% for the ilmenites. Preliminary results show that in most cases both our formulation and that of Ghiorso & Evans (2008) overestimate the temperature, especially for assemblages equilibrated under strongly reducing conditions.