Subsolidus phase relations in the system Zr-Fe-Ti-O in equilibrium with metallic iron. Implications for lunar petrology

An experimental study of the system Zr-Fe-Ti-O in equilibrium with metallic iron has been conducted at atmospheric pressure and in the temperature range 950–1300° C, with the purpose of modelling sub-solidus relations between Zr-bearing Fe-Ti oxide phases in lunar rocks. The phase relations are governed by the coexistence of the Fe-Ti oxides with ZrO₂ at temperatures below 1247° C, but with the new cubic ZFT phase at higher temperatures. ZFT, approximately Zr Fe Ti O₅, is probably related to the so-called stabilized cubic zirconia and could represent a synthetic equivalent of some lunar Zr-Fe-Ti rich oxide minerals.The zirconium contents in all Fe-, Ti and Fe-Ti-oxide phases increase with increasing temperature. At each temperature, Zr is incorporated preferentially in the Ti-oxide (up to 11 wt% ZrO₂), followed by ferropseudobrookite (up to 6 wt% ZrO₂), ilmenite (max. 4 wt%) and wüstite (max. 2.5 wt%). Ulvöspinel coexisting with ilmenite (+Fe⁰+ ZrO₂ and/or ZFT) always contains less than 1.2 wt% ZrO₂, whereas the same mineral in assemblage with wüstite-ZrO₂-Fe⁰ displays even higher ZrO₂ contents than ilmenite (above 4 wt%). Considering that the values determined here in the synthetic parageneses represent saturation concentrations at high temperatures in a simple model system, the ZrO₂ concentrations in the corresponding lunar minerals should generally be significantly lower. This is realized, except for the so-called Cr-Zr-Ca armalcolites which display ZrO₂ contents in the range 3.5–7 wt% and are thus probably related rather to Zr-rich oxide minerals than to armalcolite itself.