Redox-dependent Ti stable isotope fractionation on the Moon: implications for current lunar magma ocean models

In terrestrial magmas titanium is predominantly tetravalent (Ti4+), in contrast, lunar magmas are more reduced (IW-1) and hence approximately 10% of their bulk Ti content is trivalent (Ti3+). Changes in oxidation state and coordination number are both important parameters that can serve to drive Ti stable isotope fractionation. As such, mineral-mineral and mineral-melt Ti stable isotope fractionation factors determined for terrestrial samples may not be appropriate for lunar samples that formed under more reducing conditions. To address this issue, several experiments were carried out in gas mixing furnaces over a range of fO(2) (air to IW-1) to determine Ti stable isotope fractionation factors for minerals, such as ilmenite, clinopyroxene and rutile that are highly abundant on the Moon. Results show that the extent of Ti stable isotope fractionation significantly increases with decreasing fO(2). For example, the isotopic difference between ilmenite and residual melt (Delta Ti-49(ilmenite-melt)) is resolvably lower by similar to 0.44 parts per thousand from terrestrial-like FMQ-0.5 to lunar-like IW-1 at an intermediate precision of +/- 0.003 parts per thousand (95% c.i. OL-Ti). This confirms that fractionation factors determined for terrestrial conditions are indeed not applicable to lunar settings. Our new fractionation factors for ilmenite, clinopyroxene and silicate melt are mostly consistent with those previously determined by ab initio modelling based on density-functional theory. Using our new experimental data in conjunction with previously published high-precision HFSE data and Ti stable isotope data of lunar basalts, we modelled the solidification of the Lunar Magma Ocean (LMO). The model for LMO solidification included fractionation of Ti stable isotopes not only by Ti-oxides, but also by typical lunar silicate minerals as pyroxene or olivine. The resulting delta Ti-49 for urKREEP and ilmenite-bearing cumulates are within error of previous estimates, but also indicate that ilmenite-bearing cumulates must have contained around 15% ilmenite.

Automated summary

The fractionation factors of titanium stable isotopes in lunar magmas are different from those in terrestrial magmas due to the reduced conditions on the Moon. Experimental data obtained for common lunar minerals, such as ilmenite, clinopyroxene, and rutile, show that the extent of titanium stable isotope fractionation is significantly influenced by the oxygen fugacity. The new fractionation factors are consistent with ab initio modeling based on density-functional theory. These new data are used to model the solidification of the Lunar Magma Ocean, providing insights into the composition and formation of lunar rocks.