Trivalent titanium in high-titanium lunar ilmenite

Lunar mare basalts are often rich in titanium, hosted predominantly within the mineral ilmenite (Fe²⁺Ti⁴⁺O₃). Here, we examine ilmenite in a ~ 3.8 billion-year-old mare basalt (Apollo rock 75035) using high-resolution electron microscopy and electron energy loss spectroscopy. A key finding is that 75035 ilmenite is itself enriched in Ti, beyond the end member of the conventional solid solution series. Using energy loss near-edge spectroscopy, we determine that the excess Ti is trivalent, with Ti³⁺ accounting for 13% of the total Ti content. This discovery confirms the presence of trivalent Ti in lunar ilmenite, long hypothesized based on the Moon's reducing environment. Accounting for the change in implied stoichiometry, a review of literature data suggests that Ti³⁺ may be present in ilmenite across a wide range of lunar samples. We extrapolate known relationships from the literature to connect Ti³⁺ to redox conditions, estimating the oxygen fugacity during crystallization of 75035 to be below the iron-wüstite buffer, ΔIW≤ - 1.6. Further quantifying the relationship between Ti valence state and oxygen fugacity would allow Ti³⁺-bearing ilmenite to serve as an oxybarometer able to access the reducing conditions found on many planetary bodies.