Trivalent Titanium in High-Titanium Lunar Ilmenite

Lunar mare basalts are often rich in titanium, hosted predominantly within the mineral ilmenite (Fe ²⁺ Ti ⁴⁺ O ₃ ). Of all Apollo samples returned from the Moon, only two are olivine-free basalts, making them of interest for their potential to constrain models of lunar evolution. Here, we use electron microscopy and electron energy loss spectroscopy (EELS) to investigate ilmenite in one of these samples: Apollo rock 75035. 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 (ELNES), we determine that the excess Ti is reduced to trivalent Ti, with Ti ³⁺ mole fraction ~ 0.07. This discovery confirms a longstanding hypothesis based on the Moon's reducing environment. Its impact for lunar models comes through the implied oxygen fugacity during crystallization of 75035, which we estimate to be below the iron-wüstite buffer (IW): ∆IW ≤ −1.5. Our research also suggests that Ti ³⁺ -bearing ilmenite could serve as a single-phase oxybarometer for the reducing conditions found on many planetary bodies.