Berry, Andrew J.Doyle, Patricia M.Schofield, Paul F.Miller, Laura A.Losq, Charles LeBurnham, Antony D.Mosselmans, J. Fred W.2025-03-162025-03-16researchoutputwizard:a383154xPUB46568Scopus:85182341272https://dspace-test.anu.edu.au/handle/1885/733717895Titanium occurs as Ti3+, in addition to the more usual Ti4+, in extraterrestrial materials such as Lunar basalts and chondritic meteorites. The proportion of Ti as Ti3+ was investigated by Ti K-edge X-ray absorption near edge structure (XANES) spectroscopy for five silicate glass compositions quenched from melts equilibrated at 1400 °C, atmospheric pressure, and oxygen fugacities (fO2) in log units relative to the fayalite-magnetite-quartz (FMQ) buffer from FMQ+3.3 to FMQ-10.2 (+6.6 to −6.9 log units relative to the iron-wüstite, IW, buffer). All spectra could be well fit using a linear combination of the spectra recorded from the most oxidised and reduced samples of the same composition, indicating that the samples only contain two Ti species. Ti3+/ΣTi (where ΣTi = Ti3+ + Ti4+) = 0 for the most oxidised samples but is unknown for the most reduced. Thus, the linear combination fit results were used in a regression model in which Ti3+/ΣTi of the reduced end-member was varied to give Ti3+/ΣTi values of the other samples that best fit the thermodynamically expected dependence of Ti3+/ΣTi on fO2. The most reduced samples were found to have Ti3+/ΣTi ∼ 0.6. The resulting modified equilibrium constants of the Ti oxidation reaction, logK', are linearly correlated with the optical basicity (Λ) parameterisation of melt composition, such that as Λ increases, Ti3+/ΣTi decreases, at constant fO2. This correlation allows Ti3+/ΣTi to be predicted for other compositions and, assuming that the temperature dependence of Ti3+/Ti4+ is parallel to FMQ, a general equation relating Ti3+/Ti4+ to fO2 was obtained: log(Ti3+/Ti4+) = −0.25ΔFMQ − 0.32(19) − 3.44(32)Λ. This equation was used to predict Ti3+/ΣTi as a function of fO2 for high-Ti Mare basalt, chondrule (CV and CM), and calcium aluminium inclusion (CAI; Type A and B) compositions. For melts of these compositions Ti3+ = Ti4+ at ∼ FMQ-10.8, −9.5, −9.3, −10.6, and −10.2 (∼IW-7.5, −6.2, −6.0, −7.3, and −6.9), respectively, independent of temperature.We thank John Spratt for assistance with the EPMA, Daniel Sells and the EPSRC UK National Electron Paramagnetic Resonance Facility at the University of Manchester for enabling us to acquire the EPR spectra, Diamond Light Source for the award of beamtime for proposal SP4516, and Hugh O'Neill for providing a template for the least squares minimisation model. A.J.B. thanks the Engineering and Physical Sciences Research Council (UK) for the award of a CNA studentship, which was used to support P.M.D. We thank Michael Ackerson and two anonymous reviewers for their comments on the work.11EnglishPublisher Copyright: © 2023 The Author(s)Calcium aluminium inclusionChondruleMare basaltOxidation stateOxygen fugacitySilicate glassTitaniumXANES spectroscopy2024-02-0110.1016/j.gca.2023.11.014http://www.scopus.com/inward/record.url?scp=85182341272&partnerID=8YFLogxK