The oxidation state of titanium in silicate melts

Titanium 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.