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 degrees C, atmospheric pressure, and oxygen fugacities (fO(2)) 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-w & uuml;stite, 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+/Sigma Ti (where Sigma Ti = 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+/Sigma Ti of the reduced end-member was varied to give Ti3+/Sigma Ti values of the other samples that best fit the thermodynamically expected dependence of Ti3+/Sigma Ti on fO(2). The most reduced samples were found to have Ti3+/Sigma Ti similar to 0.6. The resulting modified equilibrium constants of the Ti oxidation reaction, logK', are linearly correlated with the optical basicity (Lambda) parameterisation of melt composition, such that as Lambda increases, Ti3+/Sigma Ti decreases, at constant fO(2). This correlation allows Ti3+/Sigma Ti 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 fO(2) was obtained: log(Ti3+/Ti4+) = -0.25 Delta FMQ - 0.32(19) - 3.44(32)Lambda. This equation was used to predict Ti3+/Sigma Ti as a function of fO(2) 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 similar to FMQ-10.8, -9.5, -9.3, -10.6, and -10.2 (similar to IW-7.5, -6.2, -6.0, -7.3, and -6.9), respectively, independent of temperature.