Ti^(3+) in meteoritic and synthetic hibonite

Abstract

Electron spin resonance has been used to make the first direct determination of Ti^(3+) in synthetic hibonite and hibonite from inclusion SH-7 of the Murchison C2 chondrite. Ti^(3+) concentrations range from 0.02 to 0.64 wt% in synthetic blue hibonite and 0.35–0.44 wt% in hibonite from SH-7. No Ti^(3+) could be detected in orange hibonite, supporting the earlier conclusion that the orange-to-blue transition is associated with the presence of Ti^(3+). At constant temperature and oxygen fugacity, Ti^(3+)/Ti^(4+) in synthetic hibonite increases with decreasing V but is not strongly dependent on bulk Ti. At the concentration levels encountered in meteoritic hibonite, Fe and Cr contents do not have a significant effect on the amount of Ti^(3+). In both synthetic and meteoritic hibonite, Ti^(3+) occupies a 5-coordinated crystallographic site, which is consistent with the formation of doubly ionized oxygen vacancies. At low oxygen fugacities, essentially all Ti^(4+) on the five-fold Al-site has been reduced to Ti^(3+). Hibonite from SH-7 equilibrated with a gas that could have been as reducing as a gas of solar composition. This is consistent with other estimates based on mineral equilibria of high temperature oxygen fugacities in Ca-Al-rich inclusions. With the possible exception of Mo-W depletions, indicators based on bulk trace element concentrations in CAIs are inconclusive. There is considerable evidence that as CAIs cooled to lower temperatures, they experienced conditions significantly more oxidizing than those of a solar gas, perhaps in planetary environments.