Ti substitution mechanisms in hibonite and the determination of Ti3+/Ti4+ by X-ray absorption spectroscopy for use as an oxybarometer with application to early Solar System processes

Quantitative determination of element species is crucial for understanding geological processes, particularly changing redox conditions. As Ti3+ is stable under very reducing conditions, Ti3+/Ti4+ oxybarometers are suitable for investigating early Solar System processes. The systematics of Ti3+/Ti4+ in Ti-bearing CaO-MgO-Al2O3-SiO2 (Ti-CMAS) glasses and both synthetic and meteoritic hibonite (Ca(Al,Ti,Mg)12O19) were investigated using X-ray absorption spectromicroscopy techniques. For this purpose suites of synthetic samples with 0-100% Ti3+/ΣTi (where ΣTi = Ti3+ + Ti4+) were prepared in a 1 atm gas mixing furnace. The samples were characterised by electron microscopy, and when relevant, by X-ray diffraction and neutron powder diffraction (NPD). The crystal structure of hibonite has five distinct Al sites onto which Ti may substitute. Structural refinements of NPD data using the Rietveld refinement method were used to determine Ti3+ and Ti4+ site occupancy in hibonite. Ti3+ was found to occupy the M4 octahedral site, whereas Ti4+ partitions between the M2 trigonal bipyramidal site and the M4 octahedral site. For hibonite with <1 Ti per formula unit Ti4+ partitions between the M2 and M4 site in a roughly constant ratio of 1:4.3, irrespective of the Ti3+ content. Ti K-edge X-ray absorption near edge structure (XANES) spectra were collected from Ti-CMAS glasses and both synthetic and meteoritic hibonite. Ti3+/Ti4+ oxybarometers were developed using the synthetic samples, and the Ti3+/Ti4+ K-edge oxybarometer was used to quantify Ti3+/ΣTi within hibonite from six different meteorites. Ti3+ hibonite was found in CAIs from the Acfer094 (unique), El Djouf001 (CR2), Murchison (CM2) and NWA1476 (R3) meteorites, whereas hibonite within CAIs from the NWA2446 (R3) and Hughes030 (R3-6) meteorites were Ti3+ poor. Furthermore, the Ti3+/ΣTi varied within hibonite grains in CAIs from Acfer094 and El Djouf001. The presence of Ti3+ within hibonite in CAIs with no alteration minerals, or those that only experienced low temperature alteration, suggests they formed and/or were reprocessed under very reducing conditions (