Atomic-scale characterization of the oxidation state of Ti in meteoritic hibonite: Implications for early solar system thermodynamics

• DOI: 10.2138/am-2022-8311
• Publication Date: 2022-04-20T20:45:04Z
• Authors: Pierre-Marie Zanetta; Venkateswara Rao ...; Yao-Jen Chang; Tarunika Ramprasad; Juliane Weber; John R. Beckett; Thomas J. Zega
• Keywords: Allende meteorite

Abstract Calcium-aluminum-rich inclusions (CAIs) in chondritic meteorites are composed of refractory minerals thought to be the first solids have formed solar nebula. Among them, hibonite, nominally CaAl12O19, holds particular interest because it can incorporate significant amounts Ti into its crystal structure both Ti3+ and Ti4+ oxidation states. The relative these cations that incorporated reflect redox conditions under which grain or last equilibrated their measurement provide insight thermodynamic landscape early Here we develop a new method for quantification states using electron energy-loss spectroscopy (EELS) an aberration-corrected scanning transmission microscope (STEM) apply hibonite. Using series Ti-bearing oxides, find onset intensity L2,3 edge decreases with increasing Ti-oxidation state, is corroborated by simulated Ti-oxide spectra first-principles density-functional theory. We test relationship on set synthetic hibonite grains known Ti4+/ΣTi values developed from compact type A inclusion Northwest Africa (NWA) 5028 CR2 carbonaceous chondrite. STEM-EELS data show zoned ratio ranging 0.78 ± 0.04 0.93 over scale 100 nm between core grain, respectively. substitution sites characterized experimental calculated high-angle annular-dark-field (HAADF) images atomic-level EEL spectrum imaging. Simulated HAADF reveal distributed M2 M4 while Mg sits M3 site. Quantitative energy-dispersive X-ray shows this also Al Ti. distribution not well correlated at nanoscale. spatial decoupling element composition suggests multistage evolution grain. hypothesize were during condensation high temperature through multiple reactions. Transient heating, presumably nebula, adds complexity chemistry potentially redistributed Mg. Concurrently, formation oxygen vacancies as result reducing gas, led reduction Ti3+. defect reactions occurring single preclude simple fO2 formation. However, moving forward, measurements fundamental inputs modeling