Characterisation of oxygen isotopic reservoirs in the early solar system

Bodénan, Jean-David André (2016). Characterisation of oxygen isotopic reservoirs in the early solar system. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f6be

Abstract

Calcium-Aluminium-rich inclusions (CAls) are the first solids to form in the early Solar System. They are often surrounded by Wark-Lovering (WL) rims, composed of sequential mineral layers, that offer a record of the conditions during the earliest stages of Solar System evolution, just after CAI formation. Analyses of oxygen and magnesium isotopes in nine CAIs and their WL rims selected in CO, CR, and CV chondrites, displaying different alteration histories, permit an investigation of the evolution of O isotope compositions in the early Solar System, and the processes that caused it.

The study of a very early corundum-hibonite CAI, A77307-COR-01, and its rim in one of the least altered COs revealed that it originally formed in a very ¹⁶O-rich reservoir (Δ¹⁷O ~ -33 ‰), before injection or homogenisation of ²⁶A1 in the Solar System. The O isotope composition of the rim, with Δ¹⁷O ~ 24 ‰, reveals that the composition of the CAI-forming region evolved between the formation of the core and rim, probably by mixing of ¹⁶O-poor dust and ¹⁶O-rich gas.

Identical O isotope compositions (Δ¹⁷O ~ 24 ‰) have been measured in rims around CAls in CR3.0, CO3.0 and CV3_Red, implying that all the rims of the CAls studied here formed by condensation in the same O isotope reservoir, most likely during a single event. Moreover, the core minerals in these CAls (with the exception of A773037-COR-01) have the same O isotope compositions, indicating that the reservoir with Δ¹⁷O ~ 24 ‰ was present during the whole duration of Type A, fine-grained CAls, and WL rim formation and that previously reported O isotope variations in WL rims are caused by secondary processes occurring after formation of these minerals. These observations suggest that self-shielding processes were temporally or spatially separated from the region of CAI and WL rim formation.