¹Jean-David Bodénan,²Natalie A.Starkey,³Sara S.Russell,²Ian P.Wright,²Ian A.Franchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.06.034]
¹ETH Zürich, Institute für Geochemie und Petrologie, Clausiusstrasse 25, 8092, Zürich, Switzerland
²Planetary and Space Sciences, School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom
³Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom
Copyright Elsevier

A ∼175 µm refractory inclusion, A-COR-01 from one of the least altered carbonaceous chondrites, ALHA 77307 (CO3.0), has been found to bear unique characteristics that indicate that it is one of the first solids to have formed at the very birth of the solar system while isotopic reservoirs were still evolving rapidly. Its core is composed mainly of hibonite and corundum, the two phases predicted to condense first from a gas of solar composition, and like many common types of Calcium-, Aluminium-rich Inclusions (CAIs) is surrounded by a rim of diopside.

Core minerals in A-COR-01 are very ¹⁶O-rich (Δ¹⁷O_Core = -32.5 ± 3.3 (2SD) ‰) while those in the rim display an O isotopic composition (Δ¹⁷O_Rim = -24.8 ± 0.5 (2SD) ‰) indistinguishable from that found in the vast majority of the least altered CAIs. These observations indicate that this CAI formed in a very ¹⁶O-rich reservoir and either recorded the subsequent evolution of this reservoir or the transit to another reservoir. The origin of A-COR-01in a primitive reservoir is consistent with the very low content of excess of radiogenic ²⁶Mg in its core minerals corresponding to the inferred initial ²⁶Al/²⁷Al ratio ((²⁶Al/²⁷Al)₀ = (1.67 ± 0.31) × 10^-7), supporting a very early formation before injection and/or homogenisation of ²⁶Al in the protoplanetary disk. Possible reservoir evolution and short-lived radionuclide (SLRs) injection scenarios are discussed and it is suggested that the observed isotope composition resulted from mixing of a previously un-observed early reservoir with the rest of the disk.