One of the earliest refractory inclusions and its implications for solar system history

A similar to 175 mu m 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 O-16-rich (Delta O-17(Core) = 32.5 +/- 3.3 (2SD) parts per thousand) while those in the rim display an O isotopic composition (Delta O-17(Rim) = -24.8 +/- 0.5 (2SD) parts per thousand) indistinguishable from that found in the vast majority of the least altered CAIs. These observations indicate that this CAI formed in a very O-16-rich reservoir and either recorded the subsequent evolution of this reservoir or the transit to another reservoir. The origin of A-COR-01 in a primitive reservoir is consistent with the very low content of excess of radiogenic Mg-26 in its core minerals corresponding to the inferred initial Al-26/Al-27 ratio ((Al-26/Al-27)(0) = (1.67 +/- 0.31) x 10(-7)), supporting a very early formation before injection and/or homogenisation of Al-26 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. (C) 2020 The Authors. Published by Elsevier Ltd.