Abstract
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 (Δ17OCore = −32.5 ± 3.3 (2SD) ‰) while those in the rim display an O isotopic composition (Δ17ORim = −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.
Highlights
It is widely accepted that the solar system formed by the collapse of a dense molecular cloud core (e.g., Boss and Goswami, 2006 and references therein) that was part of an active star formation region
While it is possible that a missing mineral phase was originally filling this void, which was later removed by secondary processes, this is considered unlikely as such a process should have affected the other phases to some extent
A-COR-01’s mineralogy, especially the presence of corundum, and extremely 16O-rich composition, suggest that it was among the very first CAIs to form in a 16O-rich environment close to the Sun
Summary
It is widely accepted that the solar system formed by the collapse of a dense molecular cloud core (e.g., Boss and Goswami, 2006 and references therein) that was part of an active star formation region. The measurement of material recovered from the Genesis mission allowed the first direct measurement of the solar oxygen-isotope composition (Δ17O = -28.4 ± 3.6 ‰; McKeegan et al (2011), where Δ17O is the departure from the Terrestrial Fractionation Line (TFL) as calculated by the formula Δ17O = δ17O – (0.52 × δ18O). This value is lighter than most CAIs and provides an important step in understanding the initial O-isotope composition of the protoplanetary disk and the processes that modified the reservoirs from which the rocky bodies we observe today formed
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