A refractory inclusion with solar oxygen isotopes and the rarity of such objects in the meteorite record

NASA's Genesis mission revealed that the Sun is enriched in O-16 compared to the Earth and Mars (the Sun's Delta O-17, defined as delta O-17-0.52x delta O-18, is -28.4 +/- 3.6 parts per thousand; McKeegan et al. 2011). Materials as O-16-rich as the Sun are extremely rare in the meteorite record. Here, we describe a Ca-Al-rich inclusion (CAI) from a CM chondrite that is as O-16-enriched as the Sun (Delta O-17 = -29.1 +/- 0.7 parts per thousand). This CAI also has large nucleosynthetic anomalies in Ca-48 and Ti-50 (delta-values are -8.1 +/- 3.3 and -11.7 +/- 2.4 parts per thousand, respectively) and shows no clear evidence for incorporation of live Al-26; (Al-26/Al-27)(0) = (0.03 +/- 0.11) x 10(-5). Due to their anomalous isotopic characteristics, the rare CAIs consistent with the Genesis value could be among the first materials that formed in the solar system. In contrast to the CAI studied here, the majority of CAIs formed in or interacted with a reservoir characterized by a Delta O-17 value near -23.5 parts per thousand. Combined with Al-26-Mg-26 systematics, the oxygen isotopic compositions of FUN (fractionation and unidentified nuclear effects), UN, and normal CAIs suggest that nebular conditions were favorable for solids to inherit this value for an extended period of time. Many later-formed materials, such as chondrules, planetesimals, and terrestrial planets, formed in reservoirs with Delta O-17 near 0 parts per thousand. The distribution could be easier to explain if the common CAI value of -23.5 parts per thousand, which is consistent with the Genesis value within 3 sigma, represented the average composition of the protoplanetary disk.