Refractory inclusions in carbonaceous chondrites: Records of early solar system processes

Chondrites consist of three major components: refractory inclusions (Ca,Al‐rich inclusions [ CAI s] and amoeboid olivine aggregates), chondrules, and matrix. Here, I summarize recent results on the mineralogy, petrology, oxygen, and aluminum‐magnesium isotope systematics of the chondritic components (mainly CAI s in carbonaceous chondrites) and their significance for understanding processes in the protoplanetary disk ( PPD ) and on chondrite parent asteroids. CAIs are the oldest solids originated in the solar system: their U‐corrected Pb‐Pb absolute age of 4567.3 ± 0.16 Ma is considered to represent time 0 of its evolution. CAI s formed by evaporation, condensation, and aggregation in a gas of approximately solar composition in a hot (ambient temperature >1300 K) disk region exposed to irradiation by solar energetic particles, probably near the protoSun; subsequently, some CAI s were melted in and outside their formation region during transient heating events of still unknown nature. In unmetamorphosed, type 2–3.0 chondrites, CAI s show large variations in the initial 26 Al/ 27 Al ratios, from <5 × 10 –6 to ~5.25 × 10 –5 . These variations and the inferred low initial abundance of 60 Fe in the PPD suggest late injection of 26 Al by a wind from a nearby Wolf–Rayet star into the protosolar molecular cloud core prior to or during its collapse. Although there are multiple generations of CAI s characterized by distinct mineralogies, textures, and isotopic (O, Mg, Ca, Ti, Mo, etc.) compositions, the 26 Al heterogeneity in the CAI ‐forming region(s) precludes determining the duration of CAI s formation using 26 Al‐ 26 Mg systematics. The existence of multiple generations of CAI s and the observed differences in CAI abundances in carbonaceous and noncarbonaceous chondrites may indicate that CAI s were episodically formed and ejected by a disk wind from near the Sun to the outer solar system and then spiraled inward due to gas drag. In type 2–3.0 chondrites, most CAI s surrounded by Wark–Lovering rims have uniform Δ 17 O (= δ 17 O−0.52 × δ 18 O) of ~ −24‰; however, there is a large range of Δ 17 O (from ~−40 to ~ −5‰) among them, suggesting the coexistence of 16 O‐rich (low Δ 17 O) and 16 O‐poor (high Δ 17 O) gaseous reservoirs at the earliest stages of the PPD evolution. The observed variations in Δ 17 O of CAI s may be explained if three major O‐bearing species in the solar system ( CO , H 2 O, and silicate dust) had different O‐isotope compositions, with H 2 O and possibly silicate dust being 16 O‐depleted relative to both the Genesis solar wind Δ 17 O of −28.4 ± 3.6‰ and even more 16 O‐enriched CO . Oxygen isotopic compositions of CO and H 2 O could have resulted from CO self‐shielding in the protosolar molecular cloud (PMC) and the outer PPD . The nature of 16 O‐depleted dust at the earliest stages of PPD evolution remains unclear: it could have either been inherited from the PMC or the initially 16 O‐rich (solar‐like) MC dust experienced O‐isotope exchange during thermal processing in the PPD . To understand the chemical and isotopic composition of the protosolar MC material and the degree of its thermal processing in PPD , samples of the primordial silicates and ices, which may have survived in the outer solar system, are required. In metamorphosed CO 3 and CV 3 chondrites, most CAI s exhibit O‐isotope heterogeneity that often appears to be mineralogically controlled: anorthite, melilite, grossite, krotite, perovskite, and Zr‐ and Sc‐rich oxides and silicates are 16 O‐depleted relative to corundum, hibonite, spinel, Al,Ti‐diopside, forsterite, and enstatite. In texturally fine‐grained CAI s with grain sizes of ~10–20 μm, this O‐isotope heterogeneity is most likely due to O‐isotope exchange with 16 O‐poor (Δ 17 O ~0‰) aqueous fluids on the CO and CV chondrite parent asteroids. In CO 3.1 and CV 3.1 chondrites, this process did not affect Al‐Mg isotope systematics of CAI s. In some coarse‐grained igneous CV CAI s, O‐isotope heterogeneity of anorthite, melilite, and igneously zoned Al,Ti‐diopside appears to be consistent with their crystallization from melts of isotopically evolving O‐isotope compositions. These CAI s could have recorded O‐isotope exchange during incomplete melting in nebular gaseous reservoir(s) with different O‐isotope compositions and during aqueous fluid–rock interaction on the CV asteroid.