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

摘要

Chondrites consist of three major components: refractory inclusions (Ca,Al-rich inclusions [CAIs] 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 CAIs 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. CAIs 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 CAIs were melted in and outside their formation region during transient heating events of still unknown nature. In unmetamorphosed, type 2-3.0 chondrites, CAIs show large variations in the initial Al-26/Al-27 ratios, from PPD suggest late injection of Al-26 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 CAIs characterized by distinct mineralogies, textures, and isotopic (O, Mg, Ca, Ti, Mo, etc.) compositions, the Al-26 heterogeneity in the CAI-forming region(s) precludes determining the duration of CAIs formation using Al-26-Mg-26 systematics. The existence of multiple generations of CAIs and the observed differences in CAI abundances in carbonaceous and noncarbonaceous chondrites may indicate that CAIs 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 CAIs surrounded by Wark-Lovering rims have uniform Delta O-17 (= delta O-17-0.52 x delta O-18) of similar to -24 parts per thousand; however, there is a large range of Delta O-17 (from similar to-40 to similar to -5 parts per thousand) among them, suggesting the coexistence of O-16-rich (low Delta O-17) and O-16-poor (high Delta O-17) gaseous reservoirs at the earliest stages of the PPD evolution. The observed variations in Delta O-17 of CAIs may be explained if three major O-bearing species in the solar system (CO, H2O, and silicate dust) had different O-isotope compositions, with H2O and possibly silicate dust being O-16-depleted relative to both the Genesis solar wind Delta O-17 of -28.4 +/- 3.6 parts per thousand and even more O-16-enriched CO. Oxygen isotopic compositions of CO and H2O could have resulted from CO self-shielding in the protosolar molecular cloud (PMC) and the outer PPD. The nature of O-16-depleted dust at the earliest stages of PPD evolution remains unclear: it could have either been inherited from the PMC or the initially O-16-rich (solar-like) MC dust experienced O-isotope exchange during thermal processing in the PPD.