New constraints on the relationship between 26Al and oxygen, calcium, and titanium isotopic variation in the early Solar System from a multielement isotopic study of spinel-hibonite inclusions

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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 26Al/27Al ratios, from <5 × 10–6 to ~5.25 × 10–5. These variations and the inferred low initial abundance of 60Fe in the PPD suggest late injection of 26Al 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 26Al heterogeneity in the CAI‐forming region(s) precludes determining the duration of CAIs formation using 26Al‐26Mg 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 Δ17O (= δ17O−0.52 × δ18O) of ~ −24‰; however, there is a large range of Δ17O (from ~−40 to ~ −5‰) among them, suggesting the coexistence of 16O‐rich (low Δ17O) and 16O‐poor (high Δ17O) gaseous reservoirs at the earliest stages of the PPD evolution. The observed variations in Δ17O 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 16O‐depleted relative to both the Genesis solar wind Δ17O of −28.4 ± 3.6‰ and even more 16O‐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 16O‐depleted dust at the earliest stages of PPD evolution remains unclear: it could have either been inherited from the PMC or the initially 16O‐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...

“…(3) In the isotopically anomalous PLACs, Δ 17 O correlates with a magnitude of isotope anomalies for 50 Ti and 48 Ca (Kööp et al. 2016b) (Fig. ).…”

Section: Oxygen Isotopic Compositions Of Chondrites and Their Componentsmentioning

confidence: 93%

Section: Origin and Distribution Of 26al In The Ppdmentioning

confidence: 99%

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

Krot

2019

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“…; Kööp et al. ; Krot et al. ), suggesting formation in a gas of approximately solar composition and indicating that 16 O‐rich compositions are primary values.…”

Section: Discussionmentioning

confidence: 98%

Oxygen and Al‐Mg isotopic compositions of grossite‐bearing refractory inclusions from CO3 chondrites

Simon

Krot

Nagashima

2019

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The distribution of the short‐lived radionuclide 26Al in the early solar system remains a major topic of investigation in planetary science. Thousands of analyses are now available but grossite‐bearing Ca‐, Al‐rich inclusions (CAIs) are underrepresented in the database. Recently found grossite‐bearing inclusions in CO3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearing CAIs in the Dominion Range (DOM) 08006 (CO3.0) and DOM 08004 (CO3.1) chondrites. All minerals in DOM 08006 CAIs as well as hibonite, spinel, and pyroxene in DOM 08004 are uniformly 16O‐rich (Δ17O = −25 to −20‰) but grossite and melilite in DOM 08004 CAIs are not; Δ17O of grossite and melilite range from ~ −11 to ~0‰ and from ~ −23 up to ~0‰, respectively. Even within this small suite, in the two chondrites a bimodal distribution of the inferred initial 26Al/27Al ratios (26Al/27Al)0 is seen, with four having (26Al/27Al)0 ≤1.1 × 10−5 and six having (26Al/27Al)0 ≥3.7 × 10−5. Five of the 26Al‐rich CAIs have (26Al/27Al)0 within error of 4.5 × 10−5; these values can probably be considered indistinguishable from the “canonical” value of 5.2 × 10−5 given the uncertainty in the relative sensitivity factor for grossite measured by secondary ion mass spectrometry. We infer that the 26Al‐poor CAIs probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in the DOM 08006 CAIs, as well as spinel, hibonite, and Al‐diopside in the DOM 08004 CAIs retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where the CO grossite‐bearing CAIs originated. Oxygen isotopic heterogeneity in CAIs from DOM 08004 resulted from exchange between the initially 16O‐rich (Δ17O ~−24‰) melilite and grossite and 16O‐poor (Δ17O ~0‰) fluid during hydrothermal alteration on the CO chondrite parent body; hibonite, spinel, and Al‐diopside avoided oxygen isotopic exchange during the alteration. Grossite and melilite that underwent oxygen isotopic exchange avoided redistribution of radiogenic 26Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ17O of the fluid can be inferred from O‐isotopic compositions of aqueously formed fayalite and magnetite that precipitated from the fluid on the CO parent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected most CAIs in CO ≥3.1 chondrites.

“…In conclusion, the abundance, size, and population of CAIs in CO chondrites show strong similarities to those in CM chondrites. This is further supported by the more likely occurrence of corundum‐bearing inclusions, SHIBs, PLACs, and hibonite‐pyroxene microspherules in the CM and CO chondrites, which are absent or extremely rare in other chondrite groups (Macdougall 1981; Hutcheon and Steele 1982; MacPherson et al 1983, 1984; Ireland 1988; Ireland et al 1991; Greenwood et al 1994; MacPherson and Davis 1994; Simon et al 1998, 2002, 2006; Liu et al 2009; Krot et al 2012; Makide et al 2013; Kööp et al 2016a, 2016b; Needham et al 2017). We note that the SHIBs and PLACs appearing to be more abundant in the CM chondrites than the COs is mainly an artifact of the sample preparation procedures, that is, “heavy liquid separation after freeze‐thaw disaggregation” (MacPherson et al 1983, 1984).…”

Section: Discussionmentioning

confidence: 99%

Petrology and oxygen isotopic compositions of calcium‐aluminum‐rich inclusions in primitive CO3.0‐3.1 chondrites

Zhang

Bonato

King

et al. 2020

The petrologic and oxygen isotopic characteristics of calcium-aluminum-rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0-3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite-bearing inclusions (spinel-hibonite spherules, monomineralic grains, hibonite-pyroxene microspherules, and irregular/nodular objects), grossite-bearing inclusions (monomineralic grains, grossite-melilite microspherules, and irregular/nodular objects), melilite-rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel-pyroxene inclusions (fluffy objects resembling fine-grained spinel-rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene-anorthite inclusions. They are typically small (98.4 AE 54.4 µm, 1SD) and comprise 1.54 AE 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca-Alsilicate during terrestrial weathering and converted melilite-rich inclusions into spinelpyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM-like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0-3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (Δ 17 O: À25 to À2&) but left those in CO3.0-3.05 chondrites mostly unchanged (Δ 17 O: À25 to À20&), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.