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

Abstract

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.

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

“…Simon et al. (2019a, 2019b) and Krot et al. () reported O‐isotope compositions of the grossite‐bearing CAIs in the DOM 08006 (CO3.0), DOM 08004 (CO3.1), and Y‐81020 (CO3.05) meteorites (Fig.…”

“…2016a, 2016b; Simon et al. 2019a, 2019b). There are several arguments suggesting that the CAIs with low ( 26 Al/ 27 Al) 0 in unmetamorphosed chondrites predate formation of the inclusions with the canonical ( 26 Al/ 27 Al) 0 .…”

“…2018b, 2018c, 2018d; Simon et al. 2019a, 2019b). The anhydrous silicates in CV chondrites (Clayton et al.…”

“…In type 2-3.0 chondrites, CAIs with approximately the canonical ( 26 Al/ 27 Al) 0 ratio (hereafter 26 Al-rich CAIs) often coexist with the CAIs having low ( 26 Al/ 27 Al) 0 (<5 9 10 À6 ; hereafter showing no evidence for late-stage thermal processing during chondrule formation, that is, surrounded by WL rims and having uniform 16 O-rich (solar-like D 17 O) isotopic compositions (Figs. 6 and 7) (Krot et al 2008a(Krot et al , 2012aMakide et al 2011Makide et al , 2013K€ o€ op et al 2016a, 2016bSimon et al 2019aSimon et al , 2019b. There are several arguments suggesting that the CAIs with low ( 26 Al/ 27 Al) 0 in unmetamorphosed chondrites predate formation of the inclusions with the canonical ( 26 Al/ 27 Al) 0 .…”

“…), and DOM 08006 (CO3.0) chondrites (Simon et al. 2019a, 2019b), the 26 Al‐rich spinel‐hibonite inclusions, and in the platy hibonite crystals (PLACs) with ( 26 Al/ 27 Al) 0 typically <5 × 10 –6 from CM chondrites (Kööp et al. 2016a, 2016b) and grossite‐bearing CAIs from CH chondrites (Gounelle et al.…”