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Oxygen and Al‐Mg isotopic compositions of grossite‐bearing refractory inclusions from CO 3 chondrites
Author(s) -
Simon Steven B.,
Krot Alexander N.,
Nagashima Kazuhide
Publication year - 2019
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.13282
Subject(s) - chondrite , melilite , geology , pyroxene , mineralogy , analytical chemistry (journal) , meteorite , geochemistry , olivine , spinel , chemistry , astrobiology , environmental chemistry , physics , paleontology
The distribution of the short‐lived radionuclide 26 Al 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 ( CAI s) are underrepresented in the database. Recently found grossite‐bearing inclusions in CO 3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearing CAI s in the Dominion Range ( DOM ) 08006 ( CO 3.0) and DOM 08004 ( CO 3.1) chondrites. All minerals in DOM 08006 CAI s as well as hibonite, spinel, and pyroxene in DOM 08004 are uniformly 16 O‐rich (Δ 17 O = −25 to −20‰) but grossite and melilite in DOM 08004 CAI s are not; Δ 17 O 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 26 Al/ 27 Al ratios ( 26 Al/ 27 Al) 0 is seen, with four having ( 26 Al/ 27 Al) 0 ≤1.1 × 10 −5 and six having ( 26 Al/ 27 Al) 0 ≥3.7 × 10 −5 . Five of the 26 Al‐rich CAI s have ( 26 Al/ 27 Al) 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 26 Al‐poor CAI s probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in the DOM 08006 CAI s, as well as spinel, hibonite, and Al‐diopside in the DOM 08004 CAI s retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where the CO grossite‐bearing CAI s originated. Oxygen isotopic heterogeneity in CAI s from DOM 08004 resulted from exchange between the initially 16 O‐rich (Δ 17 O ~−24‰) melilite and grossite and 16 O‐poor (Δ 17 O ~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 26 Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ 17 O 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 CAI s in CO ≥3.1 chondrites.