1. Mineralogy, petrography, and oxygen and aluminum-magnesium isotope systematics of grossite-bearing refractory inclusions.
- Author
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Krot, Alexander N., Nagashima, Kazuhide, Simon, Steven B., Ma, Chi, Connolly, Harold C., Huss, Gary R., Davis, Andrew M., and Bizzarro, Martin
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OXYGEN isotopes ,PETROLOGY ,MINERALOGY ,INCLUSIONS (Mineralogy & petrology) ,PROTOPLANETARY disks ,CHONDRITES - Abstract
Grossite (CaAl 4 O 7) is one of the one of the first minerals predicted to condense from a gas of solar composition, and therefore could have recorded isotopic compositions of reservoirs during the earliest stages of the Solar System evolution. Grossite-bearing Ca,Al-rich inclusions (CAIs) are a relatively rare type of refractory inclusions in most carbonaceous chondrite groups, except CHs, where they are dominant. We report new and summarize the existing data on the mineralogy, petrography, oxygen and aluminum-magnesium isotope systematics of grossite-bearing CAIs from the CR, CH, CB, CM, CO, and CV carbonaceous chondrites. Grossite-bearing CAIs from unmetamorphosed (petrologic type 2―3.0) carbonaceous chondrites preserved evidence for heterogeneous distribution of
26 Al in the protoplanetary disk. The inferred initial26 Al/27 Al ratio [(26 Al/27 Al) 0 ] in grossite-bearing CAIs is generally bimodal, ˜0 and ˜5×10−5 ; the intermediate values are rare. CH and CB chondrites are the only groups where vast majority of grossite-bearing CAIs lacks resolvable excess of radiogenic26 Mg. Grossite-bearing CAIs with approximately the canonical (26 Al/27 Al) 0 of ˜5×10−5 are dominant in other chondrite groups. Most grossite-bearing CAIs in type 2–3.0 carbonaceous chondrites have uniform solar-like O-isotope compositions (Δ17 O ˜ ‒24±2‰). Grossite-bearing CAIs surrounded by Wark-Lovering rims in CH chondrites are also isotopically uniform, but show a large range of Δ17 O, from ˜ ‒40‰ to ˜ ‒5‰, suggesting an early generation of gaseous reservoirs with different oxygen-isotope compositions in the protoplanetary disk. Igneous grossite-bearing CAIs surrounded by igneous rims of ±melilite, Al-diopside, and Ca-rich forsterite, found only in CB and CH chondrites, have uniform16 O-depleted compositions (Δ17 O ˜ ‒14‰ to ‒5‰). These CAIs appear to have experienced complete melting and incomplete O-isotope exchange with a16 O-poor (Δ17 O ˜ ‒2‰) gas in the CB impact plume generated about 5 Ma after CV CAIs. Grossite-bearing CAIs in metamorphosed (petrologic type >3.0) CO and CV chondrites have heterogeneous Δ17 O resulted from mineralogically-controlled isotope exchange with a16 O-poor (Δ17 O ˜ ‒2 to 0‰) aqueous fluid on the CO and CV parent asteroids 3–5 Ma after CV CAIs. This exchange affected grossite, krotite, melilite, and perovskite; corundum, hibonite, spinel, diopside, forsterite, and enstatite preserved their initial O-isotope compositions. The internal26 Al-26 Mg isochrons in grossite-bearing CAIs from weakly-metamorphosed CO and CV chondrites were not disturbed during this oxygen-isotope exchange. HCCJr is grateful to Klaus Keil for all his sound profession counsel and collegial friendship over the years. He has always been willing to talk and has the generous nature of listening and sharing his thoughts freely and constructively. Professor Klaus Keil has been a mentor to and played a key role in the careers of three of the authors of this paper (ANK, KN, and GRH). He has also influenced the careers of the other authors and most of the people who have worked on meteorites over the past 50+ years. We therefore dedicate this paper to Professor Keil and present it in this Special Issue of Geochemistry. [ABSTRACT FROM AUTHOR]- Published
- 2019
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