Oxygen isotope systematics of petrographically and chemically rare chondrules from the NWA 6991 (CV3) chondrite: Implications for chondrule precursors and radial transport in the protoplanetary disk

The isotopic dichotomy in the bulk meteorites suggests limited mixing between the non-carbonaceous (NC) and carbonaceous (CC) reservoirs. However, recent multi-isotope analyses of chondrules revealed mixing across this separation. Understanding the efficiency of the mixing process between the two reservoirs is crucial for revealing the diversity and evolution of solar system bodies. In this study, we analyzed the oxygen (O) isotopic compositions of fifteen chemically and petrographically rare chondrules, including FeO-rich chondrules (Mg# < 98) relative to the majority of FeO-poor chondrules (Mg# ≥ 98) in CV chondrites, dusty olivine chondrules, an enveloping compound chondrule, layered chondrules, and barred olivine (BO) chondrules, from the NWA 6991 (CV3) carbonaceous chondrite. The FeO-rich chondrules and dusty olivine chondrules constitute ∼5% of the NWA 6991 chondrule population. Among them, the O-isotopic compositions of four FeO-rich and two dusty olivine chondrules were analyzed, and all of them exhibit NC-like O-isotopic signatures with the host Δ 17 O values (=δ 17 O − 0.52 × δ 18 O) ranging from −0.5 ± 1.1‰ to +0.4 ± 0.7‰ (2σ), suggesting formation in the NC reservoir and subsequent transport outward into the CC reservoir. The other dusty olivine chondrule, Ch63, shows O-isotopic variations among constituent minerals, including NC-like dusty relict olivines in the interior (Δ 17 O = 0.5 ± 2.4‰; 2SD), CC-like olivine rims (Δ 17 O = −6.1 ± 2.0‰; 2SD), and NC-CC intermediate high-Ca pyroxene (Δ 17 O = −0.8 ± 2.2‰; 2SD). This O-isotopic characteristic suggests that the precursors of this chondrule formed in the NC reservoir and were then transported outward, where they experienced O-isotopic exchange with 16 O-richer ambient gas than the NC-like precursor dust during partial melting in the CC reservoir. The enveloping compound, layered, and BO chondrules exhibit CC-like host Δ 17 O values (−5.8 ± 0.7‰ ≤Δ 17 O ≤ −2.3 ± 0.8‰; 2σ), despite previous evidence of NC-like Cr and Si isotopic compositions. The uniformly CC-like O-isotopic compositions of layered and BO chondrules studied here also suggest that these objects experienced O-isotopic exchange between NC-like precursor dust and 16 O-rich ambient gas that was generated from refractory inclusions and/or earlier generations of CC chondrules. Importantly, the O-isotopic compositions of layered chondrules and olivine rims of the dusty olivine chondrule Ch63 are consistent with the majority of FeO-poor CV chondrules, indicating that the majority of CV chondrules formed from isotopically diverse precursors, but in a common O-isotopic reservoir that is characterized by Δ 17 O ∼ −5‰. The presence of ≥5% chondrules that are probably related to NC-like materials in the CV chondrite NWA 6991 infers that the CV chondrite parent body incorporated a larger fraction of chondrules that were transported outward from the NC reservoir than the other CC chondrite parent bodies. The higher abundance of NC-like materials compared to other CC chondrites suggests that the CV chondrite parent body accreted in a region closer to the NC reservoir, possibly near the H 2 O snowline.