¹Samuel Ebert, ³Kazuhide Nagashima, ²Alexander N. Krot, ¹Addi Bischoff
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2025.05.029]
¹Institut für Planetologie, University of Münster, Münster, Germany
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
Copyright Elsevier

Refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)] in unmetamorphosed chondrites (petrologic type ≤ 3.0) have typically uniform 16O-rich solar-like compositions. The origin of oxygen-isotope heterogeneity within individual refractory inclusions from weakly metamorphosed (petrologic type > 3.0) chondrites remains controversial. It may reflect (i) condensation from a nebular gas having variable O-isotope composition, (ii) gas–solid or gas–melt O-isotope exchange with this gas, and/or (iii) O-isotope exchange with an 16O-depleted aqueous fluid in the host chondrite parent bodies. Here, we present the mineralogy, petrology and O-isotope compositions of refractory inclusions (12 CAIs and 2 AOAs) from the Rumuruti-type (R) chondrites of petrologic type 3 – Northwest Africa (NWA) 753, NWA 1471, and Dhofar 1123. The CAIs and AOAs are extensively altered: melilite is completely replaced by secondary minerals; perovskite is largely replaced by ilmenite; spinel and olivine are enriched in FeO. The polymineralic refractory inclusions have heterogeneous O-isotope compositions: Δ17O ranges from ∼−25 ‰ to ∼5 ‰ (2σ = ±∼2‰). The only exception is a spinel-hibonite inclusion having uniform 16O-depleted composition (Δ17O ∼ −14 ‰). Hibonite, most ferroan spinel, and some olivine and Al,(Ti)-diopside grains in Rumuruti-type chondrite (RC) fragments of low petrologic type (3.15 − 3.2) retained their initial Δ 17O values, which, however, range from −25 ‰ to ∼ −14 ‰, suggesting variations in O-isotope composition of nebular gas in the CAI-forming region. Most Al,Ti-diopside and some olivine and spinel grains in RC refractory inclusions are 16O-depleted compared to minerals most resistant to O-isotope exchange (hibonite and spinel) that retained their original compositions. The most 16O-depleted compositions of Al,Ti-diopside and ferroan olivine have Δ17O of ∼ +5‰ that is similar to Δ17O of the aqueously formed grossular and ferroan olivine. We infer that the 16O-depleted Al,Ti-diopside, olivine, and spinel in isotopically heterogeneous refractory inclusions experienced post-formation O-isotope exchange with aqueous fluids in the RC parent asteroid(s).

¹Christopher R. Glein, ² Yu (余馨婷),²Cindy N. Luu
The Astrophysical Journal 985, 187 Open Access Link to Article [DOI 10.3847/1538-4357/adced4]
¹Space Science Division, Space Sector, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
²Department of Physics and Astronomy, University of Texas at San Antonio, 1 UTSA Circle, San Antonio, TX 78249, USA

The nature of sub-Neptunes is one of the hottest topics in exoplanetary science. Temperate sub-Neptunes are of special interest because some could be habitable. Here, we consider whether these planets might instead be rocky worlds with thick, hot atmospheres. Can recent James Webb Space Telescope observations of TOI-270 d be understood in terms of such a model? We perform thermochemical equilibrium calculations to infer conditions of quenching of C–H–O–N species. Our results indicate apparent CO2–CH4 equilibrium between ∼900 and ∼1100 K. The CO abundance should be quenched higher in the atmosphere where the equilibrium CO/CO2 ratio is lower, potentially explaining a lack of CO. N2 is predicted to dominate the nitrogen budget. We confirm that the atmosphere of TOI-270 d is strongly enriched in both C and Ogas relative to protosolar H, whereas N is likely to be less enriched or even depleted. We attempt to reproduce these enrichments by modeling the atmosphere as nebular gas that extracted heavy elements from accreted solids. This type of model can explain the C/H and Ogas/H ratios, but despite supersolar C/N ratios provided by solids, the NH3 abundance will probably be too high unless there is a nitrogen sink in addition to N2. A magma ocean may be implied, and indeed the oxygen fugacity of the deep atmosphere seems sufficiently low to support the sequestration of reduced N in silicate melt. The evaluation presented here demonstrates that exoplanetary geochemistry now approaches a level of sophistication comparable to that achieved within our own solar system.