¹M.E.I.Riebe et al.(>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.01.034]
¹Institute of Geochemistry and Petrology, ETH Zürich, CH-8092 Zürich, Switzerland
Copyright Elsevier

Most ureilites are melt residues from the partially melted Ureilite Parent Body. The Ureilite Parent Body was catastrophically disrupted at ∼ 5 Ma after CAI while it was still hot and the ureilites provide a unique window into early solar system magmatic processing. One ureilitic trachyandesite, one cumulate, and 16 melt residue ureilites, all from the Almahata Sitta meteorite strewn field, were analyzed for their noble gas compositions and, when such data was unavailable, for oxygen isotopes and petrology. Additionally, ureilite noble gas data from the literature was compiled together with petrology and oxygen isotope data of the same samples, this data is available in the supplementary materials. The compositions of noble gases and oxygen, as well as petrological characteristics, are similar to previously analyzed ureilites. This includes variable 36Artr/132Xe ratios of ∼ 20–––1000 correlated with variable 84Kr/132Xe ratios of ∼ 0.15–––2.5 and Xe isotopic compositions similar to the Q gases but with somewhat lower 134,136Xe/132Xe ratios. The well-established correlation between Mg-Fe olivine core composition and Δ’17O, interpreted as material mixing, is corroborated. There is no correlation between noble gas compositions and petrology or Δ’17O. Therefore, it is unlikely that the variable noble gas elemental ratios are due to mixing of noble gases from different sources, as previously suggested. We suggest that compositional variability was established during implantation of noble gases into disordered carbon prior to accretion and possibly during later processing. We discuss that partial graphitization resulted in noble gas loss, with noble gases remaining in un-graphitized organics, which were converted to diamond during the catastrophic disruption. Noble gases released during graphitization may have entered the melt. Isotopic compositions of trapped noble gases in the cumulate and trachyandesitic rocks, which crystallized from the melt are similar to those in the melt residue ureilites. The elemental noble gas composition of the cumulate shows evidence of a degassing stage and that the concentrations of noble gases in the ureilites were higher before melting. The noble gases in the trachyandesite contains radiogenic noble gases from decay of K, I, Th, and U, which were not enriched in the cumulate, showing that the trachyandesite crystallized from a more evolved melt. The cosmic-ray exposure ages of 15–––22 Ma, with mostly overlapping uncertainties, are similar to those previously determined for ureilites from the Almahata Sitta strewn field and display a limited spread in contrast to ages previously detected in Almahata Sitta chondrites.

¹Alexander N. Krot, ¹Kazuhide Nagashima, ²Tasha L. Dunn, ³Chi Ma, ⁴Michail I. Petaev
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.01.027]
¹Hawai‘i Institute of Geophysics & Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
²Department of Geology, Colby College, Waterville, ME 04901, USA
³Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
⁴Department of Earth & Planetary Sciences, Harvard University, Cambridge MA 02138, USA
Copyright Elsevier

We report on oxygen and aluminum-magnesium isotope systematics of Compact Type A (CTA), Type B (B), and Forsterite-bearing Type B (FoB) Ca,Al-rich inclusions (CAIs) from the Northwest Africa (NWA) 5343 (CK3.7) and NWA 4964 (CK3.8) chondrites that experienced metasomatic alteration in the presence of aqueous solution that resulted in replacement of primary melilite, AlTi-diopside, grossmanite, anorthite, and perovskite by secondary minerals. The primary minerals have excesses of radiogenic 26Mg (26Mg*) that correlate with 27Al/24Mg ratio; the only exception is melilite in the CTA CAI. The calculated internal Al-Mg isochrons in the CTA (excluding melilite), Type B, and FoB CAIs correspond to the initial 26Al/27Al ratios [(26Al/27Al)0] of (5.09 ± 0.58) × 10−5, (2.58 ± 3.2) × 10−5, and (5.05 ± 0.66) × 10−5, respectively. The gehlenitic melilite (Åk<1) in the CTA CAI has resolvable 26Mg* but very high 27Al/24Mg (up to ∼ 660) and does not belong to the internal isochron defined by hibonite, spinel, and grossmanite. The high 27Al/24Mg in melilite containing submicron inclusions of grossular is due to redistribution of Mg between these minerals during thermal metamorphism. Hibonite, spinel, forsterite, rhönite/louisfuchsite, and a grossmanite inclusion in spinel have 16O-rich compositions (Δ17O ∼  − 23 ± 2 ‰), whereas melilite, anorthite, and perovskite are 16O-poor (Δ17O ∼  − 3 ± 2 ‰). Grossmanite and AlTi-diopside are 16O-depleted to various degrees: Δ17O ranges from ∼  − 24 to ∼  − 3 ‰; the degree of 16O-depletion correlates with titanium content in pyroxene. On a three-isotope oxygen diagram secondary grossular, FeAl-diopside, FeMg-olivine, and plagioclase plot along mass-dependent fractionation line with Δ17O of ∼  − 3.7 ± 1.9 ‰ that corresponds to Δ 17O of metasomatic fluid in the host meteorites. This value is indistinguishable from Δ 17O of metasomatic fluid that resulted in alteration of Allende (CV > 3.6) CAIs.
Coarse-grained igneous CAIs in CKs and CVs have similar size distribution, textures and primary mineralogy, formed in a gas of approximately solar O-isotope composition (Δ 17O ∼  − 24 ± 2 ‰) and had the canonical (26Al/27Al)0, suggesting they belong to the same generation of refractory inclusions, further supporting genetic relationship between CVs and CKs. Oxygen-isotope heterogeneity in CV > 3.6 and CK3.7 − 3.8 CAIs resulted from postcrystallization O-isotope exchange with 16O-depleted metasomatic fluid (Δ 17O ∼  − 3.7 ± 1.9 ‰) on their parent asteroid(s). This exchange preferentially affected melilite, anorthite, perovskite, and AlTi-pyroxenes, whereas hibonite, spinel, rhönite/louisfuchsite, and forsterite retained their original 16O-rich compositions established during igneous crystallization in a gas of approximately solar composition. Metasomatic alteration and thermal metamorphism of CAIs from CK3.7 − 3.8 and CV > 3.6 chondrites disturbed their Al-Mg isotope systematics to various degrees.