¹N.M.Abreu,^2,3J.C.Aponte,⁴E.A.Cloutis,⁵A.N.Nguyen
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2020.125631]
¹Earth Science, Pennsylvania State University – DuBois Campus, DuBois, PA, 15801, USA
²Solar System Exploration Division, Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
³Department of Chemistry, Catholic University of America, Washington, DC 20064, USA
⁴Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB R3B 2E9, Canada
⁵Jacobs Engineering Group Inc., NASA Johnson Space Center, Houston, Texas 77058, USA
Copyright Elsevier

We present here a review of the characteristics of CR carbonaceous chondrite meteorites. Over the past three decades, our knowledge and understanding of the scientific value of the CR chondrites have increased dramatically, as more samples from cold and hot deserts have become available for analysis. Based on a variety of compositional, mineralogical, isotopic, and spectroscopic studies, we have come to understand that CR chondrites are excellent samples of asteroidal meteorites to look for virtually unaltered solar nebula material and to observe asteroidal processes in progress. This paper summarizes these investigations, their similarities, and differences with other chondritic groups, their relationships to asteroids, and the questions yet to be addressed.

^1,2Samuel Ebert,¹Kazuhide Nagashima,¹Alexander N.Krot,²Addi Bischoff
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.012]
¹School of Ocean, Earth Science and Technology, Hawai’i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, USA
²Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Germany
Copyright Elsevier

The nature of oxygen-isotope heterogeneity in refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)] from weakly metamorphosed chondrites is one of the outstanding problems in cosmochemistry. To obtain insights into possible processes resulting in O-isotope heterogeneity of refractory inclusions, we investigated the mineralogy, petrology, and oxygen isotopic compositions of six CAIs and two AOAs and aqueously formed fayalite grains within the matrix of the H3.1 chondrite Northwest Africa (NWA) 3358. Most of the refractory inclusions studied appear to be unmolten solar nebula condensates; some may have experienced partial melting and/or high-temperature annealing. The NWA 3358 refractory inclusions nearly completely avoided metasomatic alteration on the H-chondrite parent body: nepheline grains replacing anorthite and/or melilite are either very minor or absent. Five out of eight refractory inclusions studied have heterogeneous O-isotope composition: Δ17O ranges from ∼ −25‰ to ∼ 3.5±2‰ (2σ). This O-isotope heterogeneity appears to be mineralogically controlled with melilite and anorthite being systematically 16O-depleted compared to hibonite, spinel, Al,Ti-diopside, and forsterite all having similar solar-like Δ17O of ∼ −24±2‰. In contrast to NWA 3358 refractory inclusions, the previously studied AOAs and a fine-grained CAI from the LL3.00 chondrite Semarkona have uniform Δ17O of ∼ −25‰ (McKeegan et al., 1998; Itoh et al., 2007). Because the mineralogically-controlled O-isotope heterogeneity in refractory inclusions from ordinary chondrites appears to correlate with petrologic type of a host meteorite experienced by aqueous alteration, we suggest O-isotope exchange in NWA 3358 CAIs and AOAs resulted from aqueous fluid-rock interaction on the H-chondrite parent asteroids. This is supported by the presence of 16O-depleted anorthite (Δ17O ∼ 3.5±2‰) and aqueously formed fayalite similar depleted in 16O (Δ17O ∼ 4±2‰). The Δ17O of NWA 3358 fayalite is comparable to that of magnetite and fayalite in Semarkona and other weakly metamorphosed L3 and LL3 chondrites (Choi et al., 1998; Doyle et al., 2015) suggesting similar Δ17O of aqueous fluids on the H, L, and LL chondrite parent asteroids.