¹Kirsten Larsen,^1,Alexander N. Krot,¹Daniel Wielandt,²Kazuhide Nagashima,³Guy Libourel,^1,2Martin Bizzarro
Meteoritics & Planetary Society (in Press) Link to Article [https://doi.org/10.1111/maps.14325]
¹Centre for Star and Planet Formation, University of Copenhagen, Copenhagen, Denmark
²Hawaii Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawaii, USA
³Observatoire de la Côte d’Azur, UMR 7293 LAGRANGE, Nice, France
Published by arrangement with John Wiley & Sons

A coarse-grained igneous calcium-aluminum-rich inclusion (CAI) N-53, 4.3 × 5.9 mm in size, from the CR (Renazzo-type) carbonaceous chondrite Northwest Africa (NWA) 6043 is composed of two mineralogically, chemically, and isotopically distinct units—type B (B) and type C (C). Type B unit occurs in the CAI core and consists of melilite (Åk28–56), AlTi-diopside, anorthite, spinel, and minor Fe,Ni-metal. Type C unit forms islands in B (Cc) and mantle (Cm) around it and consists of Na-bearing åkermanitic melilite (Åk58–72, 0.18–0.86 wt% Na2O), anorthite, AlTi-diopside (up to 1.2 wt% Cr2O3), spinel (up to 2.1 wt% Cr2O3), perovskite, and minor wollastonite. The outermost portion of N-53 contains relict grains of olivine (Fa4) and low-Ca pyroxene (Fs4Wo5); Wark–Lovering rim is absent. Magnesian spinel in B and C is 16O-rich (Δ17O ~ −23‰); Cr-bearing spinel in Cm is 16O-depleted (Δ17O ~ −11‰). AlTi-diopside, anorthite, and melilite in B and Cc are 16O-depleted to various degrees (Δ17O ~ −22‰ to −19‰, −21‰ to −17‰, −13‰ to −8‰, respectively). AlTi-diopside, anorthite, and melilite in Cm show a range of compositions correlated with a distance from the CAI edge (Δ17O ~ −18‰ to −8‰, −16‰ to −8‰, ~ −8‰ to −2‰). Melilite in B has the heaviest Mg-isotope composition (Δ25Mg ~ 10‰); average Δ25Mg of melilite, AlTi-diopside, and spinel in C are ~9, ~8‰, and ~6‰, respectively; anorthite in both units has Δ25Mg of ~4‰. On the Al-Mg evolutionary diagram, melilite data in B oscillate around the canonical isochron. Melilite, AlTi-diopside, and spinel in C have resolvable δ26Mg* and deviate to the left of this isochron; anorthite in both units has barely resolvable δ26Mg*. Although these data are consistent with late-stage reprocessing of N-53, they provide no clear chronological information. We conclude that N-53 experienced multiple melting events. Initial melting of solid precursors took place in an 16O-rich gaseous reservoir and resulted in formation of the uniformly 16O-rich (Δ17O ~ −24‰) type B CAI. Subsequent single- or multi-stage partial melting of this CAI occurred in an 16O-depleted gaseous reservoir(s) and resulted in addition of SiO and Na to the CAI melt, O- and Mg-isotope exchange, and crystallization of C unit.

^1,2,3Harold C. Connolly Jr et al. (>10)
Meteoritics & Planetary Society (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14335]
¹Department of Geology, School of Earth and Environment, Rowan University, Glassboro, New Jersey, USA
²Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
Published by arrangement with John Wiley & Sons

The OSIRIS-REx mission returned a sample of regolith from the carbonaceous asteroid Bennu in September 2023. We present preliminary in situ investigations of the petrology and petrography of selected particles ranging in size from 0.5 to 3 mm. Using a combination of optical and electron beam techniques, we investigate whole specimens and polished sections belonging to morphologically and visually distinct categories of particles. We find that morphological differences in the particles are reflective of petrographic and petrologic differences, leading to the conclusion that we have at least two distinct major lithologies in the bulk sample. Our findings support predictions from remote sensing, suggesting that the morphological differences observed in the boulder population of Bennu correspond to petrologic differences. Our data provide insight into the geologic activity on Bennu’s parent body and the petrographic framework needed to contextualize the detailed analyses of this pristine asteroidal material.