^1,2Makoto Kimura,³Motoo Ito,²Akira Monoi,¹Akira Yamaguchi,⁴Richard C. Greenwood
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.06.002]
¹National Institute of Polar Research, 10-3 Midoricho, Tachikawa, Tokyo 190-8513, Japan
²Faculty of Science, Ibaraki University, Bunkyo 2-1-1, Mito 310-8512, Japan
³Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 200-Monobe-otsu, Nankoku, Kochi 783-8502, Japan
⁴Planetary and Space Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom
Copyright Elsevier

CI chondrites are the most significant extra-terrestrial samples for estimating the composition of primordial materials in the Solar System. However, CIs lose many primary features because of heavy parent body aqueous alteration. However, CI and CI-related Ryugu particles contain small amounts of relict anhydrous minerals, indicating primary occurrences of chondrules and refractory inclusions. In this study, we estimated the primordial abundance of chondrules in CIs from calculations of the bulk major element compositions. The constraints for the calculation were as follows: 1) CI chondrites primarily comprised chondrules, refractory inclusions, opaque minerals, and a matrix similar to other carbonaceous (C) chondrites. 2) The chemical compositions of these components were similar to those of the unaltered C chondrites. 3) The primary matrix composition of the CI was close to the mean bulk composition. 4) The alteration occurred isochemically. We used the mean major elemental compositions of chondrules and refractory inclusions in an almost unaltered chondrite, Y-81020, CO3.05. Our results were within the range of previously reported CI bulk chemical compositions in the case where chondrule abundances are ≲10 wt%. We also calculated the bulk chemical composition of Tagish Lake, ungrouped C2, which primarily contained ≲20 wt% chondrules. The CI chondrites and Tagish Lake were formed in the outer Solar System. The low primary abundance of chondrules in CIs is closely related to the formation conditions of chondrules in such regions. We suggest that dust with abundant ice and minor chondrules accreted onto the parent bodies of the CI and Tagish Lake in the outer Solar System. Primordial chondrule abundance is the key to clarifying the physical and chemical conditions and evolution of the early Solar System.

¹H. C. Bates,²R. Aspin,³C. Y. Fu,^1,4C. S. Harrison,⁵E. Feaver,^1,6E. Branagan-Harris,¹A. J. King,²J. F. J. Bryson,²S. Sridhar,²C. I. O. Nichols
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14224]
¹Planetary Materials Group, Natural History Museum, London, UK
²Department of Earth Sciences, University of Oxford, Oxford, UK
³Department of Earth Science & Engineering, Imperial College London, London, UK
⁴Department of Earth & Environmental Sciences, The University of Manchester, Manchester, UK
⁵Physics & Astronomy Department, University College London, London, UK
⁶Atmospheric, Oceanic & Planetary Physics, University of Oxford, Oxford, UK
Published by arrangement with John Wiley & Sons

Tarda is an ungrouped, hydrated carbonaceous chondrite (C2-ung) that was seen to fall in Morocco in 2020. Early studies showed that Tarda chemically resembles another ungrouped chondrite, Tagish Lake (C2-ung), which has previously been linked to the dark D-type asteroids. Samples of D-type asteroids provide an important opportunity to investigate primitive conditions in the outer solar system. We show that Tarda contains few intact chondrules and refractory inclusions and that its composition is dominated by secondary Mg-rich phyllosilicates (>70 vol%), carbonates, oxides, and Fe-sulfides that formed during extensive water–rock reactions. Quantitative assessment of first-order reversal curve (FORC) diagrams shows that Tarda’s magnetic mineralogy (i.e., framboidal magnetite) is comparable to that of the CI chondrites and differs notably from that of most CM chondrites. These traits support a common formation process for magnetite in Tarda and the CI chondrites. Furthermore, Tarda’s pre-terrestrial paleomagnetic remanence is similar to that of Tagish Lake and samples returned from asteroid Ryugu, with a very weak paleointensity (<0.6 μT) suggesting that Tarda’s parent body accreted more distally than that of the CM chondrites, possibly at a distance of >5.4–8.3 AU. An origin in the cold, outer regions of the solar system is further supported by the presence of distinct, porous clasts enriched in aliphatic-rich organics that potentially retain a pristine interstellar composition. Together, our observations support a genetic relationship between Tarda and Tagish Lake.