¹Martin R. Lee, ¹Sammy Griffin, ^2,2Ross Findlay, ³Xuchao Zhao, ³Ian A. Franchi
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2025.12.051]
¹School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK
²Department of Earth Sciences, University of Cambridge, Downing St., Cambridge CB2 3EQ, UK
³School of Physical Sciences, Open University, Milton Keynes MK7 6AA, UK
Copyright Elsevier

The CM2 meteorites Grove Mountains (GRV) 021536, Murchison, and Shidian, contain anhydrous lithic clasts that have been interpreted as fragments of a planetesimal linked to CM or CV group carbonaceous chondrites. Here we describe 57 lithic clasts in Cold Bokkeveld (CM2) that are strikingly similar to those in the other three CMs in their petrography, mineralogy, and chemical and isotopic compositions. The Cold Bokkeveld clasts are dominated by equilibrated olivine, with subordinate plagioclase feldspar (andesine), clinopyroxene (diopside), nepheline, a spinel-group oxide (ferrian chromite), pentlandite, pyrrhotite, troilite and merrillite. Their bulk chemical composition is chondritic, and olivine oxygen isotope values span a wide range, from δ18O 3.6 ‰ Δ17O −3.9 ‰ to δ18O 20.3 ‰ Δ17O 1.1 ‰. Two clusters of clasts can potentially be distinguished from the chemical composition of their olivine: Fa38 and Fa41. The Fa38 cluster includes most of Cold Bokkeveld’s clasts and is close in chemical composition to those described from GRV 021526 and Murchison. The Fa41 cluster is represented by the largest Cold Bokkeveld clast, and its olivine is compositionally comparable to that in Shidian. Anhydrous lithic clasts that occur in all four of the CM meteorites are likely to have been derived from a large planetesimal with CM and CY affinities that had undergone thermal metamorphism and metasomatism. The CV3 breccias Mokoia and Yamato 86009 contain anhydrous lithic clasts that are close in mineralogy and oxygen isotopic composition to those in the four CMs and so are likely to have been sourced from the same carbonaceous planetesimal or one with a similar geological history. The oxygen isotopic compositions of olivine in clasts from GRV 021536, Murchison, Shidian, Cold Bokkeveld, Mokoia and Yamato 86009 plot on a shared trendline in 3-oxygen isotope space that connects the CV-CK-CO, CM, and CY fields thus suggesting genetic or evolutionary links between the five carbonaceous chondrite groups. The occurrence of these distinctive clasts in four CM2 meteorites could indicate that their parent body was the same rubble pile asteroid that had been built from aqueously altered and thermally metamorphosed lithologies.

¹Bennett J.K. Wilson, ²Kazuhide Nagashima, ^3,4Thomas J. Barrett, ⁵Veronica E. Di Cecco, ^5,6Kimberly T. Tait, ¹Michael G. Daly
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.12.046]
¹Center for Research in Earth and Space Science, York University, Toronto, ON, Canada
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, 1680 East-West Road, POST602, Honolulu HI96822, USA
³Department of Earth and Environmental Sciences, The University of Manchester, UK
⁴Center for Lunar Science and Exploration, Lunar and Planetary Institute, Houston, TX, USA
⁵Department of Natural History, Center for Applied Planetary Mineralogy, Royal Ontario Museum, Toronto, ON, Canada
⁶Department of Earth Science, University of Toronto, ON, Canada
Copyright Elsevier

The Tarda meteorite is a recently recovered C2-ungrouped carbonaceous chondrite that preserves evidence of early Solar System aqueous alteration. Tarda was found to share reflectance spectra with P-type asteroids, possibly enabling these elusive asteroids to be studied in the laboratory for the first time. Furthermore, Tarda has been shown to share many petrological and isotopic affinities with Tagish Lake – a pristine C2-ungrouped chondrite that is widely considered to source a D-type asteroid. Thus Tarda, Tagish Lake, and their respective spectral classes are probably genetically related, and potentially source a shared parent body. Despite their similarities, however, Tagish Lake hosts different lithologies and carbonate species than Tarda, suggesting distinct aqueous alteration histories between the two meteorites. Here, we present in-situ oxygen, carbon, and 53Mn–53Cr isotopic analyses of dolomite and magnetite in Tarda using Secondary Ion Mass Spectrometry to (i) investigate the conditions associated with aqueous alteration on the early Tarda parent body, and to (ii) compare our findings with Tagish Lake to assess heterogeneous aqueous alteration of their unique and likely shared parent body. For dolomite, we found that δ13C ranged from 55.8 ‰ to 72.9 ‰, while δ18O ranged from 23.3 ‰ to 28.8 ‰ with an average Δ17O of 0.1 ± 1.6. Dolomite additionally contained widespread 53Cr excesses that, if interpreted to have chronological significance, corresponds to a live [(53Mn/55Mn)0] value of (
. For magnetite, the δ18O values ranged from −5.5 ‰ to 5.8 ‰ with an average Δ17O of 2.4 ‰ ± 1.7. Oxygen isotope thermometry of a co-precipitating dolomite–magnetite pair indicates alteration temperatures of
°C. Compared to carbonates in Tagish Lake, dolomite in Tarda exhibits systematically lower δ17O, δ18O, and Δ17O signatures, but similar δ13C signatures. Temporally, the carbonates in both meteorites have identical ages within uncertainty. We conclude that Tarda has experienced greater aqueous alteration than Tagish Lake, likely due to increased water–rock interaction and/or higher temperatures.