¹Daniel R. Dunlap, ¹Julia A. Cartwright, ²Piers Koefoed, ⁴Evgenii Krestianinov, ¹Stephen J. Romaniello, ³Carl Agee, ²Yuri Amelin, ¹Meenakshi Wadhwa
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2026.03.014]
¹Arizona State University, School of Earth and Space Exploration, 781 Terrace Mall, Tempe, AZ 85287, USA
²Australian National University, Research School of Earth Sciences, 142 Mills Rd, Acton ACT 2601, Australia
³University of New Mexico, Institute of Meteoritics, 1 University of New Mexico, MSC03 2050, Albuquerque, NM 871312, USA
⁴Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854 USA
Copyright Elsevier

The chronology of ungrouped achondrites provides key insights into the timeline of igneous activity in the early Solar System. Two ungrouped achondrites – Northwest Africa (NWA) 11119 (andesite-dacite) and NWA 8486 and its pair NWA 7325 (olivine gabbro) – are the focus of this study, where their chronologies are investigated using multiple high-resolution techniques. Here we report the lead-lead (207Pb-206Pb) and manganese-chromium (53Mn-53Cr) systematics of NWA 11119, as well as the 207Pb-206Pb systematics of NWA 8486 alongside aluminum-magnesium (26Al-26Mg) systematics for NWA 7325. The U-corrected 207Pb-206Pb ages of NWA 11119 and the combined ages of NWA 7325/8486 are 4566.4 ± 0.8 Ma and 4563.8 ± 1.9 Ma, respectively. Additionally, we report the 53Mn-53Cr age of NWA 11119 to be 4564.4 ± 2.5 Ma and the 26Al-26Mg age of NWA 7325/8486 to be 4563.1 ± 0.3 Ma.
The formation of NWA 11119 requires partial melting of a (likely chondritic) source reservoir leading to eruption of Si rich, alkali depleted magmas, while NWA 7325/8486 likely formed from a chemically fractionated reservoir with superchondritic Al/Mg. The clear geochemical and isotopic similarities of these achondrites, combined with the chronology reported here, is suggestive of formation of these two ungrouped achondrites on a common parent body which likely formed in the inner Solar System and experienced early differentiation under reducing conditions. If these achondrites did share a parent body, it would suggest that primary asteroids in the early Solar System commonly produced mineralogically and geochemically heterogeneous crusts. While mineralogical and geochemical heterogeneity is known to exist in the (mostly mafic) crust of asteroid Vesta, our findings show that even more significant crustal heterogeneity (representing felsic and mafic compositions) may exist on other asteroids.

¹Bianka Babrián, ²Queenie H.S. Chan, ³Jens Najorka, ¹James Malley, ⁴Lee F. White, ^1,3Martin D. Suttle
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2026.03.020]
¹School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
²Department of Earth Sciences, Royal Holloway, University of London, Egham TW20 0EX, UK
³Planetary Materials Group, Natural History Museum, Cromwell Road, London SW7 5BD, UK
⁴Electron Microscopy Suite, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
Copyright Elsevier

We performed a series of low temperature hydrothermal alteration experiments using the highly primitive CO3.00 chondrite DOM 08006 as the protolith to investigate aqueous alteration conditions on carbonaceous planetesimals. Our experiments employed a range of water-to-rock (W/R) ratios by mass (0.2, 0.4, 1.0) and a heating duration of 100 days at 80°C. We also used a short-duration experiment terminated on day 12 to investigate the earliest stages of aqueous alteration. We used isotopically doped 17O-rich water to track isotopic behaviour during alteration and ascertain the effective molar W/R ratios (i.e., the proportion of reacted water) in the experiments. Bulk O-isotope data suggest that the effective W/R ratio achieved by the experimental samples was capped at ∼0.2 at the end of the experiments. This reflects low alteration extents, which we attribute to low matrix permeability that results from the early and continuous precipitation of Fe-bearing phases, likely dominated by Fe-sulphides, into pore spaces, restricting fluid flow and hindering the progression of alteration at these short timescales. Conversely, early hydrothermal fracturing was observed which may lead to “runaway” alteration at longer timescales. Initial W/R ratio had little effect on the style of alteration, except for allowing Mg-serpentine (chrysotile) to form in addition to Fe-serpentine (cronstedtite) in the high W/R (1.0) experiment. Phyllosilicate production doubled over one order of magnitude longer durations, from 12 to 100 days, at similar effective W/R ratios. One sample (Exp-2) experienced initially oxidising conditions, which led to a marked distinction in alteration style dominated by Fe-oxides and Fe-(oxy)hydroxides. This sample also formed framboidal magnetite, showing that this morphology is possible under oxidising conditions and that the presence of ammonia may not be required, as previously suggested. This work is a strong step forward in experimentally recreating a “generic” hydrated chondrite in the nascent stages of aqueous alteration.