^1,2J.S. Gorce, ^2,3E.A. Heiny, ⁴J. Filiberto, ²C. Goodrich
Earth and Planetary Science Letters 662, 119371 Link to Article [https://doi.org/10.1016/j.epsl.2025.119371]
¹Amentum at NASA Johnson Space Center, Houston, TX, 77058, United States
²Lunar and Planetary Institute, USRA, Houston, TX 77058, United States
³Case Western Reserve University, Cleveland, OH 44106, United States
⁴Astromaterials Research and Exploration Sciences, NASA Johnson Space Center, Houston, TX 77058
Copyright Elsevier

There is a growing body of evidence that the range of planetary parent bodies sizes is greater than previously understood as new pressure and temperature (P-T) estimates of amphibolite and eclogite mineral assemblages found in chondrites are determined and subsequently used to estimate parent body sizes. Here we use thermodynamic modelling techniques to estimate that clasts containing eclogite-like minerals found in NWA 801 equilibrated at 13-15 kbars and 720°C under dry metamorphic conditions, and hydrous phases form after peak metamorphism during aqueous alteration at P∼4-6 kbars and T ∼ 200-400°C and a water/rock ratio of ∼0.006 (< 0.5 wt % H2O). Parent body size estimates are similar to previous work (2050-3700 km), but do not require that the eclogitic clasts be sampled from near the center of the parent body to achieve a peak metamorphic pressure of 13-15 kbars. The eclogitic clasts in NWA 801 are part of a growing body of evidence that imply that chondritic parent bodies could have been much larger than what has been suggested in the past (1000s vs 10s-100s km in diameter), and that the diversity of size in chondritic parent bodies is much greater than previously understood.

¹Magdalena H. Huyskens, ^2,3,4Yuri Amelin, ¹Qing-Zhu Yin, ⁵Tsuyoshi Iizuka
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.04.030]
¹Department of Earth and Planetary Sciences, University of California, Davis, Davis, CA 95616, USA
²Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia
³Korea Basic Science Institute, Ochang, Cheongwon, Cheongju, Chungbuk 28119, Republic of Korea
⁴State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry CAS, Guangzhou 510640, China
⁵Department of Earth and Planetary Science, The University of Tokyo, Japan
Copyright Elsevier

Ages of angrites, a diverse and rapidly growing group of differentiated meteorites, are important for understanding the history of their parent body, which is proposed to be an archetypal first-generation planetesimal. Angrites also commonly serve as a time reference in the early Solar System chronology. Pb-isotopic ages of angrites can be determined with high precision, and the isotopic composition of uranium thus becomes a major contributor to the age accuracy and its total uncertainty budget. Two main groups of angrites, the rapidly cooled (volcanic and/or impact-generated) and plutonic angrites, were previously found to contain uranium with different 238U/235U ratios. The variations in isotopic compositions between mineral carriers of uranium within individual angrites, which are directly relevant to calculation of accurate Pb-isotopic ages, have not been studied yet. In this study, we determined the 238U/235U for whole rocks, leachate and residue of whole rocks and mineral separates for two rapidly cooled angrites D’Orbigny and Sahara 99,555 and three plutonic angrites NWA 4801, NWA 4590 and Angra dos Reis. For the rapidly cooled angrites, all mineral separates as well as the whole rocks show consistent 238U/235U. Whole rock 238U/235U ratios for the plutonic angrites are distinctly lower than the ratios in the rapidly cooled angrites. In Angra dos Reis and NWA 4590, merrillite has higher 238U/235U than pyroxene, and both minerals have higher 238U/235U ratios than the respective whole rock, suggesting the presence of an unidentified mineral host of uranium with lower 238U/235U. These differences in U isotope composition could be possibly attributed to a combination of mass dependent and mass-independent isotope fractionation driven by the differences of oxidation state, and coordination in the crystals. We recalculated the existing Pb-isotopic dates when possible with the measured 238U/235U for the minerals that were used for the Pb-isotopic dating. The differences in U isotopic composition between cogenetic minerals point to the importance of 238U/235U determination in specific minerals that are used for Pb-isotopic dating for plutonic achondrites, rather than U isotopic data for bulk meteorites