83-10 The Hydrogen Isotopic Composition of the Ureilite Parent Body

Session: Asteroid Observations, Return Missions, and Meteoritics: Interweaving Perspectives and Data

Presenting Author:

Ben Rider-Stokes

Authors:

Rider-Stokes, Ben Gabriel ¹, Stephant, Alice², Jackson, Samuel³, Nottingham, Mark⁴, Gamblin, Julie⁵, Furi, Evelyn⁶, Russell, Sara⁷, Zhao, Xuchao⁸, Anand, Mahesh⁹, Whitehouse, Martin¹⁰, Grady, Monica¹¹

(1) The Open University, Milton Keynes, Buckinghamshire, United Kingdom, (2) INAF, Rome, Italy, (3) The Open University, Milton Keynes, United Kingdom, (4) University of Glasgow, Glasgow, United Kingdom, (5) CRPG, Nancy, France, (6) CRPG, Nancy, France, (7) Natural History Museum, London, United Kingdom, (8) The Open University, Milton Keynes, United Kingdom, (9) The Open University, Milton Keynes, United Kingdom, (10) Natural History Museum, Sweden, Sweden, (11) The Open University, Milton Keynes, United Kingdom,

Abstract:

Introduction: Evidence for silica-rich melts on the ureilite parent body (UPB) has been preserved and recorded in rare fragments of the Almahata Sitta meteorite suite, which originated from the asteroid 2008 TC3 (Bischoff et al., 2014). These unique trachyandesite fragments demonstrated that ancient crust-forming volcanism on small planetary bodies was not simply restricted to mafic lithologies. The evaluation of the water content of these fragments by Peterson et al. (2023) allowed for early reconstructions of the bulk water content of the UPB. Here, we report on two recent Northwest Africa (NWA) finds, NWA 15820 and NWA 16789, that expand the known quantity of silica-rich material from the UPB by a factor of two, allowing for a more representative investigation into the chemical and isotopic variations among these unique lithologies.

Methods: In this study, we undertook established techniques to evaluate their mineralogy and chemistry through SEM and EPMA, their Pb-Pb ages through SIMS, cosmic ray exposure histories by mass spectrometry and the water content and isotopic composition of nominally anhydrous silicates, phosphates and glass using NanoSIMS.

Results and Discussion: The bulk UPB water content, estimated based on the water content of the silicates, results in a revised estimate of 13 – 46 μg/g H₂O, marginally higher than previous estimates. Furthermore, we report the hydrogen isotopic composition of water in the UPB. While the D/H ratio of late-stage phosphates and glass is highly fractionated as a result of magmatic degassing, the nominally anhydrous silicate, pyroxene, better records the original hydrogen isotopic composition (δD) of the UPB. In particular, the pyroxene within NWA 16789 records a value of 83 ± 79 ‰. We infer this isotopic value as the best estimate for the original isotopic composition of the UPB and may be attributed to the restricted protoplanetary disk reservoir in which it accreted.

References:

Bischoff, A., Horstmann, M., Barrat, J-A., Chaussidon, M., Pack, A., Herwartz, D., Ward, D., Vollmer, C., Decker, S. 2014. Trachyandesitic volcanism in the early Solar System. Proceedings of the National Academy of Sciences 111, 12689-12692. https://doi.org/10.1073/pnas.1404799111

Peterson, L. D., Newcombe, M. E., Alexander, C. M., Wang, J., Sarafian, A. R., Bischoff, A., Nielsen, S. G. 2023. The H2O content of the ureilite parent body. Geochimica et Cosmochimica Acta. 340, 141-157. https://doi.org/10.1016/j.gca.2022.10.036

 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7083

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.