An I-Xe age 8 Myr after solar system formation in a Hayabusa2 sample records alteration on the parent planetesimal of asteroid RyuguOPEN ACCESS
S.A. Crowther, J.S. Cowpe, L. Fawcett, J.D. Gilmour, R. Okazaki, F. Kitajima, T. Yada, S. Tachibana, K. Yogata, K. Sakamoto, H. Yurimoto, T. Nakamura, T. Noguchi, H. Naraoka, H. Yabuta, S. Watanabe, Y. Tsuda, M. Nishimura, A. Nakato, A. Miyazaki, R. Uemura
Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 9 December 2025
“We report an I-Xe age of 7.94 ± 0.92 Myr after formation of calcium aluminium inclusions (CAI), and an iodine concentration of 67 ± 1 ppb for material returned from asteroid (162173) Ryugu by JAXA’s Hayabusa 2 mission. These were determined from multi-step laser heating xenon isotopic analysis of samples A0105-03 and A0105-12 (100 µg and 70 µg, respectively), the smaller of which had been neutron irradiated to convert 127I to 128Xe. The I-Xe age likely corresponds to the end of significant loss of volatiles from the parent body. A simple statistical model of all 24 xenon isotopic analyses of Hayabusa 2 material reported to date, including one sample that has an elevated concentration attributed to Xe-P7, suggests a bulk xenon content of 2.0–2.7 × 10−7 cm3 STP g−1 for such material. This is a factor of 12–60 times higher than suggested by analyses of CI chondrite meteorites that have been exposed to the terrestrial atmosphere.
The bulk xenon isotopic composition is enriched in the heavy isotopes (134, 136Xe) relative to Average Carbonaceous Chondrite (AVCC) xenon, consistent with loss of some planetary xenon (“Q-Xe”) during aqueous alteration allowing a greater relative contribution from presolar nanodiamonds than found in AVCC. The I-Xe age is within the range of I-Xe ages for aqueous alteration of CI material; it likely records either the closure of iodine-rich sites to xenon loss towards the end of a period of heating that was associated with aqueous alteration, or precipitation of iodine-bearing minerals driven by loss of water. We use a simple statistical model of xenon analyses of Ryugu material to investigate the concentration of xenon in CI material. The presence of the rare Xe-P7 component in one reported analysis increases the estimated gas concentration and so increases the discrepancy between xenon concentrations measured in Hayabusa2 samples and CI meteorites. This is consistent with a comparatively rare Xe-P7 carrier being susceptible to loss when exposed to the terrestrial atmosphere. We measured an iodine concentration of 67 ± 1 ppb, which is comparable to other analyses of CI chondrites by neutron-irradiation noble gas mass spectrometry (NI-NGMS). Our sample was sealed within a capillary tube during irradiation allowing us to monitor any gas lost from the sample; loss of iodine-derived 128Xe during the irradiation process cannot account for any discrepancy between our derived iodine concentration and those determined in carbonaceous chondrites by other methods.”
