An I-Xe Age 8 Myr After Solar System Formation In A Hayabusa2 Sample Records Alteration On The Parent Planetesimal Of Asteroid Ryugu

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.<br/>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.