The behaviour of iodine and xenon in the first asteroids

THE BEHAVIOUR OF IODINE AND XENON IN THE FIRST ASTEROIDSJennifer Louise ClaydonThe University of Manchestersubmitted for the degree of Doctor of Philosophy, September 2012.Results of I-Xe analyses have been obtained from meteorite samples that experienced different extents of thermal processing in the early Solar System in order to help characterise the movements of iodine and xenon in the early Solar System and constrain the timing of these movements using the I-Xe chronometer. Samples were irradiated to convert 127I to 128Xe* and allow simultaneous measurements of iodine and xenon isotopes. Xe isotopes were measuring using the RELAX mass spectrometer.I-Xe ages of material of different metamorphic grade from R-chondrites NWA 6492, NWA 830 and NWA 3364 suggest a link between the time of closure to Xe-loss and extent of metamorphism on the R-chondrite parent body. However, further I-Xe analyses of R5 material from NWA 6492 and R4 and R6 material from other R-chondrites are needed to confirm this. The most primitive material analysed give I? Xe ages between 4559 ? 4554 Myr, slightly later than reported Mn-Cr ages. This may support the ideal of radial heterogeneity of 53Mn in the early Solar System. However differences could also be due to variations in the samples analysed. Future analyses of I-Xe and Mn-Cr ages in mineral separates from the same R-chondrite are recommended in order to investigate this hypothesis. Closure to Xe-loss in chondrules on the R-chondrite parent body appears to have occurred ~5 ? 10 Myr later than on the ordinary and enstatite parent bodies. This implies either later accumulation of material or slower cooling in a larger body.Comparisons of I-Xe systematics in anomalous eucrites Bunburra Rockhole and Ibitira and ? nomalous? eucrites Juvinas and B�r�ba show lower 129I/244Pu ratios in the ? nomalous? eucrites. This is not due to formation on a less volatile-rich body but instead reflects extended loss of Xe on 4 Vesta. 129I/244Pu ratios indicate igneous processing continued on 4 Vesta for ~50-100 Myr after geological activity had ceased on the anomalous eucrites parent bodies. The extended processing seen in Juvinas and B�r�ba is attributed to formation on a larger body that retained heat for longer. If, as the data suggest, the anomalous eucrites formed on a separate parent body it must have been catastrophically disrupted as Vesta is thought to be the only remaining differentiated asteroid. The larger size of Vesta may explain why it has uniquely survived the impacts that destroyed its siblings. Analyses of the unique achondrite GRA 06129 show that the I-Xe system in this meteorite has no chronological significance. The data instead suggest that iodine-bearing plagioclase formed early but thermal metamorphism resulted in loss of 129Xe* from iodine bearing sites. Uranium-bearing apatite appears to be a secondary mineral that incorporated parentless 129Xe* and 129Xe*that had been redistributed during earlier metamorphism. A trapped-Xe component released at high-temperatures may be a primitive component such as Q-Xe, though terrestrial? Xe acquired during weathering cannot be ruled out by this study. If Q-Xe is present, it is most likely hosted in a primary phase other than plagioclase. During its terrestrial residence time GRA 06129 acquired iodine via Antarctic weathering. I-Xe analyses on Antarctic meteorites should therefore be carried out with caution. Further Xe analyses of mineral separates from GRA 06129 would help constrain the host phase of the trapped Xe. That the I-Xe system of the plagioclase has been completely reset make it a good candidate mineral for I-Xe dating of primary processes whereas I-Xe dating of apatite appears more problematic.