京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18805号 / 理博第4063号 / 新制||理||1584(附属図書館) / 31756 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 土`山 明, 准教授 三宅 亮, 准教授 伊藤 正一 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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 Béréba 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 Béréba 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.

Une période intéressante de l'histoire précoce du système solaire est celle du disque, i.e. la période pendant laquelle se déroule la plupart des processus qui vont conduire du mélange de gaz et de poussières nébulaires à des grains et des planétésimaux, qui seront à un stade ultérieur le matériel de départ pour la formation des embryons planétaires et des planètes. Les témoins de cette époque sont les constituants des météorites primitives (chondrites), principalement les inclusions réfractaires (CAIs) et les chondres. Une des questions centrales dans la compréhension des processus à l'origine des CAIs et des chondres est celle du temps. Les travaux récents de Johan Villeneuve ont permis de démontrer que l'26Al et les isotopes du Mg étaient distribués dans le disque d'accrétion à un niveau d'homogénéité permettant d'utiliser le système 26Al-26Mg comme le chronomètre le plus précis qui soit des évènements qui se sont déroulés lors des 2 ou 3 premiers millions d'années du disque. Le but de cette thèse a été de reprendre toute l'étude de l'26Al avec des mesures de plus haute précision que les mesures existantes, en associant les mesures in-situ (sonde ionique) et en roche totale (HR-MC-ICPMS). Les développements analytiques mis en place pour mesurer les compositions isotopiques en Mg ont été appliqués à l'étude d'olivines réfractaires et de chondres de la météorite Allende, et d'olivines de la pallasite Eagle Station. L'ensemble des données a permis d'apporter des éléments de réponse sur les âges de formation des chondres et de leurs précurseurs, et sur l'origine des olivines réfractaires riches en Mg et la possibilité que celles-ci figurent parmi les précurseurs des chondres / The disk history is a very interesting period of the early Solar System history, i.e. the period during which occurred most of the processes leading from the mixing of nebular gas and dust to grains, and then planetesimals, which will then constitute the starting material for formation of planetary embryos and terrestrial planets. The witnesses of this period that we have in the laboratory are the chondrite constituents, mostly refractory inclusions (CAIs) and chondrules. One of the central questions regarding the processes leading to the formation of CAIs and chondrules is the timing. Villeneuve et al. (2009, 2011) have shown that 26Al and Mg isotopes were homogeneously distributed in the accretion disk, at a level allowing the use of the 26Al-26Mg system as the most precise short-lived chronometer to constrain the 2 or 3 first million years of the disk history. My PhD project aimed at reconsidering the 26Al study with more precise measurements, and by combining in-situ (by MC-SIMS) and bulk (by HR-MC-ICPMS) analyses. The analytical developments we set for Mg-isotope measurements (because of the high precision needed) were then applied to a set of extraterrestrial materials, including Mg-rich isolated olivines and Mg-rich olivines in porphyritic type I chondrules from the Allende CV3 meteorite, as well as chondrules from the same meteorite, and olivines from the Eagle Station pallasite. The whole data set allowed to answer questions such as (i) the origin of chondrule precursor materials and the time of chondrule formation, as well as (ii) the origin of Mg-rich refractory olivines, and the possibility that the latters were part of chondrule precursors

The field of noble gas mass spectrometry is a rapidly developing one. There is a sustained requirement for continuous development of instrumentation in order to allow researchers to answer critical questions facing their respective fields. Within planetary science, the question of whether our solar system is 'typical', has been a driving force behind many lines of research. This project sought to build upon the strong instrumental base of the Isotope Geochemistry and Cosmochemistry group of the University of Manchester, in order to develop the Resonance Ionisation Mass Spectrometer for Krypton Isotopes. From that basis, it then sought to develop techniques and protocols for the analysis of extra-terrestrial samples, particularly those of very low mass. The thesis is broken down into three studies. The first involved the development of the mechanisms used to fill the Xe-Ar four-wave mixing cell. This section covers the development, implementation, and characterisation of the newly installed mass flow controllers. The additional control over the mixing ratio granted allowed an eightfold increase to the sensitivity of the instrument. The reproducibility of the gas ratio is consistently shown throughout, and has ultimately reduced the tuning time of the instrument from a number of weeks to a day. The second section details the required procedures involved in data reduction, and the development of protocols and software for the RIMSKI instrument. A range of approaches are assessed, including a novel approach to cosmic ray exposure age calculations. The methods are discussed and implemented on extra-terrestrial samples, the Stannern eucrite and the Bereba eucrite. Finally, the analytical limits of the RIMSKI instrument are expanded via the measurement of the krypton systems of individual calcium-aluminium-rich inclusions. It is observed that Allende CAIs show a trapped component, Br-derived Kr, as well as an anomalous 'heavy Kr' component (one that is enriched in 86Kr). It is additionally noted that there is a lack of cosmogenic krypton to be found in all but one of the measured inclusions. The implications of these findings are discussed.