abstract: Shock metamorphism in meteorites constrains the impact histories of asteroids and planets. Shock-induced high-pressure (HP) minerals can provide more precise estimates of shock conditions than shock-induced deformation effects. In this research, I use shock features, particularly HP minerals, in ordinary-chondrite samples to constrain not only shock pressures but also the pressure-temperature-time (P-T-t) paths they experienced. Highly shocked L5/6 chondrites Acfer 040, Mbale, NWA 091 and Chico and LL6 chondrite NWA 757 were used to investigate a variety of shock pressures and post-shock annealing histories. NWA 757 is the only highly shocked LL chondrite that includes abundant HP minerals. The assemblage of ringwoodite and majoritic garnet indicates an equilibration shock pressure of ~20 GPa, similar to many strongly shocked L chondrites. Acfer 040 is one of the only two chondrite samples with bridgmanite (silicate perovskite), suggesting equilibration pressure >25 GPa. The bridgmanite, which is unstable at low-pressure, was mostly vitrified during post-shock cooling. Mbale demonstrates an example of elevated post-shock temperature resulting in back-transformation of ringwoodite to olivine. In contrast, majoritic garnet in Mbale survives as unambiguous evidence of strong shock. In these two samples, HP minerals are exclusively associated with shock melt, indicating that elevated shock temperatures are required for rapid mineral transformations during the transient shock pulse. However, elevated post-shock temperatures can destroy HP minerals: in temperature sequence from bridgmanite to ringwoodite then garnet. NWA 091 and Chico are impact melt breccias with pervasive melting, blackening of silicates, recrystallization of host rock but no HP minerals. These features indicate near whole-rock-melting conditions. However, the elevated post-shock temperatures of these samples has annealed out HP signatures. The observed shock features result from a complex P-T-t path and may not directly reflect the peak shock pressure. Although HP minerals provide robust evidence of high pressure, their occurrence also requires high shock temperatures and rapid cooling during the shock pulse. The most highly shocked samples lack HP signatures but have abundant high-temperature features formed after pressure release. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2016

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18495号 / 理博第4010号 / 新制||理||1578(附属図書館) / 31381 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 平田 岳史, 准教授 伊藤 正一, 教授 土`山 明 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

[pt] Corpos relativamente pequenos do sistema solar não possuem atmosfera (ou muito rarefeita, chamada exosfera) e nem campo magnético: encontram-se praticamente desprotegidos da influência do ambiente espacial no qual estão inseridos. Tais corpos (como a maioria dos asteroides, luas e cometas distantes do Sol), estão sujeitos aos efeitos do chamado intemperismo espacial. Com o objetivo de simular e estudar este fenômeno em laboratório, três amostras de meteoritos do tipo condrito ordinário foram irradiadas por feixes de H(positivo) com energia de 1,0 MeV. Uma das amostras foi, também, irradiada por feixe de N(positivo) de 2,0 MeV. Os efeitos da irradiação foram monitorados por espectroscopias Raman e UV-VIS-NIR. Os resultados Raman mostram variações sistemáticas do número de onda, da intensidade e da largura das bandas vibracionais estudadas, evidências de modificações estruturais. As seções de choque de destruição das ligações Si-O presentes nas estruturas cristalinas foram determinadas. A espectroscopia UV-VIS-NIR por refletância de esfera integrada foi usada para monitorar as modificações nas características espectrais. Os espectros obtidos mostram escurecimento e avermelhamento progressivos das amostras irradiadas; a amplitude destas modificações depende do conteúdo inicial de ferro na estrutura. É proposto que o avermelhamento observado com o aumento da fluência (ou dose) da irradiação deve-se ao aumento do coeficiente de absorção na faixa azul que, por sua vez, decorre da diminuição do gap óptico do material. Estes resultados são relevantes para a modelagem da evolução físico-química de asteroides expostos ao vento solar. / [en] Relatively small bodies in the solar system have no atmosphere (or very thin, called the exosphere) and no magnetic field: they are virtually unprotected from the influence of the space environment in which they are inserted. Such bodies (like most asteroids, moons, and comets far from the sun), are subject to the effects of so-called space weathering. To simulate and study this phenomenon in the laboratory, three samples of ordinary chondrite meteorites were irradiated by H(positive) beams with 1.0 MeV energy. One of the samples was also irradiated by a 2.0 MeV N(positive) beam. The effects of irradiation were monitored by Raman and UV-VIS-NIR spectroscopies. The Raman results show systematic variations in the wavenumber, intensity, and width of the studied vibrational bands, evidence of structural modifications. The shock sections of the destruction of Si-O bonds present in the crystalline structures were determined. Integrated sphere reflectance UV-VIS-NIR spectroscopy was used to monitor changes in spectral characteristics. The obtained spectra show progressive darkening and reddening of the irradiated samples; The extent of these modifications depends on the initial iron content in the structure. It is proposed that the redness observed with the increase of irradiation creep (or dose) is due to the increase in the absorption coefficient in the blue band, which, in turn, results from the decrease of the optical gap of the material. These results are relevant for modeling the physicochemical evolution of asteroids exposed to the solar Wind.

Une nouvelle méthode de haute précision pour analyser les isotopes de Sn à avec double-spike 117Sn-122Sn a été développée. / A new high-precision isotope method for analyzing Sn using the 117Sn-122Sn double-spike technique was developed.