Hydratation et évolution isotopique précoce des astéroïdes carbonés : approches expérimentale et isotopique

Lionel Vacher
PhD Link to the Thesis [http://www.theses.fr/2018LORR0229]
CRPG-CNRS Université de Lorraine 15, Rue Notre Dame des Pauvres 54501 VANDOEUVRE-LÈS-NANCY
Copyright Elsevier

Carbonaceous asteroids were affected by aqueous alteration processes that have strongly modified their primary mineralogy in favour of a wide diversity of newly formed phases. Despite the numerous studies carried out on hydrated chondrites (CM chondrites), the physicochemical conditions of aqueous alteration and the identification of the water sources accreted by asteroids are still poorly constrain. From the mineralogical and isotopic survey of secondary phases, this thesis aims (i) to decipher the origin and evolution of water accreted by primitive asteroids and (ii) to retrace the physicochemical conditions of aqueous alteration using hydrothermal laboratory experiments. First of all, our results show that the pristine CM chondrite Paris contains Ca-carbonates whose O-isotopic compositions ([delta]17,18O) requires an 8-35% contribution of water ice from the outer part of the Solar System. In addition, our C-isotopic analyses conducted on these same Ca-carbonates indicate similar [delta]13C values to those of the soluble organic matter (SOM) that constitute carbonaceous chondrites. Thus, we suggest that SOM is the most probable source of carbon to form Ca-carbonates. Then, the study of different clasts in the CM chondrite Boriskino revealed that this meteorite has experienced high intensity impact events, causing the formation of fractures and the circulation of later 16O-rich fluid flow. Finally, our low temperature laboratory experiments successfully synthetized the most characteristic phases of CM chondrites: tochilinite and cronstedtite. Moreover, by comparing our results to other experimental studies, we observed a positive correlation between the nMg content in the hydroxide layer of synthetic tochilinite and temperature. This correlation suggests that the chemical composition of tochilinite represents as powerful proxy to retrace the alteration temperature experienced by CM chondrites

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