PhD fellow ORIGINS, September 2007 – September 2010, Muséum National d’Histoire Naturelle, Paris (France) September 2007, I started a PhD thesis in cosmochemistry at the Muséum National d’Histoire Naturelle (MNHN), Paris (France) under the supervision of Professeur Matthieu Gounelle. My thesis focussed on the study of Cr isotopes in carbonaceous chondrites. Chromium is a transitional metal with 4 naturally occurring isotopes 50Cr (4.35%), 52Cr (83.79%), 53Cr (9.50%) and 54Cr (2.36%). In the stellar environment, 50Cr, 52Cr and 53Cr are primarily produced during explosive oxygen and silicon burning in supernovae (Clayton, 2003) in neutron-poor environments. An additional contribution to the abundance of 53Cr comes from the decay of the short-lived 53Mn, which is also produced during explosive silicon burning in supernovae (Woosley et al., 2002). 53Mn decays in 53Cr with a half-life of 3.7 Myr. This chronological system has been widely used to constrain the chronology of early planetary formation processes (e.g. Birck and Allègre, 1985; Shukolyukov and Lugmair, 2006; Moynier et al., 2007; Yin et al., 2007), and the timescales of formation of single mineral grains (e.g. Hoppe et al., 2007; Endress et al, 1996). In contrast, 54Cr is produced in rare type 1a supernovae (Clayton, 2003) in neutron-rich environments. This feature renders this isotope particularly useful for studying mixing processes that took place prior to or during the formation of the solar system and tracing materials that formed in these regions. My thesis consists of 2 Parts. The first part of the thesis consists of a collaboration between the MNHN and UCLA (Professor Kevin McKeegan). In part I, the 53Mn-53Cr systematics was developed on the NanoSIMS 50 at MNHN and the CAMECA IMS 1270 at UCLA and applied onto different types of individual carbonate grains found in various carbonaceous chondrites. For each of the carbonate grains analysed, the 53Cr excesses measured linearly correlated with their respective 55Mn/52Cr ratios thus indicating the in-situ decay of 53Mn and demonstrating that 53Mn was still extent at the time of formation of the respective carbonate. Such internal isochrons enabled us to refine the timescales of carbonate formation on the respective parent bodies, and to constrain the temporal evolution of the chemical and isotopic compositions of the aqueous fluids. In part II, we dissolved a fragment of Tagish Lake and of Orgueil in 5 different steps by using reagents with increasing acid strength in collaboration with Université paris VI (Jussieu). The purpose of this digestion procedure was to separate and analyse the main Cr-bearing phases contained in a whole-rock sample of both meteorites. For each of these steps, the 53Cr and 54Cr isotopic compositions were measured. The present study demonstrated that the Tagish Lake meteorite displays both the highest excess and the highest deficit in 54Cr of all groups of carbonaceous chondrite meteorites. Since the magnitude of the 54Cr anomaly is proportional to the metamorphic grade of the meteorite (Rotaru et al., 1992), this result indicates that Tagish Lake may represent the least metamorphic meteorite present in the collections worldwide and is particularly adequate for finding the carrier phase of the 54Cr anomaly. The ORIGINS fellowship enabled me to have a decent living in Paris, attend international PhD schools (Les Houches 2009; Alpbach 2009), present my results at international conferences (MetSoc 2009-2010; LPSC 2008, 2009, 2010), learn how to use state-of-the-art measurement instruments (NanoSIMS and CAMECA IMS 1270), and collaborate with different Swiss (ETH Zürich), European (Münster, Jussieu) and American (UCLA) laboratories. It thus helped me to interact with some of the most important international scientists in the field of meteoritics and cosmochemistry.