Hydrogen in the high-temperature phases of chondrites: origin and contribution to the hydrogen budget of the rocky planets.
Abstract :
Chondrules, the main components of chondrites, are silicate spherules formed during episodes of high temperature in the first millions of years of our Solar System. Coming from undifferentiated meteorites, chondrules are archives of the early evolution of our Solar System. Enstatite chondrites (ECs), formed near the Sun, are often considered to be the main building blocks of the Earth due to their isotopic similarities with terrestrial rocks. To gain a better understanding of the origin of water and volatile elements on Earth, this thesis proposes a dual approach: (i) a detailed characterisation of hydrogen (H) and other volatile elements in ECs and (ii) high-temperature experiments to simulate the formation of chondrules in H-rich environments.
By combining ion probe analyses of the content of volatile elements (H, C, F, S, Cl, N) and isotopy of H and N with Raman spectroscopy on EC chondrules, we have highlighted the link between H and S in the chondrule mesostasis. We suggest that the solubilisation of H occurs during the formation of the chondrules at high temperature and reject the hypothesis of a later incorporation during parent body metamorphism or Earth weathering. Furthermore, bulk analyses by EA-IRMS (Elemental Analyzer coupled to Mass Spectrometer) allow us to propose that the H-bearing phases in ECs differ from those in other chondrites formed close to the Sun, such as rumurutites or ordinary chondrites.
To better determine the incorporation conditions of H in chondrules during their formation, we conducted high temperature experiments at 1 bar under H-rich atmospheres. Through the development of different experimental protocols, we sought to reproduce the observations in the ECs, namely a high H content and a link with S. Although our experiments were unable to reproduce the high H contents measured in the EC chondrules, we observed that the solubilisation of hydrogen decreases when the environment becomes more reduced, independently of the presence of sulphur or the silicate melt composition.
Finally, to perform isotopic characterisation of H present in anhydrous EC minerals (e.g., pyroxene or olivine) using ion probes, we initiated the development of new standards. We used the EA-IRMS for the analysis of bulk compositions and the ion probe for in-situ analyses. We encountered standardisation difficulties with systematically higher H contents with EA-IRMS. However, we have carried out analytical developments of the EA-IRMS protocols to guarantee a precision of 5‰ on the D/H isotopic ratio in minerals with low hydrogen contents (down to 250 ppm water).
Keywords : Hydrogen, Chondrites, Cosmochemistry, Experimentation
Jury Members :
Thesis supervisors :
Dr. Laurette PIANI (Centre de Recherches Pétrographiques et Géochimiques, Nancy)
Dr. Yves MARROCCHI (Centre de Recherches Pétrographiques et Géochimiques, Nancy)
Rapporteurs :
Dr. Tahar HAMMOUDA (Université Clermont Auvergne, Clermont Ferrand)
Dr. Romain TARTESE (The University of Manchester, Manchester)
Examinateurs :
Dr. Camille CARTIER (Centre de Recherches Pétrographiques et Géochimiques, Nancy)
Dr. Valérie MALAVERGNE (Université de Gustave Eiffel, Champs-sur-Marne)
Invité :
Dr. Giada IACONO-MARZIANO (Institut des sciences de la Terre d’Orléans, Orléans)