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Geochemistry and Cosmochemistry of Calcium Stable Isotopes

par Valdes, Maria
Président du jury Mattielli, Nadine
Promoteur Debaille, Vinciane
Publication Non publié, 2018-09-14

Thèse de doctorat

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Résumé :

Calcium (Ca) is the fifth most abundant element in the rocky planets. As a lithophile, refractory element, Ca does not partition into planetary cores nor is it volatilized during planetary accretion. These characteristics make Ca ideal for investigating the earliest stages of planetary formation and the subsequent chemical evolution of planetary mantles and crusts. This thesis presents observations of and explores the mechanisms involved in high-temperature mass-dependent Ca isotope fractionation in terrestrial, lunar, and meteoritic material. Chapter 1 reports Ca isotope fractionation among a co-genetic suite of samples from the Guelb el Azib ultramafic-mafic-anorthosite complex, which represents the fractional crystallization sequence of a terrestrial igneous magma chamber. The measurements imply that Ca isotope fractionation in an evolving crystallizing magma is mineralogically controlled and that the degree of fractionation can vary according to the Ca composition of the residual magma. Chapter 2 investigates ureilites, a distinctive group of achondritic meteorites, widely regarded to be mantle remnants of a disrupted asteroidal parent body. To date, it is not clear which of their features were inherited from the original chondritic body and which were created during post-accretionary igneous processes such as partial melting. This chapter presents evidence that partial melting on the ureilite parent body is responsible for two such ambiguous characteristics, Ca isotopic and magnesium number (Mg#) heterogeneity, though the possibility of mixing among heterogeneous primordial reservoirs is also discussed. Chapter 3 discusses the use of Ca isotopes to constrain the evolution of the lunar interior following the Moon’s post-formation global magma ocean stage. The results show that there is minimal Ca isotope fractionation among lunar magma ocean products. This may imply that the Ca isotopic composition of lunar materials can be overwritten by mantle overturn, source mixing, and other post-magmatic events. Taken together, the results presented in this thesis impact the way we think about the formation and isotopic evolution of magmatic reservoirs on rocky planets.