Geochemistry and nanostructures of carbons in primitive chondrites: research of pre-accretional signatures by SIMS, Raman and HRTEM

This thesis is focused on the contribution of the multi-scale organization of acapulcoites - lodranites carbons, to understand their parent-body history. Indeed, several studies carried out on silicates and metal showed that (1) acapulcoites and lodranites come from the same parent body, (2) this parent body underwent an important thermal metamorphism and a beginning of differentiation, as well as (3) an important collision with another body. In all studied acapulcoites and lodranites, carbons previously described as graphite are systematically included in metallic phases (FeNi). This strongly suggests that these carbons could have been formed by interactions between metal and organic matter. To specify this graphitization mode, we performed experimental analogues by heating a blend between saccharose based chars (a chemical and structural analogue of the chondritic insoluble organic matter (IOM)) and iron, between 650 and 1600 deg. C, that is in a range of recognized temperatures of differentiation of A-L parent-body. Then, we systematically combined observations of carbons from these meteorites with those of the experimental analogues. We developed an original methodology coupling the study of the organization at the micrometre to nanometer scales (by Transmission Electron Microscopy and by Microspectrometry Raman), and the analysis of the isotopic composition of carbons (by SIMS) on strictly the same areas of micrometre dimensions. TEM showed that Acapulco contains some 'true' graphite, among several more or less graphitized carbon phases. On the other hand, Lodran, which is recognized as more heated during the metamorphism, does not contain graphite but only a mesoporous and turbostratic carbon, systematically present within veins. The comparison with heated experimental blends of saccharose-based char - iron indicates that the Acapulco graphite was formed by a 'catalytic' graphitization mode resulting from interactions between iron and disordered carbon. The absence of graphite in Lodran, in an iron-rich and hot environment, is apparently paradoxical. This implies a late arrival of IOM, after the episode of partial differentiation, which allows a carbonization but excludes any graphitization, even 'catalytic'. All our observations can be reconciled if we consider a collision of the A-L parent body with a chondritic body bringing IOM and impact heat. The IOM is then carbonized and gives an immobile char residue and hydrocarbon effluents. The first one will be graphitized owing to the presence of iron (case of Acapulco), whereas the second tend to migrate towards colder zones, where they can be only carbonized (case of Lodran). This scenario is strengthened by the isotopic analyses which prove the chondritic origin of the A-L carbons precursor. The large range of carbon isotopic variation can be explained within this model by using a Rayleigh distillation model for each carbonization stage. The nitrogen isotopic composition reflects a different mechanism related to the IOM nitrogen speciation. Our measurements evidence a differential conservation of nitrogen-bearing moieties with effluents containing labile nitriles and chars enriched in refractory pyrroles. It is remarkable that A-L carbons were thus able to preserve pre-accretional signatures (δ¹³C, δ¹⁵N), in spite of the metamorphism and a beginning of differentiation. We also show for the first time that planetary differentiation does not necessarily induce an isotopic homogenization, but can yield large variations (in the case of carbon) or, in the case of nitrogen, reveal one, which was a pre-existent at the molecular scale. (author)