Advances in Cd and Fe isotope cosmochemistry

The question about how our solar system was formed is central in cosmochemistry. Within this thesis, I follow two different approaches that relate to this central question. First, mass-dependent Cd stable isotope fractionation was studied experimentally in order to improve our understanding of the stable isotope fractionation effects associated with evaporation and condensation processes. A thorough understanding of such stable isotope effects is required to interpret natural variations, for example in meteorites. Second, Fe isotope compositions were studied in chondrite leachates and residues at high precision in order to reveal potential isotope heterogeneities due to the uneven distribution of materials with different stellar origins within chondrite samples as well as on the planetary scale. To investigate volatile element stable isotope effects related to evaporation and condensation under reduced and oxidized conditions, two experiments were implemented, experiment 1 in CO-CO2 and experiment 2 in air. Samples were taken from evaporating silicate melts at the bottom of a furnace tube and from plates placed on the furnace wall where Cd vapor recondensed. Cadmium was separated from evaporation and condensation samples and analyzed using MC-ICPMS. For 114Cd/110Cd measurement precision (2 s.d.) was ±0.03 ‰. Results show that the loss of volatile elements during evaporation can proceed with neglectable isotope fractionation under reduced conditions. It is assumed that in experiment 1, the isotope fractionation is suppressed by a saturated boundary layer on top of the melt surface. Back reaction of evaporated material with the melt occurs and equilibrated material evaporates from the layer. Samples leached from condensation plates reveal an enrichment of light Cd isotopes closer to the melt surface and an enrichment of heavier Cd isotopes at the top of the plates. As expected from their higher collision frequency due to their higher velocity, the light Cd isotopes condensed preferentially. In experiment 2, the fractionation factor is in good agreement with what is expected from purely kinetic condensation. In experiment 1, it is likely that back reaction altered the fractionation. Deviations from terrestrial isotope ratios in solar system materials such as meteorites are called nucleosynthetic anomalies and can be detected via high-precision isotope analyses Since Fe isotope measurements increased in precision over the last decades, high-precision MC-ICP-MS Fe isotope analyses of chondritic material were implemented in this thesis. Leachates and residues of six different bulk chondrites and bulk rock samples were analyzed for their Fe isotope composition with a measurement precision (2 s.d.) of 13 ppm for 56Fe/54Fe and 16 ppm for 58Fe/54Fe. No nucleosynthetic Fe isotope anomalies were found in the analyzed samples. In contrast to homogenous Fe, other Fe-peak elements like Cr or Ti show nucleosynthetic anomalies on the bulk rock scale and planetary scale. Theories to explain this phenomenon are 1) secondary redistribution of anomalous refractory Fe-peak element carrier phases beside less refractory Fe carrier phases which could not survive high-temperature events and got homogenously mixed throughout the solar system as gas, or 2) an initially homogeneous molecular cloud or protosolar disk into which anomalous material was injected and heterogeneously distributed, while Fe is carried by thermally unstable phases that evaporated and mixed homogeneously throughout the solar system. A small amount of anomalous Fe in rare grains survived high-temperature events but cannot be detected on bulk rock scale due to the mass budget of Fe in the solar system