The Origin of Nitrogen on Earth: Implications from Isotope and Petrologic Studies of Planetary Materials at the Sub-Micrometer Scale.

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Nitrogen, the most abundant component in Earth’s atmosphere, was a key element for the evolution of Earth’s biosphere. The respective isotopic compositions of the rocky planets Earth, Mars and Venus differ from those of the Sun and the Protosolar Nebula (PSN). Thus, other components from the solar nebula must have significantly contributed to terrestrial nitrogen. Isotopic signatures of the enstatite chondrites (ECs) suggest them as the best analogue material for the building blocks of the primordial Earth, together with a smaller fraction of ordinary chondrite and carbonaceous chondrite material. Model calculations from other studies consider angrites, a group of achondrites, as best fit. Bulk chemistry indicates that our sample set of Solar System materials is far from complete, with a significant portion contributing to Earth missing from the meteorite record. This emphasizes the necessity to characterize the inventory of nitrogen-bearing phases in the available sets of meteoritic samples, to allow predictions about the missing fraction. During the first period of this project, it became evident that the known N-carriers in the ECs cannot account for the total amount of bulk nitrogen, indicating other phases as host materials. Organic material (OM) in several carbonaceous chondrites contained high enrichments in 15N compared to the terrestrial atmosphere. These enrichments were the result of fluid interaction on the meteoritic parent body, and not directly inherited from the protosolar cloud. Investigation of the chemical functionalities of these materials is still in progress, but one of the implications of this observation is that the chondritic organic material that contributed nitrogen to the primordial Earth might span an even wider (structural and chemical) range than initially anticipated. I will continue the investigation of nitrogen-bearing materials in various chondritic meteorites, with the main focus on metal, sulfides, and silicates in ECs. In addition, the quantification of the anorganic N-carriers in enstatite and ordinary chondrites will be continued, and the OM inventory of CO and CV chondrites will be studied. The sample set will be expanded to achondrites that have potential relations to the terrestrial building blocks. Sample characterization will be carried out by SEM and SEM-EDS. N-isotopic investigations will be conducted by NanoSIMS. Nitrogen is typically detected with SIMS as CN-, which normally restricts conventional N-isotopic measurements to phases that also contain carbon. Thus, some samples will be prepared by C-impregnation to enhance the CN-yield. The N isotopes of C-free phases can also be studied by using 14,15N+. Samples will be selected and prepared for structural analysis by transmission electron microscopy. This project will expand the inventory of N-bearing phases and their N-isotopic compositions, which ultimately will help to determine the nature of the carriers contributing to Earth’s nitrogen.