Origins and evolution of nitrogen on the planetesimals of the solar system : study of the nature and isotopic composition of nitrogen-bearing phases in carbonaceous chondrites

PhD defense of Lucie Laize-Generat - Thursday, October 24th, 2024 at 2.00 pm - Manuel Forestini room IPAG

The incorporation and evolution of nitrogen (N) in the early solar system, as well as on the primitive earth, are still not completely understood. Questions remain concerning the nature and distribution of N-bearing phases that were present in the protoplanetary disk and are now present in small bodies, remnants of the protoplanetary disk, and by extend in cosmomaterials. At this time in the literature, it is commonly assumed that in carbonaceous chondrites and comets, most of the total nitrogen is present in the refractory organic material. However, nitrogen in the form of ammonium (NH4+) has been identified by spectroscopy on various objects and recently has been quantified in the Ryugu samples returned on Earth by the Hayabusa2 mission. Because carbonaceous chondrites are the most volatile-rich of primitive meteorites, studying the nature and isotopic compositions of the N-bearing phases present in these meteorites are key data for exploring the origin and evolution of nitrogen in the young solar system. What are the N-bearing phases present in carbonaceous chondrites? How is the nitrogen distributed between NH4+, soluble and insoluble organic matter? Were these N-bearing phases incorporated at the same location and/or time in the parent bodies of carbonaceous chondrites?

To answer these questions, the aim of my doctoral work was to develop a protocol to analyse the nitrogen abundances and isotopic compositions of the bulk, insoluble organic matter (IOM) and NH4+ N-bearing phases present in carbonaceous chondrites.

In the studied chondrites (3 CI, 2 C2-ung, 7 CM, 6 CR), N is mainly distributed between two fractions: IOM (42 % of the total N in average) and an unanalysed N-bearing fraction, probably acid-soluble N and/or lost IOM (43 % of the total N in average). CI chondrites appear to be the richest in water-soluble NH4+, with about 1/4 of their total nitrogen being in this phase. The extracted ammonium has a d15N of +72 ± 9 ‰ in Orgueil and between +49 ‰ and +237 ‰ in Ivuna and Alais, confirming its extraterrestrial origin. The d15N of the extracted NH4+ in CMs and CRs could not be precisely constrained but might range between -4 ‰ and +184 ‰ in CMs and +121 ‰ and +1000 ‰ in CRs. The water-soluble NH4+ extracted from CIs, C2-ung, CMs and CRs could be under the form of salts and/or present in organic and/or inorganic minerals as phyllosilicates. The water-soluble NH4+ could potentially originate from the decomposition of IOM and/or amino acids and/or could be the tracer of NH3 ices and hydrates accreted on the parent bodies of carbonaceous chondrites.

Despite the experimental limitations, it would appear that the CM Winchcombe and the CI Orgueil have similar d15NH4+ values and matrix and phyllosilicate abundances. This suggests that the parent bodies of these meteorites may have incorporated their NH4+ from the same reservoir in the solar system, and/or that CI material was mixed with CM material during the evolution of the solar system. CRs appear to be distinguished from CIs, C2-ung and CMs, notably by an enrichment of the studied N-bearing phases in 15N. The N-isotopic compositions of CRs are close to those of outer solar system objects, suggesting a potential cometary origin for these meteorites. I proposed new experimental developments that will enable us to refine this research and provide new insights into the incorporation and evolution of nitrogen in the solar system.

Thesis direction

  • Lydie BONAL, Olivier POCH and Joël SAVARINO

The jury will consist of

  • Lydie Bonal, Astronomer, IPAG UGA, Thesis director
  • Evelyn Füri, Researcher, CRPG CNRS, Referee
  • Laurent Remusat, Senior researcher, IMPMC CNRS, Referee
  • Matthieu Gounelle, Professor, IMPMC CNRS, Examiner
  • Joël Savarino, Senior researcher, IGE CNRS, Thesis co-director
  • Eric Quirico, Professor, IPAG UGA, Examiner