Results

Preservation of the isotope signatures in chondritic IOM during aqueous alteration

Mighei-type carbonaceous chondrites (CM) figure among the most primitive objects in the solar system. In this experimental study, we exposed the insoluble fraction (IOM) of organic matter from Paris, the least altered CM available. This was performed in simulated hydrothermal system, at 150°C for 49 days, and the experimental was compared to recent CM2 falls Aguas Zarcas and Mukundpura. The experimental residue showed a chemical and isotopic composition similar to those of Aguas Zarcas and Mukundpura IOMs, confirming that these CMs can be seen as altered counterparts of Paris. For the first time, it was showed that population of organic radicals could be potentially produced during the asteroidal stage. In the meantime, the isotopic hotspots were not lost during the experiment, suggesting that the hotspots generally observed within the CM IOMs could be pre-accretional vestiges. 

Reference

This collaborative study (IMPMC-UMR CNRS 7290, Institut d’Astrophysique Spatiale, Chimie-ParisTech, Laboratoire d’Archéologie Structurale et Moléculaire) will be soon published in Geochemical Perspective Letters.

Polycyclic aromatic hydrocarbons in carbonaceous asteroids: molecular and isotopic properties.

Polycyclic aromatic hydrocarbons (PAHs) are organic molecules constituted by two or more fused benzene rings. We have investigated the free PAHs extracted from three carbonaceous chondrites having experienced substantial and distinct degrees of aqueous alteration: Kolang (CM1/2), Mukundpura (CM2.0) and Aguas Zarcas (CM2.2). The nature, concentration and carbon and hydrogen isotopic compositions of these molecules were determined in order to assess their origin and evolution. Our results show that PAHs in CCs likely carry information on both accretion and alteration processes. The investigated chondrites contain between 22.0 and 34.4 ppm of PAHs, with sizes ranging from 2 (naphthalene) to 5 cycles (benzopyrene). The total concentration of PAHs is not correlated to the degree of alteration experienced on the chondrite parent body. Yet, the abundance of alkylated PAHs appears correlated to the degree of alteration. The largest PAHs are also the most depleted in ¹³C, suggesting a formation by carbon addition in conditions similar to the interstellar medium. On the other hand, all the extracted PAHs are D-depleted, pointing towards hydrogen isotope re-equilibration with water having occurred during aqueous alteration.

Reference

M. Lecasble, L. Remusat, J.-C. Viennet, B. Laurent and S. Bernard (2022) Polycyclic aromatic hydrocarbons in carbonaceous chondrites can be used as tracers of both pre-accretion and secondary processes. Geochimica et Cosmochimica Acta, 355, p 243-255. doi: 10.1016/j.gca.2022.08.039.

High resolution mass spectrometry to investigate the Insoluble Organic Matter from carbonaceous chondrite


We applied laser desorption ionization - Fourier transform ion cyclotron resonance mass spectrometry (LDI-FTICR-MS) on the macromolecular carbon of recent CM carbonaceous chondrite falls, as well as Orgueil (CI) and Tarda (C2). FTICR offers a unique combination of high mass accuracy (<100 ppb) and mass resolution (>1,000,000 at /m/z /400). It enables both the detection of a diverse molecular fragments present in chondritic organic matter and the assignment of their molecular formula. Here, it appears that IOMs have retained chemical information resulting from asteroidal alteration processes as well as remaining signature of potential precursor sources. The abundance of the chemical families is correlated to the extent of aqueous alteration, which tend to promote a structural aromatisation. The weakly altered Paris has retained the largest chemical heterogeneity, whilst it is lost in more altered chondrites. Orgueil  and Tarda IOM share clear similarities; consistent an origin from the outer belt region, while all CMs tend to be related to a common precursor. FTICR-MS may represent a prime tool to study samples from Hayabusa 2 and Osiris Rex missions, and for the study of primitive organic molecules potentially preserved within asteroidal bodies.

This study will soon be published in Geochemical Perspective Letters. 


Reference

B. Laurent, J. Maillard, C. Afonso, G. Danger, P. Giusti and L. Remusat. Diversity of chondritic organic matter probed by ultra-high-resolution mass spectrometry. Geochemical Perspective Letters, in press.