Molecular and isotopic behavior of insoluble organic matter of the Orgueil meteorite upon heating

Abstract

Organic matter contained in carbonaceous chondrites may have evolved due to aqueous and/or thermal evolution on the parent body. The thermal behavior of the insoluble organic matter (IOM) of the Orgueil meteorite was investigated. The evolutions of structural and molecular properties were assessed by Raman, infrared and XANES spectroscopies, the H- and N-isotopic compositions by NanoSIMS. The starting IOM is a disordered organic macromolecule presenting a high degree of cross-linking. Hydrogen and Nitrogen isotope distributions are heterogeneous with the occurrence of numerous micron-sized hot spots enriched in heavy isotopes of H or N. After 1 hour at 300°C, there is subtle modification of the structural ordering and the isotopic compositions. After 1 hour at 500°C, the structure evolves toward condensation. Indeed, FTIR and XANES data are consistent with a continuous evolution of the molecular structure toward an increase of aromatization, starting at 300°C and becoming more intense at 500°C. The bulk D-enrichment is significantly reduced and D-rich hot spots are lost at 500°C. The experimental evolution of the δD is consistent with observations of IOM isolated from lightly altered carbonaceous chondrites. In contrast, the 15N-rich hot spots seem insensitive to high temperature up to 500°C and bulk δ15N remains constant. The thermal evolution of H- and N- isotopes is decoupled, indicating that the D-rich and 15N-rich moieties exhibit different thermal recalcitrance.