Interlayer trapping of noble gases in insoluble organic matter of primitive meteorites

Noble gases in primitive meteorites are trapped in a residue left after demineralization of bulk meteorite by HF and HCl. Most of the primordial Ar, Kr and Xe and small amounts of He and Ne are removed by oxidation of this acid-resistant residue, e.g. with HNO3. These gases, referred as P1, are trapped in a poorly characterized, presumably organic, phase labeled phase Q. In order to understand the siting of P1 noble gases, we have performed a solvation experiment on insoluble organic matter of Orgueil (CI). Pyridine was used because it presents an important swelling ratio of about 2 potentially able to change considerably the structure of phase Q without affecting the crystalline structure of nanodiamonds, chromite, spinel and metal alloys present in acid residue. noble gases are largely lost upon pyridine treatment at room temperature. However, the elemental pattern of the remaining Ar, Kr and Xe is not different from that of the starting acid residue, showing similar losses of all the heavy noble gases during solvation. Xenon stepwise heating data and deconvolution of different components based on isotopic ratios show that Xe-P1 is mainly affected by loss (≈ 60%), following by Xe-P3 (≈ 25%) and Xe-HL (≈ 12%). The xenon release is maximum at temperatures ≤ 1300 °C (≈ 70–80%) whereas only 23% is lost at 1600 °C and no further loss occurs at 2100 °C. These results suggest the existence of at least two substructures in phase Q, with Xe-P1 being trapped preferentially in the less retentive phase. As macromolecular organic matter is the only phase of acid residue sensitive to solvation, this study demonstrates the organic nature of phase Q. The behavior of heavy noble gases upon pyridine solvation supports interlayer trapping of these elements, probably within organic layers of aromatic moieties linked by short aliphatic chains.