Oxygen and magnesium mass-independent isotopic fractionation induced by chemical reactions in plasma

François Robert (1) , Marc Chaussidon (2) , Adriana Gonzalez-Cano (2) , Smail Mostefaoui (1)

1 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
2 IPGP (UMR_7154) - Institut de Physique du Globe de Paris

Enrichment or depletion ranging from −40 to +100% in the major isotopes 16 O and 24 Mg were observed experimentally in solids condensed from carbonaceous plasma composed of CO 2 /MgCl 2 /Pentanol or N 2 O/Pentanol for O and MgCl 2 /Pentanol for Mg. In NanoSims imaging, isotope effects appear as micrometer-size hotspots embedded in a carbonaceous matrix showing no isotope fractionation. For Mg, these hotspots are localized in carbonaceous grains, which show positive and negative isotopic effects so that the whole grain has a standard isotope composition. For O, no specific structure was observed at hotspot locations. These results suggest that MIF (mass-independent fractionation) effects can be induced by chemical reactions taking place in plasma. The close agreement between the slopes of the linear correlations observed between δ 25 Mg versus δ 26 Mg and between δ 17 O versus δ 18 O and the slopes calculated using the empirical MIF factor η discovered in ozone [M. H. Thiemens, J. E. Heidenreich, III. Science 219, 1073–1075; C. Janssen, J. Guenther, K. Mauersberger, D. Krankowsky. Phys. Chem. Chem. Phys . 3, 4718–4721] attests to the ubiquity of this process. Although the chemical reactants used in the present experiments cannot be directly transposed to the protosolar nebula, a similar MIF mechanism is proposed for oxygen isotopes: at high temperature, at the surface of grains, a mass-independent isotope exchange could have taken place between condensing oxides and oxygen atoms originated form the dissociation of CO or H 2 O gas.

Mots clés: MIF; cosmochemistry; isotopes; plasma

Domaines: Planète et Univers [physics]

François Robert, Marc Chaussidon, Adriana Gonzalez-Cano, Smail Mostefaoui. Oxygen and magnesium mass-independent isotopic fractionation induced by chemical reactions in plasma. Proceedings of the National Academy of Sciences of the United States of America, 2021, 118 (52), pp.e2114221118. ⟨10.1073/pnas.2114221118⟩. ⟨hal-03509544⟩