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

Abstract

Enrichment or depletion ranging from −40 to +100% in the major isotopes 16O and 24Mg were observed experimentally in solids condensed from carbonaceous plasma composed of CO2/MgCl2/Pentanol or N2O/Pentanol for O and MgCl2/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 δ25Mg versus δ26Mg and between δ17O versus δ18O 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 H2O gas.

Mass Independent Isotopic Fractionation: A Key to Plasma Chemistry

“…Appendix Appendix F) whose abundances are not negligible relative to their isotopes 24 Mg and 48 Ti used to normalize their ratios ( 25,26 Mg/ 24 Mg and 46,47,49,50 Ti/ 48 Ti). We have reported Mg and Ti MIF variations in grains condensed from plasma [37,38] for two different chemical settings: MgCl 2 /Pentanol and TiCl 4 /Pentane for Mg and Ti isotopes, respectively. Data are reported in Figures 2 and 3.…”