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

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.