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 O and Mg were observed experimentally in solids condensed from carbonaceous plasma composed of CO/MgCl/Pentanol or NO/Pentanol for O and MgCl/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 δMg versus δMg and between δO versus δO and the slopes calculated using the empirical MIF factor discovered in ozone [M. H. Thiemens, J. E. Heidenreich, III. 219, 1073-1075; C. Janssen, J. Guenther, K. Mauersberger, D. Krankowsky. 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 HO gas.