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

Enrichment or depletion ranging from 240 to +100% in the major isotopes O-16 and Mg-24 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 delta Mg-25 versus delta Mg-26 and between delta O-17 versus delta O-18 and the slopes calculated using the empirical MIF factor eta 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.

Automated summary

Experimental observations show enrichment or depletion of major isotopes O-16 and Mg-24 in solids condensed from carbonaceous plasma, with MIF effects potentially induced by chemical reactions in the plasma. Micrometer-sized hotspots of isotopic effects in NanoSims imaging are found embedded in a carbonaceous matrix, with Mg hotspots localized in carbonaceous grains exhibiting both positive and negative isotopic effects, while no specific structure is observed at O hotspots. The similarity between linear correlations of delta Mg-25 versus delta Mg-26 and delta O-17 versus delta O-18 observed and those calculated using the empirical MIF factor eta from ozone suggests a ubiquitous process.