François Robert, Marc Chaussidon, Adriana Gonzalez-Cano, and Smail Mostefaoui

PNAS December 28, 2021 118 (52) e2114221118

Significance

Both the physical effect and the chemical conditions at the origin of the oxygen isotope variations in the solar system have been puzzling questions for 50 y. The data reported here bring the MIF effect (the mass-independent fractionation originally identified on ozone) back to the center of the debate. Similar to Ti isotopes, we observe that the MIF effect for O and Mg is triggered by redox reactions in plasmas. These observations reinforce the idea of a universal mechanism observable in photochemical reactions when molecular collisions involving indistinguishable isotopes yield a symmetrical complex stabilized as a chemical product.

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.

See: https://www.pnas.org/content/118/52/e2114221118

Fig. 1.

(A) Magnesium isotopic compositions of Mg-bearing carbonaceous grains reported as δ²⁵Mg versus δ²⁶Mg variations relative to the PDCM (0:0 ± ≃20‰). Data were collected in four grains either as surface or volume variations (cf SI Appendix, Table S3.2). Error bars include statistical errors (ion counting statistic) and the reproducibility on the standard (±2σ). The 1:1 dashed line is drawn for reference but does not stand for the best fit line to the data. (B) δ²⁶Mg and δ²⁵Mg variations of the rim of the 5-μm size grain shown in Fig.1C are reported as a function of the analytical sputtering time expressed in seconds (data also reported in Fig. 1A). The total sputtering duration (6,600 s) indicates that the rim of the grain is ∼200 nm thick. The PDCM (δ²⁶Mg≃ δ²⁵Mg≃0‰) embedding the grain reappears after complete sputtering. Note the two-outlier data (which are not analytical errors) observed during the flipping from negative to positive values (also obvious in Fig. 1A). Cf. sample Robert-Juillet-2019_23.im in SI Appendix, Table S3.2. (C) Ionic image of the distribution of δ²⁶Mg shown in Fig. 1B. This image is made by the summation of the three scans collected between 3,600 and 4,800 s (cf Fig. 1B). The core of the grain (in blue) has a homogeneous isotopic composition (δ²⁶Mg≃ δ²⁵Mg≃0‰), while the rim exhibits marked enrichments in ²⁶Mg (²⁵Mg not reported here) localized in spots that do not exceed 200 nm in size. The PDCM where the δ²⁵Mg and δ²⁶Mg cannot be defined because of the too-low counting rates on ²⁵Mg and ²⁶Mg appears in black. (D) Secondary Electron Microscopy image of another Mg-bearing carbonaceous grains (not analyzed for isotopic analyses) deposited on the silicon wafers. The structure suggests that the extreme isotopic variations observed in the grain shown in (C) are concentrated in the fine-grained rim.