¹Gerardo Dominguez, ¹Caroline Caplan, ¹Lauren Tafla, ¹Joshua Lucas, ²Ming-Chang Liu, ²Kevin McKeegan, ³Jeff Gillis
Proceedings of the National Academy of Sciences, 123, e2610806123
Open Access Link to Article [DOI: 10.1073/pnas.2610806123]
¹Department of Physics, California State University, San Marcos, CA 92096-0001
²Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095-1567
³Department of Physics, Washington University of St. Louis, St. Louis, MO 63130

Mass-independent isotope fractionation (MIF) of oxygen provides key insights into atmospheric chemistry and the origin of isotopic anomalies in planetary materials. To date, MIF has been primarily associated with gas-phase processes involving symmetry-dependent reaction pathways, most notably ozone formation. Whether analogous effects can arise in condensed phases or at interfaces remains an open question. Here we report laboratory experiments demonstrating that energetic particle irradiation of icy oxide surfaces produces mass-independent oxygen isotope fractionation at temperatures as low as 10 K. Electron irradiation of SiO₂ and Al₂O₃ substrates in the presence of H₂O ice results in measurable shifts in Δ¹⁷O in the underlying solids. The resulting three-isotope relationships include slope-1-like behavior in SiO₂ and Al₂O₃ and distinct, substrate-dependent trends in Al₂O₃, indicating that irradiation-induced defect formation and interfacial chemistry govern the isotope effects. The magnitude of the observed anomalies demonstrates that irradiation-driven processes can generate substantial oxygen isotope heterogeneity in condensed phases. These results identify a previously unrecognized class of MIF and provide a quantitative framework for interpreting oxygen isotope signatures in radiation-processed environments, including interstellar grains and early solar system materials.