Mass-independent oxygen isotope fractionation driven by energetic-particle irradiation of icy solids.

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.