Silicon Isotope Effects of He Ion Irradiation on Olivine

During the early history of the Solar System or in molecular clouds, dust particles were submitted to intense irradiation by protons, and helium nuclei accelerated to MeV energies or higher. The consequence of this irradiation on the isotope composition of solids at such energies is unknown, but major element modifications of chemical compositions suggest that there could be isotope fractionation associated with irradiation. In this study, we have analyzed the Si isotope composition of olivine layers produced by sputter deposition starting from San Carlos olivine. The deposits were irradiated by He2+ with a total fluence of 1.5 × 1017. The olivine deposits were analyzed prior to and after irradiation for Si isotopes by multicollector ICPMS. The Si isotope composition after sputter deposition (prior to He irradiation) was enriched in heavy Si isotopes with a mean δ30Si value of 12.6‰ relative to the NBS-28 standard, whereas the irradiated olivine had lower δ30Si values of 10.8‰ and 8.2‰ for the 200 keV and 6 MeV He irradiation, respectively. In both cases, the Si isotope fractionation are strictly mass-dependent. First, these results show that the process of sputter deposition induces a large enrichment in heavy Si isotopes that we attribute to a difference in the sticking coefficient of SiO isotopologues. Second, the effect of He irradiation is the reverse of what is expected from theoretical studies and numerical simulations of isotope fractionation under these irradiation conditions at lower energies (keV range). The enrichment in light Si isotopes could be due to redistribution processes and segregation that take place during the irradiation. This study suggests that Si isotopes could be powerful tracers of irradiation processes in astrophysical settings.