Mapping radiation damage zoning in zircon using Raman spectroscopy: Implications for zircon chronology

Recent research has demonstrated the importance of understanding alpha radiation damage in zircon in order to effectively interpret the thermochronologic significance of (U-Th)/He dates and to appropriately select laser ablation U/Pb reference standards. Direct measurements of alpha radiation damage in zircon are most easily done by using Raman spectral response as a proxy. Current Raman microscopes provide the opportunity to quantify not just a crystal's bulk radiation damage, but to map intracrystalline variations in radiation damage related to radionuclide zoning. Here we illustrate the procedure through a detailed study of zoned Proterozoic zircon crystals from the Adirondack Mountains of New York state. Although previous Raman studies have focused on the response of the v₃(SiO₄) stretching band (1008 cm⁻¹) to radiation damage, we demonstrate that the internal v₂(SiO₄) bending band (439 cm⁻¹) and the external E_g band (357 cm⁻¹) are in some cases more useful – and in the case of the E_g band more responsive – proxies. We present curves that permit α dose estimates to be made from E_g and v₂ band widths over a range of ~2 × 10¹⁷ to 1.8 × 10¹⁸ α/g. We then illustrate how these radiation damage maps can be used to visualize and interrogate intracrystalline variations in any damage-dependent material property in zircon, using helium diffusivity as an example, and discuss the implications for (U-Th)/He and U/Pb chronology of ancient, zoned zircon.