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

Abstract

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 Eg band (357 cm−1) are in some cases more useful – and in the case of the Eg band more responsive – proxies. We present curves that permit α dose estimates to be made from Eg 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.