Quantification of radiation damage in natural and synthetic zircon by Raman spectroscopy: application to low-temperature thermochronology

Due to its ubiquitous occurrence in igneous, metamorphic, and sedimentary rocks and its wide application in geochronology and geochemistry, zircon has become the most widely used accessory mineral in the geological community. Nevertheless, the decay of U and Th causes radiation damage to the zircon structure, resulting in various degrees of metamictization, which can affect the accuracy of U-Pb dates and Hf and O isotope results. If the degree of zircon radiation damage can be quantified, the influence on geochemical analyses can be evaluated, and the results can be corrected more precisely. In this paper, synthetic and natural zircon crystals with different crystallization ages were selected for Raman spectroscopy analysis, cathodoluminescence imaging, and determination of the U and Th concentrations. The results show that Raman FWHM (full width at half band-maximum) and Raman shift correlate with alpha dose (D-alpha) of zircons following these equations, FWHM = 44.36 (+/- 2.32) x [1 - exp(- 2.74 x D-alpha)] + 1.7 (+/- 0.19), Raman Shift = - 6.53 x D-alpha + 1007.69. Analysis of synthetic zircon crystals shows that doped REEs (rare earth elements and P) can also lead to an increase in the FWHM. However, this effect can be ignored for natural zircon samples with REE contents at a normal level of hundreds to a few thousand ppm. The FWHM and Raman shift can be used as proxies to measure the degree of zircon radiation damage. Using the updated equations to calculate the latest age when zircon began to accumulate radiation damage, a more accurate and more meaningful "radiation damage age" can be obtained.