Response of Precambrian zircon to the chemical abrasion (CA-TIMS) method and implications for improvement of age determinations

Abstract

Electron backscatter imaging, Raman spectroscopy and U–Pb geochronology have been applied to Precambrian zircon grains that were annealed at 1000 and 1450 °C for various times, then leached with HF to constrain the conditions for healing radiation damage and attaining primary U–Pb zircon ages using the chemical abrasion (CA-TIMS) method. SEM images reveal a variety of textures for ZrO 2 overgrowths on 1450 °C annealed and leached zircon surfaces that depend on the degree of radiation damage and annealing history. Highly damaged zircon produces finer textures than zircon with less damage. Raman spectroscopy indicates that crystals with different levels of radiation damage are only partially restored by annealing at 1000 °C for 2–3 days. Longer annealing periods of 20 days are not noticeably more effective. Annealing at 1450 °C for 1 h results in partial breakdown of zircon but restores Raman peak widths and wave numbers to values characteristic of undamaged zircon after ZrO 2 overgrowths are removed by HF. Raman spectra are much less sensitive to polarization angle for annealed highly damaged grains than for weakly damaged zircon. U–Pb isotopic analyses of low to moderately damaged zircon (alpha fluence ranging up to 10 19/g corresponding to an amorphization volume fraction of 80% or more) yield almost concordant data (0.3–0.5% discordance) after high-temperature annealing at 1450 °C followed by HF leaching at 195 °C. Analyses of cracked zircon annealed at 1450 °C and leached may remain discordant but those of uncracked grains are concordant. Most analyses show primary 207Pb/ 206Pb ages although cracked grains annealed at 1450 °C may produce discordant data with 207Pb/ 206Pb ages that are too young after leaching. The solubility of highly damaged, very disordered zircon (amorphization level of 99%) is only slightly reduced by annealing, and analyses of leach residues are strongly discordant although primary 207Pb/ 206Pb ages are obtained. Annealing of highly damaged zircon under any conditions apparently results in a mass of randomly oriented micro-crystals that pseudomorph the original grain. This could explain the fine-scale pattern observed on etched crystal surfaces, reduced anisotropy at the 5 μm scale of the Raman laser beam and high solubility in HF. It may be impossible to restore primary U–Pb isotopic ages in such cases but precise ages can still potentially be determined from 207Pb/ 206Pb ratios or by application of the air abrasion method.