Improving the utility of detrital zircon studies through chemical abrasion [abstract only]

U-Pb isotopic dating of detrital zircon has the potential to yield important insight into a variety of geologic processes, including, but not limited to: understanding sedimentary provenance and deciphering the exhumation history of mountain belts. Most studies of detrital zircon use either SIMS or ICP-MS methods to obtain U-Pb isotopic ages on large numbers of single crystals. However, an almost ubiqutious problem with this approach is the presence of large numbers (up to 50% for random detrital zircons) of crystals that yield discordant ages. Discordance generally results from radiogenic lead loss, either through radation-damage and/or diffusion along imperfections within the crystal lattice. The presence of a significant number of discordant analyses from an individual sample complicates data interpretation and severely limits the geologic utility of this approach. Following the ‘chemical abrasion’ method developed by Mattinson [1], we investigate the potential for this preparation technique to significantly improve the overall concordance of a detrital zircon data set where Pb-loss is the cause of discordance. Previous applications of this approach to magmatic samples suggest >90% of analyses from any given sample could be moved to within 5% of concordance. Initial data indicate that annealed and non-annealed aliquots of the same sample have comparable age distributions, suggesting that this method does not introduce any additional bias into the age-population spectra. As a consqeuence of this approach, high-U and/or radiation-damaged zircon are often reduced to small irregular fragments or skeletal morphologies. In order to compensate for this, such that these types of grains are not underrepresented in any analysis, we have developed novel sampling strategies in order to obtain isotopic information without degradation of data quality. Through chemical abrasion, the proportion of detrital zircon analyses that can be included in an age spectrum could be increased dramatically without significant sampling bias. Although further work is required, the potential to improve the number of useful data points and confidence in interpreting difficult detrital zircon spectra, is significant.