INCREASED ACCURACY OF MAXIMUM DEPOSITIONAL AGE (MDA) ESTIMATIONS USING CHEMICAL ABRASION-LASER ABLATION-INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY (CA-LA-ICPMS)

U-Pb detrital zircon geochronology is commonly implemented in sediment provenance and basin analysis studies. An important component of these studies is determining the depositional age of sedimentary strata to understand temporal variations in depositional environments and sediment routing systems. Age determinations of sedimentary sequences often rely on fossil assemblages and/or dates from interbedded volcanics. However, in sequences that lack these types of age control, detrital zircon geochronology can play a crucial role in age determinations by constraining the maximum depositional age (MDA).

Recent work by Donaghy et al. (2024) shows that bulk CA of detrital zircon aliquots prior to LA-ICPMS improves the accuracy of detrital zircon spectra by selectively removing zircon domains affected by Pb-loss. We build on this study by analyzing CA-treated (N=2; n=569) and untreated aliquots (N=2; n=594) of two Eocene detrital zircon samples (SK2-535 and MT1-214) to assess the impact CA has on the accuracy of MDA estimations. Treated and untreated aliquots were first analyzed using LA-ICPMS to identify and compare peak age populations, followed by plucking of the youngest zircons in each aliquot to be tandem dated using CA-ID-TIMS. The youngest zircons from an additional CA-treated Eocene detrital zircon sample were also tandem dated.

All zircon tandem dated from CA-treated aliquots (N=3; n=14) yielded CA-LA-ICPMS dates that overlap within 2σ of their respective CA-ID-TIMS dates. We found the CA-LAICPMS dates to be between 0.6-3.6% younger than their respective CA-ID-TIMS dates, approaching the limits of single-spot precision by LA-ICPMS. In contrast, only one zircon from the untreated aliquots (N=2; n=6) overlapped within 2σ uncertainty of its CA-ID-TIMS date. The average negative bias of untreated LA-ICPMS dates was variable by sample, with most analyses between 4.5-5.8% younger than the CA-ID-TIMS dates. One zircon had a LA-ICPMS date 15.9% younger than its CA-ID-TIMS date, equating to an ~10 Myr difference. These results support the notion that Pb-loss can have a substantial effect on the accuracy of estimated MDAs, even for young samples and regardless of the method used for MDA calculation. We suggest CA-LA-ICPMS could be an important technique to mitigate the negative effects of Pb-loss and significantly improve the accuracy of MDA estimations.