Zircon U-Pb analyses by TIMS and LA-ICPMS on the same material

A number of U-Pb geochronological techniques are now available, each of which has pros and cons associated with spatial resolution, accuracy, precision and also operating cost and throughput. Zircon U-Pb age analyses using TIMS (Thermal Ionization Mass Spectrometry) can yield geological dates with uncertainties at or below the 1 permil level (precision and accuracy), but consume relatively large amounts of material (entire crystals of fragments) and are cost and time intensive. Conversely, LA-ICPMS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) geochronology is a relatively inexpensive and fast micro-sampling technique, but individual and combined uncertainties are significantly greater. The choice of a specific approach depends primarily on the nature of the geological problem; a combination of several techniques is often advisable. We present new TIMS and LA-ICPMS results on zircons from an intraformational conglomerate within the Chinle Fm (Late Triassic), and from a volcanic ash within the Late Jurassic Morrison Fm (with additional 40Ar/39Ar analyses on phenocrystic sanidine). Preliminary TIMS analyses on annealed and chemically abraded zircons from the Chinle Fm conglomerate indicate an age of ca 215.3 ± 0.9 Ma (further analyses are expected to reduce the uncertainty significantly). Previous analyses using LA-ICPMS techniques on non-polished zircons yield an age which is significantly younger (212 ± 1.5 Ma). TIMS analyses were done on the same crystals as the LA-ICPMS analyses, only one of four crystals yields an overlapping age with the ICPMS date (most of the TIMS ages are older than the corresponding LA-ICPMS ages). Part of this discrepancy may be due the fact that the LA-ICP ages preferentially suffer from surface correlated Pb loss. This effect may be mitigated by LA-ICPMS analyses on polished crystals; however, it has been shown that Pb loss also occurs within the crystal interior. Zircons from the Morrison show a similar bias in sense and magnitude when TIMS and LA-ICPMS ages are compared, and fail to agree within uncertainty (151.3 ± 0.7 Ma versus 145.6 +2.3/-3.1 Ma, (1)). A 40Ar/39Ar age is in agreement with the TIMS age (after correction for a systematic bias from a miscalibrated decay constant of 40K), again indicating that the LA-ICPMS age may be affected by open system behavior. The origin of these discrepancies remain as yet speculative. We are confident that further comparative studies will help us understand the nature of this discrepancy. We suggest that existing datasets are critically evaluated; in some cases, quoted uncertainties may fail to take complications such as Pb loss into account. Further analytical progress, for example applying the chemical abrasion technique to zircons prior to LA- ICPMS analyses, may yield more accurate ages, although potential complications arising from Pb/U fractionation due to increased surface area have to be considered. The study on the zircons from the Chinle conglomerate, which contains zircon populations from several different sources, demonstrates that a useful approach may be to apply LA-ICPMS to a large number of zircons, followed by TIMS analyses on selected crystals, to attain the highest possible precision on the depositional. 40Ar/39Ar on K-bearing minerals yields additional constraints but systematic biases have to be taken into account. 1. B. J. Kowallis, B. Britt, E. H. Christiansen, A. L. Deino, paper presented at the GSA Joint Meeting, Las Vegas 2008.