TANDEM LA-ICP-MS AND CA-ID-TIMS DETRITAL ZIRCON U-PB GEOCHRONOLOGY: ANALYSIS OF A GLOBAL DATASET

Zircon U-Pb geochronology is a commonly used dating technique with widespread application to many geologic systems. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) are among the most used methods for acquiring U-Pb dates from zircon. Although CA-ID-TIMS provides higher analytical precision, dates acquired via LA-ICP-MS are less expensive and may be acquired in sufficient quantity (i.e., >300 analyses per sample) for provenance characterization and identification of rare age modes (e.g., young zircon of near depositional age). However, recent case studies have demonstrated a tendency for the youngest LA-ICP-MS U-Pb dates from zircon to skew younger than the corresponding CA-ID-TIMS date, possibly a consequence of undetected Pb-loss that is mitigated in CA-ID-TIMS via chemical abrasion.

We conducted a systematic characterization of the degree of agreement between dates acquired via LA-ICP-MS and CA-ID-TIMS from the same zircon crystals. Our dataset consists of 1528 date pairs analyzed from 16 laboratories and 48 published studies. The lithologic makeup is 49% igneous, 40% sedimentary, and 11% metamorphic zircon. Excluding outliers and considering only < 1 Ga grains, ~69% of LA-ICP-MS ²⁰⁶Pb/²³⁸U dates are younger than their CA-ID-TIMS dates, with a typical shift of ~2.0-2.5%. Conversely, the ²⁰⁷Pb/²⁰⁶Pb dates from grains >1 Ga show no significant shift. These results highlight the potential effects of variable degrees of cryptic Pb loss on non-chemically abraded LA-ICP-MS measurements, although other complexities (e.g., inheritance, matrix mismatch effects) may also play a role. Studies using grains that were chemically abraded before LA-ICP-MS should be able to discern whether Pb-loss is a major effect. Ultimately, this research highlights a need for further research into potential effects of Pb loss on non-chemically abraded LA-ICP-MS datasets.