The leaky chronometer: Evidence for systematic cryptic Pb loss in laser ablation U-Pb dating of zircon relative to CA-TIMS

U–Pb dating of zircon via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has become a primary method for determining the age of crystalline rocks and the provenance and maximum depositional age of (meta)sedimentary rocks. Although chemical abrasion has become a standard approach for mitigating Pb loss in zircon dated via high-resolution thermal ionization mass spectrometry (CA-TIMS), laser ablation dating is rarely accompanied by chemical abrasion. We evaluate the accuracy of U–Pb dates acquired via LA-ICP-MS relative to CA-TIMS through analysis of a large database of paired analyses from the same zircon crystals. We show that laser ablation ²⁰⁶Pb/²³⁸U dates are systematically younger than their CA-TIMS counterparts, with a greater shift in detrital zircon (−2.0%) versus igneous or metamorphic zircon (−0.9%). Although systematic biases related to “matrix effects” may be a contributing factor, our analysis suggests that unmitigated cryptic Pb loss is likely widespread in laser ablation U–Pb datasets.