The LA‐ICP‐MS U‐(Th‐)Pb geochronology international community has defined new standards for the determination of U‐(Th‐)Pb ages. A new workflow defines the appropriate propagation of uncertainties for these data, identifying random and systematic components. Only data with uncertainties relating to random error should be used in weighted mean calculations of population ages; uncertainty components for systematic errors are propagated after this stage, preventing their erroneous reduction. Following this improved uncertainty propagation protocol, data can be compared at different uncertainty levels to better resolve age differences. New reference values for commonly used zircon, monazite and titanite reference materials are defined (based on ID‐TIMS) after removing corrections for common lead and the effects of excess ²³⁰Th. These values more accurately reflect the material sampled during the determination of calibration factors by LA‐ICP‐MS analysis. Recommendations are made to graphically represent data only with uncertainty ellipses at 2s and to submit or cite validation data with sample data when submitting data for publication. New data‐reporting standards are defined to help improve the peer‐review process. With these improvements, LA‐ICP‐MS U‐(Th‐)Pb data can be considered more robust, accurate, better documented and quantified, directly contributing to their improved scientific interpretation.

A Better Date: Uranium-lead (U-Pb) dating, which is one of the most commonly used methods of radiometric dating for old terrestrial materials, operates by comparing the ratio of trace levels of U with its nuclear decay product Pb. This dating method, and the similar Pb-Pb method, assumes that the ratio between the two most common U isotopes ( ²³⁸ U and ²³⁵ U) is constant. By accurately measuring the ²³⁸ U/ ²³⁵ U ratio in a suite of minerals representing a range of tectonic environments, Hiess et al. (p. 1610 ; see the Perspective by Stirling ) demonstrate that this ratio is more variable than was previously thought. The variability does not reflect any systematic bias with time, location, or temperature—suggesting that ideally ²³⁸ U/ ²³⁵ U should be determined for every sample to calculate ages. In the absence of such data, a revised ²³⁸ U/ ²³⁵ U ratio for zircon minerals could significantly modify previous age estimates using U-Pb and Pb-Pb dating techniques.

Mixed 235U-233U-205Pb(-202Pb) tracers for 12 U-Pb isotope-dilution isotope ratio mass spectrometry have been prepared under the auspices of the EARTHTIME Initiative. The methods and results for the preparation and calibration of the U/Pb ratio and isotopic abundances are given, and the various sources of uncertainty are discussed and quantified. The accuracy of the EARTHTIME U-Pb tracer isotopic composition can be traced back to SI units via a series of assay and isotopic composition reference materials combined with the experiments described herein. The parameters used in calculating U/Pb ratios (and inferentially U-Pb dates) have correlated uncertainties that result in a total uncertainty contribution to 206Pb/23820 U dates of ±<0.03% (95% confidence). For suitable terrestrial materials such as zircon, when other sources of uncertainty have been minimised (e.g., opensystem\ud behaviour, 238U/23522 U variation, intermediate daughter product disequilibrium, common Pb, etc.) the U-Pb tracer calibration uncertainty is a limiting factor in the accuracy of U-Pb geochronology – but less so than the uncertainty in the 238U and 235U decay constants (±0.11 and 0.14% 2σ). The calibration approach of the mixed EARTHTIME 235U-23325 U-205Pb(-202Pb) tracers, in addition to updated values for reference materials (e.g., mixed gravimetric reference solutions), and parameters (e.g., Pb reference material assay), can be applied to other laboratory-specific U-Pb tracers and will facilitate the generation of accurate and directly inter-comparable U-Pb data

Life Versus the Volcanoes: Correlating a specific triggering event, such as an asteroid impact or massive volcanism, to mass extinction events is clouded by the difficulty in precisely timing their occurrence in the geologic record. Based on rock samples collected in North America and Morocco, Blackburn et al. (p. 941 , published online 21 March) acquired accurate ages for events surrounding the mass extinction that occurred ∼201 million years ago, between the Triassic and Jurassic Periods. The timing of the disappearance of marine and land fossils and geochemical evidence of the sequential eruption of the Central Atlantic Magmatic Province imply a strong causal relationship.