PRECISION AND ACCURACY OF ID-TIMS U-PB GEOCHRONOLOGY APPLIED TO THE STRATIGRAPHIC RECORD (Invited Presentation)

U-Pb zircon ID-TIMS geochronology of volcanic ashbeds is a powerful and widespread method of calibrating processes recorded in the stratigraphic record, from biotic evolution to environmental change. There are more U-Pb dates pinning the geologic timescale than any other geochronologic method, and so assessing the accuracy of these dates while pushing for increased precision is an ongoing task. This contribution summarizes recent improvements in the precision and accuracy of ID-TIMS U-Pb geochronology, highlights current limitations, and offers some ways forward.

Driven by the EARTHTIME initiative, U-Pb ID-TIMS labs around the world have increased collaboration and communication to assess laboratory protocols, sample preparation techniques, and standard reproducibility. Mixing and distribution of community tracers for isotope dilution have helped eliminate sources of interlab biases and enable the traceability of U-Pb dates to first principle experiments, permitting more accurate comparison to other dating techniques and cyclostratigraphy. Synchronous increases in analytical precision have been facilitated by improved lab techniques and better mass spectrometry. Dates from single ashbed zircons and zircon fragments are commonly far more precise than the timescales of zircon growth within upper crustal magmatic systems that source eruptions. Thus, uncertainty in relating such data to the process we actually want to date – ashbed eruption – limits the accuracy of timescale calibration and global correlation of events.

There is currently no consensus on how to interpret a dataset where zircon dates span over 10,000 to 1,000,000 years, given recognition of both prolonged zircon growth and anticipated analytical scatter, but new tools are emerging that continue to increase the precision and accuracy of dates and age models. These include imaging and microsampling of zircon, integration of geochemical information from zircon into age interpretations, characterization of plutonic and volcanic zircon systematics to build a database of common age distributions from each, and application of Bayesian methods to assess zircon growth records and develop more robust age models for stratigraphic successions.