COLLECTING LARGE-N U-PB DETRITAL GEOCHRONOLOGY DATA VIA RAPID ACQUISITION (300–1,200 ANALYSES/H) LASER ABLATION MULTICOLLECTOR ICP-MS

Detrital zircon (DZ) U-Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has revolutionized the way geologists approach questions of source-to-sink sediment provenance, paleogeography, magmatic arc dynamics, to name only a few. LA-ICP-MS provides excellent spatial resolution, low-cost, high-throughput acquisition of zircon U-Pb age dates. One of the most important aspects of DZ geochronology data is sample size (i.e., the number of ages in a single sample, n). To improve the efficiency of increasing n, recent research has focused on increasing sample acquisition rate through a variety of means including automation, rapid ablation rate, and foregoing steps in traditional sample preparation. However, acquisition rates are still limited to 100–300 analyses/h. We present a new method to routinely acquire zircon U-Pb age dates at rates of 120, 300, 600 and 1,200 analyses/h (30, 12, 6, and 3 s/analysis). We demonstrate the efficacy of our method by analyzing twelve zircon standard materials ranging in age from ~28 Ma to ~3465 Ma in round-robin fashion for comparison to high precision age dates. Mean offset from accepted ages is between 0.9% and 1.1% for individual analyses. Mean internal uncertainties for individual analyses increase from 0.6% to 1.3% for 120 and 1,200 analyses/h. We test the new rapid acquisition method by analyzing DZs from the Upper Cretaceous Wahweap Formation (near Henrieville, UT) previously characterized by large-n DZ geochronology. We tested the four rates of acquisition by analyzing the same n as the hourly rate itself (e.g., 1,200 unknowns at 1,200 analyses/h). Results show stabilization in the number of age modes when age distributions comprise > 300 ages. Quantitative comparison using Cross-correlation, Likeness, and Similarity yields a consistent increase from 0.30, 0.59, and 0.79 to 0.88, 0.85, and 0.97, respectively. This new method holds significant promise for DZ geochronology because (1) there is only a minor sacrifice of accuracy and precision; (2) it requires no major changes to hardware, but rather to software and data reduction methods; (3) it yields robust age distributions well-suited for quantitative analysis and ascribing maximum depositional ages; and (4) there is no additional burden to the user in terms of time investment or analytical cost.