Rapid (1,200 analyses/hour) U-Pb geochronology and applications to quantitative analysis of high-n detrital data sets

Presenter: Kurt Sundell (Assistant Professor, Idaho State University)

Topic: Rapid (1,200 analyses/hour) U-Pb geochronology and applications to quantitative analysis of high-n detrital data sets

Time/Date: 14 September 2022, noon-1 pm.

Location: ESB 5106

Abbreviated abstract:

We tested a recently developed rapid (3s/analysis) data acquisition method by laser ablation multicollector inductively coupled plasma mass spectrometry (LA-ICP-MS). The method incorporates an automated sample selection algorithm that calculates two-dimensional (2D) grain geometry from polished sample surfaces. The method was applied to eleven samples from below and above the Blackhawk-Castlegate unconformity of the Book Cliffs, Utah, in a down-depositional-dip transect including Price, Horse, Tusher, and Thompson canyons. 12,448 new concordant dates (n > 950 per sample) were generated over two approximately eight-hour measurement sessions. Results show that samples with larger grains and a larger range in grain size are more easily biased based on grain size. However, biased age distributions can be mitigated by gathering large-n data sets (n ≈ 300); however, extremely large sample sizes (n >> 300) are not necessary for most complex samples, such as those found at the Book Cliffs, Utah. These “large-n” data sets confirm previous conclusions that samples of n ≤ 100 are inadequate to characterize age distributions as complex as those commonly found in North America, n = 300 results in more consistent age distributions unless only the largest grains are analyzed, and n ≈ 1000 is not necessary unless seeking extremely rare ages.