What happens when n= 1000? Creating large-n geochronological datasets with LA-ICP-MS for geologic investigations

Abstract

The direct age dating of individual mineral components in sedimentary rocks through the analysis of radiogenic parent and daughter isotopes has been routinely applied to better understand sediment provenance and dispersal patterns for several decades. Time, labor, and financial cost—sadly, not scientific inquiry—are typically the determining factors in the number of analyses run for a sedimentary rock sample during provenance investigations. The number of observations reported for detrital zircon provenance investigations using secondary ion mass spectrometers SIMS and laser-ablation inductively-coupled-plasma mass-spectrometers LA-ICP-MS typically range from n = 60–120. In this range, minor, but commonly geological relevant, age components are commonly not identified from the sample aliquot. In addition, the relative proportions of zircon ages from within an age component are typically unreliable for intersample comparisons because the relative proportions of ages from aliquots of n = 60–120 may poorly reflect the ‘true’ proportions of ages from a sample. This study investigates the practicality and usefulness of generating large-n (n = 300–1000) datasets. A LA-MC-ICP-MS and LA-SC-ICP-MS were used to generate four n ≈ 1000 datasets. We show that precision large-n U–Pb detrital zircon datasets can be created using LA-ICP-MS with total sample-run analysis times that are on par with more traditional studies. At best, most provenance investigations based on n = 60–100 have been statistically limited to identifying principle age components. The statistical robustness on n = 1000 datasets not only significantly increase the probability that exotic or low abundance age components (i.e., f < 0.05) are identified in detrital samples, but it allows for the quantitative comparisons between relatively high abundance age components in samples. This potentially transformative outcome of large-n has the potential to stimulate new avenues of research in sedimentology and tectonics.