Zircon U-Pb dating of Pleistocene tephra beds in northwestern North America

Abstract:

Tephrochronology has emerged as an important tool for correlating Quaternary deposits across large distances, but it can be challenging to obtain direct dates on Quaternary tephra beds, particularly beyond the limit of radiocarbon dating. Zircon U-Pb dating by LA-ICP-MS is emerging as an important technique for dating increasingly younger tephra. We demonstrate that LA-ICP-MS zircon dating can produce accurate and precise ages for key tephra beds found in Yukon and Alaska. Zircon U-Pb ages for Gold Run, Old Crow, and HP tephras—three key tephra beds in the region—are consistent with previously established chronology based mainly on glass fission track dating techniques. Our results show that data processing and associated corrections must be treated with caution in order to find the youngest zircon crystals within a population to estimate the timing of crystallization. For Pleistocene tephra, corrections for ²³⁰Th and, in some cases, ²³¹Pa can have a substantial influence on the Pleistocene zircon age calculation, as do corrections for the common-Pb. We compare several practical approaches to the corrections for ²³⁰Th disequilibrium and the common-Pb. We also discuss several strategies for optimizing the likelihood of calculating tephra maximum deposition ages (MDA) from heterotemporal populations of U-Pb ages that yield results closest to the true deposition age, including weighted mean of analyses with overlapping uncertainties, the single youngest grain, and the youngest “peak” of the probability density plot.