Needles in a haystack: Detrital zircon U[sbnd]Pb ages and the maximum depositional age of modern global sediment

Abstract

The practice of using the youngest detrital grains from a sedimentary deposit to constrain its depositional age has grown rapidly over the past two decades. Researchers that use the maximum depositional age (MDA) as a proxy for a deposit's true depositional age (TDA) assume that processes of mineral crystallization, exhumation, and transport are rapid, such that minimal time elapses between the time recorded by a mineral's age and time of deposition. However, this assumption is in many cases untestable, as it requires independent age constraints that would preclude the need to interpret MDAs as TDAs. This study uses a global compilation of >70,000 detrital zircon UPb ages from 792 modern and Holocene sediment samples to evaluate the frequency with which various MDA methods approximate the TDA (~0 Ma) and to determine the geologic factors that control the global distribution of Earth's youngest detrital zircon. We show that young (< 2 Ma) zircon are rare (~0.4% of the total dataset) and are largely restricted to areas associated with active volcanism. Dilution of the youngest, volcanically sourced grains may preferentially occur in large sediment routing systems (i.e., river catchments >106 km2), causing the youngest grains to be missed during routine provenance analysis. Methods of calculating the MDA that rely on just one or two grains yield results that are closest to the TDA in the modern-Holocene dataset. However, use of more conservative MDA methods that rely on multiple, overlapping age measurements are likely necessary for avoiding calculation of an MDA that is younger than the TDA in ancient samples. A number of strategies can be used to increase the likelihood of finding young grains, if present, including conservative mineral separation, ‘high-n’ sampling strategies, and depth-profiling of whole zircon grains. Future efforts to maximize the benefit of MDA analysis could include increased sampling of modern sedimentary systems in underrepresented tectonic settings and depositional environments, development of improved methods for recognizing Pb loss in detrital zircon, and improvements to the precision and accuracy of high-throughput detrital geochronology.