PRECISION ISN’T EVERYTHING: A CONSIDERATION OF THE DETERMINATION OF MAXIMUM DEPOSITIONAL AGES USING DETRITAL GEOCHRONOLOGY

An essential piece of information for any detrital sedimentary deposit is the age of deposition. This is most often obtained from fossils or the isotopic dating of interbedded volcanic rocks. When these techniques fail or are unavailable, a third method can be attempted: dating detrital minerals within the rock (the most-popular version of this is U-Pb dating of zircon but the concepts discussed here are applicable to any geochronologic system). Assuming no post-depositional modification of the isotopic system in question, the age of such deposits must be younger than the youngest mineral date obtained and this date is referred to as the Maximum Depositional Age (MDA). The simplicity of this approach is burdened by the fact that the analytical uncertainty of an age obtained from a single grain is greater than the uncertainty when multiple grains are considered together. In addition to the straightforward definition of the MDA using the youngest single grain (YSG), a variety of rubrics have been proposed (e.g., YC1σ (2+), YC2σ (3+), τ age, δ age, Σ age) for the inclusion of grains other than the youngest one in determining the MDA, sometimes to the exclusion of the actual youngest grain. These approaches have been characterized as being “more conservative” and “statistically rigorous” but it is not conservative to get the wrong answer and there is no statistical rigor in averaging materials that were not originally part of the same population. Exclusion of grains from consideration in defining the MDA can be appropriate but only based on characteristics of individual grains (e.g., age uncertainty, % discordance) and not by comparison to other grains. When using detrital geochronology to estimate the time of deposition, YSG is the way to go; all other ways will usually produce values with greater precision but choosing to value precision above accuracy is a mistake that will not increase our understanding of Earth history.