LIKENESS AMONG DETRITAL ZIRCON POPULATIONS- AN APPROACH TO THE COMPARISON OF AGE FREQUENCY DATA IN TIME AND SPACE

The advent of new procedures for the rapid collection of large amounts of detrital zircon age data has created significant challenges to the representation, quantitative interpretation, and analysis of these results. Many studies adopt a subjective-graphic approach to these tasks, primarily through the examination of vertically stacked probability density plots. Although this methodology allows for quick visual assessment of changes in zircon age frequencies in space and/or time, it is fundamentally qualitative in aspect, and becomes rather impractical as numbers of analyses become large. Here, we develop a ‘likeness’ metric that quantifies the degree of overlap between PDP pairs. We compare this metric to several others that have been proposed previously, and we evaluate its usefulness in describing source-to-sink changes in sample age populations by analyzing published data on Paleozoic samples from the Grand Canyon and on modern fluvial sands from along the Amazon River. Between-sample likeness decreases with increasing stratigraphic and geographic separation; as might be expected, greater temporal and/or spatial disjunction results in greater dissimilarity. In contrast, spatially-contiguous fluvial samples exhibit down-system increases in likeness, suggesting that sediment grain ages may undergo increasing homogenization with transport. The degree to which these results are representative of other ancient and modern sedimentary systems awaits further evaluation, but do demonstrate that the determination of sample-pair likenesses, and evaluation of their geographic and/or stratigraphic variation, serve to effectively highlight those aspects of differences in detrital zircon age frequencies that ultimately record to the histories of geologic and geomorphic processes in both space and in time.