Gaining from loss: Detrital zircon source-normalized α-dose discriminates first-versus multi-cycle grain histories

Abstract

Detrital zircon U–Pb ages are widely employed as an archive of geological processes through time. Changes in detrital zircon age patterns within sediments reflect changes in source areas that are often related to tectonic and/or climatic processes. However, discrimination of first-cycle and multi-cycle detrital zircon with primary crystalline and secondary sedimentary sources, respectively, can be challenging using only crystallisation age constraints. Here, we present U–Pb geochronology of detrital zircon from modern fluvial and littoral environments on the Scott Coastal Plain in Western Australia to investigate the use of α-dose to identify sedimentary recycling. The majority of 1032 concordant U–Pb ages are interpreted to be ultimately sourced from the local basement. However, U–Pb ages do not reflect the areal extent of source rocks and indicate significant reworking of coastal plain sediments. A novel metric – source-normalized α-dose – demonstrates predominant detrital zircon routing via recycling through intermediate storage. This metric is defined as the ratio of the average α-dose (a measure of metamictization) of detrital zircon belonging to a characteristic age group and the average α-dose of zircon grains within the corresponding source crystalline basement. Average values of source-normalized α-dose of detrital zircon populations <1 are interpreted to reflect selective removal of more labile (metamict) grains via attrition and diagenesis, indicating greater grain transport and recycling, whereas values of c. 1 signify shorter transport and a first-cycle origin. Application of this approach to ancient clastic systems is supported by consistency of results with independent indicators of progressive sedimentary recycling and/or transport. Source-normalized α-dose is an internal measure using zircon grain chemistry (U and Th), and avoids bias associated with multi-mineral measures of sediment recycling that may be related to source fertility. Additionally, source-normalized α-dose uses measures typically captured during routine U–Pb geochronology. Source-normalized α-dose of detrital zircon provides an additional method to address sedimentary source-to-sink transport and recycling, and ultimately allows more robust interpretation of U–Pb zircon data.