BEYOND DETRITAL ZIRCON: COMBINING U-TH-PB DATING WITH ND ISOTOPE COMPOSITION OF DETRITAL MONAZITE TO FURTHER ENHANCE SEDIMENT SOURCE IDENTIFICATION

The use of detrital zircon (DZ) dating in provenance studies has become so common that the current database of tabulated U-Pb dates has surpassed 500,000 (Puetz, 2018). While this is an invaluable data set it emphasizes the recycled nature of DZ and suggests that DZ ages more likely reflect ultimate source age of zircon rather than the immediate source of detritus. This is exemplified by the ubiquity of ~ 1 Ga detrital zircon in N. American sedimentary rocks, even those located > 1,000 kilometers from the Grenville Front. This is demonstrated well in Modern alluvium from the French Broad River, a major river draining part of the SE USA. The majority of the watershed consists of bedrock of Ordovician, Devonian, and Permian ages (corresponding to the classic Appalachian orogenies). Only about 12% of the watershed drains ~ 1 Ga bedrock (i.e. Grenvillian crust) yet the majority of detrital zircon in the alluvium is ~ 1 Ga reflecting a clear bias in the areal extent of exposed rock. In contrast, the age frequency of detrital monazite (DM) more accurately reflects the areal distribution of exposed bedrock (Hietpas et al., 2010) and thus appears as a more robust tool for sedimentary provenance compared to zircon. But even more information can be gleaned from the source of detrital monazite if in addition to its age other characteristics, such as its Nd isotopic composition, are determined. Simultaneously determining the age and Nd isotopic composition of DM is analogous to determining the age of DZ and its Hf isotopic composition. We have demonstrated this enhanced provenance utility by reanalyzing DM grains from French Broad (FB) alluvium for Nd isotopic composition and comparing those results to ages and Nd composition of bedrock samples within the FB watershed via laser ablation split stream ICP-mass spectrometric analysis (LASS-ICPMS). DM grains of similar age would normally be grouped together if no other information were available, but several grains of overlapping age have different mean Nd isotopic compositions. Unfortunately, the LASS-ICPMS data are of insufficient precision to assure the grains have distinct initial ¹⁴³Nd/¹⁴⁴Nd values. Several of these dual analyzed grains, however, have recently been triply analyzed via high precision TIMS measurements and have confirmed their distinct Nd isotope composition. Thus we have established that a monazite tripartite analytical scheme, although difficult, can shed substantially more light on determining immediate provenance than any single analytical technique on any single mineral species.