Trace and rare-earth element composition of 2480 Ma detrital zircons in Proterozoic metapsammites from northwestern Arizona

Detrital zircon grains in the ~1740-1750 Ma Vishnu Schist and similar rock units in northwestern Arizona consist of up to 30% grains dated by U-Pb isotopic analysis at 2470-2490 Ma. These zircon grains are distributed over ~40,000 km2 and define an age peak at 2480.0 ± 27.3 Ma (2SE). These grains have yielded unusually consistent 207Pb/206Pb dates, with generally smaller analytical uncertainty and greater concordance to ideal U-Pb evolution than grains of other ages. A weighted mean age of 2480 ± 0.9 Ma (2SE) for this zircon population reflects consistent analytical results and high analytical precision but not the accuracy of the age. The source of these zircons has not been identified. To better characterize the unidentified source, we analyzed 45 of these grains for trace and rare-earth elements by laser-ablation mass spectrometry and scanned 16 grains with an electron microprobe to identify mineral inclusions. Mass spectrometer determinations of Sc/Yb and Nb/Sc support derivation fr..., Two samples of Paleoproterozoic psammite were collected for previous studies and the zircons in them were analyzed for U-Pb geochronology.  The grain mounts that were used for U-Pb analysis of zircons from Cottonwood Cliffs and Jerome Canyon (Doe, 2014, Colorado School of Mines, Ph.D. dissertation; Spencer et al., 2016, Geosphere) were re-used to determine the trace-element and rare-earth-element (TREE) content of 45 zircon grains that had been dated at ~2480 Ma. Analyses were conducted in 2022 at the Arizona Laserchron Center at the University of Arizona with a Photon Machines Analyte G2 laser (193 nm) connected to an Element2 multi-collector LA-ICP-MS (laser-ablation – inductively coupled plasma – mass spectrometer) equipped with a Jet pump and interface (Pullen et al., 2018; Gehrels, 2020). Primary calibration standards SL and FC were analyzed intermittently during sample analyses (set-up with sequential analyses of standards FC-SL-FC-SL-FC-SL, then 5 unknowns in groups separated by ..., , # 2480 Ma zircon TREE analyses

[https://doi.org/10.5061/dryad.6djh9w189](https://doi.org/10.5061/dryad.6djh9w189)

Trace and rare-earth element analysis of 45 zircons dated at 2470-2490 Ma indicate several unusual properties of the source igneous rock, as follows: (1) The titanium content of ~2480 Ma zircons, typically ~5-25 ppm, reflects the temperature of crystallization. Calculation of crystallization temperature requires the presence of rutile, however, otherwise only a minimum temperature can be determined (Watson & Harrison, 2005; Watson et al., 2006). Titanium concentrations in the analyzed 2480 Ma zircon grains, none of which are known to contain rutile inclusions, indicate minimum crystallization temperature of ~740-800 °C, a temperature range that is more characteristic of mafic rather than felsic magmas (Fu et al., 2008). (2) The 2480 Ma zircon population in our two schist samples has large europium depletions. Thirty-seven of forty-five 2480 Ma zircon grains analyzed fo...