DOI: 10.5061/dryad.6djh9w189
title: Trace and rare-earth element composition of 2480 Ma detrital zircons in Proterozoic metapsammites from northwestern Arizona

description: 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 from an
 oceanic-island igneous source. Electron microprobe scans revealed quartz
 in 5 of 16 grains, indicating a felsic source. The low variability in
 207Pb/206Pb dates and a generally linear relationship between U and Th
 support zircon derivation from a single igneous unit or closely related
 set of units without xenocrystic zircons. A literature search for other
 zircon populations with similar age and U/Th ratios identified ~2480 Ma
 zircons in a Mesoproterozoic(?) metapsammite and conglomerate in
 southwestern Montana. This sandstone was deposited near the margin of the
 Wyoming craton and contains almost entirely 2400-3600 Ma zircons, unlike
 zircon grains in Vishnu Schist which include a large population of
 1730-1900 Ma zircons. From this relationship, we infer that the 2480 Ma
 zircons in both areas were derived from a source in the Wyoming craton. We
 conclude that the 2480 Ma Vishnu zircons were derived from a felsic
 batholith that formed above and from hotspot magma related to the
 ~2450-2480 Ma Matachewan Large Igneous Province, that this batholith
 formed by mixing between a mantle-derived hotspot magma and assimilated
 Archean continental crust, and that the source rock was emplaced during
 initial rifting between the Wyoming craton and the Superior province.
description: 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 FC-SL, then ending with FC-SL-FC-SL). Secondary standards R33
 and NIST612 were also analyzed occasionally during sample analyses. The
 precision of measurements of element concentrations is &lt;5% for a
 typical zircon grain (Gehrels, 2020). The expected stoichiometric
 concentration of Si in zircon (153,230 ppm) is shown for all zircons
 because this value was assumed and then used to calibrate signal
 intensities for all other analyzed elements (Gehrels, 2020).
description: # 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 for rare-earth elements contain 7-16% of the Eu that would be expected given the measured concentrations of Sm and Gd, while the other 8 grains contain 20-50% of the expected Eu. If the zircons crystallized from intermediate to moderately felsic magmas, then crustal thickness was ~30-40 km during the time of magma ascent and crystallization of most of the zircons (using the calibration of Tang et al., 2020). (3) Element ratios that are most effective in identifying magma sources include U/Yb, Sc/Yb, Nb/Yb, Nb/Sc, and U/Nb. The 2480 Ma zircon grains analyzed for this study are plotted on five graphs of element ratios with fields identified for each of the three major magma sources as determined by Grimes et al. (2015; mid-ocean ridge, oceanic island, and magmatic arc). The 2480 Ma zircons are not associated with mid-ocean ridge magmas in any of the graphs, but in three graphs these zircons plot well within the field for oceanic island zircons. ##### Description of the data and file structure The data table lists elements across the top. Each row are the data produced from a single laser-ablation spot on a zircon grain. Abbreviations "FC", "NIST612", "SL', "R33", "FC", and "SL" represent zircon TREE standards. Sample numbers are 20120803 (Cottonwood Cliffs) and 2-15-11-3 (Jerome Canyon). Numbers appended to sample number are analysis numbers and generally correspond to a single zircon grain.