DOI: 10.5281/zenodo.5747227
title: CA-TIMS U-Pb analysis of IGSN samples IEMM10001, IEMM10002, IEMM10003, IEM10004

description: Single zircon U-Pb analyses were performed at the Berkeley Geochronology Center. The samples were crushed and mineral concentrates were then purified using standard mineral separation techniques, including sieves, magnetic separation and density separation. If present, euhedral, clear grains devoid of optically recognizable cores were chosen for analysis. All zircons were pretreated using thermal annealing at 850°C for 48 hrs, followed by chemical abrasion with concentrated HF in pressurized dissolution capsules at 220°C for 6-8 hrs. Prior to dissolution the crystals were cleaned in ultrasonically agitated aqua regia followed by multiple steps of rinsing in clean HNO<sub>3</sub>. Zircon crystals were then spiked with <sup>205</sup>Pb-<sup>233</sup>U- <sup>235</sup>U tracer solution mixed and calibrated at BGC, and dissolved by vapor transfer in HF using miniature PTFE capsules at 220°C for 6 days. After dissolution, large crystals were loaded in 2 to 3 aliquots (see table) to optimize beam stability (which typically is significantly decreased for large, unextracted zircon samples) and test reproducibility. Isotope ratios were determined on a Micromass Sector 54 mass spectrometer using a Daly-type ion counter positioned behind a WARP filter. Pb (as Pb<sup>+</sup>) and U (as UO<sup>2+</sup>) were run sequentially on the same filament. The accuracy of the used mixed tracer was tested repeatedly against solutions derived from certified standards of isotopically pure <sup>206</sup>Pb and natural U (NIST SRM-991 and CRM-145, respectively), as well as age solutions distributed by the Earthtime initiative (agreement was found to be &lt;0.1%, additional details regarding tracer calibration are in Irmis et al., 2011; Mundil et al., 2004; Black et al., 2004). An additional subset of 5 zircons of the KHB01 sample was analyzed using the Earthtime 535 tracer solution. The resulting mean age of 97.80 ± 0.19 Ma (MSWD 2.0) from this batch is slightly younger, but within uncertainty, compared to our reported age of 98.00 ± 0.14 Ma, even without taking into account an augmented uncertainty resulting from U/Pb in the tracer solutions. The elevated MSWD may be a result of excess scatter from true age dispersion within the zircon population of KHB01. Repeat measurements of the total procedural blank averaged 0.82 ± 0.36 pg Pb (U blanks were indistinguishable from zero), with <sup>206</sup>Pb/<sup>204</sup>Pb = 18.40 ± 0.46, <sup>207</sup>Pb/<sup>204</sup>Pb = 15.64 ± 0.25, <sup>208</sup>Pb/<sup>204</sup>Pb = 38.04 ± 0.75 (all 2σ of population), and a <sup>206</sup>Pb/<sup>204</sup>Pb-<sup>207</sup>Pb/<sup>204</sup> Pb correlation of +0.47 (ratios and uncertainties were propagated into the age and age-error calculations). The total procedural blank including ion exchange chemistry is also as low as 1 pg but scatter in total common Pb concentration and possibly common Pb composition suggests that results from unextracted analyses of zircon yield better reproducibility. Deficient radiogenic <sup>206</sup>Pb in zircon due to initial deficit of <sup>230</sup>Th is accounted for by assuming a partition coefficient ratio DTh/DU of 0.2 (as applied in Wotzlaw et al., 2014), or by assuming Th/U in the parent magma to be 3.5 where the ET535 tracer solution was used (the resulting difference in age from either approach is negligible). Mass fractionation of U during analysis was controlled by the U double spike, whereas Pb mass fractionation was corrected by 0.15 ± 0.6 %/AMU (based on multiple analyses of NBS 981). The photo-micrograph shows crystals extracted from KHB03 and illustrates that the zircon population is homogeneous.
description: Samples collected under US National Park Service permit PORE-2015-SCI-0025 to Michael Manga
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