Single zircon grains were loaded into 200-μl Savillex “micro”-capsules with 100 μl of 29 M HF + 15 μl of 3N HNO3 for a single leaching step in high-pressure Parr bombs at 185°C for 12 hours to remove crystal domains affected by Pb loss (51). Grains were rinsed after leaching in 6 N HCl, MQ H2O, 3N HNO3, and 29 M HF before spiking with EARTHTIME (202Pb-)205Pb-233U-235U tracer and addition of 100 μl of 29 M HF + 15 μl of 3N HNO3 (52, 53). Zircons were then dissolved to completion in Parr bombs at 210°C for 48 hours. Dissolved zircon solutions were subsequently dried down, dissolved in 100 μl of 6N HCl, and converted to chlorides in Parr bombs at 185°C for 12 hours, after which solutions were dried again and brought up in 50 μl of 3N HCl. The U-Pb and trace element aliquots were then separated by anion exchange chromatography using 50-μl columns and AG-1 X8 resin (200 to 400 mesh, chloride from Eichrom) (54) and dried down with a microdrop of 0.015 M H3PO4. The dried U and Pb aliquot was loaded in a silica gel emitter (55) to an outgassed zone-refined Re filament.

Isotopic determinations were performed using an IsotopX PhoeniX-62 TIMS at Princeton University, with Pb analysis performed in peak-hopping mode on a Daly photomultiplier ion-counting detector. A correction for mass-dependent Pb fractionation was applied in one of two ways. For double-Pb spiked analyses (202Pb-205Pb, ET2535), a cycle-by-cycle fractionation correction was calculated from the deviation of measured 202Pb/205Pb from the known tracer 202Pb/205Pb [0.99924 ± 0.00027 (1σ)]. For single-Pb spiked analyses (205Pb, ET535), a Pb fractionation of 0.182 ± 0.041% per atomic mass unit (amu) was used, as determined by repeat measurements of NBS982 at Princeton. A Daly photomultiplier dead time of 28.8 ns was used, as determined by repeat measurements of National Bureau of Standards (NBS) standards. Corrections for interfering isotopes under masses 202, 204, and 205 were made cycle by cycle by measuring masses 201 and 203 and assuming that they represent 201BaPO4 and 203Tl and using natural isotopic abundances to correct for 202BaPO4, 204BaPO4, 205BaPO4, and 205Tl.

UO2 measurements were performed in static mode on Faraday cups with a bulk U fractionation correction calculated from the deviation of measured 233U/235U from the known tracer 233U/235U [0.995062 ± 0.000054 (1σ)], and an oxide composition of 18O/16O of 0.00205 was used (56). Data reduction was performed using the programs Tripoli and U-Pb Redux (57, 58) and the decay constants of Jaffey et al. (59). All Pbc was attributed to laboratory blank with a mean isotopic composition determined by total procedural blank measurements (see table S1 for values). Two different blank models were generated to assess data collected before [Outliers Culled (OC)] and after [Side Filaments (SF)] January 2017, when the laboratory began heating side filaments before collecting data on the mass spectrometer, which was found to reduce interferences. Uncertainties in reported U-Pb zircon dates are at the 95% confidence level and exclude tracer calibration and decay constant uncertainties. Correction for initial 230Th disequilibrium in the 206Pb/238U system was made on a fraction-by-fraction basis by estimating (Th/U)magma using (Th/U)zircon determined by TIMS and a mean (Th/U)zircon-magma partition coefficient ratio of 0.19 ± 0.11, which encompasses the range of values for (Th/U)zircon-magma partition coefficients obtained from glasses from a variety of volcanic settings (60). Uncertainties for the resulting (Th/U)magma were also calculated on a fraction-by-fraction basis, propagating the uncertainty in the (Th/U)zircon-magma partition coefficient. Overall, these corrections for 230Th disequilibrium affected our results by no more than ±10 ka compared to an alternative approach using a constant (Th/U)magma of 3.5 ± 1.0 (see the Supplementary Materials).

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