Sample Analyses: Four samples of silica sinter from Giant and Castle Geysers compo sedof opal-A or opal-A/C were analyzed for U-Th isotopes at USGS laboratories in Denver, CO (https://www.usgs.gov/centers/gecsc/science/denver-radiogenic-isotope-lab?qt-science_center_objects=0#qt-science_center_objects). Small fragments, weighing approximately 10 g, were cut and polished to enable microsampling using carbide dental burrs. Sub-samples consisted of small pits or trenches that cut across microfabric elements and integrate material weighing 0.08–0.13 g. Resulting powders were digested using concentrated hydrofluoric acid (HF) after spiking with known amounts of a mixed 229Th-233U-236U tracer solution. After evaporation, residues were re-dissolved with 7M nitric acid and centrifuged at 10,000 rpm to ensure that all material went into solution in order to avoid laboratory U/Th fractionation. Purified salts of U, and Th were obtained from a single digest using piggy-backed ion-chromatography columns containing Biorad™ AG1×8 anion resin in the upper column for U and Th. Columns were separated after the first 3 resin volumes of nitric acid wash (see Paces et al., 2019 supplemental data repository at https://www.geosociety.org/datarepository/2020/2020023.pdf for additional details). U and Th separates required 2 column passes to obtain sufficiently pure material. Total U- and Th-process blanks for digestion and chemistry are 5–10 pg. All U-Th isotope measurements were made on a Thermo Finnigan™ Triton mass spectrometer in peak-jumping mode using a single discrete-dynode secondary electron multiplier behind a retarding potential quadrupole (RPQ) energy filter. Primary standard NIST 4321B with a certified 234U/238U value of 0.0000529±0.0000011 was used to normalize measurements for instrument bias and drift. Secondary standards were used to evaluate total chemical and data-reduction procedures. Long-term monitoring of a USGS in‐house secular equilibrium standard (69‐Ma‐old uranium ore from the Schwartzwalder Mine; Ludwig et al., 1985) including an analysis performed along with sinter samples yield results that are analytically indistinguishable from the secular equilibrium value of 1.0000 ([234U/238U] = 0.9989±0.0038, 2×standard deviation for N=79 {square brackets denote activity ratios}; and [230Th/238U] = 0.9995±0.0041, 2×standard deviation for N=73). Analyses of a USGS late Pleistocene Acropora coral dating standard (mean age of 119.6 ±1.9 ka for N=17 presented in Watanabe and Nakai, 2006) processed in an identical manner as the unknown samples yielded an average age of 119.4 ±1.2 ka and an initial 234U/238U activity ratio of 1.154±0.006 (N=16). All uncertainties presented herein are given at the 95% confidence level (±2σ). Measurable amounts of common thorium (232Th) present in sinter samples implies that some 230Th not associated with the in-situ decay of parent isotope 234U is present. To avoid calculating erroneously old 230Th/U ages, any initial 230Th originating from detrital sources was eliminated mathematically based on the measured [232Th/238U] and an assumption about the isotopic composition of the common-thorium‐bearing detrital component (Ludwig and Paces, 2002). The detrital component is assumed to be uniform with an atomic Th/U of 4 and the following activity ratios and 2σ errors: [232Th/238U] = 1.276±0.64; [234U/238U] = 1.0±0.1; and [230Th/238U] = 1.0±0.25. Isotope ratios corrected for detrital components are used to calculate 230Th/U ages, initial [234U/238U] and associated errors using conventional U‐series age equations (Ludwig, 2012). Age uncertainties are propagated such that samples with little or no detrital Th typically have small age errors essentially defined by analytical uncertainties, whereas samples with large corrections will have large age errors due to the substantial uncertainty (±50%) in the “true” composition of the detrital component. Database Contents: The data file (U_Th_Supplementary.csv) contains the Uranium-Thorium measured activity ratios, detritus-corrected activity ratios, and the first derivative solution to the age equation. References Ludwig, K.R., Wallace, A.R., and Simmons, K.R., 1985, The Schwartzwalder uranium deposit, II: age of uranium mineralization and Pb-isotope constraints on genesis: Economic Geology, v. 80, p. 1858–1871, https://doi.org/10.2113/gsecongeo.80.7.1858. Paces, J.B., Long, A.J., and Koth, K., 2015, Potential Application of Radiogenic Isotopes and Geophysical Methods to Understand the Hydrothermal System of the Upper Geyser Basin, Yellowstone National Park. National Park Service, Natural Resource Report NPS/YELL/NRR—2015/1077, 57 p. https://irmadev.nps.gov/DataStore/DownloadFile/530782 Paces, J.B., Palmer, M.V., Palmer, A.N., Long, A.J., and Emmons, M.P., 2019, 300,000 yr history of water-table fluctuations at Wind Cave, South Dakota, USA—Scale, timing, and groundwater mixing in the Madison Aquifer: GSA Bulletin, https://doi.org/10.1130/B35312.1 Watanabe, Yumiko, and Nakai, Shun’ichi, 2006, U-Th radioactive disequilibrium analyses for JCp-1, coral reference distributed by the Geological Survey of Japan: Geochemical Journal, v. 40, p. 537-541, https://doi.org/10.2343/geochemj.40.537.