Sample Collection:Field work was conducted in November 2018 in the Upper Geyser Basin. Three sinter samples were collected from Castle Geyser and one sinter sample was collected from Solitary Geyser. At Castle Geyser, an effort was made to identify stable surfaces associated with the oldest exposed stratigraphic levels of sinter deposition, in order to target material emplaced during early stages of hydrothermal activity in the basin. At Solitary Geyser, material was collected from an exposure on the upper surface of a sinter deposit. All four sinter sample locations were selected to coincide with collection sites for material dated by radiocarbon methods, to enable a cross-comparison of results. Sinter was collected with hammer and chisel from the uppermost few centimeters of stable deposits located on smooth-surfaced topographic highs, so as to minimize issues with erosion and to reduce potential for soil and snow cover. Geographic coordinates and altitudes of each collection site were obtained using a handheld GPS, and topographic measurements and strike and dip of rock surfaces were used to derive shielding corrections for each sample site. All sites were thoroughly characterized and photographed. Sample Analyses: Beryllium was extracted from milled rock to produce BeO target material for accelerator mass spectrometry (AMS) analysis, following established geochemical procedures (Licciardi, 2000; Corbett et al., 2016). All chemical work was conducted in clean laboratory facilities at the University of New Hampshire. Sinter samples collected for 10Be exposure dating were first sawed to a measured uniform thickness (1.25 to 1.75 cm) and milled to a 600–250 μm size fraction. Magnetic mineral phases were removed with a hand magnet. Sample material was sonicated in a dilute 1% HF solution and non-opaloid mineral phases were removed by froth flotation (Herber, 1969). Milled samples were sonicated in 6 N HCl for pre-cleaning, and opal was then isolated and purified by repeated etching in a 2% HF/1% HNO3 solution (Kohl and Nishiizumi, 1992). Sample amounts were reduced considerably during progressive etchings, which involved a sequence of one 24-hour etch followed by successive 48-hour etches. For sample YGT18-27, the initial 100 g of opal was reduced to 2 g after two etches in 2% HF/1% HNO3 solution. All initial 200 g of sample YGT18-30 dissolved completely after six etches. For AMS measurements, 9Be carrier (~0.2 mg) was added to the purified opal before digestion in concentrated HF. Beryllium was extracted from samples using ion-exchange chromatography, selective precipitation, and oxidation to BeO. The 10Be/9Be analyses were performed at the Lawrence Livermore National Laboratory Center for Accelerator Mass Spectrometry (LLNL-CAMS). Cosmogenic 10Be exposure ages are calculated using a calibration of regional in situ 10Be production rates based on measurements at Promontory Point, Utah (Lifton et al., 2015). A nuclide- and time-dependent scaling model formulated by Lifton et al. (2014) and designated as “LSDn” is implemented in the age calculations. Exposure ages were derived from the CRONUS-Earth calculator version 3 (Balco et al., 2008). Snowpack data are not available for the Upper Geyser Basin field area; hence the exposure ages are not corrected for snow shielding. Sinter surface erosion rates are also not known in the field area, and no erosion corrections were applied in the age calculations. Database Contents: The data file (10Be_Data_Supplementary.csv) contains the steps taken in order to calculate beryllium-10 radiometric ages for three samples collected from the shield of Castle Geyser, and one sample from Solitary Geyser. The entries in the data file appear in the following columns: A. Sample ID B. Location C. Latitude (°N) D. Longitude (°W) E. Altitude (m.a.s.l.), a F. Thick (cm), b G. Shield factor, c H. Opal (g) I. Carrier (mg 9Be), d J. 10Be/9Be (10-14 ± 1σ), e K. [10Be] (104 at g-1 ± 1σ) L. Age (ka ± 1σ)