Sample Analysis: Cation concentration data was obtained as a part of the beryllium isotope analyses. Samples collected from the Upper Geyser Basin were first treated in clean laboratory facilities at the University of New Hampshire following the methods laid out in the “Beryllium Isotope Data” portion of this data release. Following the hydrofluoric acid (HF) etches, and prior to Accelerated Mass Spectroscopy (AMS) analyses of 10Be, opal purity was assessed by measurement of major cation concentrations via inductively coupled plasma-optical emission spectrometry (ICP-OES) using a Perkin Elmer Optima 3300DV at the Analytical and Technical Services lab at SUNY College of Environmental Science and Forestry in Syracuse, New York. The cations were run on a ~0.2 g dry aliquot of the purified opal, which was removed from a homogenized sample (the same material analyzed for 10Be with AMS). The opal aliquot was digested in a few mL concentrated HF, dried down, and then redissolved in a solution of 5% nitric acid (HNO3)/1% sulfuric acid (H2SO4) prior to ICP-OES measurements. The first etch ran for 24 hours, and all subsequent etches ran for 48 hours. Sample YGT18-30D dissolved completely after 6 etches, hence no ICP-OES measurements were possible. Sample cation concentrations were determined with reference to in-house standard solutions prepared from commercially purchased certified single-element ICP standards and diluted to concentration ranges customized for each measured element. Several blanks were run in each ICP-OES measurement batch, and average blank values for each batch were subtracted from measured sample concentrations to derive blank-corrected cation values. ICP-OES analyte wavelengths for each element are as follows: Al (396.153, 308.215), Be (313.107, 313.042, 234.861), Ca (317.933, 315.887), Fe (238.204, 259.939), K (766.490), Ti (334.940, 336.121). All reported cation concentrations are corrected for blanks run concurrently with samples. Maximum blank corrections were negligible for all elements, as follows: Al (0.04%), Be (0.1%), Ca (0.05%), Fe (0.3%), K (0.01%), Ti (0.5%). Database Contents: The data file (ICPOES Data.csv) contains the inductively coupled plasma-optical emission spectrometry measured cation concentrations for samples collected from Castle and Solitary Geyser. A. Sample ID B. Location C. # Etches D. Al (ppm) E. Be (ppm) F. Ca (ppm) G. Fe (ppm) H. K (ppm) I. Ti (ppm)