Sample Analyses: Samples were analyzed for their trace element chemistry using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at the Corman Center for Mass Spectrometry, Rensselaer Polytechnical Institute, New York. After XRF concentrations were determined following the methods laid out in the “X-ray Fluorescence (XRF)” portion of this data release, a small piece (~5 mm by 5-8 mm) was cut from the wavelength dispersive (WDXRF) pellets and mounted on a one-inch glass puck with 25-30 other samples. Samples were polished down to ~0.5 micron with diamond paste and placed in an ablation chamber that holds two sample pucks, 18 reference standards, and a drift monitor on a third puck. Samples were then ablated using Iolite software. Aerosol is carried from the laser cell toward the Varian 820 ICPMS by He carrier gas, mixing with Ar make-up gas in a mixing volume (20ml) just before the torch. Argon plasma is run under ‘hot’ plasma conditions with no collision mode. The ICPMS is run in peak-hopping, time-resolved mode for 54 major and trace analytes. Dwell times are ~10ms per mass. The detector is run in medium attenuation mode for major elements and none/auto attenuation for traces. Gas blank is also acquired between samples for background subtraction. Background corrected intensities were converted to concentrations using the method described by Conrey et al. (2019). Eighteen fused low dilution glass reference materials (made with common USGS, GSJ, etc. standards) and all sample glasses were each ablated four times during a run, with 1000 μ by 150 μ tracks. A custom fused low dilution drift monitor was ablated frequently throughout the approx. seven hour run to correct for drift. Many major element concentrations are near trace element levels in these sinters, and the overall uncertainty in LA-ICP-MS determinations for both “major” and trace elements lies around ±10% RSD. Database Contents: The data file (LAICPMS_Data_Supplementary.csv) contains the major and trace element concentrations measured from three sinter samples from Giant Geyser and three sinter samples from Castle Geyser. Error ± values are based solely on counting error; major elements are given in wt% oxide; trace elements are given in ppm. The entries in the data file appear in the following columns: A. Sample ID B. Location C. TiO2 (wt% oxide) D. TiO2 +/- (wt% oxide) E. Al2O3 (wt% oxide) F. Al2O3 +/- (wt% oxide) G. FeO* (wt% oxide) H. FeO +/- (wt% oxide) I. MnO (wt% oxide) J.MnO +/- (wt% oxide) K. MgO (wt% oxide) L. MgO +/- (wt% oxide) M. K2O (wt% oxide) N. K2O +/- (wt% oxide) O. P2O5 (wt% oxide) P. P2O5 +/- (wt% oxide) Q. Ag (ppm) R. Ag +/- (ppm) S. As (ppm) T. As +/- (ppm) U. Ba (ppm) V. Ba +/- (ppm) W. Bi (ppm) X. Bi +/- (ppm) Y. Cd (ppm) Z. Cd +/- (ppm) AA. Cs (ppm) AB. Cs +/- (ppm) AC. Ga (ppm) AD. Ga +/- (ppm) AE. Ge (ppm) AF. Ge +/- (ppm) AG. Hf (ppm) AH. Hf +/- (ppm) AI. Mo (ppm) AJ. Mo +/- (ppm) AK. Nb (ppm) AL. Nb +/- (ppm) AM. Pb (ppm) AN. Pb +/- (ppm) AO. Rb (ppm) AP. Rb +/- (ppm) AQ. Sb (ppm) AR. Sb +/- (ppm) AS. Sc (ppm) AT. Sc +/- (ppm) AU. Sn (ppm) AV. Sn +/- (ppm) AW. Sr (ppm) AX. Sr +/- (ppm) AY. Ta (ppm) AZ. Ta +/- (ppm) BA. Th (ppm) BB. Th +/- (ppm) BC. Tl (ppm) BD. Tl +/- (ppm) BE. U (ppm) BF. U +/- (ppm) BG. V (ppm) BH. V +/- (ppm) BI. Y (ppm) BJ. Y +/- (ppm) BK. Zr (ppm) BL. Zr +/- (ppm) BM. La (ppm) BN. La +/- (ppm) BO. Ce (ppm) BP. Ce +/- (ppm) BQ. Pr (ppm) BR. Pr +/- (ppm) BS. Nd (ppm) BT. Nd +/- (ppm) BU. Sm (ppm) BV. Sm +/- (ppm) BW. Eu (ppm) BX. Eu +/- (ppm) BY. Gd (ppm) BZ. Gd +/- (ppm) CA. Tb (ppm) CB. Tb +/- (ppm) CC. Dy (ppm) CD. Dy +/- (ppm) CE. Ho (ppm) CF. Ho +/- (ppm) CG. Er (ppm) CH. Er +/- (ppm) CI. Tm (ppm) CJ. Tm +/- (ppm) CK. Yb (ppm) CL. Yb +/- (ppm) CM. Lu (ppm) CN. Lu +/- (ppm) References Conrey, R.M., Bailey, D.G., Singer, J.W., Wagoner, L., Parfitt, B., Hay, J. and Keh, O., 2019, March. Optimization of internal standards in LA-ICPMS analysis of geologic samples using lithium borate fused glass. In Northeastern Section-54th Annual Meeting-2019. GSA.