Abstract

We report the radiocarbon age, chemistry, and mineralogy of sinter deposits from Castle and Giant Geyser in the Upper Geyser Basin of Yellowstone National Park. At each geyser, we sampled following the stratigraphy of the older terrace and younger cone. We analyzed 15 samples with x-ray diffraction, x-ray tomography, x-ray fluorescence, scanning electron microscopy, thin section microscopy, 3D imaging, and loss on ignition. Castle Geyser's terrace and Giant Geyser's cone are composed of more increasingly more mature sinter. The concentrations of Na, K, Cs, and Ga decrease in the sinter with decreasing H2O and increasing SiO2, therefore water and trace element concentrations correlate with stratigraphic position, and sinters exhibit progressive dehydration with increasing age. However, radiocarbon dates resulted in stratigraphically out-of-sequence ages. This suggests both physical mixing and the influence of magmatically dead carbon. By isolating and dating individual pieces of externally sourced organic material, we report ages as upper bounds---increasing the known age of Castle Geyser cone to 2,000 yr BP. Additionally, we demonstrate that Castle Geyser's shield is mineralogically distinct from and at least 1,000 years older than it's cone.