SILICON ISOTOPE VARIATIONS IN SILICEOUS SINTER DEPOSITS IN HOT SPRINGS: EXAMPLES FROM YELLOWSTONE NATIONAL PARK, WYOMING, U.S.A

The δ³⁰Si values of silica precipitates down flow from Cistern Spring, Norris Geyser Basin, Yellowstone National Park, decrease systematically from -2.25±0.04‰ to -4.79±0.04‰. This is similar to other hot spring systems that are dominated by abiotic silica precipitation. The formation of siliceous sinter deposits in these types of hot spring systems are controlled by rapid cooling of fluids, degassing, and evaporation, and are accompanied by significant Si isotopic fractionation of the precipitates. The water-solid Si isotopic fractionation is a kinetic process and the degree of fractionation correlates inversely with temperature and precipitation rate. In contrast, the δ³⁰Si values of silica precipitates collected along the flow path of Deerbone Spring (Lower Geyser Basin) show a different pattern, with two sinter samples showing anomalously high δ³⁰Si values. The dissolved silica concentrations of spring waters from both springs, although different, remain relatively constant along their flow paths. However, Deerbone spring waters is significantly less supersaturated with respect to amorphous silica than that of Cistern, indicating that the silica precipitation in Deerbone may have experienced different kinetic processes. In addition, the two anomalous samples were in close association of biofilms and microoganisms. Silica accumulation process is controlled by the aggregation of silica nanoparticles. Although recent field and experimental studies suggest that microorgamisms play a passive role in silica biomineralization, their presence has a profound influence in the surface area and nucleation sites and thus may have caused different degrees of Si isotope fractionation compared with purely abiotic precipitation.