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Geochemical and mineralogical studies of hydrothermal deposits and altered vent muds from the floor of Yellowstone Lake indicate that these features form due to hydrothermal fluid quenching in shallow flow conduits or upon egress into bottom waters. Siliceous precipitates occur as conduits within the uppermost sediments, as tabular deposits that form along sedimentary layers, and as spires as much as 8 m tall that grow upward from crater-like depressions on the lake bottom. These deposits are enriched in As, Cs, Hg, Mo, Sb, Tl, and W.
Variations in major-element geochemistry indicate that subaerial sinters from West Thumb and spire interiors are nearly pure SiO2, whereas sublacustrine conduits are less SiO2 rich and are similar in some cases to normal Yellowstone Lake sediments due to incorporation of sediments into conduit walls. Vent muds, which are hydrothermally altered lake sediments, and some outer conduit walls show pervasive leaching of silica (~63 weight percent silica removal). This hydrothermal leaching process may explain the occurrence of most sublacustrine vents in holes or vent craters, but sediment winnowing by vent fluids may also be an important process in some cases.
Stable-isotope studies indicate that most deposits formed at temperatures between 78°C and 160°C and that vent fluids had oxygen-isotope values of –3.2 to –11.6 per mil, significantly higher than lake waters (–*16.5 per mil). Sulfur-isotope studies indicate that vent waters and lake waters are dominated by sulfur derived from volcanic rocks with δ34S ~ 2.5 per mil.
Geochemical reaction modeling indicates that spires form from upwelling hydrothermal fluids that are saturated with amorphous silica at temperatures 80°–96°C. Reaction calculations suggest that silica precipitation on the lake bottom is initially caused by mixing with cold bottom waters. Once a siliceous carapace is established, more rapid silica precipitation occurs by conductive cooling. Silicification of thermophilic bacteria is a very important process in building spire structures.