Depositional and diagenetic facies in siliceous hotsprings: Examples from Yellowstone National Park, Wyoming, United States of America.

摘要

Siliceous hotspring systems in Yellowstone National Park provide insights into spring geometries, depositional facies, hydrochemistry, and lithofacies associated with modern hotsprings. Analyses of active and inactive siliceous hotsprings has facilitated construction of a generic model for siliceous hotspring deposits. Yellowstone's siliceous springs tend to group into four broad categories: siliceous spires/cones, domal mounds, terraced mounds, and ponds. The major spring morphotypes identified include eight cumulative hotspring depositional facies: (1) vent (>95°C), (2) proximal vent (95°C), (3) pool (∼80–90°C), (4) pool margin (∼80°C), (5) pool eddy (80°C), (6) discharge channel/flowpath (80°C to ambient), (7) debris apron (variable temperatures), and (8) geyser (variable temperatures).; Siliceous sinter precipitation within hotsprings has been attributed to a variety of processes: evaporative concentration, cooling, changes in pH, and cation effects. Repetitive in situ (T, pH, alkalinity, etc.) and laboratory (major, minor, and trace elemental, stable isotopic) analyses of the waters, plus observations of silica precipitation on natural (e.g. twigs, pine cones) as well as artificial substrates (glass slides, copper plates) in the waters substantiate that subaqueous precipitation is occurring throughout the vent to distal end of flow in Cistern and Deerbone springs. Based on a suite of measured and theoretical saturation indices downflow changes in the system (e.g. Cl increases 10%), changes in pH (e.g. 5.6 to 7.1), and cation effects (Al and Fe) are of negligible importance in the subaqueous precipitation of opal-A. Modeling of the two active siliceous sinter precipitating systems indicates that cooling (e.g. 80 to 17°C) is the predominant process governing subaqueous mineral precipitation.; Older siliceous precipitates from an 11m long core through a relict siliceous hotspring deposit provide insights into diagenesis and long-term preservation potential of organic matter in these environments. Diagenetic alteration is a rapid process in these environments, and pervasive alteration can be observed at shallow depths in the subsurface (e.g. centimeters). Significantly, little soft organic matter is preserved in the core despite the abundance of silicified microbial remains. This apparent lack of preserved biochemical remains indicates rapid dissolution and/or replacement of soft organic matter, and has significant consequences in the search for extraterrestrial organic remains at hotspring sites.