Depositional facies and aqueous-solid geochemistry of travertine-depositing hot springs (Angel Terrace, Mammoth Hot Springs, Yellowstone National Park, USA)

Petrographic and geochemical analyses of travertine depositing hot springs at Angel Terrace, Mammoth Hot Springs, Yellowstone National Park, have been used to define five depositional facies along the spring drainage system. Spring waters are expelled in the vent facies at 71 to 73 degrees C and pre cipitate mounded travertine composed of aragonite needle botryoids. The apr on and channel facies (43-72 degrees C) is floored by hollow tubes composed of aragonite needle botryoids that encrust sulfide-oxidizing Aquificales b acteria. The travertine of the pond facies (30-62 degrees C) varies in comp osition from aragonite needle shrubs formed at higher temperatures to ridge d networks of calcite and aragonite at lower temperatures. Calcite "ice she ets", calcified bubbles, and aggregates of aragonite needles ("fuzzy dumbbe lls") precipitate at the air-water interface and settle to pond floors. The proximal-slope facies (28-54 degrees C), which forms. the margins of terra cette pools, is composed of arcuate aragonite needle shrubs that create sma ll microterracettes on the steep slope face. Finally, the distal-slope faci es (28-30 degrees C) is composed of calcite spherules and calcite "feather" crystals. Despite the presence of abundant microbial mat communities and their observ ed role in providing substrates for mineralization, the com positions of sp ring-water and travertine predominantly reflect abiotic physical and chemic al processes, Vigorous CO2 degassing causes a +2 unit increase in spring wa ter pH, as well as Rayleigh type covariations between the concentration of dissolved inorganic carbon and corresponding delta(13)C. Travertine delta(1 3)C and delta(18)O are nearly equivalent to aragonite and calcite equilibri um values calculated from spring water in the higher-temperature (similar t o 50-73 degrees C) depositional facies. Conversely, travertine precipitatin g in the lower-temperature (< similar to 50 degrees C) depositional facies exhibits delta(13)C and delta(18)O values that are as much as 4%0 less than predicted equilibrium values, This isotopic shift may record microbial res piration as well as downstream transport of travertine crystals, Despite th e production of H2S anti the abundance of sulfide-oxidizing microbes, preli minary delta(34)S data do not uniquely define the microbial metabolic pathw ays present in the spring system. This suggests that the high extent of CO2 degassing and large open-system solute reservoir in these thermal systems overwhelm biological controls on travertine crystal chemistry.