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 precipitate mounded travertine composed of aragonite needle botryoids. The apron and channel facies (43-72 degrees C) is floored by hollow tubes composed of aragonite needle botryoids that encrust sulfide-oxidizing Aquificales bacteria. The travertine of the pond facies (30-62 degrees C) varies in composition from aragonite needle shrubs formed at higher temperatures to ridged networks of calcite and aragonite at lower temperatures. Calcite "ice sheets", calcified bubbles, and aggregates of aragonite needles ("fuzzy dumbbells") precipitate at the air-water interface and settle to pond floors. The proximal-slope facies (28-54 degrees C), which forms the margins of terracette pools, is composed of arcuate aragonite needle shrubs that create small microterracettes on the steep slope face. Finally, the distal-slope facies (28-30 degrees C) is composed of calcite spherules and calcite "feather" crystals. Despite the presence of abundant microbial mat communities and their observed role in providing substrates for mineralization, the compositions of spring-water and travertine predominantly reflect abiotic physical and chemical processes. Vigorous CO2 degassing causes a +2 unit increase in spring water pH, as well as Rayleigh-type covariations between the concentration of dissolved inorganic carbon and corresponding delta 13C. Travertine delta 13C and delta 18O are nearly equivalent to aragonite and calcite equilibrium values calculated from spring water in the higher-temperature (approximately 50-73 degrees C) depositional facies. Conversely, travertine precipitating in the lower-temperature (< approximately 50 degrees C) depositional facies exhibits delta 13C and delta 18O values that are as much as 4% less than predicted equilibrium values. This isotopic shift may record microbial respiration as well as downstream transport of travertine crystals. Despite the production of H2S and the abundance of sulfide oxidizing microbes, preliminary delta 34S data do not uniquely define the microbial metabolic pathways 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.

中文翻译：

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石灰华沉积温泉的沉积相和水固地球化学（天使台，猛犸温泉，黄石国家公园，美国）

黄石国家公园猛犸温泉天使台的石灰华沉积温泉的岩石学和地球化学分析已被用于定义沿泉排水系统的五个沉积相。泉水在 71 至 73 摄氏度的喷口相中被排出，并沉淀出由文石针状葡萄石组成的丘状石灰华。围裙和通道相（43-72 摄氏度）由空心管组成，这些空心管由包裹硫化物氧化 Aquificales 细菌的文石针状葡萄石组成。池塘相（30-62 摄氏度）的石灰华在成分上有所不同，从在较高温度下形成的文石针状灌木到在较低温度下形成的方解石和文石脊状网络。方解石“冰盖”、钙化气泡和文石针聚集体（“模糊哑铃”）) 在空气-水界面沉淀并沉淀到池底。近坡相（28-54 摄氏度）形成梯田池的边缘，由弓形文石针灌木组成，在陡坡面上形成小的微梯田。最后，远端斜坡相（28-30 摄氏度）由方解石球粒和方解石“羽毛”晶体组成。尽管存在丰富的微生物垫群落并观察到它们在为矿化提供基质方面的作用，但泉水和石灰华的成分主要反映了非生物物理和化学过程。剧烈的 CO2 脱气导致泉水 pH 值增加 +2 个单位，以及溶解的无机碳浓度与相应的 delta 13C 之间的瑞利型协变。钙华 delta 13C 和 delta 18O 几乎相当于从较高温度（约 50-73 摄氏度）沉积相中的泉水计算的文石和方解石平衡值。相反，在较低温度（< 大约 50 摄氏度）沉积相中沉淀的石灰华显示 delta 13C 和 delta 18O 值，比预测的平衡值少 4%。这种同位素变化可能会记录微生物呼吸以及钙华晶体的下游运输。尽管产生了 H2S 和丰富的硫化物氧化微生物，但初步的 delta 34S 数据并不能唯一定义泉水系统中存在的微生物代谢途径。