Geological Survey Bulletin 1444 Geology and Thermal History of Mammoth Hot Springs

Chemical and isotopic analyses of hot-spring waters supply important information about the history of the thermal water. Cold, dilute meteoric water (rain or snow) has a distinctive hydrogen and oxygen isotopic composition that distinguishes it from water that comes from magmas deep in the earth. Such information suggests that over 95 per cent of the hot-spring water emerging in Yellowstone Park probably originates as meteoric water (White, 1969).

Water from rain and snowmelt percolates into the ground and gradually descends to depths of a kilometer or more beneath the Earth's surface, where it is heated to very high temperatures and becomes enriched in several chemical constituents. The dissolved chemicals in the deep hot water change according to the composition and temperature of the surrounding rock. If the water remains in an aquifer at a uniform temperature for a relatively long period of time, a chemical equilibrium between the water and the rock will be reached. When the deep circulating water eventually moves back toward the surface, its temperature drops and its composition may undergo additional change by continued reaction with the surrounding rock and by mixing with shallow dilute meteoric water.

The chemical composition of water from Soda Spring, shown in table 2, is typical of cold, dilute, shallow ground water that has not been heated to high temperatures during deep circulation. In contrast, the chemical composition of water from Little Whirligig Geyser in Norris Geyser Basin is typical of water coming directly to the surface from a hot aquifer with little or no dilution during the ascent. Fournier, White, and Truesdell (1976) estimated that the aquifer feeding the Norris hot springs and geysers has a temperature of 270°C. Large amounts of dissolved chloride and silica and small amounts of calcium and bicarbonate are typical of high-temperature Yellowstone waters.

Average chemical composition of Mammoth Hot Spring water compared with acid chloride water from Norris Geyser Basin and cold spring water from Snow Pass (table 2)
[Analyses given in parts per million]

The distinctive chemical composition of Mammoth Hot Springs water shown in table 2 (very high calcium, bicarbonate, and sulfate, and moderate silica, chloride, and sodium) possibly results during movement of Norris Basin acid-chloride water along a fault between Norris Geyser Basin and Mammoth Hot Springs (A. H. Truesdell and R. O. Fournier, oral commun., 1976). Along the route, three major events may occur: (1) The water reacts with sedimentary rock that is rich in calcium carbonate, liberating carbon dioxide gas (CO₂), (2) the hot water is cooled and diluted by mixing with water similar to that coming from Soda Spring, and (3) the mixed water reaches a new chemical equilibrium with the surrounding rock in an aquifer at about 73°C.⁴

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⁴Research drill hole Y—10, one of 13 holes drilled in Yellowstone National Park by the U.S. Geological Survey in 1967 and 1968, lies just east of Bath Lake (pl. 1). Drill core recovered from the 113-m hole shows travertine (intermixed with glacial sediments in the middle part of the section) down to 77.3 m, below which are Mesozoic sedimentary rocks. Temperature measurements made during drilling of Y—10 show a remarkably constant temperature of about 73°C at all depths below 15 m (White and others, 1975).

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The source of heat that gives rise to the Yellowstone Park hot springs is partly molten rock in a gigantic magma chamber situated beneath the Yellowstone caldera with its top about 5-10 km below the surface of the ground (Eaton and others, 1975). Alinement of fairly young volcanic rocks along the Norris-Mammoth fault zone (see fig. 2) suggests that the thermal water that eventually reaches Mammoth may be heated by partly molten magma within the fault zone (D. E. White, oral commun., 1976).

The thermal water beneath the Mammoth travertine deposits contains a large amount of dissolved gas, mainly carbon dioxide. Measurements (White and others, 1975) in research hole Y—l0, drilled through the travertine terrace into thermal water, showed that the confining pressure necessary to keep the gas dissolved in the water is greater than 6 kg/cm². As the water flows upward through a labyrinth of channels in the old fractured terraces (fig. 4), the confining pressure gradually decreases, and hot gas, consisting mainly of CO₂ (table 3), separates and escapes at the surface. The effect is similar to removing the cap from a carbonated soft drink bottle. The escape of CO₂ causes the water to become supersaturated with calcium carbonate (CaCO₃), which precipitates out of solution to form travertine, mainly in the form of the mineral calcite. An average chemical analysis of the travertine is given in table 4.

Steeply dipping fractures cutting horizontally bedded travertine deposits of Highland Terrace (fig. 4). Numerous channels have been carved along the fractures by thermal water flowing toward the surface. Pencil (circled) is about 15 cm long.

Average chemical composition of gas exsolved from the Mammoth Hot Springs (table 3)
[----, values not reported]

Average chemical composition of travertine from Mammoth Hot Springs (table 4)
[Average of ten analyses reported by Clarke (1904)]

Allen (1934) estimated that the travertine at Mammoth Hot Springs is being deposited at a rate varying from about 2.8 to 56.5 cm per year and averaging about 21.1 cm per year. According to Allen and Day (1935), the porous travertine (fig. 5) that forms the massive Mammoth terrace deposits is a product of rapid deposition. Allen and Day also suggest that the formation of denser travertine, found locally about a meter below the surface, may result from either slow deposition or precipitation of calcite into the pore spaces of older deposits. The cross-section through part of an old fissure ridge (fig. 6) also supports the idea that dense travertine is a product of slow deposition. The right half of the figure shows horizontally layered porous travertine that forms the bulk of the fissure-ridge deposit. Vertically banded layers in the left half of the figure are composed of nonporous dense travertine that precipitated (presumably over a long period of time) along the interior walls of the fissure ridge. Figure 7 shows a sample of porous travertine in which the pore spaces have been partially filled.

Porous travertine of drill core (fig. 5). Specimen is from 73 cm below the surface, drill hole Y—10.

Dense vertically banded travertine (fig. 6). Specimen lines an old channel in a partially collapsed fissure ridge of the Highland Terrace area. Coin (circled) is 1.8 cm across.

Dense travertine of drill core (fig. 7). Specimen is from 14.1 m below the surface, drill hole Y—10. Channel (c) is partly filled by younger lighter colored travertine (arrow).