Carbonate Precipitation from Moderately to Extremely Supersaturated Waters: An Example of Significance of Microenvironmental Parameters

Henry S. Chafetz, Patrick F. Rush, Nancy M. Utech

Calcite and aragonite are precipitating from springs along highway U.S. 550 between Durango and Silverton, Colorado. Spring waters issue at 33°C; pH of 6.1; approximately 590 ppm calcium, 78 ppm magnesium, 5.5 ppm strontium, and 0.58 ppm manganese; a P_CO2 of 1.3; and approximately 2.5-4.5 times saturation with respect to calcium carbonate (I_sat = 2.5-4.5). Temperature downflow ranged between 23° and 36°C, depending on the time of day; pH increased to 8.0; P_CO2 decreased to 0.01; and I_sat reached a high of 77. Degassing of CO₂ drives this system to its extremely high states of supersaturation.

Calcium carbonate precipitation ranged downflow from well-developed 2 to 4-µm calcite rhombs (I_sat = 2-5) to spectacular 20-µm "skeletal" calcite crystals (I_sat = 50-77) to hemispheroid, dumbbell-and stellate-shaped accumulations of microsize aragonite needles at extremely high saturation levels.

Calcite and aragonite precipitated on the travertine mound as floating crusts, encrustations around gas bubbles (spring peas), copper pennies, etc. Formation of spring peas illustrates the significance of microenvironments in controlling the carbonate precipitate. Peas are initiated as a coating of 5-µm stellate clusters of aragonite needles around oxygen bubbles; the aragonite clusters are overlain by rhombs of calcite. We believe the highest level of supersaturation exists in the thin film of water immediately adjacent to the bubbles, where there is a maximum gradient for degassing of CO₂ from the water into the "pure" bubble of oxygen. Once CO₂ degasses into the oxygen, the gradient is diminished, the microenvironment immediately adjacent to the bubble is no longer as extremely supersaturated, and calcite precipitates rather than aragonite.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.