EVAPORATIVE CHEMICAL EVOLUTION OF NATURAL WATERS FROM VOLCANIC TERRAINS

We report results from a series of open-system (i.e., constant atmospheric conditions) laboratory experiments on the evaporative chemical evolution of two different waters, representative of natural waters from volcanic terrains in the western United States: a Na-HCO3-CO3 water and a Ca-Cl-SO4-rich water. Our experiments indicate the evolution of high pH, Na-HCO3-CO3 brines from the Na-HCO3-CO3 water and the evolution of near-neutral pH brines from the Ca-Cl-SO4-rich water. The minerals recovered after complete evaporation of the Na-HCO3-CO3 experiments include amorphous silica, aragonite, calcite, halite, niter, thermonatrite and, possibly, gypsum, anhydrite and hectorite. Tachyhydrite and gypsum were observed to form in the experiments with the Ca-Cl-SO4-rich water. Limitations in theory and data regarding chemical speciation in highly-concentrated saline solutions, which restrict the usefulness of conventional thermodynamic modeling studies as a tool to assess the evaporative evolution of these waters, will also be discussed.