3 Source of Solutes
In general, most terrestrial evaporative environments are the result of regional discharges of groundwater and surface water that transport solutes and water to where the water is removed by evaporation, thus concentrating the solutes. In some cases, deep geologic brines are discharged directly to the surface. These brines originate in a variety of ways (Figure 8) including dissolution of relic salt minerals left from evaporating seas (e.g., CaSO4, Figure 8a), release of fluid inclusions (Figure 8b) and water-rock interactions along mineral crystal boundaries (Figure 8c) (Roedder, 1984; Kamineni, 1987, Nordstrom et al., 1989; Frape and Fritz, 1987; Lowenstein and Risacher, 2009). Fluid inclusions are often abundant in granitic rocks. The fluid can range from 0 to 70 percent NaCl (Roedder, 1984) with an average composition of perhaps 10 percent NaCl (Nordstrom and Olsson, 1987). Sulfuric acid generated by pyrite oxidation dissolves the host rock generating a source of solutes (Edmunds et al., 1987) as shown in Figure 8c. The acid dissolves oligoclase, plagioclase, and biotite in density-driven convective hydrothermal-flow systems where hot, less dense water, is transported to the cooler oxygenated surface where its density increases and it sinks to repeat the cycle of dissolution and transport.
Figure 8 – Illustration of processes that add solutes to groundwater and form brine when the water evaporates: a) mineral dissolution; b) release of fluid inclusions; and, c) water-rock interactions.
In polar environments, water freezes and excludes most of the solutes so that ice is essentially free of solutes. If there is a topographic depression where the excluded solutes collect, brines can form through multiple freezing and thawing events (Frank and Gui, 2010; Starinsky and Katz, 2003). The process is analogous to the increase of solute concentration resulting from evaporation.
Radiolysis has been proposed as a source of brines in crystalline rocks. It is the process by which water is decomposed by alpha, beta and gamma rays from radioactive material thus concentrating the solutes (Vovk, 1981). More recent work, however, suggests that this is an unlikely source of most brines (Frape and Fritz, 1987).