4.1 Chemically Closed Systems

In a closed system, the solute concentration increases with evaporation eventually exceeding the K_iap/K_eq ratio of the lowest solubility minerals (K_iap is the ion activity product of the solutes; K_eq is the equilibrium constant of the minerals in the water). Those minerals precipitate, changing the remaining solute concentration and ionic ratios controlling the next mineral that will precipitate. This process is conceptually analogous to the well-studied process of solidus and liquidus in a cooling igneous magma. For a closed system at a given temperature, the evolution of solutes and minerals can be predicted for any known solute input ratio. This results in a set order of mineral deposits as was first demonstrated for sea water by Usiglio in 1849. Hardie and Eugster (1970) stated this concept in a modern thermodynamic paradigm.

Consider the evolution of calcium (Ca⁺²) in seawater in a closed system at 25 °C and atmospheric CO₂ pressure. As evaporation proceeds, the least soluble mineral calcite (CaCO₃) precipitates from solution removing Ca⁺² ions. As evaporation proceeds further, calcite continues to precipitate removing Ca⁺² and CO₃^–2 ions until either all the Ca⁺² ions or the CO₃^–2 ions are exhausted. In the case of seawater, the carbonate ion is completely consumed before the Ca⁺² ion. Then, as evaporation proceeds the next least soluble mineral gypsum (CaSO₄^. 2H₂O) will precipitate. Gypsum continues to precipitate until either Ca⁺² or SO₄^–2 is completely consumed. In the case of seawater at 25 °C there is more SO₄^–2 than Ca⁺² thus, all calcium is consumed marking the end of precipitation of calcium bearing minerals. Evaporation proceeds to the next least soluble common mineral, halite (NaCl), and it precipitates until all of the Na⁺¹ or Cl^–1 ions are consumed. This process yields the evolution of minerals and solutes during the evaporation of seawater observed by Usiglio (1849). That is, the specific and characteristic assemblage of mineral deposits is controlled by mineral solubility. If the temperature is cooler than the 25 °C used in the example above, the mineral mirabilite will form rather than gypsum as gypsum exhibits retrograde solubility, meaning it becomes more soluble with decreasing temperature.

Several problems are commonly encountered with this closed system model when brines or evaporate sediments are examined in detail. The first is the lack of mass balance (Eugster and Jones, 1979). The mass balance of a closed system requires that the sum of any dissolved solutes entering a basin must equal the amount of precipitated mineral phase in the basin plus the amount remaining in solution. Observations of lacustrine systems suggest there is almost always missing mass (Alderman, 1985; Spencer et al., 1985). That is, the mass of some constituents in the evaporite deposits is significantly less than that calculated using reasonable input fluxes. This problem is frequently addressed in the literature by changing the input solute with time to match the observed chemistry. This, however, is scientifically unacceptable unless there is independent evidence for the changing input.

The second problem is the lack of an equilibrium mineral assemblage. If a volume of water of known solute chemistry evaporates, a definite sequence and amount of evaporite deposition can be predicted based upon thermodynamic considerations. The predicted relative abundances of the evaporites, however, are seldom observed (King, 1947). In fact, one factor that makes many of these evaporite deposits economically interesting is the accumulation of thick deposits of one or a few selected minerals, rather than the collection of the many different minerals that would be expected from repeated basin fillings and subsequent evaporations. This is an enigma for a closed system but is resolved if one considers a partially open “leaky” system.