5 Temperature Control of Mineral Deposition

Groundwater in an unconfined aquifer is separated into the saturated zone, capillary fringe, and unsaturated zone (Figure 15). In both the saturated and capillary zone, the pore spaces are completely filled with water; in the unsaturated zone, both gas and water fill the pore spaces. Relative humidity is 100 percent in the portion of the unsaturated zone that is more than a few centimeters below land surface, so evaporation seldom occurs from the unsaturated zone. The flux of water evaporated from the unsaturated zone declines exponentially with depth from the surface to the top of the capillary zone (Ripple et al., 1970). If depth to the top of the capillary zone is more than a few meters, then there is almost no upward flux of water. There is, however, some direct water vapor removed from the unsaturated zone by diurnal barometric pumping that moves air and water vapor into and out of the subsurface.

Figure 15 – Illustration of relative position of water table, capillary fringe, and unsaturated zone (modified from Alley et al., 1999).

The capillary fringe is typically on the order of 0.01 m thick in gravel, up to 1 m thick in fine sand, and 3 m or more in clay-sized material. Evaporation of water occurs at a much greater rate from the water table or capillary zone where they intersect the surface. The area of water surface exposed to the atmosphere is a function of porosity. As a first approximation, we can say that evaporation rate from the soil will be on the order of porosity time the evaporation rate from a Class A evaporation pan in the area. That is, where the saturated or capillary zone intersect the surface in a material with 25 percent porosity, evaporation will be approximately 25 percent of the amount evaporated from a Class A evaporation pan.

Authigenic minerals such as calcite, dolomite, gypsum and anhydrite that commonly occur in the capillary zone (Figure 16) are referred to as evaporites. This suggests that they reach thermodynamic supersaturation by removal of water that concentrates the solutes. However, the pore spaces in the capillary zone are filled with water so evaporation only occurs at the top of the capillary fringe. Thus, mineral precipitation a meter or so below the capillary surface is likely not a result of concentration due to evaporation. It is proposed that mineral precipitation in the capillary zone is controlled by seasonal temperature and solute flux variation, not evaporation.

Figure 16 – Authigenic gypsum, anhydrite and minor carbonate minerals forming in the capillary zone of a coastal sabkha in the Emirates of Abu Dhabi, United Arab Emirates. This nearly homogeneous aeolian sediment (with grain sizes ranging from 0.16 to 22 mm) comprised of 65 percent detrital carbonate and 35 percent quartz that was deposited between 8000 and 9000 years ago, has been receiving an upward brine flux of 4 mm/year from underlying deep geologic formations. Conservative solutes have been concentrated over geologic time by evaporation (Wood, et al., 2002) and recycled by density driven convection (Van Dam et al., 2009 and 2014; Wood and Böhlke, 2017).

The conceptual model for this thermal induced precipitation in the Abu Dhabi sabkha is as follows:

• solutes and water are transported upward toward the surface from deeper formations in response to the low topographic position of the sabkha on the ocean edge;
• evaporation from the surface concentrates the solutes by removing the water; and,
• during transport through the capillary zone, the solutes experience large changes in both seasonal temperature and transport rates. Transport rate varies as a function of varying evaporation rates. Temperature variation changes their solubility resulting in precipitation or solution depending on the current conditions.

Thus, the interplay between changing temperature and changing solute flux, determines the type of mineral precipitation and its spatial distribution. There is a net mineral accumulation in the capillary zone, but it is a dynamic system with both mineral precipitation and solution occurring at different times. The sabkha of Abu Dhabi experience a seasonal variation of at least 20 °C (Figure 17) and evaporation is limited to summer months (Figure 5) leading to time-dependent mineral deposition and brine formation. Figure 17 illustrates the annual temperature variation at different depths that control the dissolution/precipitation of minerals and are particularly important to the behavior of retrograde minerals (minerals that become less soluble with increasing temperature). Mineral equilibrium values are a function of temperature (Figure 18) so the rate of precipitation and dissolution varies with depth from season to season as the surface temperature propagates downward in the system. In a warmer phase of the annual heat fluctuation, retrograde minerals precipitate; in a cooler phase, they dissolve. Mineral equilibrium coefficients do not vary linearly with temperature. When annual temperature variations combine with a seasonal water flux that is greater during warm periods (when thermodynamic activity of atmosphere is lower), a net accumulation of retrograde minerals can occur (e.g., deposition of minerals such as calcite, aragonite, dolomite, gypsum, and anhydrite).

Figure 17 – Annual temperature variation for six depths in the sabkha of the Emirate of Abu Dhabi illustrating the potential for thermal control of retrograde minerals.

Figure 18 – The equilibrium constants as a function of temperature illustrate the retrograde solubility of calcite, aragonite, dolomite, gypsum, and anhydrite compared to normal solubility of halite with increasing concentration with temperature (data from PHREEQC as reported in Parkhurst and Appelo, 2013).

Retrograde mineral formation is controlled by temperature, not by concentration due to evaporation, as is commonly assumed. Thus, calcite, aragonite, dolomite, gypsum, and anhydrite should be called thermalites not evaporites. Normally soluble minerals such as halite and sylvite are concentrated on the surface as true evaporites (Figure 19). Correct thermodynamic interpretation of the origin of these deposits is important for understanding the millions of hectares of land around the world that are currently undergoing salinization and for accurate reconstruction of the tectonic, sedimentological, geochemical, and paleo-climatic conditions of the geologic past.

Figure 19 – Sabkha surface showing halite (NaCl) forming in large polygonal features on the surface as a result of evaporation from a coastal sabkha in the Emirate of Abu Dhabi (Wood et al., 2002).