Evaporites deposition

Resources for Potash Fertilizers. Potassium is the seventh most abundant element in the earth s cmst. The raw materials from which postash fertilizer is derived are principally bedded marine evaporite deposits, but other sources include surface and subsurface brines. Both underground and solution mining are used to recover evaporite deposits, and fractional crystallization (qv) is used for the brines. The potassium salts of marine evaporite deposits occur in beds in intervals of haUte [14762-51-7] NaCl, which also contains bedded anhydrite [7778-18-9], CaSO, and clay or shale. The K O content of such deposits varies widely (see Potassium compounds). 

Boron is found in two underground ores, ulexite and colemanite. Research and pilot-plant studies have been completed to solution mine these ores, but as yet it is not done commercially. Boron is found in many different evaporite deposits (17). 

Sodium chloride, by far the most abundant compound of chlorine, occurs in extensive evaporite deposits, saline lakes and brines, and in... 

Chlorine is the twentieth most abundant element in crustal rocks where it occurs to the extent of 126 ppm (cf. nineteenth V, 136 ppm, and twenty-first Cr, 122 ppm). The vast evaporite deposits of NaCl and other chloride minerals have already been described (pp. 69, 73). Dwarfing these, however, are the inconceivably vast reserves in ocean waters (p. 69) where more than half the total average salinity of 3.4 wt% is due to chloride ions (1.9 wt%). Smaller quantities, though at higher concentrations, occur in certain inland seas and in subterranean brine wells, e.g. the Great Salt Lake, Utah (23% NaCl) and the Dead Sea, Israel (8.0% NaCl, 13.0% MgCU, 3.5% CaCU). 

Evaporite deposition is a much more episodic process and thus difficult to quantify. Because seawater is significantly undersaturated with respect to common evaporitic minerals, like gypsum and halite, evaporites are only formed when restricted circulation develops in an ocean basin in which evaporation exceeds precipitation. A geologically recent example is the Mediterranean Sea of 5-6 Myr ago. At this time excess evaporation exceeded the supply of ocean water through shallow inlet(s) from the Atlantic Ocean. As salinity increased, first CaS04, then NaCl precipitated. Over time, salt deposits 2-3 km thick formed. This thickness represents about 40 desiccations of the entire... 

However, not all the chloride is cyclic, a fact first appreciated in recent years. An example comes from a detailed study of river geochemistry conducted in the Amazon Basin. In the inland regions, rains typically have a chloride content of 10 pM, while major inland tributaries have chloride contents of 20-100 pM. These data suggest that only 25% of the Cl is cyclic, whereas 75% is derived by weathering of evaporites. Indeed, 90% of this 75% can be shown to have its origin in the Andean headwaters, derived from evaporites that make up only 2% of the area of the Amazon Basin (Stallard and Edmond, 1981). As the ratio of sulfate to chloride in evaporite deposits is generally much higher... 

Solution of halite present as bedded or domal Na-Cl salt-evaporite deposits. Na-Ca-Cl... 

Butler, G.P., 1969, Modem evaporite deposition and geochemistry of coexisting brines, the sabkha, Trucial Coast, Arabian Gulf. Journal of Sedimentary Petrology 39,70-89. 

Perthuisot, J. P, 1980, Sebkha el Melah near Zarzis a recent paralic salt basin (Tunisia). In G. Busson (ed.), Evaporite Deposits, Illustration and Interpretation of Some Environmental Sequences, Editions Technip, Paris, pp. 11-17, 92-95. 

Figure 4. Histogram of 6 Cl in evaporite deposits fields relative to Cl/ Cl in SMOC (vertical dashed line at 0%o). N is the number of analyses, and references (bracketed numbers) are as in Figure 3 and [12] Eggenkamp et al. 1995 [13] Eastoe and Peryt 1999 and [14] Eggenkamp and Schuiling 1995.

Figure 12. Models and data showing variations in the concentration of Ca in the oceans over Phanerozoic time. Figure adapted from Horita et al. (2002), who used the mineralogy of evaporite deposits to infer the values shown as filled circles and bars.

The formation of substantial evaporite deposits requires two conditions (1) some mechanism by which salt ion concentrations are kept at supersaturated levels, and (2) a steady resupply of salt ions. The hydrogeologic setting that is most likely to meet these two criteria are shallow-water embayments located in arid climates where sea level is relatively stable and terrestrial runoff is very low or absent. 

Schematic longitudinal profile through a semi-isolated basin located in a hot, arid climate and separated from the open sea by a narrow portal. The sill depth, although shallow, is still great enough to permit some two-way flow of surface water. The lines show inferred seawater density (g/cm ) and the arrows show current directions. The pattern of evaporite deposition is based on the relationships between brine density and precipitate composition as shown in Figure 17.1, assuming that salt particles accumulate on the seafloor through the process of pelagic sedimentation. Source-. From Scruton, P. C. (1953). American Association of Petroleum Geologists Bulletin, 37, 2498-2512.

Timeiine of marine evaporite deposition during the Phanerozoic. Shown are the volumes of NaCI (halite, dark line) and CaS04 (gypsum and anhydrite, dashed line) deposited over time in km . The arrows mark the current volumes of NaCi and CaS04 contained in modern ocean water. These are approximately 1.8 x 10 and 9 x 10 km , respectively. Source After Holser, W. T. (1984). Patterns of Change in Earth Evolution, Springer, pp. 123-143. 

Evaporite deposition also occurred in barred basins whose shallow sills restricted water exchange with the open ocean. Several modes of formation have been proposed for these basinal evaporites as illustrated in Figure 17.10. First, if sea level dropped... 

Thus, the Mediterranean Sea must have been refilled in an episodic fashion such that conditions favoring shallow-water evaporite deposition were rapidly reattained. Some geologists have proposed that this was achieved via periodic inflows of seawater from the Atlantic Ocean over the exposed Gibraltar Sill into a nearly dry Mediterranean Sea basin. This must have taken the form of a waterfeU hundreds of meters in height The episodic nature of this process is reflected in the repeating evaporite sequences foimd throughout the Messinian deposits. 

Considerable geographic variability exists in the distribution of the source rocks contributing salts to river and groundwaters. As shown in Table 21.3, most of the evaporites, which are the dominant natural source of Na and Cl in river water, lie in marginal and endorheic (internal) seas. Some of these subsurfece evaporite deposits dissolve into groundwaters, which eventually carry Na and Cl into the ocean. Carbonates are the prevalent rock type between 15°N and 65°N. Precambrian-age crustal rocks and meta-morphic minerals predominate between 25°S and 15°N and north of 55°N. Shales and sandstones represent on average 16% of the terrestrial surfece lithology. 

Salts Atmospheric seasalt cycling, pore water burial, evaporite deposition -6.1 C -5.7 -0.3 -0.3 -0.06 -0.1 ... 

From the above observations, it is certain that corrensite is a mineral which will form in normal sedimentary rocks and weathering environments. Thus it will be stable throughout the full range of clay mineral physical environments. It forms in sedimentary rocks which contain important quantities of iron (divalent) or magnesium but is not necessarily related to evaporite deposits and thus alkaline conditions of formation. 

Salt deposits or evaporites precipitate from evaporating seawater that becomes trapped in semi-isolated marine basins. Salty desert lakes, such as Great Salt Lake, Utah, USA, or those in Death Valley, California, USA, are also sites of evaporite deposition. Common salt minerals include halite (NaCl), sylvite (KC1),... 

Salts seem to be nearly as widespread as ice (though not as abundant) in the Solar System, being present on nearly every object where ice has also been observed and where indications of a watery past are in evidence (Enceladus is the key exception so far). There are very few instances (Venus is the most notable one) where salts occur on worlds where H2O is absent or nearly absent. This is a striking correlation, given the terrestrial situation of most evaporites, which are mostly associated with hot and arid areas of Earth. Even so, notable instances of ice-associated evaporites also occur on Earth. In detail, each world and the paragenesis of its salts tells a different story. The key point here is that in this Solar System, salts commonly are associated directly with ice. Hence, phase equilibria generally are described by FREZCHEM, even if on Earth some of the most important evaporite deposits were not formed in the presence of ice or icy-cold conditions. 

The same phenomenology must be important locally on Earth, too, where thick evaporite deposits of hydrated salts and local thick beds of methane clathrate in permafrost or seafloor sediments should influence the thermal environment of the crust. The predicted control on the crust s thermal state by hydrate deposits should have consequences for the localization of hydrothermal springs around and within evaporite basins, hydrothermal metamorphism... 

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