Salty Matters

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Non-solar thick salt masses: Part 1 - Mixing zone and cryogenic (freeze-dried) salts

John Warren - Saturday, May 18, 2019

Introduction

Conceptually, what defines evaporite sediment is broad, within the general theme that the rock was precipitated initially during solar heating of a brine. Heating drives the loss of water as vapour and concentrates of the residual brine to elevated salinities where a suite of evaporite salts precipitate (Babel and Schreiber, 2014; Warren 2016). In a modern marine-fed brine pan the precipitative sequence evolves from carbonate through gypsum, to halite and on into the various bittern salts (Figure 1).


But, evaporite salts are also highly reactive. Even in a simple single-pan evaporation scenario, crystallising salts tend to alter, backreact, replace, dissolve and reprecipitate. Then, as bedded sedimentary salts attain the subsurface and are exposed to increasing temperature and pressure, they continue to alter. Hydrated minerals, such as gypsum or carnallite transform into anhydrous phases such as anhydrite or sylvite. At the same time, thick halite beds tend to flow into halokinetic salt structures with a complete loss of depositional textures.

Some authors restrict the term "evaporite" to sediments forged by evaporation and use "saline deposit" or "salt deposit" for units formed not only by evaporation but also by ongoing alteration, cooling, heating or salting out in hydrothermal and diagenetic settings. Hence, the term salt structure rather than evaporite structure to describe many subsurface features encompassing the outcomes of halokinesis (salt flow). Some authors have suggested other names for the various bedded salt rocks precipitated by mechanisms other than solar heating of a brine; however, these names (burial salt, hydrothermal salt, reactionite, mixing precipitate, thermalite, replacementite, etc.) are not in general use in the sedimentological community.

In the subsurface sedimentary realm, most of the diagenetic processes described in preceding paragraphs have acted on a mass of salt first deposited by solar heating of brine and so are considered "true evaporites". This is so even though textures, total volumes of salts and mineralogies may have changed. Diagenetic processes acting on evaporite masses may also have transposed portions of the original salt mass into nearby saline cements within adjacent non-evaporite lithologies. These diagenetic or burial salt sediments lie outside the focus of this present series of articles, which will discuss mechanisms capable of precipitating large volumes of salts on or near the earth's surface without solar-driven heating of a brine (see Warren, 2016; Chapter 1 and 8 for a discussion of diagenetic or burial salts).

Cryogenesis, brine-mixing and mantle-driven thermal processes are the main surface and shallow-subsurface processes capable of precipitating significant volumes of non-evaporite salts. Many of the salt bodies precipitated in this way have similar mineralogies to those found in evaporite successions. So this, and the next Salty Matters article focus on mechanisms and products created by non-evaporite precipitation. I will attempt to define criteria that allow their separation from "true" evaporites. In this first article, we focus on mechanisms of brine mixing and cryogenesis, in the second we shall look at salt masses crystallising from fluids created and driven by mantle heating and cooling.

Some basic salt chemistry

Before that, we need to discuss a few basic chemical properties that define a crystal's response to heating and cooling of an enclosing brine. The questions we must ask are; in the ambient conditions are we dealing with influencing a prograde or retrograde salt phase and does the particular salt of interest undergo congruent or incongruent dissolution?

Prograde or retrograde salt?

A prograde salt crystallises as a brine cools and dissolve as brine is heated. With prograde salts, the average kinetic energy of the molecules in brine increases with temperature. The increase in kinetic energy allows solvent molecules to more effectively break apart the solute molecules; hence solubility increases with temperature and decreases with cooling. Halite and sylvite are both prograde salts (Figure 2a). When a shallow surface brine rich in either NaCl or KCl at say 45-50°C cools overnight, it will precipitate halite or sylvite-carnallite on the pan floor. If it sinks into an underlying porous salt bed and cools, it will precipitate as intercrystalline cement. This is happening beneath the salt flats of Dabuxum Lake in the Qaidam Basin of China where intercrystalline carnallite fill pores in a halite bed. In the deeper crust, with ongoing magmatic heating, the solubility of sodium chloride increases to where just below its critical point (≈400°C), salt content in a NaCl brine could be as high as 40 weight percent (see next article).

 

A retrograde (temperature-inverse) salt precipitates with increasing temperature and dissolves if it sits in a cooling saturated brine. Anhydrite is a retrograde salt and like all retrograde salts produces heat when dissolved in water (exothermic reaction). In an exothermic reaction the resulting additional heat shifts the equilibrium towards the reactants (Ca + SO4 --> CaSO4) (Figure 3b). In contrast, the hydrated form of CaSO4, gypsum, follows a prograde solubility curve, whereby in the temperature range where gypsum is the stable CaSO4 phase (<40-45°C) it is increasingly soluble with increasing temperature (Figure 3a). At higher temperatures (>45-50°C) both gypsum (where stable) and anhydrite possess retrograde solubility. Thus, when two anhydrite-saturated waters of different temperatures mix in the subsurface, the result is supersaturated brine with a propensity to precipitate anhydrite. A combination of retrograde solubility and brine mixing helps explain why anhydrite is a commonplace hydrothermal precipitate in mid-oceanic ridges and smoker chimneys (see next article).

On land, beneath the immediate surface of Abu Dhabi and Saudi sabkhas, the heating of desert playa and sabkha waters, rising through the capillary zone, to temperatures above 35°C drives the precipitation of CaSO4 (as nodular anhydrite) in a fashion that Wood et al. (2002, 2005) terms a sabkha thermalite (see Warren, 2016 for sabkha discussion). A reverse thermal gradient (upward cooling) in the winter sabkha permits dissolution of some of the previously precipitated retrograde minerals. Because the negative thermal gradient is less in the winter than the positive thermal gradient of the summer, but with nearly the same water flux, the cooling water cannot dissolve all of the mineral mass that precipitated in the previous summer. Thus, there is a net accumulation of retrograde (thermalitic) anhydrite nodules and layers in the capillary zone of an Abu Dhabi sabkha (Wood et al. 2005 ).

Brine mixing drives precipitation or dissolution of salts

Van’t Hoff’s work in physical chemistry, which won him the first Nobel Prize in Chemistry, showed that salts would precipitate at chemically-suitable brine interfaces. At that interface, all that is needed is a mixing of waters of two saturation states, with respect to the mineral of interest (Figure 3a). When two waters that are saturated with a particular salt phase are mixed, the resulting solution can be undersaturated or supersaturated with respect to that particular phase (Figure 3a). The saturation state during mixing depends on the convex or concave shape of the solubility curve for the mineral phase of interest and the parameter of interest (ion concentration, temperature, salinity, etc.). The only requirement is that the solubility curve for that particular component is nonlinear.

 

Raup (1970, 1982) in several experiments showed how halite and gypsum could be precipitated by the mixing of two seawater brines of differing salinity and densities. Figure 3b plots experimental results for the mixing of various low and high-density seawater brines and the resulting amount of gypsum precipitated (Raup, 1982). A similar plot can be drawn for the mixing of more saline NaCl (seawater) brines (Figure 4a, b; Raup, 1970).

Hence blending brines with different temperatures or salinities can be an important salting-out mechanism in the calcium sulphate (gypsum/anhydrite) salt system where gypsum follows a nonlinear solubility trend, as do saturated brines in a halite-MgCl2 bittern system (Raup, 1970, 1982). Figure 2b shows the solubility curves of both gypsum and anhydrite plotted with respect to increasing temperature in pure water. For gypsum, it clearly shows that when two gypsum-saturated waters with different temperatures mix in its stability range then the resulting solution is undersaturated and so gypsum tends to dissolve (Upper left curve in Figure 2b). In contrast, gypsum drops out of solution when two brines of differing density mix, with the amount of gypsum dependent on the density contrast between the two brines (Figure 3b).

Anhydritic solubility drives complex diagenetic effects when CaSO4-saturated brines mix via dispersion into adjacent less saline brines (Figure 2b). This happens in brine reflux systems where dense CaSO4-saturated brine plumes, derived at the surface at halite saturation, sink into and interact with less saline brines held in underlying or adjacent anhydritic carbonates. If the temperature remains near constant the tendency in this zone of dispersion or mixing is to dissolve gypsum, creating vugular porosity in the interval below or adjacent to a thick salt sequence. If temperature decreases and the brine plume cools, with little change in ionic proportions due to mixing, then the tendency is to precipitate anhydrite. It is not a simple system, with temperature and mixing processes pulling the brine chemistry in opposite directions.

A similar salting-out of halite occurs when halite-saturated brine mix with either MgCl2 or CaCl2 brine (Figure 4a, b, respectively). In potash basins, MgCl2-saturated brines are created by the incongruent dissolution of carnallite, while CaCl2 brines can typify the brine products of basinal hydrothermal waters. In both cases, the result is a sparry, relatively inclusion-free halite cement. Depending or the location where this mixing occurred, the cement can be part of a bedded carnallite unit that is converting to sylvite, or it can be localised in a fracture fill or form an intergranular cement in a non-evaporite host lithology.


Salt precipitation, driven by brine mixing, occurs in many stratified at-surface and shallow subsurface diagenetic interfaces in evaporitic settings. But, it is in the context of the mixing of deeply circulating mantle brines that it may be capable of precipitating significant volumes of salt and it in this context we shall discuss it further in the next article.

 

Cryogenic salts

Cryogenic brines and associated salts require temperatures at or below the freezing point of the liquid phase. These salts crystallise from a cold, near-freezing, residual brine as it concentrates via the loss of its liquid phase, which is converting/solidifying to ice (Figure 5a). As brine concentration increases, the freezing temperature decreases and minerals such as ikaite, hydrohalite, mirabilite, epsomite, potash bitterns and antarcticite can crystallise from the freezing brine (Figure 5a, b; Table 1; Warren, 2016; Chapter 12). Brine freezing ends when the phase cehemistry attains the eutectic point is reached. This is the point when all compounds (including H2O) pass to the solid state. Depending on the initial mineralization and compostion of the brine, the eutectic point is reached between -21 and -54 °C (Marion et al., 1999; Strakhov, 1970).


Cryogenic concentration of seawater precipitates mirabilite at four times seawater salinity and hydrohalite at eight times. In contrast, evaporating seawater precipitates gypsum at 4-5 times the original concentration and halite and 10-11 times (Figure 6). Evaporative gypsum precipitation decreases the relative proportions of both Ca and SO4 in the brine, while cryogenic precipitation of mirabilite decreases the sulphate proportion and drives the inflexion of the Na cryogenic curve slightly earlier than Na inflexion created by the precipitation of evaporative halite. In both the freezing and the evaporation situations, the brine remains chloride dominant prior to bittern crystallisation. Freezing seawater becomes increasingly sulphate enriched to where sulphate levels exceed sodium around 20 times the initial concentration. Evaporating seawater remains a Na-Cl dominant brine until the bittern stage is reached around 60 times the initial concentration (Warren, 2016, Chapter 2).


Mirabilite (NaSO4.10H2O) is one of several sodium sulphate salts and is stable in sulphate brines at temperatures lower than a few centigrade degrees (Figure 7a). Hence, it is a commonplace cold-climate lacustrine precipitate (Figure 8a-d; Table 1). Mirabilite beds are commercially exploited in colder climates; their latitudinal and altitudinal occurrences illustrate an interesting climatic dichotomy inherent to economic deposits of the various sodium sulphate salts. One sodium sulphate grouping of exploited deposits is characterised by mirabilite precipitated via brine freezing, as in the Great Salt Lake, Karabogazgol and Hedong (Yucheng) salt lake (Table 1 and illustrated in Figure 8a-d). The other sodium sulphate salt grouping is characterised by varying combinations of glauberite (CaSO4.Na2SO4)/bloedite-astrakanite (Na2SO4.MgSO4.4H2O) salts, which crystallised at higher temperatures via the evaporation of continental brines in saline groundwater sumps in warm to hot arid climates (as discussed in Warren, 2016, Chapter 12).


The climatic dichotomy reflects the fact that sodium sulphate solubility in water changes as a nonlinear function of temperature (Figure 7a). Below 1.2°C, ice and mirabilite tend to precipitate as seawater or a sodium sulphate brine freezes. As the temperature increases above 0°C, increasing amounts of hydrous sodium sulphate (as the decahydrate, mirabilite) become soluble, while the anhydrous form (thenardite- NaSO4) becomes the precipitative phase in brines saturated with respect to the sodium sulphate. At 32.4°C in pure water, a transition point on the solubility curve is reached, whereby mirabilite melts in its water of crystallisation and thenardite crystallises. Presence of other dissolved salts changes the transition temperature and solubility characteristics of sodium sulphate due to the double salt effect.


Cryogenesis in saline lacustrine sumps

Every year, once the water temperatures drop below 5.5-6°C in late November in Karabogazgol, mirabilite precipitates as transparent crystals on the embayment bottom. The crystals are then transported by wind and wave action, especially during winter storms, into low dunes lining shore-zone (Figure 8b). By mid-March when the bay waters are heated to over 6°C, the mirabilite on the bay floor begins to redissolve. By July–August the entire precipitated mirabilite crop in the bay has redissolved. Historically, the period from November through March was a period of ‘‘harvesting’’ mirabilite on the bay shores. In summer with its arid climate, any remaining strandzone salt converts to thenardite (Na2SO4), and this too was gathered at the end of each summer.


Below the floor of Karabogazgol are four NaSO4 beds that are likely cryogenic remnants from colder climatic periods over the last 10,000 years (Figure 9; Karpychev, 2007). Back then, large amounts of mirabilite formed each winter, much like today but, unlike today, the colder more humid glacial climate meant the bay was not as subject to summer desiccation and warming. Dense residual bottom brines were perennially ponded and so preserved a summer-halite sealing bed. This allowed the underlying mirabilite/epsomite winter precipitates to be preserved across the lake floor. During the following winter, the process was repeated as mirabilite/epsomite/halite couplets stacked one atop the other to create a future ore horizon. In time, the combination of groundwater and exposure, especially nearer the Gulf’s strandzone, converted most of the mirabilite, along with epsomite, to astrakanite, and then both phases to glauberite in the upper three beds. This explains the association of the richer glauberite zones with the lake edges (Figure 9a; Strakhov, 1970). Water of crystallisation released by the subsequent mirabilite to thenardite conversion, slightly diluted any strong residual brines, facilitating a dominant sodium-sulphate mineral and brine composition across the bay (Kurilenko et al., 1988).

Winter mirabilite also crystallises cryogenically from Kuchuk Lake brines on the Kulunda Steppe, southwest of Novosibirsk, western Siberia (Kurilenko, 1997; Garrett, 2001. The lake area is 170 km2, and brine depth is around 3.2 metres. In 1938 that lake was estimated to contain some 540 million mt of equivalent sodium sulfate. Thick, glassy mirabilite occurs as two crystalline layers, the upper one is around 3 m thick, and both layers are pure containing <1% other soluble salts. In total, the crystalline mirabilite interval ranges up to 7 m thick, covers around 133 km2 and is overlain by a 0.05- to 2-m thick unconsolidated interval of mud and salt oozes. The ooze typically contains some 40.5% salts, 20.6% water, and 38.9% insolubles (including considerable gypsum). Much of the mirabilite in the upper ooze layer has been transformed into thenardite, with the previous water of crystallisation supply much of the dense brine held in the ooze, which also contains halite, glauberite, hydrohalite and epsomite.

Mirabilite crystallises from the lake brine during the winter and cool summer evenings (volume estimated to be ≈ 340-580 thousand mt/yr of mirabilite). Then, during the warm summer months, some of it converts to thenardite. A limited amount of insoluble accumulates with the crystals, forming thin layers of mud with the thenardite. Brine in the lake has a 10-31% soluble salt content, depending upon the season and lake level. Every every three years, at the end of summer this brine is pumped to solar ponds to allow cryogenic mirabilite to crystallise during the autumn (a process similar to the production of mirabilite in the Canadian Salt Lake (Warren 2016, Chapter 12). Residual brine in the ponds is returned to the lake before winter sets in, and the ponds are harvested as needed for the production of sodium sulfate (Charykova et al., 1996). For most of a year, the lake's surface brine is a magnesium chloride water, but during the summer it changes to a sodium sulfate base, because of the dissolving of the underlying mirabilite, thenardite, and glauberite held in the lake floor oozes.

Ebeity (Ebeyty) Lake, located 110 km west of Omsk, is another cryogenic salt lake with a cyclic pattern of mirabilite crystallising in the winter and having it dissolve in the summer (Garrett, 2001; Kolpakova et al., 2018). Brine concentration can reach 30-31% total salts by the end of summer and can begin to crystallise halite (which usually dissolves in the spring). Mirabilite deposition starts when the brine temperature in the lake is less than 18-19°C (which can be as early as August or September). At 0°C, 70% of the sodium sulfate has crystallised from the lake brine. At -10°C, 85% has been deposited, and at -15°C 98%. At -7°C some ice crystallizes with the mirabilite, and at -21.8°C hydrohalite forms. The lake does not freeze solid because of the insulating effect of surface layers of snow on top of floating mirabilite rafts, but brine temperatures of -23.5°C have been recorded (Strakhov, 1970). The winter deposit of mirabilite, with some hydrohalite, covers the entire lake bottom is 25-30 cm thick and is quite pure. Laboratory tests have shown the soluble salts in this mirabilite, including hydrohalite, can be almost completely removed (i.e., reduced to 0.08%) by a single stage of washing.

Hydrohalite is a stable precipitate in a freezing brine only when the water temperature is below 0°C. In a NaCl–H2O system in the laboratory, hydrohalite is a stable phase that begins to precipitate cryogenically at temperatures below 0.12 °C, forming hydrohalite and a brine solution until it reaches the eutectic point for a solution saturated with NaCl at −21.1°C, where the remaining solution freezes (Figure 7b). Above 0.12 °C hydrohalite melts incongruently and decomposes to NaCl and a NaCl-saturated solution, losing 54.3% of its volume (Craig et al., 1974, 1975; Light et al., 2009). Hydrohalite (NaCl.2H2O) crystals have pseudo-hexagonal cross sections and are found in a number of modern cold saline lakes and springs (Table 1, Figure 8e-f).

Cryogenesis in salty springs

The mutual occurrence and downdip evolution of mirabilite/thenardite and hydrohalite/halite in brine spring encrustations (barrage structures) downdip of Stolz diapir on Axel Heiberg Island in the Canadian Archipelago illustrate the ephemeral nature of cryogenic salts on the Earths surface, even in extremely cold settings (Fox-Powell et al., 2018; Ward and Pollard, 2018). The halite-exposed core of the Stolz dome rises some 250 m above the adjacent flood plain, while the salt/hydrohalite deposit occurs autochthonously within a narrow, steep-sided tributary valley carved by a small stream fed mainly by perennial groundwater discharge emanating from the base of the diapir. The host valley begins abruptly at the spring outlet and is incised through surficial colluvial and glacial sediments into steeply dipping bedrock (shale). The Stolz diapir is the only diapir within the archipelago where the halite core is exposed, and the surface is extensively karstified with a suffusion cover, along with several large sinkholes and collapse structures.


The cryogenically influenced part of the at-surface salt deposit is approximately 800 m long and is thickest at the spring outlet (≈4.0 m) and gradually thins down-valley until it fans out, creating a salt pan that extends 300 m into the Whitsunday River floodplain. The morphology of the deposit is characterised by a series of salty barrage structures that staircase down the valley until the dispersing and dissolving at the valley opening. The barrages are constructed of salts but morphologically resemble typical fluvial travertines and tufas and are predominantly curvilinear with a downstream convexity, particularly the larger dams in the upper valley.

Sub-zero air temperatures persist at the site for at least ten months of the year, and the spring’s outlet temperature is relatively constant at −1.9°C ±0.1°C, confirming the presence of permafrost and so facilitating the precipitation of mirabilite and hydrohalite via freezing of spring waters (Figure 8f). In July air temperatures reach 5° - 6°C.

Although initial precipitates in the permafrost zone of the spring outflow are cryogenic (mirabilite and hydrohalite) the presence of these cryogenic salts in the spring precipitates is ephemeral (Figure 10).

The main body of a deposit is layered (Figure 4a, b) with alternating light and dark bands interpreted by Ward and Pollard (2018) as alternating periods of winter hydrohalite deposition and periods of summer pool drainage, when hydrohalite decomposes, and halite/thenardite sediment is deposited (Figure 11). As layers likely reflect an annual couplet cycle, then single winter accumulations can be as thin as a few millimetres in some parts of the deposit and as much as half a meter in others. It appears the accumulation phase ends as pools drain in early May corresponding with mean daily temperatures rising above 0°C. The darker layers are generated during summer as the hydrohalite decomposes leaving a granular halite crust with a veneer of fine clastic sediment transported into the deposit by wind, rain and runoff from adjacent slopes (during the spring snowmelt). The contact between the primary sediment and halite is abrupt and unconformable. The excavation of the deposit revealed numerous thin frozen layers (Figure 4c). These layers first appear ≈47cm below the surface, and overlying unfrozen material is considered analogous to an active layer in permafrost (Ward and Pollard, 2018). Samples of frozen salt collected from this layer and exposed to ambient air temperatures in summer reverted to a mixture of brine and halite grains (Figure 4d-e). It is not clear if these are residual hydrohalite layers preserved by thicker precipitate accumulations or if they represent secondary hydrohalite formation.


The lack of mirabilite or thernadite within the upper portion of the stream/spring deposit is thought to be due to the low kinetic rates of precipitation for sulfate salts (Ward and Pollard, 2018). Below the halfway point, thernadite is present (Figure 8f). This is also reflected in the SO4 concentrations along the spring in winter, as the precipitation of mirabilite removes sulfate ions from solution beyond the halfway point. Based on the morphology of the crystals observed in the deposit, the low concentration of sulfate ions compared to chloride ions, as well as the extensive identification of halite in the XRD samples, the deposit is dominated by hydrohalite (not mirabilite), which decomposes to halite in the summer and at the same time the mirabilite that is present dehydrates to thenardite (Figure 11).

Glacial and sea-ice cryogenesis

Whenever polar seawater freezes, salts precipitate in the increasingly dense residual brines held in inclusions or fissures in the ice (Butler et al., 2016). Salts that are known to precipitate within the freezing brine include CaCO3.6H2O (ikaite), Na2SO4.10H2O (mirabilite), NaCl.2H2O (hydrohalite), KCl (sylvite), and MgCl2.12H2O (magnesium dichloride dodecahydrate), while hydrohalite is the most abundant salt to precipitate in sea ice (Light et al., 2009). In the seawater system, hydrohalite begins to precipitate at -22.9C, and further cooling results in additional precipitation until the source of Na is exhausted at the eutectic (Figure 7b).

Salts do not only accumulate in sea ice. As dense brines and inclusion waters in flowing glacial ice sink into underlying rocks they can accumulate in ice sheet fissures at the base of the ice, or in load-induced fractures in the ice understory (Herut et al., 1990). Residual dense interstitial saline brines are found in pore waters extracted from deep cores sampling submarine sediments in McMurdo Sound, Antarctica (Frank et al., 2010). It seems that when ice sheets retreat, the at-surface cryogenic salts dissolve into a freshening at-surface hydrology, but dense hypersaline brines remain behind in deep fissures, held and preserved in the rock fractures (Starinsky and Katz, 2003).

In the extreme setting of at-surface brine freezing in small saline depressions in the Dry Valleys of Antarctica, a solid form of calcium chloride, antarcticite, grows cryogenically in what is probably the most saline perennial natural water mass in the world (47% salinity in Don Juan Pond, Antarctica; Figure 8g-h; Horita et al., 2009).


Mirabilite beds are known to be exposed atop ice floes of the Ross Ice Shelf near immediately down dip of the Hobbs glacier, on Cape Barne on Ross Island and in the vicinity of Cape Spirit, Black Island (Figure 12a; Brady and Batts, 1981). The bed is made up of relatively pure mirabilite that in places is more than a metre thick (Figure 12b). It is exposed in three coast-parallel ice pressure ridge systems and may not be continuous between the three sampling sites. According to Brady and Batts (1981), the mirabilite formed in response to a recent retreat of the Ross Ice Shelf that began some 840 years ago.

The upper contact of the ice beneath the McMurdo mirabilite bed is not conformable as there are often small irregularities and undulations in its surface ranging from 2 to 6 cm high (Figure 12b). These undulations control the thickness of a silty sand interval (0-8cm thick) separating the ice from the mirabilite. The upper surface of the basal sediment layer defines a sharp conformable contact with the overlying mirabilite bed. This basal sediment layer is devoid of internal bedding and consists of 80% glacial flour mixed with sand and small pebbles. The sediment also contains many ice-crushed fragments of marine diatoms and sponge spicules (< 10 µm long).

The overlying mirabilite bed is massive up to 1/2m thick and primarily made up of transparent cm-scale crystal clusters. Locally crystals can aggregate into large granules up to 95 mm across, that when exposed to air are coated by an anhydrous sodium sulphate powder rind Although there is no apparent bedding in the mirabilite bed, small pods and layered stringers of pebbly sand do occur. These are usually parallel or sub-parallel to the salt bed layer itself and vary in thickness from 0 to 12 cm. Broken shell fragments occur as rare isolated individual fragments in the salt. When the mirabilite is dissolved in distilled water, some fine mud and rare sand grains are recovered, as well as a perfectly preserved flora of non-marine diatoms.

A lag of sediment, pebbles, cobbles, and boulders covers the mirabilite bed. The majority of the class are erratics, some of which are striated and come from the McMurdo alkaline volcanic province. There are also some erratics of gneiss, granite, and sandstone from continental suites. This lag is mostly overlain by a non-marine microbial mat (0-26 cm thick) but, in some places, the mat underlies or is mixed with the boulder lag. One mat sample collected 4 m above the level of the pool-and-channel systems on a pressure ridge at site 1, yielded a radiocarbon age of 870±70 years n.p. This single date cannot be used to date the whole mat since algae are still growing in pools in small depressions atop the salt deposit. The algal mat contains non-marine diatoms, but these are not as numerous as in nearby pools on the present-day ice shelf.

Debenham (1920) suggested that the mirabilite he had observed on the Ross Ice Shelf was formed under the ice shelf by precipitation from brines. But it is unlikely that extensive linear pods and beds of friable salt could be brought directly to the surface by anchor ice; furthermore, the salt beds contain non-marine diatoms that indicate surface precipitation (Brady and Batts, 1981). Since non-marine diatoms have only settled in the salt itself, it would seem that the basal sediment layer was formed immediately after the injection of a sub-ice-shelf brine and before non-marine algal production in the brine pools.

Hence, Brady and Batts (1981) conclude that mother brines first formed underneath an ice shelf from freezing sea-water. These brines were displaced to the ice-shelf surface by the weight of a large area of thick ice shelf as it grounded. Fine marine sediment carried in suspension by these brines settled to form an irregular thin discontinuous basal sediment layer containing marine diatoms. Mirabilite then crystallised cryogenically from this brine. During precipitation of the massive mirabilite beds, some non-marine diatoms, which can tolerate the high salt content of Antarctic saline lakes, were deposited with the salt. After the deposition of the mirabilite, massive non-marine algal production occurred, forming a thick irregular mat up to 26 cm thick on the mirabilite surface.

Mirabilite beds lying on the Ross Island coast near Cape Barne and on the mainland near Hobbs Glacier likely formed in the same manner as those at Cape Spirit. That is, they were stranded on the coast as the ice shelf retreated to its present position in the south of McMurdo Sound during the last interglacial period.


Extraterrestial cryogenic salts

Cryogenic gypsum is released via ice ablation and is spread widely by katabatic winds across the circumpolar Martian dunefields (Table 2). Hydrated and calcium perchlorate cryogenic salts grow seasonally in soils of Mars and typify slope lineae on parts of the Marian surface Lineae activity is possibly tied to periodic release of liquid water in paces on the Martian surface. More than 3 billion years ago, evaporitic halite once precipitated in impact sumps on the surface of Mars.

Cryogenesis explains the presence of hydrated magnesium sulphate salts in megapolygonal ice-crack fissures that crisscross the icy surfaces of Europa and Ganymede (moons of Jupiter) (Table 2; (Figure 13). The presence of hydrated magnesium sulphate and sodium carbonate salts indicates the presence of liquid salty oceans up to 100 km deep below an icy crust that is tens of kilometres thick (McCord et al., 1998; Craft et al., 2016. The fissures indicate an icy type of plate tectonics driven by strong tides in response to the varying gravitational pull of nearby Jupiter. Similar plumes of icy saline water escape from cracks and cryovolcanoes on the surface of Enceladus, an icy moon that circle Saturn. Spectral analysis shows the escaping plumes of Enceladus contain a variety of sodium and potassium salts (Postberg et al., 2011.


Recognition of ancient terrestrial cryogenic salts

Outside of relatively unaltered cool-temperature Quaternary lacustrine, permafrost and ice examples, as listed in Table 1, dehydration linked to the heating inherent to burial diagenesis will create mineralogical and textural difficulties in reliably interpreting the remains of ancient cryogenic salt beds. This is because all ionic salts forming in ambient surface conditions become metastable in the subsurface as they experience increased temperatures, pressures and evolving pore-fluid chemistries inherent to the diagenetic realm (Warren, 2016).

Across all subsurface and re-exhumed ancient examples, the original low-temperature cryogenic salts (mirabilite, hydrohalite) will be long gone. Anhydrous salty remnants (thenardite, halite) may be preserved but are typically altered, replaced and dissolved. This makes it more challenging to assign a depositional setting to a cryogenic salt than it is to a typical evaporite.

In their study of Miocene lacustrine thenardite in the Tajo Basin, Spain, Herrero et al. (2015) used three main criteria to suggest the conversion from a cold climate mirabilite precursor They were; 1) inclusion chemistry in the thenardite, 2) cooler climate mammal fauna synchronous with deposition of sodium sulphate salts and, 3) widespread dewatering structures tied to a burial transition from mirabilite to thenardite. A perusal of the depositional settings in the Quaternary examples listed in Table 1, underlines the conclusion that a glaciogenic association should be added as a fourth recognition criterion. Almost every case is either underlain or overlain by varying combinations of glacial till, glacial flour or glacial laminites with dropstones (Table 3).


In terms of pre-Quaternary deposits of sodium sulphate salts, it was noted by Warren (2010, 2016), that the greater majority of economic deposits are Neogene sediments. Pre-Neogene cryogenic deposits are typically so diagenetically altered that no significant volumes of the original metastable NaSO4 cryogenic salts remain. Across all pre-Neogene greenhouse climate settings, that is across times that lack permanent polar ice sheets, there is little documentation of preserved volumes of cryogenic salts. Worldwide warmer temperatures mean it is unlikely there were extensive cryogenic salt beds. This leaves past periods in Earth history when the planet was in ice-house mode as possible times of cryogenic salt deposits were possible. As yet, no pre-Neogene cryogenic salt beds are known. Mirabilite/hydrohalite deposits have been inferred to be reasonable at tropical latitudes during the Neoproterozoic snowball period (Light et al. 2009; Carns et al., 2015)). Their presence has bee inferred to have increased albedo in tropical ice sublimation regions and so modify climate models, but as yet no evidence of their existence is known either then or in younger Ordovician or Permo-Carboniferous ice-age sediments. Likewise, the presence of glauberite in Permian lithologies is not diagnostic; glauberite forms from evaporating marine waters at times of MgSO4 -enriched oceans (Hardie 1985; Warren, 2016).

Cryogenic salts are an extreme end-member of the concept of "the salt that was." Across past geological time, beyond Neogene remnants, even with isotopic and inclusion techniques, it is next to impossible to identify a cryogenic evaporite reliably.

As W. Edwards Deming (Engineer and statistician, 1900-1993) once said "...Without data, you're just another person with an opinion."

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Brine evolution and origins of potash: primary or secondary? SOP in Quaternary saline lakes: Part 2 of 2

John Warren - Friday, November 30, 2018

 


Introduction

This, the second in this series of articles on potash brine evolution deals with production of sulphate of potash in plants that exploit saline hydrologies hosted in Quaternary saline sumps. There are two settings where significant volumes of sulphate of potash salts are economically produced at the current time; the Ogden salt pans on the northeast shore of the Great Salt Lake in Utah and Lop Nur in China. Although potassium sulphate salts precipitate if modern seawater is evaporated to the bittern stage, as yet there is no operational SOP plant utilising seawater. This is due to concurrent elevated levels of magnesium and chlorine in the bittern, a combination that favours the precipitation of carnallite concurrently with the precipitation of double sulphate salts, such as kainite (Figure 1). Until now, this makes the processing of the multi-mineralogic precipitate for a pure SOP product too expensive when utilising a marine brine feed.

 

Potash in Great Salt Lake, USA (SOP evolution with backreactions)

Today sulphate of potash fertiliser is produced via a combination of solar evaporation and brine processing, using current waters of the Great Salt Lake, Utah, as the brine feed into the Ogden salt pans, which are located at the northeastern end of the Great Salt Lake depression in Utah (Figure 2a). A simpler anthropogenic muriate of potash (MOP) brine evolution occurs in the nearby Wendover salt pans on the Bonneville salt flats. There, MOP precipitates as sylvinite in concentrator pans (after halite). The Bonneville region has a bittern hydrochemistry not unlike like the evolved Na-Cl brines of Salar de Atacama, as documented in the previous article, but it is a brinefield feed without the elevated levels of lithium seen in the Andean playa (Figure 3b).

Great Salt Lake brine contains abundant sulphate with levels sufficiently above calcium that sulphate continues to concentrate after most of the Ca has been used up in the precipitation of both aragonite and gypsum. Thus, as the brines in the anthropogenic pans at Ogden approach the bittern (post halite) stage, a series of sulphate double salts precipitate (Figure 4), along with carnallite and sylvite.


Great Salt Lake brines

The ionic proportions in the primary brine feed that is the endorheic Great Salt Lake water depends on a combination of; 1) the inflow volumes from three major rivers draining the ranges to the east, 2) groundwater inflow, 3) basin evaporation, and 4) precipitation (rainfall/snowfall) directly on the lake (Jones et al., 2009). Major solute inputs can be attributed to calcium bicarbonate-type river waters mixing with sodium chloride-type springs, which are in part hydrothermal and part peripheral recycling agents for NaCl held in the lake sediments. Spencer et al. (1985a) noted that prior to 1930, the lake concentration inversely tracked lake volume, which reflected climatic variation in the drainage. However, since that time, salt precipitation, primarily halite and mirabilite, and dissolution have periodically modified lake brine chemistry and led to density stratification and the formation of brine pockets of different composition.

Complicating these processes is repeated fractional crystallisation and re-solution (backreaction) of lake mineral precipitates. The construction of a railway causeway has restricted circulation, nearly isolating the northern from the southern part of the lake, which receives over 95% of the inflow. Given that Great Salt Lake waters are dominated by Na and Cl, this has led to halite precipitation in the north (Figures 2a, 3a; Gwynn, 2002). Widespread halite precipitation also occurred before 1959, especially in the southern area of the lake, associated with the most severe droughts (Jones et al., 2009; Spencer et al., 1985a). Spencer et al. (1985a) also described the presence of a sublacustrine ridge, which probably separated the lake into two basins at very low lake stands in the Quaternary. Fluctuating conditions emphasise brine differentiation, mixing, and fractional precipitation of salts as significant factors in solute evolution, especially as sinks for CaCO3, Mg, and K in the lake waters and sediments. The evolution of these brine/rock system depends on the concentration gradient and types of suspended and bottom clays, especially in relatively shallow systems.


Brine evolution across the Ogden pans

Figure 3a plots the known hydrochemistry of the inflow waters to the Great Salt Lake and their subsequent concentration. Evolving lake waters are always Na-Cl dominant, with sulphate in excess of magnesium in excess of potassium, throughout. Any post-halite evaporite minerals from this set of chemical proportions will contain post-halite potash bittern salts with elevated proportions of sulphate and magnesium and so will likely produce SOP rather than MOP associations. Contrast these hydrochemical proportions with the inflow and evolution chemistry in the pore brines of the Bonneville salt flat (Figure 3b) the Dead Sea and normal marine waters. Across all examples, sodium and chlorine are dominant and so halite will be the predominant salt deposited after aragonite and gypsum (Figure 4). Specifically, there are changes in sulphate levels with solar concentration (Figure 3b). In brines recovered from feeder wells in the Bonneville saltflat, unlike the nearby bajada well waters, the Bonneville salt flat brines show potassium in excess of sulphate and magnesium. In such a hydrochemical system, sylvite, as well as carnallite, are likely potassium bitterns in post-halite pans. The Wendover brine pans on the Bonneville saltflat produce MOP, not SOP, along with a MgCl2 brine, and have done so for more than 50 years (Bingham, 1980; Warren, 2016).


The mineral series in the Ogden pans

Figure 5 illustrates a laboratory-based construction of the idealised evolution of a Great Salt Lake feed brine as it passes through the various concentration pans. Figure 5 is a portion of the theoretical 25°C sulphate-potassium-magnesium phase diagram for the Great Salt Lake brine system and shows precipitates that are in equilibrium with brine at a particular concentration. Figures 4 and 5 represent typical brine concentration paths at summertime temperatures (Butts, 2002). Importantly, these figures do not describe the entire brine concentration story and local variations; mineralogical complexities in the predicted brine stream are related to thermal stratification, retention times and pond leakage. Effects on the chemistry of the brine due to the specific day-by-day and season-by-season variations of concentration and temperature, which arise in any solar ponding operation, require onsite monitoring and rectification. Such ongoing monitoring is of fundamental import when a pilot plant is constructed to test the reality of a future brine plant and its likely products.

 

Figure 6 illustrates the idealised phase evolution of pan brines at Ogden in terms of a K2SO4 phase diagram (no NaCl or KCl co-precipitates are shown; Felton et al., 2010). Great Salt Lake brine is pumped into the first set of solar ponds where evaporation initially proceeds along the line shown as Evap 1 until halite reaches saturation and is precipitated. Liquors discharged from the halite ponds are transferred to the potash precipitation ponds where solar evaporation continues as line Evap 2 on the phase diagram and potassium begins to reach saturation after about 75% of the water is removed. Potassium, sodium levels rise with further evaporation and schoenite precipitates in the schoenite crystalliser. After some schoenite precipitation occurs, the liquor continues to evaporate along the Evap 3 line to the point that schoenite, sylvite, and additional halite precipitate. Evaporation continues as shown by line Evap 3 to the point that kainite, sylvite and halite become saturated and precipitate. From this plot, the importance of the relative levels of extraction/precipitation of sulphate double salts versus chloride double salts is evident, as the evaporation plot point moves right with increasing chloride concentrations. That is, plot point follows the arrows from left to right as concentration of chloride (dominant ion in all pans) increases and moves the plot position right.


Production of SOP in the Great Salt Lake

To recover sulphate of potash commercially from pan bitterns fed from the waters of Great Salt Lake, the double salts kainite and schoenite are first precipitated and recovered in post-halite solar ponds (Figures 6, 7). The first salt to saturate and crystallise in the concentrator pans is halite. This is successively followed by epsomite, schoenite, kainite, carnallite, and finally bischofite. To produce a desirable SOP product requires ongoing in-pan monitoring and an on-site industrial plant whereby kainite is converted to schoenite. The complete salt evolution and processing plant outcome in the Ogden facility is multiproduct and can produce halite, salt cake and sulphate of potash and a MgCl2 brine product. Historically, sodium sulphate was recovered from the Great Salt Lake brines as a byproduct of the halite and potash production process, but ongoing low prices mean Na2SO4 has not been economically harvested for the last decade or so.

The complete production and processing procedure is as follows (Figure 7; Butts, 2002, 2007; Felton et al., 2010): 1) Brine is pumped from the Great Salt Lake into solar evaporation ponds where sodium chloride precipitates in the summer. 2) When winter weather cools the residual (post-halite) brine in the pans to -1 to -4°C, sodium sulphate crystals precipitate as mirabilite in a relatively pure state. Mirabilite crystals can be picked up by large earth-moving machinery and stored outdoors each winter until further processing takes place. 3) The harvested mirabilite can be added to hot water, and anhydrous sodium sulphate precipitated by the addition of sodium chloride to the heated mix to reduce sodium sulphate solubility through the common ion effect. The final salt cake product is 99.5% pure Na2SO4. 4) To produce SOP, Great Salt Lake brines are allowed to evaporate in a set of halite ponds, until approaching saturation with potassium salts. The residual brine is then transferred to a mixing pond, where it mixes with a second brine (from higher up the evaporation series, that contains a higher molar ratio of magnesium to potassium. 5) This adjusted brine is then allowed to evaporate to precipitate sodium chloride once more, until it is again saturated with respect to potassium salts. 6) The saturated brine is then transferred to another pond, is further evaporated and precipitates kainite (Figures 5, 6). Kainite precipitation continues until carnallite begins to form, at which time the brine is moved to another pond and is allowed to evaporate further to precipitate carnallite. 6) Some of the kainite-depleted brine is recycled to the downstream mixing pond to maintain the required molar ratio of magnesium to calcium in this earlier mixing pond (step 4). 7) Once carnallite has precipitated, the residual brine is transferred to deep storage and subjected to winter cooling to precipitate additional carnallite as it is a prograde salt. 8) Cryogenically precipitated carnallite can be processed to precipitate additional kainite by mixing it with a kainite-saturated brine. 9) MgCl2-rich end-brines in the post-carnallite bittern pans are then further processed to produce either MgCl2 flakes or a 32% MgCl2 brine. These end-bitterns are then used as a feedstock to make magnesium metal, bischofite flakes, dust suppressants, freeze preventers, fertiliser sprays, and used to refresh flush in ion exchange resins.

Some complexities in the observed mineral precipitation series in the Ogden Pans

Under natural solar pond conditions in the Ogden Pans, the brine temperature fluctuates with the air temperature across day-night and seasonal temperature cycles, and there is a lag time for temperature response in waters any brine pan, especially if the pan is heliothermic. Atmosphere-driven fluctuations in temperature results in changes in ion saturations, which can drive selective precipitation or dissolution of salts in the brine body. Air temperature in the Ogden pans may be 35°C during the day and 15°C at night. Brine at point A in figure 5 may favour the formation of kainite during the daytime and schoenite at night. The result of the diurnal temperature oscillation is a mixture of both salts in a single pond from the same brine. In terms of extracted product, this complicates ore processing as a single pan will contain both minerals, produced at the same curing stage, at the same time, yet one double salt entrains KCl, the other K2SO4, so additional processing is necessary to purify the product stream (Butts, 2002).

The sulphate ion in the pan waters is particularity temperature sensitive, and salts containing it in GSL pans tend to precipitate at cooler temperatures. Surficial cooling during the summer nights can cause prograde salts to precipitate, but the next day's heat generally provides sufficient activation energy to cause total dissolution of those salts precipitated just a few hours before (Butts, 2002). It is not unusual to find a 0.5 cm layer of hexahydrite (MgSO4.6H2O) at the bottom of a solar pond in the morning, but redissolved by late afternoon.

Under controlled laboratory conditions, brine collected from the hypersaline north arm of the Great Salt Lake will not crystallise mirabilite  until the brine temperature reaches 2°C or lower. Yet, in the anthropogenic solar ponds, mirabilite has been observed to crystallise at brine temperatures above 7°C. During the winter, as the surface temperature of the GSL pan brine at night becomes very cold (2°C or lower), especially on clear nights, and mirabilite rafts will form on and just below the brine surface and subsequently sink into the somewhat warmer brine at the floor of the pond. Because there is insufficient activation energy in this brine to completely redissolve the mirabilite, it remains on the pan floor, until warmer day/night temperatures are attained. However, it is also possible for salts precipitated by cooling to be later covered by salts precipitated by evaporation, which effectively prevents dissolution of those more temperature-sensitive salts that would otherwise redissolve (Butts, 2002).

There are also longer terms seasonal influences on mineralogy. Some salts deposited in June, July, and August (summer) will convert to other salts, with a possible total change in chemistry, when they are exposed to colder winter temperatures and rainfall. Kainite, for example, may convert to sylvite and epsomite and become a hardened mass on the pond floor; or if it is in contact with a sulphate-rich brine, it can convert to schoenite. Conversely, mirabilite will precipitate in the winter but redissolve during the hot summer months.

The depth of a solar pond also controls the size of the crystals produced. For example, if halite (NaCl) is precipitated in a GSL pond that is either less than 8 cm or more than 30cm deep, it will have a smaller crystal size than when precipitated in a pond between 8 and 30 cm deep. Smaller crystals of halite are undesirable in a de-icing product since a premium price is paid for larger crystals.

In terms of residence time, some salts require more time than others to crystallise in a pan. Brine that is not given sufficient time for crystallisation before it is moved into another pond, which contains brine at a different concentration, will produce a different suite of salts. For example, if a brine supersaturated in ions that will produce kainite, epsomite, and halite (reaction I), is transferred to another pond, the resulting brine mixture can favour carnallite (reaction 2), while kainite salts are eliminated.

Reaction 1: 9.75H2O + Na+ + 2Cl- + 2Mg2+ + K+ + 2SO42+ —> MgSO4.KCl +2.75H2O + MgSO4.7H2O + NaCl

Reaction 2: 12H2O + Na+ + 4Cl- + 2Mg2+ + K+ + 2SO42+ —> MgCl2.6H2O +MgSO4.6H2O + NaCl

Reaction 1 retains more magnesium as MgCl2 in the brine; reaction 2 retains more sulphate. In reaction 2, it is also interesting to note the effect of waters-of-hydration on crystallization; forcing out salts with high waters of crystallization results in higher rates of crystallization. The hydrated salts remove waters from the brine and further concentrate the brine in much the same way as does evaporation.

Pond leakage and brine capture (entrainment) in and below the pan floor are additional influences on mineralogy, regardless of brine depth or ponding area. As mentioned earlier, to precipitate bischofite and allow for MgCl2 manufacture, around ninety-eight percent of the water from present North Arm brine feed must evaporate. If pond leakage causes the level of the ponding area to drop too quickly, it becomes near impossible to reach saturation for bischofite (due to brine reflux). Control of pond leakage in the planning and construction phases is essential to assure that the precipitated salts contain the optimal quantity of the desired minerals for successful pond operation.

The opposite of leakage is brine retention in a precipitated layer; it can also alter brine chemistry and recovery economics. Brine entrained (or trapped) in the voids between salt crystals in the pond floor is effectively removed from salt production and so affects the chemistry of salts that will be precipitated as concentration proceeds and can also drive unwanted backreactions. The time required to evaporate nearly ninety percent of the water from the present north arm Great Salt Lake brine in the Ogden solar pond complex, under natural steady state conditions, is approximately eighteen months.

Summary of SOP production procedures in Great Salt Lake

Sulphate of potash cannot be obtained from the waters of the Great Salt Lake by simple solar evaporation (Behrens, 2002). As the lake water is evaporated, first halite precipitates in a relatively pure form and is harvested. By the time evaporative concentration has proceeded to the point that saturation in a potash-entraining salt occurs, most of the NaCl has precipitated. It does, however, continue to precipitate and becomes the primary contaminant in the potassium-bearing salt beds in the higher-end pans.

Brine phase chemistry from the point of potassium saturation in the evaporation series is complicated, and an array of potassium double salts are possible, depending on brine concentration, temperature and other factors. Among the variety of potash minerals precipitated in the potash harvester pans, the majority are double salts that contain atoms of both potassium and magnesium in the same molecule, They are dominated by kainite, schoenite, and carnallite. All are highly hydrated; that is, they contain high levels of water of crystallisation that must be removed during processing. SOP purification also involves removal of the considerable quantities of sodium chloride that are co-precipitated, after this the salts must be chemically converted into potassium sulphate.

Controlling the exact mineralogy of the precipitated salts and their composition mixtures is not possible in the pans, which are subject to the vagaries of climate and associated temperature variations. Many of the complex double salts precipitating in the pans are stable only under fixed physiochemical conditions, so that transitions of composition may take place in the ponds and even in the stockpile and early processing plant steps.

While weathering, draining, temperature and other factors can be controlled to a degree, it is essential that the Great Salt Lake plant be able to handle and effectively accommodate a widely variable feed mix (Behrens 2002). To do this, the plant operator has developed a basic process comprising a counter-current leach procedure for converting the potassium-bearing minerals through known mineral transition stages to a final potassium sulfate product (Figure 7). This set of processing steps is sensitive to sodium chloride content, so a supplemental flotation circuit is used to handle those harvested salts high in halite. It aims to remove the halite (in solution) and upgrade the feed stream to the point where it can be handled by the basic plant process.

Solids harvested from the potash ponds with elevated halite levels are treated with anionic flotation to remove remaining halite (Felton et al., 2010). To convert kainite into schoenite, it is necessary to mix the upgraded flotation product with a prepared brine. The conversion of schoenite to SOP at the Great Salt Lake plant requires that new MOP is added, over the amount produced from the lake brines. This additional MOP is purchased from the open market. The schoenite solids are mixed with potash in a draft tube baffle reactor to produce SOP and byproduct magnesium chloride.

The potassium sulfate processing stream defining the basic treatment process in the Great Salt Lake plant is summarised as Figure 7, whereby once obtaining the appropriate chemistry the SOP product is ultimately filtered, dried, sized and stored. Final SOP output may then be compacted, graded, and provided with additives as desired, then distributed in bulk or bagged, by rail or truck.


Lop Nur, Tarim Basin, China (SOP operation)

Sulphate of potash (SOP) via brine processing (solution mining) of lake sediments and subsequent solar concentration of brines is currently underway in the fault-bound Luobei Hollow region of the Lop Nur playa, in the southeastern part of Xinjiang Province, Western China (Liu et al., 2006; Sun et al., 2018). The recoverable sulphate of potash resource is estimated to be 36 million tonnes from lake brine (Dong et al., 2012). Lop Nur lies in the eastern part of the Taklimakan Desert (Figure 8a), China’s largest and driest desert, and is in the drainage sump of the basin, some 780 meters above sea level in a BSk climate belt. The Lop Nur depression first formed in the early Quaternary, due to the extensional collapse of the eastern Tarim Platform and is surrounded and typically in fault contact with the Kuruktagh (to the north), Bei Shan (to east) and Altun (to the south) mountains (Figure 8b).

The resulting Lop Nur (Lop Nor) sump is a large groundwater discharge playa that is the terminal point of China’s largest endorheic drainage system, the Tarim Basin, which occupies an area of more than 530,000 km2 (Ma et al., 2010). The Lop Nur sump is the hydrographic base level to local and regional groundwater and surface water flow systems, and thus collectively captures all river and subsurface flow originating in the surrounding mountainous regions. The area has been subject to ongoing Quaternary climate and water supply oscillations, which over the last few hundred years has driven concentric strandzone contractions on the playa surface, to form what is sometimes called the “Great Ear Lake" of the Lop Nur sump (Liu et al., 2016a).

Longer term widespread climate oscillations (thousands of years) drove precipitation of saline glauberite-polyhalite deposits, alternating with more humid lacustrine mudstones especially in fault defined grabens with the sump. For example, Liu et al. (2016b) conducted high-resolution multi-proxy analyses using materials from a well-dated pit section (YKD0301) in the centre of Lop Nur and south of the Luobei depression. They showed that Lop Nur experienced a progression through a brackish lake, saline lake, slightly brackish lake, saline lake, brackish lake, and playa in response to climatic changes over the past 9,000 years.

Presently, the Lop Nur playa lacks perennial long-term surface inflow and so is characterised by desiccated saline mudflats and polygonal salt crusts. The upward capillary flux from the shallow groundwater helps to maintain a high rate of evaporation in the depression and drives the formation of a metre-thick ephemeral halite crust that covers much of the depression (Liu et al., 2016a).

Historically, before construction of extensive irrigation systems in the upstream portion of the various riverine feeds to the depression and the diversion of water into the Tarim-Kongqi-Qargan canal, brackish floodwaters periodically accumulated in the Lop Nur depression. After the diversion of inflows, terminal desiccation led to the formation of the concentric shrinkage shorelines, that today outline the “Great Ear Lake” region of the Tarim Basin (Figure 8b; Huntington, 1907; Chao et al., 2009; Liu et al., 2016a).

The current climate is cool and extremely arid (Koeppen BSk); average annual rainfall is less than 20 mm and the average potential evaporation rates ≈3500 mm/yr (Ma et al., 2008, 2010). The mean annual air temperature is 11.6°C; higher temperatures occur during July (>40°C), and the lower temperatures occur during January (<20°C). Primary wind direction is northeast. The Lop Nor Basin experiences severe and frequent sandstorms; the region is well known for its wind-eroded features, including many layered yardangs along the northern, western and eastern margins of the Lop Nur salt plain (Lin et al., 2018).


Salinity and chemical composition of modern groundwater brine varies little in the ‘‘Great Ear” area and appears not to have changed significantly over the last decade (Ma et al., 2010). Dominant river inflows to the Lop Nor Basin are Na-Mg-Ca-SO4-Cl-HCO3 waters (Figure 9). In contrast, the sump region is characterised by highly concentrated groundwater brines (≈350 mg/l) that are rich in Na and Cl, poor in Ca and HCO3+CO3, and contain considerable amounts of Mg, SO4 and K, with pH ranging from 6.6 to 7.2 (Figure 9). When concentrated, the Luobei/Lop Nur pore brines is saturated with respect to halite, glauberite, thenardite, polyhalite and bloedite (Ma et al., 2010; Sun et al., 2018).

Groundwater brines, pooled in the northern sub-depression, mostly in the Luobei depression, are pumped into a series of pans to the immediate south, where sulphate of potash is produced via a set of solar concentrator pans. Brines in the Luobei depression and adjacent Xingqing and Tenglong platforms are similar in chemistry and salinity to the Great Ear Lake area but with a concentrated saline reserve due to the presence of a series of buried glauberite-rich beds (Figure 9; Hu and Wang, 2001; Ma et al., 2010; Sun et al., 2018).

K-rich mother brines in the Luobei hollow also contain significant MgSO4 levels and fill open phreatic pores in a widespread subsurface glauberite bed, with a potassium content of 1.4% (Liu et al., 2008; Sun et al., 2018). Feed brines are pumped from these evaporitic sediment hosts in the Luobei sump into a large field of concentrator pans to ultimately produce sulphate of potash (Figure 8a).

Brine chemical models, using current inflow water and groundwater brine chemistries and assuming open-system hydrology, show good agreement between theoretically predicted and observed minerals in upper parts of the Lop Nor Basin succession (Ma et al., 2010). However, such shallow sediment modelling does not explain the massive amounts of glauberite (Na2SO4.CaSO4) and polyhalite (K2SO4MgSO4.2CaSO4.2H2O) recovered in a 230 m deep core (ZK1200B well) from the Lop Nor Basin (Figure 9a).


Hydrochemical simulations assuming a closed system at depth and allowing brine reactions with previously formed minerals imply that widespread glauberite in the basin formed via back reactions between brine, gypsum and anhydrite and that polyhalite formed via a diagenetic reaction between brine and glauberite. Diagenetic textures related to recrystallisation and secondary replacement are seen in the ZK1200B core; they include gypsum-cored glauberite crystals and gypsum replacing glauberite. Such textures indicate significant mineral-brine interaction and backreaction during crystallisation of glauberite and polyhalite (Liu et al., 2008). Much of the glauberite dissolves to create characteristic mouldic porosity throughout the glauberite reservoir intervals (Figure 10, 11b)


Mineral assemblages predicted from the evaporation of Tarim river water match closely with natural assemblages and abundances and, in combination with a model that allows widespread backreactions, can explain the extensive glauberite deposits in the Lop Nor basin (Ma et al., 2008, 2010). It seems that the Tarim river inflows, not fault-controlled upwelling hydrothermal brines, were the dominant ion source throughout the lake history. The layered distribution of minerals in the more deeply cored sediments documents the evolving history of inflow water response to wet and dry periods in the Lop Nor basin. The occurrence of abundant glauberite and gypsum below 40 m depth, and the absence of halite, polyhalite and bloedite in the same sediment suggests that the brine underwent incomplete concentration in the wetter periods 10b).

In contrast, the increasing abundance of halite, polyhalite and bloedite in the top 40 m of core from the ZK1200B well indicate relatively dry periods (Figure 10a), where halite precipitated at lower evaporative concentrations (log Concentration factor = 3.15), while polyhalite and bloedite precipitated at higher evaporative concentrations (log = 3.31 and 3.48 respectively). Following deposition of the more saline minerals, the lake system once again became more humid in the later Holocene, until the anthropogenically-induced changes in the hydrology over the last few decades, driven by upstream water damming and extraction for agriculture (Ma et al., 2008). These changes have returned the sump hydrology to the more saline character that it had earlier in the Pleistocene.

The Lop Nur potash recovery plant/factory and pan system, located adjacent to the LuoBei depression (Figures 8, 11a), utilises a brine-well source aquifer where the potash brine is reservoired in intercrystalline and vuggy porosity in a thick stacked series of porous glauberite beds/aquifers.

Currently, 200 boreholes have been drilled in the Lop Nor brine field area showing the Late-Middle Pleistocene to Late Pleistocene strata are distributed as massive, continuous, thick layers of glauberite with well-developed intercrystal and mouldic porosity, forming storage space for potassium-rich brine (Figure 11b; Sun et al., 2018). However, buried faults and different rates of creation of fault-bound accommodation space, means there are differences in the brine storage capacity among the three brinefield extraction areas; termed the Luobei depression, the Xingqing platform and the Tenglong platform areas (Figures 9a, 11a).

In total, there are seven glauberitic brine beds defined by drill holes in the Luobei depression, including a phreatic aquifer, W1L, and six artesian aquifers, W2L, W3L, W4L, W5L, W6L, and W7 (Figure 10b; Sun et al., 2018). At present, only W1L, W2L, W3L, and W4L glauberite seams are used as brine sources. There are two artesian brine aquifers, W2X and W3X, exposed by drill holes in the Xinqing platform and there are three beds in the Tenglong extraction area, including a phreatic aquifer, W1T, and two artesian aquifers, W2T and W3T (Figure 10b).

W1L is a phreatic aquifer with layered distribution across the whole Luobei depression, with an average thickness of 17.54 m, water table depths of 1.7 to 2.3 m, porosities of 6.98% to 38.45%, and specific yields of 4.57% to 25.89%. Water yield is the highest in the central and northeast of the depression, with unit brine overflows of more than 5000 cubic meters per day per meter of water table depth (m3/dm). In the rest of the aquifer, the unit brine overflows range from 1000 to 5000 m3/dm (Sun et al., 2018). The W2L artesian aquifer is confined, nearly horizontal with a stratified distribution, and has an average thickness of 10.18 m, unit brine overflows of 10 to 100 m3/dm, water table depths of 20 to 40 m, porosities of 4.34% to 37.8%, and specific yields of 1.08% to 21.04%. The W3L artesian aquifer is confined, with stratified distribution and an average thickness of 8.50 m, unit brine overflows of 10 to 100 m3/dm, water table depths of 40 to 70 m, porosities of 2.85% to 19.97%, and specific yields of 1.10% to 13.37%. The W3L aquifer is also confined with stratified distribution, with an average thickness of 7.28 m, unit brine overflows of 10 to 100 m3/dm, water table depths of 70 to 100 m, porosities of 5.22% to 24.72%, and specific yields of 1.03% to 9.91%. The lithologies of the four brine storage layers are dominated by glauberite, and occasional lacustrine sedimentary clastic rocks, such as gypsum (Figure 10a).

The Xinqing platform consists of two confined potassium-bearing brine aquifers (Figure 10b). Confined brines have layered or stratified distributions. The average thicknesses of the aquifers are 4.38 to 7.52 m. Due to the F1 fault, there is no phreatic aquifer in the Xinqing platform, but this does not affect the continuity of the brine storage layer between the extraction areas. The W2X aquifer is confined, stratified, and distributed in the eastern part of this ore district with a north-south length of 77.78 km, east-west width of 16.82 km, and total area of 1100 km2. Unit brine overflows are 2.25 to 541.51 m3/dm, water table depths are 10 to 20 m, porosities are 3.89% to 40.69%, and specific yields are 2.01% to 21.15%. The W3X aquifer is also confined and stratified, with a north-south length of 76.10 km, east-west width of 18.81 km, and total area of 1444 km2. Unit brine overflows are 1.67 to 293.99 m3/d m, water table depths are 11.3 to 38 m, porosities are 4.16% to 26.43%, and specific yields are 2.11% to 14.19%23.

The Tenglong platform consists of a phreatic aquifer and two confined aquifers. W1T is a phreatic, stratified aquifer and is the main ore body, and is bound by the F3 fault (Figure 10b). It is distributed across the northern part of the Tenglong extraction area, with a north-south length of about 33 km, east-west width of about 20 km, and total area of 610 km2. Water table depths are 3.26 to 4.6 m, porosities are 2.03% to 38.81%, and specific yields are 22.48% to 1.22%. On the other side of the F3 fault, in the southern part of the mining area, is the W2T confined aquifer (Figure 10b). Water table depths are 16.91 to 22 m, porosities are 3.58% to 37.64%, and specific yields are 1.35% to 18.69%. W3T is also a confined aquifer, with a stratified orebody distributed in the southern part of the mining area, with a north-south length of about 29 km, east-west width of about 21 km, and total area of 546 km2. Water table depths are 17.13 to 47 m, porosities are 2.69% to 38.71%, and specific yields are 1.26% to 17.64%.

Lop Nur is an unusual potash source

The glauberite-hosted brinefield in the Luobei depression and the adjacent platforms makes the Lop Nur SOP system unique in that it is the world's first large-scale example of brine commercialisation for potash recovery in a Quaternary continental playa aquifer system with a non-MOP brinefield target. Elsewhere, such as in the Dead Sea and the Qarhan sump, Salar de Atacama and the Bonneville salt flats, the brines derived from Quaternary lacustrine beds and water bodies are concentrated via solar evaporation in semi-arid desert scenarios. Potash plants utilising these Quaternary evaporite-hosted lacustrine brine systems do not target potassium sulphate, but process either carnallitite or sylvinite into a commercial MOP product

 Glauberite is found in a range of other continental Quaternary evaporite deposits around the world but, as yet,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       outside of Lop Nur is not economically exploited to produce sulphate of potash. For example, glauberite is a significant component in Quaternary cryogenic beds in Karabogazgol on the eastern shore of the Caspian Sea, in Quaternary evaporite beds in Laguna del Rey in Mexico, in saline lacustrine beds in the Miocene of Spain and Turkey, and in pedogenic beds in hyperarid nitrate-rich soils of the Atacama Desert of South America (Warren, 2016; Chapter 12).

In most cases, the deposits are commercially exploited as a source of sodium sulphate (salt cake). In a saline Quaternary lake in Canada, SOP is produced by processing saline lake waters. This takes place in Quill Lake, where small volumes of SOP are produced via mixing a sylvite feed (trucked into the site) with a cryogenic NaSO4 lake brine.

The Lop Nur deposit is mined by the SDIC Xinjiang Luobupo Hoevellite Co. Ltd, and the main product is potassium sulfate, with a current annual production capacity of 1.3 million tons. Pan construction began in 2000, and the plant moved in full -cale operation in 2004 when it produced ≈50,000 tons. The parent company, State Development and Investment Corporation (SDIC), is China’s largest state-owned investment holding company. The company estimates a potash reserve ≈ 12.2 billion tons in the sump. This makes Lop Nur deposit the largest SOP facility in the world, and it is now a significant supplier of high premium fertiliser to the Chinese domestic market.

Implications

A study of a few of the Quaternary pans worldwide manufacturing economic levels of potash via solar evaporation shows tha,t independent of whether SOP or MOP salts are the main product, all retain abundant evidence that salt precipitates continue to evolve as the temperature and the encasing brine chemistry change. As we shall see in many ancient examples discussed in the next article, ongoing postdepositional mineralogical alteration dominates the textural and mineralogical story in most ancient potash deposits.

As we saw in the previous article, which focused on MOP in solar concentrator plants with brine feeds from Quaternary saline lakes, SOP production from brine feeds in Quaternary saline lakes is also related strongly to cooler desert climates (Figure 12). The Koeppen climate at Lop Nur is cool arid desert (BWk), while the Great Salt Lake straddles cool arid steppe desert and a temperate climate zone, with hot dry summer zones (BSk and Csa)


Outside of these two examples, there are a number of other Quaternary potash mineral occurrences with the potential for SOP production, if a suitable brine processing stream can be devised (Warren, 2010, 2016). These sites include intermontane depressions in the high Andes in what is a high altitude polar tundra setting (Koeppen ET), none of which are commercial (Figure 12b).

Similarly, there a number of non-commercial potash (SOP) mineral and brine occurrences in various hot arid desert regions in Australia, northern Africa and the Middle East (Koeppen BWh). Today, SOP in Salar de Atacama is currently produced as a byproduct of lithium carbonate production, along with MOP, as discussed in the previous article in this series.

As for MOP, climatically, commercial potash brine SOP systems are hosted in Quaternary-age lacustrine sediments are located in cooler endorheic intermontane depressions (BWk, BSk). The association with somewhat cooler desert and less arid cool steppe climates underlines the need for greater volumes of brine to reside in the landscape in order to facilitate the production of significant volumes of potash bittern.

Put simply, in the case of both MOP and SOP production in Quaternary settings, hot arid continental deserts simply do not have enough flowable water to produce economic volumes of a chemically-suitable mother brine. That is, currently economic Quaternary MOP and SOP operations produce by pumping nonmarine pore or saline lake brines into a set of concentrator pans. Mother waters reside in hypersaline perennial lakes in steep-sided valleys or in pores in salt-entraining aquifers with dissolving salt compositions supplying  a suitable ionic proportions in the mother brine. In terms of annual volume of product sold into the world market, Quaternary brine systems supply less than 15% The remainder comes from the mining of a variety of ancient solid-state potash sources. In the third and final article in this series, we shall discuss how and why the chemistry and hydrogeology of these ancient potash sources is mostly marine-fed and somewhat different from the continental hydrologies addressed so far.

References

Behrens, P., 2002, Industrial processing of Great Salt lake Brines by Great Salt Lake Minerals and Chemical Corporation, in D. T. Gywnne, ed., Great Salt Lake: A scientific, historical and economic overview, Utah Geological and Mineral Survey, Bulletin 116, p. 223-228.

Bingham, C. P., 1980, Solar production of potash from brines of the Bonneville Salt Flats, in J. W. Gwynn, ed., Great Salt Lake; a scientific, history and economic overview. , v. 116, Bulletin Utah Geological and Mineral Survey, p. 229-242.

Butts, D., 2002, Chemistry of Great Salt Lake Brines in Solar Ponds, in D. T. Gywnne, ed., Great Salt Lake: A scientific, historical and economic overview, Utah Geological and Mineral Survey, Bulletin 116, p. 170-174.

Butts, D., 2007, Chemicals from Brines, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., p. 784-803.

Chao, L., P. Zicheng, Y. Dong, L. Weiguo, Z. Zhaofeng, H. Jianfeng, and C. Chenlin, 2009, A lacustrine record from Lop Nur, Xinjiang, China: Implications for paleoclimate change during Late Pleistocene: Journal of Asian Earth Sciences, v. 34, p. 38-45.

Dong, Z., P. Lv, G. Qian, X. Xia, Y. Zhao, and G. Mu, 2012, Research progress in China's Lop Nur: Earth-Science Reviews, v. 111, p. 142-153.

Felton, D., J. Waters, R. Moritz, D., and T. A. Lane, 2010, Producing Sulfate of Potash from Polyhalite with Cost Estimates, Gustavson Associates, p. 19.

Hu, G., and N.-a. Wang, 2001, The sand wedge and mirabilite of the last ice age and their paleoclimatic significance in Hexi Corridor: Chinese Geographical Science, v. 11, p. 80-86.

Huntington, E., 1907, Lop-Nor. A Chinese Lake. Part 1. The Unexplored Salt Desert of Lop: Bulletin of the American Geographical Society, v. 39, p. 65-77.

Jones, B., D. Naftz, R. Spencer, and C. Oviatt, 2009, Geochemical Evolution of Great Salt Lake, Utah, USA: Aquatic Geochemistry, v. 15, p. 95-121.

Lin, Y., L. Xu, and G. Mu, 2018, Differential erosion and the formation of layered yardangs in the Loulan region (Lop Nur), eastern Tarim Basin: Aeolian Research, v. 30, p. 41-47.

Liu, C., W. Mili, J. Pengcheng, L. I. Shude, and C. Yongzhi, 2006, Features and Formation Mechanism of Faults and Potash-forming Effect in the Lop Nur Salt Lake, Xinjiang, China: Acta Geologica Sinica - English Edition, v. 80, p. 936-943.

Liu, C.-A., H. Gong, Y. Shao, Z. Yang, L. Liu, and Y. Geng, 2016a, Recognition of salt crust types by means of PolSAR to reflect the fluctuation processes of an ancient lake in Lop Nur: Remote Sensing of Environment, v. 175, p. 148-157.

Liu, C. L., M. L. Wang, P. C. Jiao, W. D. Fan, Y. Z. Chen, Z. C. Yang, and J. G. Wang, 2008, Sedimentary characteristics and origin of polyhalite in Lop Nur Salt Lake,Xinjiang: Mineral Deposits.

Liu, C. L., J. F. Zhang, P. C. Jiao, and S. Mischke, 2016b, The Holocene history of Lop Nur and its palaeoclimate implications: Quaternary Science Reviews, v. 148, p. 163-175.

Ma, C., F. Wang, Q. Cao, X. Xia, S. Li, and X. Li, 2008, Climate and environment reconstruction during the Medieval Warm Period in Lop Nur of Xinjiang, China: Chinese Science Bulletin, v. 53, p. 3016-3027.

Ma, L., T. K. Lowenstein, B. Li, P. Jiang, C. Liu, J. Zhong, J. Sheng, H. Qiu, and H. Wu, 2010, Hydrochemical characteristics and brine evolution paths of Lop Nor Basin, Xinjiang Province, Western China: Applied Geochemistry, v. 25, p. 1770-1782.

Spencer, R. J., H. P. Eugster, and B. F. Jones, 1985b, Geochemistry of Great Salt Lake, Utah II: Pleistocene-Holocene evolution: Geochimica et Cosmochimica Acta, v. 49, p. 739-747.

Spencer, R. J., H. P. Eugster, B. F. Jones, and S. L. Rettig, 1985a, Geochemistry of Great Salt Lake, Utah I: Hydrochemistry since 1850: Geochimica et Cosmochimica Acta, v. 49, p. 727-737.

Sun, M.-g., and L.-c. Ma, 2018, Potassium-rich brine deposit in Lop Nor basin, Xinjiang, China: Scientific Reports, v. 8, p. 7676.

Warren, J. K., 2010, Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits: Earth-Science Reviews, v. 98, p. 217-268.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3): Berlin, Springer, 1854 p.

 

Evaporites and climate: Part 2 of 2 - Ancient evaporites

John Warren - Saturday, February 25, 2017

Introduction

Evaporites, along with coal and bauxites, are sediments considered to be climate sensitive. Ancient evaporite distribution and associated paleolatitudes are used to reconstruct the distribution of the world's arid belts across time. As we saw in Part 1 (Salty Matters, Tuesday, January 31, 2017), thick, widespread evaporite deposit are essentially a result of the atmospheric circulation of the Hadley cells. That is, locations of subtropical dry zones and tropical/subtropical deserts of the globe are mostly determined by the positions of subsiding branches of cool, dry descending air in a Hadley cells (aka Trade Wind Belt; Lu et al., 2007; Crowley and North, 1991) within low-lying regions tied a sufficient supply of mother brine. Thus, climate plus ongoing brine supply are the underlying factors controlling locales of significant evaporite deposition (Ziegler et al., 1981). The previous article focused on regional and local climatic controls across the Quaternary (Salty Matters, Tuesday, January 31, 2017). This article extends the time frame for the evaporite/climate association across the Phanerozoic and into the Precambrian.


As we move back in time, we move out of an icehouse-dominated world climate, with permanent ice caps waxing and waning at the world's poles, into greenhouse-dominant world climates. In greenhouse times there are no permanent polar ice sheet and glaciers occurred only in some high-altitude mountainous belts (Figure 1a, b). The transition into greenhouse climate changes the dominant 4th-order eustatic style from 100m amplitude changes every 100,000 years or so into 4th-order responses with much lower 3-5 m amplitude oscillations every 100,000 years (Figure 2a, b). Lack of polar caps raises world sealevel on the order of 40-50 metres. Thus, even without tectonic considerations, there is less continental freeboard in greenhouse times and increased the potential for significant cratonic coverage by epicontinental and pericontinental seaways (Warren, 2016; Chapter 5).


Looking back in time, beyond the last few million years of the Quaternary, means our conceptual models must encompass a broader range of tectonic settings as well as changes in the rates of seafloor spreading, supercontinents, and times of significant igneous outpourings (superplumes). All these additional world-scale variables across a longer set of available time explain the greater range of climate potentials on the earth, compared to anything that has occurred in the time-limited base seen in icehouse-dominant Quaternary climate spreads. The last few million years of the current icehouse mode is inhabited by the human species or its primate ancestors. The previous Icehouse dominant mode was in the Carboniferous-Mid Permian when the dominant land animals were amphibians and primitive reptiles.
One the first questions this broader ancient climate spectrum, tied to evaporites over deep time, offers up is; "How do the positions of Hadley cells vary across geological time frames?" In part 1, we saw how the rise of the Himalayas deflected a belt of cool, dry descending air much further south toward the Equator. Moving back in time creates a broader scaffolding for documenting climate variation, in part driven by the rise and fall of mountain ranges, but also influenced by increases in the rate of seafloor spreading driving ocean basin shallowing and by changes in atmospheric/seawater compositions and temperatures.


Hadley cells and latitudinal variability over time
According to Chen Xu et al., 2013 and Boucot et al., 2013, much of the world-scale Phanerozoic distribution of significant bedded evaporite accumulations indicates the ongoing presence of two mid-latitude arid belts, presumably situated beneath Hadley Cells. There are exceptions in locales generated in local rain shadows with orographic control provided by neighbouring mountain ranges. However, in the later Permian, through the Triassic, and much of the Jurassic the two formerly mid-latitude Hadley Cells merged over the more central Pangaeanic regions of Africa, Europe and the adjacent Americas, to form an arid belt that also encompassed low-latitude, equatorial arid regions (Figure 3). Across the continental interior of the Pangean supercontinent, this arid to hyperarid equatorial belt in the supercontinent interior prevented the formation of climate-sensitive sediments that are more typical of humid equatorial conditions that deposit coals, kaolinites, lateritic materials and bauxites. However, in this same time interval, these more humid sediment products are typically present at low latitudes of these time slices adjacent to the Panthalassic ocean.
That is, in the absence of an equator-spanning supercontinent, low latitudes, typically imply humid and non-seasonal tropical conditions throughout much of the Phanerozoic, as we see today. But the assembly of the Pangaean supercontinent disrupted this latitudinally-zoned atmospheric circulation, replacing it with a progressively more monsoonal (seasonal) circulation and more arid, at times hyperarid, conditions in the equatorial continental interior of Pangaea (Parrish, 1993). The Pangean supercontinent reached its maximum areal extent in the Triassic and was associated with what is known as the Pangaean Megamonsoon. There were immense arid regions across the interior regions of the supercontinent that were nearly uninhabitable, with scorching days and frigid nights. However, Panthalassian coasts still experienced seasonality, transitioning from rainy weather in the summer to dry conditions during the winter and the associated accumulation of humid sediments (Figure 3b' Boucot et al., 2013). Megamonsoon aridity is evidenced not just in the accumulation of low-latitude bedded evaporite deposits. Low latitude continental aridity also drove the accumulation of thick, widespread low-latitude desert redbeds, sourced by eolian, not fluvial, detrital transport (Sweet at al., 2013) and the precipitation of bedded salt crusts in ephemeral saline lakes under exceptionally-high surface temperatures of up to 73°C (Zambito and Benison, 2013).
Paleolatitude reconstructions Chen-Xu et al. (2013) show these continental interior arid belts in low latitude tropical-subtropical regions persisted from the Permian to the Early Cretaceous. Reunion of the humid regions from both sides of Pangaea by the early Late Cretaceous formed a through-going low latitude humid tropical-subtropical belt. This coincides with the disaggregation of Pangaean supercontinent, as the initial stages of a modern latitudinal climate belt distribution pattern emerged, tied to latitudinally-restricted evaporites that continue to the present (Figure 4).


Tectonism and eustacy in arid climates drive the formation of mega-evaporite basins
Within this Phanerozoic climatic framework, there are times when significant volumes of evaporites form what are know as saline giants, or megahalite/megasulphates deposits. These massive accumulations of salts formed beneath arid climates that can span both greenhouse and icehouse climates (Figure 1; Warren, 2010, 2016). Ancient marine saline giants (megahalites and megasulphates) accrued in either of two plate-scale settings, which at times merged into one another, namely; 1) Platform evaporites (Figure 5) and, 2) Basinwide evaporites (Figure 6).

The first major contrast with nonmarine continental dominance in Quaternary evaporite settings is the fact that platform evaporites require greenhouse eustasy a and a marine feed, the second is that basinwide evaporites require tectonically- and hydrographically-isolated widespread subsealevel depressions, typically found along plate edges with continent-continent proximity in regions with a marine seepage feed and/or periodic marine overflows (Figure 6). Neither platform or basinwide conditions are present on the current earth surface. For basinwides, suitable hydrologic conditions were last present during the Messinian Salinity Crisis in the Mediterranean region, and platform evaporite settings were last present on earth across large parts of the Middle East carbonate platform during the Eocene (Tables 1, 2). There is a third group of ancient evaporite deposits; it encompasses all nonmarine lacustrine beds past and present (Table 3). This group has same-scale modern-ancient counterparts, unlike ancient marine platform and basinwide evaporites (Warren, 2010, 2016). Interestingly, the lacustrine depositional style for bedded salt accumulation dominates in the icehouse climate that is the Quaternary, and so biases a strictly uniformitarian view of the past with respect to the relative proportions of nonmarine versus marine evaporite volumes (see Part 1; Salty Matters, Tuesday, January 31, 2017).


Platform evaporites
Are made up of stratiform beds, usually <50 m thick and composed of stacked <1 to 5 m thick parasequences or evaporite cycles, with a variably-present restricted-marine carbonate unit at a cycle base (Table 1). Salts were deposited as mixed evaporitic mudflat and saltern evaporites, sometimes with local accumulations of bittern salts. Typically, platform salts were deposited in laterally extensive (>50-100 km wide), hydrographically-isolated, subsealevel marine-seepage lagoons (salterns) and evaporitic mudflats (sabkhas and salinas). These regions have no same-scale modern counterparts and extended as widespread depositional sheets across large portions of hydrographically isolated marine platform areas that passed seaward across a subaerial seepage barrier into open marine sediments (Figure 5). In marine margin epeiric settings, such as the Jurassic Arab/Hith and Permian Khuff cycles of the Middle East or the Cretaceous Ferry Lake Anhydrite in the Gulf of Mexico, these platform evaporites are intercalated with shoalwater marine-influenced carbonate shelf/ramp sediments, which in turn pass basinward across a subaerial sill into open marine carbonates. Landward they pass into arid zone continental siliciclastics or carbonate mudflats.

 

Platform evaporite deposition occurred in both pericontinental and epicontinental settings, at times of low-amplitude 4th and 5th order sealevel changes, which typify greenhouse eustasy (Figure 5; Warren, 2010). Platform evaporites also typify the saline stages of some intracratonic basins. Platform evaporites cannot form in the high-amplitude, high-frequency sealevel changes of Icehouse eustasy. The 100m+ amplitude oscillations of Icehouse times mean sealevel falls off the shelf edge every 100,000 years, so any evaporite that had formed on the platform is subaerially exposed and leached. Fourth order high-amplitude icehouse eustatic cycles also tend to prevent laterally-continuous carbonate sediment barriers forming at the top of the shelf to slope break, and so icehouse evaporite systems tend not to be hydro-graphically isolated (drawdown) at the platform scale. Rather icehouse eustasy favours nonmarine evaporites as the dominant style, along with small ephemeral marine-margin salt bodies, as seen today in the bedded Holocene halites and gypsums of Lake Macleod in coastal West Australia (Part 1; Salty Matters, Tuesday, January 31, 2017).
Ancient platform evaporite successions may contain halite beds, especially in intracratonic basinwide settings, but periplatform settings, outside of intracratonic basins, are typically dominated by 5–40 m thick Ca-sulphate beds intercalated with normal-marine platform carbonates (Table 1). The lateral extent of these epeiric platform sulphate bodies, like the Middle Anhydrite Member of the Permian Khuff Fm. of Saudi Arabia and the UAE, with a current area of more than 1,206,700 sq. km., constitute some of the most aerially-extensive evaporite beds ever deposited.


Basinwide evaporites
Are made up of thick evaporite units >50–100 m thick made up of varying combinations of deepwater and shallow water evaporites (Figure 1; Table 2). They retain textural evidence of different but synchronous local depositional settings, including mudflat, saltern, slope and basin (Figure 6). When basinwide evaporite deposition occurs, the whole basin hydrology is evaporitic, holomictic, and typically saturated with the same mineral phase across vast areas of the basin floor, as seen on a much smaller scale today in the Dead Sea basin. The Dead Sea currently has halite forming simultaneously as; 1) decimeter-thick chevron-dominated beds on the saline-pan floor of the shallow parts around the basin edge in waters typically less than 1-10 metres deep, and 2) as coarse inclusion-poor crystal meshworks of halite on the deep basin floor that sits below a halite-saturated brine column up to hundreds of metres deep. Ancient basinwide successions are usually dominated by thick massive salt beds, generally more than 100-500 m thick. Deposits are made up of stacked thick halite beds, but can also contain substantial volumes of thick-bedded Ca-sulphate and evaporitic carbonate, as in the intracratonic basinwide accumulations of the Delaware and Otto Fiord Ba-sins (Table 2).
Owing to inherent purity and thickness of the deposited halite, many halite-dominant basinwide beds are also remobilized, via loading or tectonics, into various halokinetic geometries (Hudec and Jackson, 2007). Some basinwide systems (mostly marine-fed intracratonic settings) entrain significant accumulations of marine-fed potash salts, as in the Devonian Prairie Evaporite of western Canada. In contrast, all Quaternary examples of commercial potash deposits are accumulating in continental lacustrine systems (Warren 2016; Chapter 11).

 

Basinwide evaporite deposits are the result of a combination of tectonic and hydrological circumstances that are not currently active on the world’s surface (Figure 1). They were last active in the Late Miocene (Messinian), in association with soft-suture collision basins tied to the Alpine-Himalaya orogenic belt, and in Middle Miocene (Badenian) basins developed in the early rift stages of the Red Sea. Basinwide systems will be active again in the future at sites and times of appropriate plate-plate interaction, when two continental plate edges are nearby, and the intervening seafloor is in or near a plate-edge rift or suture and is both subsealevel and hydrographically isolated (Figure 6). Unlike most platform evaporites, basinwides do not require greenhouse eustacy, only the appropriate association of arid climate and tectonics. The latter sets up a deep hydrographically-isolated subsealevel tectonic depression with a geohydrology that can draw on a huge reserve of marine mother brine in the nearby ocean. For this reason, saline giants tend to form at times of plate-scale continent-continent proximity and so occur mostly in craton-margin settings.

 

Lacustrine (nonmarine) evaporites
Quaternary continental playa/lacustrine are constructed of stratiform salt units, with the greater volume of saline sediment accumulating in lower, more-saline portions of the lacustrine landscape. Beds are usually dominated by nodular gypsum and displacive halite, deposited in extensive evaporitic mudflats and saltpans with textures heavily overprinted by capillary wicking, rather than as bedded bottom-nucleated layers on the subaqueous floors of perennial brine lakes (Ruch et al., 2012). In ancient counterparts, the total saline lacustrine thickness ranged from meters to hundreds of meters, with lateral extents measured in tens to hundreds of kilometres (Table 3). Lacustrine salt beds are separated vertically, and usually surrounded by, deposits of lacustrine muds, alluvial fans, ephemeral streams, sheet floods, eolian sands, and redbeds. As today, ancient lacustrine salts accumulated in endorheic or highly restricted discharge basins, with perennial saline water masses tending to occur in the drainage sumps of steep-sided drainage basins (Warren, 2010, 2016). Saline lake basins accumulating gypsum, or more saline salts like halite or glauberite, typically have a shallow water table in peripheral saline mudflat areas and so are dominated by continental sabkha textures. Nearby is the lowermost part of the lacustrine depression or sump where deposition is typified by ephemeral ponded brine pan deposits, rather than permanent saline waters.
Saline lacustrine mineralogies depend on compositions of inflowing waters, so depositional sumps in regions with non-marine ionic proportions in the feeder inflow, accumulate thick sequences of nonmarine bedded salts dominated by trona, glauberite, and thenardite. In contrast, nonmarine areas with thalassic (seawater-like) inflows tend to accumulate more typical sequences of halite, gypsum, and anhydrite.
Across the Pliocene-Quaternary icehouse, less-saline perennial saline-lake beds tend to occur during more humid climate periods in the same continental-lacustrine depressions where saline-pan beds form (e.g., Lake Magadi, Great Salt Lake, Lake Urmia). On a smaller scale, in some modern saline lake basins, parts of the lake floor can be permanently located below the water surface (Northern Basin in the Dead Sea or Lake Asal). In some modern saline sumps dominated by mudflats, a perennial saline lake water mass is located toward the edge of a more central salt-flat zone, forming a perennial water filled “moat” facies surrounding a seasonally desiccated saline pan (as in Salar, de Atacama, Salar de Uyuni, Lake Magadi, Lake Natron). These permanent to near-permanent saline water “moat” regions are typically created where fresher inflows encounter saltier beds of the lake centre, dissolve them, and so form water-filled peripheral depressions. Bottom sediment in the moats tends to be mesohaline carbonate laminites, which can contain TOC levels as high as 12%.
High-water stage perennial saline lacustrine sediments tend to be carbonate-rich or silica-rich (diatomaceous) laminites. Ancient examples of large saline lacustrine deposits made up of alternating humid and desiccated lacustrine units include the Eocene Green River Formation of Wyoming and the Permian Pingdiquan Formation of the Junggar Basin, China (Table 4). Evaporites deposited in a suprasealevel lacustrine basin (especially Neogene deposits) have numerous same-scale Quaternary analogues, unlike the more voluminous ancient marine platform and basinwide evaporites (Figure 7). Clearly, across the Quaternary, saline continent lacustrine settings possess areas of bedded salt accumulation that are far greater than those of any contemporaneous marine-fed salt sumps (Part 1; Salty Matters, Tuesday, January 31, 2017). But in ancient climes, especially during in the continental interior of the Pangean supercontinent (mid Permian to Triassic), regions of continental interior sabkhas and saline pans had areas far greater than any seen in Quaternary continental saline sumps (Zambia and Benison, 2013).


Is the present-day climate the key to evaporite understanding?
This short answer is yes, Hadley Cells across the Phanerozoic are mostly tied to climate belts that maintain sub-tropical positions, but to this notion, we must add a geological context. Today we are living in an icehouse climate mode and have been for the last 12 Ma. It is tied to the presence of polar ice-sheets that wax and wane over 100,000-year time frames, so moving the position of the Hadley cells and changing the intensity of atmospheric circulation. In this icehouse climate, large eustatically-controlled marine-fed evaporite deposits are not preserved, as sea level falls off the continental shelf every 100,000 years or so. The world's largest bedded salt deposits formed sometime in the last 2 million years, are found in continental interiors, typically in endorheic tectonic sumps in either hot arid or steppe climate settings, often with salt diapirs outcropping or subcropping in the drainage basin and the basin floor can be located at elevations well above sealevel (Part 1; Salty Matters, Tuesday, January 31, 2017).
As we move back into much of Phanerozoic time, we see world climates dominated by greenhouse modes, with shorter episodes of polar ice-sheets and icehouse climates in the Carboniferous-Early Permian ≈ 50 million years long, and the Ordovician, some 15 million years long (Figure 1). Greenhouse climate lacks permanent polar ice sheets, so sea-levels are higher, and 4th-order eustatic amplitudes in sea level are much less (a few meters versus hundred meters plus changes). Greenhouse sets up epeiric and intercontinental seaways that when hydrographically isolated, but still marine-fed, can deposit huge areas of platform evaporites centred in isolated seepage-fed sub-sealevel sumps. These platform deposits can also form outside of Greenhouse times in marine-fed tectonically-induced intracratonic sumps.
Basinwide evaporite deposits span icehouse and greenhouse mode arid belts, whenever a marine-fed subsealevel tectonic sump forms at positions of continent-continental proximity in an arid belt. Across much of the Phanerozoic, basinwide deposits typically accumulated beneath subtropical belts of cool, dry descending air set up in a Hadley cell, and so are located north and south of a tropical equatorial belt. But the accretion of the Pangaean supercontinent (Carboniferous to Jurassic) set up conditions of continentality and orographic shadowing that allowed an arid saline belt to span the hyperarid interior of the supercontinent.
 
References

Boucot, A., Chen-Xu, and C. Scotese, 2013, Phanerozoic Paleoclimate: An Atlas of Lithologic Indicators of Climate: Concepts in Sedimentology and Paleontology, v. 11: Tulsa, OK, SEPM, 32 p.

Chen-Xu, A. J. Boucot, C. R. Scotese, F. Junxuan, W. Yuan, and Z. Xiujuan, 2012, Pangaean aggregation and disaggregation with evidence from global climate belts: Journal of Palaeogeography, v. 1, p. 5-13.

Crowley, T. J., and G. R. North, 1991, Paleoclimatology: New York, Oxford University Press, 339 p.

Hudec, M. R., and M. P. A. Jackson, 2007, Terra infirma: Understanding salt tectonics: Earth-Science Reviews, v. 82, p. 1-28.

Lu, J., G. A. Vecchi, and T. Reichler, 2007, Expansion of the Hadley cell under global warming: Geophysical Research Letters, v. 34.

Parrish, J. T., 1993, Climate of the Supercontinent Pangea: Journal of Geology, v. 10.

Ruch, J., J. K. Warren, F. Risacher, T. R. Walter, and R. Lanari, 2012, Salt lake deformation detected from space: Earth and Planetary Science Letters, v. 331-332, p. 120-127.

Sweet, A. C., G. S. Soreghan, D. E. Sweet, M. J. Soreghan, and A. S. Madden, 2013, Permian dust in Oklahoma: Source and origin for Middle Permian (Flowerpot-Blaine) redbeds in Western Tropical Pangaea: Sedimentary Geology, v. 284–285, p. 181-196.

Warren, J. K., 2010, Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits: Earth-Science Reviews, v. 98, p. 217-268.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3): Berlin, Springer, 1854 p.

Zambito, J. J., and K. C. Benison, 2013, Extremely high temperatures and paleoclimate trends recorded in Permian ephemeral lake halite: Geology, v. 41, p. 587-590.

Ziegler, A. M., S. F. Barrett, C. R. Scotese, and B. W. Sellwood, 1981, Palaeoclimate, Sedimentation and Continental Accretion [and Discussion]: Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, v. 301, p. 253-264.


 

 

What is an evaporite? Solar versus cryogenic (freeze-dried) salts

John Warren - Tuesday, February 24, 2015

The term evaporite is usually used to describe sediment precipitated during the solar-driven desiccation of a standing water body in a saltern or salina, or a near-surface pore brine in an evaporitic mudflat or sabkha. Almost all the modern and ancient examples of bedded salts that we work with in the rock record are thought to have crystallised via this process of solar evaporation. The chemical dynamics of solar evaporation are simple; on average, water molecules within a standing at-surface brine lake or in near-surface pore spaces, near a water-table and its associated capillary zone, do not have enough kinetic energy to escape the liquid phase and so cross the surface tension barrier (figure 1). Otherwise, liquid water would turn to vapour spontaneously and any at-surface liquid phase would spontaneously disappear, while recharge to an underlying water-table would be an impossibility.

 

Every so often in this situation, the level of solar energy transfer (heat absorption) at the molecular collision site is sufficient to give a water molecule (near the water-air interface) the heat energy necessary to pass into the vapour phase and so exit the liquid water mass (figure 1). That is, for a water molecule to escape into the vapour phase it must absorb heat energy, be located near the liquid surface, be moving in the proper direction and have sufficient energy to overcome liquid-phase intermolecular forces and then pass through the surface tension interface. As the concentration of the residual brine increases, the specific heat capacity decreases, and the density increases (the effects of specific heat and density increases on evaporite mineralogy and distribution in the depositional setting will be the topic of a future blog).

But salts, some with the same mineralogy as solar evaporation salts, can also form as a water or brine body freezes to leave behind cryogenic salt layers (aka the "freeze-dried" salts). This is the process that forms significant volumes of the sodium sulphate salts in various cold-zone brine lakes and saline ice-sheets around the world. Unlike solar evaporites, cryogenic brines and associated salts require temperatures at or below the freezing point of the liquid phase. Cryogenic salts, such as mirabilite (Na2SO4.10H2O), hydrohalite (NaCl.2H2O), antarcticite (CaCl2.6H2O) and epsomite (MgSO4.7H2O), can then accumulate. These cryogenic salts crystallise in cold, near-freezing, residual brines as they concentrate via the loss of the liquid water phase as it converts/solidifies to ice. As the volume of ice grows, the various anions and cations are excluded from the expanding ice lattice. Hence, concentration of the residual brine increases until it reaches saturation with a salt phase that then precipitates (figure 2). There are a number of well-documented cryogenic salt beds in various Quaternary-age cold-continental lacustrine settings. Probably the best known are the sodium sulphate salts in Karabogazgol, Turkmenistan, where strand-zone stacks of cryogenic mirabilite form each winter. Beneath the lake centre there are subsurface beds (meters thick) of Quaternary-age cryogenic glauberite-halite.

 

In the Turkmen language, Karabogazgol means “lake of the black throat,” so named because the gulf is continually gulping down the waters of the Caspian Sea, via a narrow connecting natural channel (figure 3). Ongoing evaporation in Karabogazgol keeps the water surface in the perennial brine lake depression around a metre below that of the Caspian. It is one of our few natural examples of evaporative drawdown occurring via a hydrographic (surface) connection to the mother water body. Groundwater seepage connections with the mother water mass are more typical, especially in hot arid basins.

 

Only since the end of the Soviet era and the re-opening of the lake’s natural connection to the Caspian Sea in 1992, by a newly independent Turkmen government, did Karabogazgol re-fill with perennial brines. Since then, a natural cryogenic mirabilite winter cycle has returned the Karabogaz hydrology to its longterm natural state. Today, the main open water body in the centre of Karabogaz is a Na-Mg-Cl brine, sourced via gravitationally-driven inflow of Caspian Sea waters. Perennial Karabogaz brines today have a density of 1.2 g/cm3, and pH values that range between 7.2 and 9. Surface water temperatures in the lake centre range from around 4°C in December (winter) to 25°C in July (summer). Temperature fluctuations and the cool arid steppe climate (Koeppen BSk) of Karabogazgol combine to drive the precipitation of different mineral phases during the year. Calcite, aragonite and perhaps hydromagnesite usually precipitate from saturated lake surface waters in spring, gypsum and glauberite in summer (via solar evaporation), while rafts of cryogenic mirabilite form at the air brine interface in the winter. These winter rafts are then blown shoreward, to form stacked strand-zone-parallel accumulations of cryogenic mirabilite and halite. By the following summer much of the strand-zone mirabilite has deliquesced or converted to glauberite. In the 1920s and 30’s, prior to the damming of the connecting channel between Karabogaz and the Caspian Sea, the strand-zone salts were harvested by the local peasantry. From the 1950s to 1980s there was a significant Soviet chemical industry operational in the basin, focused on older buried salt bed targets.


Beneath Karabogaz bay there are 4 beds dominated by various NaSO4 salts (figure 4). These cryogenic beds are likely the result of cooler climatic periods over the last 10,000-20,000 years. Back then, under a glacial climate, large amounts of mirabilite formed each winter, much like today. But, unlike today, a cooler more-humid glacial climate meant that the bay was not as subject to as an intense summer desiccation as it is today. Dense residual bottom brines were perennially ponded and so preserved a summer-halite sealing bed atop the winter NaSO4 layer. This allowed the underlying mirabilite/epsomite    precipitates to be preserved across the lake floor. During the following winter the process was repeated as mirabilite/epsomite/halite beds stacked one atop the other to create a future NaSO4 bedded-ore horizon. In time, a combination of groundwater and exposure, especially nearer the Gulf’s strand-zone, converted most of the mirabilite, along with epsomite, to astrakanite, and then both phases to glauberite in the upper three beds. This explains the association of the richer glauberite zones with the lake edges (figure 4). Whenever water of crystallisation is released by a mirabilite to thenardite conversion, it then slightly dilutes any strong residual brine; in Korabogazgol this facilitated the high sodium-sulphate mineral and brine compositions seen in modern and ancient waters across the bay.

There are similar cryogenic salt beds preserved in perennial saline lakes across the cold arid portions of the Great Plains of Canada and there is also a mirabilite bed preserved beneath Holocene sediments in Great Salt Lake, Utah. Last century, some of the richer subaqueous salt beds in the Canadian lakes were sources of commercial NaSO4 salts (figure 5). However, extraction of brine and solid salts and an increasingly expensive product meant this area no longer competes with cheaper product from the solar evaporite NaSO4 lakes of Mexico and Turkey. Only one (Big Quill Lake) of the Canadian sites remains operational.


Hydrohalite is another common cryogenic salt, it quickly redissolves as brine temperatures rise above 0 degrees centigrade and so is said to indicate halite cryogenesis (figure 6). Hydrohalite crystals have distinctive pseudo-hexagonal cross sections (c.f. typical cubic forms of halite) and crystals or NaCl-filled pseudomorphs have been recognised in a number of modern cold saline lake settings. For example, hydrohalite has been extracted from the lake bottom sediments in saline Lake Bonney in Antarctica, where the bottom water temperatures vary between +2.0 and -2.0°C. It also can precipitate in winter in the Baskunchak salt lake, located some 300 km northwest of the Caspian Sea (48°N latitude). There hydrohalite was directly observed on two occasions when formative brine temperatures were between-3° and -23°C. In summer, halite precipitates via solar evaporation in the same saline lake. Hydrohalite also occurs in bottom sediments in salt-saturated cryogenic lakes in Saskatchewan, at about 51°N latitude, and has been observed in nearby saline springs sediments of the Northern Great Plains. Hydrohalite pseudomorphs occur as halite crystals with hexagonal cross sections in cores some 100-140m deep, in Death Valley, California, indicating NaCl cryogenesis occurred in the Pleistocene Death Valley Lake at a time when brine temperatures were less that 0°C.


When polar seawaters freeze on Earth, hydrohalite and mirabilite precipitate from the residual marine brines and accumulate in ice sheet fissures, or in load-induced fractures in the ice understory wherever an increasingly saline brine sinks into rock fractures beneath the growing ice sheets. For example, there are mirabilite layers on the ice floes of the Ross Ice Shelf near Black Island. Likewise, there are dense residual saline brines in interstitial waters extracted from deep cores in sediments of McMurdo Sound. It seems that when ice sheets retreat, the at-surface cryogenic salts dissolve in the freshened at-surface hydrology, but dense hypersaline brines can remain behind in deep fissures, held and preserved in the rock fractures. In the extreme setting of at-surface brine freezing in some of the small saline depressions in the Dry Valleys of Antarctica, a solid form of calcium chloride, antarcticite, grows cryogenically. Antarcticite precipitates today in Don Juan Pond, Antarctica, in what is probably the most saline perennial natural water mass on Earth (47% salinity). Although the Don Juan pond is often cited in the saline literature as a most impressive example of an extremely-hypersaline modern closed-basin cryogenic hydrology, it should be pointed out that this cryogenic salt pond measures some 100 by 300 metres across and is tens of centimetres deep (figure 7). 

 

On Earth, the volume of salt beds formed by cryogenesis is much less than the volumes that result from solar evaporation. Extraterrestially, in planets and moons of our solar system with liquid water and located further out than the earth’s orbit, there are likely, at least locally, volumes of cryogenic salts that are significant. Cryogenesis explains sulphate salt (epsomite-dominant) phases that typify ice crack fissures crisscrossing the surface of Europa (a moon of Jupiter). Sulphate salts also grow seasonally in soils of Mars where, for example, widespread gypsum forms via ice ablation in the circumpolar Martian dune-field. But, for now, I will leave the discussion of the significance of these extraterrestial cryogenic salts, it will be the topic of a future blog dealing with liquid water indications in and on a variety of planets and moons located beyond the earth’s orbit.

On Earth, as ground temperatures increase, cryogenic salts tend to deliquesce or convert to their higher temperature daughter salts (thenardite, glauberite and halite). But worldwide, in appropriate cold climatic settings, there are numerous examples of cryogenic salt beds; the volumes grow even larger if we include sediments containing cryogenic hydrated calcite (ikaite-glendonite; CaCO3.6H2O). Across deep time, the volume of cryogenic salts increases during glacial episodes, their susceptibility to deliquescence and conversion as temperatures increase means a low propensity for significant preservation other than as pseudomorphs. If such pseudomorphs are to be found across the rock record, then there will be a greater likelihood of their retention and recognition in sediments of the late Tertiary, the Permo-Carboniferous, the Ordovician and the late Neoproterozoic, which are all times of an icehouse climate.


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