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, and 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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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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Messinian MgSO4 enriched hydrological indicator lunette well log interpretation methanotrophic symbionts well logs in evaporites chert NPHI log doline Zaragoza Stebnyk potash Salar de Atacama brine evolution CO2 halogenated hydrocarbon evaporite karst H2S salt mine sulphur Hyperarid phreatomagmatic explosion bedded potash Badenian NaSO4 salts hydrothermal potash Hadley Cell evaporite-hydrocarbon association High Magadi beds wireline log interpretation vestimentiferan siboglinids Realmonte potash cryogenic spring salts mirabilite Warrawoona Group retrograde salt Deep seafloor hypersaline anoxic lake RHOB Dead Sea caves trona circum-Atlantic Salt Basins Kalush Potash Pilbara collapse doline gas in salt Ceres methanogenesis lazurite Five Island salt dome trend brine pan stevensite causes of glaciation stable isotope MOP Koppen climate salt trade Corocoro copper palygorskite End-Permian cryogenic salt LIP Pangaea epsomite hydrogen natural geohazard phreatic explosion Mesoproterozoic seal capacity halocarbon nitrogen potash ore price Ethiopia perchlorate waste storage in salt cavity saline giant Dead Sea saltworks brine lake edge deep seafloor hypersaline anoxic basin Proterozoic base metal Paleoproterozoic Oxygenation Event sepiolite Neutron Log Zabuye Lake Quaternary climate snake-skin chert black salt salt leakage, dihedral angle, halite, halokinesis, salt flow, nuclear waste storage Ripon tachyhydrite sedimentary copper bischofite CO2: albedo carbon oxygen isotope cross plots geohazard Gamma log halophile basinwide evaporite causes of major extinction events sulfate silicified anhydrite nodules methane Dallol saltpan McArthur River Pb-Zn Thiotrphic symbionts Ure Terrace saline clay halokinetic mummifiction salt seal Density log Bathymodiolus childressi jadarite carbon cycle Precambrian evaporites salt periphery sinkhole Patience Lake member meta-evaporite North Pole York (Whitehall) Mine Phaneozoic climate Sumo Stolz diapir endosymbiosis halite Catalayud Lop Nur capillary zone 18O SedEx End-Triassic GR log Lake Peigneur hydrothermal karst anomalous salt zones Great Salt Lake Jefferson Island salt mine McMurdo Sound kainitite lithium brine DHAL silica solubility Prairie Evaporite extraterrestrial salt Sulphate of potash namakier Calyptogena ponderosa authigenic silica Platform evaporite sulphate supercontinent Neoproterozoic anthropogenically enhanced salt dissolution crocodile skin chert extrasalt hydrohalite Belle Isle salt mine African rift valley lakes Danakhil Depression, Afar Ingebright Lake Musley potash Hadley cell: Enceladus allo-suture Ganymede Noril'sk Nickel Belle Plain Member gem sulfur 13C enrichment water in modern-day Mars astrakanite evaporite magadiite Seepiophila jonesi well blowout salt suture Turkmenistan cauliflower chert 18O enrichment water on Mars eolian transport ozone depletion sinjarite hectorite Deep Red Sea Europe Prograde salt recurring slope lines (RSL) Mixing zone evaporite dissolution halotolerant auto-suture venice deep meteoric potash seawater evolution Kara bogaz gol salt ablation breccia Lake Magadi Large Igneous Magmatic Province blowout Weeks Island salt mine DHAB vanished evaporite gassy salt Crescent potash Mars antarcticite gypsum dune lot's wife SO2 SOP MVT deposit source rock lithium battery alkaline lake Karabogazgol nacholite solikamsk 2 Magdalen's Road dark salt climate control on salt rockburst sodium silicate vadose zone knistersalz potash Lamellibrachia luymesi subsidence basin solar concentrator pans oil gusher End-Cretaceous halite-hosted cave zeolite lapis lazuli Mega-monsoon Muriate of potash Neoproterozoic Oxygenation Event intersalt evaporite-metal association dissolution collapse doline ancient climate Dead Sea karst collapse K2O from Gamma Log Atlantis II Deep Stebnik Potash Mulhouse Basin anthropogenic potash HYC Pb-Zn marine brine Archean salt tectonics Clayton Valley playa: organic matter CaCl2 brine Koeppen Climate freefight lake MgSO4 depleted salt karst dihedral angle Boulby Mine Hell Kettle gas outburst intrasalt lithium carbonate 13C mass die-back carnallitite flowing salt Evaporite-source rock association Lomagundi Event mine stability Schoenite Lop Nor potash ore

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