Salty Matters is a monthly article discussing something salty that has caught John's interest. You can also check out the archive page for access to older Salty Matter articles
Hypersaline brine layers below a less-saline and less-dense brine layer characterise depositional settings across many evaporite and cryogenic salt deposits. The halocline that separates the layers tends to prevent convective mixing and slows heat loss from the lower brine layer. If the halocline resides at depths where sunlight penetrates, then it can set up a long-term reverse or heliothermal stratification. In any perennial brine body (evaporitic or cryogenic) the lower hypersaline brine layer (monimolimnion) tends to persevere, while the upper layer (mixolimnion) comes and goes. Evaporite and cryogenic textures accumulating on the bottom of a perennial brine lake or seaway reflect the presence or absence of meromictic or holomictic conditions in the column.
As an evaporite bed infills its accommodation space, there is a predictable upward transition in crystallisation textures, indicative of evolution in hydrogeochemistry and permanence of formative brine layers and the level of freshening that can reach down to an accreting sediment surface. Primary evaporite textures in all evaporites are direct indicators of the hydrological state and the presence or absence of brine layering at the time of salt deposition.
Evaporite sediments precipitate in a brine and can dissolve into a brine. The formative brine is always saline (more saline than freshwater) and the levels of salinity in the brine will increase or decrease depending on the relative rate of fluid input to output in the location where the evaporite mass is accumulating at the earth's surface or residing in the subsurface. That is, rates of fluid input to loss to and from an evaporite geobody change both in the at-surface depositional environment and in the subsurface environment. More saline brines have higher concentrations of ionic constituents in the aqueous host than less saline brines. Accordingly, more saline brines have somewhat higher densities and lower specific heat capacities than less saline brines.
The higher density of a free-standing brine or porosity-held brine compared to an overlying fresher, less-dense free-standing brine layer or surrounding fresher pore fluids in non-evaporite sediment can drive geochemical stability, interaction and alteration across time frames measured in months to hundreds of millions of years, from the time of deposition through diagenesis and uplift. The distinct properties of evaporite-associated brine also influence the generation of viable oil and gas accumulations and ore deposits. Over the next four articles, we consider brine in this context of short-term and longterm pore fluid stability and alteration, starting with this article discussing various physical properties of evaporite and cryogenic brines.
Major commercial borate deposits occur in a limited number of Neogene to Holocene non-marine evaporitic settings that are generally closely tied to volcanic rocks and pyroclastic deposits leached into closed-basin alkaline lakes or playas fed by hydrothermal sources. Four minerals comprise more than 90% of the borate ore salts extracted by the borate industry; the sodium borates - borax (tincal), kernite, the calcium borate - colemanite, and the sodium-calcium borate - ulexite. Water content of the various borate salts tends to decrease with burial and to increase with exposure and weathering.
Borax, and to a lesser extent kernite, is open-pit mined today at the Kramer mine near Boron in California, the Sarikaya mine in Kirka ore district (Eskisehir Province, in Turkey), and Tincalayu in northern Argentina. These three saline lacustrine regions, plus the brines from Searles Lake, California, furnish the majority of the sodium borates used by the world’s chemical industry in the last 100 years.
Sodium borates ores are readily soluble in water, making them preferable feedstock for many end uses. Most of the larger extractive facilities also produce boric acid at nearby refineries. Colemanite is mined from several deposits in the Emet and Bigadic basins of western Turkey, one former Death Valley deposit, and small deposits in northern Argentina. This calcium borate is used in boron endproducts that have a low sodium requirement, and in making boric acid. Ulexite is the common playa (salar) or hypersaline marsh borate. It is produced commercially from numerous salars in South America and playas in the Provinces of Quinghi and Xizang (Tibet) in western China. Ancient saline lacustrine deposits 5–20 million years old— are the source of Turkish ulexite, it is found also in Death Valley and some of the ores produced in Argentina.
All gourmet salt are composed of more that 95% NaCl or halite, but varieties such as Fleur de Sel, Sel Gris, Himalayan Pink Salt, Persian Blue Salt, Hawaiian Red Salt and Bamboo Salt are marketed at a price premium. The markup can be more than 500% of the price of ordinary table salt. I look at what scientifically defines the various types of gourmet salt and if they are exclusive compared to the processes that can form a variety of salt textures and colours in the natural world. I also document some of the range of methods used to manufacture the various types of gourmetand flavoured salt on the market and attempt to analyse just what you pay for when purchasing salt from a speciality supplier.
From a long-term or geological time perspective, the combination of salt's (NaCl) physical, chemical and thermal properties make it idiosyncratic when compared to the responses of most non-evaporitic sedimentary minerals and rocks in a basin. Its distinctive features mean thick subsurface salt beds in the diagenetic realm tend to dissolve or flow while carbonates and siliciclastics do not. In fact, evaporites, especially thick pure halite units (>50-80 m thick), are the weakest rocks in most deforming geosystems. Some of halites microstructural responses to stress in the diagenetic realm are more akin to structural responses in other sediments in the metamorphic realm.
However, this axiom applies only over geologic time scales, in large dimensions, and at depth (Jackson and Hudec, 2017). Time is central to understanding salt deformation at all scales from the micro to the macro. Like an ice glacier, a salt glacier (extruding sheet or namakier) is solid enough to walk, over but flows under its own weight over geologic time scales. The slower the deformation, the weaker is rock salt compared with other sedimentary rocks.
Gypsum (CaSO4.2H2O) is a common evaporite salt, first precipitated as a primary-textured bedded deposit at the initial eogenetic end of the burial cycle. With burial into the mesogenetic realm, gypsum dehydrates and compresses into various forms of nodular anhydrite (CaSO4) showing variable textural retention of original gypsum textures. Later, at the other of the burial cycle during uplift into the telogenetic realm, exhumed mesogenetic anhydrite rehydrates as secondary gypsum precipitates. During telogenetic rehydration, the most common process forming secondary gypsum takes place where once impervious mesogenetic nodular anhydrite beds move into regions of increasing phreatic undersaturation and meteoric crossflow, before finally entering the vadose zone. In more humid settings, a newly-formed telogenetic gypsum unit can redissolve before reaching the land surface. Another common telogenetic rehydration scenario is where exhuming halite beds or diapiric masses undergo differential dissolution on their way to the surface. This leaves behind layers of coalesced residual nodular anhydrite (aka fractionated dissolution residue). Residual CaSO4 then converts to gypsum as the ascending anhydrite interval interacts first with more-deeply circulating then shallower meteoric waters. This is how a caprock forms around the upper and outer edges of a diapiric and bedded salt masses.Read More
Precipitative mechanisms and settings are discussed along with the association of saddle dolomite with burial evaporites. Such occurrences are divided into examples where; 1) the saddle dolomite is encased in anhydrite, 2) saddle dolomite crystals form along with burial anhydrite (and its destruction) in regions of thermochemical sulphate reduction (TSR), 3) where the saddle dolomites occurs in a variety of MVT deposits in proximity to sparry anhydrite or other saline burial indicators.
During saltern deposition and the precipitation of reflux dolomites, there is a hydrological system of basinward-flushing dense saline brines. Major substrate flushing occurs in the early stages of saltern deposition and shallow burial (<100 m burial) and prior to the loss of intrasalt and subsalt permeability. This is especially obvious where a gypsum saltern is accumulating atop a marine platform carbonate and prior to the burial alteration of gypsum to anhydrite. Circulation is driven by gravitational instability, set up by an updip dense brine plume or brine curtain below accumulating evaporites. At its periphery this refluxing plume mixes with and displaces less dense subsurface and connate waters to create or enhance the reservoir properties of adjacent subsalt limestones via reflux dolomitisation.
As dense Mg-rich brines seep downdip from the platform saltern or mudflat they create intervals of porous dolomites in more distal zones, located seaward of overdolomitised evaporite-plugged areas. In some cases the most extensively dolomitised carbonates in these more distal zones are high-energy, shallow water grainstones and packstones. These were the sediments with the higher permeabilities at the time of dolomitisation. However, in areas of pervasive syndepositional marine cementation, some originally porous carbonates may have already lost much of their intergranular permeability via overgrowths of isopachous rim cements. In this situation the less cemented platform mudstones act as aquifers to the refluxing brines and so are preferentially dolomitised.
In this the second of four articles on dolomite in saline settings, we look at dolomite as a possible byproduct of microbial metabolism. Dolomite is a complex mineral and, due to an inherent requirement of lattice order, is not found in significant volumes in many Holocene marine-margin carbonate depositional settings. Climatically, these sites are diverse, ranging from humid to semi-arid to arid deserts, geographically from deep offshore marine to coastal and hydrologically from eogenetic marine to shallow meteoric capillary and phreatic to hypersaline continental seepage. A prevailing association across most of the sites is that most can be tied to zones of active microbial metabolism, especially with saline anoxic settings exemplified the presence of a flourishing community of sulphate-reducing bacteria. The article discusses these occurrences and the concept of microbial facilitation as well as crystal ageing in modern saline dolomite occurrences.Read more
This series of four articles looks at dolomite in saline settings, ranging from the modern lacustrine to ancient evaporitic platforms. In the first article, we look at dolomite in saline marine-margin settings, focusing mostly on the nature of dolomite mineralogy and distribution in the Salt Creek lakes, Coorong region, South Australia. In the next article, we focus on the bacterial association with dolomites in some brackish to hypersaline saline settings in Brazilian carbonate lagoons and Abu Dhabi sabkhas and compare these Holocene mineralogies with the Coorong, while in the third, we will have a look if these Holocene occurrences relate to brine reflux and a fourth article focuses other ancient saline dolomitization models and occurrences related to burial diagenesis.
Most terrestrial non-solar salt precipitates can be considered hydrothermal salts, which is a broader descriptor than burial salts, making a broader grouping encompassinga higher temperature range compared to the diagenetic realm. One group of such hydrothermal salts, mostly composed of anhydrite, with lesser baryte, typically develop along oceanic seafloor ridges within heated subsurface fractures or at seafloor vents. There seawater-derived hydrothermal waters are heating, mixing, degassing, escaping and ultimately cooling. Active deep seafloor hydrothermal hydrologies create a specific group of sulphide ore deposits known as volcanic-hosted massive sulphide deposits (VHMS), with anhydrite as the primary-salt driving mineralisation. The other non-solar salt grouping we shall discuss are salting-out precipitates, mostly halite, created when brines reach supercritical temperatures of 400-500°C. Some proponents of this mechanism postulate this halite sources much of the hydrothermal halite in rifts such as the Red Sea or the Danakhil Depression.Read More
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 cryo-genesis; in the second, we shall look at salt masses crystallising from fluids created and driven by mantle heating and cooling. Cryogenesis on extraterrestial planets is one of the main ways significant quantities of salt accumulate on other planets, and their presence is one of the main ways we know liquid water and possibly oceans are present below their icy surfaces.Read More
Three of the Earth's five major Phanerozoic extinction events have an evaporite association. The article starts with the most intense extinction event of the Phanerozoic; the end-Permian and a link to LIP (Large Igneous Province) emplacement into two separate sequences of massive bedded evaporite (Cambrian versus Devonian mega-salts) in the Tunguska Basin, Siberia. Likewise the end Triassic extinction ties to a LIP emplacement in the intracratonic Amazon Salt Basin. The third ties to the impact of a large bolide into the Jurassic megasalt basin in the Gulf of Mexico, off the Yucatan Peninsula. Interestingly, two other events on the list of the "big five;" the Emeishan and late Devonian events also have possible associations with heated evaporites. The Emeishan LIP intersects the edge of the anhydrite-rich Sichuan basin, while the 120km-diam., Late Devonian, Woodleigh bolide impacted the intracratonic Silurian Yaringa Fm. salts (including potash beds) on the coast of West Australia.
The article discusses general mechanisms of earth-scale volatile entry into the ancient atmosphere during events that involved rapid and widespread heating of saline giants. It develops this notion by looking at whether volumes of volatiles escaping to the atmosphere are enhanced by either the introduction of vast quantities of molten material to a saline giant or the thermal disturbance of that salt basin by bolide impacts. This begins a discussion of the contribution of heated evaporites in two (or three if the Captitanian is counted as a separate event) of the world's five most significant extinction events. It also looks at possible evaporite associations with a substantial bolide impact that marks the end of the Cretaceous. The next article presents the geological details and implications of the various magma-evaporite-volatile associations tied to major extinction events.
Styles of evaporites interactions with magma are a spectrum, with two endmember situations; 1) orthomagmatic (salt-assimilative and internal to the magma), and 2) paramagmatic (salt-interactive and external to the magma). Both encompass outcomes that can include a variety of substantial ore deposits). Only in situations where igneous sills and dykes have intruded salt masses, with contacts preserved, can direct effects of magma-salt interaction be documented. Even then, determining the timing of the evaporite igneous interaction can be problematic; one must ask if the chemistry and texture indicate, 1) syn-igneous emplacement, or 2) post-emplacement alteration and deeply circulating groundwater flushing, or 3) a combination. Historically, ignous melt interactions are considered in terms of the dominant subsurface evaporite phases - halite and anhydrite - both anhydrous salt minerals. In this article we will considered also igneous interactions with hydrous salts, like carnallite, kainite, polyhalite and gypsum; any of which can be locally significant bed constituents in a halite-dominant basin fill .