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Saturday, 26 November 2011

Hydrothermal Salt

Hydrothermal Salt

   Most part of geologists still believe that salt rocks as halite NaCl, anhydrite CaSO4, gypsum CaSO4·2H2O, sylvite KCl, tachyhydrite CaMg2Cl6·12H2O, carnallite KMgCl3·6H2O, among others would be formed intrinsically by evaporation processes of shallow saline waters at surface environments and thus would configure sequences formed by the so-called "evaporites" which would be a class of chemical sedimentary rocks. The paradigm of the model related to formation of evaporites dates since 1849 by Italian chemist Usiglio and after proposals by Bischof  and Ochsenius, respectively in 1854 and 1877 based on previous ideas would take place through the model of restriction by a barrier (Barrier Model) of saline water bodies where the salt to precipitate and form layers according to its solubility, concentration and evaporation rates that are influenced by topography and arid climates.

   Would be traditional evaporite model the only real  explanation for the genesis of salt deposits? The traditional evaporite model is, however, not the only way of explaining all the salts appearing on Earth today. Thus, a group of Norwegian scientists have, over the past decade developed the new Hydrothermal Salt Theory (Hovland et al., 2006). Unlike the traditional model, this theory is more physically and chemically consistent, and over time, may take over parts of the current view. The new theory is not only valid on Earth, but also on planet Mars, where probably there are salt domes and salt deposits (Hovland et al., 2009)

   According to this new theory, Salt is originated from hydrothermal systems deep-through where supercritical water acts on stage at certain conditions of pressures and temperatures in a superior range of the superficial environments. The molecules of supercritical water has no polarity  and, different of non-critical water (normal water), indeed supercritical water can not dissolved salts and carrier them from deep sources brines to surface environments where then the effects of climate dry and restrictions areas in lakes or marine environments such salts are accumulated, reworked, precipitated, redissolved, reprecipitate forming spetacular sequences of salt rocks.

   It is noteworthy that there is no rock-forming minerals in the crust of the Earth with high content of chlorine or abundance to justify the occurrence of huge deposits of salt halite (NaCl). It makes sense then think that salt would not be derived primarily only from the dissolution of surface rocks. By the other hand, salt usually occurs associated with volcanic environments and hydrothermal systems as mud volcanism, for instance. There are many examples in the world, in marine, estuarine and continental environments and also a wider range of altitudes. 

   Good example of non-marine salt association is the salt of the Salar de Uyuni, in Bolivia. This occurrence is situated over 3,650 m altitude in the Andes, wide more than 100 km in area about 3,000 square miles. The salar is surrounded by several volcanic buildings. Another example is the Danakil Depression, in Ethiopia, Eritrea and the salts that occur in the Afar Triangle in Djibouti, where there is also intense volcanism. White Sands National Monument, in New Mexico, USA is another good example of salt related to volcanism. In this place there are spetacular dunes, not sand dunes, but gypsum dunes and the Carrizozo Malpais lava flow, over 70 km long, occurs closely towards north of White Sands. There also many similar occurences in the world.

Salar de Uyuni, Bolivia, South America (NASA GFSC Modis Terra image)
Salar de Uyuni is the world's largest salt flat, at larger than 3000 square miles in size. It is located near the crest of the Andes mountains, at an altitute of 3650 meters. It is estimated that Salar de Uyuni contains 10 billion tons of salt; its mineral content is mainly gypsum and halite. Around salar there are several volcanoes. 

Dallol is a unique crater in the world, located in Afar territory, in the Danakil desert (one of the most inhospitable deserts in the world), north-eastern Ethiopia, about fifteen kilometers from the border of Eritrea. This site is volcanic in the north end of a saline lake, Lake Karoum, in which salt is still mined today by the Afar. It follows from the explosion of a large magma chamber of the Great Rift Valley, over a large area west of saline Red Sea, and is - 136.8 meters below the level Sea, in the Danakil Depression. The heat regularly reaches 45 degrees in the shade. 

This vast area is known for its unusual geological formations: acidic hot springs, mountains of sulfur, salt columns, small gas geysers, pools of acid isolated by cornices of salt concretions and evaporites, sulfur chloride magnesium, sodium hydroxide and brine solidified. All on a white background, yellow, green and red ocher, due to the strong presence of sulfur, iron oxide, salt and other minerals. 

The site, like the volcanoes surrounding this area (Erta Ale volcano in Kenya, etc..) Is the result of the separation of the Arabian Plate and the African plate and the creation of the Red Sea rift. 

The large white area near the bottom of this image of New Mexico, USA, is the sand dune field known as the White Sands National Monument. The field has a surface area of 710 km² (275 mi²). White Sands is the world’s largest gypsum dunefield. The thick, dark brown line just north of the dunes is the Carrizozo Malpais, a large lava flow, one of the youngest volcanic features in the state of New Mexico. The 75 kilometer long Malpais, is composed of basaltic lava flows. (from Earth Snapshot)

   The questions are: where does Chlorine (Cl) come from for  to form the chloride salts  and sulfur (S) to sulphate salts? It is likely that chlorine migrate from great depths as organochlorine compounds, i.e., through its connection with primordial hydrocarbon molecules of the mantle. Salt  cations such as sodium, calcium and magnesium are abundant in the crust and mantle, but not chlorides and sulphates. Elements such as halogens Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) can have its origin related to primordial volatiles , including sulphur (S), which is likely to combine with oxygen to form sulphate compounds.

   Hydrothermal Salt Theory offers plausible explanation on the origin and evolution for the end-members of these salt sequences,  but  the dogmas of  geology, as the principle of actualism, are still very influential and difficult paradigm shift to an unconventional theory. This is due the difficult of most part of geologists still do not have comprehension of physical, chemical laws and mass balance of natural processes because they still remaining prisoner with their reasoning in context of restricted "a priori" theories.

   Indeed, understanding the whole process - from the origin to the formation of salt deposits is not  simple, since the elements and compounds have high chemical reactivity with many changes. Maybe Hydrothermal Salt Theory is the great light to the knowledge of salt rocks formation and evaporites are only extrinsic processes  of salt rework at surface shallow environments.

   Why are the oceans salty? That is a question that even children always ask and geologists and scientists still do not get an apropriated answer, obviously because they do not have enough understanding about what is salt and its origin but the Hydrothermal Salt Theory is probably a right way to answer that and other many questions about salt and its real origin.

   The traditional evaporite model could explains only certain situations after salt formed originally by hydrothermal systems reach to surface and reworked at this enviroment by normal evaporation processes. On the other hand, Hydrothermal Salt Theory also explain existence of deep-water salt sequences.

   Recently NASA found evidence of salt in Mars (see link below). Hydrothermal Salt Theory is suitable to explain origin of salt in Mars. We know that there was extensive volcanism in Mars. There, the highest volcan of solar system is present - The Olympus Mons (Mount Olympus) is a martian shield volcano which has base diameter about 624 km (374 mi) and 25 km (16mi) high. Volcanic outgassing brings methane and is oxidize to carbon dioxide to Mars atmosphere. Hydrothermal brines synchronous with volcanism could provide salt reach near surface of Mars. Natural oil-spills or oil seeps, seepages, are interpreted from Mars images also associated with salt. This phenomenon is very common on Earth.

Mons Olympus - Mars 
(ESA/DLR/FU Berlin - G. Neukum)

Hovland, M., Rueslåtten, H., Johnsen, H.K., Kvamme, B., Kutznetsova, T., 2006. Salt formation associated with sub-surface boiling and supercritical water. Marine and Petroleum Geology 23, 855-869.

Hovland, M., Rueslåtten, H., ,Johnsen, H.K., Fichler, C., 2009.(Abstract) Hydrothermal evaporites - from the Conrad Deep, via Dallol, to Elysium Planitia. International Association of Sedimentologists (IAS) Annual meeting, Alghero, Sardinia, Book of Abstracts.

Links about Hydrothermal Salt