Large salt accumulations as a consequence of hydrothermal processes associated with ‘Wilson cycles’:
A review, Part 2: Application of a new salt-forming model on selected cases
Martin Hovland, Håkon Rueslåtten, Hans Konrad Johnsen
The new hydrothermal salt model predicts that salt may accumulate in the sub-surface by hydrothermal circulation of seawater and brines in locations of high heat-flow. Such conditions are primarily found along tectonic plate boundaries, with processes of subduction and rifting, associated with the Wilson cycles. Modern knowledge of the physicochemical and thermodynamic properties of salt solutions at elevated pressures and temperatures, allows numerical modeling of fluid behavior at relevant conditions in the deep crust. This modeling shows how seawater that migrates down towards hot magma bodies in a rift situation (e.g. in the Red Sea) is subject to phase transitions, where low-saline (distilled) water vapor migrates out of the system, while still saltier brine continues to migrate further down towards the heat source, until solid salt precipitates. Similarly, in a subduction situation, the seawater confined in the subducting oceanic plate is subjected to an ever increasing pressure and temperature during the descent towards the mantle, which leads to similar phase behavior of the brine as in the rifting situation. The salts forming in the deep of a subduction zone are not readily transported up to the surface due to thick overburden of mantle- and crustal rocks. Hence, much of the salt formed during subduction remains hidden from human observation in the roots of the mountains. The formation of solid salt is therefore an inevitable process associated with the Wilson cycles, even without solar evaporation. Recent observations of thick carpets of mobile salt slurries on the Red Sea floor (Salt Flows) and huge accumulations of salts in the subsea/subsurface (Salt Walls and Salt Ridges), associated with topographic lows (Deeps), suggest that the Red Sea currently produces new volumes of brines and solid salts underground. The different solubilities of sea salts lead to a refining of the salt types. When reaching the seafloor, the warm brines are cooled down in brine pools, eventually becoming oversaturated with salts, which results in precipitation onto the seafloor. The dense brine layers protect seafloor salts from re-dissolution by normal seawater. Magmatic and volcanic processes associated with hydrothermal activity, e.g. connected to the subduction under the Andes Mountains are associated with enormous deposits of salts. The brines are transported hydrothermally up from the subduction zone and are venting out at 3500–4000 m above sea level. At the surface, the brines evaporate in the dry climate, which is also protecting the salt from re-dissolution. The brine feeding the hydrothermal systems above the subduction zone originates from dewatering of the subducting slabs. This water is causing fractional melting of the mantle wedge above, initiating volcanism. The hydrothermal water is also transporting salts to the surface, thus, explaining the close relationship between some types of volcanism and salt accumulations. It is assumed that this new understanding of hydrothermal salt formation will have a profound impact on interpretation of geological phenomena involving salt and salt formation, including the early development and behavior of salt basins.
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