About this Research Topic
Geophysical investigations and core-drill recoveries indicated that several salt deposits occur in the sub-seafloor of deep marine basins all over the world. Some of these deposits have salt volumes in the order of 1.0 – 2.5 mln. cubic km, depending on the assessments, and are therefore called "salt-giants".
Salt deposits are generally considered precipitates from saturated solutions, driven by solar evaporation of seawater ("evaporites"). Studies on water–rock interaction provided alternative explanations on the origins of salt-giants. Anhydrous mafic silicate minerals from basalts and mantle rocks cannot reach thermodynamic equilibrium with aqueous solutions in the range of pressures and temperatures compatible with a wide section of the oceanic crust. Water dissolves some minerals, yielding secondary products. Such hydration reactions do not require external energy inputs, and hence they can develop spontaneously over time. For example, Mg-rich olivine, and/or Mg-rich orthopyroxene, will be transformed into serpentine group minerals, hence the newly formed rocks are called serpentinites. In case the reactant is a saline solution derived from seawater, the salinity increases as the reaction proceeds, because the formation of serpentine consumes water from the solution, hence rejecting most of the seawater solutes. Thermodynamic calculations indicated that brine composition and salt assemblages are dependent on the temperature and carbon dioxide partial pressure, also giving reason to the presence and sustainability of highly soluble salts, such as tachyhydrite and bischofite. Brine droplets and salt particles ("dehydratites") can be trapped in fractures and cavities in serpentinites, where they remain as long as no perturbation events will occur. Successive thermal dehydration of buried serpentinites, due, for instance, to igneous intrusion, can mobilize and accumulate the brines, forming highly-saline hydrothermal solutions. These can migrate upwards and erupt onto the seafloor as "saline geysers", which may form salt saturated water pools, as are currently observed in the Red Sea seafloor and elsewhere.
Alternatively, brines could be slowly expelled from fractured serpentinites by buoyancy gradients giving rise to salt (± mud) diapirs. Serpentinization is an ubiquitous, long-lasting process throughout geological times. Serpentinite-related salt deposits can therefore occur buried in many areas of the world, including continents, as well as on Mars. Moreover, gaseous and condensate hydrocarbons can be originated by catalytic reduction of aqueous carbon dioxide and carbonate by dihydrogen produced from serpentinization reactions. This fact can be accounted for the ubiquitous association of salts and hydrocarbon deposits, even those with gigantic size. In addition, gaseous hydrocarbons (mostly methane) released by deep-seated serpentinites, or produced by other processes, can migrate upward, being eventually trapped as gas-hydrates in the near-seafloor, seawater drenched, clayey mud. Salt is an inhibitor for hydrate structure and will form a salinity gradient capable of forming saline brines, even dehydratites, if the hosting mud acts as an efficient sealing layer.
Contributions addressing diverse geological, geochemical and geophysical aspects on salt-giants are welcome. Some suggested topics are given below:
• Formation mechanisms of salt giants;
• Climate-driven evaporation;
• Hydrothermal salination;
• Salt-giant study cases;
• Tectonic setting and evolution;
• Geophysical investigations;
• Hydrocarbon potential of salt-giants;
• Salt deposits: The "bitter salts" perspective;
• Salt deposits: The sulfate perspective.
Keywords: seawater, salt deposits, desalination, serpentinite, hydrothermal brines
Important Note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.