Origin of salt giants in abyssal serpentinite systems

Abstract Worldwide marine salt deposits ranging overthe entire geological record are generally considered climate-related evaporites, derived from the precipitation ofsalts (mainly chlorides and sulfates) from saturated solutionsdriven by solar evaporation of seawater. This explanationmay be realistic for a salt thickness ≤100 m, beingtherefore inadequate for thicker (>1 km) deposits. Moreover,sub-seafloor salt deposits in deep marine basins aredifficult to reconcile with a surface evaporation model.Marine geology reports on abyssal serpentinite systemsprovide an alternative explanation for some salt deposits.Seawater-driven serpentinization consumes water andincreases the salinity of the associated aqueous brines.Brines can be trapped in fractures and cavities in serpentinitesand the surrounding ‘country’ rocks. Successive thermal dehydration of buried serpentinites can mobilizeand accumulate the brines, forming highly saline hydrothermalsolutions. These can migrate upwards and eruptonto the seafloor as saline geysers, which may form saltsaturatedwater pools, as are currently observed in numerousdeeps in the Red Sea and elsewhere. The drainage ofdeep-seated saline brines to seafloor may be a long-lasting,effective process, mainly occurring in areas characterizedby strong tectonic stresses and/or igneous intrusions. Alternatively,brines could be slowly expelled from fracturedserpentinites by buoyancy gradients and, hence, separatedsalts/brines could intrude vertically into surrounding rocks,forming salt diapirs. Serpentinization is an ubiquitous, exothermic,long-lasting process which can modify large volumesof oceanic lithosphere over geological times. Therefore,buried salt deposits in many areas of the world can bereasonably related to serpentinites.