Abyssal serpentinites as gigantic factories of marine salts and oil

Serpentinites occur worldwide in various geological settings, such as in magma-starved oceanic basins. Abyssal serpentinites are long-term storage for chlorine and carbon, the latter forming dominantly reduced and subordinately oxidized compounds. Hence, serpentinites, salts, and reduced carbon phases coexist in an “abyssal association”. Although huge reservoirs of salts and reduced carbonaceous matter in abyssal serpentinites are thought to be lacking, this may be more a reflection of intrinsic limits in research methods than an indication of their absence. On the other hand, shallow-seated hydrocarbon-rich mud volcanoes and diapirs, consisting of salts, hydrocarbons (oils and gases), and clayey muds (including mudded serpentines) can provide evidence for such an oceanic association. Here, we search for scientific data in support of the “oceanic association”, and hence we propose a reappraisal of exploitable worldwide salt-hydrocarbon reservoirs, even gigantic in size. Abyssal serpentinites are well-known source rocks of abiotic gaseous hydrocarbons via Fischer-Tropsch-type reaction (FTT), whereas macromolecular carbonaceous matter (mostly bitumen) found in serpentinites is postulated to be biogenic. This inference is based mainly on three assumptions: (i) FTT in nature does not yield oils; (ii) hydrocarbons derive essentially from the thermal alteration of microbial molecules; (iii) the presence of biomarkers, traditionally considered evidence of the biological origin of oils. Nevertheless, some lines of strong evidence disprove these accepted conventions: (i) Lab experiments demonstrated that FTT can synthesize significant quantities of liquid hydrocarbons (oils) under conditions compatible with the deeper and more reduced level of serpentinite-hosted hydrothermal systems; (ii) macromolecules, generally thought to derive from microbial activity, can be abiotically generated as documented in extraterrestrial bodies; (iii) biomarkers are evidence for the proliferation of microbial communities, which subsist on serpentinization and FTT products, thus they are biological pollutants. Moreover, seawater-driven serpentinization consumes water producing huge amounts (∼11 kg halite·m−3 of peridotite) of salts that can be stored in the deeper zone of abyssal-type hydrothermal systems. Thus, the origin of shallow-sited salt deposits, even gigantic in size, can be related to either the advective upwelling of hot hydrothermal brines, due to the dehydration of abyssal serpentinites, or to the passive upwelling of buoyant saline geobodies. Piercement structures are indeed seismically imaged in several localities, such as South Atlantic, the Gulf of Mexico, the Serpentinite belt extending from Cuba to Hispaniola, the North Sea trench, and the Zagros orogenic belt. Similar lines of evidence at the microscopic scale have been found in some Hyblean serpentinite xenoliths (south-eastern Sicily), being representative of the unmetamorfosed in-situ relic of the Palaeo-Tethys Ocean, upon which Sicily and its off-shore areas lie. In spite of the accepted stereotype of hydrocarbons as fossil fuels, and hence finite resources, reduced carbon compounds abiotically produced in abyssal serpentinites may be copious, protracted over time, and thus renewable