Mediterranean salt giants beyond the evaporite model: The Sicily perspective.

Mediterranean salt giants, occurring both in sub-seafloor and in onshore settings (the “Gessoso Solfifera Group”),are traditionally explained by repeated cycles of desiccation and replenishment of the entire basin. However, suchhypotheses are strongly biased by mass balance calculations and geodynamic considerations. In addition, anyhypothesis without full desiccation, still based on the evaporite model, should consider that seawater brines startto precipitate halite when 2/3 of the seawater has evaporated, and hence the level of the basin cannot be the sameas the adjacent ocean.On the other hand, hydrothermal venting of hot saline brines onto the seafloor can precipitate salt in a deep marinebasin if a layer of heavy brine exists along the seafloor. This process, likely related to sub-surface boiling orsupercritical out-salting (Hovland et al., 2006), is consistent with geological evidence in the Red Sea "Deeps"(Hovland et al., 2015).Although supercritical out-salting and phase separation can sufficiently explain the formation of several marinesalt deposits, even in deep marine settings, the Mediterranean salt giant formations can also be explained by theserpentinization model (Scribano et al., 2016). Serpentinization of abyssal peridotites does not involve seawatersalts, and large quantities of saline brines accumulate in pores and fractures of the sub-seafloor serpentinites.If these rocks undergo thermal dehydration, for example, due to igneous intrusions, brines and salt slurries canmigrate upwards as hydrothermal plumes, eventually venting at the seafloor, giving rise to giant salt deposits overtime.These hydrothermal processes can take place in a temporal sequence, as it occurred in the "Caltanissetta Basin"(Sicily). There, salt accumulation associated with serpentinization started during Triassic times (and even earlier),and venting of heavy brines onto the seafloor eventually occurred in the Messinian via the hydrothermal plumemechanism (Scribano et al, 2016).This innovative model arises from the study of xenoliths in the Hyblean diatremes (southeastern Sicily), suggestingthat a widely serpentinized Tethyan basement lies beneath the entire Sicily island and its offshore areas (Scribanoet al., 2006; Manuella et al., 2015), according to geophysical, geological and petrological aspects.In conclusion, our viewpoint represents a plausible explanation for the origin of giant salt deposits in theMediterranean area and elsewhere, albeit this hypothesis should be constrained by further studies.Hovland et al. (2006), Mar. Petrol. Geol., 23, 855–869.Hovland et al. (2015), In: Rasul, N.M.A., and Stewart, I.C.F., eds., The Red Sea. Springer, 187-203. DOI:10.1007/978-3-662-45201-1_11Manuella et al. (2015), Int. J. Earth Sci., 104, 1317–1336.Scribano et al. (2006), Mineral. Petrol., 86, 63–88.Scribano et al. (2016), Int. J. Earth Sci., submitted.