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Extremely low solubility of typical seawater salts within certain supercritical sections of their pressure^temperature composition space is a proven experimental fact. Its consequences are often referred to as either 'shock crystallization' or 'out-salting' . Our alternative model for the formation of salt deposits hypothesizes that high temperatures and pressures characteristic for the high heat-£ow zones of tectonically active basins may bring submarine brines into the out-salting regions and result in the accumulation of geological-scale salt depositions.To con¢rm the laboratory observations, molecular-scale simulations (molecular dynamics) have been employed to study structural changes in a model seawater system where temperature increased from ambient to near-critical and supercritical. Fluid properties and phase transition regions extracted from the simulations were then used as input parameters for a reservoir simulator program to model out-salting in a simple hydrothermal geological system. Both numerical simulations and laboratory experiments con¢rm that supercritical out-salting is a viable process of geological signi¢cance for the formation and accumulation of evaporites.We suggest two regions where hydrothermally associated salts may be depositing today: Atlantis II Deep, in the Red Sea, and Lake Asale, Ethiopia. Supercritical waterThe critical point (CP) for distilled water is 374.15 1C and 221.2 bar. Beyond this point, the physical and chemical properties of water change fundamentally; e.g. the dielec-Correspondence: Martin Hovland,

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