Abstract
The evaporation of seawater in arid climates is currently the main accepted driving mechanism for the formation of ancient and recent salt deposits in shallow basins. However, the deposition of huge amounts of marine salts, including the formation of tens of metres of highly soluble types (tachyhydrite and bischofite) during the Aptian in the South Atlantic and during the Messinian Salinity Crisis, are inconsistent with the wet and warm palaeoclimate conditions reconstructed for these periods. Recently, a debate has been developed that opposes the classic model of evaporite deposition and argues for the generation of salt by serpentinization. The products of the latter process can be called “dehydratites”. The associated geochemical processes involve the consumption of massive amounts of pure water, leading to the production of concentrated brines. Here, we investigate thermodynamic calculations that account for high salinities and the production of soluble salts and MgCl2-rich brines through sub-seafloor serpentinization processes. Our results indicate that salt and brine formation occurs during serpentinization and that the brine composition and salt assemblages are dependent on the temperature and CO2 partial pressure. Our findings help explain the presence and sustainability of highly soluble salts that appear inconsistent with reconstructed climatic conditions and demonstrate that the presence of highly soluble salts probably has implications for global tectonics and palaeoclimate reconstructions.
Highlights
Change during the Messinian Salinity Crisis (MSC) and average annual conditions with a temperature of 22 °C (Δ = 15.6–24.7 °C), a relative humidity of 19% (Δ = 17.5–60%) and a rainfall of 575 mm (Δ = 355–872 mm)[19]
The conditions were more humid than the current Danakil climate and would have prevented preservation of solid soluble salts due to seasonal variations and day/night humidity alternations[21,22,23]
The observation of highly soluble salts has led to the conclusion that the climate during their deposition was arid on the basis of the evaporation theory[24]
Summary
Emerging theories propose deep salt formation by supercritical fluids[27] or exothermic serpentinization in mantle exhumation zones[28,29,30]. Post-serpentinization processes produce Mg-rich, high-salinity fluids that can be effective agents in the metasomatism of country rocks. Some authors disregard the large-scale formation of salt by serpentinization arguing, without thermodynamic calculations, that the fluid resulting from the interaction between seawater and mantle rocks would be depleted in Mg, enriched in K and Ca and converted to CaCl2-rich brine[37]. To investigate this process, we calculate the processes by using the Pitzer formalism[38], which is suited to describing salt solubility in complex mixtures of aqueous electrolytes
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