Na-carbonate waters of extreme composition: Possible origin and evolution

抄録

Na-carbonate (NaCW) waters are not concentrated in well defined areas, but usually widespread in areas where other water types (e.g., Ca-carbonate) are dominant. NaCW are the product of long-term water-rock interaction with dissolution of Na-silicates in presence of phyllosilicates, silica phases, and calcite. NaCW circulating in calcite-bearing sediments very probably have a Ca-carbonate parent with moderate to low P_CO2, which changes its composition assuming increasing Na character as the water-rock interaction proceeds. However, the moderate to low P_CO2 values of the potential parent Ca-carbonate waters do not account for the high Na content of many NaCW. The higher P_CO2 required may be due to oxidation of organic matter of the sediments and, perhaps, to further addition of CO₂ coming from deeper crustal levels, from the mantle, or from other sources. Na-Ca exchange involving a Na-exchanger could be an alternative genetic hypothesis. At present, however, at least for some areas (e.g., Northern Apennines, Italy), this hypothesis is not supported by mineralogical evidence.