Separation of Salts NaCl and CaCl2 in Aqueous-Carbon Dioxide Deep Fluids

Abstract

The possibility of changing the ratio of the concentrations of NaCl and CaCl₂ salts in fluid phases formed as a result of heterogenization of the H₂O–CO₂–NaCl–CaCl₂ fluid with a decrease in P-T parameters has been studied. A well-known experimental fact regarding the ternary systems H₂O–CO₂–NaCl and H₂O–CO₂–CaCl₂ is the greater tendency of the H₂O–CO₂–CaCl₂ system to separate into coexisting predominantly aqueous-salt and aqueous-carbon dioxide phases compared to the similar system H₂O–CO₂–NaCl. This experimental fact can be interpreted as a greater affinity of NaCl for CO₂ compared to CaCl₂. Using a recently developed numerical thermodynamic model of the H₂O–CO₂–NaCl–CaCl₂ quaternary fluid system, it was possible to identify geologically significant consequences of this difference in the interaction of NaCl and CaCl₂ with CO₂. Multistage heterogenization of the H₂O–CO₂–NaCl–CaCl₂ fluid with a significant decrease in P-T parameters ultimately leads to the formation of aqueous-carbon dioxide fluid phase f2, the salt component of which is significantly enriched in NaCl and depleted in CaCl₂ compared to the initial fluid. The fluid phase f1 formed at each stage of heterogenization has a predominantly water-salt composition with the ratio of the mole fractions of NaCl and CaCl₂ salts, differing little from that in the initial fluid. However, the total mole fraction of salt in the f1 phase, as a rule, significantly exceeds that in the original fluid. The density of phase f1 significantly exceeds the density of phase f2. During the process of multistage heterogenization of the fluid phase f1, there is no formation of a fluid with a significant enrichment of CaCl₂ compared to the initial ratio of the mole fractions of NaCl and CaCl₂. At the same time, successive multiple separation of the f2 phase leads to the enrichment of its salt component in NaCl. Under favorable conditions, this process can lead to the formation of a fluid with almost pure NaCl salt. Changes in the salt composition of the fluid H₂O–CO₂–NaCl–CaCl₂ are considered in application to the evolution of fluid composition along the regressive branch of the P-T trend of HP metamorphism and syngranulite metasomatism in the Lapland granulite belt.