MELTING BEHAVIOR OF FLUID INCLUSIONS IN LABORATORY-GROWN HALITE CRYSTALS IN THE SYSTEMS NACL-H2O, NACL-KCL-H2O, NACL-MGCL2-H2O, AND NACL-CACL2-H2O

The chemical composition of aqueous fluid inclusions in crystals of halite can be accurately determined from observed melting behaviors of ice, hydrohalite, and sylvite. Some fluid inclusion melting behaviors observed in laboratory-grown halite crystals (systems NaClH2O, NaClKClH2O, NaClMgCl2 H2O, and NaClCaCl2H2O) differ from predicted stable equilibrium relations. In the NaClH2O and NaClKClH2O systems, observed first melt temperatures are up to 15°C below the equilibrium eutectic temperatures of -21.2° and -22.9°C, respectively. The final melting temperature of ice, in the presence of hydrohalite, and the final melting temperature of hydrohalite are reproducible and match predicted melting temperatures. The limit of detection of sylvite daughter crystals in the NaClKClH2O system is approximately 5 wt% (≈ 1 molal) KCl. Final melting temperatures of sylvite match published equilibrium data to within 0.3°C. In the NaClKClH2O system at halite saturation, mKCl can be determined from the final sylvite dissolution temperature or from the final melting temperature of hydrohalite. Fluid inclusions in the NaClMgCl2H2O and NaClCaCl2H2O systems that form stable salt hydrates (MgCl2 · 12H2O and CaCl2 · 6H2O) during freezing first melt within 3°C of predicted eutectic temperatures (-37° and -52°C). However, fluid inclusions with MgCl2 or CaCl2 may also start melting at temperatures as low as -80°C. Such low first melt temperatures indicate the presence of metastable salt hydrates (presumably MgCl2 · 8H2O, MgCl2 · 6H2O or CaCl2 · 4H2O). The formation of metastable phases during freezing of fluid inclusions can lead to misinterpretation of the chemical composition of fluid inclusions in natural samples. This is especially true for fluid inclusions with first melt temperatures below -37°C which may be erroneously interpreted as being rich in CaCl2. The final melting of ice in the presence of hydrohalite may vary by more than 15°C in fluid inclusions of different size but identical bulk composition, and occurs at lower temperatures than predicted in fluid inclusions from the NaClMgCl2H2O and NaClCaCl2H2O systems. However, the final melting temperature of ice in inclusions which fail to nucleate hydrohalite, and the final melting temperature of hydrohalite are reproducible to within ±0.1 °C and can be used to determine MgCl2 and CaCl2 molalities. © 1990.