LIQUID-ABSENT AQUEOUS FLUID INCLUSIONS AND PHASE-EQUILIBRIA IN THE SYSTEM CACL2-NACL-H2O

Raman spectra and microthermometric data are reported for a new class of aqueous fluid inclusions in quartz from the Bushveld Complex. The inclusions, which are approximated by the system CaCl2NaClH2O, hold special interest because they display the following properties: 1. (1) The inclusions do not contain a liquid phase at room temperature, but they homogenize to an aqueous liquid at elevated temperatures; 2. (2) Initial melting occurs at a reaction point at +29°C, rather than at the CaCl2NaClH2O eutectic point at -52°C, as is generally the case for fluid inclusions approximated by this system; 3. (3) Ice is absent in the subsolidus assemblage despite the high-H2O contents of the inclusions. An aqueous liquid is generated upon heating through decomposition of incongruently melting hydrate daughter minerals; 4. (4) The most abundant minerals in the subsolidus assemblages are antarcticite (CaCl2 · 6H2O) and a second hydrate that may be a new mineral, probably a polymorph of CaCl2 · 4H2O; and 5. (5) The fluid compositions fall outside the compositional limits defined by previous studies of natural fluid inclusions. The inclusions are characterized by a high Ca Na ratio and a very high concentration of total dissolved solids (greater than 52 wt%). Aqueous fluid inclusions that do not contain a liquid phase at ambient temperature may be commonly overlooked or misinterpreted as mineral inclusions. The liquid-absent aqueous inclusions from the Bushveld Complex are spatially associated with a second type of fluid inclusion comprised of antarcticite, halite, vapor, and liquid at 20°C. The two types of inclusions have similar bulk compositions, but they differ radically in melting behavior because their compositions lie on opposite sides of a subsolidus join. Given the topology of the CaCl2NaClH2O phase diagram, small variations in bulk composition in the vicinity of a phase boundary or subsolidus join can result in large differences in the initial melting temperature, equilibrium melting sequence, and phase proportions at 20°C. Conversely, significant constraints on fluid composition may be derived from qualitative observations of the equilibrium melting sequence. © 1990.