Liquid-absent aqueous fluid inclusions and phase equilibria in the system CaCl 2 ---NaCl---H 2 O

Details Liquid-absent aqueous fluid inclusions and phase equilibria in the system CaCl 2 ---NaCl---H 2 O Liquid-absent aqueous fluid inclusions and phase equilibria in the system CaCl 2 ---NaCl---H 2 O

Mar 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54..611s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 54, Issue 3, pp.611-619

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Scientific paper

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 CaCl 2 ---NaCl---H 2 O, 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 CaCl 2 ---NaCl---H 2 O 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-H 2 O 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 (CaCl 2 · 6H 2 O) and a second hydrate that may be a new mineral, probably a polymorph of CaCl 2 · 4H 2 O; 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 CaCl 2 ---NaCl---H 2 O 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.

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