LIQUID INCLUSIONS IN MINERALS AS A GEOLOGIC BAROMETER

Abstract

The understanding and determination of pressures at the time of mineral formation is an important factor in the study of mineral-forming environments; environmental pressures are derived through investigation of physicochemical properties of gaseous-liquid inclusions. The pressuretemperature (P-T) diagram for H2O with specific-volume isograds, is used as a basis of computation. Complex CO2-inclusion samples for experimentation were taken from quartz crystals found in the Nogolnyy range (Donbass). A basic premise of pressure estimation involved ascertaining that conditions were identical for capture of both CO2 and H2O phases by minerals from a system in its heterogeneous state. The capture process may have involved adhesion of CO2 bubbles to crystal faces as carbonic acid moved (as an emulsion) with the mother liquors. Two types of syngenetic inclusion are investigated: Homogeneous inclusions consisting of aqueous, or gaseous, solution of ‘pure’ carbonic acid were used for temperature determinations; and homogeneous inclusions, for pressure determinations. The latter required estimation of specific weight (specific volume) for inclusion contents at the time of inclusion sealing. Complex relationships of included matter with respect to liquid and gaseous phase-volume ratios to aqueous-solution content and to solubility of carbonic acid in water and of H2O in CO2, necessitated careful compensation for error throughout the investigation. Inclusion cavity volumes were expressed as two hexagonal pyramids plus one prism; and liquid carbonic-acid and gas-bubble inclusion volumes, as spheres standard for the computation of actual volumes for deformed gas bubbles. The author describes a method and apparatus for measurement of gaseous-bubble volumes and for transfer of included materials to determine deformation effects on gaseous-liquid inclusion volumes. Dimensions of gaseous bubbles are compared to those of ideal geometric figures. It was found that gaseous bubbles up to 0.4 mm in diameter were not deformed, i. e. spherical and those 0.4 to 2.0 mm in diameter approached the shape of an ellipsoid of rotation; in this case, 10 percent reduction in volume was introduced as compensation for error. Pressure calculated for Volynian pegmatites at the time of crack healing in morion, was based on two, largely gaseous, inclusions. Pressures established for the time of mineral formation were a minimum of 222 atmospheres for the first sample, and 292 atmospheres for the second. D. D. Fisher