Mineralogical Thermometry

a set of methods that make possible recreation of the physical and chemical conditions of endogenous formation of minerals. Very small systems of gaseous, liquid, and solid inclusions in minerals are used as a source of genetic information in mineralogical thermometry.

The homogenization method is one of the main techniques in mineralogical thermometry. The method entails the restoration (homogenization) of the original uniform state (a liquid water-salt or gaseous solution and magmatic melt) by heating the inclusions under a microscope. The homogenization method is used to determine the temperature at which the original volume of the vacuole is filled by the liquid. The temperature at which the gas bubble disappears determines the temperature at which microscopic portions of the melt or hydrothermal salt solution are captured by the mineral, which is at a minimum under pressure. Gaseous-liquid inclusions of various growth zones of crystals reveal the course of changes with temperature, and those in mineral parageneses of various stages of the formation of veins of ore make possible establishment of the thermodynamics of the formation of a deposit as a whole in relative values of the homogenization temperatures (T_h).

The homogenization method is usually combined with two auxiliary methods: the visual method (empirical curves) and the decrepitation method (disruption of inclusions). The visual method is based on the determination, under a microscope, of the percentage ratios of the volumes of gas and the liquid that was hot and homogeneous upon conservation and, upon subsequent cooling to ordinary temperatures, condensed with the formation of a gas (vapor) bubble of volume V that is proportional to the extent of heating of the aqueous solution. This temperature (T_v) is determined up to 200°C with good accuracy according to empirical temperature-volume curves. Above T_h, the inclusions are disrupted because of a sharp increase in internal pressure, and the effect of their disruption (decrepitation) is determined by means of a pulse counter on an oscillograph or automatic recorder, using electron amplifiers. The temperature of onset of massive disruptions (T_d) is assumed to be greater than T_h, but the values are taken as being to some extent close to the temperatures of mineral formation. The decrepitation method is inferior in accuracy to the homogenization method but is suitable for both transparent and opaque minerals. The temperature points obtained by these methods for abyssal processes of the geological past (T_h, T_v, and T_d) have their own significance, apart from discrepancies with any temperature scale adopted for the surface of the earth. In temperature experiments with inclusions, the relative course of changes in energy levels in endogenous mineral formation processes is revealed by a group of methods.