A system of mineralogical geothermobarometers for deep parageneses based on the phase diagram of the basic petrological system CaO-MgO-Al2O3-SiO2

Based on comparison of phase relationships in the system CaO-MgO-Al2O3-SiO2, the author considers mineralogical geothermometers and geobarometers which are used in geothermobarometry of basic and ultrabasic rocks for reconstructing the structure of the upper mantle, stratifying deep-seated xenoliths, searching for new diamond deposits, as well as for global tectonic constructions. The paper discusses the possibilities and restrictions of geothermobarometry for parageneses of garnet harzburgites, pyroxenites, Iherzolites, garnet pyroxenites, garnet Iherzolites, eclogites, garnet wehrlites, grospydites, and quartz eclogites, since they are similar to phase associations of the system CaO-MgO-Al2O3-SiO2. Also, the problems of developing geothermobarometers for studying the majorite component in deep-seated garnets are considered. It is shown that phase relationships in the system CaO-MgO-Al2O3-SiO2 inherit the character of phase volume boundaries and the peculiarities of changes of solid phase solutions on introducing additional components into the system. A change in the content of any cation with temperature or pressure depends on the character of the cation position in a mineral structure rather than on the chemical nature of this cation. Temperature affects cation replacement in the octahedral and other positions between the main lattice sites or in the structural positions of the aluminosilicate framework itself. Pressure has an effect on the structural positions of mineral framework, namely, on distribution of aluminum cations between the four- and six-coordinated positions in the pyroxene structure and on distribution of silicon cations as a majorite component between the four- and six-coordinated positions in the garnet structure.