2025/10/04 by Viktorovna Banushkina, Sofja, Friedrichovna Golitsyna, Zoia, Anatolyevich Kirdyashkin, Alexey
#CMAS system #CaEs end member #Clinopyroxene solid solution #Experimental petrology #Geothermobarometry
paper · doi:10.57647/j.ijes.2025.17558
The possibility of ‘excess’ silica dissolution in clinopyroxene (Cpx) as the calcium molecule of Eskola (CaEs) end member has been known for a long time. Currently, attempts to determine the dependence of non-stoichiometric Cpx compositions on (P, T)-parameters are ongoing. This article presents data on the analysis of Cpx compositions and the structural and textural features of a phase association obtained from an experimental study of the Di-CaEs cross-section in a ‘dry’ CMAS system. At atmospheric pressure, the temperature range is 960-1240±10 oC. At pressures of 1.0-3.0 GPa, the temperature range is 1170-1550±10 oC. Experiments with synthetic compositions were carried out in a vertical shaft electric resistance furnace with silicon carbide heaters (at atmospheric pressure) and by the quenching method on a piston-cylinder apparatus (at high pressure). It is noted that in the investigated cross-section, two independent stable pyroxene phases are formed: an aluminum-bearing Cpx and a high-magnesian aluminum-free diopside (Di). The composition of clinopyroxene solid solution Cpx(ss) is represented by a quad series of diopside (Di), enstatite (En), calcium molecule of Tschermak (CaTs), calcium molecule of Eskola (CaEs) end members. The diopside phase belongs to the Di-En series. The influence of P-T parameters and bulk chemical composition on the content of CaEs in Cpx is considered. The trend of eutectic equilibria exists in the temperature range from 1137±10 оС (at 1 atm) to 1300±10 оС (at 3.0 GPa). The experimental data obtained can be used in the future to build a physicochemical model of the evolution of quartz-normative rocks.