Experimental determination of phase relations of pyroxenes in the system Na2O-CaO-MgO-Al2O3-SiO2 (NCMAS) at 2.5 GPa and high temperatures

摘要

Phase relations of clinopyroxene (Cpx) and orthopyroxene (Opx) in the system Na2O-CaO-MgO-Al2O3-SiO2 (NCMAS) were studied experimentally at 2.5 GPa and a range of temperatures from 1000 to 1300°C using a piston-cylinder apparatus. The compositions of the coexisting Opx and Cpx solid solutions were determined by electron microprobe analysis. The results show that Cpx and Opx solid solutions coexist with a large miscibility gap. The concentrations of Mg2Si 2O6 in Cpx increase substantially with increasing temperatures but the content of NaAlSi2O6 and CaMgSi2O 6 in Opx is very limited. Based on electron microprobe data from this study and the data from previous studies for the three binary systems NaAlSi 2O6-Mg2Si2O6, Mg2Si 2O6-CaMgSi2O6, and NaAlSi2O 6-CaMgSi2O6, the ternary diagram of Mg2Si2O6 - NaAlSi2O6 - CaMgSi2O6 has been constructed. These results can be used empirically as a geothermometer. However, an attempt to develop a thermodynamic model based on the experimental data failed and the problems are discussed.