ABSTRACT

Partition of Al, Mg and Si in coexisting Ca-rich and Ca-poor pyroxenes is expressed by the equation: Mg₂Si₂O₆^Opx + CaAl₂SiO₆^Cpx = MgAl₂SiO₆^Opx + CaMgSi₂O₆^Cpx. Enthalpy- and volume differences in the reaction have been estimated on the basis of available high-temperature experimental data and unitcell parameters of the pyroxene solid solutions as follows: ΔH (at 0kb) = -11.2 (kcal/mol), ΔV = 7.52 X_Al^M1-1.72 (ml/mol) where X_Al^M1 is the mole fraction of MgAl₂SiO₆ in Ca-poor pyroxene in Mg₂Si₂O₆-MgAl₂SiO₆ join. From the thermodynamic data and the calculation of the apparent distribution coefficient, K_D', = (X_MgAl2SiO6/X_Mg2Si2O6)^Opx·(X_CaMgSi2O6/X_CaAl2SiO6)^Cpx for natural pyroxene pairs of various modes of occurrence, the following conclusions have been obtained: (1) The apparent distribution coefficient, K_D' depends on the Fe/Mg ratio of pyroxene as well as equilibration temperature. (2) The normalized apparent distribution coefficient, K_D* can be used as a geothermometer. (3) The distribution coefficient, K_Al (=(^ZAl/Si)^Cpx/(^ZAl/Si)^Opx) defined by GREEN (1964)) may not be an appropriate geobarometer.