Partition of Al, Mg and Si in coexisting Ca-rich and Ca-poor pyroxenes is expressed by the equation: Mg
2Si
2O
6Opx + CaAl
2SiO
6Cpx = MgAl
2SiO
6Opx + CaMgSi
2O
6Cpx. 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: Δ
H (at 0kb) = -11.2 (kcal/mol), Δ
V = 7.52
XAlM1-1.72 (ml/mol) where
XAlM1 is the mole fraction of MgAl
2SiO
6 in Ca-poor pyroxene in Mg
2Si
2O
6-MgAl
2SiO
6 join. From the thermodynamic data and the calculation of the apparent distribution coefficient,
KD', = (
XMgAl2SiO6/
XMg2Si2O6)
Opx·(
XCaMgSi2O6/
XCaAl2SiO6)
Cpx for natural pyroxene pairs of various modes of occurrence, the following conclusions have been obtained: (1) The apparent distribution coefficient,
KD' depends on the Fe/Mg ratio of pyroxene as well as equilibration temperature. (2) The normalized apparent distribution coefficient,
KD* can be used as a geothermometer. (3) The distribution coefficient,
KAl (=(
ZAl/Si)
Cpx/(
ZAl/Si)
Opx) defined by GREEN (1964)) may not be an appropriate geobarometer.
View full abstract