Intensive parameters of enstatite chondrite metamorphism

The Enstatite-Oldhamite Geothermometer (EOG) developed by Larimer and Buseck (1974) makes use of the five-phase assemblage kamacite, quartz, enstatite, oldhamite, troilite to determine metamorphic temperatures in enstatite chondrites. A re-evaluation of the thermodynamic foundation of the EOG shows that several of the assumptions and approximations can be significantly improved with new thermodynamic data. The geothermometer has been modified to reflect, among other things, the solution behavior of Si in kamacite, the solution behavior of a calcium silicate component in enstatite, and metal sulfide fusion as a function of temperature and composition. An analysis of the EOG shows that the activity coefficient of Si in kamacite and the Ca-rich component of enstatite are the model's most important parameters. For Si, the activity coefficient data of Sakao and Elliot (1975) have been used. The activity coefficient of a Ca-rich component of enstatite presents a unique problem since data on this parameter do not exist for one-pyroxene systems and two-pyroxene solution models do not provide unique activity coefficients in the absence of diopside. The model, therefore, has been formulated using CaMgSi2O6 as the Ca-rich component of enstatite and assumes that it behaves as a symmetric regular Margules solution. The Margules interaction parameter WG is then used as an adjustable constant and is varied such that the resulting temperatures fall between minimum and maximum temperatures determined independently. Temperature minima were determined from the enstatite-diopside solvus of Carlson (1986) and indicate that the EL6 chondrites have been metamorphosed to temperatures exceeding 900–1000°C. The temperature of the enstatite-plagioclase-quartz eutectic (1090°C), appropriate for EL6 plagioclase compositions, was used as the metamorphic maxima. With these constraints, a WG value of 52 kJ/mole was estimated as appropriate (within the context of the model) for EL6 metamorphic conditions. EL6 temperatures determined via this method range from 981°C for Yilmia to 1068°C for Blithfield. Temperature estimates of greater than 1000°C for the highly equilibrated EL6 chondrites require the formation of a metal-sulfide liquid during metamorphism. Liquid fractionation of this predominantly Fe-FeS liquid could explain the siderophile, chalcophile, and volatile element depletion in the EL6 class relative to CI and EH chondrites. Liquid fractionation, however, is not required by the