Olivine cooling speedometers

Several kinetic models of zoning in olivines are discussed at length. The effects on predicted cooling rates of various assumptions used in the analyses are evaluated. It is concluded that the models of Walker et al. (1977) and Taylor et al. (1977) both provide underestimates of the cooling rate required to preserve a given compositional profile, and that both models as well as the model of Taylor et al. (1978) can be used to provide order-of-magnitude estimates of cooling rates. A new model is described which considers diffusion in both solid and liquid during crystallization as well as diffusion in the solid after crystallization is complete. The model provides a description of the compositional gradients which develop during crystallization as well as after cooling at various rates. Applied to olivine crystals nucleated at 1272 C in a high-iron analogue to Lunar Composition 15555, the model predicts only slight compositional gradients - in accord with electron beam microprobe measurements on crystals grown isothermally at this temperature.