Cooling rates of group IVA iron meteorites determined from a ternary Fe-Ni-P model

Cooling rates have been determined for twelve group IVA iron meteorites using a ternary (Fe-Ni-P) model that simulates the growth of the Widmanstätten pattern. The new ternary model is governed by a set of differential diffusion equations that are coupled through the phase growth velocity and elemental concentration profiles. Measured ternary diffusivities and phase diagram solubilities were extrapolated below 500°C for use in the model. The model is more sophisticated than previous ones in that P as well as Ni gradients are calculated, ternary α-γ tie lines are allowed to vary, and ternary diffusivities are used.Output from the simulation is used to create a family of cooling rate curves on plots of central taenite Ni vs. log taenite half-width for each meteorite. A comparison of measured data to the cooling rate curves yields unique meteorite cooling rates. The measured cooling rates for the twelve IVA irons vary inversely with Ni content by over an order of magnitude (4–200°C/Myr). It is proposed that the group IVA irons were accommodated at various depths in an asteroidal-sized body.