Solidification zoning and metallographic cooling rates of chondrites

Abstract

The cooling rates of meteorites provide important clues to the sizes of their bodies. The cooling rates of chrondritic meteorites have been determined by measuring the concentration of nickel in the interiors of taenite grains of various sizes and by comparing these data with computer models for the Ni distribution as a function of cooling rate¹. It has been recently suggested^2,3 that in some meteorites the measured Ni gradients in taenite are not entirely produced by solid-state diffusion but partly reflect Ni gradients produced during solidification coring. If this is correct, Wood's cooling rate method¹ would be invalidated for these meteorites. We have investigated the effect of zoning produced during solidification on the formulation of the Wood model, and report here that solidification zoning is erased during kamacite growth and has no influence on the resultant taenite Ni gradients for all cases in which the cooling rate is sufficiently slow (<1,000K Myr⁻¹) to allow Ni to diffuse to the taenite grain centre.