Cooling rates and thermal histories of iron and stony-iron meteorites

Abstract

Cooling rates of meteorite parent bodies are calculated on the basis of theoretical thermal models. These calculated cooling rates are compared with meteorite cooling rates derived by diffusion-growth analysis of the metal phase of iron and stony-iron meteorites. Previous thermal calculations are extended to include effects of melting, redistribution of radioactive heat sources, and surface heating. For a range of assumed conditions, the cooling rates depend strongly on the extent of internal redistribution of radioactive elements. Cooling rates calculated for uniform and fractionated parent bodies exceeding 100 km in radius can differ by a factor of more than 3. Since the distribution of radioactive materials is uncertain, size estimates for bodies with a radius exceeding 100 km can only be narrowed to a range of values bounded by the uniform and the fractionated case. This range of body sizes is still compatible with an origin of iron and stony-iron meteorites in asteroidal bodies. The present calculations are not consistent with the development of iron meteorites by surface heating of parent bodies. The core model appears to be appropriate for some iron meteorites. However, the variable cooling rates for iron meteorite groups IIIa and IVa are explained by a model where small iron bodies are distributed throughout a large region of the parent body from the deep interior out to a fractional radius of 0.90 to 0.96. It is proposed that such a distribution of iron meteorites could be produced by a zone melting process