A comparison of metallographic cooling rate methods used in meteorites

Abstract

The primary objective of this study was to test the postulate that cooling rates acquired from metal grains in chondrites are consistent with those from iron meteorites. Both types of metal occur in some Group IAB meteorites, which are mixtures of massive metal with well-developed Widmanstätten structures and chondritic inclusions with dispersed metal grains. The grains have textures and compositions similar to chondritic metal, including negligible P. The meteorites studied show little or no sign of shock reheating and textural evidence indicates that silicates and metal were mixed before Widmanstätten patterns formed during cooling. Cooling rates were obtained by comparing measured to modeled taenite grain or lamellae dimensions and central Ni contents. Modeling entails solving diffusion equations using experimental diffusion coefficients, phase relations, and bulk or local Ni and P contents, taking into account geometry, undercooling, and impingement. There is one set of parameters for grains and another, quite different set for Widmanstätten lamellae, including a factor of 30 difference in diffusion coefficients. Yet cooling rates obtained from Widmanstätten structures and metal grains in chondritic inclusions of the same meteorite are consistent; uncertainties in the best data are ±10° /Ma, equivalent to a factor of 1 ± 0.25. This agreement implies that the data and models are correct or contain fortuitously offsetting errors, which is quite unlikely. Cooling rates range from 40°/Ma to 70° /Ma in IAB meteorites that contain both grains and Widmanstätten structures. Rates based on grains in Ni-poor and Ni-rich meteorites lacking Widmanstätten patterns expand the range from 30°/Ma to perhaps 200°/Ma. Cooling rates correlate with Ni content; Ni-poor meteorites have slower rates than Ni-rich ones. Evidently, IAB meteorites were radially distributed over >30km in a body with a radius >50km. A comparison of the available Ar ages with cooling times inferred from the cooling rates suggests that the parent body cooled more slowly after the metallographic cooling rates were established.