Cooling rates of 27 iron and stony-iron meteorites

Abstract

A method for the precise determination of the cooling rates of iron and stony-iron meteorites has been developed and applied to 18 irons, 8 pallasites, and the siderophyre Steinbach. The cooling rates are determined by comparing the measured Ni gradients in several kamacitetaenite areas which result from the growth of the Widmanstätten structure between 700 and 300°C with gradients calculated by a theoretical growth analysis. The theoretical diffusion-growth analysis model shows quantitatively that the imperfect correlation between kamacite band width and bulk Ni content is due to variations in cooling rates. It is suggested that a classification system based on both kamacite band width and bulk Ni content should be used so that variations in cooling rates from one meteorite to another can be clearly seen. A very wide range of cooling rates was found for the 27 meteorites studied (0.4–40°/10 6 yr). Three approximate groupings were found: (1) fine octahedrites, Ge (IVa), 9–40°/10 6 yr, (2) fine and medium octahedrites, 1–5°/10 6 yr, (3) pallasites, 0.4–1.0°/10 6 yr. It can be seen from these groupings that various iron and stony-iron meteorites developed their Widmanstatten structure in different thermal environments, although each meteorite was insulated within a parent body to a temperature less than 350°C. The differences in cooling rates may be due to either varying depth of burial within one or several parent bodies or within the core of several parent bodies. However, not all of the iron meteorites could have cooled within the core of one parent body.