| Abstract: | Abstract— Olivine in the angritic meteorite Lewis Cliff (LEW) 86010 contains abundant exsolution lamellae of kirschsteinite. Compositional gradients adjacent to the interface in both host and lamellae were formed by diffusion of chemical components into and out of the lamellae during cooling and growth. We have compared these gradients with compositional profiles calculated from diffusion and heat flow equations to estimate the cooling rate and burial depth of the sample. The resulting values for cooling rate and burial depth depend on which values are used for the diffusion rate of Ca in olivine, and how measured diffusivities are extrapolated to the lower temperatures at which the lamellae grew. If the highest diffusion coefficients are used, the cooling rates obtained from seven different lamellae range from 30 to 52 °C/year, with an average of 42 °C/year, and burial depths (assuming an overburden with a thermal diffusivity typical of solid rock) range from 14 to 17 m, with an average of 15 m. If the lowest reasonable diffusion coefficients are used, the cooling rates range from 1.4 to 2.2 °C/year, with an average of 1.7 °C/year, and the depths range from 68 to 83 m, with an average of 75 m. For the highest Ca diffusivities, details of the compositional profiles near the olivine/kirschsteinite interface suggest that continuous cooling was greatly accelerated at a temperature near 600–700 °C. The simplest physical explanation for such an acceleration is excavation of the sample from its original burial depth by an impact event. If Ca diffusivities are lower, a two-stage cooling history is not required. |
