THE COOLING HISTORY OF THE LEWIS-CLIFF-86010 ANGRITE AS INFERRED FROMKIRSCHSTEINITE LAMELLAE IN OLIVINE

Olivine in the angritic meteorite Lewis Cliff (LEW) 86010 contains abu ndant exsolution lamellae of kirschsteinite. Compositional gradients a djacent to the interface in both host and lamellae were formed by diff usion of chemical components into and out of the lamellae during cooli ng and growth. We have compared these gradients with compositional pro files calculated from diffusion and heat flow equations to estimate th e cooling rate and burial depth of the sample. The resulting values fo r cooling rate and burial depth depend on which values are used for th e 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 obtaine d from seven different lamellae range from 30 to 52 degrees C/year, wi th an average of 42 degrees C/year, and burial depths (assuming an ove rburden with a thermal diffusivity typical of solid rock) range from 1 4 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 degree s C/year, with an average of 1.7 degrees C/year, and the depths range from 68 to 83 m, with an average of 75 m. For the highest Ca diffusivi ties, details of the compositional profiles near the olivine/kirschste inite interface suggest that continuous cooling was greatly accelerate d at a temperature near 600-700 degrees C. The simplest physical expla nation for such an acceleration is excavation of the sample from its o riginal burial depth by an impact event. If Ca diffusivities are lower , a two-stage cooling history is not required.