Evaluation of a crust model of eucrites from the width of exsolved pyroxene

Abstract

An attempt has been made to locate known eucrites within a proposed crust model by estimating the absolute cooling rates from the width of exsolution lamellae of augite in pigeonite. We simplified the model of exsolution phenomena such that the growth of the exsolution lamella will be controlled by the diffusion of Ca atom perpendicular to the lamella. By taking into account temperature dependency of Ca atomic diffusion coefficient in pigeonite and the temperature-time variations at a given depth of the crust, we integrated the equation derived from diffusion equation, with respect to temperature (1, 200-850°C) by the numerical method to match the observed width of lamellae for the bulk Ca content of pigeonite. For a proposed achondrite parent body of 1, 000km in diameter, the depth estimated for Moama is 12km below the surface, Moore County 9km, Juvinas 1.7km and Mt. Padbury 0.5km. The absolute cooling rate for the first two meteorites is about 1°C/10⁴ years. The eucritic crust of the parent body may be thinner than about 15 km.