Abstract
Cooling rates for seven hexahedrites, (Uwet, Coahuila, Walker County, Lombard, Quillagua, Hex River Mountains and Tocopilla) have been determined using a ternary diffusion controlled phase growth analysis developed by the authors. The model is applied to the exsolution and growth of plate phosphides in the kamacite phase of hexahedrites during cooling of the meteorite in its parent body. The effects of cooling rate, bulk composition, nucleation temperature and diffusion field length are considered. A unique cooling rate was determined by comparing the Ni content and width of several phosphides in a given hexahedrite to computer-generated curves of Ni content vs phosphide width. Six hexahedrites have cooling rates of approximately 2°C/10 6yr. One hexahedrite, Coahuila, has a somewhat higher cooling rate of 10°C/10 6 yr. These cooling rates fall within the range calculated by an independent method for octahedrites. The cooling rate analysis indicates that the hexahedrites, except for one possible exception, were formed in or close to the core of a parent body ~ 150 km in radius.
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