THERMAL HISTORY OF MESOSIDERITES - QUANTITATIVE CONSTRAINTS FROM COMPOSITIONAL ZONING AND FE-MG ORDERING IN ORTHOPYROXENES

We have derived mathematical relations to calculate cooling rates from the extent of compositional zoning developed during cooling across th e interface of a natural diffusion couple. These relations were used t o calculate the high temperature cooling rates of the mesosiderites Lo wicz and Clover Springs from the available data on compositional zonin g across core-overgrowth interface of orthopyroxene grains. We have al so determined the cation ordering in four selected orthopyroxene cryst als from Bondoc and Estherville mesosiderites with very high precision , and used these data to calculate their low temperature cooling rates . The compositional zoning of orthopyroxene crystals reflects extremel y rapid cooling rates, at least approximately 1-degrees-C/100 years in the temperature range 850-1150-degrees-C. However, simultaneous consi deration of both metallographic and cation ordering data for Esthervil le, within the framework of either an asymptotic or an exponential coo ling model, requires a cooling rate of approximately 1-degrees-C/Ma ne ar 250-degrees-C. The cation ordering data for Bondoc, for which no me tallographic data are yet available, are suggestive of even slower coo ling rate, which implies excavation from a somewhat greater depth in t he parent body. However, within the limits of their uncertainties, the measured site occupancies of the orthopyroxene crystals from Bondoc c an be reconciled with a cooling rate similar to that of Estherville. T he calculated cooling rates at both high- and low-temperatures have be en used to develop a thermal evolution model of mesosiderites. The sug gested model is not incompatible with an asteroidal parent body for th ese meteorites. Further, it is shown that the closure temperature of A r-Ar age must be tied to the slow cooling rate below 500-degrees-C.