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 the interface of a natural diffusion couple. These relations were used to calculate the high temperature cooling rates of the mesosiderites Lowicz and Clover Springs from the available data on compositional zoning across core-overgrowth interface of orthopyroxene grains. We have also determined the cation ordering in four selected orthopyroxene crystals 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 extremely rapid cooling rates, at least ~1°C/100 years in the temperature range 850-1150°C. However, simultaneous consideration of both metallographic and cation ordering data for Estherville, within the framework of either an asymptotic or an exponential cooling model, requires a cooling rate of ~l°C/Ma near 250°C. The cation ordering data for Bondoc, for which no metallographic data are yet available, are suggestive of even slower cooling rate, which implies excavation from a somewhat greater depth in the parent body. However, within the limits of their uncertainties, the measured site occupancies of the orthopyroxene crystals from Bondoc can be reconciled with a cooling rate similar to that of Estherville. The calculated cooling rates at both high- and low-temperatures have been used to develop a thermal evolution model of mesosiderites. The suggested model is not incompatible with an asteroidal parent body for these meteorites. Further, it is shown that the closure temperature of Ar-Ar age must be tied to the slow cooling rate below 500°C.