Diffusion kinetics of Cr in olivine and 53Mn-53Cr thermochronology of early solar system objects

We have determined the diffusion coefficient of Cr in olivine as function of temperature, oxygen fugacity (fO(2)), and crystallographic orientation and used these data to develop a quantitative understanding of the resetting of the short-lived Mn-53-(53) Cr decay system in olivine during cooling within meteorite parent body. The diffusion of Cr in olivine was found to be anisotropic, and effectively independent Of fO(2) between wustite-iron buffer and two orders of magnitude above this buffer. The diffusion data were used to calculate the spatially averaged mean closure temperature of the Mn-53-(53) Cr decay system in olivine as function of the initial temperature, cooling rate and grain size, and also the closure age profile of this system in olivine single crystal as function of radial distance and a dimensionless parameter that incorporates the effects of various parameters that affect the closure age. We also present a thermochronolgic formulation that permits retrieval of cooling rates from the extent of resetting of the bulk Mn-53-(53) Cr closure age of olivine during cooling. This method was applied to determine the cooling rate of the pallasite Omolon, which showed Mn-53-(53) Cr bulk age of olivine that is 10 Myr younger than the age of the solar system. The calculated cooling rate, which is 20-40 degrees C/Myr at similar to 985-1000 degrees C, is in good agreement with the metallographic cooling rate at similar to 500 degrees C, when the two results are considered in terms of a cooling model in which the reciprocal temperature increases linearly with time. The inferred cooling rate of Omolon, which seems to be a sample from the core-mantle boundary, yields a burial depth of similar to 30 km in a parent body of at least similar to 100 km radius. (c) 2005 Elsevier Inc. All rights reserved.