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 (fO₂), and crystallographic orientation and used these data to develop a quantitative understanding of the resetting of the short-lived ⁵³Mn-⁵³Cr 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 fO₂ between wüstite-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 ⁵³Mn-⁵³Cr 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 ⁵³Mn-⁵³Cr closure age of olivine during cooling. This method was applied to determine the cooling rate of the pallasite Omolon, which showed ⁵³Mn-⁵³Cr bulk age of olivine that is 10 Myr younger than the age of the solar system. The calculated cooling rate, which is 20-40 °C/Myr at ∼985-1000 °C, is in good agreement with the metallographic cooling rate at ∼500 °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 ∼30 km in a parent body of at least ∼100 km radius.