Cr diffusion in orthopyroxene: Experimental determination, 53Mn– 53Cr thermochronology, and planetary applications

Abstract

We have determined Cr diffusion coefficients ( D) in orthopyroxene parallel to the a-, b-, and c-axial directions as a function temperature at f(O 2) corresponding to those of the wüstite–iron (WI) buffer. Diffusion is found to be significantly anisotropic with D(// c) > D(// b) > D(// a), conforming to an earlier theoretical prediction. Increase of f(O 2) from WI buffer conditions to 4.5 log unit above the buffer at 950 and 1050 °C leads to decrease of D(Cr) by a factor of two to three, possibly suggesting significant contribution from an interstitial diffusion mechanism. We have used the diffusion data to calculate the closure temperatures ( T c) of the Mn–Cr decay system in orthopyroxene as a function of initial temperature ( T 0), grain size ( a) and cooling rate for spherical and plane sheet geometries. We also present graphical relations that permit retrieval of cooling rates from knowledge of the resetting of Mn–Cr ages in orthopyroxene during cooling, T 0 and a. Application of these relations to the Mn–Cr age data of the cumulate eucrite Serra de Magé yields a T c of 830–980 °C, and cooling rates of 2–27 °C/Myr at T c and ∼1–13 °C/Myr at 500 °C. It is shown that the cooling of Serra de Magé to the closure temperature of the Mn–Cr system took place at its original site in the parent body, and thus implies a thickness for the eucrite crust in the commonly accepted HED parent body, Vesta, of greater than 30 km. This thickness of the eucrite crust is compatible only with a model of relatively olivine-poor bulk mineralogy in which olivine constitutes 19.7% of the total asteroidal mass.