Cooling history of some Antarctic ureilites

Abstract

To obtain some constraints on the cooling histories of ureilites, we have studied the chemical zoning of olivines and the compositional variations of pyroxenes in the PCA82506, Y790981, and ALH77257 ureilites using an electron microprobe. Fayalite components and CaO contents in olivines show a gradual decrease toward the rims. The MnO/FeO variation of ureilite pyroxenes shows a subtle anticorrelation that is probably related to the redox reaction during their original crystallization. The chemical variation within a grain of Y790981 pigeonites may be attributed to partial melting due to shock heating. Cooling rates of 2–6°C/hour have been obtained from diffusional calculations based on the reverse zoning profile of Fa components at the grain boundary of ureilite olivines. We assume that iron diffusion controls the reduction reaction, although the reduction process may be more complex. Because iron diffusion is slower in pyroxene than in olivine, ureilite pigeonite would not show chemical zoning by reduction if their cooling rates were faster than about 1°C/hour. This result is consistent with impact‐produced excavation of the ureilite parent mass.