Olivine coronas, metamorphism, and the thermal history of the Morristown and Emery mesosiderites

Coronas are present on all millimeter-sized mineral clasts of olivine in the Emery and Morristown mesosiderites and are a manifestation of high-temperature (T ≈ 850-1100°C) metamorphism. These coronas formed by reaction and diffusion between olivine and a mesosiderite-like matrix assemblage. The bulk composition of the coronas can be approximated by a mixture of ≈ 10-25 wt% olivine and as 90-75 wt% metal-free matrix, except for P and Cr, which are significantly enriched in coronas. Phosphorus and Cr diffused relatively rapidly to coronas and were derived from a large volume of matrix, most likely from metal that was originally enriched in these elements prior to metamorphism. The coronas in both meteorites show a similar zone sequence, but are systematically thicker in Emery (≈800 μm wide) than in Morristown (≈350 μm wide), suggesting that Emery experienced more grain growth and more intensive metamorphism than Morristown. Textural relationships suggest that corona formation and high-temperature metamorphism occurred largely after intensive millimeter-scale brecciation and after or during metal-silicate mixing. A local equilibrium model can explain many features of the coronas, but chemical equilibrium was maintained only on a very small scale. Overgrowths are present on plagioclase in the coronas of both mesosiderites and probably formed during high-temperature metamorphism. The compositional interface between core and overgrowth plagioclase is extremely sharp, suggesting that cooling rates were ≥0.1°C/y at the peak temperature of metamorphism, consistent with high-temperature metamorphism occurring in a near-surface region of the parent body.