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 h igh-temperature (T almost-equal-to 850-1100-degrees-C) metamorphism. T hese coronas formed by reaction and diffusion between olivine and a me sosiderite-like matrix assemblage. The bulk composition of the coronas can be approximated by a mixture of almost-equal-to 10-25 wt% olivine and almost-equal-to 90-75 wt% metal-free matrix, except for P and Cr, which are significantly enriched in coronas. Phosphorus and Cr diffus ed relatively rapidly to coronas and were derived from a large volume of matrix, most likely from metal that was originally enriched in thes e elements prior to metamorphism. The coronas in both meteorites show a similar zone sequence, but are systematically thicker in Emery (almo st-equal-to 800 mum wide) than in Morristown (almost-equal-to 350 mum wide), suggesting that Emery experienced more grain growth and more in tensive metamorphism than Morristown. Textural relationships suggest t hat corona formation and high-temperature metamorphism occurred largel y after intensive millimeter-scale brecciation and after or during met al-silicate mixing. A local equilibrium model can explain many feature s of the coronas, but chemical equilibrium was maintained only on a ve ry small scale. Overgrowths are present on plagioclase in the coronas of both mesosiderites and probably formed during high-temperature meta morphism. The compositional interface between core and overgrowth plag ioclase is extremely sharp, suggesting that cooling rates were greater -than-or-equal-to 0.1-degrees-C/y at the peak temperature of metamorph ism, consistent with high-temperature metamorphism occurring in a near -surface region of the parent body.