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

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

Jun 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.2725r&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 12, pp.2725-2741

Computer Science

7

Scientific paper

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 m wide) than in Morristown ( 350 m 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.

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