A. Ruzicka et al., OLIVINE CORONAS, METAMORPHISM, AND THE THERMAL HISTORY OF THE MORRISTOWN AND EMERY MESOSIDERITES, Geochimica et cosmochimica acta, 58(12), 1994, pp. 2725-2741
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.