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

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)2725-2741
Number of pages17
JournalGeochimica et Cosmochimica Acta
Volume58
Issue number12
DOIs
StatePublished - 1994

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mesosiderite
corona
olivine
metamorphism
history
Metals
Temperature
Meteorites
Silicates
matrix
Grain growth
metal
plagioclase
Phosphorus
Minerals
Hot Temperature
Cooling
parent body
clast
meteorite

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

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title = "Olivine coronas, metamorphism, and the thermal history of the Morristown and Emery mesosiderites",
abstract = "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.",
author = "Alex Ruzicka and Boynton, {William V.} and Jibamitra Ganguly",
year = "1994",
doi = "10.1016/0016-7037(94)90140-6",
language = "English (US)",
volume = "58",
pages = "2725--2741",
journal = "Geochmica et Cosmochimica Acta",
issn = "0016-7037",
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number = "12",

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TY - JOUR

T1 - Olivine coronas, metamorphism, and the thermal history of the Morristown and Emery mesosiderites

AU - Ruzicka, Alex

AU - Boynton, William V.

AU - Ganguly, Jibamitra

PY - 1994

Y1 - 1994

N2 - 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.

AB - 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.

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U2 - 10.1016/0016-7037(94)90140-6

DO - 10.1016/0016-7037(94)90140-6

M3 - Article

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VL - 58

SP - 2725

EP - 2741

JO - Geochmica et Cosmochimica Acta

JF - Geochmica et Cosmochimica Acta

SN - 0016-7037

IS - 12

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