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Relict silicate inclusions in extraterrestrial chromite and their use in the classification of fossil chondritic material

Alwmark, Carl LU and Schmitz, Birger LU (2009) In Geochimica et Cosmochimica Acta 73(5). p.1472-1486
Abstract
Chromite is the only common meteoritic mineral surviving long-term exposure on Earth, however, the present study of relict chromite from numerous Ordovician (470 Ma) fossil meteorites and micrometeorites from Sweden, reveals that when encapsulated in chromite, other minerals can survive for hundreds of millions of years maintaining their primary composition. The most common minerals identified, in the form of small (<1-10 mu m) anhedral inclusions, are olivine and pyroxene. In addition, sporadic merrillite and plagioclase were found. Analyses of recent meteorites, holding both inclusions in chromite and corresponding matrix minerals, show that for olivine and pyroxene inclusions, sub-solidus re-equilibration between inclusion and host... (More)
Chromite is the only common meteoritic mineral surviving long-term exposure on Earth, however, the present study of relict chromite from numerous Ordovician (470 Ma) fossil meteorites and micrometeorites from Sweden, reveals that when encapsulated in chromite, other minerals can survive for hundreds of millions of years maintaining their primary composition. The most common minerals identified, in the form of small (<1-10 mu m) anhedral inclusions, are olivine and pyroxene. In addition, sporadic merrillite and plagioclase were found. Analyses of recent meteorites, holding both inclusions in chromite and corresponding matrix minerals, show that for olivine and pyroxene inclusions, sub-solidus re-equilibration between inclusion and host chromite during entrapment has led to an increase in chromium in the former. In the case of olivine, the re-equilibration has also affected the fayalite (Fa) content, lowering it with an average of 14% in inclusions. For Ca-poor pyroxene the ferrosilite (Fs) content is more or less identical in inclusions and matrix. By these studies an analogue to the commonly applied classification system for ordinary chondritic matrix, based on Fa in olivine and Fs in Ca-poor pyroxene, can be established also for inclusions in chromite. All olivine and Ca-poor pyroxene inclusions (>1.5 mu m) in chromite from the Ordovician fossil chondritic material plot within the L-chondrite field, which is in accordance with previous classifications. The concordance in classification together with the fact that inclusions are relatively common makes them an accurate and useful tool in the classification of extraterrestrial material that lacks matrix silicates, such as fossil meteorites and sediment-dispersed chromite grains originating primarily from decomposed micrometeorites but also from larger impacts. (C) 2008 Elsevier Ltd. All rights reserved. (Less)
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type
Contribution to journal
publication status
published
subject
in
Geochimica et Cosmochimica Acta
volume
73
issue
5
pages
1472 - 1486
publisher
Elsevier
external identifiers
  • wos:000263714300017
  • scopus:59649120900
ISSN
0016-7037
DOI
10.1016/j.gca.2008.12.006
language
English
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yes
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e402dc1b-f030-4c24-b864-f5ae98fd1e99 (old id 1371159)
date added to LUP
2009-05-08 16:23:53
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2017-01-01 04:20:54
@article{e402dc1b-f030-4c24-b864-f5ae98fd1e99,
  abstract     = {Chromite is the only common meteoritic mineral surviving long-term exposure on Earth, however, the present study of relict chromite from numerous Ordovician (470 Ma) fossil meteorites and micrometeorites from Sweden, reveals that when encapsulated in chromite, other minerals can survive for hundreds of millions of years maintaining their primary composition. The most common minerals identified, in the form of small (&lt;1-10 mu m) anhedral inclusions, are olivine and pyroxene. In addition, sporadic merrillite and plagioclase were found. Analyses of recent meteorites, holding both inclusions in chromite and corresponding matrix minerals, show that for olivine and pyroxene inclusions, sub-solidus re-equilibration between inclusion and host chromite during entrapment has led to an increase in chromium in the former. In the case of olivine, the re-equilibration has also affected the fayalite (Fa) content, lowering it with an average of 14% in inclusions. For Ca-poor pyroxene the ferrosilite (Fs) content is more or less identical in inclusions and matrix. By these studies an analogue to the commonly applied classification system for ordinary chondritic matrix, based on Fa in olivine and Fs in Ca-poor pyroxene, can be established also for inclusions in chromite. All olivine and Ca-poor pyroxene inclusions (&gt;1.5 mu m) in chromite from the Ordovician fossil chondritic material plot within the L-chondrite field, which is in accordance with previous classifications. The concordance in classification together with the fact that inclusions are relatively common makes them an accurate and useful tool in the classification of extraterrestrial material that lacks matrix silicates, such as fossil meteorites and sediment-dispersed chromite grains originating primarily from decomposed micrometeorites but also from larger impacts. (C) 2008 Elsevier Ltd. All rights reserved.},
  author       = {Alwmark, Carl and Schmitz, Birger},
  issn         = {0016-7037},
  language     = {eng},
  number       = {5},
  pages        = {1472--1486},
  publisher    = {Elsevier},
  series       = {Geochimica et Cosmochimica Acta},
  title        = {Relict silicate inclusions in extraterrestrial chromite and their use in the classification of fossil chondritic material},
  url          = {http://dx.doi.org/10.1016/j.gca.2008.12.006},
  volume       = {73},
  year         = {2009},
}