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Igneous graphite in enstatite chondrites

Published online by Cambridge University Press:  05 July 2018

Alan E. Rubin
Alan E. Rubin*
Affiliation:
Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA
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Abstract

Igneous graphite, a rare constituent in terrestrial mafic and ultramafic rocks, occurs in three EH and one EL enstatite chondrite impact-melt breccias as 2–150 µm long euhedral laths, some with pyramidal terminations. In contrast, graphite in most enstatite chondrites exsolved from metallic Fe-Ni as polygonal, rounded or irregular aggregates. Literature data for five EH chondrites on C combusting at high temperatures show that Abee contains the most homogeneous C isotopes (i.e. δ13C = −8.1 ± 2.1‰); in addition, Abee's mean δ13C value is the same as the average high-temperature C value for the set of five EH chondrites. This suggests that Abee scavenged C from a plurality of sources on its parent body and homogenized the C during a large-scale melting event. Whereas igneous graphite in terrestrial rocks typically forms at relatively high pressure and only moderately low oxygen fugacity (e.g., ∼ 5 kbar, logfO2 ∼ −10 at 1200°C), igneous graphite in asteroidal meteorites formed at much lower pressures and oxygen fugacities.

Keywords

igneous graphiteenstatite chondriteimpact-melt brecciaoxygen fugacity
Type
Research Article
Information
Mineralogical Magazine , Volume 61 , Issue 408 , October 1997 , pp. 699 - 703
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1997

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