Igneous Graphite in Enstatite Chondrites

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

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 graphite • enstatite chondrite • impact-melt breccia • oxygen fugacity

Mineralogical Magazine; October 1997 v. 61; no. 408; p. 699-703; DOI: 10.1180/minmag.1997.061.408.09
© 1997, The Mineralogical Society
Mineralogical Society (www.minersoc.org)