Igneous graphite in enstatite chondrites

University of California at Los Angeles, Institute of Geophysics and Planetary Physics, Los Angeles, CA, United States

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 pm 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. delta ¹³ C = -8.1+ or -2.1 per mil); in addition, Abee's mean delta ¹³ C 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., approximately 5 kbar, log f _O2 approximately -10 at 1200 degrees C), igneous graphite in asteroidal meteorites formed at much lower pressures and oxygen fugacities.

This record provided courtesy of AGI/GeoRef.