Survival of isotopically heterogeneous graphite in a differentiated meteorite

Abstract

PRIMITIVE meteorites (carbonaceous chondrites and unequilibrated ordinary chondrites) are isotopically heterogeneous, indicating that the material from which the Solar System formed was not com-pletely homogenized^1-4. On the other hand, isotopically hetero-geneous material is not expected to survive in thermally 'processed' planetary or asteroidal objects. The achondrite meteorite Acapulco is a remnant of one such object; its petrographic and trace-element characteristics suggest that the parent body experienced pervasive heating and partial melting⁵. Here we report the discovery of graphite grains in the Acapulco meteorite that have a wide range of carbon and nitrogen isotopic compositions (δ ¹³C ranging from -34 to -8%o and δ ¹⁵N from -154 to -67%o). The graphite is associated with metal, and in some cases, graphite grains associ-ated with the same metal grain have very different isotopic compos-itions. These findings suggest that the graphite grains retain the isotopic signatures of a diverse range of precursor materials, despite the high temperatures reached in the parent asteroid.