SNC meteorites: Evidence against an asteroidal origin

The 1.3 b.y. crystallization ages and other features of the nine SNC meteorites (shergottites, nakhlites,and chassignites) distinguish them from all other meteorites which apparently formed more than 3 b.y. earlier. Proposed models for the origin of the young SNC meteorites include internal melting, either within a large planet such as Mars or within a large asteroid, and impact melting on one or more asteroids. Although an asteroidal source is dynamically more reasonable, we discuss petrologic, geochemical, and isotopic observations that strongly argue against an asteroidal origin for SNC meteorites, either as impact melts or as endogenically derived melts. Cumulate textures, as displayed by the SNC meteorites, have never been observed in terrestrial or lunar impact melts. Even the largest impact melt sheets cooled relatively quickly by thorough mixing with cool clastic debris, and the resulting rocks show obvious lithic and/or mineral clasts derived from the target materials. No clasts or remnants of lasts are present in SNC meteorites. The chemical and isotopic homogeneity of impact melt rocks is not displayed by SNC meteorites. Differences in initial 87Sr/86Sr among some of the shergottites, which are otherwise chemically similar, preclude an origin in the same impact melt sheet. These relationships indicate that the SNC meteorites did not form within an impact melt sheet, either on an asteroid or anywhere else. Instead, their parental magmas probably formed by internal melting within their parent body. Although thermal models of large asteroids ( > 800 km diameter) can be devised to allow endogenic melting as young as 1.5 b.y. ago, isotopic and geochemical data for SNC meteorites preclude them from having formed in this way. Specifically, the isotopic signature of the ancient (∼4.6 b.y.) large‐scale heating event which established the shergottite sources is inconsistent with such asteroidal thermal models which require cold accretion of materials with initially low thermal conductivities. The thermal models further require that the young melts were restricted to the asteroid's core; total fragmentation of such asteroids would be required to liberate these rocks into Earth‐crossing orbits. Destruction of an 800 km asteroid would have produced an immense swarm of asteroids with similar orbits. No Hirayama family with sufficient mass has been identified in the asteroidal belt. If retention of residual garnet in the shergottite sources is a requirement, as indicated by geochemical modeling, this would directly imply a parent body larger than the largest asteroid (Ceres, diameter = 1020 km). A planet‐sized parent body such as Mars still remains the most likely candidate for the SNC meteorites.