Strangways Crater, Northern Territory, Australia: Siderophile element enrichment and lithophile element fractionation

The Strangways Crater, Northern Territory, Australia (15°12′S, 133°35′E), has a central core, about 10 km in diameter, of shocked granitic gneiss and amphibolite, and some remnants of a melt rock sheet, surrounded by outer rings of quartzite and siltstone to a diameter of 20–25 km. Seven samples of melt rock (six granitic melts, one shale melt clast) and four samples of country rock (granitic gneiss, amphibolite, shale, quartzite) were analyzed by neutron-activation analysis: for Sc, Cr, Fe, Co, Zn, Rb, Zr, Sb, Cs, Ba, rare earth elements, Hf, Ta, Th, and U, the samples were analyzed instrumentally; and for Ni, Se, Pd, Ag, Cd, Re, Os, Ir, and Au, they were analyzed radiochemically. Siderophile elements are significantly enriched in the granitic melt rocks relative to country rocks; for example, the Ir enrichments range from 0.6 to 2.8 ppb. The low Ir/Ni ratio (∼0.16 relative to C1 chondrites) excludes a chondritic impacting body, and Cr enrichment argues against impact by an iron meteorite. The Strangways Crater may have been formed by the impact of an olivine-rich achondrite and melt rocks appear to contain about 3 wt.% of projectile material. The composition of the granitic melt rocks cannot be reproduced by any simple mixture of analyzed country rock types and chemical fractionation by selective shock melting appears to have taken place.