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Geochemical evidence for an impact origin for a Late Archean spherule layer, Transvaal Supergroup, South Africa.
Bruce M. Simonson et al.
Thin layers rich in sand-size spherules of former silicate melt have been identified in five different sedimentary formations in Western Australia and South Africa. The layers range in age from roughly 2.63 to 2.49 billion years old and have all been interpreted as reworked ejecta from impacts by large extraterrestrial bodies such as asteroids or comets. This was originally based on geologic comparisons with younger layers of impact ejecta, especially the Cretaceous-Tertiary boundary layer, but an impact origin for two of the four spherule layers found in Australia is supported by anomalously high concentrations of iridium and other platinum group elements (PGEs) (reported in Simonson et al., Geology, 1998, v. 26, p. 195-198). New analyses of samples from the one late Archean layer found to date in South Africa indicate that it, too, is enriched in iridium and other siderophile elements relative to associated tuffs, carbonates, and shales. Moreover, the relative abundance of the PGEs in the spherule layer is very similar to those in chondritic meteorites. This suggests up to about 1% extraterrestrial material is present in the South African spherule layer and provides support for an impact origin.