Geochemistry of 2.63–2.49 Ga impact spherule layers and implications for stratigraphic correlations and impact processes

Thin layers rich in spherules formed during impacts by large extraterrestrial objects have the potential to shed new light on impact processes and aid in the long-distance stratigraphic correlation of Precambrian successions. Seven formations in Western Australia and South Africa clustered around the Archean-Proterozoic boundary each contain a single spherule layer, all of which were deposited between ca. 2.49 and 2.63 Ga. Analyses of 25 samples from 6 of the 7 spherule layers and 23 samples from closely associated strata free of spherules revealed an extraterrestrial component (ETC) in all six layers, based on PGE ratios and/or Cr isotopic composition. The amount of ETC varies from marginally detectable to clear and obvious; it generally amounts to ca. a few percent. Both PGE ratios and Cr isotopic anomalies indicate most if not all of the impactors were ordinary chondritic in composition. In contrast, all spherule layers older than ca. 3.0 Ga that have been analyzed were produced by carbonaceous chondritic impactors. Normalized rare earth element patterns suggest most target rocks were basaltic in composition and variability in PGE ratios favors ballistic emplacement of melt droplets rather than spherule formation via vapor condensation. The geochemical data also provide a means to test proposed intra- and intercontinental stratigraphic correlations. Cr isotopic compositions are consistent with the formation of the oldest layers on both continents by a single impact event about 2.63 Ga. In contrast, clear geochemical differences between the middle layers, both deposited ca. 2.54 Ga, suggest they were not produced by the same impact event. The youngest, banded iron formation-hosted layers on both continents have also been correlated, but no geochemical data are available on the African layer to test this at present.