Abstract

Three Early Archean spherule beds from Barberton, South Africa, have anomalous Cr isotope compositions in addition to large Ir anomalies, confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites. The extraterrestrial components in beds S2, S3, and S4 are estimated to be ∼1%, 50%–60%, and 15%–30%, respectively. These beds are probably the distal, and possibly global, ejecta from major large-body impacts. These impacts were probably much larger than the Cretaceous-Tertiary event, and all occurred over an interval of ∼20 m.y., implying an impactor flux at 3.2 Ga that was more than an order of magnitude greater than the present flux.

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