Frank T. Kyte, Alex Shukolyukov, Günter W. Lugmair, Donald R. Lowe, Gary R. Byerly; Early Archean spherule beds: Chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type. Geology ; 31 (3): 283–286. doi: https://doi.org/10.1130/0091-7613(2003)031<0283:EASBCI>2.0.CO;2
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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.