Archean spherule layers in the Barberton Greenstone Belt, South Africa: A discussion of problems related to the impact interpretation

This review of the current knowledge of impact spherule layers in the Barberton greenstone belt, together with new petrographic, geochemical, and field data on a number of impact and volcanic spherical particle horizons, highlight a number of problems with the proposition of frequent and large meteorite impacts during mid-Archean times. Field data indicate that some of the four previously proposed impact spherule layers may be laterally correlative units that may have formed from the same impact event. Petrographic work reveals the presence of volcaniclastic particles associated with some spherule layers, while other layers not regarded as impact deposits contain clasts commonly observed in the spherule beds. Major and trace element compositions of spherule layers reflect the composition of the immediate host rocks. The existing platinum group element and chromium isotope data are difficult to reconcile with the current knowledge of the composition of meteoritic debris. A thorough discussion of these problems is necessary before meaningful estimates of Archean impact flux and bolide diameters should be attempted.