Geochemical evidence for an impact origin for a Late Archean spherule layer, Transvaal Supergroup, South Africa

Abstract

A Late Archean layer rich in sand-sized spherules of former silicate melt in the Monteville Formation (Transvaal Supergroup, South Africa) has Ir concentrations as high as 6.4 ppb and is clearly enriched in Ir relative to associated tuffs, carbonates, and shales. The Monteville spherule layer is also enriched in other siderophile elements, including the platinum group elements (PGEs). The PGEs in the spherule layer produce a flat (meteorite like) pattern when they are normalized to chondritic abundances. The abundances of Ir and other siderophile elements are similar to broadly contemporaneous spherule layers in the Hamersley basin of Western Australia. That the mineral compositions, textures, and sedimentary structures of the spherule layers in the Transvaal Supergroup and Hamersley basin are also very similar suggests that they were all formed by the same processes. We think that the best way to explain the high Ir concentrations and other characteristics of the Monteville spherule layer is that it represents distal impact ejecta. There are, however, significant differences between the Monteville spherule layer and Early Archean spherule layers in the Barberton greenstone belt, including much higher average and maximum concentrations of Ir in the latter. The data presented here clearly show that each Precambrian spherule layer is unique and needs to be characterized individually, as is true for the impact spherule layers of the Phanerozoic.