Detection of extraterrestrial material in ca. 2.49 Ga Kuruman spherule layer, South Africa

Impacts by at least 4 large extraterrestrial bodies generated layers consisting largely of formerly molten spherules in a time span of ~140 million years straddling the Archean-Proterozoic Boundary (APB) at 2.5 Ga. Remnants of these layers are preserved in the Hamersley Basin (Western Australia) and the Griqualand West Basin (South Africa). The youngest layers found to date in both Basins occur in large banded iron formations (BIFs) that have been correlated to one another via a combination of sequence stratigraphic analysis and isotopic age dating. Both of these large BIFs, the Kuruman (South Africa) and the Dales Gorge (Western Australia), were deposited ca. 2.48-2.49 Ga and contains a single spherule layer. Based on strong similarities in stratigraphic position, mode of deposition, and petrography of the spherules, the Kuruman (KSL) and Dales Gorge (DGSL) spherule layers have been suggested to correlate. In every APB spherule layer tested to date, a component of extraterrestrial (ET) material has been detected in the form of anomalously high concentrations of the platinum group elements (PGEs: Ru, Rh, Os, Ir, Pt, Pd) with roughly chondritic ratios and/or chromium isotopic ratios. Here we report evidence of ET material in the one APB layer that had not previously been analyzed. The KSL has only been identified in 3 drill cores, all with a relatively low abundance of spherule-rich material. In this work, two samples from the KSL in Agouron core GKP-1 were characterized geochemically and found to be almost identical to some of the DGSL samples in terms of major and trace element contents. The mean Ir concentration of ~11.9 ppb and the PGE inter-element ratios corresponding to the presence of about 1-3% ordinary chondrite for the KSL samples are very comparable to the average of ~11.5 ppb Ir of 5 DGSL samples reported in Simonson et al. (2009, Precamb. Res. 175, p. 62). The main significance of these new data is that they 1) confirm the presence of ET material in the KSL, thereby proving it is yet another example of a spherule-rich distal impact ejecta layer near the APB, and 2) increase the likelihood that the proposed correlation between the KSL and DGSL is correct, i.e., that the two layers on different continents are geographically separated remnants of distal ejecta from a single large impact that happened somewhere on Earth ca. 2.49 Ga.