Saltpan impact crater, South Africa: Geochemistry of target rocks, breccias, and impact glasses, and osmium isotope systematics

Abstract

The Pretoria Saltpan crater is a well-preserved 220,000 year-old, 1.13 km-diameter, simple impact crater. The crater was formed in Nebo granites of the Bushveld Complex. Some minor intrusions thought to be younger than the Nebo granite are present at the crater and have earlier been believed to support a volcanic origin of the structure, but recent geological studies showed them to be part of the regional geology and of Proterozoic age. We studied the petrology and geochemistry of fourteen target granite samples, three suevitic breccias, nine intrusive rocks, as well as melt agglutinates, handpicked impact glass fragments and sulfide spherules from the Saltpan impact crater. Unconsolidated suevitic breccias recovered from different depths in the crater were found to contain abundant evidence of shock metamorphism. The target rock granites show only limited compositional variability. The major and trace element composition of the bulk breccia is very similar to that of average basement granite. Impact glass fragments recovered from the unconsolidated suevitic breccia have a CIPW normative composition similar to that of the basement granites. No evidence for admixture from any of the minor intrusions was found. The similarity of trace element abundances and ratios, and REE patterns between impact glasses and granites favors derivation of the glasses from the granites. The impact glass fragments show considerable enrichments of Mg, Cr, Fe, Co, Ni, and Ir, compared to the basement granites. The abundances of these elements in the glasses (after correction for indigenous concentrations) can be explained by admixture of about ≤ 10% of a chondritic component. High Ir concentrations (≈ 100 ppb) have been found in sulfide spherule samples, which may complement the (lower) Ir abundances in the glasses and could indicate some fractionation during impact. Re-Os isotopic studies were applied to further investigate the presence of a meteoritic component in the suevitic breccia. The target granite shows very low osmium abundances of about 7 ppt and high 187Os 188Os ratios of about 0.72 that would be expected for old continental crust. In contrast, the breccia samples were found to have much higher osmium abundances (≈80 ppt) and lower 187Os 188Os ratios of about 0.205. These values can be explained by mixing of target rocks with a chondritic component.