Iridium anomalies and shocked quartz in a Late Archean spherule layer from the Pilbara craton: New evidence for a major asteroid impact at 2.63 Ga

Abstract

A thin (1‐5 mm) spherule layer in ca. 2.63 Ga shale from the Jeerinah Formation (Pilbara craton, northwestern Australia) has been identified at two new localities. The layers have Ir concentrations as high as 15.5 ppb, significantly higher than the surrounding carbonaceous shale (,1.5 ppb). The sand-sized spherules display quench and devitrification textures and are interpreted as former silicate melt droplets that were replaced by K-feldspar, carbonate, and sulfide during diagenesis. In one spherule-layer sample, an angular quartz grain (;100 mm in size) with planar deformation features was found, which represents the oldest known shocked grain in distal ejecta by .2000 m.y. The survival of shocked quartz in ca. 2.63 Ga rocks, which have undergone multiple metamorphic events, suggests that their absence in other impact ejecta layers may not only be a question of preservation. The presence of shocked quartz and anomalously high Ir contents in a layer containing melt spherules provides compelling evidence for an extraterrestrial impact with a target area that was at least partly silicic, favoring a continental impact site. Estimates based on geochemical data suggest that the spherule layer comprises as much as 2‐3 wt% of a chondritic meteorite component. If proposed correlations with the Carawine (eastern Pilbara craton) and Monteville (South Africa) layers are correct, then the combined ejecta blanket represents fallout from a single major impact with an areal distribution of .32,000 km 2 , among the largest yet documented in the Precambrian rock record.