Iridium anomaly but no shocked quartz from Late Archean microkrystite layer: Oceanic impact ejecta?

摘要

Samples from a ca. 2.54 Ga layer rich in microkrystites (sand-size spherules of former silicate melt) in the Hamersley Group (Western Australia) are enriched in Ir up to 50 fold over associated carbonates and shales, but few of the other siderophile elements display significant anomalies or chondritic interelement ratios. However, similar concentrations and interelement ratios are observed in ejecta from the ca. 590 Ma Acraman impact structure and have been attributed to diagenetic redistribution. The microkrystite layer also contains sand-size quartz grains with internal textures typical of regionally metamorphosed rocks rather than evidence of shock metamorphism. We suggest that the microkrystites were created by an impact that took place in a deep ocean basin rather than on a continent, and that the associated quartz is epiclastic detritus brought in by unusually high energy waves and/or currents, not as ballistic ejecta.