IRIDIUM ANOMALY BUT NO SHOCKED QUARTZ FROM LATE ARCHEAN MICROKRYSTITELAYER - OCEANIC IMPACT EJECTA

Samples from a ca. 2.54 Ga layer rich in microkrystites (sand-size sph erules of former silicate melt) in the Hamersley Group (Western Austra lia) are enriched in Ir up to 50 fold over associated carbonates and s hales, but few of the other siderophile elements display significant a nomalies or chondritic interelement ratios, However, similar concentra tions and interelement ratios are observed in ejecta from the ca, 590 Ma Acraman impact structure and have been attributed to diagenetic red istribution. The microkrystite layer also contains sand-size quartz gr ains with internal textures typical of regionally metamorphosed rocks rather than evidence of shock metamorphism. We suggest that the microk rystites were created by an impact that took place in a deep ocean bas in rather than on a continent, and that the associated quartz is epicl astic detritus brought in by unusually high energy waves and/or curren ts, not as ballistic ejecta.