Multiple 3.47-Ga-old asteroid impact fallout units, Pilbara Craton, Western Australia

Abstract

A new microkrystite spherule-bearing diamictite is reported from below the impact spherule-bearing 3.47 Ga Antarctic Chert Member (ACM) at the base of the Apex Basalt, central Pilbara Craton, Western Australia [1,5]. The diamictite, defined as ACM-S2, consists of 0.6-0.8-m-thick spherule-bearing pebble to cobble-size chert-intraclast conglomerate separated from the main ACM-S3 by a ∼200-m-thick dolerite and ∼30-m-thick felsic hypabyssals. The microkrystite spherules are discriminated from angular to subangular detrital volcanic fragments by their high sphericities, inward-radiating fans of sericite pseudomorphs after K-feldspar, relic quench textures and Ni-Cr-Co relations. Scanning Electron Microscopy coupled with E-probe (EDS) and laser ICPMS analysis indicate high Ni and Cr in sericite-dominated spherules, suggesting mafic composition of source crust. Ni/Cr and Ni/Co ratios of the spherules are higher than in associated Archaean tholeiitic basalts and high-Mg basalts, rendering possible contamination by high Ni/Cr and Ni/Co chondritic components. The presence of multiple bands and lenses of spherules within chert and scattered spherules in arenite bands within S3 may signify redeposition of a single impact fallout unit or, alternatively, multiple impacts. Controlling parameters include: (1) spherule atmospheric residence time; (2) precipitation rates of colloidal silica; (3) solidification rates of colloidal silica; (4) arenite and spherule redeposition rates, and (5) arrival of the tsunami. The presence of spherule-bearing chert fragments in S3 may hint at an older spherule-bearing chert (?S1). Only a minor proportion of spherules is broken and the near-perfect sphericities of chert-hosted spherules and arenite-hosted spherules constrain the extent of shallow water winnowing of the originally delicate glass spherules. It is suggested that the spherules were either protected by rapid burial or, alternatively, disturbance was limited to a short term high energy perturbation such as may have been affected by a deep-amplitude impact-triggered tsunami wave.