Iridium anomalies and fractionated siderophile element patterns in impact ejecta, Brockman Iron Formation, Hamersley Basin, Western Australia: evidence for a major asteroid impact in simatic crustal regions of the early Proterozoic earth

Abstract

A stratigraphically consistent <20-cm-thick unit of microkrystite spherule and microtektite-bearing impact fallout ejecta overlying volcanic tuff of the 4th Shale Macroband (DGS4) of the Dales Gorge Member (2.47-2.50 Ga), Brockman Iron Formation, Hamersley Group, Western Australia, displays anomalous platinum group element (PGE) and other trace metal patterns. The unit has high Ir (13 ppb) and Pt (35 ppb), and low Pd (2.7 ppb) and Au (1.55-1.88 ppb). The low Pd/Ir ratios and low Cr/V suggest depletion in volatile PGE and metals relative to refractory PGE and V, contrasted to the ubiquitous high Pd/Ir of most terrestrial rocks. Marked depletion in the volatile Rare Earth Element (REE) abundances in stilpnomelane spherule cores is consistent with this model. The loss of volatile PGE, analogous to relations in 3.24 Ga impact fallout units of the Barberton greenstone belt (S3 and S4), suggests fractionation related to atmospheric spherule condensation. The microkrystite spherule unit locally incorporate fragments and up to meter-scale boulders of banded chert and stromatolite carbonate, suggesting tsunami transport postdating spherule deposition. DGS4 microkrystite spherules are dominated by stilpnomelane mantled by K-feldspar shells, which consist of inward-radiating fibrous feldspar aggregates suggestive of devitrification. The K and REE enrichment of spherule margins are contrasted to flat REE patterns of the stilpnomelane cores, suggesting adsorption of lithophile elements during settling of the spherules through the hydrosphere. K-feldspar shells contain submicron-scale Ni metal, oxide, sulfide and arsenide grains and euhedral needles of feldspar-exsolved ilmenite. Associated magnetite may have high nickel (<1.25% NiO). The generally mafic composition of the spherules and high Ni/Cr and Ni/Co are consistent with a target mafic-ultramafic crust, consistent with the lack of shock-metamorphosed quartz. Mixing calculations suggest a contribution of 2.5-3% projectile component to the impact-generated volatile cloud. Conservative mass balance estimates derived from the Ir and Pt flux, assuming global extent of a 10-cm-thick spherule unit and chondritic projectile composition, suggest an asteroid diameter on the scale of ∼30 km. Similar estimates are obtained from spherule sizes, which in DGS4 reach a mean diameter of ∼2.0 mm in aerodynamically elongate spherules. The evidence implies formation of an impact basin on the scale of 400 km in simatic/oceanic regions of the early Proterozoic crust.