Geochemical and isotopic signatures of Archaean to Palaeoproterozoic extraterrestrial impact ejecta/fallout units

Abstract

Criteria allowing diagnostic identification of asteroid and comet impact fallout units (impactites), including fragmental ejecta, microtektites and microkrystite spherules (impact vapour condensates) comprise: (i) unique mineral fallout phases-shocked quartz grains, coesite and nano-diamonds; (ii) unique intra-microkrystite phases - Ni-chromite, Ni-nanonuggets and Ir-nanonuggets, condensed from vapour enriched in meteoritic components; (iii) geochemical features such as high abundance and unique ratios of the platinum group (PGE) and other siderophile elements (Ni, Co); (iv) meteoritic isotopic ratios including ε 53Cr (53Cr/52Cr), ε 54Cr (54Cr/52Cr), ε 182w (182W/183W) or (182W/184W), ε()s (187Os/188Os), ε17O (17O/16O); ε18O (18O/16O); and (v) cometary seeding of 3He/4He and racemic organic molecules (AIB) and possibly fullerenes (C60). Relic nickel chromites and metasomatically derived sulfides may contain PGE nanonuggets. Alteration, burial metamorphism and open-system mobility of uranium in hydrous terrestrial environments renders preservation of meteoritic ε207Pb (207Pb/204Pb) and ε 206Pb (206Pb/204Pb) values unlikely. Where least affected, PGE patterns of microkrystites and microtektite-bearing impact fallout units (impactites) are the reverse of terrestrial PGE patterns, including low Pd/Pt ratios, which provide some of the more readily identified and analytically practical criteria for identifying a meteoritic component. εCr-ε Cr relations in Barberton Greenstone Belt impact fallout units (3.26-3.24 Ga) Identify a carbonaceous chondrite composition of the parental asteroids. PGE abundances (Ir, Pt) and εCr isotope values allow mass-balance estimates of parental projectiles in the order of 20-30 km diameter. Ir and Pt mass-balance estimates correspond to projectiles ∼20 km in diameter or larger in the Pilbara Craton for the JIL (> 2.63 Ga) and DGS4 (2.5-2.47 Ga) impact fallout units. The Ni, Co and Cr enriched composition of most Precambrian microkrystite spherules militates for mafic/ultramafic target crust which, coupled with the estimated diameter of the ensuing craters, implies the existence of maria-scale impact basins in oceanic-type crustal regions during the Archaean and Palaeoproterozoic.