Abstract

Early Precambrian impact ejecta units—consisting of vapor-condensate spherules (microkrystites), microtektites, rip-up clasts and fragmental tsunami deposits—display high siderophile element (Ni, Co, platinum group elements [PGEs]) abundances and, in some instances, high V and Cr levels. The data allow an indirect insight into the composition of crustal regions from which the ejecta were derived, including the contribution of extraterrestrial components and the fractionation history of impact ejected liquid and vapor plumes. The absence of shocked quartz grains in recorded early Precambrian ejecta and the largely ferromagnesian compositions of the microkrystite spherules, except where heavily altered, are consistent with impacts into mafic to ultramafic crust. The PGE data coupled with stratigraphic data are used to estimate the PGE flux and the size of projectiles. PGE patterns relative to chondritic values are mostly depleted in volatile, low-boiling-point species (Au, Pd) and enriched in refractory species (Ir, Ru, Rh), with consequently lower than chondritic Pd/Ir and Pd/Pt ratios, providing a useful tracer of microkrystite-rich impact ejecta units. First approximations of asteroid and crater sizes based on Ir mass balance and on spherule size frequencies suggest impact by 20–30 km asteroids, scaled to oceanic impact basins several hundred kilometers in diameter. A high ratio of sima to sial crust during the Archean is consistent with positive εNd, εHf, and low 87Sr/86Srinitial. The evidence suggests post–3.8 Ga geotectonic systems consisting of small sialic granite-greenstone nuclei surrounded by extensive sima crust in which transient Maria-scale impact basins formed during 3.47, 3.26– 3.24, 2.63, 2.56, 2.50–2.47 Ga and as yet unrecorded impact events.

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