Effects of Archaean to early Proterozoic asteroid impact clusters on crustal evolution

Andrew Glikson

1 Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
School of Archaeology and Anthropology The Australian National University, Canberra, ACT 0200, Australia
(joint with, and logistically assisted by, the Pilbara project of the Geological Survey of Western Australia).

 

The role of asteroid and comet impacts as triggers of mantle-crust processes poses one of the fundamental questions in Earth science. Evidence has been documented for close association between impact ejecta/fallout units with major unconformities and lithostratigraphic boundaries in early Precambrian terrains, including abrupt changes in the nature of volcanic and sedimentary environments across stratigraphic impact boundaries, with implications for the composition of provenance. In the Barberton Greenstone Belt, eastern Kaapvaal Craton, South Africa, 3.26-3.24 Ga asteroid mega-impact units are juxtaposed with abrupt break between mafic-ultramafic volcanic crust and an overlying association of turbidites, banded iron formations, felsic tuff and conglomerates.

Contemporaneous stratigraphic relationships are identified in the Pilbara Craton, Western Australia. Evidence for enrichment of seawater in ferrous iron in the wake of major asteroid impacts reflects emergence of new source terrains, likely dominated by mafic compositions, attributed to impact triggered oceanic volcanic activity. Relationships between Impact and volcanic activity are supported by the onset of major mafic dyke systems associated with ~2.48 Ga and possibly the 2.56 Ga mega-impact events. Abrupt breaks at 3.26-3.24 Ga between ~12 km-thick mafic-ultramafic volcanic sequences of Archaean greenstone belts and overlying felsic volcanic-turbidite-banded iron formation assemblages in the Transvaal and the Pilbara cratons are accompanied by 4 asteroid ejecta units. 

Mass balance calculations based on Ni/Cr, PGE and 52/53Cr isotope data indicate mafic-ultramafic target crust and parent asteroid on a scale of 20 - 50 km diameter. Kinematic models of impact by such asteroids on thin geothermally active Archaean crust and lithosphere suggest consequent reorganization of mantle convection cell systems, accounting for contemporaneous peak igneous activity.

The onset of ferruginous sedimentation immediately following the impacts, indicated by occurrence of BIF above ejecta units, indicates increased supply rates of soluble ferrous iron to the oceans under the low Eh conditions of the Archaean hydrosphere, indirectly suggesting the erosion of mafic volcanics possibly triggered by the impacts. A new impact crater discovered by Dr A.H. Hickman and documented by the author is reported in the current issue to the Australian Journal of Earth Science (see Figure 1). The results of this study are reported in 16 papers in international and national scientific journals and in books during 2004 - 2008.