Untitled Document
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 newly found Hickman
Crater, Ophthalmia Range, Western Australia, reported in Glikson
et al., Australian Journal of Earth Science, December, 2008
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.