Mineralogy and chemistry of Archaean and Early Proterozoic asteroid
impact ejecta, Pilbara and Transvaal, may imply existence of large
oceanic impact basins on the early Precambrian Earth.

Andrew Glikson
Research School of Earth Science
Australian National University
Canberra, ACT 0200


Asteroid impact fallout units, consisting of microkrystite (impact
condensate) spherules and microtektites (see list below), increasingly allow
the deciphering of the early impact history of Earth. In a paper of key
importance, B.M. Simonson, D. Davies, M. Wallace, S. Reeves, and S.W.
Hassler, (1998, Iridium anomaly but no shocked quartz from Late Archie
microkrystite layer: oceanic impact ejecta?, Geology, 26:195-198) point out
the likely oceanic (mafic-ultramafic) crustal source of early Proterozoic
impact ejecta in the Pilbara Craton, Western Australia. Studies of mainly
chloritic microkrystite spherules from the Barberton greenstone belt,
Transvaal, are consistent with a mafic derivation of impact condensates
(Lowe et al., 1989; Byerly and Lowe. 1994; Shukloyukov et al., 2000; Kyte et
al., 2003; Lowe et al., 2003). Recent field and geochemical studies of
Archaean to early Proterozoic impact units in the Pilbara Craton (Glikson
and Vickers, 2003) lend support to Simonson et al.'s (1998) suggestion, on
the following basis:

[1]  Siderophile element (Ni, Co), ferroan elements (Cr, V) and Platinum
Group Element (PGE) patterns of least-altered microkrystite
(impact-condensate) spherules and microtektites from Archaean and early
Proterozoic impact fallout in the Pilbara Craton (northwestern Australia)
and the Kaapvaal Craton (Transvaal) (Table 1) indicate a mafic/ultramafic
composition of impact target crust.

[2]  No shocked quartz grains are observed in the impact fallout units.

Estimates of asteroid and crater sizes based on (a) Mass balance
calculations of asteroid masses based on the flux of Iridium and Platinum as
measured from impact fallout units, and (b) spherule size-frequency
distribution using the method of Melosh and Vickery (1991), provide evidence
for asteroids several tens of kilometer in diameter (Byerly and Lowe,1994;
Shukloyukov et al., 2000; Kyte et al.; Glikson and Vickers, 2003) and
consequent oceanic (sima crust-located) impact basins with diameters on a
scale of several hundred kilometers

The implications of these observations for the nature of the early Earth are
inconsistent with strict uniformitarian geodynamic models based exclusively
on plate tectonic processes. It is suggested the evolution of the early
crust represents the combined effects of mantle-driven convection, modified
plate tectonic regimes, and large extraterrestrial impacts which triggered
deep faulting and adiabatic mantle melting. The latter resulted, in turn, in
a feedback mechanism which temporally and spatially controlled the onset and
loci of long term dynamic plate tectonic patterns.

A picture emerges of a post-3.8 Ga early Precambrian Earth, i.e. postdating
the Late Heavy Bombardment of 3.9-3.8 Ga, which consisted of sialic
(SiAl-dominated) continental nuclei composed of multiple superposed
greenstone-granite cycles interspersed within extensive tracts of simatic
(SiMg-dominated) oceanic crust. The latter included maria-like impact basins
on scales of up to several hundred kilometer, i.e. similar in size to the
lunar Mare Crisium impact basin (~3.2 Ga; Ds ~ 400 km) or even Mare
Serenitatis (Ds ~ 600 km).

References: Byerly, G.R., Lowe, D.R., 1994, Geochim. Cosmochim. Acta, 58,
3469-3486; Glikson, A.Y., Vickers, J., 2003, Geol. Surv. West Aust. Report;
Kyte, F.T., Shukloyukov, A., Lugmair, G.W., Lowe, D.R., Byerly, G.R., 2003,
Geology, 31, 283-286; Lowe, D.R., Byerly, G.R., Asaro, F., Kyte, F.T., 1989,
Science 245, 959-962; Lowe, D.R., Byerly, G.R., Kyte, F.T., Shukloyukov, A.,
Asaro, F., Krull, A., 2003, Astrobiology, 3, 7-48; Melosh, H.J., Vickery,
A.M., 1991, Nature, 350, 494-497; Shukolyukov, A., Kyte, F.T., Lugmair,
G.W., Lowe, D.R. and Byerly, G.R. (2000), Springer, Berlin, pp. 99-116;
Simonson, B.M., Davies, D., Wallace, M., Reeves, S., Hassler, S.W., 1998,
Geology, 26, p. 195-198;

List of Precambrian microkrystite-bearing asteroid impact fallout units.
(stratigraphic unit; age in Ga [billion years]; location; impact unit
symbol; reference).

A. Apex Basalt, Antarctic Chert Member,  3.47 Ga,  North Pole Dome, central
Pilbara Craton;  AMC-1, AMC-2;  Lowe and Byerly, 1986; Glikson and Vickers,
B. Hooggenoeg Formation, 3.47 Ga, Barberton greenstone belt, Hoog-1, Hoog-2;
Lowe and Byerly, 1986; Lowe et al., 2003.
C. Base Fig Tree Group, Mapepe Formation, 3.258-3.243 Ga; Barberton
greenstone belt, Eastern Transvaal, S2-S3-S4;  Lowe and Byerly, 1986; Lowe
et al., 2003.
D. Jeerinah Impact Layer, 2.63-2.68 Ga, Central Pilbara, Western Australia;
Simonson et al., 2000.
E. Monteville Formation; ?2.63 Ga; West Griqualand, Transvaal; Simonson et
al., 1999
F. Bee Gorge Member, Wittenoom Formation, 2.56 Ga, Hamersley Group,
Hamersley Basin, Western Australia,  SMB-1, SMB-2; Simonson, 1992; 2003;
Glikson and Vickers, 2003.
G. Carawine spherule-bearing Megabreccia, ?2.56-2.54, East Hamersley Group;
Simonson, 1992, 2003, Glikson and Vickers, 2003.
H. DG4 Shale Macroband Dales Gorge Formation, Hamersley Group; 2.50-2.47 Ga,
Hamersley Basin, Western Australia; DS4; Simonson, 1972.
I. base Kuruman Iron Formation, ?2.47 Ga, Griquastad, Western Transvaal;  F.
McDonald, pers.comm., 2002.
J. DG7 Shale Macroband Dales Gorge Formation, Hamersley Group, Hamersley
Basin, Western Australia; Glikson, 2003
K. Vallen Group, 1.8-2.1 Ga, Graenseso, South Greenland; Chadwick et al.,
L. Bunyeroo Ejecta, 0.59 Ga, Adelaide and Officer basins, South Australia;
Gostin et al., 1992.

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