PLEASE NOTE:
*
CCNet ESSAY: EARLY TERRESTRIAL MARINA-LIKE IMPACT BASINS?
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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
Andrew.glikson@anu.edu.au
EXTENDED SUMMARY
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,
2003.
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.,
2001
L. Bunyeroo Ejecta, 0.59 Ga, Adelaide and Officer basins, South
Australia;
Gostin et al., 1992.
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