CCNet-ESSAY, 22 March 2000


By Andrew Glikson <>

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

The Late Heavy Bombardment (LHB) in the Earth-Moon system, broadly
defined at 4.2-3.8 * 10^9 years [1], may represent the tail-end of
planetary accretion or, alternatively, include a distinct
3.95-3.80*10^9 years bombardment episode [2]. Here I point out that
combined evidence from terrestrial Archaean terrains and from the
Moon militate for a major impact cataclysm in the Earth-Moon system
about 3.2+/-0.1*10^9 years. Older less-well-defined impact events in
the Earth-Moon system are also marked about 3.47-3.46*10^9 years - a
time of maximum greenstone-granite formation. The question is whether
these events signify an extension of the LHB or represent temporally
distinct episodes.

Some of the largest lunar maria basins contain low-Ti basalts which
likely represent impact-triggered volcanic activity, including Mare
Imbrium (3.86+/-0.02*10^9 year) and associated KREEP-basalts (K, REE,
and P-rich) (3.85+/-0.03*10^9 year) (Apollo 15) [3,4]. Similar
genetic impact-volcanic relationships may pertain in Oceanus
Procellarum (Apollo 12) (low-Ti basalts - Rb-Sr and Ar-Ar ages -
3.29-3.08*10^9 year) and in Hadley Apennines (Apollo 15) (low-Ti
basalts - Rb-Sr ages - 3.37-3.21*10^9 year; Ar-Ar ages -
3.35-3.10*10^9 year) [1]. The likelihood of impact-volcanic
relationships on the Moon gains support from the recent laser
40Ar/39Ar analyses of lunar impact spherules from sample 11199 (Fra
Mauro Formation - Apollo 14) by Muller [5] and Culler et al. [6] -
showing a significant age spike at 3.18*10^9 year, ie. near the
boundary between the Late Imbrian lunar era (3.9-3.2*10^9 year) and
the post-mare Eratosthenian lunar era (3.2-1.2*10^9 year) as defined
by the cratering record [7]. Some 34 lunar impact spherules yield a
mean age of 3188+/-198 Ma and a median age at 3181 Ma, whereas 7
lunar spherule ages with 1* < 100 m.y. yield a mean age of
3178+/-80*10^6 years and a median at 3186*10^6 years.

Since 1986 Don Lowe and Gary Byerly [8-12] perceptively recognised
several impact fallout spherule horizons in the Barberton Mountain
Land, Transvaal. These are defined by U-Pb zircon age determinations
of underlying and overlying pyroclastic volcanic units as: S1 -
3474-3445*10^6 years; S2 - 32434*10^6 years; S3, S4 -
3243-32274*10^6 years. They also detected a 3465-3458*10^6 years
spherule units in the Warrawoona Group, Pilbara Craton, Western
Australia [8]. The extraterrestrial impact origin has been questioned
on textural basis [13] and the PGE-rich composition of the spherules
[10,11] was interpreted as due to secondary processes [14], with
ensuing literature discussion [15,16]. However, these spherule
horizons are now established as undoubted impact condensate fallout
deposits, on the following basis:

(1) Occurrence within the spherules of quench-textured and octahedral
Ni-chromites with extreme values of Ni (NiO<23%), Co, Zn and V,
unknown in terrestrial chromites [12,17], and which contain PGE
nano-nuggets compositionally distinct from terrestrial PGE  nuggets
[17]; (2) 53Cr/52Cr isotopic indices (* = -0.32) corresponding to
values of carbonaceous chondrites and values of K-T boundary impact
fallout deposits, but distinct from terrestrial values [18]; (3) PGE
chondrite-normalised patterns displaying marked depletion in the
volatile species (Pd, Au) relative to refractory species (Ir, Pt),
distinct from terrestrial PGE profiles, excepting depleted mantle
harzburgites; (4) diagnostic textural features, including
inward-radiating quench textures and offset central vesicles, as
defined by B.M. Simonson [19,20].

Mass balance calculations based on Ir and Cr levels and
thermodynamic-based correlations of spherule sizes of 1-4 mm-diameter
[12,18], suggest impact by asteroids on the order of 30-50
km-diameter, scaled to 400-800 km-diameter terrestrial impact basins.
The Fe-Mg-rich spherule composition and the absence of shocked quartz
in the units suggest the impact basins formed in simatic/oceanic
regions of the Archaean Earth, which from geochemical and isotopic
evidence [21] occupied over 90 percent of the Earth surface before
about 3.0*10^9 years. The occurrence of the Barberton S2-S4 spherule
units immediately above the top of a >12 km thick mafic-ultramafic
volcanic sequence (Onverwacht Group) and at the base of a partly
granite-shed clastic sedimentary sequence (Fig Tree Group), hints at
the onset of fundamentally different tectonic/magmatic regimes about
c.3.24-3.227*10^9 years [10].  An analogous break is observed in the
Pilbara craton along the boundary between a 3.24*10^9 years volcanic
sequence (Strelley Group) and an unconformably overlying clastic
(partly granite-shed) -banded ironstone sequence (Gorge Creek Group).
A search for impact spherules within these units is in progress.
Estimates of impact incidence rates and crater/size frequency
distribution, and modelling of the tectonic and magmatic consequences
of mega-impacts on thin thermally active oceanic crust [22,23],
suggest these processes were of fundamental importance during
Archaean crustal evolution. 

It is suggested that the period 3.2+/-0.1*10^9 years represents a
major cataclysm in the Earth-Moon system, resulting in extensive
volcanic activity in lunar maria basins, with a possibility that some
of the largest craters formed at that time. On Earth the bombardment
resulted in formation of terrestrial maria on a scale of several
hundred km-diameter, major volcanic activity, strong vertical
movements, and formation of faulted trough/rift structures
accumulating clastic sediments from uplifted terrains [22, 23].

[1] Basaltic Volcanism of the Terrestrial Planets, Pergamon, New
    York, 1981.
[2] G. Ryder, 1991. Lunar Planet. Instit. Contrib. 746, 42-43.
[3] G. Ryder, 1997. Lunar Planet. Instit. Contrib. 790, 60-61.
[4] G. Ryder, D. Dalrymple, 1997. Lunar Planet. Instit. Contrib. 790,
[5] R.A. Muller, 1993. Tech. Report LBL-34168, Lawrence Berkeley
    National Laboratory, Berkeley, CA.
[7] T.S. Culler, T.A. Becker, R.A. Muller, P.R. Renne, 2000. 
    Science, 287, 1785-1789.
[8] D.E. Wilhelms, 1987. U.S. Geol. Surv. Prof. Pap. 1348.
[9] D.R.. Lowe, G.R. Byerly, 1986.  Geology, 14, 83-86.
[10] D.R. Lowe, G.R. Byerly, F. Asaro, F.T. Kyte, 1989.  Science,
     245, 959-962.
[11] F.T. Kyte, L. Zhou, D.R. Lowe, 1992.  Geochim. et Cosmochim.
     Acta, 56, 1365-1372.
[12] G.R. Byerly, D.R. Lowe, 1994.  Geochim. et Cosmochim. Acta, 58,
[13] R. Buick, 1987. Geology, 15, 178-179.
[14] C. Koeberl, W.U. Reimold, R.H. Boer, 1993.  Earth Planet. Sci.
     Lett., 119, 441-452.
[15] A.Y. Glikson, 1994, Earth Planet. Sci. Lett., 126, 497-499.
[16] C. Koeberl, W.U.Reimold, 1993. Earth Planet. Sci. Lett., 126,
[17] W. Taylor, A.Y. Glikson, G.R. Byerly, in preparation.
[18] A. Shukolayukov, A., F.T. Kyte, G.W. Lugmair, D.R. Lowe, 1998. 
     Abstract, Cambridge meeting on Impacts and the Early Earth.
[19] B.M. Simonson, 1992. Geol. Soc. Am. Bull. 104, 829–839.
[20] B.M. Simonson, S.W. Hassler, 1997.  Aust. J. Earth Sci., 44,
[21] M.T. McCulloch, V. Bennett, 1996.  Geochim. et Cosmochim. Acta,
     58, 4717-4738.
[22] A.Y. Glikson, 1996. Aust. Geol. Surv. Org. J. Aust Geol.
     Geohys., 16/4, 587-608.
[23] A.Y. Glikson, 1999. Geology, 27, 387-341.

Copyright 2000, Andrew Glikson

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