THE PRESENT WORKS ARE PUBLISHED WITH THE PERMISSION OF THE AUTHOR

 

TECOS - February 2001

 

 

ASTEROID/COMET IMPACTS AND EARLY CRUSTAL EVOLUTION

2000 progress report, Andrew Glikson, RSES/ANU


As predicted from the cratering flux in the solar system and increasingly
evidenced by the geological record, terrestrial crustal evolution has been
severely perturbed by post-Late Heavy Bombardment (post-3.8 Ga) impact by
large asteroids and comets.  The research reported below includes (1) a
search for distal impact fallout deposits in Archaean terrains, identified
by magnesian and siderophile trace element signatures in sediments and by
microkrystite spherule condensates from impact-released silicate vapour;
(2) study of Australian multi-ring impact structures, notably the newly
proven 120 km-diameter Woodleigh impact structure, and their environmental
consequences, and (3) identification of the origin of circular structures
under the Timor Sea.
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Evidence for major 3.2 0.1 Ga asteroid bombardment in the Earth-Moon
system, with implications for early crustal evolution.

A.Y. Glikson, G. R. Byerly1, D.R. Lowe2, W. Taylor, C. Allen, I.H. Campbell,

1 - University of Louisiana;
2 - Stanford University

Evidence for up to mare-scale impact basins in the mid-Archaean from c.
3.24 - 3.225 Ga-old impact-produced vapour condensation-fallout spherule
layers (microkrystites) in the Barberton greenstone belt, Transvaal (D.R.
Lowe, G.R. Byerly, and F. Kyte) is demonstrated by (1) PGE anomalies
showing depletion in volatile species (Pd, Au) consistent with atmospheric
condensation; (2) Ni-rich chromites (up to 23% NiO) distinct from
terrestrial spinels; (3) 53Cr/52Cr ratios depleted relative to terrestrial
values (A. Shukloyukov and G.W. Langmuir).  Mass balance calculations based
on abundance of Ir and Cr isotopes and correlations based on spherule size
distribution suggest asteroid diameters in the range of 20-50 km.  The
chlorite-dominated spherule composition and lack of shocked quartz-bearing
sial-derived fallout suggests an oceanic setting of the impacts, scaled at
400 - 800 km diameter.  This impact cluster may correspond to c. 3.18 Ga
impact peak documented by laser Ar-Ar ages of lunar spherules (T.S. Culler
et al.).  A peak in lunar volcanism about 3.2 Ga, suggested by Rb-Sr and
K-Ar lunar volcanic age frequencies, may hint at a late Imbrian -
Eratosthenian boundary (c. 3.2 Ga) impact-triggered basaltic volcanism
within the lunar mare (Fig. 1).  The terrestrial and lunar data are
consistent with a mid-Archaean impact cataclysm in the Earth-Moon system.
An abrupt lithological change in both the Kaapvaal and Pilbara cratons from
pre-3.24 Ga simatic crustal assemblages to post 3.24 Ga felsic volcanic -
turbidite - granite clast-bearing conglomerates indicates a change in
crustal conditions involving strong differential vertical movements
expressed by rifting and unroofing of granitic batholiths.  Models of
terrestrial crustal evolution need to take into account the inevitable
magmatic and tectonic consequences of large impacts, not least where they
occur in thin crustal geothermally active oceanic regimes.

EDS and laser ICPMS sections across spherules (sample SA306-1) indicate the
latter are dominated by micro crystalline assemblages of Cr-rich (~0.2-0.3%
Cr2O3) ferrous chlorite  and apatite (~7-10% P2O5; ~8-12% CaO).  EDS
sections across Ni-chromite grains indicate substitution of Cr and Fe but
little zonation of Ni and Co.  laser ICPMS sections indicate that the PGE
are concentrated in nano-nuggets, showing peak Ir, Os< Pt and Ru values
(Fig. 2[a]), and are depleted in Ir, Os, Ru and Pd relative to Pt and Rh
(Fig. 2[b]).  These patterns, which differ from most terrestrial PGE
patterns, probably reflect fractionation of the PGE during condensation,
including loss of the volatile Pd, also reflected by depletion in Pd and Au
in whole rock PGE analyses (F. Kyte).

A search was made for equivalents of the ~3.24-3.225 Ga Barberton spherules
at the base of a turbidite-felsic volcanic sequence of the c.3.24 Ga base
of the Gorge Creek Group, Pilbara Block, Western Australia.  The abundance
of spherulitic volcanic textures, including accretionary lapilli and
varioles, in felsic and mafic volcanic rocks in the Gorge Creek Group and
in the c.2.78 - 2.63 Ga Fortescue Group complicates identification of
impact spherules in these sequences.

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Age and implications of the 120 km-diameter Woodleigh impact structure,
Carnarvon Basin, Western Australia.

A.Y. Glikson, T. Uysal1, A.J. Mory2, R. Iasky2, F. Pirajno2, S.P. Kelley3 ,
S. Eggins, R.A. Armstrong.

1 - University of Queensland;
2 - Geological Survey of Western Australia;
3 - Open University, Milton Keynes, U.K.

A determination of the age of the Woodleigh impact - now the world's the
4th largest - was attempted using several isotopic methods.  A pre-Jurassic
age is indicated by overlying Early Jurassic sediments of the Woodleigh
Formation, restricted to the ring synclines as well as onlapping the
margins of the central uplift.  The youngest deformed sediments include
lower Devonian units, setting an older age limit to the impact.  The
isotopic systems of biotite and zircon in the basement uplift remained
stable in the samples analysed, as shown by (1) a model Rb-Sr age of 835
Ma, and (2) K-Ar isotopic age of ~800 Ma, with marginal rim resetting to
about 700 Ma.  However, isotopic K-Ar analysis of illite and interleaved
illite/smectite grains separated from granitoid of the central uplift and
from basal diamictite of the inner ring syncline suggest upper Devonian
ages.  Thus, core sample W149148 yielded an age of 362 Ma, and K-Ar ages of
illite and smectite separated from the basal diamictite at Woodleigh 2A
yield late to end-Devonian ages.  Infra-red laser spot fusion analysis
indicates loss of potassium in pseudotachylite veins in shocked granitoid,
and altered K-feldspar yielded ages mostly in the range 180-130 Ma,
reflecting post-impact burial. The diamictite contains shale clasts
containing early Permian palynomorphs, probably representing sedimentary
mixing in the ring syncline.  It follows that the Woodleigh impact
structure is of late to end-boundary age.

Woodleigh thus constitutes the largest member of a late Devonian impact
cluster, which also includes Charlevoix (Quebec), Siljan (Sweden), Ternovka
(Ukraine), Kaluga (Russia), Ilynets (Ukraine), and Elbow (Saskatchewan).
Iridium anomalies related to the late Devonian (Frasnian-Fammenian
boundary) events are known from the north Canning Basin, Western Australia,
microtektites are found along this boundary in south China, and marked
13/12C excursions occur in Alberta.  This extinction saw the elimination of
rugose coral reefs, trilobites, ammonoids, brachiopods and conodont
species, including destruction of the Devonian reefs which fringe the
Kimberley Block, north Western Australia.  A search is planned for tsunami
deposits along the late Devonian boundary in the Gascoyne Province.

Laser-ICPMS and SEM/EDS studies of pseudotachylite and impact melt-injected
shock-metamorphosed granitoid from the uplifted basement core of Woodleigh
confirm earlier indications of anomalous siderophile and magnesian element
abundances (Mg, Fe, Ni, Cr, Co, V) in the pseudotachylite and diaplectic
glass inclusions within feldspar.  In the absence of ultramafic rocks in
the vicinity, it is suggested the anomalous ultramafic chemistry is related
to injection of melt and condensation of vapour phases derived from the
exploding projectile.


Publications

Gorter, J.D. and Glikson, A.Y., 2000.  A late Eocene to pre-Miocene buried
crater and breccia lens at Fohn-1, North Bonaparte Basin, Timor Sea: a
probable extraterrestrial connection. Meteoritics and Planetary Science,
35: 381-392.

Glikson, A.Y., 2000.  Spherulitic microkrystites and early terrestrial
maria.  The Australian Geologist, March, 2000.

Mory, A.J, Iaski, R., Glikson, A.Y., Pirajno, F., 2000.  The Woodleigh
structure, Carnarvon Basin, Western Australia: a multi-ring impact
structure of 120 km diameter.  Earth Planet. Sci. Lett. 177, 1-2: 119-128

Mory, A.J, Iaski, R., Glikson, A.Y., Pirajno, F., 2000. Reply to discussion
of "The Woodleigh structure, Carnarvon Basin, Western Australia: a
multi-ring impact structure of 120 km diameter" (Earth Planet. Sci. Lett.,
177, 1-2: 119-128) by U.W. Reimold and C. Koeberl.  Earth Planet. Sci.
Lett. 184, 359-365.

Glikson, A.Y., 2000.  The world's fourth largest impact and the tale of two
craters.  Meteorite  6: 18-20.

Glikson, A.Y., 2000.  Early asteroid impacts and the origin of terrestrial
life.  Meteorite  6: 8-15.

Glikson, A.Y., 2000.  On early life and panspermia: reflections at an
outcrop of 3.46 Ga Pilbara stromatolites.  The Australian Geologist,
December, 2000.

Glikson, A.Y., 2000.  The astronomical connection of crustal evolution:
effects of post-3.8 Ga impacts, with particular reference to 3.2 Ga
bombardment in the Earth-Moon system. Journal of Geodynamics  (in press).