Progress in meteoritic impact and crustal evolution research
Research School of Earth Science,
Australian National University
Canberra, ACT 0200
Evidence is accumulating that terrestrial crustal evolution has been
severely perturbed by post-Late Heavy Bombardment (LHB - post-3.8 Ga)
impact by large asteroids and comets, as predicted from the cratering flux
in the solar system and increasingly evidenced by the geological record.
The research program summarised below includes (1) a search for distal
impact fallout deposits in Archaean terrains, identified by microkrystite
spherule condensates from impact-released silicate vapour; (2)
mineralogical and geochemical relics of the LHB in early Archaean
sediments; (2) elucidation of the origin of the Permian-Triassic boundary
extinction, with reference to impact event/s; (3) study of Australian
multi-ring impact structures, notably the newly proven 120 km-diameter
Woodleigh impact structure, of possible Permian-Triassic or late Triassic
age, and (4) study of strewn crater fields - the late Eocene Carolyn crater
field, Timor Sea, possibly signifying a cometary breakdown event.
1. Early Archaean extra-terrestrial impact signatures
A search is in progress for impact fallout components in early Archaean
detrital sediments from the Western Gneiss Terrain of the Yilgarn Craton
(with D.R. Nelson), southwest Greenland (with A. Nutman), and the Slave
Province, Northwestern Territories, Canada (with W. Bleeker). Principal
search criteria include (1) occurrence of shocked quartz and zircon; (2)
Cr-rich chondritic spinels; (3) Ni-rich chromites (with high Co, V, Zn),
where Ni levels are one to two orders of magnitude higher than terrestrial
Ni-chromites associated with Ni-sulphides; (3) PGE anomalies showing
enrichment in refractory Ir and Ru and depletion in the volatile Pd and Au
- as contrasted with the volatile PGE-enriched terrestrial profiles,
excepting depleted mantle harzburgite.
Three 3.26-3.24 Ga-old units of spherulitic microkrystite condensates
identified at the base of the Fig Tree Group, Barberton Mountain Land,
eastern Transvaal, are characterised by marked PGE anomalies,
quench-textured and octahedral resorbed Ni-rich chromites, iridium
nano-nuggets, and negative 53Cr/52Cr ratios diagnostic of C1 chondrites.
These units define the oldest well-defined mega-impact cluster known in the
Archaean record to date. Scanning electron microscopy coupled with energy
dispersive spectrometry of Ni-chromites-bearing spherules (with G.R. Byerly
and D.R. Lowe) indicate internal Ni zonation of the chromites, to be
followed by ICPMS study of the PGE. Assuming global distribution of
fallout layers, mass balance calculations of Ir and Cr can be used to
estimate projectile composition and dimensions. For spherule unit S4 the
model suggests projectile diameter >30 km, consistent with estimates based
on recent vapour-liquid equilibria by J. Melosh and by O'Keefe and Aherns
(1982). Crater size scaling suggest a >600 km-diameter-large structure,
likely to have formed in a simatic crustal environment in view of the lack
of shocked quartz fragments, thus constituting an Archaean terrestrial mare
3.26-3.24 Ga events are well recorded in the Pilbara Craton of Western
Australia, including the Sulphur Springs Group which consists of komatiite,
andesite, dacite and chert interbeds. This sequence is overlain by a
yet-undated granitoid clast-bearing siltstone-banded ironstone sequence of
the Gorge Creek Group. This break and the contemporaneous break between
the Onverwacht Group (komatiite-tholeiite-felsic volcanic assemblage) and
Fig Tree Group (turbidite-felsic volcanic assemblage) in the Barberton
Mountain Land, representi fundamental changes from simatic to sialic
volcanic-sedimentary facies. The onset of rifting and exposure of
granitoids following the impacts suggest strong vertical tectonic
movements, with implications for the potential consequences of Archaean
mega-impacts. Modelling of the effects of very large impacts on thin
thermally active oceanic crust overlying shallow asthenosphere predict
development of propagting lithospheric faults and regional to global
magmatic and tectonic effects. The post-LHB impact flux indicated by lunar
cratering data and the modern asteroid and comet flux underpin the
significance of these events for crustal evolution.
2. Woodleigh structure, Carnarvon Basin, Western Australia: a 120
km-diameter multi-ring impact basin of Late Triassic or Permian-Triassic
The relations between the extinction of ssome 85 percent of species
(trilobites, rugose and tabulate corals) at 251 Ma, Siberian Norilsk flood
volcanism (248.4+/-2.4 Ma), contemporaneous oceanic anoxia, and observed
extraterrestrial impacts (c. 250 Ma-old 40 km-diameter Araguinha crater,
Brazil; shocked quartz fragments in sediments from Antarctic and New South
Wales) poses an enigma. Candidate mega-impacts based on geophysical
anomalies in the Falkland Plateau (M.Rampino) and Bedout rise, Northwest
Australian shelf (J. Gorter), remain unconfirmed. The discovery of the 120
km-diameter large Woodleigh multi-ring impact structure, south Carnarvon
Basin, Western Australia, provides a potential resolution of this problem.
In view of its size, it is likely that Woodleigh, by analogy with other
mega-impacts (Manicouagan - late Triassic; Morokweng - J-K boundary;
Chicxulub - K-T boundary; Popigai and Chesapeake Bay - late Eocene) was
associated with extinction. The pre-lower Jurassic post mid-Permian age of
Woodleigh therefore suggest it may be of either late Triassic or of
Permian-Triassic boundary age. A hint supporting the latter possibility
may be provided by a thermal high at 280-250 Ma indicated by apatite
Following a suggested impact origin (Mory and Iasky, 1998; Glikson, RSES
Annual Report for 1998), Woodleigh has been proven by drilling into (1) the
~25 km-diameter central granitoid uplift, and (2) breccia-bearing >600
m-deep rim syncline. Petrological, SEM, EDS, and Laser Raman (T.P.
Mernagh) studies of samples from the shocked granitoid core demonstrate
planar deformation features (PDF) in quartz and feldspar, microbrecciation,
pseudotachylite veining, and pervasive vitrification of feldspars.
Pseudotachylite veins contain highly refractory micron-scale comminuted
breccia and (?) glass enriched in the refractory elements (Mg, Al, Ca) and
depleted in relatively volatile elements (K, Si) (Fig. X). Experimental
laser fusion/volatilisation studies (Yu.P. Dikov) and relations between
chemical fractionation and boiling points militate for a role of
shock-induced volatilisation. High Mg levels (<3.0% MgO) in glass
inclusions within MgO-free feldspars remain unexplained. (with A.J. Mory,
R.P. Iasky, F. Pirajno, T.P. Mernagh).
ICPMS analyses of shocked pseudotachylite-injected granitoid samples
indicate that, while the lithophile and chalcophile trace metals (Pb, Sn,
Wo, Mo, Bi, Ag, Sb, Zn, Cu, Au) vary by factors in the range of X0.1 to
X10.0 relative to average granite, the siderophile trace metals are
strongly enriched, including V (< 390 ppm; X6-20AG), Cr (< 125 ppm; X30AG),
Co (< 48 ppm; X20-50AG), and Ni (< 66 ppm; X70-120AG). These enrichments
are consistent with EDS analysed MgO levels of < 3.0 % in the
pseudotachylites. Ni/Cr ratios fall in the range 0.12 - 0.53 and are more
similar to typical mantle pyrolite values (Ni/Cr ~ 0.75) than chondritic
values (Ni/Cr ~ 4.0). Ni sulphide collection ICPMS analysis of the PGE
suggests Ni/Ir values in the range of 17 000 - 33 000, more similar to
chondrites (Ni/Ir ~ 23 000) than to mantle pyrolite (Ni/Ir ~ 600 000). The
evidence tentatively suggests the introduction of a chondrite-contaminated
component through volatile condensation and melt transport, represented by
the pseudotachylites. Selective volatilisation of the projectile and mixing
with granitoid components may not allow direct identification of original
meteoritic chemical parameters.
3. The Carolyn crater field: Asteroid breakdown or SL9-type cometary
Seismic reflection array studies of the North Bonaparte Basin, Timor Sea,
disclose an ENE-striking 120x25 km-large swathe of more than 40 circular
features excavated in the pre-Miocene - post-Eocene erosional surface and
buried by Langhian sediments. The larger craters include central uplifts,
circular troughs and raised rims, whereas smaller circular features (Dc <
2.0 km) consist of crater-form and bulge-form structures. An impact origin
of many of these craters is suggested by their similarity to Fohn impact
structure, located in the north-eastern part of the crater field, where 350
meter-thick PGE-rich melt breccia lens was drilled within the ring
syncline. The smaller circular features are interpreted in terms of deeper
erosion of analogous structures. Limits on the degree of erosion are
allowed at Fohn impact structure from stratigraphic and morphometric
estimates. The Timor Sea crater field may alternatively represent (1)
high preservation rate of the 37.5-24 Ma impact flux, 2) an atmospheric
breakup of a low-angle asteroid, or (3) a cometary fragmentation event
similar to the Shoemaker-Levy-9 event on Jupiter.
Glikson, A.Y., 1999. Oceanic mega-impacts and crustal evolution, Geology,
Gorter, J.D. and Glikson, A.Y., 1999. Origin of a late-Eocene to
pre-Miocene buried crater and breccia lens at Fohn-1, North Bonaparte
Basin, Timor Sea: a probable extraterresrial connection. Meteoritics and
Planetary Science (in press).
Glikson, A.Y., 1999. Can shatter cones and penetrative deformation lamella
form due to explosive volcanic processes? A discussion of "Argument
supporting explosive igneous activity for the origin of "cryptoexplosion"
structures in the mid-continent, United States. Geology, March, 1999.
Glikson, A.Y., 2000. Spherulitic microkrystites and early terrestrial
maria. The Australian Geologist (in press).
Gorter, J.D. and Glikson, A.Y., A terrestrial analogue of the
Shoemaker-Levy-9 comet fragmentation event: the late Eocene - pre-Miocene
strewn crater field, Timor Sea, northern Australia. Science (in press).
Mory, A.J, Iaski, R., and Glikson, A.Y. The Woodleigh structure, Carnarvon
Basin, Western Australia: a multi-ring impact structure of 120 km diameter.
Earth Planetary Science Letters (in press).