Larry Klaes ( )
Fri, 07 May 1999 12:23:56 -0400

>From: Benny J Peiser <>
>Date: Fri, 7 May 1999 10:16:20 -0400 (EDT)
>Priority: NORMAL
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>>From Andrew Glikson <>
>In 1986 Don Lowe (sedimentologist) and Gary Byerly (petrologist) - both
>from the US - reported spherulitic units - similar to those observed
>along the K-T impact boundary - from 3.26-3.24 * 10^9 years-old
>Archaean sediments in the Barberton greenstone belt (eastern
>Transvaal), and from the Pilbara Craton (Western Australia) [1,2]. A
>meteoritic fallout origin by condensation of impact-produced silicate
>vapor was supported by platinum group element (PGE) patterns studied by
>Frank Kyte and colleagues [2] and by the occurrence of relic Ni-rich
>chromites of chondritic trace metal affinities [3] More recently a
>meteoritic origin has been confirmed by Chromium isotopic studies by A.
>Shukolayukov, F.T. Lugmair and colleagues [4]. For spherule unit S4 -
>mass balance calculations based on Cr and Ir abundances suggest a
>chondritic projectile larger than 30 km in diameter, and possibly as
>large as 40-50 km in diameter, implying cratering on the scale larger
>than 600 km in diameter. A probable oceanic setting of the crater/s is
>suggested by the lack of shocked quartz fragments in the spherulitic
>sediments [2-4]. It is likely that the craters themselves were
>subsequently detroyed by subduction of the oceanic crust.
>The spherulitic condensate units include: unit S2 - 32434 * 10^6 years;
>units S3 and S4 - 32274 to 32434 * 10^6 years. The Barberton spherule
>units are marked by PGE anomalies showing chondrite-normalized profiles
>which are depleted in the volatile species (Pd, Au) [2] - which is the
>reverse trend from terrestrial PGE patterns (excepting those of residual
>mantle peridotite). The spherules include relic quench-textured and
>resorbed Ni-rich chromites with high siderophile and chalcophile element
>abundances (Co, Zn, V) [3], Iridium nano-nuggets in sulphides [2], and
>53Cr/52Cr ratios which are diagnostic of C1 chondrites [4].
>Lowe and Byerly [1] remarked on the potential significance of these impact
>records for the change from a simatic (oceanic) volcanic assemblage
>(komatiite and basalt-dominated Onverwacht Group - 3.55-3.3 * 10^9
>years-old) to a turbidite, felsic volcanic and conglomerate-dominated
>sequence (Fig Tree Group, Moodies Group - less than 3.24 * 10^9 years-old).
>The period 3.26-3.24 * 10^9 years is well represented in the Pilbara Craton
>of Western Australia [5,6]. In the central Pilbara Craton it is represented
>by a volcanic and sedimentary sequence (the Sulphur Springs Group) [7],
>which includes high-Mg komatiite volcanics, andesite and dacite, dated by
>U-Pb zircon as about 3.24 * 10^9 years-old [6]. These volcanics are capped
>by felsic volcanic lenses and olistostromes, unconformably overlain by a
>yet-undated sequence of siltstone-banded ironstone sequence (Gorge Creek
>Group), and is intruded by the comagmatic Strelley Granite. In both the
>Barberton and the Pilbara the first occurrence of granite-derived detrital
>sediments above the 3.26-3.24 * 10^9 years-old units signifies the onset of
>uplift and erosion of granitic batholiths associated with differential
>vertical movements.
>The temporal juxtaposition between major impacts and the onset of magmatic
>and rifting events which involve the uplift and exposure of granite
>batholiths provides the first test case for potential relationships between
>mega-impacts and Precambrian magmatic/rifting episodes, suggested by
>Glikson (1993, 1996) [8,9]. Modelling of the effects of very large impacts
>on thin thermally active oceanic crust overlying shallow asthenosphere
>predict regional to global tectonic and magmatic effects, consistent with
>observations in the Barberton Mountain Land and the Pilbara Craton.
>References: [1] Lowe, D.R., Byerly, G.R., Asaro, F. and Kyte, F.T., 1989,
>Geological and geochemical record of 3400 m.y.-old terrestrial meteorite
>impacts. Science 245:959-962; [2] Kyte, F.T., Zhou, L., and Lowe, D.R.,
>1992, Noble metal abundances in an early Archaean impact deposit.
>Geochimica et Cosmochimica Acta, 56:1365-1372; [3] Byerly, G. R., and Lowe,
>D. R., 1994, Spinels from Archean impact spherules: Geochimica et
>Cosmochimica Acta, 58:3469-3486; [4] Shukolayukov, A., Kyte, F.T., Lugmair,
>G.W. and Lowe, D.R., The oldest impact deposits on Earth - first
>confirmation of an extraterrestrial component, European Science Foundation
>Impact Project, Cambridge Meeting on Impacts and the Early Earth, 1998; [5]
>Sun, S.S. and Hickman, A.H., 1998, New Nd isotopic and geochemical data
>from the West Pilbara - implications for Archaean crustal accretion and
>shear zone movement: Australian Geological Survey Organization Research
>Newsletter, 28:25-28; [6] Vearncombe, E.S., Barley, M.E., Groves, D.I.,
>McNaughton, N.J., Micucki, E.J., and Vearncombe, J.R., 1995, 3.24 Ga black
>smoker-type mineralization in the Strelley belt, Pilbara Craton, Western
>Australia: J. Geological Society of London, 152:587-590; [7] Van
>Kranendonk, M.J. and Morant, P.,1998, Revised Archaean stratigraphy of the
>North Shaw 1:100 000 sheet, Pilbara Craton: Geological Survey Western
>Australia Annual Review 1997-98, 55-62; [8] Glikson, A.Y., 1993, Asteroids
>and early crustal evolution. Earth Science Reviews, 35: 285-319; [9]
>Glikson, A.Y., 1996, Mega-impacts and mantle melting episodes: tests of
>possible correlations. Australian Geological Survey Organisation Journal,
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