Early Archean spherule beds; chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type

Three Early Archean spherule beds from Barberton, South Africa, have anomalous Cr isotope compositions in addition to large Ir anomalies, confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites. The extraterrestrial components in beds S2, S3, and...

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Bibliographic Details
Published in:Geology (Boulder) 2003-03, Vol.31 (3), p.283-286
Main Authors: Kyte, Frank T, Shukolyukov, Alex, Lugmair, Günter W, Lowe, Donald R, Byerly, Gary R
Format: Article
Language:English
Subjects:
Africa
Archean
Barberton greenstone belt
carbonaceous chondrites
chondrites
chromium
Cr-53/Cr-52
ejecta
Fig Tree Group
geochemical anomalies
geochemistry
Geological time
impacts
iridium
isotope ratios
Isotopes
metals
metamorphic rocks
meteorites
Paleoarchean
platinum group
Precambrian
Southern Africa
spherules
stable isotopes
stony meteorites
Stratigraphy
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Summary:Three Early Archean spherule beds from Barberton, South Africa, have anomalous Cr isotope compositions in addition to large Ir anomalies, confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites. The extraterrestrial components in beds S2, S3, and S4 are estimated to be ∼1%, 50%-60%, and 15%-30%, respectively. These beds are probably the distal, and possibly global, ejecta from major large-body impacts. These impacts were probably much larger than the Cretaceous-Tertiary event, and all occurred over an interval of ∼20 m.y., implying an impactor flux at 3.2 Ga that was more than an order of magnitude greater than the present flux.
ISSN:0091-7613
1943-2682
DOI:10.1130/0091-7613(2003)031<0283:EASBCI>2.0.CO;2