Computer Science
Scientific paper
Jul 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.1933d&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 7, pp.1933-1951
Computer Science
4
Scientific paper
Samples from sediments of the Fig Tree Group located in the central part of the circa 3.2 to 3.5 Ga Barberton greenstone belt (BGB) have been analyzed by the 40 Ar / 39 Ar laser step-heating technique. This technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include: reworked chemical sediments (barite), clastic sediments (sandstone/shale), and one stromatolite. In most cases the apparent age plateaux can be identified by corresponding Ca / K and Cl / K ratio plots as belonging to separate mineral phases. Most of the samples exhibit simple Ar diffusion loss during the low temperature part of the experiments while occasionally showing excess Ar, or possible 39 Ar recoil, effects. The various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 Ma. One barite sample gave T 0 (original blocking age) = 2673 ± 3 Ma (1 ) which is close to a calculated model Ar diffusion-loss age for the same sample of 2688 ± 3 Ma (1 ). This age is interpreted as representing final granitoid activity adjacent to the BGB, and/or craton-scale tectonism associated with the Limpopo Orogeny. Two samples gave T 0 ages of ~2350-2400 Ma which may reflect increased thermal gradients associated with the formation of the thick Transvaal sedimentary basin that may once have covered the BGB. The dominant apparent age plateaux together with modelled diffusive Ar loss ages of 2025-2090 Ma, are thought to represent regional thermal anomalies related to large-scale tectono-thermal activity in the Kaapvaal craton, of which the Bushveld Complex (which covers a surface area of ~ 60,000 km 2 ) and formation of the Vredefort Structure are obvious manifestations. The strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere (i.e., 40 Ar / 36 Ar ratios < 295.5) signatures.
de Ronde Cornel E. J.
Hall Chris Michael
Spooner T. C. E.
York Derek
No associations
LandOfFree
LASER step-heating 40 Ar / 39 Ar age spectra from early Archean (~3.5 Ga) Barberton greenstone belt sediments: A technique for detecting cryptic tectono-thermal events does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with LASER step-heating 40 Ar / 39 Ar age spectra from early Archean (~3.5 Ga) Barberton greenstone belt sediments: A technique for detecting cryptic tectono-thermal events, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and LASER step-heating 40 Ar / 39 Ar age spectra from early Archean (~3.5 Ga) Barberton greenstone belt sediments: A technique for detecting cryptic tectono-thermal events will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-862235