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GEOREF RECORD

Rb-Sr systematics of Holocene pelitic sediments and their bearing on whole-rock dating

Umberto G. Cordani, Ana M. Mizusaki, Koji Kawashita and Antonio Thomaz Filho
Rb-Sr systematics of Holocene pelitic sediments and their bearing on whole-rock dating
Geological Magazine (March 2004) 141 (2): 233-244

Abstract

This investigation of the behaviour of Rb and Sr during sedimentation and early diagenesis, involves study of more than 60 samples of fine-grained recent sediments from selected coastal localities of Brazil. The results indicate that pelitic samples from some recent to present-day transitional or shallow marine environments, such as the Jacarepagua tidal flat and the Amazonas River mouth, where halmrolysis could have occurred, may produce nearly horizontal best-fit lines in a Rb-Sr isochron diagram. Moreover, the initial isotopic (super 87) Sr/ (super 86) Sr ratios appear to be well above 0.710. In open marine environments, such as the Campos Basin, where sampling was spread over more than 100 metres, exchanges between the argillaceous sediments and seawater potentially happen at low T, inducing a reduction of the (super 87) Sr/ (super 86) Sr isotopic composition of the sediments to nearly 0.710. An almost horizontal best-fit line is produced for the entire stratigraphic section, independent of the sedimentation age. The above described studies on fine-grained and recent argillaceous sediments may confirm the production of horizontal best-fit lines in isochron diagrams, justifying the application of the Rb-Sr whole-rock method to pelitic sedimentary rocks. The nature of the material is critical. The pelitic sediments to be analysed should contain only very limited amounts of coarse clastic material (especially feldspars and mica fragments) and consist mainly of fine clay minerals in which smectites, illite and mixed layers illite-smectite predominate.


ISSN: 0016-7568
EISSN: 1469-5081
Coden: GEMGA4
Serial Title: Geological Magazine
Serial Volume: 141
Serial Issue: 2
Title: Rb-Sr systematics of Holocene pelitic sediments and their bearing on whole-rock dating
Affiliation: Universidade de Sao Paulo University, Instituto de Geociencias, Sao Paulo, Brazil
Pages: 233-244
Published: 200403
Text Language: English
Publisher: Cambridge University Press, London, United Kingdom
References: 27
Accession Number: 2004-066141
Categories: Quaternary geologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch maps
S22°15'00" - S21°45'00", W42°30'00" - W41°30'00"
S05°30'00" - S05°00'00", W45°15'00" - W44°30'00"
Secondary Affiliation: Universidad Federal do Rio Grande do Sul, BRA, BrazilUniversidade Estadual do Rio de Janeiro, BRA, Brazil
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United KingdomTwickenhamUKUnited Kingdom
Update Code: 200419
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