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

Abstract

-In many cases, when dealing with argillaceous fine-grained sedimentary rocks, the isotopic ages obtained from Rb-Sr whole-rock isochron calculations are of geological significance, despite the fact that the initial conditions of Sr isotopic homogenization are not fulfilled. To explain this, a mechanical mixing during deposition has been suggested, leading to an aggregate of mixed material with fairly uniform 87 Sr/ 86 Sr values, whatever Rb/Sr ratio is found in the analysed samples. 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 Jacarepaguá tidal flat and the Amazonas River mouth, where halmirolysis could have occurred, may produce nearly horizontal best-fit lines in a Rb-Sr isochron diagram. Moreover, the initial isotopic 87 Sr/ 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 temperature, inducing a reduction of the 87 Sr/ 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. We believe that in similar environments such 'zero age isochrons' are maintained for some time, testifying to exchanges between the sediment and the interstitial fluids, before the onset of burial diagenesis. The above described studies on fine-grained and recent argillaceous sediments seem to 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 largely predominate.

Isotopic and thermochronologic evidence of extremely cold lithosphere associated with a slab flattening in the Central Andes of Argentina

“…As illite clay minerals contain Rb, the Rb–Sr system is also frequently used for dating. The partial or complete homogenization of the Rb–Sr system during the burial history of a succession takes place in the anchizone–epizone interval (200–300°C; Cordani et al ., , ). Consequently, below the anchizone this system behaves like the K–Ar system.…”

“…Consequently, below the anchizone this system behaves like the K–Ar system. To obtain diagenetic ages, the best results are supplied by “internal isochronous”, constructed from analyses of authigenic fractions (<2 μm), leachates (with dilute HCl) and residues (Cordani et al ., ).…”

Depositional ages of Paleozoic and Mesozoic pre-rift supersequences of the Recôncavo Basin in northeastern Brazil: A Rb–Sr radiometric study of sedimentary rocks

“…If the assumption of Sr isotope homogenization during sediment deposition is valid, successful RbeSr of siliciclastic sedimentary rocks depends on the selection of samples with high contents of expansive clays (see Mizusaki, 1992;Mizusaki et al 1998;Cordani et al, 2004) such as smectite and interstratified illite-smectite sampled according to the recommendations of Thomaz Filho and Lima (1979).…”

“…However, since these units are formed of very finegrained siltstones and sandstones, and in view of the chosen criteria, they were not deemed suitable for Rb dating (Mizusaki, 1992;Cordani et al, 2004).…”

Caracterização das fontes dos sedimentos marinhos da Plataforma Continental da região sudeste do Brasil, durante o Holoceno médio e superior com base na composição isotópica de Sr, Nb e Pb: implicações nos estudos paleoclimáticos e de hidrodinâmica marinha

“…Foram realizados testes, envolvendo os isótopos de Sr e Nd, visto que diversas técnicas vêm sendo empregadas na dissolução do carbonato de cálcio precipitado do oceano (bioclastos), que integra o material coletado (e.g. Asahara, 1999;Asahara et al, 1995;1999;Bayon et al, 2002;Bentahila et al, 2008;Cordani et al, 2004;Parra et al 1997). Também foram realizados testes para verificar a necessidade de remoção da contaminação das amostras por metais pesados antes das análises isotópicas de Pb, devido à contaminação dos sedimentos, superficiais (e.g.…”