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

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.