Therefore, dating of such material should give an average of the provenance ages of the sedimentary constituents.

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Analysis of detrital sediments has moved towards the analysis of fine-grained, almost authigenic, minerals such as illite, in order to escape the effects of the detrital component.

In contrast, analysis of authigenic minerals has been focussed on the sub-authigenic mineral glauconite, since the truly authigenic Rb-bearing evaporite minerals are too susceptible to burial metamorphism to be viable geochronometers.

Detrital Rb-bearing minerals (mica, K-feldspar, clay minerals etc.) can be expected to contain inherited old radiogenic Sr.

Absolute dating of the time of deposition of sedimentary rocks is an important problem, but one that is very difficult to solve.

Accurate dates depend on thorough re-setting of isotopic clocks.

In the case of Rb Sr dating of sediments, this rests on the assumption that Sr isotope systematics in the rock were homogenized during deposition or early diagenesis, and thereafter remained as a closed system until the present day.

However, we will see that these two requirements may be mutually exclusive.

In principle, sedimentary rocks may be divided into two groups according to the nature of the Rb-bearing phase present.

Allogenic (detrital) minerals are moderately resistant to open-system behaviour during burial metamorphism, but problems arise from inherited isotopic signatures.