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Subsequent work on the dating of shales sought to avoid problems of contamination with detrital micas and feldspars by analysing separated clay-mineral fractions, whose purity is checked by x-ray diffraction (XRD). XRD analysis of illites can also yield information about the nature and origin of clay minerals in a shale which is to be dated.
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However, two whole rocks with traces of microcline (2 & 3), lay slightly above it, while one with 15% microcline (1) was displaced well above the isochron.
It appears from this example that whole-rock Rb Sr dating of shales is an unreliable geochronometer, but that analysis of separated illite fractions may give meaningful ages of diagenesis or low-grade metamorphism.
Glauconite is a micaceous mineral similar to illite which is best developed in macroscopic pellets (called glauconies by Odin and Dodson, 1982).
These are probably formed by the alteration of a very fine-grained clay precursor intermixed with organic matter in a faecal pellet.
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 practice, the two distinct dating approaches associated with these sediment types have tended to converge.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.Glauconies form near the sedimentwater interface in the marine environment. However, by studying pellets on the present day ocean floor, Odin and Dodson (1982) have shown that glauconitisation is a slow process which may take hundreds of thousands of years to reach completion. Variation in the strontium isotopic composition of seawater (8 Ma to present): implications for chemical weathering rates and dissolved fluxes to the oceans. The latter reflect a detrital component, or post-diagenetic metamorphism.). These define an array which is colinear with associated dolomites, yielding an age of 860 35 Myr and an initial ratio of 0.7088, characteristic of Precambrian seawater. Redistribution of strontium and rubidium isotopes during metamorphism, World Beater complex, Panamint Range, California.