Rb sr dating
Much further sampling is needed, as well as the combination of several methods of dating.To facilitate this, modern geological and tectonic maps of medium scale are very desirable. Although the unreliability of the Rb-Sr method for dating is readily demonstrated, it is important that alternative models are developed to explain the observed patterns of strontium isotopes. When diffusive isotope fractionation of Sr isotopes is repeated a sufficient number of times by within-mantle processes on isotopically-anomalous mantle xenoliths (each of which originally had low Sr ratios.When mixed with variable amounts of terrestrial nonradiogenic strontium, these remnants generate mixing lines indistinguishable from age-indicative Rb-Sr isochrons.
Included are “Older Precambrian” gneiss(es); Cambrian shale-siltstone; and sheared, mylonitized, and brecciated (fault) equivalents of these rocks. Among the tentative conclusions reached are: (1) Some activity, probably the development of frictional heat that accompanies faulting, may cause adjustments of isotopic ratios so that the resulting “ages” indicate time of the faulting; and (2) isotopic data on rocks from such fault zones may also give clues as to the depth of the faulting.’Actually, the question of whether Sr isotopes can be fractionated by nature or not is irrelevant, because all Sr isotope analysis are routinely corrected for all isotopic fractionation effects, be they natural or laboratory-induced.The possibility of isotopic fractionation as an explanation for differences in Sr has been proposed to both authors on many occasions.In proper usage, the phrase ‘isotope fractionation’ refers exclusively to the physical separation of one isotope of an element from another isotope of the same element.It has been known for several decades that the lighter elements (notably carbon, oxygen, hydrogen and sulphur) undergo isotopic fractionation under the influence of their physical and chemical environment, and much research has been conducted to understand this behaviour.