The role of different isotopic systems in identifying reservoirs and processes

1 The role of different isotopic systems in identifying reservoirs and processes 

The different elements used in radiogenic isotope studies in geology vary greatly in their chemical and ph cal properties, so much so that ffifferent isotope systems vary in their sensitivity to particular petrological processes. This variability may show itself in two ways. Firsdy, the parent and the daughter elements may under certain drcumstances behave in different ways so that the two become fractionated. The order of incompatibility for elements of interest is 

These values indicate the extent to which an element is fractionated into the crust relative to the depleted mande. For example, Rb is the element most concentrated in the crust reladve to the depleted mande whereas Sr, Sm and Lu are the least concentrated. Alternatively, a parent-daughter element-pair may behave coherendy and not be fractionated and yet behave in a very different manner from the parent-daughter pair of another isotopic system. A good example is the contrast between the Sm-Nd system, in which both elements share very similar chemical and physical characteristics, and the Rb-Sr system, in which the elements are 

Upper crust High Rb/Sr low Sm/Nd High U/Pb High U/Pb High Th/Pb  

Middle aust Mod. high Unieplet ed U-dejpleted Mod. high Th  

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