Trace elements and mantle end-member compositions

Continental The isotopic composition of rocks from the continental crust is extremely variable crustal sources and isotope ratios are only strictly comparable if the samples are all of the same age. 

Thus compositional differences are best considered relative to a normalizing parameter which takes into account the age of the sample. One such reference point is the chondritic model of Earth composition (CHUR) and its evolution through time. Thus crustal samples may be plotted on a graph of isotopic composition vs time relative to a reference line such as the CHUR reservoir (see Section 6.3.3). For 

Nd isotopes this inay be quantified in tenns of the epsilon notation (see Section 6.3.4). The general isotopic characteristics of crustal reservoirs are given in Table 6.5 and typical isotopic compositions of crustal rocks are presented in Table 6.6. 

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