Assumptions made in the calculation of model ages

It is important when calculating model ages to remember the assumptions upon which they are based, for these are not always fulfilled. Firstly, assumptions are made about the isotopic composition of the reservoir which is being sampled — either CHUR or depleted mantle. This aspect of model age calculations in itself raises three further problems. 

A second assumption used in model age calculations is that the Sm/Nd ratio of the sample has not been modified by fractionation after its separation from the mantle source. In the case of Nd isotopes this is a reasonable assumption to make. 

Thirdly, it is assumed that all material came from the mantle in a single event Table 6.4 shows model Nd-CHUR and depleted mantle ages for granulites and 

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