The Samarium – Neodymium System:

Some background on Samarium and Neodymium:
Samarium-147 decays to Neodymium-143 through alpha decay. Samarium and Neodymium are both incompatible elements, however Nd is more incompatible. This produces the reverse of the Rb-Sr system where the parent was more incompatible because here the daughter is more incompatible. As partial melting occurs, the melt will become relatively enriched in Nd compared to Sm, while the mantle residue will be depleted in Nd relative to Sm.

We can write a similar equation to the Rb-Sr system to express the ratio of radiogenic to non-radiogenic Nd over time.

Because the Sm/Nd ratio of the bulk earth is the same as that of chondrites, another, more useful equation can be written to compare the ratio of a sample to that of chondrites.


This Epsilon-Neodymium value varies between -20 and +10 in most rock samples. A negative value indicates continental crust whereas a positive value is represented by MORB or OIB (Reed, et al, 1993). The following figure shows eNd values relative to Sm/Nd values.

(White, 2005)

A powerful tool is to use the Rb-Sr system in conjunction with the eNd value for a sample. The following figure shows the relationship between the two systems as they vary inversely.

(White, 2005)

From White, 2005 and Dickin, 2002

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