Sm and Nd Geochemistry
The concentration of Sm and Nd in silicate minerals increase with the order in which they crystallise from a magma according to Bowen's reaction series. Samarium is accommodated more easily into mafic minerals, so a mafic rock which crystallises mafic minerals will concentrate neodymium in the melt phase faster relative to samarium. Thus, as a rock undergoes fractional crystallization from a mafic to a more felsic composition, the abundance of Sm and Nd changes, as does the ratio between Sm and Nd.
Thus, ultramafic rocks have low Sm and Nd and high Sm/Nd ratios. Felsic rocks have high concentrations of Sm and Nd but low Sm/Nd ratios (komatiite has 1.14 parts per million (ppm) Sm and 3.59 ppm Nd versus 4.65 ppm Sm and 21.6 ppm Nd in rhyolite).
The importance of this process is apparent in modeling the age of continental crust formation.
Read more about this topic: Samarium-neodymium Dating