Sm-Nd Dating

The Sm-Nd system is useful for determining crystallization ages of igneous and metamorphic rocks. It has proven especially valuable for dating mafic and ultramafic rocks that are, in general, difficult to date using other methods. The principles of Sm-Nd dating are identical to those of Rb-Sr dating, which have been discussed in detail elsewhere (e.g. Lanphere et al. 1963; Faure 1977). The underlying assumption is that at some time Tx in the past, all the minerals in a rock have identical values of the ratio 143Nd/144Nd, but have different ratios of 147Sm/144Nd. This starting point is shown in Fig. 2.1 by the open circles representing minerals A, B, and C. The open circle labelled “TR” represents the whole rock, which is just the sum of the minerals, and thus must also have the same 143Nd/144Nd. As time progresses 147Sm atoms decay to 143Nd atoms, so 147Sm/144Nd decreases and 143Nd/144Nd increases. The positions of the minerals and rock on this diagram move along the arrows shown, which have a slope of -1.