Samarium–neodymium dating is a radiometric dating method useful for determining the ages of rocks and meteorites, based on the alpha decay of the long-lived samarium isotope (147Sm) to the stable radiogenic neodymium isotope (143Nd). Neodymium isotope ratios together with samarium–neodymium ratios are used to provide information on the age and source of igneous melts. It is sometimes assumed that at the moment when crustal material is formed from the mantle the neodymium isotope ratio depends only on the time when this event occurred, but thereafter it evolves in a way that depends on the new ratio of samarium to neodymium in the crustal material, which will be different from the ratio in the mantle material. Samarium–neodymium dating allows the determination of when the crustal material was formed. The usefulness of Sm–Nd dating stems from the fact that these two elements are rare earth elements and are thus, theoretically, not particularly susceptible to partitioning during sedimentation and diagenesis. Fractional crystallisation of felsic minerals changes the Sm/Nd ratio of the resultant materials. This, in turn, influences the rate at which the 143Nd/144Nd ratio increases due to production of radiogenic 143Nd. In many cases, Sm–Nd and Rb–Sr isotope data are used together.
Sm–Nd radiometric dating Samarium has seven naturally occurring isotopes, and neodymium has seven. The two elements are joined in a parent–daughter relationship by the alpha decay of parent 147Sm to radiogenic daughter 143Nd with a half-life of 1.066(5)×1011 years and by the alpha decay of 146Sm (an almost-extinct radionuclide with a half-life of 9.20(26)×107 years) to produce 142Nd. To find the date at which a rock (or group of rocks) formed one can use the method of isochron dating. The Sm–Nd isochron plots the ratio of radiogenic 143Nd to non-radiogenic 144Nd against the ratio of the parent isotope 147Sm to the non-radiogenic isotope 144Nd. 144Nd is used to normalize the radiogenic isotope in the isochron because it is a quasi-stable (with a half-life of 2.29(16)×1015 years) and relatively abundant neodymium isotope. The Sm–Nd isochron is defined by the following equation:
(
143 N d
144 N d ) p r e s e n t = (
143 N d
144 N d ) i n i t i a l + (
147 S m
144 N d ) ⋅ ( e λ t − 1 ) , {\displaystyle \left({\frac {{}^{143}\mathrm {Nd} }{{}^{144}\mathrm {Nd} }}\right)_{\mathrm {present} }=\left({\frac {{}^{143}\mathrm {Nd} }{{}^{144}\mathrm {Nd} }}\right)_{\mathrm {initial} }+\left({\frac {{}^{147}\mathrm {Sm} }{{}^{144}\mathrm {Nd} }}\right)\cdot (e^{\lambda t}-1),}
where:
t is the age of the sample, λ is the decay constant of 147Sm, (eλt−1) is the slope of the isochron which defines the age of the system. Alternatively, one can assume that the material formed from mantle material which was following the same path of evolution of these ratios as chondrites, and then again the time of formation can be calculated (see #The CHUR model).
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 relative to samarium. Thus, as a melt 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 high Sm and low Nd and therefore high Sm/Nd ratios. Felsic rocks have low concentrations of Sm and high Nd and therefore low Sm/Nd ratios (for example komatiite has 1.14 parts per million (ppm) Nd and 3.59 ppm Sm versus 4.65 ppm Nd and 21.6 ppm Sm in rhyolite). The importance of this process is apparent in modeling the age of continental crust formation.
… excerpt ends here. Continue reading the full article.
