Samarium-neodymium dating

Samarium–neodymium dating is a radiometric dating method useful for determining the ages of rocks and meteorites, based on radioactive decay of a long-lived samarium (Sm) isotope to a radiogenic neodymium (Nd) isotope. Neodymium isotope ratios together with samarium-neodymium ratios are used to provide information on the source of igneous melts, as well as to provide age information. 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 us to determine when the crustal material was formed. Samarium–neodymium dating is a radiometric dating method useful for determining the ages of rocks and meteorites, based on radioactive decay of a long-lived samarium (Sm) isotope to a radiogenic neodymium (Nd) isotope. Neodymium isotope ratios together with samarium-neodymium ratios are used to provide information on the source of igneous melts, as well as to provide age information. 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 us to determine when the crustal material was formed. The usefulness of Sm–Nd dating stems from the fact that these two elements are rare earths 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. Samarium has five naturally occurring isotopes, and neodymium has seven. The two elements are joined in a parent–daughter relationship by the alpha decay of 147Sm to 143Nd with a half-life of 1.06×1011 years and by the alpha decay of 146Sm (an almost-extinct nuclide with a half-life of 1.08×108 years) to produce 142Nd. (Some of the 146Sm may itself have originally been produced through alpha-decay from 150Gd, which has a half-life of 1.79×106 years.) To find the date at which a rock (or group of rocks) formed one can use the method of isochron dating. This involves making a graph of 143Nd:144Nd ratio versus 147Sm:144Nd ratio for various minerals or rocks. From the slope of the 'isochron' line through these points the date of formation can be determined. 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). 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) 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. Through the analysis of isotopic compositions of neodymium, DePaolo and Wasserburg (1976) discovered that terrestrial igneous rocks at the time of their formation from melts closely followed the 'chondritic uniform reservoir' or 'chondritic unifractionated reservoir' (CHUR) line – the way the 143Nd:144Nd ratio increased with time in chondrites. Chondritic meteorites are thought to represent the earliest (unsorted) material that formed in the Solar system before planets formed. They have relatively homogeneous trace-element signatures, and therefore their isotopic evolution can model the evolution of the whole Solar system and of the 'bulk Earth'. After plotting the ages and initial 143Nd/144Nd ratios of terrestrial igneous rocks on a Nd evolution vs. time diagram, DePaolo and Wasserburg determined that Archean rocks had initial Nd isotope ratios very similar to that defined by the CHUR evolution line.