The Sm-Nd system

The equation of radioactive decay for the Sm-Nd system in the sediment reads... 

The method is based on Equation 5-10, with one simple but necessary modification. Use the Sm- Nd system as an example. " Sm decays to " Nd with a half-life of 106 billion years. The growth equation of " Nd is (Equation 5-10)... 

The choice of the normalizing isotope (such as Nd for the Sm- Nd system) is by convention, although many considerations went into the choice. The most important criterion is that the normalizing isotope is stable and non-radiogenic. Other considerations include the mass difference between the radiogenic isotope and the normalizing isotope, and the convenience to make the... 

Another coupled system is the Sm-Nd system, with two Sm isotopes ( Sm and Sm) undergoing ot-decay to become two Nd isotopes ( Nd and Nd). The half-life of Sm is 106 billion years and that of is 103 million years. In principle, the concepts for the U-Pb system (such as concordia and discordia, Nd-Nd isochron) can also be applied to the Sm-Nd system. However, the Sm-Nd coupled system has not found many applications. One reason is that the half-life of " Sm is so short that it is an extinct nuclide. Secondly, the half-lives of Sm and " Sm are very different, by a factor of 1000 (in contrast, the half-lives of and 235 differ only by a factor of 6.3). Hence, the coupled system has found only limited applications to very old rocks, such as meteorites and very old terrestrial rocks. 

Interpretation of chronological data on ureilites is contentious. There appear to be three groups in the Sm-Nd system. Several fall along a line with a slope consistent with an age of 3.79 Ga. A second set defines a line with a slope corresponding to an age of 4.23 Ga. A third set plots along a 4.55 Ga chondritic evolution line. The three lines intersect at 0.51. Torigoye-Kita... 

The Lu- Hf isotopic system (half-life —37 Gyr) is, in many ways, chemically similar to Sm- Nd. In both isotopic schemes the parent and daughter elements are refractory lithophile elements, such that their relative abundances in the Earth were probably not modified during accretion, nor did they participate in core formation. Thus, as for the Sm-Nd system, the compositions of chondritic meteorites can, in principle, be used to establish bulk silicate Earth isotopic compositions and Lu/Hf ratios directly. The potential, therefore, exists for establishing a precise isotopic baseline to use for recognizing fine-scale deviations in isotopic compositions, which can then be used to reveal... 

Scherer E. E., Cameron K. L., and Blichert-Toft J. (2000) Lu-Hf garnet geochronology closure temperature relative to the Sm-Nd system and the effects of trace mineral inclusions. Geochim. Cosmochim. Acta 64(19), 3413-3432. 

Mezger K., Essene E. J., and HalUday A. N. (1992) Closure temperatures of the Sm-Nd system in metamorphic garnets. Earth Planet. Sci. Lett. 113, 397-409. 

BOX 1.1 FIGURE 1 Isochron diagram for the Sm-Nd system showing a suite of 13 samples which formed at the same time, but which have slightly different Sm/Nd ratios. The calculated age for the 13 samples is 3,540 30 Ma. 

Thus isochron calculations require a suite of cogenedc samples, formed from the same parental material, and assume that there has been no exchange of parent-daughter isotopes other than through radioactive decay. An example of an isochron drawn for the Sm-Nd system from data in Table 6.2 is shown in Figure 6.2. 

These values indicate the extent to which an element is fractionated into the crust relative to the depleted mande. For example, Rb is the element most concentrated in the crust reladve to the depleted mande whereas Sr, Sm and Lu are the least concentrated. Alternatively, a parent-daughter element-pair may behave coherendy and not be fractionated and yet behave in a very different manner from the parent-daughter pair of another isotopic system. A good example is the contrast between the Sm-Nd system, in which both elements share very similar chemical and physical characteristics, and the Rb-Sr system, in which the elements are... 

LU Hf but differ markedly from Rb-Sr, U-Pb and Th-Pb. Sm-Nd are immobile under hydrothermal condidons and so their isotopic composition reflects the actual proportions of rock or magma involved in specific petrological processes. The Sm- Nd system, however, has the disadvantage that small amounts of recycled crust mixed with a large proportion of a mantle component become isotopicaliy in Hisibte. 

The evolution of Nd/ isotopes with rime in the mande, the continental crust and the bulk earth. Relative to the bulk Earth (CHUR) in which the fractionation of Sm/Nd is normalized to unity, the depleted mande (DM) has a high Sm/Nd ratio and shows higher Nd. The continental crust has lower Sm/Nd and shows a retarded Nd/ " Nd evolution with time. Enriched mantle (EM) shows some affinity with the continental crust inasmuch as it also has a retarded evolution. Note that in the Sm-Nd system... 

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