Sm-Nd isotopic system

Pearson D. G. and Nowell G. M. (2003) Dating mantle differentiation a comparison of the Lu-Hf, Re-Os and Sm-Nd isotope systems in the Beni Bousera peridotite massif and constraints on the Nd-Hf composition of the lithospheric mantle. Geopkys. Res. Abstr. 5, 05430. 

In addition to the long-lived " Sm- " Nd pair, the Sm-Nd isotopic system also contains the short half-life (103 Myr) Sm- " Nd decay scheme. All " Sm has now decayed such that it is no longer possible to generate variations in i" Nd/i" Nd compositions. Any deviation from modem compositions must, there-... 

Vervoort J. D., Patchett P. J., Blichert-Toft J., and Albarede F. (1999) Relationships between Lu-Hf and Sm-Nd isotopic systems in the global sedimentary system. Earth Planet. Sci. Lett. 168(1-2), 79-99. 

We can now turn to isotopic ratios. The Nd isotope composition 143Nd/144Nd and the 147Sm/144Nd ratio are noted y and x, respectively, with the m and c subscripts denoting mantle and crust. Using this notation, the chronometric equation of the Sm-Nd closed-system reads... 

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. 

Goldstein S. L., O Nions R. K., and Hamilton P. J. (1984) A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems. Earth Planet. Sci. Lett. 70, 221-236. 

The geochemistry of marine sediments is a major source of information about the past environment. Of the many measurements that provide such information, those of the U-series nuclides are unusual in that they inform us about the rate and timescales of processes. Oceanic processes such as sedimentation, productivity, and circulation, typically occur on timescales too short to be assessed using parent-daughter isotope systems such as Rb-Sr or Sm-Nd. So the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides (principally or the U-series nuclides. This makes the U-series nuclides powerful allies in the quest to understand the past ocean-climate system and has led to their widespread application over the last decade. 

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... 

Note that this is similar to the approach adopted earlier with Sm- " Nd upon which many ideas of Hadean mantle depletion, melting processes and early cmst were based (Chase and Patchett, 1988 Galer and Goldstein, 1991). However, the difficulty with insuring closed-system behavior with bulk rock Sm-Nd in metamorphic rocks and achieving a robust age correction of long-lived " Sm over four billion years has meant that this approach is now viewed as suspect (Nagler and Kramers, 1998). The isotopic system... 

Figure 3 Crust-mantle differentiation patterns for the decay systems Rb-Sr, Sm-Nd, Lu-Hf, and Re-Os. The diagram illustrates the depletion-enrichment relationships of the parent-daughter pairs, which lead to the isotopic differences between continental crust and the residual mantle. For example, the Sm/Nd ratio is increased, whereas the Rb/Sr ratio is decreased in the residual mantle. This leads to the isotopic correlation in mantle-derived rocks plotted in Figure 4(a). The construction is similar to that used in Figure 2, but D values have been adjusted slightly for greater clarity.

Because of the numerous secondary effects that modify the whole-rock elemental and isotopic compositions, our discussion below will focus solely on mineral-separate analyses for the Rb-Sr, Sm-Nd, Lu-Hf, and U-Pb isotope systems. In comparison, the Re-Os system is much more robust to the effects of... 

The robustness of the Lu-Hf isotope system in some mantle environments is demonstrated by the precise Lu-Hf isochron of 1,413 67 Myr dehned by clinopyroxene separates from the Beni Bousera peridotite massif (Pearson and Nowell, 2003). This age probably dates the time of melt extraction from these rocks and is considerably more precise than the Sm-Nd isochron or the scattered Re-Os isotope systematics of these rocks. This indicates the potential power of this system in dating mantle rocks. The initial results from the Lu-Hf isotope system indicate that of the incompatible element isotope systems, it is the more robust to metasomatic effects, with signatures frequently recording the time-integrated response to melt depletion. 

---