Nd isotopes

Wasserburg, GJ, Jacobsen SB, DePaolo DJ, McCullock MT, Wen T (198L) Precise determination of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochim Cosmo-chim Acta 45 2311-2323. 

Macdongall JD, Lugmair G (1986) Sr and Nd isotopes in basalts from the East Pacific Rise significance for mantle heterogeneity, Earth Planet Sci Lett 77 273-284 McKenzie D (1984) The generation and compaction of partially molten rock. J Petrol 25 713-765 McKenzie D (1985) °Th- U disequilibrium and the melting processes beneath ridge axes. Earth Planet Sci Lett 72 149-157... 

The loading matrix FA1/2 given by equation (4.4.26) can be found in Table 4.14. Lead isotopes have strong correlation coefficients on the first component. They are decoupled from Sr and Nd isotopes which strongly correlate and anticorrelate, respectively, with the second component. On a global scale, Pb isotopic variations in oceanic islands seem to be decoupled from Sr and Nd isotopic variations. 

Figure 4.13 Principal component analysis of the mean isotopic data for oceanic islands (courtesy of Vincent Salters). In the top left corner, the plane of the first two components (the Mantle Plane of Zindler et al, 1982) explains 93 percent of the variance. Component 1 is dominated by lead isotopes, component 2 by Sr and Nd isotopes. Other components are plotted for reference. In the top right corner, the Mantle Plane is viewed sideways along the direction of the second component, so the distance of each point to the plane can be easily seen. In the bottom left corner, it is viewed along the axis of the first component. The bottom right corner shows how little variance is left with components 3 and 4.

For the sake of illustration, we will calculate the Nd isotope composition of continents in a very simple model of crustal evolution. A newly formed crustal segment results from the addition of both juvenile mantle and material recycled from the... 

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... 

Allegre, C. J. Rousseau, D. (1984). The growth of the continents through geological time studied by Nd isotope analysis of shales. Earth Planet. Sci. Letters, 67, 19-34. 

Juteau, M., Michard, A. Albarede, F. (1986). The Pb-Sr-Nd isotope geochemistry of some recent circum-Mediterranean granites. Contrib. Mineral. Petrol., 92, 331-40. 

Michard, A., Gurriet, P., Soudan, M. Albarede, F. (1985). Nd isotopes in French Phanerozoic shales external vs. internal aspects of crustal evolution. Geochim. Cosmochim. Acta, 49, 601-10. 

Vance D, Thirlwall ME (2002) An assessment of mass discrimination in MC-ICP-MS using Nd isotopes. Chem Geol 185 227-240... 

Stevenson R, Upton BGJ, Steenfelt A (1997) Crust-Mantle interaction in the evolution of the Ilimaussaq Complex, South Greenland Nd isotopic studies. Lithos 40 189-202 Stewart MA, Boudreau AE, Spivack AJ (1996) Cl isotopes of the Stillwater Complex, Montana, and the formation of high-Cl Bushveld and Stillwater parent liquids during mantle metasomatism by Cl-rich fluids. Eos Trans, AGU 77 F829... 

Albarede F, Goldstein SL, Dautel D (1997b) The Nd isotopic composition of Mn-nodules from the Southern and Indian Oceans, the global oceanic Nd budget, and their bearing on deep ocean circulation. Geochim Cosmochim Acta 61 1277-1291... 

Sm = 0.2672, and Sm = 0.2271. Three of these isotopes undergo a decay, being transformed into Nd isotopes ... 

Sandeman, H.A., Hanmer, S., Davis, W.J., Ryan, J.J., Peterson, T.D. 2004. Whole-rock and Nd isotopic geochemistry of Neoarchaean granitoids and their bearing on the evolution of the Central Hearne supracrustal belt. Western Churchill Province, Canada. Precambrian Research, 134, 143-167. 

Figure 3-24 Calculated diffusion-couple profiles for trace element diffusion and isotopic diffusion in the presence of major element concentration gradients using the approximate approach of activity-based effective binary treatment. The vertical dot-dashed line indicates the interface. The solid curve is the Nd trace element diffusion profile (concentration indicated on the left-hand y-axis), which is nonmonotonic with a pair of maximum and minimum, indicating uphill diffusion. The dashed curve is the Nd isotopic fraction profile. Note that the midisotopic fraction is not at the interface.

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. 

In isotope geochemistry, ( Th/ U) reflects U-Th disequilibrium. This ratio is not measured directly from mass spectrometers. In stead, isotope composition measurement provides nd isotope dilution... 

Steinmann, M., Stille, P., Bernotat, W. Knipping, B. 1999. The corrosion of basaltic dykes in evaporites Ar-Sr-Nd isotope and rare earth elements evidence. Chemical Geology, 153, 259-279. 

In order to trace the migration of basalt-derived REE in the salt, REE distribution patterns (Fig. 7) and Nd isotopic compositions (Fig. 8) have been determined in a salt horizon adjacent to a basalt dyke (Fig. 2). The flat REE distribution patterns and the almost basaltic Nd isotopic composition of the salt samples collected at the basalt-salt contact point to a basaltic origin of the REE for this sample. With increasing distance from the contact, the patterns are more and more depleted in Ce, Pr, Nd, Sm, and Eu and the Nd isotopic compositions are slightly shifted towards lower eNd values, which, however, still remain above values typical for continental crust or Permian seawater (Stille et al. 1996, and citations therein). This evolution of the REE distribution patterns and the Nd isotopic compositions could basically be due to mixing between a basalt and a salt end member or, alternatively, it could have been fractionation of the REE during migration in the salt that modified the REE patterns. 

Fig. 8. The evolution of the Nd isotopic compositions with distance in the salt profile together with the compositions of average basalt and average continental crust. The error bars represent 2 sigma mean values of the individual measurements.

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