Nd isotopic composition

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... 

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... 

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.

Bouvier, A., Vervoort, J. D. and Patchett, P. J. (2008) The Lu-Hf and Sm-Nd isotopic composition of CHUR constraints from unequilibrated chondrites and implications for the bulk compositions of the terrestrial planets. Earth and Planetary Science Letters, 273, 48-57. 

Mafic rocks from the Sicily Province display variable abundances of incompatible elements, which increase from tholeiites to alkali basalts and nephelinites. However, all the rocks show low LILE/HFSE ratios, typical of intraplate basalts, in contrast with the high LILE/HFSE compositions of volcanism from the Aeolian arc and the Italian peninsula. They have a relatively restricted range of Sr-Nd isotopic compositions, but variable Pb isotopic ratios, which fall along a trend connecting HIMU and intermediate compositions between DMM and EMI mantle. 

To interpret xenolith data, however, it is important to consider the general question of how well they can be expected to preserve the record of their mantle environment independently of the record in their host magmas (cf. Hofmann Hart, 1978, for solid elements Trull Kurz, 1993, for noble gases and Kellog, 1992, for a general discussion). Studies of Sr, Pb, and Nd isotopic compositions in various xenoliths indicate that these elements generally have not equilibrated with their host lavas. Experimental studies of diffusion coefficients of Sr, Mg, Al, and other cations indicate that this is reasonable, and that isotopic disequilibrium on a centimeter scale could persist for periods at least of the order of 10s to 109 year in dry crystalline... 

Figure 1 Nd isotopes in an aggregate of meteoritic SiC from Murchison. The deviation of the Nd isotopic composition from normal is given in per mil (6). All ratios are normalized to Full symbols are...

Figure 39 Cross-section of typical cratonic and noncratonic lithospheres as sampled hy xenoliths showing the variation in ohvine Mg, density (p). Os and Nd isotope compositions with depth.

Macdougal J. D. and Haggerty S. F. (1999) Ultradeep xenoliths from African Mmberhtes Sr and Nd isotopic compositions. Earth Planet. Sci. Lett. 170, 73 —82. 

Beard B. L. and Glazner A. F. (1995) Trace element and Sr and Nd isotopic composition of the mantle xenoliths from the big pine volcanic field, California. J. Geophys. Res. 100(B3), 4169-4179. 

Samson S. D., D Lemos R. S., Blichert-Toft J., and Vervoort J. D. (2003) U-Pb geochronology and Hf-Nd isotope compositions of the oldest Neoproterozoic crust within the Gadomian Orogen new evidence for a unique juvenile terrane. Earth Planet. Sci. Lett. 208, 165-180. 

Lahaye Y., Arndt N., Byerly G., Chauvel C., Fourcade S., and Gmau G. (1995) The influence of alteration on the trace-element and Nd isotopic compositions of komatiites. Chem. Geol. 126, 43-64. 

Tricca A., Stille P., Steinmann M., Kiefel B., Samuel J., and Eikenberg J. (1999) Rare earth elements and Sr and Nd isotopic compositions of dissolved and suspended loads from small river systems in the Vosges mountains (France), the river Rhien and groundwater. Chem. Geol. 160, 139-158. 

Figure 6 Sr and Nd isotopic compositions of granitoid bedrock, minerals, waters, and sediments from the Strengbach Catchment, France. A mixing calculation is shown with the percentage apatite in apatite-plagioclase mixtures. Sediments are dominated by plagioclase, whereas the waters and sediment leachate derive 10-30% of their Nd and Sr from the trace mineral apatite (source Aubert et al., 2001).

Ohlander B., Ingri J., LandM., and Schoberg H. (2000) Change of Sm-Nd isotope composition during weathering of till. Geochim. Cosmochim. Acta 64, 813-820. 

Amakawa H., Alibo D. S., and Nozaki Y. (2(X)0) Nd isotopic composition and REE pattern in the surface waters of the eastern Indian Ocean and its adjacent seas. Geochim. Cosmochim. Acta 64 10), 1715-1727. 

Ionov, D. A. Jagoutz, E. 1989. Sr and Nd isotopic composition in minerals of garnet and spinel peridotite xenoliths from the Vitim Highland first data for mantle inclusions in the USSR. Transactions Doklady J of the USSR Academy Sciences, Earth Science Section, 301, 232-236. 

Wooden, J. L. Mueller, P. A. 1988. Pb, Sr, and Nd isotopic compositions of a suite of Late Archaean, igneous rocks, eastern Beartooth Mountains implications for crust-mantle evolution. Earth and Planetary Science Letters, 87, 59-72. 

FIGURE 4.6 A plot of eNd versus time for samples of Proterozoic crust from North America and Europe (grey boxes). The age of the samples was fixed from their U-Pb zircon age. The Nd-isotope compositions are interpreted as mixing between a depleted mantle source, indicative of juvenile crust, and recycled Archaean crust. The % curves indicate the proportion of reworked Archaean crust incorporated into Proterozoic crust and hence the proportion of truly juvenile crust formed (data from Patchett Arndt, 1986) after Rollinson (2006). 

Three different crustal growth curves are shown in Fig. 4.8. The Nagler and Kramers (1998) curves are based upon the Nd-isotopic composition of the mantle. The Collerson and Kamber (1999) curve is based upon the Nb/U ratio of the mantle and the Condie (2000) curve is based upon U-Pb zircon ages in juvenile crust. All show slightly different rates of crustal growth, particularly in the late Archaean and early Proterozoic. 

---