Neodymium/strontium isotopic ratios

Isotopic heterogeneities are also observed on much smaller scales than so far discussed. For example, the region around 14 °N on the MAR shows a sharp spike in neodymium and strontium isotope ratios (see Figures 7 and 8) with an amplitude in e(Nd) nearly as large as that of the entire Atlantic MORE variation, even though there is no obvious depth anomaly or other physiographic evidence for the possible presence of a mantle plume. 

Radiogenic isotope ratios of OIE are shown on Figure 15. These diagrams display remarkably similar topologies as the respective MORE data shown in Figures 4-6. Strontium isotope ratios are negatively correlated with neodymium and hafnium isotopes, and correlations between strontium, neodymium, and hafnium isotopes and lead... 

The fractionated igneous and sedimentary rocks in Figure 20 have Rb/Sr and Eu/Sr ratios that are much higher than those in current estimates for the upper continental crust. The latter can be constrained by the strontium isotope ratios of continental run-off (—0.712), and its model neodymium age (—1.8 Ga). According to this method, a minimum time-integrated upper crustal Rb/Sr ratio of 0.14 is indicated. 

Earth value (s d = —1.5 to —4.3). They concluded correctly that neodymium in the oceans is mainly derived from the continents, and incorrectly that neodymium-isotope ratios in the oceans are like strontium isotopes in the sense that they are globally well-mixed and display this restricted range. In order to reach this conclusion they ignored a sample from the Indian Ocean having high strontium-isotope ratios and a lower, even more continent-like neodymium-isotope ratio of 8n(J = —8.5. 

Strontium-isotope ratios are not particularly diagnostic of source terrain ages because Rb/Sr ratios are easily disturbed. However, Sm/Nd ratios tend to be conservative and neodymium-isotope ratios are effective tracers of the mean cmstal age of sediment sources. Neodymium-isotope ratios of Heinrich layers HI, H2, H4, and H5 are consistent with derivation from a source with Archean heritage, and Grousset et al. (1993)... 

Isotope Ratios of Strontium, Neodymium, Hafnium, and Lead... 

Unlike. 208pb/204pb or 2 pb/204pb, which depend on Th/Pb and U/Pb, respectively, 208p 5 /206p]3 reflects the Th/U ratio integrated over the history of the Earth. The existence of global correlations between neodymium, strontium, and hafnium isotope ratios and, and the absence... 

Brimhall et al. (1991) used lead isotopes in zircons within a bauxite profile from Western Australia to differentiate between zircons derived from the underlying bedrock and zircons of eolian origin. Borg and Banner (1996) applied both neodymium and strontium isotopes to constrain the sources of soil developed on carbonate bedrock. Using these isotopes and Sm/Nd ratios, they were able to delineate the importance of atmospheric versus bedrock contributions in controlling the composition of the soil. Kurtz et al. (2001) used neodymium and strontium isotopes to determine the amount of Asian dust in a Hawaiian soil chronosequence. They found that the basaltic bedrock isotope signatures in soils had, in many cases, been completely overprinted by dust additions, demonstrating the profound effect of Asian dust on soil nutrient supplies. 

The first neodymium-isotopic analyses aimed at characterizing the oceans closely followed the initial development of neodymium isotopes as a chronometer and tracer (Richard et al., 1976 DePaolo and Wasserburg, 1976a O Nions et al., 1977). O Nions et al. (1978) was the first to report neodymium (along with lead and strontium) isotopes in manganese nodules and hydrothermal sediments. They confirmed the distinction between continental and mantle provenances of lead in hydrogenous and hydrothermal manganese sediments, respectively. Consistent with the previous studies, they found that strontium in these deposits is derived from seawater. All of the neodymium-isotope ratios in their samples from the Pacific were similar and lower than the bulk... 

Major-element compositions of most lithospheric peridotites reflect an origin as melt-residues. However, as with parent-daughter isotope ratios, compilation of their strontium and neodymium mineral isotopic compositions reveals that very few samples show the characteristics of ancient melt residues (Figure 37). Osmium isotopes are the exception and dominantly reflect ancient extraction of high Re/Os melts, leaving rhenium-depleted residues to develop time inte-... 

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