Compositional evolution neodymium isotopes

Mantle evolution curves for neodymium isotopic compositions in the Earth prior to 2.7 Ga are much less well defined than for younger time periods. Many studies have elected simply to extrapolate a linear evolution from 2.7 Ga to the composition of the primitive mantle (snu = 0) at 4.56 Ga (e.g., Goldstein et al., 1984). In contrast, other studies have attempted a more precise definition of the neodymium mantle evolution curve for the Archean based on observed compositions (e.g., Bennett et al., 1993 Bowring and Housh, 1995 Vervoort and Blichert-Toft, 1999). The justification of these efforts is that Archean mantle chemistry, particularly for the long-lived isotopic systems, provides a window into the first 700 Myr of Earth s history, prior to establishment... 

The rather constant fractionation of Sm/Nd ratios in upper continental cmstal rock reservoirs is the basis for the widely applied neodymium model age that is illustrated in Figure 3. The Sm-Nd systematics of chondritic meteorites serve as a reference for the parent/daughter ratio of the undifferentiated Earth (Jacobsen and Wasserburg, 1984), labeled as CHUR for chondritic uniform reservoir. The evolution of this undifferentiated Earth is the basis for calculation of CHUR model ages (McCulloch and Wasserburg, 1978), while the neodymium isotopic evolution of the depleted upper part of the mantle is a more valid reference for most cmstal materials, resulting in the DM model age (DePaolo, 1981). Neodymium isotopic compositions are usually given by s d, where the deviations of Nd/ Nd above or below... 

The Sm-Nd method can be used to date mafic igneous rocks (basalt and gabbro) which are not suitable for dating by the Rb-Sr method. Magma of basaltic chemical composition originates by decompression melting in the mantle of the Earth which contains radiogenic Nd that has formed by decay of Sm. The isotopic evolution of Nd in the mantle of the Earth is represented by a model in Fig. 3.19 that is based on the isotope composition of neodymium in chondrite meteorites and which is therefore known as the Chondritic Uniform Reservoir (CHUR). The present value of the Nd/ Nd ratio in CHUR is ... 

In this section mantle evolution curves are presented for neodymium (Nd), hafnium (Hf), lead (Pb) and osmium (Os) isotopes. A summary of these isotopic systems is given in Text Box 3.2. Earlier studies based upon the study of Sr isotopes in the mantle (e.g. Bell et al., 1982) are now known to be unreliable because of its high geochemical mobility (Goldstein, 1988). The significance of the mantle evolution curves described here is that they demonstrate that the mantle does not operate as an isolated system but that it has evolved in its composition over time, in response to core formation, crust extraction, and the recycling of crustal material. 

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