Neodymium isotopes model ages

TABLE III Age Province Distribution Based on Neodymium Isotope Model Ages... 

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... 

TrCHUR model ages The CHUR model assumes that the Earth s primitive mantle had the same isotopic composition as the average chondritic meteorite at the formation of the Earth, which in this case is taken to be 4.6 Ga. For neodymium isotopes CHUR is synonymous with the composition of the bulk Earth. A model age calculated reladve to therefore is the time in the past at which the... 

Table 6.3 Parameters used in the nornializatiDii of neodymium isotope, data for the calculation of model ages and epsilon valuesf...

Volcanics from the Onverwacht Group, Swaziland and South Africa, comprise the basal part of the Early Precambrian Barberton greenstone belt found both in Swaziland the Transvaal. Data from them gave an age of about 3.54 Ga calculated using an initial [ Nd/ Nd]o ratio of 0.50809 00004 (Hamilton et aL 1979). The strontium and neodymium isotopic record of Apollo-12 basalts from the Moon were examined. Isochrons were derived for an achondrite (eucrite) and another meteorite and gave ages of about 4.58 and 4.562 Ga, respectively, with primordial [ Nd/ Nd]p ratios of 0.50684 0.00008 and 0.506664. An initial [ Sr/ Sr]o ratio was termed ADOR. The isotopic evolution of neodymium in the Earth has been described by means of a model termed CHUR (GHondritic Uniform Reservoir). And the present value of the [ Nd/ Nd] ratio is 0.512638 relative to a [ Nd/ Nd] ratio of 0.7219. The present value of the [ Sm/ Nd] ratio of CHUR is 0.1967, which permits calculation of [ Nd/ Nd] ratio in any past time interval. 

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. 

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