Chondritic uniform reservoir

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 first approach was based on the observed complementarity with respect to Sm/Nd ratios of continental crust and depleted mantle. With time, the latter reservoir develops radiogenic Nd/ Nd ratios relative to the undifferentiated Chondritic Uniform Reservoir (CHUR see Jacobsen Wasserburg 1984), i.e. positive Nd values, where Nd = [( " Nd/ Nd,amp,e)/ ( Nd/ NdcHUR) f] 10" - Positive initial CNd... 

Figure 6.3 The evolution of Nd/ Nd with rime in an individual sample (Tabic 6.4, No. 234J compared with two models of the mantle — CHUR (the chondritic uniform reservoir) and depleted mantle, A model Nd age Is the time at which the sample had the same Nd/ " d ratio as its mantle source. In this case there are two possible solutions, depending upon which mande model Is preferred. The CHUR model age is approximately 1.67 Ga and the DM model age is approximately 1.98 Ga. A similar diagram can be constructed using the epsilon notation (see Section 6,3.4) in place of...

We used the " Nd/ Nd and Sm/ Nd ratios of one whole-rock basalt sample and three magnetic splits reported by Rowell et al. (1993) to determine their initial Nd/ Nd ratio by means of the Sm-Nd isochron diagram in Fig. 3.18. An unweighted least-squares regression of the data yields a value of 0.511594 for the initial Nd/ Nd ratio which we used to calculate the time in the past when the Nd/ Nd ratio of the chondritic uniform reservoir (CHUR) of the Earth in Fig. 3.19 had this value. The resulting Sm-Nd model date of 809 10 Ma is similar to the date obtained by Rowell et al. (1993) who reported t = 700 to 800 Ma. 

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 ... 

Tasmanian Dolerites (open circles) cluster in quadrant 4 of the Sr-Nd isotopic mixing diagram which is partitioned based on the isotopic composition of the Chondritic Uniform Reservoir (CHUR) described by Faure and Mensing (2005). The diagram also contains data points (crosses) of contact sediment which define an... 

The geochronological information that can be obtained from such Hf isotopic analysis is a so-called model age (Figure 9.6), representing the moment at which the zircon would have formed from a reservoir with an Hf isotopic composition similar to the depleted mantle (DM) or chondritic uniform reservoir (CHUR), the latter being a model for the whole Earth. 

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