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radiogenic isotopes1 - Granite from mixed SOUI’CC(a...

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Unformatted text preview: Granite from mixed SOUI’CC (a) Figure 6.8 6.3 Radiogenic isotopes in petrogenesis 231 0 EM! Granite from mixed source . . . 1.0 1.8 2.0 3.0 t(Ga) " ’ (b) t(Ga) The evolution of 143Nd/ 144Nd (expressed as epsilon units — see Section 6.3.4) with time, for depleted mantle and continental crust. Relative to CHUR the depleted mantle shows increasing 143Nd/ 144Nd increasingly positive e—values) with time, whereas the continental crust shows retarded 14 Nd/ 144Nd evolution (increasingly negative e—values). (a) The depleted mantle model age for a granite formed at 2.7 Ga from a mixture of a juvenile Proterozoic (1.9 Ga) and Archaean (3.0 Ga) crustal sources at 1.8 Ga is 2.6 Ga. The model ages have little real meaning for they neither reflect the crystallization age of the granite nor the age of the crustal source (Arndt and Goldstein, 1987). of the basaltic precursor to the granite. This is usually assumed to be very close in time to the crystallization age of the granite. Granitoidsgwhich are derived by the melting of older continental crust will give model ages which are indicative of the age of the crustal source. This is possible because the intracrustal fractionation process does not greatly disturb the Sm/Nd ratio of the source. Often, however, granites are a mixture of crustal and mantle sources. When this is the case, calculated model ages may give very misleading results (Arndt and Goldstein, 1987, and Figure 6.8). Model ages for elastic sedimentary rocks are thought to provide an estimate of their crustal residence age (O’Nions at £11., 1983), for minimal fractionation of Sm/ Nd accompanies their generation. Since, however, many continental sediments are a mixture of materials from different sources, model Nd ages are the average model age of the sediment and provide a minimum estimate of the crustal residence age or an average crustal residence age. Radiogenic isotopes in petrogenesis NM ...
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