Mantle evolution diagrams

The evolution of mantle reservoirs with time — mantle evolution diagrams... 

There are many examples in the literature of authors who have used mantle evolution diagrams of the type illustrated in Figure 6.15 to plot the initial strontium isotope ratios of measured samples relative to mantle and crustal evolution curves, in order to determine their likely source region. For example, it is easy to see from Figure 6.15 that a suite of rocks produced by partial melting of the mantie at 1.0 Ga wUl have a very different initial ratio ( Sr/ Sr = 0.7034) from rocks produced by partial melting of the crust at this time ( Sr/ Sr = 0.7140). It is this principle which may be used to identify the source of magmatic rocks of known age. 

Fig. 2. Os isotope evolution diagram showing details of Re-Os model age systematics. The Trd age assumes that all Re was removed during a single stage of melting whereas the Tma age uses the measured Re concentration to calculate a model age with respect to the mantle evolution line. The equations for each age calculation are ...

FIGURE 3.29 eH time diagrams for the evolution of the Earth s mantle, (a) The mantle evolution curve of Vervoort and Blichert-Toft (1999) showing pre-3.0 Ga fractionation followed by a less extreme fractionation event. The mantle curve indicates that the mantle was depleted in Lu relative to Hf. Negative eHf-values would imply derivation from an enriched source such as enriched mantle or ancient continental crust, (b) A proposed eH -mantle evolution curve based upon eH -values calculated using the fast decay sheme of Bizzarre et al. (2003) - A = 1.983 X 10-11. This dataset supports the mantle evolution curve proposed by Vervoort and Blichert-Toft (1999) shown in (a). The data are taken from Blichert-Toft and Arndt (1999), Vervoort and Blichert-Toft (1999), Amelin et al. (1999), and Amelin et al. (2000). Symbols diamonds, most radiogenic samples Arc, modern arc samples ... 

Brevart tfr a/. (1986) use Pb isotope initial ratios to demonstrate mantle heterogen ty at 2.7 Ga and have constructed a depleted mantle curve from 2.7 Ga to the present day from the initial ratios of komatiites and basalts on a isotope evolution diagram. 

Radiogenic isotopes have proved a most powerful tool for understanding mantle processes. By studying mantle peridotites and mantle-derived melts from a variety of different geological time periods it is possible to define evolutionary curves for the different isotopic systems within the mantle. These curves, when plotted on isotope ratio versus time diagrams, can be used to characterize the chemical evolution of the mantle over time. Deviations from the chondritic trend are used to identify chemical fractionation events in the mantle during Earth history. Of particular... 

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...

The evolution of Sr/ Sr with time in the continental crust and mantle. At 2.7 Ga mande differentiadon led to the formadon of new continental crust. The new crust inherited an inidal Sr/ Sr rado of 0.7014 from its parent mande but acquired a substantially different Rb/Sr rado (0.17) compared with 0.03 in the mande. The higher Rb/Sr ratio in the continental crust led to an accelerated growth of Sr/ Sr with time in the continental crust relative to that in the mantle so that the present-day measured value in the orust is 0.7211 compared with 0,7045 in the mande. The Sr/ Sr rados shown on the right-hand side of the diagram indicate the initial ratios in melts formed from continental crust at 1.0 Ga (0.7140) and from the mantle at I.OGa (0.7034). 

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