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radiogenic isotopes4 - 234 6.3.2 Using radiogenic isotope...

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Unformatted text preview: 234 6.3.2 Using radiogenic isotope data Lu—Hf but differ markedly from Rb—Sr, U—Pb and Th—Pb. Sm—Nd are immobile under hydrothermal conditions and so their isotopic composition reflects the actual proportions of rock or magma involved in specific petrological processes. The S'm-—N d system, however, has the disadvantage that small amounts of recycled crust mixed with a large proportion of a mantle component become isotopically invisible, Lead isotopes are more complex because of the three different decay schemes employed (Box 6.1) and because they do not define linear trends on lead isotOpe evolution diagrams. In general, uranium and lead are relatively mobile in crustal processes, particularly in magmatic/ hydrothermal situations, whereas thorium is highly insoluble. Both U and Pb are incompatible elements in silicates although U ' enters a melt more readily than Pb. In detail, the two isotopes of lead produced from uranium, 206Pb and 207Pb, show contrasting behaviour as a consequence of their differing radioactive decay rates. Early in the history of the Earth 235U decayed rapidly relative to 238U so that 207Pb evolved rapidly with time. 207Pb abundances therefore are an extremely sensitive indicator of an old source. Today, however, 235U is largely extinct so that in the recent history of the earth Z38U decay is more prominent and consequently 206Pb abundances show a greater spread than 207Pb (Figure 6.18a and Table 6.6). The difference in behaviour between the different isotopes of lead allows the identification of several isotopic reservoirs (Table 6.5). The crustal reservoirs are best sampled by studying the isotopic composition of a mineral such as feldspar with a low U/ Pb or low Th/ Pb ratio and which preserves the ‘initial’ Pb isotopic composition of the source. This approach was developed by Doe and Zartman (1979) and is discussed in the section on ‘plumbotectonics’ (Section 6.3.6). Strontium is relatively immobile under hydrothermal conditions, although Rb is more mobile. Sr therefore reflects fairly closely the original bulk composition of a suite of rocks, and Rb less so. In addition the Rb—Sr system shows the most extreme differences in incompatability between the parent and daughter elements. Rb and Sr are easily separated, so that there is extreme fractionation between crust and mantle leading to the accelerated strontium isotope evolution of the continental crust relative to the mantle (see Figure 6.15). Within the continental crust Rb and Sr are further separated by remelting, metamorphism, and sedimentation, for Sr is partitioned into and retained by plagioclase whereas Rb is preferentially partitioned into the melt or fluid phase. Recognizing isotopic reservoirs Taylor et a]. (1984) recognize three isotopic reservoirs in the continental crust which they characterize with respect to Nd, Sr and Pb isotopes. Zindler and Hart (1986) have delineated five end—member compositions in the mantle which by a variety of mixing processes can explain all the observations on mid—ocean ridge and ocean-island basalts. The composition of each of these sources is summarized in Table 6.5 and plotted on a series of generalized isotope correlation diagrams (Figures 6.9 to 6.12). In addition Table 6.6 summarizes the present—day compositional ranges of Sr, Nd and Pb isotopes in oceanic and crustal rock types. In the following section each of the important mantle and crustal reservoirs is described and its particular isotopic character highlighted. ...
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