Isotopic signatures of the depleted MORBsource mantle

A comparison of the concentrations of the parent and daughter elements in the depleted MORB-source mantle and in the continental crust (Table 17.1) gives the enrichment factors reproduced in Table 27.1. These enrichment factors show that the continental crust, with a mass about 0.6% of the total mass of the silicate Earth, is the most important reservoir of Rb, Pb, U and Th, and the depletion of these elements in the MORB-source mantle is extreme (Fig. 24.2).

The Pb-, Nd- and Sr-isotope data of the depleted MORB-source mantle (DMM) are summarized in Figs. 27.1(a), (b) and 27.2(a). The Nd-isotope compositions are well above the BSE value and the Sr-isotope ratios are low, as expected in a long-lived depleted reservoir. This, and the inverse correlation between the 143Nd/144Nd and 87Sr/86Sr ratios seen in Fig. 27.2, confirm the predictions, from the relative incompatibilities of parent-daughter pairs, of how these ratios should develop with time in depleted reservoirs.

The MORB Pb isotope compositions further allow two important observations. First, in the 207Pb/204Pb versus 206Pb/204Pb plot in Fig. 27.1(b) the relatively compact data field is clearly well to the right of the meteorite isochron (grey line), showing that the DMM has not evolved as a closed system since the origin of the Earth. More precisely, its ^ value has increased in the course of geological time. Second, in the 208Pb/204Pb versus 206Pb/204Pb plot in Fig. 27.1(a), the data indicate development from primordial Pb with a k value of 3.8 (the assumed BSE value) in an apparently closed system, in contrast with the actual value, k — 2.6 to 2.8,

16.5 41

40 39 38 37

15.3 16


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