Pb204 Pb

19.5

20.5

Fig. 27.1 Plots of 208Pb/204Pb (a) and 207Pb/204Pb (b) vs. 206Pb/204Pb for MORB samples. After Hofmann (2003). The average compositions for GLOSS and MORBs are from Plank and Langmuir (1998) and Kramers and Tolstikhin (1997), respectively, © Elsevier Science 2003, reproduced by permission.

0.702 0.703 0.704

87Sr/86Sr

0.702 0.703 0.704

87Sr/86Sr

0.5132

Cook-Austral St. Helena Marquesas Samoa Societies Pitcairn-Gambier Tristan Hawaii Iceland

Cook-Austral St. Helena Marquesas Samoa Societies Pitcairn-Gambier Tristan Hawaii Iceland

EMII

0.702

0.703

0.704

0.705

87Sr/86Sr

0.706

0.707

0.708

Fig. 27.2 Neodymium-strontium isotope plot for MORBs (a) and OIBs (b) samples. (a) The Sr and Nd compositions of Pacific (the most depleted) and Indian (the least depleted) MORBs are distinct, whereas the Atlantic MORB composition overlaps both. (b) The OIBs show much more heterogeneous isotope signatures than the MORBs. Model-derived BSE compositions are shown (grey lines) for comparison. The positions of the labels HIMU, EMI and EMII show distinct mantle domains that are sources of mantle plumes (Zindler and Hart, 1986). After Hofmann (2003), © Elsevier Science 2003, reproduced by permission.

for the DMM. This is known as the mantle Th/U paradox and is also proof of the open-system behaviour of the DMM (Galer and O'Nions, 1985; O'Nions and McKenzie, 1993). Both problems may be solved via the (reasonable) assumption that U became soluble owing to weathering after ~ 2.2 Gyr, when the atmosphere had become oxidizing (Section 27.9). Uranium from sea water is incorporated in altered oceanic basalt and is largely subducted, while Pb is mainly released from the slab by fluids (Kelley et al., 2005). The ^ value of the mantle is thereby raised and its k value lowered. Meanwhile, Pb isotope ratios in the mantle retain a memory of pre-oxygen times, when U was not preferentially recycled (Kramers and Tolstikhin, 1997; Elliott et al., 1999).

Without doubt the relatively large total melt fraction and polybaric melt region involved in MORB generation has helped to iron out small-scale heterogeneities in the source rocks. Also, the similarity of the isotope ratios over thousands of kilometres of ridge length characterizes the depleted MORB-source mantle (DMM) as a relatively homogeneous reservoir. However, a closer look reveals some differences between the major ridge systems, Pacific, Atlantic and Indian, and slight heterogeneities within them, for all three isotope systems. While MORBs from the East Pacific Rise are especially homogeneous (e.g. Meibom and Andersen, 2003), the Indian MORBs plot away from these in Fig. 27.2(a) and the Atlantic MORBs overlap both. Within the Atlantic oceanic basin, the 87Sr/86Sr and 143Nd/144Nd ratios vary coherently between 0.7022 and 0.7035 and from 12 to 5 s143 units, respectively (e.g. Hofmann, 2003 and references therein), and the spread in the Pb isotopes is also greater than for the two other ridge systems. In the 208Pb/204Pb versus 206Pb/204Pb plot (Figure 27.1(a)), the Indian MORB field does not overlap that of the Pacific MORBs, and the Indian 206Pb/204Pb ratios are clearly lower than those in both the Atlantic and Pacific. The source region of the Indian MORBs thus contains an ancient component with slightly higher Rb/Sr and lower Sm/Nd ratios than the other basins, and relatively high k and low ^ values.

Isotopic signatures and sources of OIB magmatism: the importance ofsubduction

The mantle provinces that act as sources for OIB magmas show far greater isotopic variations than those of MORBs (Figs. 27.2(b), 27.3 and 27.4), which must reflect long-lived mantle heterogeneities, as first pointed out by Gast et al. (1964). In the Nd versus Sr as well as in the Pb-isotope plots, the points appear to radiate in different directions from a single compositional field, which is best represented by the large Hawaii and Iceland plumes. This field is very similar to the MORB source in its Pb-isotope compositions and overlaps with MORBs in its Nd and Sr isotopes, although the most depleted MORB Sr and Nd compositions are not represented in it. Although this compositional field is only poorly resolvable from the DMM in terms of Sr, Nd and Pb isotopes, the two reservoirs have not generally been considered identical and a number of names have been used for this component, mainly PREMA (prevalent mantle) or FOZO (focal zone; see Hofmann, 2003, for a review).

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