Basalts radiogenic isotopes

White, W. M. (1985). Sources of oceanic basalts radiogenic isotopic evidence. Geology,... 

Figure 9. Plots of Li and radiogenic isotopes for mantle rocks, (a) 5 Li vs. Sr/ Sr (b) 5 Li vs. Nd/ Nd (c) "Sr/ Sr vs. Pb/ Pb (d) 5"Li vs. Pb/ Pb (Nishio et al. 2003, 2004). Symbols + = south Pacific island basalts (six islands) O = Iherzolite xenolith, Bullenmerri, Australia = Iherzolite xenolith, Sikhote-Alin, Russia (three localities) A = dunite-peridotite-pyroxenite xenolith, Kyushu, Japan (two localities) V = Iherzolite xenolith, Ichinomegata, Japan. The ocean island data are from bulk rocks, the xenolith data are clinopyroxene separates. For explanations of the derivation of radiogenic isotope fields (DM, EMI, EM2, HIMU), see Zindler and Hart (1986). The estimate for Li isotopes in DM is based on MORE. The Li isotopic ranges for the other mantle reservoirs are based on Nishio et al. (2004) and Nishio et al. (2003), but these will require further examination (hence the use of question marks).

The formation of basalts by partial melting of the upper mantle at mid-oceanic ridges and hot spots provides the opportunity to determine mantle composition. Early studies of radiogenic isotopes in oceanic basalts (e.g., Eaure and Hurley, 1963 Hart et al, 1973 Schilling, 1973) showed fundamental chemical differences between OIBs and MORBs (see Chapter 2.03). This led to the development of the layered mantle model, which consists essentially of three different reservoirs the lower mantle, upper mantle, and continental cmst. The lower mantle is assumed primitive and identical to the bulk silicate earth (BSE), which is the bulk earth composition minus the core (see also Chapters 2.01 and 2.03). The continental cmst is formed by extraction of melt from the primitive upper mantle, which leaves the depleted upper mantle as third reservoir. In this model, MORB is derived from the depleted upper mantle, whereas OIB is formed from reservoirs derived by mixing of the MORB source with primitive mantle (e.g., DePaolo and Wasserburg, 1976 O Nions et al., 1979 Allegre et al., 1979). 

Geochemical studies of aUcah basalts have been aided over the past 10-15 yr by an explosion in the hterature of ICP-MS-based trace-element and radiogenic-isotope data for continental alkali basalts. Representative chemical analyses of the two main aUcah basalt groups, sodic and potassic (KjO/NajO <1 and 1, respectively), are listed in Tables 2(a) and (b), respectively, derived from the average composition of lavas at each locality having <51 wt.% SiOa and >5 wt.% MgO. No attempt was made to account for crystal... 

A study of the radiogenic isotope memory of the Earth s mantle clearly shows that the mantle is not an independent part of the Earth system, nor has it been for a long time. But rather, it records a history of the extraction and recycling of both basaltic and continental crust. Because of the relatively slow mixing rates and rates of diffusion, compositional heterogeneities within the mantle produced by these processes may be preserved for >1.0 Ga so that significant parts of the Earth s prehistory can be seen in recent mantle melts. Mantle melts from early Earth history (basalts and komatiites), therefore, have the potential to record very early mantle heterogeneities. 

Figure 12. Comparison of 5 O values with Srr Sr ratios in representative ocean-island basalts, organized by sub-type as defined by radiogenic isotope systematics (see Fig. 11 data from Filer et al. 1996a). Low- HeAle lavas are a special case discnssed in Filer et al. (1996a) and in the text of this chapter and are exclnded here. The heavy dashed craves mark the range of mixing hyperbolae between depleted mantle (based on data for NMORBs Figs. 9 and 10) and high-o 0 siliciclastic sediments.

Oxygen isotope variations are closely related to variations in radiogenic-isotope ratios and abundances of trace and major elements in many basaltic systems. Interpretations of these relationships vary from place to place and from time to time as debates evolve however, the observations themselves are better than would have been imagined five or more years ago. 

Kurz MD (1991) Noble gas isotopes in oceanic basalts controversial constraints on mantle models. In Short Course Handbook on Applications of Radiogenic Isotope Systems to Problems in Geology. L Heamar, JN Ludden (eds) Mineral Assoc Canada, p 259-286 KyserTK (1986) Stable isotope variations in the mantle. Rev Mineral 16 141-164 Kyser TK, O Neil JR, Carmichael ISE (1981) Oxygen isotope thermometry of basic lavas and mantle nodules. Contrib Mineral Petrol 77 11-23... 

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