Oceanic island basalts composition

Figure 8. ° Pb7 Pb vs. Th/Us (derived using Eqn. 5 in the text) diagram for mid-ocean ridge and ocean island basalt based on a recent data set with mostly mass spectrometry measurements (Turner et al. 1997 Bourdon et al. 1996 Dosso et al. 1999 Claude-lvanaj et al. 1998, 2001 Sims et al. 2002). The data show a relatively well defined array that intersect a closed-system hne for the bulk Earth starting with an initial lead isotope composition equal to Canyon Diablo (T = 4.55 Ga). This intersect was used by Allegre et al. (1986) to define the Th/U ratio of the Earth.

Mantle reservoirs. The only quasi-systematic studies of igneous materials have centered on the mantle in particular mid-ocean ridge basalts (MORE), ocean island basalts, and mantle peridotites. After reporting one MORE analysis in Chan and Edmond (1988), the first full study of MORE (Chan et al. 1992) reported three apparently imaltered Atlantic basalts and one from the East Pacific Rise, with a range in 8 Li of +3.4 to +4.7 (Fig. 5). Subsequent studies have increased the global range of samples, the diversity of bulk compositions analyzed. 

Weaver BL (1991) The origin of ocean island basalt end-member compositions trace element and isotopic constraints. Earth Planet Sci Lett 104 381-397... 

Figure 37 Frequency distribution plots for Os, Nd, and Sr isotope compositions of cratonic and noncratonic peridotite xenoliths. Upper right plots give the range for ocean island basalts (OIB) and arrows show the direction of isotopic evolution for melt depletion and enrichment events. Data compiled from sources cited in Menzies (1990b), Pearson (1999a,b), and those given in Figure 21 (after Pearson and Nowell, 2002).

The mantle is the Earth s largest chemical reservoir comprising 82% of its total volume and 65% of its mass. The mantle constitutes almost all of the silicate Earth, extending from the base of the crust (which comprises only 0.6% of the silicate mass) to the top of the metallic core at 2,900 km depth. The chemical compositions of direct mantle samples such as abyssal perido-tites (Chapter 2.04) and peridotite xenoliths (Chapter 2.05), and of indirect probes of the mantle such as basalts from mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) (Chapter 2.03), and some types of primitive... 

TABLE II Estimates of the Average Chemical Composition of the Mid-Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB) Reservoirs... 

FIGURE 8 Plot of K20 versus Si02 for various models of bulk continental crustal composition, superimposed on the classification of calc-alkaline volcanic rocks. The model favored here is shown in the heavy black circle. A variety of other compositions have been proposed on the basis of plate tectonic models (e.g., andesite, ocean island basalt open crosses) and various seismic and geological models (open squares). These various models predict a wide range of heat-producing element abundances (K, Th, U) and thus can be tested from heat-flow data. 

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