Continental crust basaltic

Richter FM, Davis AM, Ehel DS, Hashimoto A (2002) Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions Experiments, theoretical considerations, and constraints on their thermal evolution. Geochim Cosmochim Acta 66 521-540 Richter FM, Davis AM, DePaolo DJ, Watson EB (2003) Isotope fractionation by chemical diffusion between molten basalt and rhyolite. Geochim Cosmochim Acta 67 3905-3923 Rudnick RL, Fountain DM (1995) Nature and composition of the continental crust—a lower crustal perspective. Rev Geophys 33 267-309... 

Mantle-derived basalts, on the other hand, have a relatively uniform composition with 8 Li values of 4 2%o (Tomaszak 2004 Elliott et al. 2004). The continental crust generally has a lighter Ei isotope composition than the upper mantle from which it was derived (Teng et al. 2004). Considering the small Li isotope fractionation at high temperature igneous differentiation processes (Tomaszak 2004), pristine... 

Continental basalts tend to be enriched in relative to oceanic basalts and exhibit considerably more variability in O-isotope composition, a feature attributed to interaction with 0-enriched continental crust during magma ascent (Harmon and Hoefs 1995 Baker et al. 2000). 

In order to trace the migration of basalt-derived REE in the salt, REE distribution patterns (Fig. 7) and Nd isotopic compositions (Fig. 8) have been determined in a salt horizon adjacent to a basalt dyke (Fig. 2). The flat REE distribution patterns and the almost basaltic Nd isotopic composition of the salt samples collected at the basalt-salt contact point to a basaltic origin of the REE for this sample. With increasing distance from the contact, the patterns are more and more depleted in Ce, Pr, Nd, Sm, and Eu and the Nd isotopic compositions are slightly shifted towards lower eNd values, which, however, still remain above values typical for continental crust or Permian seawater (Stille et al. 1996, and citations therein). This evolution of the REE distribution patterns and the Nd isotopic compositions could basically be due to mixing between a basalt and a salt end member or, alternatively, it could have been fractionation of the REE during migration in the salt that modified the REE patterns. 

Fig. 8. The evolution of the Nd isotopic compositions with distance in the salt profile together with the compositions of average basalt and average continental crust. The error bars represent 2 sigma mean values of the individual measurements.

The Earth s crust and, indeed, the crusts of all differentiated bodies, are enriched in incompatible elements relative to their mantles. This reflects the partial melting of mantle material and extraction and transport of the basaltic melt to the surface. On Earth, further partial melting of the basaltic crust in the presence of water produces magma compositions even richer in silica (andesite and granite), which form the bulk of the continental crust. Because other differentiated bodies are effectively dry, this second level of differentiation did not occur. 

Differentiation of other terrestrial planets must have varied in important ways from that of the Earth, because of differences in chemistry and conditions. For example, in Chapter 13, we learned that the crusts of the Moon and Mars are anorthosite and basalt, respectively - both very different from the crust of the Earth. N either has experienced recycling of crust back into the mantle, because of the absence of plate tectonics, and neither has sufficient water to help drive repeated melting events that produced the incompatible-element-rich continental crust (Taylor and McLennan, 1995). The mantles of the Moon and Mars are compositionally different from that of the Earth, although all are ultramafic. Except for these bodies, our understanding of planetary differentiation is rather unconstrained and details are speculative. 

Halides (fluorine, chlorine, bromine, and iodine). Fluorine as F substitutes readily for OH in hydroxy minerals, implying that it probably occurs in all nominally anhydrous minerals in the same way as OH. Fluorine is not significantly soluble in seawater. Both these properties make its geochemical behavior quite different from the other halides. F/K and F/P ratios in basalts are reasonably constant (Smith et al, 1981 Sigvaldason and Oskarsson, 1986) with ratios of 0.09 0.04 and 0.29 0.1, respectively. Both ratios yield the same value of 25 ppm for the PM abundance which is listed in Table 4. However, the F/K ratio in the continental crust appears distinctly higher and the F/P ratio lower (Gao et al, 1998), indicating that the incompatibility of these elements increases in the order P < F < K. 

Silver. The common oxidation state is Ag, which has an ionic radius between Na and and nearer to the former. Rather gratifyingly, the ratio Ag/Na is quite constant between peridotites (BVSP), MORE and other basalts (Laul et al, 1972 Hertogen et al., 1980), and continental crust (Gao et al., 1998) at (1.6 1.0) X 10 , giving 4 ppb in the PM. Silver reported by Garuti et al. (1984) in massive peridotites of the Ivrea zone is much higher and does not appear realistic. 

These basalts represent the oceanic subclass of so-called intraplate basalts, which also include continental varieties of flood and rift basalts. They will be collectively referred to as OIE, even though many of them are not found on actual oceanic islands either because they never rose above sea level or because they were formed on islands that have sunk below sea level. Continental and island arc basalts will not be discussed here, because at least some of them have clearly been contaminated by continental crust. Others may or may not originate in, or have been contaminated by, the subcontinental lithosphere. For this reason, they are not considered in the present chapter, which is concerned primarily with the chemistry of the sublitho-spheric mantle. 

The above explanation does not account for the elevated ° Pb/ °" Pb ratios of continental rocks and their sedimentary derivatives relative to mantle-derived basalts (Figures 5(a) and 21). This special feature can be explained by a more complex evolution of continents subsequent to their formation. New continental crust formed during Archean time by subduction and accretion processes must have initially possessed a U/Pb ratio slightly higher than that of the mantle. At that time, the terrestrial ratio was signifi-... 

Figure 54 Comparison of present-day osmium isotopic compositions of eclogite xenoliths from Udachnaya, Yakutia (Pearson et ah, 1995c) and S. Africa (Pearson et al, 1992 Menzies et al, 1999 Shirey et ah, 2001) with continental crust, oceanic basalts (Shirey and Walker, 1998), and Archean komatiites and basalts (Walker et al, 1989b). Udachnaya peridotite data from Pearson et al (1995a).

Circum-cratonic mantle beneath continental crust is sampled as xenoliths in alkalic basalt... 

Figure 5 Average Ce/Pb and Nb/U from selected kimberlites, continental alkali basalts, and continental flood basalts. Present-day continental crust values from Condie (1993), and MORE and OIB values from Hofmann (1997)...

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