Elemental abundances mantle

The historical background is presented for the asteroid-impact theory that is based on the iridium anomaly found in rocks frm the Cretaceous-Tertiary boundary. Recent measurements of Ir, Pt, and Au abundances from such rocks in Denmark have shown that the element abundance ratios are different from mantle-derived sources and agree with values for chondritic meteorites within one standard deviation of the measurement errors (7-10%). Rare-earth patterns for these rocks are... 

Frey F. A. (1982). Rare earth element abundances in upper mantle rocks. In Rare Earth Element Geochemistry, R Henderson, ed. Elesevier, Amsterdam. 

Mars bulk silicate (mantle + crust) composition, estimated from Martian meteorites by Wanke and Dreibus (1988). This composition differs from the bulk silicate of Earth, because of differences in volatile element abundances and core differentiation. 

Mafic Plio-Quatemary rocks in Italy show very variable trace element and isotopic compositions. Incompatible2 trace element abundances and ratios are best illustrated by mantle-normalised diagrams (spiderdiagrams), where concentrations of single elements in the rocks are divided by the abundances of the same elements in the mantle (Wood 1979). 

REE patterns are fractionated silicic rocks contain negative Eu anomalies (Fig. 7.14a), which are much smaller than observed for the Lipari and Vulcano rhyolites. Mantle normalised incompatible element patterns of mafic rocks show high LILE/HFSE ratios and a positive anomaly of Pb a small positive Sr spike is observed in the calc-alkaline basaltic andesites (Fig. 7.14b). HKCA and shoshonitic rocks have higher incompatible element abundances than the associated CA products. 

Trace element abundances of rocks dredged from the Sicily Channel seamounts are scarce (Beccaluva et al. 1981 Calanchi et al. 1989). They show variable concentrations, with incompatible element abundances increasing from tholeiitic to alkaline basalts and basanites (Fig. 8.17). Mantle normalised incompatible elements define bell-shaped patterns (not shown), which resemble those for the exposed rocks in the Sicily Channel. 

Figure 6.4 Elemental abundance patterns for noble gases in mantle-derived samples (cf. Figure 6.3 and Table 6.1). The ordinate is (G/36Ar)s/(G/36Ar)a, where subscripts s and a designate sample and atmosphere, respectively, and G represents any noble gas isotope.

Rehkamper, M., Halliday, A. N., Barfod, D., Fitton, J.G., and Dawson, J.B. (1997a) Platinum group element abundance patterns in different mantle environments. Science 278, 1595-1598. 

The major problem presented by the Earth s chemical composition and core formation models is providing mechanisms that predict correctly the siderophile element abundances in the Earth s upper mantle. It long has been recognized that siderophile elements are more abundant in the mantle than expected if the sihcate Earth and the core were segregated under low-pressure and moderate-temperature equilibrium conditions (Chou, 1978 Jagoutz et al, 1979). Several explanations for this siderophile excess have been proposed, including ... 

Arculus R. J. and Delano J. W. (1981) Siderophile element abundances in the upper mantle evidence for a sulfide signature and equilibrium with the core. Geochim. Cosmochim. Acta 45, 1331-1344. 

Figure 8 Comparison of elemental abundances in the mantles of Mars and Earth, based on the Mars geochemical model of Wanke and Dreibus (1988) (after Halliday et al., 2001).

The six most abundant, nonvolatile rock-forming elements in the Sun are Si (100), Mg (104), Fe (86), S (43), Al (8.4), and Ca (6.2). The numbers in parentheses are atoms relative to 100 Si atoms. They are derived from element abundances in Cl-meteorites which are identical to those in the Sun except that Cl-abundances are better known (see Chapter 1.03). From geophysical measurements it is known that the Earth s core accounts for 32.5% of the mass of the Earth. Assuming that the core contains only iron, nickel, and sulfur allows us to calculate the composition of the silicate fraction of the Earth by mass balance. This is the composition of the bulk silicate earth (BSE) or the primitive earth mantle (PM). The term primitive implies the composition of the Earth s mantle before crust and after core formation. 

Figure 7 shows the abundances of the four refractory lithophile elements—aluminum, calcium, scandium, and vanadium—in several groups of undilferentiated meteorites, the Earth s upper mantle and the Sun. The RLE abundances are divided by magnesium and this ratio is then normalized to the same ratio in Cl-chondrites. These (RLE/Mg)N ratios are plotted in Figure 7 (see also Figure 1). The level of refractory element abundances in bulk chondritic meteorites varies by less than a factor of 2. Carbonaceous chondrites have either Cl-chondritic or higher Al/Mg ratios (and other RLE/Mg ratios), while rumurutiites (highly oxidized chondritic meteorites), ordinary chondrites, acapulcoites, and enstatite chondrites are depleted in refractory elements. The (RLE/Mg)N ratio in the mantle of the Earth is within the range of carbonaceous chondrites.

Garuti G., Gorgoni C., and Sighinolli G. P. (1984) Sulfide mineralogy and chalcophile and siderophile element abundances in the Ivrea-Verbano zone mantle peridotites (Western Italian Alps). Earth Planet. Sci. Lett. 70, 69-87. 

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