Chondrites magnesium/silicon ratio

The only in situ chemical data for asteroids are from the NEAR Shoemaker spacecraft, which orbited 433 Eros in 2000-1, and from the Japanese Hayabusa spacecraft, which visited 25143 Itokawa in 2003. NEAR obtained numerous measurements of the surface composition using X-ray fluorescence and gamma-ray spectrometers, and Hayabusa carried an XRF. The magnesium/silicon and aluminum/siUcon ratios for both asteroids are consistent with the compositions of chondrites. However, sulfur is depleted in Eros relative to chondritic compositions, possibly due to devolatilization by impacts or small degrees of melting. 

Figure 12 Cl chondrite-normalized element to silicon ratios for CS and CP IDPs. The solid line represents frequency of CS IDPs and the dotted line frequency of CP IDPs. Numbers in upper right of each histogram are the number of CS and CP IDPs, respectively, with element to silicon ratios >3 CL CS IDPs are systematically depleted in calcium and magnesium while CP IDPs are only slightly depleted in calcium, aluminum, sulfur, and iron relative to Cl (vertical dotted line) (source Schramm et al., 1989).

The four elements—magnesium, silicon, iron, and oxygen—contribute more than 90% by mass to the bulk Earth. As stated above, magnesium, silicon, and iron have approximately similar relative abundances (in atoms) in the Sun, in chondritic meteorites, and probably also in the whole Earth. On a finer scale there are, however, small but distinct differences in the relative abundances of these elements in chondritic meteorites. Figure 10 shows the various groups of chondritic meteorites in an Mg/Si versus Al/Si plot. As discussed before, the Earth s mantle has Al/Si and Al/Mg ratios within the range of... 

In Figure 13 the Earth s mantle seems to extend the trend of the moderately volatile elements to lower abundances, at least for sodium, manganese, and zinc (zinc behaves as a lithophile element in the Earth s mantle (see Dreibus and Palme, 1996)). The elements lithium, potassium, and rubidium which are not plotted here, show similar trends. The carbonaceous chondrite trend of iron is not extended to the Earth, as most of the iron of the Earth is in the core. The magnesium abundance of the Earth shows a slightly different trend. If the core had 5% silicon (previous section) and if that would be added to the bulk Earth silicon, then the bulk Mg/Si ratio of the Earth would be the same as that of carbonaceous chondrites (Eigure 10) and the silicon abundance of the Earth s mantle in Figure 13 would coincide with the magnesium abundance. 

For this review the Earth s composition will be considered to be more similar to carbonaceous chondrites and somewhat less like the high-iron end-members of the ordinary or enstatite chondrites, especially with regard to the most abundant elements (iron, oxygen, silicon, and magnesium) and their ratios. However, before reaching any firm conclusions about this assumption, we need to develop a compositional model for the Earth that can be compared with different chondritic compositions. To do this we need to (i) classify the elements in terms of their properties in the nebula and the Earth and (2) establish the absolute abundances of the refractory and volatile elements in the mantle and bulk Earth. 

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