Volatile elements mantle

Trace element measurements in lunar basalts also indicate that the Moon is depleted in highly volatile elements (Taylor et al., 2006a). Estimates of some of the Moon s volatile element concentrations are compared with the Earth in Figure 13.11 a. The absence of water in lunar basalts suggests that the mantle is dry. The Moon may also be enriched in refractory elements (Fig. 13.11b). Volatile element depletion and refractory element enrichment are expected consequences of the giant impact origin and subsequent high-temperature accretion of the Moon. 

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. 

Figure 13 Major and moderately volatile elements in carbonaceous chondrites and in the Earth s mantle. All data are normalized to the RLE Ti. There is a single trend for RLE, Mg-Si, and moderately volatile elements. The Earth may be viewed as an extension of the carbonaceous chondrite trend. The low Cr content in the present mantle (full symbol) is the result of Cr partitioning into the core. The open symbol is plotted at the extension of the carbonaceous chondrite trend. Data for ordinary chondrites are plotted for comparison. Similar chemical trends in carbonaceous chondrites and the Earth are evident. H-chondrites are very different (sources Wolf and Palme, 2001 Wasson and...

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. 

Figure 15(a) plots Cl and magnesium-normalized abundances of lithophile moderately volatile elements against their condensation temperatures. The trend of decreasing abundance with increasing volatility is clearly visible. As mentioned above only few elements can be used to define this trend most of the moderately volatile elements are siderophile or chalcophile and their abundance in the Earth s mantle is affected by core formation. 

Figure 15 Abundances of moderately volatile elements in the Earth s mantle versus condensation temperatures (a) lithophile elements define the volatility trend (b) siderophile elements have variable depletions reflecting the process of core formation and (c) chalcophile elements. The difference between siderophile and chalcophile elements is not well defined, except for S and Se. The large depletions of S, Se, and Te are noteworthy (see text) (after...

Figure 16 Mn/Na versus Mn/Al in chondritic meteorites. The two moderately volatile elements Na and Mn have the same ratio in all chondritic meteorites and in the primitive Earth s mantle, here designated as BSE. The low Mn/Al content of the Earth s mantle reflects enrichment of A1 and depletion of Mn. Because of the chondritic Mn/Na ratio of the Earth s mantle, it is unlikely that a significant fraction of the Earth s inventory of Mn is in the core (source...

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. 

Figure 4 The relative abundances of the lithophile elements in the primitive mantle (or silicate Earth) plotted versus the log of the 50% condensation temperature (K) at 10 atm pressure. The relative abundances of the lithophile elements are reported as normalized to Cl carbonaceous chondrite on an equal basis of Mg content. The planetary volatility trend (negative sloping shaded region enclosing the lower temperature elements) establishes integrated flux of volatile elements at 1 AU. Data for condensation temperatures are from Wasson (1985) chemical data for the chondrites are from Wasson and Kelleme3m (1988) and for the Earth are from Table 2.

Potassium is a moderately volatile element and is depleted by a factor of —8 in the bulk silicate Earth compared to Cl chondrites, but a precise and unambiguous concentration is difficult to obtain. Estimates have been made by comparison with uranium, which like potassium is highly incompatible during melting and so is not readily fractionated between MORE and the upper mantle. There is little debate regarding the concentration of uranium, which is obtained from concentration in carbonaceous chondrites and, by assuming that refractory elements (e.g., calcium, uranium, thorium) are unfractionated from solar values... 

The aim of this study is to identify the volatile (H, C, N, rare gases) components present at the Earth s surface and in its interior. We then use such compositions to infer the potential contributing sources and processes that resulted in the surface and the mantle inventory of volatile elements. 

As a result of plate tectonics, plume activity and volcanism, volatile elements are continuously exchanged between the surface and the mantle. 

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