Silicon bulk Earth composition

As regards the rock-forming elements, the bulk composition of the Earth is basically chondritic (i.e., solar) with approximately equal abundances of magnesium, silicon, and iron atoms. In detail, however, there are some variations in chemistry among chondritic meteorites, and from a detailed comparison with meteorites it is concluded that the bulk Earth composition has similarities with the chemical composition group of carbonaceous chondrites. 

The effect of silicon partitioning into the core is shown in Figure 10. If the core has 5% silicon, the bulk Earth Mg/Si and Al/Si ratios would match with CV-chondrites which define a fairly uniform Mg/Si ratio of 0.90 0.03, by weight (Wolf and Palme, 2001). Such a composition is not unreasonable in view of the similarity in moderately volatile element trends of carbonaceous chondrites and the Earth, as shown below. 

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. 

In contrast to their rather low dissolved concentrations in seawater, some of the trace metals, e g., iron and aluminum, along with oxygen and silicon, comprise the bulk of Earth s crust. Some trace elements are micronutrients and, hence, have the potential to control plankton species composition and productivity. This provides a connection in the crustal-ocean-atmosphere fectory to the carbon cycle and global climate. 

In addition to making comparisons with chondrites, the bulk composition of the Earth also has been defined in terms of a model mixture of highly reduced, refractory material combined with a much smaller proportion of a more oxidized volatile-rich component (Wanke, 1981). These models follow on from the ideas behind earlier heterogeneous accretion models. According to these models, the Earth was formed from two components. Component A was highly reduced and free of all elements with equal or higher volatility than sodium. All other elements were in Cl relative abundance. The iron and siderophile elements were in metallic form, as was part of the silicon. Component B was oxidized and contained all elements, including those more volatile than sodium in Cl relative abundance. Iron and all siderophile and lithophile elements were mainly in the form of oxides. 

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