Silicon's Travels

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Science  16 Feb 2001:
Vol. 291, Issue 5507, pp. 1159
DOI: 10.1126/science.291.5507.1159a

Early in the evolution of Earth, the iron-rich core separated from the silicate-rich mantle. Exactly how this differentiation occurred is not understood, although the segregation of metal-rich parcels from a partially molten mantle (a magma ocean) is favored. The current composition of Earth's core also is uncertain because seismic evidence indicates that some lighter elements must be present.

Gessmann et al. measured the solubility of Si in liquid Fe metal and found that it increases with increasing pressure at upper-mantle conditions, and that it then decreases with increasing pressure at lower-mantle conditions (due to the increase in Si coordination from 4 to 6 in solid silicates). Thus, an 850 kilometer-thick magma ocean (providing approximately upper-mantle pressures with higher temperatures) would allow as much as 7% by weight of Si to be added to the core. These results are consistent with models of differentiation that start from a bulk chemistry like that of primitive meteorites (CI chondrites). After this initial differentiation, the liquid outer core could have begun to expel Si back into the lower mantle because of its lower solubility. This flux would help to explain the formation of the D'' layer at the core-mantle boundary, supporting the proposal that this layer is chemically distinctive. — LR

Earth Planet. Sci. Lett.184, 367 (2001).

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