Bulk Silicate Earth Element Composition and Depletion
Channel: Dominik Hezel
Date: 2023-04-12

Bulk silicate Earth (BSE) is the composition of the Earth minus its metal core. Lithophile refractory and main elements up to about Mg have CI chondritic normalised concentrations. Elements with lower T50% condensation temperatures than Mg are increasingly depleted in the BSE. Siderophile refractory and main elements abundances are depleted relative to CI (and Mg). The degree of depletion depends on the partition of the according elements between core and mantle, i.e., between metal and silicate. This partition depends on the pressure, O-fugacity and changing valence state of some of these elements, which, of course, also depend of the ambient conditions such as pressure or O-fugacity. All highly siderophile elements (HSE) are solely expected in the metal core, i.e., their normalised abundances should essentially be 0. This is not the case, and although the normalised HSE abundances are still very low in BSE, this cannot be the remnant from Earth’s differentiation into core and mantle. It is therefore commonly suggested that a late and large meteorite impact (called ›late veneer‹) with CI chondritic composition dissolved in the Earth’s mantle, thereby adding and enriching it in HSE. The volatile depletion trend is generally independent of the litho-/sidero-/chalco-/atmophile character of an element. This could be explained by a general volatile depletion of the reservoir from which Earth formed. However, individual elements (e.g., In, Pb) or groups of elements (e.g., S/Se/Te) deviate from the volatile depletion. These deviations are not yet fully understood, but likely as well related to ambient and/or varying element states such as pressure, O-fugacity or valence.