Chemical stratification in the silicate Earth

Abstract

The largest density contrast between magmas and common ferromagnesian minerals in the mantle occurs at two locations; these are nearest the Earth's surface and at about 650–700 km depth where the lower mantle mineral assemblage becomes stable at solidus temperatures. The large density contrast between magmas and olivine nearest the surface diminishes to zero somewhere between the middle and lower stratigraphic sections of the upper mantle. It is also here where the composition of all anhydrous magmas is most similar to that of an undepleted garnet-iherzolite composition. For an Earth that has differentiated by fractional crystallization and melting processes which depend on the separation of crystalline phases from silicate liquids having different densities, the 650-km discontinuity could represent a chemical boundary layer; the chemical change in the early Archean could have been from primordial upper mantle compositions above the boundary layer to differentiated lower mantle compositions below. A primary stratigraphic model is offered for the distribution of depleted, enriched, and primordial isotopic mantle components formed by the transport fate of magmas in the beginning of Earth history. Depleted MORB source regions were established in the uppermost part of the upper mantle. Primordial mantle was located at the middle to the deepest regions of an upper mantle which was processed by anhydrous melting and crystallization. Enriched and depleted components could have been formed at these levels also by igneous processing in the presence of H 2O or CO 2 and involving highly compressible magmas. The lower mantle became heterogeneous, with enrichments at the top and depletions elsewhere. It is suggested that the present-day isotopic structure of the mantle has inherited elements of this primary stratification. Primordial 143Nd/ 144Nd and 87Sr/ 86Sr observed in oceanic islands basalts are located in the middle of upper mantle convection cells. Enriched ratios found in both oceanic islands basalt and kimberlite could have originated near and on either side of the 650-km chemical boundary layer.