Melting of subducted basalt at the core-mantle boundary

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Science  23 May 2014:
Vol. 344, Issue 6186, pp. 892-895
DOI: 10.1126/science.1250466

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The geological materials in Earth's lowermost mantle control the characteristics and interpretation of seismic ultra–low velocity zones at the base of the core-mantle boundary. Partial melting of the bulk lower mantle is often advocated as the cause, but this does not explain the nonubiquitous character of these regional seismic features. We explored the melting properties of mid-oceanic ridge basalt (MORB), which can reach the lowermost mantle after subduction of oceanic crust. At a pressure representative of the core-mantle boundary (135 gigapascals), the onset of melting occurs at ~3800 kelvin, which is ~350 kelvin below the mantle solidus. The SiO2-rich liquid generated either remains trapped in the MORB material or solidifies after reacting with the surrounding MgO-rich mantle, remixing subducted MORB with the lowermost mantle.

Delving deeper into the lower mantle

Earth's lower mantle is an enigmatic region, a transition zone between slowly churning solids and a liquid outer core. Large seismic structures and discontinuities in this region are probably due to sharp gradients in temperature, composition, or mineralogy. Teasing apart the precise effects of these factors requires experiments at lower mantle temperatures and pressures (see the Perspective by Williams). Zhang et al. found that the major mineral phase of the lower mantle decomposes into two minerals. Andrault et al. show how the melting of subducted basalt from the oceanic crust will form pile-like structures on top of the core/mantle boundary.

Science, this issue p. 877, p. 892; see also p. 800.

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