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Melting in the deep mantle

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MIMW03 - From the continuum to the tectonic: the magma/mantle dynamics of planet earth

Melting in the shallow mantle is well documented. It is caused by the adiabatic ascent of a material or by the addition of “flux” such as water and/or carbon dioxide that reduces the solidus. Melt density and melt morphology (i.e., the dihedral angle) are well known. Consequently, it is possible to interpret some geophysical observations in terms of the presence of melt: in most cases, geophysical anomalies are difficult to attribute to the presence of melt unless the melt geometry is unusual (e.g., zero dihedral angle).            
Melting can also occur in the deep mantle particularly across the mantle transition zone and in the D” layer. Recent experimental studies show that melting is ubiquitous in the deep mantle (deep upper mantle and the lower mantle), but the geochemical and geophysical consequence of melting in the deep mantle is largely unknown. In most cases, melting in the deep mantle is “flux melting” assisted by the volatiles. I will summarize the current status of studies on melting in the deep mantle with the focus on the conditions for melting, chemical composition of the melt and the melt density with the focus on the role of water. Water-induced melting in the lower mantle is extensive and in almost all areas in the lower mantle melting is difficult to avoid unless other materials that dissolve volatiles exists (e.g., metallic Fe). The composition of the melt produced in the lower mantle is (Mg,Fe)O-rich as opposed to the melt produced in the shallow mantle (SiO2-rich). Consequently, the deep mantle melting will affect the chemical evolution of Earth quite differently than the shallow mantle melting. However, two key parameters, namely the density and the dihedral angle, are poorly constrained. A review of current status and a discussion on the future directions will be provided.

This talk is part of the Isaac Newton Institute Seminar Series series.

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