This session focuses on the physio-chemical controls involved in generation of melt from the mantle, including mid-ocean ridges, intraplate and subduction settings. Questions to be discussed include: whether primary melts form in the convecting and/or lithospheric mantle; the temperature and composition of melt source regions; volatile contents; the role of upwelling mantle plumes and/or lithospheric mantle and the overlying crust as a source or control on the nature of surface magmatism. Contributions from the fields of geochemistry, petrology, volcanology, geophysics and geodynamics will be welcome.