The nature of magma generation, crustal construction, and hydrothermalism beneath ocean ridges at extremely low spreading rates is currently a hot topic in marine Earth sciences. Partial melting (10–20%) and magma mixing experienced by most mid-ocean ridge basalts hampers our understanding of the mantle and melting processes. This is because the lowest-degree partial melts (containing essentially all the incompatible trace elements) and their still-fertile peridotite residues are never observed; their compositions can only be inferred. Thermal models predict a sharp drop in partial melting at spreading rates significantly under 16 mm/yr (full rate) [Reid and Jackson, 1981].