Abstract: Carbon, water and other volatile components are transported in the mantle principally as partial melts. The first melts that form and move around in the mantle are volatilerich and most of them solidify before they can leave the mantle, forming dykes rich in amphiboles, mica and clinopyroxene known as the hydrous pyroxenite suite. Our experiments show that the melting points of all hydrous pyroxenites are below that of peridotite and high degrees of melting occur quickly so that melts are mobile at temperatures below or close to the melting point of peridotite. Hydrous minerals exert strong control on many firstrow transition elements (especially Ni and Cr) and may sequester these in mantle dykes to a greater extent than olivine does in peridotite.

Carbonaterich melts from either subduction or melting of the uppermost asthenosphere trap carbon by redox freezing or as carbonaterich dykes at the base of the lithosphere. Melting releases carbon preferentially at steps in lithosphere thickness. This results in carbonaterich melts, explaining the spatial association of carbonaterich magmatism and metal deposits with craton edges. Variations in rock types and oxidation state may be very local and exert strong influences on carbon storage and release mechanisms. Global concepts such as average sediment compositions and a uniform mantle oxidation state are not appropriate for smallscale processes, particularly in the thermal boundary layer. An increased focus on local variations will help to refine carbon budget models.