RSES Seminar Series 2024-07-04 Stephen Foley (Macquarie University)

Incipient melts of the mantle and the deep carbon cycle
Stephen Foley (Macquarie University)
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 volatile-rich 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 first-row transition elements (especially Ni and Cr) and may sequester these in mantle dykes to a greater extent than olivine does in peridotite.
Carbonate-rich melts from either subduction or melting of the uppermost asthenosphere trap carbon by redox freezing or as carbonate-rich dykes at the base of the lithosphere. Melting releases carbon preferentially at steps in lithosphere thickness. This results in carbonate-rich melts, explaining the spatial association of carbonate-rich 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 small-scale processes, particularly in the thermal boundary layer. An increased focus on local variations will help to refine carbon budget models.