The petrological and geochemical interrogation of mantle diamonds and their inclusions provide the best empirical method to study the deep carbon cycle over geological time. But what do diamonds teach us about the history of the mantle? About the crust to the deep mantle interaction over time? However, it is not known if the present models of formation are all correct, if some are mutually-exclusive, or if some are simply wrong. For example, it is agreed that diamonds are metasomatic, but the specific nature of these fluids are still debatable. In this session we propose to address what we know, and what we need to know about diamond-formation in the mantle and the cycling of volatiles throughout the Earth. We welcome all petrological, geochemical, experimental, and theoretical contributions.