DIAMOND — PREMIER MINERAL FOR UNDERSTANDING THE DEEP GEOLOGY OF EARTH’S MANTLE

Diamond is the only mineral that samples the mantle at up to 800 km depths and preserves included minerals. Diamond formation and exhumation is a product of plate tectonics and recycling. Therefore, extraordinary information about the deepest extent of the plate tectonic cycle and its geochemical effect on the mantle comes from the study of superdeep (sublithospheric) diamonds and their inclusions. Diamond crystallizes from mobile C-O-H-bearing fluids or melts and can display remarkable zonation -- giving it the ability to record carbon-bearing fluids they move through the deep mantle. Through fortuitous co-crystallization or entrapment of silicate, sulfide, metal, and carbide inclusions, information about fluid sources, host lithologies, diamond ages, mantle mineralogy, and mantle redox state can be obtained. Information on diamond age, C composition, and inclusion mineralogy gleaned from many single diamonds can be combined into large-scale patterns that can be related to continental geologic structure and/or the geodynamics of mantle convection. Since diamonds from the continental lithospheric mantle have great antiquity (up to 3.5 Ga) diamond has great potential to examine these processes in the geologic past. Direct studies on diamond or studies considering its role in mantle mineralogy by members of the DMGC* and others have pinpointed the initiation of subduction, traced recycling (including water) into the mantle transition zone, recorded the passage of fluids into the continental lithosphere, preserved the signature of carbonatitic fluids that trigger deep mantle melting, and revealed the change in mantle redox with depth. In the future, better knowledge of fluid compositions, diamond forming reactions, C and N fractionation factors, and new analyses of the major deep mantle minerals hold the promise of changing our understanding of the geology of the deep Earth.

*Diamonds and the Mantle Geodynamics of Carbon (DMGC) is part of the Reservoirs and Fluxes community of the Deep Carbon Observatory