Diamonds form in the upper mantle during episodic events and have been transported to the Earth’s surface from at least the Archean to the Phanerozoic. Small diamonds occur as inclusions in robust minerals in tectonically activated, ultrahigh-pressure metamorphosed crustal rock, establishing an association with subduction processes and recycled carbon, but providing no economic deposits. Diamonds in economic deposits are estimated to be mainly (99%) derived from subcontinental lithospheric mantle and rarely (approx. 1%) from the asthenosphere. Harzburgite and eclogite are of roughly equal importance as source rocks, followed by lherzolite and websterite. Diamonds which provide evidence of extensive residence time in the mantle are, with minimal exceptions, smooth-surfaced crystalline diamonds (SCD) with potential commercial value. The oldest prolific SCD formation event documented on the world’s major diamond producing cratons occurs in Archean lithospheric mantle harzburgite, metasomatized by likely subduction-related potassic carbonatitic fluids.
Disaggregation of the diamondiferous carbonated peridotite on decompression during volcanic transit gives rise to the association between diamonds, G10 garnets, and diamond inclusion-type chromites, well used in diamond exploration. Within the mantle domains of diamond stability, there have been repeated episodes of further diamond crystallization and/or growth. These are associated with old, often Proterozoic, subduction-related melt generation, metasomatic fluid migration, and reaction with preexisting mantle eclogite, websterite, and peridotite. Using improved methods of isotope analysis, diamond formation ages can be correlated with specific major processes such as craton accretion, craton edge subduction, and magmatic mantle refertilization. Fibrous cuboid diamond and fibrous coats on SCD are rough-surfaced diamonds with abundant fluid inclusions. They have low mantle residence time, forming rapidly from late stage metasomatic fluids in diamond stable domains that may already contain SCD. The symbiotic relationship between formation of fibrous diamond and magmatic sampling and transport of diamonds into the crust suggest that the associated fluids contribute diamond-friendly volatile loading of the deep lithospheric mantle shortly before the triggering of a volcanic eruption, continuing a process of volatile enrichment in the lithospheric mantle already identified in the Archean harzburgite diamond event. Mantle-derived SCD commonly shows evidence of resorption, illustrating that diamond-unfriendly processes, including lamproite and kimberlite generation, are also active and may have a substantial negative effect in extreme cases on SCD crystal size. Exposure of SCD to a long period of changing conditions during mantle residence contributes to the difficulty of assigning specific parageneses and ages to individual inclusion-free diamonds with our current state of knowledge.