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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 crystallizatian 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 hare 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 votatile 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. [ABSTRACT FROM AUTHOR]

Abstract: Lithospheric domains in the Slave Province, inferred from regional variations in chemistry of mantle-derived garnets, are parallel with northeasterly structural trends in Neoarchean surface rocks and were used previously to infer Late Neoarchean crust–mantle coupling and argue against the presence of diamondiferous Mesoarchean lithosphere under the western Slave craton prior to its late Neoarchean orogenic climax. A review of more recent evidence does not support this view but concludes that a Mesoarchean lithospheric root was coupled to Mesoarchean crust of the Central Slave superterrane (CSST) prior to accretion of adjacent juvenile Neoarchean terranes. A significant part of this root survived the Neoarchean break-up of the CSST, Neoarchean terrane accretion and a complex sequence of post-accretion deformation, granitoid plutonism and metamorphism. It was least modified in the ultra-depleted harzburgitic layer (UDL) under the central part of the Contwoyto terrane, where it is trapped above the deepest part of an underplated Paleoproterozoic slab which stabilized the remnants of the Archean root and caused downward younging of the lithosphere. Upper mantle domains recognized on the basis of the distribution of peridotitic garnets are not primary Neoarchean features but were either inherited from the original Mesoarchean root or resulted from Neoarchean and Paleoproterozoic tectonic erosion and/or metasomatism of this root. The Slave Province resembles other diamondiferous cratons in that its diamond endowment consists of both Mesoarchean and Paleoproterozoic stones. [Copyright &y& Elsevier]

Abstract: The longevity of the continental lithosphere mantle is explained by its unusual composition. This part of the mantle is made up mainly of forsterite-rich olivine (Fo92–94), with or without orthopyroxene, and it is essentially anhydrous. The former characteristic makes it buoyant, the latter makes it viscous, and the combination of these features that allow it to remain isolated from the convecting mantle. Highly forsteritic olivine is not normally produced during mantle melting. Possible explanations for its abundance in old Archean subcontinental lithospheric mantle include: (a) high-degree mantle melting in a plume or at an Archean ocean ridge; (b) accretion of this material to older lithosphere and its reworking in a subduction zone; (c) redistribution of material to eliminate high-density, low-viscosity lithologies. Following an evaluation of these models based on petrological and numerical modeling, we conclude that the most likely explanation is the accumulation of the residues of melting of one or more mantle plumes following by gravity-driven ejection of denser, Fe-rich components. [Copyright &y& Elsevier]