Plume related kimberlites and carbonatites

The origin of the alkaline magmatism, including kimberlites and carbonatites, is believed to be related to deep-seated mantle plumes. A chondritic Earth’s mantle contains very low amounts of alkaline elements, with Na prevailing over K. Consequently, the source of the alkaline rocks cannot be the ‘chondritic’ mantle and most likely a mantle modified by subducted crustal materials. Alkaline magmas and carbonatites appear first in the Mesoarchean (~ 3 Ga) and possibly coincided with the onset of plate tectonics. Melting and degassing of subducted slabs into the deep mantle caused widespread metasomatism and formation of reservoirs enriched in the alkaline and lithophile trace elements. These served as sources of alkaline and carbonatitic magmas, and from ~ 2 Ga onwards of kimberlite magmas. Theoretical and experimental modeling predict the lower mantle and transition zone to be largely composed of bridgmanite, ferropericlase, Ca-Si-perovskite, ringwoodite, wadsleyite, majorite, NAL (a hexagonal aluminous phase of the lower mantle containing Na, Al and K), breyite and carbonates. The alkaline elements, isomorphic in CaSi-perovskite, bridgmanite and NAL, can be released during the ascent of mantle plume and transferred to the melt/fluid-enriched reservoir of carbonatites and alkaline magmas. At ~ 600 km depth where majorite is stable, an extensive fractionation of K and Na occurs, as the partitioning coefficient of Na is an order of magnitude larger than that of K. This results in K enrichment of the metasomatic melt/fluid that contribute to prospective sources of kimberlitic and other deep-mantle K magmas.