[Widdekind]

Diamond-bearing Kimberlite comes from deep below the Earth's surface. Kimberlite is rapidly propelled up to the surface, from a depth of several kilometers, in a matter of minutes to at most 1 hour, by Carbon-Dioxide disolved in deep Mantle Magmas (~250 km)*. Indeed, Kimberlite stems from deep "volatile charged" Mantle Magmas**. Kimberlites "are found exclusively in Archaean [3.8 to 2.5 Ga] Cratons"***.

* National Geographic Channel Naked Science -- The Earth's Crust (TV)
** http://en.wikipedia.org/wiki/Diatreme

Now,

The oldest parts of Continental Lithosphere underlie Cratons, and the Mantle Lithosphere there is thicker and less dense than typical; the relatively low density of such mantle "roots of Cratons" helps to stabilize these regions*.

And,

Cratons are generally found in the interiors of Continents and are characteristically composed of ancient crystalline basement Crust of lightweight felsic igneous rock such as granite. They have a thick Crust and deep Roots that extend into the Mantle beneath to depths of 200 km.

Furthermore,

Continental Cratons have deep Roots that extend down into the Mantle... Cratons are underlain by anomalously cold Mantle corresponding to Lithosphere more than twice the approximately 100 km thickness of mature Oceanic or Noncratonic Continental Lithosphere. Thus, at that depth, it could be argued that some Cratons might even be anchored in the Asthenosphere. Mantle Roots must be chemically distinct because Cratons have a neutral or positive buoyancy, and a low intrinsic density that is required to offset any density increases due to geothermal contraction. Rock samples of Mantle Roots contain Peridotites, and have been delivered to the surface as Inclusions in Diamond-bearing Subvolcanic Pipes called Kimberlite Pipes. These Inclusions have densities consistent with Craton composition and are composed of Mantle material residual from high degrees of partial melt. Peridotites are important for understanding the deep composition and origin of Cratons because Peridotite nodules are pieces of Mantle rock modified by partial melting... An associated class of Inclusions called Eclogites, consists of rocks corresponding compositionally to Oceanic Crust (Basalt), but that metamorphosed under deep mantle conditions. Isotopic studies reveal that many Eclogite Inclusions are samples of ancient Oceanic Crust subducted billions of years ago to depths exceeding 150 km into the deep Kimberlite Diamond areas. They remained fixed there within the drifting Tectonic Plates until carried to the surface by deep-rooted magmatic eruptions. If Peridotite and Eclogite Inclusions are of the same temporal origin, then Peridotite must have also originated from... Mantle affected by Subduction of Oceanic Crust [billions of years ago]. During the early years of Earth's existence, when the planet was much hotter, greater degrees of melting, at oceanic spreading ridges, generated Oceanic Lithosphere with thick Crust, much thicker than 12 miles (20 km), and a highly depleted Mantle. Such a Lithosphere would not sink deeply or Subduct because of its buoyancy, and because of the removal of denser melt that in turn lowered the density of the residual Mantle. Accordingly, Cratonic Mantle Roots are probably composed of buoyantly Subducted slabs of a highly depleted Oceanic Lithosphere. These deep Mantle Roots increase the stability, anchoring and survivability of Cratons and makes them much less susceptible to Tectonic Thickening by collisions, or destruction by Sediment Subduction...

Surviving Archean Crust [Cratons] is from regions of cooler, and more depleted, Mantle, wherein greater stability permitted uncommonly thick volcanic accumulations from which voluminous partial-melt, low-density felsic rocks could be generated**.

Note that Kimberlite originates from depths of between 150 to 450 km***.

* http://en.wikipedia.org/wiki/Lithosphere#Oceanic_lithosphere
** http://en.wikipedia.org/wiki/Craton
*** http://en.wikipedia.org/wiki/Kimberlite

CONCLUSION (?):

Kimberlite, being both buoyant and gas charged, "pools" at the bottom of ultra-thick Cratonic Mantle Roots, at depths of over 200 km. There, it is trapped by the overlying Continental Craton, thereby being "bottled up", until it penetrates upwards, and erupts, as a Kimberlite Pipe Volcano. Everywhere else on Earth, thinner and weaker Crusts permit the disolved Volatiles to dissipate by other means.

[Geosciences-Forum.com]

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[Widdekind]

Perhaps it is because the Mantle Roots of Archaean-Eon Cratons are so anomolously cold, that they have trapped Volatiles like Carbon-Dioxide and Water, whose eruptions produce Kimberlite Pipes ?? To wit, those Mantle Roots may have been "flash frozen", trapping the Volatiles.

[Geosciences-Forum.com]

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