# What is it about ancient craton geology that results in associated kimberlite pipes containing economic diamond deposits?

According to the Australian Geological Survey Organisation paper Kimberlite and lamproite diamond pipes (Jaques, 1998), economic deposits of volcanically-formed diamonds are

restricted to ancient cratons (typically >2.Ga) or cratonised provinces older than 1.8 Ga

Ga = billion years

and that rarely do younger accreted cratonised belts contain economic deposits.

This relationship can be seen in the Greenland Mineral Exploration Newsletter map below:

What is it about the ancient craton geology (and geochemistry) that results in the associated kimberlite pipes containing economically important diamond deposits?

• Which is untrue. There are also impact diamonds. – Deer Hunter Sep 21 '14 at 5:11
• True, however, the question is about the types of kimberlites and lamproites that produce economic deposits - the question and post is reworded to clarify that – user889 Sep 21 '14 at 5:18

The Science Daily article How diamond-bearing kimberlites reach the surface of Earth: Acidification provides the thrust (Ludwig-Maximilians-Universitaet Muenchen (LMU)) reports of research completed by an international research team led by Professor Donald Dingwell, Director of the Department of Geo- and Environmental Sciences at LMU.

They state that ancient cratonic crust provides silicate minerals that result in the originally basic magma to become more buoyant and as they are forced through the pipes, it causes the buoyant magma to move fast. With the authors describing that as the magma ascent increases in velocity, the more silicates it gathers (entrains), increasing the dissolved silicate concentration - until the amount of released $\mathrm{CO}_2$ and $\mathrm{H_2O}$ greatly increasing the velocity of ascent, as the authors state 'like a rocket'.

A more thorough geochemical description is provided in the article Kimberlite ascent by assimilation-fuelled buoyancy (Russell et al. 2012), the proposed model mechanism as the original kimberlite melt to be carbonatite-like - poor in silica content, this magma assimilates materials during its accent through the upper mantle and crust, becoming more silicic, causing the $\mathrm{CO}_2$ solubility to decrease, reducing density and increasing buoyancy, causing the magma to accelerate.

In terms of why cratonic crust is the locale of many diamond rich kimberlite pipes is described from Professor Dingwell as being due to the silica rich cratonic crust is thick enough to provide the magma to make contact with the silicate rich minerals therein.

• So why is it only in Archean crust? The amount of silicic crust has been growing steadily since then, so why didn't the kimberlitic magmatism become more common with time? – Gimelist Nov 29 '14 at 11:16
• True, but asides from the Igwisi Hills, Tanzania kimberlitic pipes - which erupted through the Tanzanian Craton, there seems to be a reduction in the amount of economic-diamond bearing kimberlites erupting over time. Could be a good separate question. – user889 Nov 29 '14 at 11:23