Kimberlite pipes, diamonds, and zircons

If you or your spouse have a diamond, then it probably came from a kimberlite pipe. Only found on continental cratons, kimberlite pipes offer us a geologic window into the deep earth. These pipes are geological features created by an unusual form of volcanic eruption that drags bits of the earth’s interior to the surface from depths of about 200 kilometers (125 miles). Pressures and temperatures at those depths are high enough to turn carbon into diamonds. But, we cannot retrieve these diamonds. No wells or mines can reach those depths, so the diamonds must come to us.  

Diamonds are not the only minerals that kimberlite pipes bring to the surface. Garnet, olivine, rutile, zircon, and more are also associated with kimberlite formations, and not all kimberlite deposits contain diamonds. Kimberlite is an igneous rock formed from the cooling of olivine-rich magma. The kimberlite matrix rock is classified as ultrabasic, meaning it doesn’t contain any quartz or feldspar.

This ultrabasic magma develops in the earth’s mantle about 200 kilometers below the surface, where the olivine-rich rock peridotite occurs in large quantities. When temperatures are high enough, and the peridotite melts, then the process of forming a kimberlite pipe starts.

How to build a kimberlite pipe  

Kimberlite pipes are only found on the cratons of earth’s continents. A craton is the core or nucleus of a continent, and it represents the thickest and most stable portion of the continental mass. But most importantly, a craton represents a thick layer of silica-rich rock that extends deep into the earth. So, when the peridotite melts and tries to rise, it quickly encounters silica, which then becomes incorporated in the magmatic melt.

The silica changes the chemical composition of the magma, thus making it more acidic. Also, an increase in dissolved carbon dioxide accompanies acidification. So, when the magma becomes fully saturated with carbon dioxide and can hold no more, the additional CO2 starts forming bubbles. The introduction of free gas in the magma lowers its total density, and then buoyancy starts to drive the magma upward, like an air balloon released at the bottom of a pool.

Some geological processes are slow, but the formation of a kimberlite pipe is not. As the magma rises, it breaks apart the overlying rock and incorporates it into the flow. The buoyant volume of magma increases in size and moves through the crust at speeds of up to 250 kilometers per hour. Once the process gains momentum from buoyancy, the eruption is almost instantaneous from a practical perspective. 

Zircons count too

The need to mix silica with ultrabasic magma explains why kimberlite deposits are only found on ancient cratons. Because of their thickness the cratons are the only locations where the required silica-rich continental crust is deep enough to participate in the formation process. The end result of this unusual type of igneous eruption is a Kimberlite pipe. Physically the pipe looks like a carrot with the narrow tip in the mantle and the wide top at the earth’s surface. 

Diamonds are of value commercially, but zircons provide a wealth of scientific information. Recent work on zircons from West Greenland kimberlite deposits offered new insights into the deep geology of the North American Craton. Chronological dating and compositional analysis of individual zircons provided information on the age of deep crustal rocks. The research also provided insights into the formation conditions for those rocks deep in the roots of the craton.  

Scientific investigations of kimberlite pipes allow a physical view of the inner workings of the planet earth. These inner workings also allow for vast wealth when diamonds are part of the mix.

Sources:

Kimberlite Pipes: What They Are and How They’re Formed (By Priscila Barrera; Investing News) – https://investingnews.com/daily/resource-investing/gem-investing/diamond-investing/kimberlite-pipes-south-africa-australia-borneo-brazil-russia/ Also:

What are Kimberlites? (Kansas Geological Survey) – http://www.kgs.ku.edu/Publications/pic16/pic16_2.html Also:

How diamond-bearing kimberlites reach the surface of Earth: Acidification provides the thrust (Science Daily) – https://www.sciencedaily.com/releases/2012/01/120123094523.htm Also:

James K. Russell, Lucy A. Porritt, Yan Lavallée, Donald B. Dingwell. Kimberlite ascent by assimilation-fuelled buoyancy. Nature, 2012; 481 (7381): 352 DOI: 10.1038/nature10740 Also:

The Crazy Eruptions That Spit Up Diamonds (By Erik Klemetti; Wired) – https://www.wired.com/2017/05/crazy-eruptions-spit-diamonds/ Also:

North Atlantic Craton architecture revealed by kimberlite-hosted crustal zircons (Nicholas J. Gardiner, Christopher L. Kirkland, Julie A. Hollis, Peter A. Cawood, Oliver Nebel, Kristoffer Szilas, Chris Yakymchuk; Earth and Planetary Science Letters) – https://govmin.gl/publications/north-atlantic-craton-architecture-revealed-by-kimberlite-hosted-crustal-zircons/?ind=1580236442876&filename=Gardiner_etal_2019b_Maniitsoq.pdf&wpdmdl=7643&refresh=5e3dab2a22e431581099818 Also:

Feature Image: Vintage diamond engagement ring (By CatherineLewis) (Modified) – https://commons.wikimedia.org/wiki/File:Vintage_diamond_engagement_ring.jpg –  Also this file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license. – https://creativecommons.org/licenses/by-sa/4.0/deed.en