Kimberlite the source for Diamonds

Kimberlite is a potassic volcanic rock best known for sometimes containing diamonds.  It was originally discovered in Kimberly, South Africa in 1871 hence its name.  The miners digging in the “big hole” found a large diamond weighing 83.5 carets that spawned the South African Diamond Rush.  There have been literally thousands of kimberlites that have been discovered since the initial discovery in 1871.  Of all the discoveries however only about one kimberlite in three hundred actually become diamond mines.  Diamondiferous kimberlites are actually quite rare.

In cross section a kimberlite is a vertically standing feature called a diatreme in the earth’s crust that is shaped like a big carrot.  Where in the earth’s mantle kimberlites originate has been a matter of controversy for decades the one thing we are sure of however that they come from a depth greater then the so-called diamond window at more then 140 Km below the earth’s surface. 

The highly complex volcanics that solidify into kimberlite or lamproite are not the source of diamonds they are the elevator that brings them to the surface.  The volcanics rise through a complicated set of cracks and fissures until they near the surface where their extreme pressure causes them to blast an opening through the surface called a pipe.  In the case of kimberlites the pipe is carrot shaped, and in lamproites it is shaped like a champagne glass.

Cross section of a kimberlite chimney
By Heriberto Arribas Abato

Kimberlite and lamproite are similar magmas full of debris from the mantle of the earth as xenoliths with the actual magma acting as glue to stick the mass together.  Diamonds found in kimberlites or lamproites are part of the debris brought to the surface.  The magmas are rich in magnesium and volatile components such as water and carbon dioxide as these volatiles near the earth’s surface they explode into the characteristic carrot or champagne craters.  At the time of eruption they also form small conelike features on the surface of the earth that are quickly eroded away by the process of weathering.  Both types of eruption are quite rare and small in size.  They may also occur as dikes or sills in the country rock which undergoes considerable fracturing as the magma reaches towards the surface.  The rocks in the lithosphere are very brittle making it easy for kimberlites to reach the surface.

There are several different compositions related to kimberlites and they are named after the predominant mineral.  Normally the most abundant mineral associated with the primary magma is olivine making the kimberlite with olivine known as a periodotic kimberlite.  Other types are eclogitic, websteritic, sublithospheric and uncertain.  Any of these may contain diamonds.    

References:

Mineralogical Association of Canada, Editor Lee A. Groat, Geology Of Gem Deposits, Short Course Series, Volume 37, Yukon Geological Survey, © 2007, Yellowknife, Northwest Territories
Gem Deposits, http://amonline.net.au/geoscience/earth/gem.htm
Geology of Gem Deposits, Mineralogical Association of Canada, Editor Lee A. Groat, http://www.mineralogicalassociation.ca/doc/promo_SC37.pdf  Volume 37 © 2007
Industrial Minerals and Rocks, Page 418, http://books.google.com/books?id=zNicdkuulE4C&pg=PA417&lpg=PA417&dq=kimberlites+and+the+occurance+of+diamonds&source=bl&ots=Nhqev_Ebqc&sig=WQyRiIxdpBMtnaZWwhGwKC5o1sY&hl=en&ei=csyvSbDZAojWnQfAmpTNBQ&sa=X&oi=book_result&resnum=9&ct=result#PPA418,M1
Kimberlite, Wikipedia the free encyclopedia, http://en.wikipedia.org/wiki/Kimberlite
The Nature of diamonds, Kimberlites and Lamproites, American Museum of Natoual History, http://www.amnh.org/exhibitions/diamonds/kimberlite.html