In the last two days I been reading extensively on the processes behind diamond formation and how diamonds are transported to the earths surface. I thought I'd share a little of what I have learnt and try and translate some of the geological story into language that the average person should be able to understand. A lot of this information will ultimately have a bearing on our current exploration at Lulo and also when the kimberlite exploration becomes more advanced and the company starts reporting information on what they've found in the kimberlite pipes.
Here are some basic facts to start with that most scientists are now agreed on (apologies to any geologists out there, please feel free to correct any facts if you feel they needed correcting) .
1) The kimberlite magmas that bring diamonds up from the deep mantle layers of the earth DO NOT form the diamonds themselves.
2) The kimberile magmas are ONLY transporters of the diamonds to the surface. Scientist refer to the diamonds as xenocrysts; meaning a single individual foreign crystal within an igneous body.
3) The diamonds themselves are much older than the kimberilte magmas that transport them. Most kimberlites where in placed within the last 350 million years and most diamonds where created 1G to 3G years ago (that's 1,000 to 3,000 billion years ago).
3) The kimberilte magma can also transport with it fragments of rocks from the lower mantle layers within which the diamonds were created and also other fragments of rock that are torn away from higher levels in the earths crust as the magma rises to the surface. Scientist call these fragments xenoliths.
4) The diamonds can only exist deep within the earth under certain combinations of temperature and pressure. Scientist refer to these temperature and pressure conditions as the diamonds stability field or region. Below the stability region carbon can exist as diamond and above the stability region it can only exist as graphite or other forms of carbon. The zones in which diamonds normally reside are found under the oldest and thickest parts of continents called cratons and the diamonds are stored in what scientists refer to as the keels of the craton or mantle keels. Most diamonds are stored at depths of between 100km and 250km but they can also originate from within deeper mantle depths (greater than 660km). Below is a schematic drawing showing the relationship between a craton, the lithospheric mantle, the stability field of diamonds and the lower mantle layers.
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The lithospheric mantle is a term used by scientists to describe the earths crust and upper most solid part of the earths mantle. Below is a diagram showing the lithosphere against the earths cross section.
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As mentioned in point 4) above diamonds can only exist within certain temperature and pressure stability conditions so for them to survive the trip towards the earths surface they need to travel within the conveying magma at considerable speed. Scientists have calculated the speeds at which diamonds need to ascend to survive the journey at between 4 and 20 meters per second.
The diamonds that we find at the surface can give us a lot of clues about the types of rocks that they were formed in and they can also tell us about what happened to them on their trips to the surface. If a diamond spends to long travelling up to the earths surface and sits in a hot magma (outside of its stability field) for too long it can get resorbed into the magma and turn into graphite or even just disappear completely as carbon dioxide. Other diamonds that don't get fully resorbed can become rounded and their commercial value may also become effected. Below is a photo showing a resorbed 4.82ct octahedron diamond next to a naturally occurring morphology.
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Hopefully this might help a few like myself who up until a few weeks ago knew practically nothing about how and where diamonds are formed. I'll leave the story here for the moment and call this Part I. I'll try and expand a little on the story in Part II and I'll also try and weave a few facts in that are more specific to our story at Lulo. Feel free to ask questions and I'll try and give an answer if I can or add to the story.
Eshmun
(20min delay)