The discussion of formation of primary diamond deposit is based upon an idealized cross section of the earth through a craton and subcratonic areas, to a depth of about 250 km, characterized by long term stability. The following sequence of events is envisaged.
1. Diamonds form anywhere in the diamond stability field from either peridotite, which is a major constituent of the upper mantle, or eclogite, which is subducted from the earth's surface to a depth of 150 (or more) km. But because of the direction and angle of subducted plates, eclogitic diamonds are more likely to be found on the flanks of the diamond stability field.
2. The diamonds formed from either of the two sources ideally should be moved into the diamond storage area (stippled zone) for long term stability. It is also possible that some diamonds form initially in the storage area, in which case no migration is necessary. Diamonds in the storage area may have multiple origins (either peridotite or eclogite, or both).
3. Kimberlite is generated below the storage area, perhaps at a depth of 200 - 300 km, and, in the case of a first kind of a pipe, it migrates directly through the storage area, bringing to the surface diamonds of both types. A 2nd Pipe is formed by kimberlite that samples primarily the eclogite area with the result that most of its diamonds will be of this type. A 3rd Pipe is composed of kimberlite that did not pass through the storage area of the zone of formation of eclogitic diamond and, therefore, it is likely to be barren of diamonds. For one reason or another, probably about 75% of the world's kimberlite pipes do not contain diamonds.
A 4th Pipe is possibly representative of the formation of the Argyle, Australia, lamproite pipe. Because of the complex nature of the fracture system through which the lamproite magma moved upward, it will contain diamonds of both the peridotitic and eclogitic types, and it will appear on the surface in a "mobile belt" that is displaced from the most ancient (central) part of the craton.
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