diamonds-where to look

Where to look for diamonds

The search for diamonds has determined that most are derived from kimberlite pipes in the oldest, nuclear portions of the continents, where the basement rocks are older than 1.5 billion years.
 

The oldest parts of continents are called cratons, and can be divided into two terranes: Archean-age archons, which are older than 2,500 million years, and Proterozoic-age protons, which are 1,600 -- 2,500 million years old. The distribution of these terranes is shown on the map.

 

 



Kimberlite pipes occur in many parts of the continental crust, but most diamond-rich ones are found in archons. This fact suggests that most diamonds were formed and stored deep below the cratons, in the area shown in the lower figure, and were later transported to the surface by kimberlite and lamproite magmas that extracted them and other samples from the mantle.

Kimberlite and Lamprolite

The complex volcanic magmas that solidify into kimberlite and lamproite are not the source of diamonds, only the elevators that bring them with other minerals and mantle rocks to Earth's surface. Although rising from much greater depths than other magmas,

these pipes and volcanic cones are relatively small and rare, but they erupt in extraordinary supersonic explosions.

 Kimberlite and lamproite are similar mixtures of rock material. Their important constituents include fragments of rock from Earth's mantle, large crystals, and the crystallized magma that glues the mixture together. The magmas are very rich in magnesium and volatile compounds such as water and carbon dioxide. As the volatiles dissolved in the magma change to gas near Earth's surface, explosive eruptions create the characteristic carrot- or bowl-shaped pipes.
 

 

Kimberlite magma rises through Earth's crust in networks of cracks or dikes. The pipes only form near Earth's surface. This cross-section of a kimberlite pipe shows the carrot-shaped profile produced by explosive eruption. The root zone starts in fissures, where gases are released from the rising magma and drive the eruption; they blow out the fragment-laden kimberlite to form the volcano's tuff ring and fill the pipe.









Depth measurements show the level of erosion for various kimberlite pipes in South Africa. Adapted from Hawthorne (1975).
 

 

These drawings illustrate the formation and filling of the typical champagne-glass shape of a lamproite pipe. The initial stage of the eruption, powered by gases either from the lamproite magma or from boiling ground water, corrodes the hosting rock to form the champagne-glass shape (top). The eruption then produces particles of ash, lapilli, and pumice that partially fill the crater and form a tuff ring (middle). Finally, the crater fills with a lava pond from the degassed lamproite magma (bottom). Adapted from a sketch by Barbara Scott-Smith












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