Lamproite (a source rock for diamonds)

Collection Highlights | Updated 6 years ago

A large slab of a rock ore of diamonds
Lamproite (a common source rock for diamonds)
Image copyright of WA Museum

In Western Australia diamonds are produced from mines at Argyle in the east Kimberley and Ellendale in the west Kimberley.

Diamonds are made of carbon and crystallise naturally under the immense pressure found deep below the Earth’s surface, more than 150 km down.

A special mechanism is required to rapidly transport diamonds from their source rocks in the Earth's mantle to the surface without allowing them to transform to the most common low-pressure form of carbon, graphite.

This transport mechanism is provided by some unusual volcanic rocks called kimberlite and lamproite, both of which are rocks rich in potassium and magnesium, and poor in silica.

Kimberlite and lamproite are two of the deepest-sourced volcanic rocks that occur at the Earth’s surface. Each may have the opportunity to sample diamond-bearing mantle rocks during their ascent to the surface.

Both kimberlites and some lamproites may contain large volumes of rounded olivine crystals (macrocrysts) that originated from the upper mantle underlying the Earth's crust. Rocks containing high volumes of these olivine crystals are often the most rich in diamonds. In the Kimberley region of Western Australia, only the lamproites containing high volumes of olivine crystals have proved to contain economic quantities of diamond.

Kimberlites and lamproites have different chemical compositions and contain different assemblages of minerals. These differences are probably related to variations in their depth of origin, as well as the degree of melting and the composition of the rocks that melted to produce them.

Rock Collection