Kimberlites are unusual, ancient igneous rocks that originate deep within the Earth. These rare formations provide insights into the planet’s interior. Their distinct composition and formation processes set them apart from most other rock types found on the surface.
Formation and Geological Features
Kimberlites form at extreme depths within the Earth’s mantle, typically between 150 and 450 kilometers below the surface. This deep origin is possible due to specific conditions in the mantle, where volatile components like water and carbon dioxide are abundant. The magma then ascends rapidly through the Earth’s crust in powerful, explosive volcanic eruptions, creating carrot-shaped structures known as kimberlite pipes. These eruptions are thought to be far more violent than modern volcanic events.
Kimberlites are ultramafic rocks, rich in magnesium and iron. Their composition typically includes minerals such as olivine, phlogopite (a type of mica), and various garnets like chromium pyrope. Kimberlites also frequently contain xenoliths, which are fragments of mantle rock brought to the surface during the rapid eruption, offering geologists direct samples of the deep Earth.
Where Kimberlites Are Found
Kimberlites are globally distributed but are predominantly found on ancient, stable continental landmasses called cratons. These cratons possess deep, thick “roots” that extend into the mantle, providing the necessary conditions for kimberlite formation and ascent. The presence of kimberlites in these areas offers valuable information about the history and composition of these deep continental roots.
Significant kimberlite-bearing regions include parts of Africa (such as South Africa and Botswana), Siberia, Canada, and Australia. The oldest known kimberlites date back to the Archean Eon, around 2.8 billion years ago.
The Connection to Diamonds
Kimberlites are the primary natural transport mechanism for diamonds. Diamonds form much deeper within the Earth’s mantle, typically at depths of 150 to 200 kilometers, under immense pressure and temperatures ranging from 900 to 1,300 degrees Celsius, where carbon atoms crystallize into their rigid structure.
The rapid ascent of kimberlite magma to the surface is crucial for preserving diamonds. If the ascent were slow, the diamonds would revert to graphite, their more stable form at lower pressures. Kimberlites act as a high-speed elevator, bringing these deeply formed diamonds to the surface relatively quickly, sometimes in a matter of hours. While kimberlites are the main source, not all kimberlite pipes contain diamonds, and only a small percentage are rich enough to be mined.
Identifying Kimberlites
Identifying kimberlites involves observing physical characteristics and searching for specific indicator minerals. Unaltered kimberlite is typically dull gray-green to bluish, known as “blue ground” among miners. When weathered and oxidized, it can appear yellowish-brown, called “yellow ground,” due to the alteration of minerals like magnetite to limonite. The rock often has a fine-grained texture and can contain visible fragments of other rocks.
Prospectors look for specific “indicator minerals” that are commonly found alongside kimberlites and are more resistant to weathering. These include distinct types of pyrope garnet (often reddish-purple), chromite (black), and magnesian ilmenite (black). These minerals indicate the potential presence of a kimberlite pipe nearby.