Kimberlite

Kimberlite is a unique type of igneous rock that serves as a primary carrier for diamonds.

It is a rare variant of a rock called peridotite. This rock is scientifically significant because it transports diamond xenocrysts from the deep mantle to the Earth's surface. By studying kimberlite, geologists can learn about the composition of the deep mantle. They can also observe melting processes near the interface of the cratonic continental lithosphere and the asthenospheric mantle.

The formation of kimberlite begins deep within the Earth's mantle. This occurs at extreme depths between 150 and 450 kilometers. At these depths, the magma is highly enriched with volatiles. Volatiles are substances like carbon dioxide and water that can turn into gases. The magma undergoes a rapid and violent eruption due to this high gas content. As the magma rises, it fractures the surrounding rock. This explosive process creates vertical structures known as kimberlite pipes.

Kimberlite structures appear in several different shapes and forms. The most famous are the carrot-shaped kimberlite pipes. These vertical intrusions are formed by a deep explosive boiling stage. These pipes can range from 75 meters to 1.5 kilometers in diameter. Besides pipes, kimberlite can also form as igneous dykes or horizontal sills. A dyke is a thin, vertically dipping sheet of rock. These dykes can be as narrow as 1 to 4 meters.

The history of kimberlite is closely tied to the discovery of diamonds in South Africa. The rock is named after the town of Kimberley. In 1869, a massive diamond called the Star of South Africa was discovered there. This diamond weighed 83.5 carats, which is about 16.70 grams. This discovery caused a major diamond rush. It also led to the excavation of the Big Hole, a famous open-pit mine.

Geologists classify kimberlites into different groups based on their chemistry. Historically, they were called "basaltic" and "micaceous." Later, scientist C. B. Smith renamed these Group I and Group II based on isotopic affinities. These are chemical signatures involving lead, strontium, and neodymium. Roger Mitchell later suggested that Group II rocks might be different from Group I. He reclassified Group II as orangeites. It was previously an error to call olivine lamproites "Kimberlite II."

Kimberlite is chemically distinct from most other igneous rocks. It is classified as an ultramafic rock because it has very high magnesium oxide content. This content usually exceeds 12% and often goes above 15%. These rocks are also ultrapotassic, meaning they have a high ratio of potassium to aluminum. They are rich in primitive elements like nickel, chromium, and cobalt. Kimberlites also contain many rare earth elements. These elements help scientists understand how the magma evolved as it rose.

Finding these deposits requires advanced exploration techniques. Geologists look for indicator minerals, which are specific minerals that only form under high pressure. These include chromium diopside, chromium spinels, and pyrope garnets. Scientists use sediment sampling to find these minerals in soil or streams. In areas covered by ice, they use esker or till sampling. By analyzing these minerals, geologists can estimate if a diamond-bearing pipe is nearby. This helps them decide where to drill for valuable resources.