Formation of Diamonds

Canada is a world leader in diamond production. Diamond exploration is ongoing in its territories and several provinces.

How Diamonds Are Made

When carbon material is subjected over time to extremely high temperature and pressure, it crystallizes as diamonds. The diamonds we find today were formed millions of years ago in molten rock, more than 160 kilometres (km) below the Earth’s surface. They were carried to the surface in rising magma (molten rock) by volcanic activity.

When the magma cooled, cone-shaped pipes of bluish to dark green rock, known as kimberlite, were left behind. Over years of wind, rain, snow and ice, the kimberlites eroded, releasing diamonds and indicator minerals (small particles that are more widespread and can indicate the presence of diamonds). Sometimes, advancing and receding glaciers can disperse and transport these eroded materials to locations thousands of kilometres away.

Locating Canada’s Diamonds

Diamonds can sometimes be found in these secondary deposits of washed-away materials, which are called alluvial deposits. But the main source of diamonds is the kimberlite itself. To find kimberlites, geologists combine information gathered through surveys. Surveys are done by mapping the presence of indicator minerals in soils, regional ice advance studies, and geophysical analysis.

To date, more than 500 kimberlites have been found in Canada, most in remote northern areas (see the location of kimberlites in Canada).

Video: Formation of a Kimberlite Pipe

Kimberlite Pipe Animation

 

 
Video Description

Video Description:

Formation of a Kimberlite Pipe Animation

A craton is a part of the Earth's crust that has been stable for at least one billion years. The Slave Craton of the Northwest Territories and Nunavut is a prime geological target for diamond exploration.

A cross section of the Earth shows it is composed of the lithosphere (includes the crust and very top of the upper mantle), the asthenosphere (a partially molten part of the upper mantle), the mesosphere (lower mantle), a fluid outer core, and a solid inner core.

Diamonds formed 47 to 540 million years ago under conditions of high temperature and pressure (1,000°C and 55 kbar).

The animation shows molten rock (magma) from the Earth's mantle rising 250 km in the lithosphere and passing through a diamond stability field at progressively faster speeds—from 20 km per hour initially to 1,200 km per hour. The magma erupts at the surface of the Earth after about nine hours.

The diamond-bearing kimberlites were weathered by wind and water over a span of some 60 million years. Between 20,000 and 13,000 years ago, they were further eroded by glaciers. As a result, diamonds are now found in glacial deposits and sometimes spread hundreds of kilometres from where they originated.

Diamond Exploration Process

Indicator Mineral Train

When kimberlite erupts, it ejects rock and ash debris onto the Earth’s surface, forming a tuff ring around the volcanic vent. As glaciers form, ice sheets scour the surface, moving the debris from within and around the source in a direction parallel to the direction in which they are travelling. The debris is deposited as the ice sheets melt, leaving a trail of minerals and rock fragments. Geologists carefully sample these glacial deposits in search of indicator minerals. The location of these minerals maps out what are known as “indicator mineral trains,” which geologists can follow back to the kimberlite source. The minerals that commonly show up in indicator mineral trains include pyrope garnet, chrome-rich diopside, picroilmenite, chromite, olivine, and diamond.

Crater Zone

As the kimberlite volcano erupts, debris is also deposited into the crater zone. The crater zone is composed of two types of kimberlite: pyroclastic kimberlite (tuffs deposited on the surface and within the pipe vent) and epiclastic kimberlite (eroded pyroclastic kimberlite that has been redeposited either outside of or within the pipe vent). Sedimentary features, such as bedding and sorting, are common in the crater zone. There are often xenoliths (rock fragments of the bedrock that was intruded by the kimberlite), vegetation, and terrestrial organisms (such as microfossils) that existed at the time of emplacement. Usually in the Slave Craton, all of the tuff ring of epiclastic kimberlite and part or all of the pyroclastic kimberlite have been eroded by glaciation. Diamonds are found in this zone.

Diatreme Zone

Explosive events originating at deeper levels in the root zone initiate the formation of the diatreme zone. As the kimberlite rises to the surface, variations in its composition and the physical properties of the surrounding host rock determine the size and shape of the diatreme zone. Most often, this is the thickest zone in a kimberlite pipe and underlies the crater zone. It is typically longer than one kilometre, steep-sided, and carrot-shaped. The diatreme zone usually has a fragmented appearance, containing variable amounts of xenoliths, deeper mantle rocks, and kimberlite. Diamonds are found in this zone.

Root Zone

The root zone is formed before and during the kimberlite eruption, when magma crystallizes beneath the diatreme zone. Kimberlite in the root zone often shows crystalline rock textures and contains varying amounts of xenoliths and diamonds.

Dyke

Kimberlite dykes are typically narrow, level bodies comprised of hypabyssal kimberlite and are thought to originate from the root zone. Kimberlite dykes contain varying amounts of xenoliths and diamonds.

Kimberlite

Kimberlite is a volatile-rich, potassic, ultrabasic igneous rock that occurs as small volcanic pipes, dykes, and sills.

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