Almost all commercial diamonds are formed deep in the Earth’s mantle in a slow process stretching over nearly 1 to 3.3 billion years. 87 to 118 miles under the Earth’s continental crust, temperatures of 1,650 and 2,370 °F combine with pressures forty-five thousand to sixty thousand times atmospheric pressure to create zones that are just right for turning carbon into diamonds. Most diamonds are formed from carbon that was trapped in the planet’s interior at the time of its formation. Some, however, are formed from organic material that was naturally pushed down from the Earth’s surface to the depths suitable for diamond formation.
Diamonds that are formed at this great depth exist within rocks called xenoliths. But something else must happen to bring the xenoliths to the surface where they can be mined. Deep volcanic eruptions, more than three times deeper than average, bring the xenoliths to the surface. In doing so, they create deep but thin craters called volcanic pipes. The diamonds containing xenoliths are then trapped in the volcanoes’ ignaceous rock as it cools. Natural processes such as wind and erosion scatter some of the surface diamond containing rocks to surrounding areas.
Not all volcanic pipes contain diamonds and not all that do have enough precious stone to be profitable for mining. Prospectors look for pipes that have a large supply of marketable diamonds that will make their work profitable.
The formation of natural diamonds requires very high temperatures and pressures. These conditions occur in limited zones of Earth’s mantle about 90 miles (150 kilometers) below the surface where temperatures are at least 2000 degrees Fahrenheit (1050 degrees Celsius)
primarily in the mantle beneath the stable interiors of continental plates
Diamonds formed and stored in these “diamond stability zones” are delivered to Earth’s surface during deep-source volcanic eruptions. These eruptions tear out pieces of the mantle and carry them rapidly to the surface
The carbon source for these mantle diamonds is most likely carbon trapped in Earth’s interior at the time of the planet’s formation.
chunks of diamond-containing rocks called xenoliths. These eruptions create pipes in the upper layers of Earth’s crust called kimberlite pipes.
diamond has to be created in a “diamond stability zone”, located in the upper mantle of Earth. In order for diamonds to form, a temperature of over 1000 degrees Celsius and very high pressure is required. In the natural environment, these conditions are only found deep beneath the surface (est~ 150km below the Earth’s crust).
Most natural diamonds are formed at high temperature and pressure at depths of 140 to 190 kilometers (87 to 118 mi) in the Earth’s mantle. Carbon-containing minerals provide the carbon source, and the growth occurs over periods from 1 billion to 3.3 billion years (25% to 75% of the age of the Earth).
conditions for diamond formation to happen in the lithospheric mantle occur at considerable depth
The correct combination of temperature and pressure is only found in the thick, ancient, and stable parts of continental plates where regions of lithosphere known as cratons exist
Some diamonds, known as harzburgitic, are formed from inorganic carbon originally found deep in the Earth’s mantle. In contrast, eclogitic diamonds contain organic carbon from organic detritus that has been pushed down from the surface of the Earth’s crust through subduction
Diamonds that have come to the Earth’s surface are generally quite old, ranging from under 1 billion to 3.3 billion years old.
Diamond-bearing rock is carried from the mantle to the Earth’s surface by deep-origin volcanic eruptions. The magma for such a volcano must originate at a depth where diamonds can be formed—150 km (93 mi) or more (three times or more the depth of source magma for most volcanoes). This is a relatively rare occurrence. These typically small surface volcanic craters extend downward in formations known as volcanic pipes. The pipes contain material that was transported toward the surface by volcanic action, but was not ejected before the volcanic activity ceased. During eruption these pipes are open to the surface, resulting in open circulation; many xenoliths of surface rock and even wood and fossils are found in volcanic pipes. Diamond-bearing volcanic pipes are closely related to the oldest, coolest regions of continental crust (cratons). This is because cratons are very thick, and their lithospheric mantle extends to great enough depth that diamonds are stable