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How Do Diamonds Form?

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1) Diamond Formation in Earth's MantleGeologists believe that the diamonds in all of Earth's commercial diamond deposits were formed in the mantle and delivered to the surface by deep-source volcanic eruptions. These eruptions produce the kimberlite and hat are sought after by diamond prospectors. Diamonds weathered and eroded from these eruptive deposits are now contained in the sedimentary (placer) deposits of streams and coastlines.The formation of natural diamonds requires very high temperatures and pressures. These conditions occur in limited zones of Earth's bout 90 miles (150 kilometers) below the surface where temperatures are at least 2000 degrees Fahrenheit (1050 degrees Celsius) [1]. This critical temperature-pressure environment for diamond formation and stability is not present globally. Instead it is thought to be present primarily in the mantle beneath the stable interiors of continental plates [2].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 [3]. See Location 1 in the diagram at the top of the page. This type of volcanic eruption is extremely rare and has not occurred since scientists have been able to recognize them.Is coal involved? Coal is a sedimentary rock, formed from plant debris deposited at Earth's surface. It is rarely buried to depths greater than two miles (3.2 kilometers). It is very unlikely that coal has been moved from the crust down to a depth well below the base of a continental plate. The carbon source for these mantle diamonds is most likely carbon trapped in Earth's interior at the time of the planet's formation.2) Diamond Formation in Subduction ZonesTiny diamonds have been found in rocks that are thought to have been subducted deep into the mantle by plate tectonic processes — then returned to the surface [4]. (See Location 2 in the diagram at the top of the page.) Diamond formation in a subducting plate might occur as little as 50 miles (80 kilometers) below the surface and at temperatures as low as 390 degrees Fahrenheit (200 degrees Centigrade) [1]. In another study, diamonds from Brazil were found to contain tiny mineral inclusions consistent with the mineralogy of oceanic crust [8]. Others have inclusions that suggest that subducted seawater was involved in their formation [9].Is coal involved? Coal is a possible carbon source for this diamond-forming process. However, oceanic plates are more likely candidates for subduction than continental plates because of their higher density. The most likely carbon sources from the subduction of an oceanic plate are carbonate rocks such as limestone, marble, and dolomite, and possibly particles of plant debris in offshore sediments.For more details: Data Management Video Examples

Автор: Robert Hans


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