Diamonds are the most precious of all stones, adorning many engagement rings, necklaces, and other jewelry. They are admired for their hardness, clarity, beauty, and ability to divide light into its component colors. Diamonds, it is said, are forever. Diamonds are stable crystalline forms of pure carbon that form only at high pressures in cool locations in the Earth's mantle. Their origin is restricted, therefore, to places in the subcontinental mantle where these conditions exist, between 90 and 125 miles (150-200 km) below the surface. The vast majority of diamonds that make their way back to the surface are brought up from these great depths by rare and strange explosive volcanic eruptions known as kimberlites.
Kimberlites and related rocks found in diatremes are rare types of continental volcanic rock, produced by generally explosive volcanism with an origin deep within the mantle. They form pipelike bodies extending vertically downward and are the source of most of the world's diamonds. Kimberlites were first discovered in South Africa during diamond exploration and mining in 1869, when the source of many alluvial diamonds on the
Vaal, Orange, and Riet Rivers was found to be circular mud "pans," later appreciated to be kimberlite pipes. In 1871 two diamond-rich kimberlite pipes were discovered on the Vooruitzigt Farm in South Africa, owned by Nicolas de Beer. These discoveries led to the establishment of several large mines and one of the most influential mining companies in history.
Kimberlites are complex volcanic rocks with mixtures of material derived from the upper mantle and water-rich magma of several different varieties. A range of intrusive volcanic styles, including some extremely explosive events, characterizes kimberlites. True volcanic lavas are only rarely associated with kimberlites, so volcanic styles of typical volcanoes are not typical of kimberlites. Most near-surface kimberlite rocks are pyroclastic deposits formed by explosive volcanism filling vertical pipes, and they are surrounded by rings of volcanic tuff and related features. The pipes are typically a couple hundred yards wide, with the tuff ring extending another hundred yards or so beyond the pipes. The uppermost part of many kimberlite pipes includes reworked pyroclastic rocks, deposited in lakes that filled the kimber-lite pipes after the explosive volcanism blasted much of the kimberlite material out of the hole. Geologic studies of kimberlites have suggested that they intrude the crust suddenly and behave differently from typical volcanoes. Kimberlites intrude violently and catastrophically, with the initial formation of a pipe filled with brecciated material from the mantle, sometimes including diamonds, reflecting the sudden and explosive character of the eruption. As the eruption wanes, a series of tuffs fall out of the eruption column, forming the tuff ring around the pipes. Unlike most volcanoes, kimber-lite eruptions are not followed by the intrusion of magma into the pipes. The pipes simply get eroded by near-surface processes, lakes form in the pipes, and nature tries to hide the very occurrence of the explosive event.
Below these upward-expanding craters are deep vertical pipes known as diatremes that extend down into the mantle source region of the kimberlites. Many diatremes have features that suggest the brecciated mantle and crustal rocks were emplaced at low temperature capped by a chert-shale sequence. This partial ophio-lite is pervasively hydrothermally altered and shows chemical evidence for interaction with seawater with high heat and fluid fluxes. silicon dioxide (sio2) and magnesium dioxide (Mgo) alteration and black smokerlike mineralization is common, with some hydrothermal vents traceable into banded iron formations and subaerial mudpool structures. These features led maarten de Wit and others in 1992 to suggest that this ophiolite formed in a shallow sea and was locally subaerial, analogous to the Reyk-janges ridge of Iceland. In this sense Archean oceanic lithosphere may have looked very much like younger oceanic plateaux lithosphere.
several partial or dismembered ophiolites have been described from the slave Province of northern Canada. A fault-bounded sequence on Point
Lake grades downward from shales and chemical sediments (umbers) into several kilometers of pillow lavas intruded by dikes and sills, locally into multiple dike/sill complexes, then into isotropic and cumulate-textured layered gabbro. The base of this partial Archean ophiolite is marked by a 3,000-foot (1-km) thick shear zone composed predominantly of mafic and ultramafic mylonites, with less deformed domains including dunite, websterite, wherlite, ser-pentinite, and anorthosite. syn-orogenic conglomerates and sandstones were deposited in several small foredeep basins, and are interbedded with mugearitic lavas (and associated dikes), all deposited/intruded in a foreland basin setting.
A complete but dismembered and metamorphosed 2.5-billion-year-old ophiolite complex from the North China craton has been described. This nonviolently, presenting a great puzzle to geologists. How can a deep source of broken mantle rocks passively move up a vertical pipe to the surface, suddenly explode violently, then disappear beneath a newly formed lake?
Early speculations on the intrusion and surface explosion of the diamond-containing kimberlites suggested that they rose explosively and catastrophically from an origin in the mantle. Subsequent studies revealed that the early deep parts of their ascent did not seem to be explosive. It is likely that kimberlite magma rises from deep in the upper mantle along a series of cracks and fissures until it gets to shallow levels, where it mixes with water and becomes extremely explosive. Other diatremes may be more explosive from greater depths, and they may move as gas-filled bodies rising from the upper mantle. As the gases move into lower-pressure areas, they would expand and the kimberlite would move faster until it explodes at the surface. Still other ideas for the emplacement of kimberlites and diatremes invoke hydrovolcanism, or the interaction of the deep magma with near-surface water. Magma may rise slowly from depth until it encounters groundwa-ter in fractures or other voids, leading to an explosion when the water mixes with the magma. The resulting explosion could produce the volcanic features and upward-expanding pipe found in many kimberlites, spewing the kimberlite magma and diamonds across a wide area on the surface.
It is likely that some or all of the processes discussed here play a role in the intrusion of kimberlites and diatremes, the important consequence being a sudden, explosive volcanic eruption at the surface, far from typical locations of vol-canism, and the relatively rapid removal of signs of this volcanism. The initial explosions are likely to be so powerful that they may blast material into the stratosphere, though other kimberlite eruptions may form only small eruptions and ash clouds.
Diamonds are the hardest substance known and are widely used as gemstones. Uncut varieties may show many different crystal shapes, and many show striated crystal faces. They crystallize in isometric tetrahedral forms, exhibit concoidal fracture, have a greasy luster, and may be clear, yellow, red, orange, green, blue, brown, or even black. Triangular depressions are common on some crystals, and others may be preserved as elongate or pear-shaped forms. Diamonds have been found in alluvial deposits such as gravel, and some mines have been located by tracing the source of the gravel to the kimberlite pipe from where the diamonds were brought back to the surface. Some diamond-mining operations such as those of the Vaal River, South Africa, proceeded for many years before it was recognized that the source was in nearby kimberlites.
Dating the age of formation of small mineral inclusions in diamonds has yielded important results. All diamonds from the mantle appear to be Precam-brian, with one type being up to 3.2 billion years old, and another 1.0 to 1.6 billion years old. Since diamonds form at high pressures and low temperatures, their very existence shows that the temperature deep in the Earth beneath the continents in the Precambrian was not much hotter than today. The diamonds were stored deep beneath the continents for billions of years before being erupted in the kimberlite pipes. Diamonds really are forever.
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