Ultrapotassic rocks

Igneous rocks, from which ultrapotassic rocks derive, form when magma from within the earth’s mantle rises toward the earth’s surface and cools to form solid rock. Magma pockets lead to a variety of igneous deposits, including batholiths, sills, dikes, and volcanic pipes. Ultrapotassic rocks are a type of igneous rock that form deep within mantle rock contained within a craton, a stable portion of the mantle.

Intrusive Igneous Rocks

Igneous rocks are formed from magma, a mixture of molten rock, gases, and other trace chemicals that form in pockets within the earth’s mantle. The mantle is a layer of rock heated by radioactive energy from the earth’s core and pressurized by the weight of rock above it.

The mantle shifts in response to thermal currents from the core. As this occurs, localized disturbances cause part of the mantle to destabilize. The molecules within the disturbed mantle move freely, generating energy that melts the surrounding rock. Magma is lower in density than surrounding rock and therefore rises to the surface, pushing through the solid rock of the mantle and the crust.

When magma cools in the crust or on the surface, it solidifies into igneous rocks. Magma that breaks through the surface of the crust before cooling is called lava and gives rise to volcanic rock or extrusive igneous rock. When magma cools beneath the earth’s surface, insulated within pockets of solid rock, it develops into intrusive igneous rock or plutonic rock. Plutonic rock cools slowly, in millions of years, which causes the development of larger crystals. For this reason, intrusive igneous rock tends to be characterized as phaneritic in texture, meaning that the grains of the rocks can be discerned by the naked eye.

Igneous rocks also are characterized by their chemical composition. The primary components of igneous rocks are silica, a molecular combination of silicon and oxygen, and feldspar, which is another silica-based crystal that contains a variety of included materials. Some igneous rocks, called felsic rocks, are composed primarily of silica (55 to 70 percent) and frequently contain a variety of potassium-rich feldspars. By contrast, mafic rocks have lower levels of silica (less than 50 percent) and are rich in iron and magnesium. Ultramafic rocks have even lower concentrations of silica (less than 45 percent) and correspondingly higher concentrations of included metals.

Igneous Rock Deposits

Magma that cools within the earth’s crust forms into plutonic rock deposits. The largest plutonic deposits are called batholiths, which result when large pockets of magma join as they cool. Batholiths may exist deep within the mantle, within the continental cratons, which are the deep, solid portions of the lithosphere least affected by the collisions between tectonic plates.

Batholiths give rise to dikes, which are vertical or diagonal segments of plutonic rock that rise through the surrounding strata toward the surface. Dikes also may give rise to horizontal layers of igneous rock, called sills, that form when magma infiltrates layers of rock parallel to the surface.

At times, magma pockets deep within the earth may erupt because of a buildup of volcanic gases, passing rapidly through the mantle and crust to erupt onto the surface. These formations, called volcanic pipes, result in vertical, cone- or carrot-shaped deposits of igneous rock connected to deep subterranean deposits. Volcanic pipes may result from magma pockets that develop hundreds of kilometers beneath the earth’s surface.

Ultrapotassic Igneous Rocks

Ultrapotassic igneous rocks are rocks that contain high levels of potassium in the form of potassium oxide (K₂O). A rock is considered ultrapotassic if it contains more than 3 percent potassium oxide by weight and contains a minimum of two times more potassium oxide than sodium oxide (NA₂O).

In addition, ultrapotassic rocks must contain more than 3 percent magnesium oxide (Mg₂O) by weight. Ultrapotassic rocks are usually mafic or ultramafic in composition and are a relatively rare form of igneous rock.

The formation of potassic and ultrapotassic rocks begins with the enrichment of magma deposits with potassium oxide-rich minerals from the surrounding rock. When magma of this type is subjected to intense pressures, such as those that arise from the collision of continental plates, ultrapotassic rocks may develop. Potassic rock sediments uncovered in Mediterranean and Italian strata appear to have resulted from the collision of continental plates.

In rare occasions ultrapotassic rocks may result from magma that forms above an active subduction zone, an area in which one tectonic plate collides with and is forced under another tectonic plate. Ultrapotassic sediments from the Sunda arc in Indonesia, Papua New Guinea, and Fiji originated from this tectonic environment.

The most uncommon environment for ultrapotassic rock formation is through intraplate magmatism, or magma that forms somewhere in the central portion of a tectonic plate. This type of magmatism is known as a hot spot, where a deep magma pocket pushes through the mantle and crust to create a volcano or a volcanic pipe. Ultrapotassic rock deposits from South Africa and western Uganda developed from intraplate magmatism surrounding the development of a hot spot.

Ultrapotassic rocks are generally divided into three types: 1, 2, and 3. Type 1 ultrapotassic rocks, called lamproites, have relatively low percentages of calcium oxide (CaO), aluminum oxide (Al₂O₂), and sodium oxide (NA₂O₂) coupled with high percentages of potassium and magnesium oxides. Type 2 rocks, called kamafugites, have low levels of silicate and high levels of calcium oxide. Type 3 rocks typically occur in areas in which mountain formation has taken place and usually include high levels of calcium and aluminum oxide.

Lamproites

Lamproites are ultrapotassic rocks in which the ratio of potassium oxide to sodium oxide exceeds 5 percent by weight. Most forms of lamproite have between 6 and 8 percent potassium oxide by weight.

Lamproites generally have high levels of magnesium oxide and are composed of between 45 and 50 percent silica, which categorizes them as mafic igneous rocks. Lamproites have been discovered connected to dikes and sills and also to batholith deposits. One of the most common locations for lamproites is within volcanic pipe deposits.

Lamproites generally develop only when a certain set of specific geoformational processes have taken place. The first stage occurs in the Benioff zone, which is an area of deep seismic activity that occurs beneath a subduction zone. Millions of years later the minerals created by subduction over a Benioff zone are subjected to a process in which two continental plates collide, leading to orogenesis, or mountain formation. In these conditions, lamproites may develop in the deep sediment, near the edges of the continental craton. Generally, lamproites will form only when a portion of the mantle melts at depths greater than 150 kilometers (93 miles) and at temperatures of between 1,100 to 1,500 degrees Celsius (2,012 to 2,732 degrees Fahrenheit).

Lamproites are of particular interest in geology because of the number of rare minerals the rocks contain. As lamproite magma rises to the surface, it may carry with it a variety of xenoliths, which are incompatible minerals or pieces of other rock types that become included in the magma as it hardens. Inclusions in lamproites often include rocks that are found only in deep mantle strata. These mineral types include a variety of iron-rich and titanium-rich minerals. Deposits of aluminum, nickel, quartz, and chromium also are commonly found in lamproites.

Kimberlites are ultrapotassic rocks derived from intraplate magma activity, generally confined to portions of the crust in which the plate overlies old portions of a continental craton. Kimberlites appear to be related to hot spots that develop within the core of a continental craton. Most kimberlite deposits occur within five degrees of a known hot spot, while others that are not close to a recent hot spot may have originated along the moving path of an ancient hot spot.

Kimberlites are volatile rocks because they contain high levels of carbon dioxide (CO₂) and water (H₂O) within their structure, leading them to fracture easily at sea-level pressure and temperature. Kimberlites have low sodium oxide levels and frequently contain a large variety of incompatible elements and minerals incorporated into their matrices.

Kimberlites occur in dikes and sills but are best known from large deposits uncovered from volcanic pipes. Most kimberlite deposits contain inclusions of ultramafic rocks from the upper mantle and may also contain xenoliths and xenocrysts, a type of xenolith consisting of individual crystals embedded within the matrix of another rock type.

Kimberlites are generally divided into two groups, based on the level of mica, a mineral silicate containing aluminum, potassium, magnesium, iron, and silicon. Both types of kimberlite may occur together in the same deposit.

Kimberlites are derived from magma pockets that originated at depths of between 100 and 200 km (62 to 124 mi) beneath the surface and are created by partial melting of mantle rock containing high levels of water and carbon dioxide. Kimberlites are usually formed in situations where intraplate magma rises to the surface rapidly, in less than ten years. Most known kimberlite deposits originated in strata that developed before the Cenozoic period (65 million years ago) indicating that the tectonic and geologic processes involved in the formation of kimberlite may have been more common in the ancient Earth’s lithosphere.

Ultrapotassic Rocks and Diamonds

Ultrapotassic rocks are economically important because of the discovery of diamonds as xenocrysts in deposits of kimberlite and lamproite. Kimberlite is named after the town of Kimberly, in the Northern Cape region of South Africa, where diamonds were first discovered in kimberlite deposits in the 1870s. The first known lamproite diamond deposit was discovered in 1979 in Western Australia.

Lamproite diamonds are often of poor quality in comparison with kimberlite inclusions. However, lamproite deposits sometimes contain rare-colored diamond inclusions and a variety of other precious stones.

Diamonds form within cratons at depths of between 140 and 300 km (87 and 186 mi) beneath the surface, a depth that is known as the diamond stability zone. At deeper or shallower depths, diamonds will not form. Diamonds may therefore be included as xenocrysts in deposits of kimberlite and lamproite that developed from magma that originated within the mantle below or within the diamond stability zone.

Diamonds most frequently occur in kimberlite and lamproite that hardens within volcanic pipes. These explosions of magma and gas quickly rise to the surface, thereby dragging inclusions of various mantle rocks, xenoliths, and xenocrysts, which may include diamonds. Many of the world’s most productive diamond mines are part of ancient kimberlite pipes.

Cratons occur in two general types, based on the age of material contained within the craton. Those composed of mantle rock and crust that are more than 2.5 billion years old are called Archean age cratons, while those arising from mantle rock that is between 1.6 and 2.4 billion years old are called Proterozoic age cratons. Diamonds are usually found in kimberlite and lamproite derived from Archean age cratons containing strata that is 2.4 billion years old or more.

Kimberlite and lamproite pipes, sills, and dikes are derived from magma chambers that develop deep within the center of a craton. These pipes, sills, and dikes, therefore, provide geologists with samples of rocks and minerals from the ancient earth in addition to precious and semiprecious stones. For this reason, kimberlite and lamproite are important from both an economic and a scientific perspective. Minerals and mantle rocks included within kimberlite and lamproite deposits give geologists data regarding the composition and environmental conditions of the ancient Earth at the time when these rocks first formed or became incorporated into the mantle.

Principal Terms

craton: stable portion of the earth’s mantle and associated crust that forms the inner core of the lithospheric components

diamond: mineral formed from carbon atoms organized in a characteristic structure known as a diamond lattice

diamond stability zone: area within the earth’s mantle where pressures and temperatures are sufficient for the formation of diamonds from deposits of carbon

dike: igneous rock formation that develops in a vertical or diagonal direction from the surface of the earth and often connects to a magma chamber within the earth’s crust or mantle

igneous rock: type of rock formed when magma that develops within the earth’s mantle rises to the surface, where it cools and solidifies into solid rock structures

magma: molten rock, gas, and liquid minerals that develop within the earth’s mantle from decompression, molecular excitation, and heat from the earth’s core

volcanic pipe: deposit of igneous rock formed when a deep magma pocket produces an explosion of gas and magma that rapidly rises to the surface and forms a cone-shaped deposit

xenocryst: crystal that differs from the surrounding rock found in igneous rock deposits

xenolith: inclusion of rock in the body of a different type of rock; often occurs in igneous rock formations

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