Pages in category "Ultramafic rocks"
The following 13 pages are in this category, out of 13 total. This list may not reflect recent changes (learn more).
The following 13 pages are in this category, out of 13 total. This list may not reflect recent changes (learn more).
1. Komatiite – Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content, komatiite was named for its type locality along the Komati River in South Africa. True komatiites are very rare and essentially restricted to rocks of Archean age and this restriction in age is thought to be due to cooling of the mantle, which may have been up to 500 °C hotter during the early to middle Archaean. The early Earth had much higher heat production, due to the heat from planetary accretion. Geographically, komatiites are restricted in distribution to the Archaean shield areas, Komatiites occur with other ultramafic and high-magnesian mafic volcanic rocks in Archaean greenstone belts. The youngest komatiites are from the island of Gorgona on the Caribbean oceanic plateau off the Pacific coast of Colombia, magmas of komatiitic compositions have a very high melting point, with calculated eruption temperatures in excess of 1600 °C. Basaltic lavas normally have eruption temperatures of about 1100 to 1250 °C, the higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archean Earth. Komatiitic lava was extremely fluid when it erupted, the major komatiitic sequences preserved in Archaean rocks are thus considered to be lava tubes, ponds of lava etc. where the komatiitic lava accumulated. Komatiite chemistry is different from that of basaltic and other common mantle-produced magmas, Komatiites are considered to have been formed by high degrees of partial melting, usually greater than 50%, and hence have high MgO with low K2O and other incompatible elements. There are two classes of komatiite, aluminium undepleted komatiite and aluminium depleted komatiite, defined by their Al2O3/TiO2 ratios. These two classes of komatiite are often assumed to represent a real petrological source difference between the two related to depth of melt generation. Komatiites probably form in extremely hot mantle plumes, boninite magmatism is similar to komatiite magmatism but is produced by fluid-fluxed melting above a subduction zone. Boninites with 10–18% MgO tend to have higher large-ion lithophile elements than komatiites, the pristine volcanic mineralogy of komatiites is composed of forsteritic olivine, calcic and often chromian pyroxene, anorthite and chromite. A considerable population of komatiite examples show a cumulate texture and morphology, the usual cumulate mineralogy is highly magnesium rich forsterite olivine, though chromian pyroxene cumulates are also possible. Volcanic rocks rich in magnesium may be produced by accumulation of olivine phenocrysts in basalt melts of normal chemistry, the often rarely preserved flow top breccia and pillow margin zones in some komatiite flows are essentially volcanic glass, quenched in contact with overlying water or air. Because they are cooled, they represent the liquid composition of the komatiites. The spinifex texture is named after an Australian grass that grows in clumps with similar shapes, primary mineral species also encountered in komatiites include olivine, the pyroxenes augite, pigeonite and bronzite, plagioclase, chromite, ilmenite and rarely pargasitic amphibole. Secondary minerals include serpentine, chlorite, amphibole, sodic plagioclase, quartz, iron oxides and rarely phlogopite, baddeleyite, all known komatites have been metamorphosed, therefore should technically be termed metakomatiite though the prefix meta is inevitably assumed
2. Lherzolite – Lherzolite is a type of ultramafic igneous rock. It is a rock consisting of 40 to 90% olivine along with significant orthopyroxene. Minor minerals include chromium and aluminium spinels and garnets, plagioclase can occur in lherzolites and other peridotites that crystallize at relatively shallow depths. At greater depth plagioclase is unstable and is replaced by spinel, at approximately 90 km depth, pyrope garnet becomes the stable aluminous phase. Garnet lherzolite is a constituent of the Earths upper mantle. Partial melting of spinel lherzolite is one of the sources of basaltic magma. The name is derived from the Lherz Massif, an alpine peridotite complex, at Étang de Lers, near Massat in the French Pyrenees, the Lherz massif also contains harzburgite and dunite, as well as layers of spinel pyroxenite, garnet pyroxenite, and hornblendite. The layers represent partial melts extracted from the host peridotite during decompression in the mantle long before emplacement into the crust, the Lherz massif is unique because it has been emplaced into Paleozoic carbonates, which form mixed breccias of limestone-lherzolite around the margins of the massif. The Moons lower mantle is said to be composed of lherzolite, blatt, Harvey and Robert J. Tracy,1996, Petrology, Igneous, Sedimentary and Metamorphic, 2nd ed. Freeman, ISBN 0-7167-2438-3
3. Peridotite – Peridotite is a dense, coarse-grained igneous rock consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica and it is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from the Earths mantle, either as solid blocks and fragments, the compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole. Peridotite is the dominant rock of the part of the Earths mantle. The word peridotite comes from the gemstone peridot, which consists of pale green olivine, classic peridotite is bright green with some specks of black, although most hand samples tend to be darker green. Peridotitic outcrops typically range from bright yellow to dark green in color. While green and yellow are the most common colors, peridotitic rocks may exhibit a range of colors such as blue, brown. Dunite, more than 90% olivine, typically with Mg/Fe ratio of about 9,1, wehrlite, mostly composed of olivine plus clinopyroxene. Harzburgite, mostly composed of olivine plus orthopyroxene, and relatively low proportions of basaltic ingredients, lherzolite, most common form of peridotite, mostly composed of olivine, orthopyroxene, and clinopyroxene, and have relatively high proportions of basaltic ingredients. Partial fusion of lherzolite and extraction of the melt fraction can leave a residue of harzburgite. Magnesium-rich olivine forms a proportion of peridotite, and so magnesium content is high. Layered igneous complexes have more varied compositions, depending on the fractions of pyroxenes, chromite, plagioclase. Minor minerals and mineral groups in peridotite include plagioclase, spinel, garnet, amphibole, in peridotite, plagioclase is stable at relatively low pressures, aluminous spinel at higher pressures, and garnet at yet higher pressures. Peridotite is the dominant rock of the Earths mantle above a depth of about 400 km, below that depth, olivine is converted to the higher-pressure mineral wadsleyite. Oceanic plates consist of up to about 100 km of peridotite covered by a thin crust, the crust, commonly about 6 km thick, consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, abyssal peridotite, is found on the walls of rifts in the sea floor. Oceanic plates are usually subducted back into the mantle in subduction zones, peridotites also occur as fragments carried up by magmas from the mantle. Among the rocks that commonly include peridotite xenoliths are basalt and kimberlite, certain volcanic rocks, sometimes called komatiites, are so rich in olivine and pyroxene that they also can be termed peridotite
4. Lamprophyre – Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide and high nickel, lamprophyres occur throughout all geologic eras. Archaean examples are commonly associated with gold deposits. Cenozoic examples include magnesian rocks in Mexico and South America, modern science treats lamprophyres as a catch-all term for ultrapotassic mafic igneous rocks which have primary mineralogy consisting of amphibole or biotite, and with feldspar in the groundmass. They are classified under the IUGS Nomenclature for Igneous Rocks separately, for example, the TAS scheme is inappropriate due to the control of mineralogy by potassium, not by calcium or sodium. Classification schemes which include information, may be required to properly describe lamprophyres. Rock considered lamprophyres are part of a clan of rocks, with similar mineralogy, textures, lamprophyres are similar to lamproites and kimberlites. While modern concepts see orangeites, lamproites and kimberlites as separate, Mitchell considered the lamprophyres as a facies of igneous rocks created by a set of conditions. Either scheme may apply to some, but not all, occurrences and variations of the group of rocks known as lamprophyres. Rock considered lamprophyres to be derived from deep, volatile-driven melting in a subduction zone setting, others such as Mitchell consider them to be late offshoots of plutons, etc. though this can be difficult to reconcile with their primitive melt chemistry and mineralogy. Lamprophyres are a group of rocks containing phenocrysts, usually of biotite and amphibole, and pyroxene and they are thus distinguished from the porphyries and porphyrites in which the feldspar has crystallized in two generations. They are essentially dike rocks, occurring as dikes and thin sills and they are usually dark in color, owing to the abundance of ferro-magnesian silicates, of high specific gravity and liable to decomposition. For these reasons they have defined as a melanocrate series. Biotite and amphibole are panidiomorphic, all are euhedral, well formed, feldspar is restricted to the ground mass. In many lamprophyres the pale quartz and felspathic ingredients tend to occur in rounded spots, or ocelli and these spots may consist of radiate or brush-like feldspars or of quartz and feldspar. A central area of quartz or of analcite probably represents an original miarolitic cavity infilled at a later period, the presence or absence of the four dominant minerals, orthoclase, plagioclase, biotite and hornblende, determines the species, Minette contains biotite and orthoclase. Each variety of lamprophyre may and often contain all four minerals but is named according to the two which predominate. These rocks contain iron oxides, apatite, sometimes sphene, augite
5. Kimberlite – Kimberlite is an igneous rock best known for sometimes containing diamonds. Kimberlite occurs in the Earths crust in vertical structures known as kimberlite pipes as well as igneous dykes, Kimberlite also occurs as horizontal sills. Kimberlite pipes are the most important source of mined diamonds today, the consensus on kimberlites is that they are formed deep within the mantle. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts, despite its relative rarity, kimberlite has attracted attention because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earths surface. Many kimberlite structures are emplaced as carrot-shaped, vertical intrusions termed pipes, Kimberlite classification is based on the recognition of differing rock facies. These differing facies are associated with a style of magmatic activity, namely crater, diatreme. The morphology of kimberlite pipes, and their classical carrot shape is the result of explosive volcanism from very deep mantle-derived sources. These volcanic explosions produce vertical columns of rock rise from deep magma reservoirs. The morphology of kimberlite pipes is varied but includes a sheeted dyke complex of tabular, within 1. 5–2 km of the surface, the highly pressured magma explodes upwards and expands to form a conical to cylindrical diatreme, which erupts to the surface. The surface expression is rarely preserved but is similar to a maar volcano. The diameter of a pipe at the surface is typically a few hundred meters to a kilometer. Two Jurassic kimberlite dikes exist in Pennsylvania, one, the Gates-Adah Dike, outcrops on the Monongahela River on the border of Fayette and Greene Counties. The other, the Dixonville-Tanoma Dike in central Indiana County, does not outcrop at the surface and was discovered by miners, similarly aged kimberlite is found in several locations in New York Both the location and origin of kimberlitic magmas are subjects of contention. The mechanism of enrichment has also been the topic of interest with models including partial melting, historically, kimberlites have been classified into two distinct varieties termed basaltic and micaceous based primarily on petrographic observations. This was later revised by Smith who renamed these divisions Group I and Group II based on the affinities of these rocks using the Nd, Sr. Mitchell later proposed that these group I and II kimberlites display such distinct differences and he showed that Group II kimberlites show closer affinities to lamproites than they do to Group I kimberlites. Hence, he reclassified Group II kimberlites as orangeites to prevent confusion, olivine lamproites were previously called Group II kimberlite or orangeite in response to the mistaken belief that they only occurred in South Africa. Their occurrence and petrology, however, are identical globally and should not be referred to as kimberlite
6. Dunite – Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with amounts of other minerals such as pyroxene, chromite, magnetite. Dunite is the olivine-rich end-member of the group of mantle-derived rocks. Dunite and other rocks are considered the major constituents of the Earths mantle above a depth of about 400 kilometers. It is also found in alpine peridotite massifs that represent slivers of sub-continental mantle exposed during collisional orogeny, Dunite typically undergoes retrograde metamorphism in near-surface environments and is altered to serpentinite and soapstone. Dunite may also form by the accumulation of crystals on the floor of large basaltic or picritic magma chambers. These cumulate dunites typically occur in layers in layered intrusions, associated with cumulate layers of wehrlite, olivine pyroxenite, harzburgite. Small layered intrusions may be of any age, for example, the Triassic Palisades Sill in New York. The largest layered mafic intrusions are tens of kilometers in size and almost all are Proterozoic in age, e. g. the Stillwater igneous complex, the Muskox intrusion, and the Great Dyke. Cumulate dunite may also be found in complexes, associated with layers of wehrlite, pyroxenite. Dunite was named by the German geologist, Ferdinand von Hochstetter in 1859 after Dun Mountain near Nelson, Dun Mountain was given its name because of the dun colour of the underlying ultramafic rocks. This color results from surface weathering that oxidizes the iron in olivine in temperate climates, a massive exposure of dunite in the United States can be found as Twin Sisters Mountain, near Mount Baker in the northern Cascade Range of Washington. In southern British Columbia, Canada dunite rocks form the core of an ultramafic rock complex located near the community of Tulameen. The rocks are enriched in platinum group metals, chromite and magnetite. Dunite could be used to sequester CO2 and help mitigate climate change via accelerated chemical rock weathering. This would involve the mining of dunite rocks in quarries followed by crushing and grinding as to create fine ground rock that would react with the carbon dioxide. The resulting products are magnesite and silica which could be commercialized. Mg 2 SiO4 +2 CO2 ⟶2 MgCO3 + SiO2 Dunite Blatt, Harvey and Robert J. Tracy,1996, Petrology, 2nd ed. W. H. Freeman, ISBN 0-7167-2438-3