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. 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
2. 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
3. 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
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. 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
6. Orthopyroxenite – Orthopyroxenite is an ultramafic and ultrabasic rock that is almost exclusively made from the mineral orthopyroxene, the orthorhombic version of pyroxene and a type of pyroxenite. It can have up to a few percent of olivine and clinopyroxene, orthopyroxenites can also occur on other planets. ALH84001 is a Martian meteorite that can be classified as an orthopyroxenite and it is the only meteorite found with that composition and the only member of the Martian orthopyroxenite group of meteorites
7. 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
8. Picrite basalt – Picrite basalt, picrobasalt is a variety of high-magnesium olivine basalt that is very rich in the mineral olivine. It is dark with yellow-green olivine phenocrysts and black to dark brown pyroxene, the compositions of these rocks are well represented by mixes of olivine and more typical tholeiitic basalt. Picrites and komatiites are somewhat similar chemically, but differ in that komatiite lavas are products of more magnesium-rich melts, in contrast, picrites are magnesium-rich because crystals of olivine have accumulated in more normal melts by magmatic processes. Komatiites are largely restricted to the Archean, when the term oceanite was apparently first proposed by Antoine Lacroix, he used the term to apply only to basalts with more than 50% olivine content. Picrite basalt is found in the lavas of Mauna Kea and Mauna Loa in Hawaiʻi, Curaçao, in the Piton de la Fournaise volcano on Réunion Island, Picrite basalt has been erupted in historical times from Mauna Loa during the eruptions of 1852 and 1868. Picrite basalt with 30% olivine commonly erupts from the Piton de la Fournaise, the term was coined by Antoine Lacroix in 1923. Olivine basalt is commonly used by foundries, boilermakers and boiler users to protect the area around a burner tip or to protect a floor from molten metal and its use in this fashion is appropriate since olivine is a highly refractory, high-melting-temperature mineral. ^ Carmichael, Ian S. E. Turner, Francis J. and Verhoogen, John, Igneous Petrology, McGraw-Hill, what is the difference between a komatiite and a picrite. ^ Le Maitre, L. E. ed. Igneous Rocks, A Classification and Glossary of Terms 2nd edition, the 1852 and 1868 Mauna Loa Picrite Eruptions Geophysical Monograph Series, vol. 92, AGU, Abstract retrieved 18 February 2006, ^ Wilkenson, J. F. G. and Hensel, H. D.1988, The petrology of some picrites from Mauna Loa and Kilauea volcanoes, Hawaii, Contrib. Mineralogy and Petrology, v.98, pp. 326–345, ^ Williams, Howel, Francis J. Turner, and Charles M. Gilbert,1954, Petrography W. H. Freeman, pp.40 –41
9. Pyroxenite – Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both types of pyroxenes, closely allied to this group are the hornblendites, consisting essentially of hornblende and other amphiboles. They are essentially of igneous origin, though some pyroxenites are included in the metamorphic Lewisian complex of Scotland and this connection is indicated also by their mode of occurrence, for they usually accompany masses of gabbro and peridotite and seldom are found by themselves. They are often very coarse-grained, containing individual crystals which may be several inches in length, the principal accessory minerals, in addition to olivine and feldspar, are chromite and other spinels, garnet, magnetite, rutile, and scapolite. Pyroxenites can be formed as cumulates in ultramafic intrusions by accumulation of crystals at the base of the magma chamber. Here they are associated with gabbro and anorthite cumulate layers and are typically high up in the intrusion. They may be accompanied by layers, ilmenite layers. Pyroxenites are also found as layers within masses of peridotite and these layers most commonly have been interpreted as products of reaction between ascending magmas and peridotite of the upper mantle. The layers typically are a few centimeters to a meter or so in thickness, pyroxenites that occur as xenoliths in basalt and in kimberlite have been interpreted as fragments of such layers. Although some mantle pyroxenites contain garnet, they are not eclogites, as clinopyroxene in them is less sodic than omphacite, purely pyroxene-bearing volcanic rocks are rare, restricted to spinifex-textured sills, lava tubes and thick flows in the Archaean greenstone belts. This is in similar to the formation of olivine spinifex textures in komatiite lava flows. They frequently occur in the form of dikes or segregations in gabbro and peridotite, in Shetland, Cortland on the Hudson River, North Carolina, Baltimore, New Zealand and they are also found in the Bushveld Igneous Complex in South Africa and Zimbabwe. The pyroxenites are often subject serpentinization under low temperature retrograde metamorphism, under pressure-metamorphism hornblende is developed and various types of amphibolite and hornblende-schist are produced. Sobolev, A. V. and others,2007, The amount of recycled crust in sources of mantle-derived melts, Science 316, media related to Pyroxenite at Wikimedia Commons Flett, John Smith
10. Ultramafic rock – Ultramafic are igneous and meta-igneous rocks with a very low silica content, generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals. The Earths mantle is composed of ultramafic rocks, ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks. Intrusive ultramafic rocks are found in large, layered ultramafic intrusions where differentiated rock types often occur in layers. Such cumulate rock types do not represent the chemistry of the magma from which they crystallized, the ultramafic intrusives include the dunites, peridotites and pyroxenites. Other rare varieties include troctolite which has a percentage of calcic plagioclase. Gabbro and norite often occur in the portions of the layered ultramafic sequences. Hornblendite and, rarely phlogopite, are also found, subvolcanic ultramafic rocks and dykes persist longer, but are also rare. Many of the lavas being produced on Io may be ultramafic, mercury also appears to have ultramafic volcanic rock. Examples include komatiite and picritic basalt, komatiites can be host to ore deposits of nickel. Ultrapotassic, ultramafic rocks such as lamprophyre, lamproite and kimberlite are known to have reached the surface of the Earth. Although no modern eruptions have been observed, analogues are preserved, most of these rocks occur as dikes, diatremes, lopoliths or laccoliths, and very rarely, intrusions. Most kimberlite and lampproite occurrences occur as volcanic and subvolcanic diatremes and maars, vents of Proterozoic lamproite, and Cenozoic lamproite are known, as are vents of Devonian lamprophyre. Kimberlite pipes in Canada, Russia and South Africa have incompletely preserved tephra and these are generally diatreme events and as such are not lava flows although tephra and ash deposits are partially preserved. These represent low-volume volatile melts and attain their ultramafic chemistry via a different process to typical ultramafic rocks, metamorphism of ultramafic rocks in the presence of water and/or carbon dioxide results in two main classes of metamorphic ultramafic rock, talc carbonate and serpentinite. When such metamorphic fluids have less than 10% molar proportion of CO2, reactions favor serpentinisation, the majority of ultramafic rocks are exposed in orogenic belts, and predominate in Archaean and Proterozoic terranes. Ultramafic magmas in the Phanerozoic are rarer, and there are very few recognised true ultramafic lavas in the Phanerozoic, many surface exposures of ultramafic rocks occur in ophiolite complexes where deep mantle-derived rocks have been obducted onto continental crust along and above subduction zones. Serpentine soil is a rich, calcium, potassium and phosphorus poor soil that develops on the regolith derived from ultramafic rocks. Ultramafic rocks also contain elevated amounts of chromium and nickel which may be toxic to plants, as a result, a distinctive type of vegetation develops on these soils
11. Wehrlite – Wehrlite is an ultramafic and ultrabasic rock that is a mixture of olivine and clinopyroxene. It is a subdivision of the peridotites, the nomenclature allows up to a few percent of orthopyroxene. Accessory minerals include ilmenite, chromite, magnetite and an aluminium phase, wehrlites occur as mantle xenoliths and in ophiolites. Another occurrence is as cumulate in gabbro and norite layered intrusions, some meteorites can also be classified as wehrlites. Wehrlite is named after Alois Wehrle and he was born 1791 in Kroměříž, Czech Republic and was a professor at the Ungarische Bergakademie in Banská Štiavnica, Slovakia