Rochas Ultramáficas

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<p>Enc</p><p>Ultramafic Rocks</p><p>Hilary Downes, Department of Earth and Planetary Sciences, Birkbeck University of London, London, United Kingdom</p><p>© 2020 Elsevier Inc. All rights reserved.</p><p>Introduction 1</p><p>Ultramafic Rocks Forming the Mantle 1</p><p>Ultramafic Rocks Formed by Cumulate Processes 5</p><p>Ultramafic Meteorites 6</p><p>Altered Ultramafic Rocks 6</p><p>Other Ultramafic Rocks 6</p><p>Further Reading 6</p><p>Glossary</p><p>Dunite A coarse-grained rock composed almost entirely of olivine.</p><p>Eclogite A high grade metamorphic ultramafic rock composed mainly of Na-rich clinopyroxene and garnet, with a chemical</p><p>composition similar to basalt.</p><p>Glimmerite A coarse-grained rock composed almost entirely of dark mica (phlogopite or biotite).</p><p>Harzburgite A coarse-grained peridotite consisting of >40% olivine, the remainder being mostly orthopyroxene, and with</p><p>40% olivine, the remainder being orthopyroxene and</p><p>clinopyroxene (> 5% of each).</p><p>Mafic A mnemonic term for minerals rich in magnesium and iron. Such minerals are also often described as ferromagnesian.</p><p>Ophiolite A complex of different rock-types, which can include mantle peridotites and cumulus ultramafic rocks, generally</p><p>considered to be a tectonically emplaced fragment of oceanic lithosphere.</p><p>Peridotite A general term for a coarse-grained rock consisting of >40% olivine, with variable amounts of orthopyroxene and</p><p>clinopyroxene.</p><p>Pyroxenite A general term for a coarse-grained ultramafic rock consisting of >60% pyroxene.</p><p>Websterite A coarse-grained pyroxenite rock consisting of orthopyroxene and clinopyroxene in variable proportions.</p><p>Wehrlite A coarse-grained peridotite rock consisting mostly of olivine and clinopyroxene.</p><p>Introduction</p><p>Ultramafic rocks are usually defined by their modal mineralogy. They have a color index greater than 90, where the term “color</p><p>index” refers to the percentage of mafic minerals such as olivine and pyroxene present in the rock. The term “ultrabasic” is applied to</p><p>igneous rocks which contain 45 wt% SiO2. Many pyroxenites are also ultramafic,</p><p>being composed mostly of orthopyroxene and/or clinopyroxene, but these rocks are also not ultrabasic.</p><p>The range of ultramafic rocks is very wide because they are formed in several different ways. This article will cover ultramafic</p><p>rocks that form part of the mantle and, to a lesser extent, those that occur as layered ultramafic rocks formed by cumulus processes</p><p>from magmas in the oceanic and continental crust. Ultramafic rocks are also found as different types of meteorites which tell us</p><p>about processes early in Earth history and on other planets and asteroids.</p><p>Ultramafic Rocks Forming the Mantle</p><p>Most coarse-grained ultramafic rocks are associated with the Earth’s mantle and by analogy therefore with the mantles of other</p><p>terrestrial planets. The Earth’s mantle was formed during the phase of planetary differentiation when the primitive Earth separated</p><p>into an iron-rich core and an ultramafic mantle. The upper mantle consists largely of peridotite which is the most abundant rock</p><p>type down to the transition zone at 400 km depth. At this depth, orthorhombic olivine transforms to the spinel-structured (cubic)</p><p>mineral wadsleyite, which is denser than olivine despite having the same chemical composition. Other ultramafic rocks found in the</p><p>upper mantle include pyroxenites (which have various origins, both magmatic and metamorphic, and which mostly occur in</p><p>yclopedia of Geology, 2nd edition https://doi.org/10.1016/B978-0-12-409548-9.12478-9 1</p><p>https://doi.org/10.1016/B978-0-12-409548-9.12478-9</p><p>2 Ultramafic Rocks</p><p>layers), and a variety of minor rock types such as hornblendites and glimmerites (rocks dominated by micas such as biotite or</p><p>phlogopite) which often occur as veins.</p><p>The major minerals commonly found in ultramafic rocks are olivine (forsterite Mg2SiO4), orthopyroxene (enstatite Mg2Si2O6)</p><p>and clinopyroxene (diopside (Ca, Mg)2Si2O6); hence the two most common ultramafic rock types in the mantle are peridotites and</p><p>pyroxenites (Fig. 1). Accessory minerals include plagioclase, spinel or garnet in peridotites, with garnet, spinel, biotite and</p><p>amphibole being found in pyroxenites. Hornblendites (with more than 50% igneous amphibole) and glimmerites (more than</p><p>50% phlogopite or biotite mica) are also ultramafic mantle rocks but are much rarer than peridotites and pyroxenites.</p><p>The Earth’s mantle where most ultramafic rocks exist is almost inaccessible to geologists. Some outcrops of mantle rocks occur at</p><p>the Earth’s surface at slow-spreading oceanic ridges; e.g. Zabargad (St John’s island) in the Red Sea. These are also known as “abyssal</p><p>peridotites” or “oceanic core complexes.” Other ultramafic massifs are slices of the sub-continental upper mantle which have been</p><p>tectonically emplaced onto the continents (examples include Lherz in the French Pyrenees, Ronda in the Betics of Spain, and</p><p>Balmuccia in the Italian Alps). They are also known as “Alpine peridotites.” These outcrops reveal the complexity of the sub-</p><p>continental upper mantle (Fig. 2), including black and green pyroxenite layering, cross-cutting hornblendite dykes, and areas of</p><p>increased concentrations of clinopyroxene, amphibole and biotite. Some outcrops reveal folding of pyroxenite layers. Tectonic slices</p><p>of the oceanic crust occur as ophiolites and include sections of the sub-oceanic mantle and ultramafic cumulates (e.g. the Troodos</p><p>ophiolite in Cyprus and the Semail ophiolite in Oman). The mantle rocks are usually highly depleted harzburgites (Fig. 1) which</p><p>tend not to show the same variety of additional ultramafic rock-types as sub-continental mantle outcrops.</p><p>Ultramafic xenoliths are commonly found in primitive alkali basalts and are pieces of the Earth’s upper mantle that have reached</p><p>the surface of the Earth in a rapid eruption. Such xenoliths range from as small as a few crystals (or even single xenocrysts), to several</p><p>tens of centimeters in diameter. They are usually spinel peridotite or, more rarely, spinel or garnet pyroxenite. Such mantle xenoliths</p><p>(Fig. 3) are well known from many intraplate volcanic regions. However, in analyzing mantle xenoliths, care must be taken to</p><p>consider whether interaction with the host basalt has taken place during eruption and cooling.</p><p>The mineralogical composition of upper mantle spinel peridotites depends on the extent to which magma has previously been</p><p>extracted from themantle. The least depleted spinel peridotites would contain�19% clinopyroxene, but this value rapidly decreases</p><p>with increasing degree of magma extraction, and most spinel peridotite xenoliths contain no more than 11% clinopyroxene.</p><p>Kimberlites also frequently carry ultramafic xenoliths including both spinel peridotite and garnet peridotite. Kimberlite magmas</p><p>are sourced from deeper in the mantle than alkali basalts and so bring up xenoliths from depths of up to 200 km. Undepleted garnet</p><p>peridotites should contain around 13% garnet, but this proportion decreases with increasing depletion of the mantle and can be as</p><p>little as 2–3% in garnet harzburgites. Some garnet peridotites which have low equilibration temperatures and granular textures are</p><p>derived from the lithospheric part of the upper mantle, whereas others with higher equilibration temperatures and more sheared</p><p>textures are thought to be derived from the deeper convecting asthenosphere. In addition to peridotite xenoliths, kimberlite</p><p>eruptions have brought a wider variety of mantle-derived rocks to the surface, including eclogites which may be remnants of</p><p>subducted oceanic crust, and mica-amphibole-rutile-ilmenite-diopside</p><p>(MARID) rocks and glimmerites (mica-dominated rocks</p><p>with some diopside and ilmenite) which are both derived from highly metasomatized regions of the sub-continental mantle</p><p>beneath ancient continental crust (cratons). The deepest “super-deep” xenoliths are considered to have come from depths of</p><p>>300 km. Samples from even deeper parts of the mantle are sometimes found as mineral inclusions within diamonds.</p><p>Peridotites in the upper mantle occur in three facies depending on the pressure under which they have equilibrated: plagioclase</p><p>peridotite is the shallowest facies, spinel peridotite occurs at higher pressures, and garnet peridotite is the deepest, extending down</p><p>to the transition zone (Fig. 4). Textures in mantle peridotites are complex but are most commonly coarse-grained and granular.</p><p>Some are much finer-grained, foliated, and even mylonitic in appearance. Some rare spinel peridotites contain clusters of</p><p>vermicular-textured spinel with pyroxene; these “spinel-pyroxene clusters” are most probably the result of a retrogression from</p><p>olivine</p><p>orthopyroxene clinopyroxene</p><p>peridotites</p><p>pyroxenites</p><p>lherzolite</p><p>olivine websterite</p><p>harzburgite</p><p>olivine</p><p>orthopyroxenite</p><p>websterite</p><p>wehrlite</p><p>olivine</p><p>clinopyroxenite</p><p>40</p><p>9090</p><p>40</p><p>10 10</p><p>dunite</p><p>orthopyroxenite clinopyroxenite</p><p>M = 90 to 100</p><p>Fig. 1 Triangular diagram showing nomenclature for coarse-grained ultramafic igneous rocks with color index M > 90. Reproduced from Streckeisen A (1976) To</p><p>each plutonic rock its proper name. Earth-Science Reviews 12: 1–33.</p><p>Fig. 2 Outcrops of ultramafic rocks showing field relationships: (A) folded pyroxenite layering in the Lherz massif; (B) hornblendite veins cutting peridotite in the</p><p>Lherz massif; (C) green pyroxenite veining in the Balmuccia massif; (D) black pyroxenite veining in the Balmuccia massif.</p><p>Fig. 3 Mantle xenoliths in alkali basaltic pyroclastic deposits (A, B) and an alkali basalt dyke (C): (A) Maar de Borée; (B) Ray Pic; (C) Guiraud’s dyke. All localities in</p><p>Massif Central, France.</p><p>Ultramafic Rocks 3</p><p>garnet peridotite, probably by decompression. A few mantle xenoliths contain both spinel and garnet so they must have been</p><p>derived from close to the garnet-peridotite-spinel peridotite boundary (Fig. 4).</p><p>Ultramafic rocks from the mantle show a commonmineralogical trend from lherzolite to harzburgite (Table 1), which is usually</p><p>considered to be a result of partial melting and the removal of basaltic magma from the mantle. This results in increasing MgO</p><p>content and decreasing CaO and Al2O3 contents in their chemical compositions (Fig. 5), because basalt magmas have lower MgO</p><p>contents and higher CaO and Al2O3 thanmantle rocks. Parts of the sub-oceanic mantle can be ultra-depleted harzburgites as a result</p><p>of extensive partial melting of an already depleted mantle. Mantle rocks from which magma has been removed are often called</p><p>“depleted mantle.” The term “fertile mantle” is commonly given to mantle samples which have the highest concentrations of CaO</p><p>and Al2O3. The amount of melting can be modeled from fertile mantle or a theoretical “undepleted mantle,”which is considered to</p><p>have the composition of the Bulk Silicate Earth and was formed during the early evolution of the planet. No remnant of the original</p><p>“undepleted mantle” has been identified but its composition can be calculated from the composition of chondritic meteorites.</p><p>Depletion of the mantle is also reflected in the compositions of constituent minerals. Olivines in undepleted or fertile mantle</p><p>usually have Mg# of 89 (where Mg# ¼ 100 Mg/(Mg + Fe) as atomic proportions in the mineral composition), whereas those in</p><p>depleted mantle can reach Mg# ¼ 92 (i.e. an increase in the MgO relative to FeO content of the mineral). The Mg#s of</p><p>10</p><p>20</p><p>30</p><p>40</p><p>50</p><p>60</p><p>70</p><p>80</p><p>90</p><p>D</p><p>ep</p><p>th</p><p>in</p><p>k</p><p>ilo</p><p>m</p><p>et</p><p>re</p><p>s</p><p>P</p><p>re</p><p>ss</p><p>ur</p><p>e</p><p>in</p><p>k</p><p>ilo</p><p>b</p><p>ar</p><p>s</p><p>10</p><p>20</p><p>30</p><p>15</p><p>25</p><p>5</p><p>1000 1100 1200 1300 1400 1500 1600</p><p>Temperature, �C</p><p>garnet peridotite</p><p>spinel peridotite</p><p>plagioclase peridotite</p><p>crystals</p><p>+</p><p>liquid</p><p>so</p><p>lid</p><p>us</p><p>Fig. 4 Different mineralogical facies of the upper mantle depend mainly on pressure of equilibration (i.e. depth within the mantle). After Yoder HS (1976).</p><p>Generation of Basaltic Magma. National Academy of Sciences; 265 pp.</p><p>Table 1 Mantle peridotite bulk rock compositions.</p><p>1 2 3 4 5 6 7 8 9 10 11</p><p>SiO2 45.00 45.05 44.16 43.80 46.6 45.9 44.3 42.2 44.2 42.6 45.5</p><p>TiO2 0.20 0.13 0.09 0.08 0.06 0.05 0.04 0.09 0.04 0.00 0.16</p><p>Al2O3 4.45 2.89 2.25 0.56 1.4 1.2 1.0 0.6 1.9 1.8 3.8</p><p>Cr2O3 0.38 0.37 0.39 0.38 0.35 0.27 0.37 0.37 0.44 0.44 0.44</p><p>FeO 8.05 8.05 8.14 7.71 6.6 6.4 7.6 7.4 7.6 8.4 8.2</p><p>MnO 0.13 0.13 0.14 0.12 0.11 0.09 0.13 0.10 0.13 0.13 0.14</p><p>MgO 37.80 40.25 41.05 46.30 43.5 45.2 45.2 47.8 43.5 44.7 38.1</p><p>NiO 0.25 0.26 0.27 0.35 0.28 0.27 0.29 0.31 0.29 0.26 0.25</p><p>CaO 3.55 2.51 2.27 0.75 1.0 0.5 1.0 1.0 1.6 1.4 3.3</p><p>Na2O 0.36 0.20 0.21 0.07 0.10 0.09 0.07 0.07 0.05 0.06 0.25</p><p>Data for 1–4 from McDonough and Rudnick (1998): 1 ¼ primitive mantle; 2 ¼ lherzolite massifs; 3 ¼ off-craton lherzolite xenoliths; 4 ¼ on-craton harzburgite xenoliths; data for</p><p>5–11 from Griffin et al. (2009): 5 ¼ median Kaapvaal (S. Africa) lherzolite xenoliths; 6 ¼ median Kaapvaal harzburgite xenoliths; 7 ¼ median Daldyn (Russia) lherzolite xenoliths;</p><p>8 ¼ median Daldyn (Russia) harzburgite xenoliths; 9 ¼ median E Australia lherzolite xenoliths; 10 ¼ median Obnazhennaya (N. Russia) lherzolite xenoliths; 11 ¼ median E. China</p><p>lherzolite xenoliths.</p><p>0</p><p>1</p><p>2</p><p>3</p><p>4</p><p>0 1 2 3 4 5</p><p>C</p><p>aO</p><p>(w</p><p>t.</p><p>%</p><p>)</p><p>Al2O3 (wt.%)</p><p>Bulk silicate Earth</p><p>re-</p><p>en</p><p>ric</p><p>hmen</p><p>t</p><p>dep</p><p>let</p><p>ion</p><p>Fig. 5 Chemical variation diagram for wt% CaO and Al2O3 in a variety of ultramafic mantle rock compositions (data points from Table 1). Star indicates composition</p><p>of undepleted mantle derived from studies of chondritic meteorites. Arrows shows the effect of partial melting on mantle composition (reducing CaO and Al2O3) and</p><p>the effect of re-fertilization (increasing CaO and Al2O3).</p><p>4 Ultramafic Rocks</p><p>Ultramafic Rocks 5</p><p>orthopyroxene and clinopyroxene behave in a similar way. Spinels, with the general formula (Mg, Fe)(Al, Cr)2O4, show a decrease</p><p>in Al2O3 content and an increase in Cr2O3 content such that their Cr#s (Cr# ¼ 100 Cr/(Cr + Al)) increase from �20 to �80 as the</p><p>extent of mantle depletion increases. It is probably the case that lithospheric mantle formed in Archean times, which often has very</p><p>high olivine Mg#s of �93, resulted from extreme depletion, possibly by extraction of komatiite magmas. Multiple episodes of</p><p>partial melting through time also create extreme depletion of the mantle.</p><p>Subsequent to mantle depletion by partial melting, passage of magmas or fluids can enrich the mantle in a variety of processes</p><p>generally described as “metasomatism.” These metasomatizing agents can include tholeiitic and/or alkali basalts, subduction-</p><p>related water-rich fluids, and even carbonatite magmas. Each metasomatic agent is considered to have a different effect on the</p><p>depleted mantle; for example carbonatite melt can convert mantle orthopyroxene into clinopyroxene, producing wehrlites (Fig. 1).</p><p>Different styles of metasomatism have been recognized, including: (a) addition of new minerals such as amphibole and phlogopite</p><p>to the pre-existing mantle rocks (generally called “modal”metasomatism), (b) a more cryptic variety in which the compositions of</p><p>the pre-existing minerals are modified (such as becoming enriched in trace elements such as the light rare earth elements, and (c) as</p><p>“stealth” metasomatism in which the added minerals are compositionally similar to the pre-existing minerals and it is only the</p><p>unusual abundance which may be a clue to the effects of metasomatism. Such metasomatism may turn a harzburgite back into a</p><p>lherzolite and is also referred to as a process of “refertilization” (Fig. 5). Some rare mantle xenoliths have an excess of orthopyroxene</p><p>in them, which may be a result of interaction of peridotite with Si-rich hydrous fluids.</p><p>Fluid inclusions in minerals from mantle peridotite xenoliths reveal</p><p>that the Earth’s upper mantle contains volatile species,</p><p>particularly CO2 and, particularly above subduction zones, H2O. Other volatiles present in much smaller amounts include N2, CO,</p><p>SO2 and noble gases such as He. Minor minerals such as carbonates (dolomite) and phosphates (apatite) can also be found</p><p>occasionally in mantle xenoliths, as well as occasional zircon which can be used to date mantle metasomatism. Evidence for</p><p>compositionally unusual mantle-derived fluids such as fluoride-rich melts can sometimes be found in mantle xenoliths.</p><p>The shallow sub-continental lithospheric mantle is thought to be of similar age to the overlying continental crust (e.g. the mantle</p><p>beneath Archean cratons is generally Archean in age) and therefore it has experienced many of the same magmatic and tectonic</p><p>events as the overlying crust. Only rarely does the lithospheric mantle delaminate and new material from the convecting</p><p>asthenospheric mantle upwells to take its place. Therefore, the record of magmatic and tectonic processes is most complete in the</p><p>old subcontinental mantle, reflected in the widespread heterogeneity of the subcontinental mantle which displays pyroxenite</p><p>layering and metasomatic veining (Fig. 2), whereas the much younger sub-oceanic lithospheric mantle displayed in abyssal</p><p>peridotites or ophiolites generally shows the least effect of metasomatism.</p><p>Pyroxenites usually form a few per cent of mantle rocks in outcrop, although parts of the Cabo Ortegal complex in NW Spain are</p><p>strongly enriched in pyroxenite layers. Pyroxenites can have several different origins, including being cumulates from magmas that</p><p>have traversed the lithospheric upper mantle, interaction between such magmas and the host peridotite, incipient melting of the</p><p>mantle peridotite with segregation of melts, and metamorphic (solid state) segregation. In some ultramafic massifs such as Ronda</p><p>(SE Spain) and Beni Bousera (northern Morocco), garnet pyroxenites are related to recycled oceanic crust or magmas derived from</p><p>melting of such crust. They can contain graphite pseudomorphs after diamond, showing that they have been subducted to high</p><p>pressure in the mantle, during which time graphite converted to diamond, prior to slow exhumation to the surface of the Earth</p><p>when the diamond turned back to graphite.</p><p>Ultramafic Rocks Formed by Cumulate Processes</p><p>Ultramafic rocks are also found in association with layered intrusions such as the Bushveld intrusion in Southern Africa, where they</p><p>occur as dunites, peridotites and pyroxenites near to the base of the layered sequences. These are cumulate rocks that formed from</p><p>magmas, but they do not represent the chemical composition of their parental magma. They are often associated with ore deposits</p><p>of chromite and magnetite. Their original textures are clearly magmatic but can be modified by sub-solidus cooling. The layered</p><p>suite in Rum (Inner Hebrides, Scotland) not only contains layers of peridotite, but also tongues and plugs of peridotite which may</p><p>have been emplaced as olivine-rich crystal mushes. Very rarely, such as in Bute (Scotland), ultramafic xenoliths brought to the</p><p>surface in eruptions of alkali basalt magma can show poikilitic (cumulus) textures indicating that they were derived from an</p><p>unexposed layered ultramafic intrusion at depth in the crust.</p><p>Ultramafic cumulates also occur in the deeper parts of the oceanic crust as “layered peridotites,” formed by similar processes as</p><p>those that form the stratiform peridotites in intrusions in the continental crust. Such cumulus rocks can often be recognized in</p><p>ophiolite complexes. They are formed by separation of olivine from the tholeiitic picritic magma that enters the magma reservoir</p><p>beneath the mid-ocean ridge and undergoes fractional crystallization. These cumulate rocks form part of the oceanic crust and are</p><p>separated from the underlying tectonite peridotites of the oceanic upper mantle by a “petrological Moho.”</p><p>A group of poorly understood ultramafic intrusive complexes first described from Alaska often show concentric zoning in</p><p>outcrop and clear cumulus textures in hand specimen or thin-section, but without showing the mineral layering associated with</p><p>layered intrusions. These “Alaskan-type ultramafic complexes”may be related to intrusion of hydrous magma, as they often contain</p><p>abundant hornblende (a mineral with OH− in its structure). They are also much smaller (1–2 km diameter) than the large layered</p><p>plutons such as Bushveld. They occur not only in Alaska but also in the Urals, the Andes and eastern Australia, all regions associated</p><p>with subduction- and collision-related magmatism, and it has been suggested that they may represent cumulates from such</p><p>magmas.</p><p>6 Ultramafic Rocks</p><p>Cumulus ultramafic rocks are also associated with alkaline massifs such as those of the Kola alkaline province in NW Russia and</p><p>the Gardiner complex in East Greenland. These include alkali clinopyroxenites, wehrlites, glimmerites and olivinites. Russian</p><p>authors have subdivided dunites into mantle-derived dunites and those that are found in alkaline ultramafic massifs which they</p><p>term “olivinites.” Ultramafic cumulate xenoliths are frequently hosted in alkali basalt eruptions; these are often hornblendites or</p><p>hornblende-pyroxenites and may result from fractional crystallization of alkaline mafic or ultramafic magmas at depth within the</p><p>crust or upper mantle.</p><p>Ultramafic Meteorites</p><p>Many groups of meteorites are ultramafic, including most chondritic meteorites which are derived from asteroidal parent bodies</p><p>and which have not experienced planetary differentiation (separation into a layered structure including an iron-rich core, an</p><p>ultramafic mantle, and basaltic crust). Ordinary chondrites (the most abundant meteorites that fall to Earth) are composed of a</p><p>variety of components including abundant (60–80 vol%) chondrules which are millimeter-sized spherical objects consisting largely</p><p>of olivine and low-Ca pyroxene, showing textural evidence of having cooled rapidly from a melt. Since the Bulk Earth has a</p><p>composition similar to that of chondritic meteorites, it is not surprising that the mantle of our planet is largely composed of</p><p>ultramafic rocks.</p><p>Many ultramafic cumulate meteorites have been formed by impacts on the crust of Mars; these include chassignites (olivine-rich</p><p>cumulates), nakhlites (composed mostly of cumulus augite and olivine) and orthopyroxenites. Sparse examples of dunite rocks</p><p>(almost pure olivine cumulates) have also been found among the lunar samples returned by Apollo astronauts. Diogenite</p><p>meteorites are orthopyroxene-rich cumulates thought to come from the deep crust of the asteroid Vesta. Brachinites are also</p><p>olivine-rich meteorites but whether they are mantle fragments from a small FeO-rich asteroid or were formed by cumulate processes</p><p>is not clear. Ureilite meteorites are also ultramafic, being composed largely of olivine and pyroxene; their textures and mineral</p><p>compositions indicate that they are mostly remnants of the mantle of an unknown asteroid or planetesimal, rather than being</p><p>igneous cumulates.</p><p>Altered Ultramafic Rocks</p><p>Serpentinite is the most common alteration product of ultramafic rocks such as peridotites. Hydrothermal alteration changes the</p><p>olivine crystals to serpentine, often releasing iron oxides which form inclusions and trails in the serpentine pseudomorphs after</p><p>olivine. Lower temperature alteration in an oxidizing environment, such as subaerial weathering and alteration because of the</p><p>effects of steam around volcanic vents, can alter olivine to a brick-red amorphous mixture of minerals generally referred to as</p><p>“iddingsite.”</p><p>Other Ultramafic Rocks</p><p>Eclogites are plagioclase-free ultramafic rocks formed by high pressure metamorphism of basalts. There are several types of eclogites,</p><p>defined by the compositions of their constituent minerals. Their mineralogy is dominated by garnet and clinopyroxene, although</p><p>some can also contain relics of coesite (a high-pressure polymorph of quartz) or kyanite (a high-pressure polymorph</p><p>of Al2SiO5).</p><p>They occur as xenoliths in many kimberlite pipes where they are usually interpreted as being the high-grade metamorphic</p><p>equivalent of subducted oceanic crust. These eclogites often contain diamonds. They can be distinguished from garnet pyroxenites</p><p>because their clinopyroxenes are omphacitic (Na-rich) whereas those in garnet pyroxenites are not. Eclogites also occur as small</p><p>bodies within crustal rocks which have experienced high-pressure metamorphism of basaltic or gabbroic rocks, followed by</p><p>exhumation and return to the Earth’s surface.</p><p>Further Reading</p><p>Andersen T and Neumann E-R (2001) Fluid inclusions in mantle xenoliths. Lithos 55: 301–320.</p><p>Dilek Y and Furnes H (2014) Ophiolites and their origins. Elements 10: 93–100.</p><p>Bodinier J-L and Godard M (2007) Orogenic, ophiolitic and abyssal peridotites. Treatise on Geochemistry. 2: pp. 1–73. Amsterdam: Elsevier.</p><p>Downes H (2001) Formation and modification of the shallow sub-continental lithospheric mantle: A review of geochemical evidence from ultramafic xenolith suites and tectonically</p><p>emplaced ultramafic massifs of western and central Europe. Journal of Petrology 42: 233–250.</p><p>Downes H (2007) Origin and significance of spinel and garnet pyroxenites in the shallow lithospheric mantle: Ultramafic massifs in orogenic belts in Western Europe and NW Africa.</p><p>Lithos 99: 1–24.</p><p>Downes H, Upton BGJ, Connolly J, Beard AS, and Bodinier J-L (2007) Petrology and geochemistry of a cumulate xenolith suite from Bute: evidence for late Paleozoic crustal</p><p>underplating beneath SW Scotland. Journal of the Geological Society 164: 1217–1231.</p><p>Gill RCO (2010) Igneous Rocks and Processes—A Practical Guide. Chichester: John Wiley and Sons. 428 pp.</p><p>Griffin WL, O’Reilly SY, Abe N, Aulbach S, Davies RM, Pearson NJ, Doyle BJ, and Kivi K (2003) The origin and evolution of Archean lithospheric mantle. Precambrian Research</p><p>127: 19–41.</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0010</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0015</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0020</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0025</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0025</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0030</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0030</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0035</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0035</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0040</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0045</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0045</p><p>Ultramafic Rocks 7</p><p>Griffin WL, O’Reilly SY, Afonso JC, and Begg GC (2009) The composition and evolution of lithospheric mantle: a re-evaluation and its tectonic implications. Journal of Petrology</p><p>50: 1185–1204.</p><p>Haggerty SE (1995) Upper mantle mineralogy. Journal of Geodynamics 20: 331–364.</p><p>Harte B (2010) Diamond formation in the deep mantle: the record of mineral inclusions and their distribution in relation to mantle dehydration zones. Mineralogical Magazine</p><p>74: 189–215.</p><p>Jacob DE (2004) Nature and origin of eclogite xenoliths from kimberlites. Lithos 77: 295–316.</p><p>McDonough WF and Rudnick RL (1998) Mineralogy and composition of the upper mantle. In: Hemley RJ (ed.) Ultrahigh-Pressure Mineralogy, Reviews in Mineralogy. 37: pp. 139–164.</p><p>Washington D.C.: Mineralogical Society of America.</p><p>Mittlefehldt DW (2003) Achondrites. Treatise on Geochemistry. 1st edn, vol. 1, pp. 291–324. Oxford: Elsevier.</p><p>O’Reilly SY and Griffin WL (2013) Mantle Metasomatism. In: Metasomatism and the chemical transformation of rock, 471–533. Springer Lecture Notes in Earth System Sciences .</p><p>Pearson DG, Canil D, and Shirey SB (2003) Mantle samples included in volcanic rocks: xenoliths and diamonds. Treatise on Geochemistry. 1st edn, vol 2, pp. 171–275. Amsterdam:</p><p>Elsevier.</p><p>Sautter V, Haggerty SE, and Field S (1991) Ultradeep (>300 kilometers) ultramafic xenoliths: petrological evidence from the Transition Zone. Science 252: 827–830.</p><p>Simon NSC, Neumann E-R, Bonadiman C, Coltorti M, Delpech G, Gregoire M, and Widom E (2008) Ultra-refractory domains in the oceanic mantle lithosphere sampled as mantle</p><p>xenoliths at oceanic islands. Journal of Petrology 49: 1223–1251.</p><p>Streckeisen A (1976) To each plutonic rock its proper name. Earth-Science Reviews 12: 1–33.</p><p>Yoder HS (1976) Generation of Basaltic Magma. Washington: National Academy of Sciences. 265 pp.</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0050</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0050</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0055</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0060</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0060</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0065</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0070</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0070</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0075</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0080</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0085</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0085</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0090</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0090</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0095</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0095</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0100</p><p>http://refhub.elsevier.com/B978-0-12-409548-9.12478-9/rf0105</p><p>UltramaficRocks</p><p>Glossary</p><p>Introduction</p><p>Ultramafic Rocks Forming the Mantle</p><p>Ultramafic Rocks Formed by Cumulate Processes</p><p>Ultramafic Meteorites</p><p>Altered UltramaficRocks</p><p>Other UltramaficRocks</p><p>Further Reading</p>