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68-457: The spreading rate along the SWIR varies: the transition between slow (30 mm/yr) and ultra-slow (15 mm/yr) spreading occur at magnetic anomaly C6C (ca. 24 Ma). This occurs between 54°–67°E, the deepest, and perhaps coldest and most melt-poor, part of Earth's mid-ocean ridge system. Crustal thickness decreases quickly as spreading rates drop below c. 20 mm/yr and in the SWIR there

136-496: A monoclinic crystal structure). This distinction is important in the classification of pyroxene peridotites since clinopyroxene melts more easily than orthopyroxene or olivine. The most common orthopyroxene is enstatite , Mg 2 Si 2 O 6 , in which iron substitutes for some of the magnesium. The most important clinopyroxene is diopside , CaMgSi 2 O 6 , again with some substitution of iron for magnesium ( hedenbergite , FeCaSi 2 O 6 ). Ultramafic rock in which

204-485: A tsunami , but the SWIR is located far away from any continental margin and rocks of similar age have been reported from the Mid-Atlantic Ridge. If the rocks came directly out of the mantle it would have lost most of its isotopic lead . Ice-rafted dropstones commonly show sign of rounding. Hydrothermal circulation at mid-ocean ridges can, however, bring intrusive rocks into the shallow mantle, and it

272-660: A base station repeatedly or by having another magnetometer that periodically measures the field at a fixed location. Second, since the anomaly is the local contribution to the magnetic field, the main geomagnetic field must be subtracted from it. The International Geomagnetic Reference Field is usually used for this purpose. This is a large-scale, time-averaged mathematical model of the Earth's field based on measurements from satellites, magnetic observatories and other surveys. Some corrections that are needed for gravity anomalies are less important for magnetic anomalies. For example,

340-560: A discontinuity at 16°E axial depth drops 500 m and there is an abrupt change in morphology and magnetism. In the western end of this area (9°30'–11°45') a short magmatic ridge segment intersects the Shaka FZ. The rough topography here obscures the SWIR which runs into the western flank of the Joseph Mayes Seamount, one of few volcanic centres along the oblique supersegment. The seamount splits an old peridotite block,

408-430: A full spreading rate of 14.5 mm/a (0.57 in/year) during the last 3 Ma. The axial valley is a kilometres deep, typical of slow-spreading ridges, and 16 km wide, which is unusually wide. The zero-age axis lies 2,000 m (6,600 ft) below sea level in the central segment, but deeper closer to the two transforms: This is roughly a kilometre shallower than similar slow-spreading ridges, probably because of

476-610: A location 60 km (37 mi) south of the SWIR in 2010. This age is comparable to that of the break-up of Gondwana , the opening of the Indian Ocean, and emplacement of the Karoo Large Igneous Province (179-183 Ma) — in sharp contrast the Neogene age of the ocean floor near the SWIR. It can be assumed the rocks were deposited near the SWIR by an external force, such as an ice-rafting or

544-461: A low-cost, safe and permanent method of capturing and storing atmospheric CO 2 as part of climate change -related greenhouse gas sequestration . It was already known that peridotite reacts with CO 2 to form a solid carbonate -like limestone or marble mineral; and this process can be sped up a million times or more by simple drilling and hydraulic fracturing to allow injection of the CO 2 into

612-621: A mixed Ocean Island Basalt (OIB)/Mid-ocean ridge basalt (MORB) signature). The Crozet hotspot/Bank is, however, located more than 1000 km from the SWIR and ridge-hotspot interaction at distances beyond 500 km is, theoretically, supposed to be insignificant. The Kerguelen and Réunion hotspots are, however, probably interacting with the Southeast Indian Ridge and Central Indian Ridge over similar distances, as suggested by volcanic chains and lineaments connecting those ridges and hotspots. The absence of such lineaments between

680-588: A more rugged terrain. Since then, the Atlantis II transform fault has grown while the offsets west and east of it have begun to disappear. About 40 Ma in the future the Gallieni, Atlantis II, and Melvilles transform faults will continue to grow while the SWIR segments between them will keep most of their present length and shape. East of the Indomed FZ (south of Madagascar) the SWIR is the product of

748-424: A process in which the pyroxenes and olivines are converted to green serpentine . This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures. Serpentinites are mechanically weak and so flow readily within the earth. Distinctive plant communities grow in soils developed on serpentinite, because of the unusual composition of the underlying rock. One mineral in

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816-455: A regional survey of deeper rocks. In shipborne surveys, a magnetometer is towed a few hundred meters behind a ship in a device called a fish . The sensor is kept at a constant depth of about 15 m. Otherwise, the procedure is similar to that used in aeromagnetic surveys. Sputnik 3 in 1958 was the first spacecraft to carry a magnetometer. In the autumn of 1979, Magsat was launched and jointly operated by NASA and USGS until

884-490: A remanent magnetization or remanence. This remanence can last for millions of years, so it may be in a completely different direction from the present Earth's field. If a remanence is present, it is difficult to separate from the induced magnetization unless samples of the rock are measured. The ratio of the magnitudes, Q = M r / M i , is called the Koenigsberger ratio . Interpretation of magnetic anomalies

952-599: A section affected by the Marion hotspot. Between Indomed and Gallieni FZs the SWIR is more shallow and has a higher magma supply than neighbouring deeper sections; the crust is also thicker and/or the mantle hotter. This is probably due to the interaction with the Crozet hotspot, the increased magmatism of which resulted in the large Crozet volcanic plateau at c. 10 Ma. The hotspot also triggers thermal plumes and incorporates small amounts of lower mantle material (resulting in

1020-450: A series of parallel runs at a constant height and with intervals of anywhere from a hundred meters to several kilometers. These are crossed by occasional tie lines, perpendicular to the main survey, to check for errors. The plane is a source of magnetism, so sensors are either mounted on a boom (as in the figure) or towed behind on a cable. Aeromagnetic surveys have a lower spatial resolution than ground surveys, but this can be an advantage for

1088-468: A series of stations, typically 15 to 60 m apart. Usually a proton precession magnetometer is used and it is often mounted on a pole. Raising the magnetometer reduces the influence of small ferrous objects that were discarded by humans. To further reduce unwanted signals, the surveyors do not carry metallic objects such as keys, knives or compasses, and objects such as motor vehicles, railway lines, and barbed wire fences are avoided. If some such contaminant

1156-439: A variety of size scales. Layered peridotites may form the base layers of layered intrusions. These are characterized by cumulate textures , characterized by a fabric of coarse (>5mm) interlocking euhedral (well-formed) crystals in a groundmass of finer crystals formed from liquid magma trapped in the cumulate. Many show poikilitic texture in which crystallization of this liquid has produced crystals that overgrow and enclose

1224-422: Is ultramafic , as the rock contains less than 45% silica . It is high in magnesium (Mg ), reflecting the high proportions of magnesium-rich olivine, with appreciable iron . Peridotite is derived from Earth's mantle , either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting

1292-448: Is a magnesium orthosilicate containing some iron with the variable formula (Mg,Fe) 2 SiO 4 . The magnesium-rich olivine of peridotites is typically olive-green in color. Pyroxenes are chain silicates having the variable formula (Ca,Na,Fe ,Mg)(Cr,Al,Fe ,Mg,Mn,Ti,V)Si 2 O 6 comprising a large group of different minerals. These are divided into orthopyroxenes (with an orthorhombic crystal structure) and clinopyroxenes (with

1360-433: Is almost perfectly orthogonal relative to the spreading direction and is composed of magmatic accretionary segments linked by short non-transform offsets. Where the obliquity of the SWIR increases so does its length. This lengthening results in a decrease in mantle upwelling and a ridge geometry characteristic of ultra-slow spreading ridges (<12 mm/yr). The orthogonal supersegment is similar to larger ridge segments of

1428-438: Is an absence of volcanic activity along 100 km (62 mi) stretches of ridge axis. Along large sections, the SWIR runs obliquely relative to the spreading direction, typically about 60°. Because obliquity increases ridge length while decreasing mantle upwelling rates, the SWIR is transitional between slow and ultra-slow ridges. The slow-spreading sections of the SWIR have magmatic segments linked by transform faults, while

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1496-421: Is an older term for an ophiolite emplaced in a mountain belt during a continent-continent plate collision. Peridotites also occur as fragments ( xenoliths ) carried up by magmas from the mantle. Among the rocks that commonly include peridotite xenoliths are basalt and kimberlite . Although kimberlite is a variant of peridotite, kimberlite is also considered as brecciated volcanic material as well, which

1564-488: Is bright green with some specks of black, although most hand samples tend to be darker green. Peridotitic outcrops typically range from earthy bright yellow to dark green; this is because olivine is easily weathered to iddingsite . While green and yellow are the most common colors, peridotitic rocks may exhibit a wide range of colors including blue, brown, and red. Coarse-grained igneous rocks in which mafic minerals (minerals rich in magnesium and iron ) make up over 90% of

1632-491: Is characterised by deep, sub-parallel, and well-delineated fracture zones , sometimes deeper than 6,000 km (3,700 mi), delineated by elevated rims, sometimes reaching up to 2,000 m (6,600 ft) below sea level. These fracture zones are very long and often align with older structures near the continental shelves. These fracture zones, and their extensions into the Agulhas Basin , are flow-lines describing

1700-403: 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 deep sea floor. Oceanic plates are usually subducted back into

1768-481: Is named TMA-2, and one in the Olduvai Gorge is found in 2513 and retroactively named TMA-0 because it was first encountered by primitive humans. Peridotite Peridotite ( US : / ˈ p ɛ r ɪ d oʊ ˌ t aɪ t , p ə ˈ r ɪ d ə -/ PERR -ih-doh-tyte, pə- RID -ə- ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene . Peridotite

1836-442: Is no volcanism along this section. The flanks of the ridge axis are wide and lack a volcanic crustal layer. These flanks are rounded and smooth and lack the corrugated pattern associated with oceanic core complexes . This non-volcanic sea-floor is made of seawater-altered mantle-derived rocks brought to the surface by large-scale detachment faults. During the last 10 Ma these detachment faults have flipped back and forth across

1904-444: Is overlooked, it may show up as a sharp spike in the anomaly, so such features are treated with suspicion. The main application for ground-based surveys is the detailed search for minerals. Airborne magnetic surveys are often used in oil surveys to provide preliminary information for seismic surveys. In some countries such as Canada, government agencies have made systematic surveys of large areas. The survey generally involves making

1972-533: Is possible, rather than evident, that these rock found their way to the SWIR this way. Because spreading in the SWIR is ultra-slow, the mantle beneath should be abnormally cool, which could prevent melting of the rocks. The western end of the SWIR, known as the Bouvet Ridge, is bounded by the Bouvet and Moshesh transforms north and south of it respectively. The Bouvet Ridge is 110 km (68 mi)-long with

2040-633: Is possibly a good candidate in this case. Most rocks in Africa facing the SWIR are Archean cratons. The Neoproterozoic Pan-African Orogenic Belt , however, was accreted during the closure of the Mozambique Ocean and some rocks from eastern Africa, Madagascar, and Antarctica are associated with this event. During the break-up of Gondwana the Karoo volcanics intruded the Pan-African rocks and it

2108-434: Is present in peridotite mostly as a consequence of alteration by hydrous fluids. Although peridotites are classified by their content of olivine, pyroxenes, and hornblende, a number of other mineral families are characteristically present in peridotites and may make up a significant fraction of their composition. For example, chromite is sometimes present in amounts of up to 50%. (A chromite composition above 50% reclassifies

Southwest Indian Ridge - Misplaced Pages Continue

2176-405: Is primarily permanent magnetization carried by titanomagnetite minerals in basalt and gabbros . They are magnetized when ocean crust is formed at the ridge. As magma rises to the surface and cools, the rock acquires a thermoremanent magnetization in the direction of the field. Then the rock is carried away from the ridge by the motions of the tectonic plates . Every few hundred thousand years,

2244-425: Is removing short-term variations in the field from external sources; e.g., temporal variations which include diurnal variations that have a period of 24 hours and magnitudes of up to 30 nT, probably from the action of the solar wind on the ionosphere . In addition, magnetic storms can have peak magnitudes of 1000 nT and can last for several days. Their contribution can be measured by returning to

2312-434: Is usually done by matching observed and modeled values of the anomalous magnetic field. An algorithm developed by Talwani and Heirtzler(1964) (and further elaborated by Kravchinsky et al., 2019) treats both induced and remnant magnetizations as vectors and allows theoretical estimation of the remnant magnetization from the existing apparent polar wander paths for different tectonic units or continents. Magnetic surveys over

2380-403: Is why it is referred to as a source of peridotite xenoliths. Peridotite xenoliths contain osmium and other elements whose stable isotope ratios provide clues on the formation and evolution of the Earth's mantle. Such xenoliths originate from depths of up to nearly 200 kilometers (120 mi) or more. The volcanic equivalent of peridotites are komatiites , which were mostly erupted early in

2448-570: The Mozambique Basin indicate this is the dominant spreading direction for the past 80 Ma. Major changes at Discovery FZ (42°E), Galliene FZ (52°E), and Melville FZ (60°E) define large-scale segmentation of the SWIR. Mean axial depth varies between 4,730 m (15,520 ft) between Melville FZ and Rodrigues TJ, a section underlain by either thin crust or cold mantle, to 3,050 m (10,010 ft) between Andrew Bain FZ and Discovery FZ,

2516-591: The 64 million years of eastward propagation of the Rodriguez triple junction. This section is composed of regularly spaced non-transform discontinuities, short oblique amagmatic segments, and the Atlantis II, Novara, and Melville transforms. An increase in axial depth east of 49°E reflects non-magmatic extension. The segmentation and morphology in the axial valley of the easternmost SWIR is unique to ultra-slow spreading ridges. 3000 m-high ridge segments are linked by more than 100 km-long axial segments. There

2584-430: The Earth's history and are rare in rocks younger than Archean in age. Small pieces of peridotite have been found in lunar breccias. The rocks of the peridotite family are uncommon at the surface and are highly unstable, because olivine reacts quickly with water at typical temperatures of the upper crust and at the Earth's surface. Many, if not most, surface outcrops have been at least partly altered to serpentinite ,

2652-657: The Eocene, much younger than could be assumed from their length. 42°S 41°E  /  42°S 41°E  / -42; 41 Magnetic anomaly In geophysics , a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying material. The magnetic variation ( geomagnetic reversals ) in successive bands of ocean floor parallel with mid-ocean ridges

2720-652: The Marion hotspot is located, lies 250 km (160 mi) from the SWIR on 28  Ma crust. Bouvet Island, located 300 km (190 mi) from the Bouvet triple junction and 55 km (34 mi) from the SWIR, is located on 7 Ma crust, though the exact location of the Bouvet hotspot has not been determined. Between the Marion hotspot and Gallieni FZ there is an irregular segmentation with relatively shallow axial depth. Between Prince Edward FZ and Atlantis II FZ (35–57°E), all major transform faults (and their 35 Ma associated magnetic anomalies ) are increasingly trending more directly north–south. Magnetic anomalies in

2788-572: The Mid-Atlantic Ridge. A series of fracture zones — Du Toit, Andrew Bain, Marion, and Prince Edward  — offsets the SWIR 1,230 km (760 mi) between 45°S,35°E—53°S,27°E. The largest of these, the 750 km long-long Andrew Bain FZ, is where the Nubia-Somalia boundary intersects the SWIR. The active section of the Andrew Bain TF represents the largest age-offset (65 Ma) of any oceanic transform fault and it's also

Southwest Indian Ridge - Misplaced Pages Continue

2856-399: The SWIR and Crozet can be explained by plate age and thickness — plates older than 25 Ma are thought to be to thick for plumes to penetrate. Between the Gallieni and Melville FZs the SWIR was originally roughly perpendicular to the spreading direction with few and small offsets. About 40 Ma a clock-wise change in spreading direction quickly resulted in evenly spaced offsets and

2924-654: The SWIR is in fact divided into three plates: the Nubian, Lwandle , and Somalian plates. The location on the SWIR of this "diffuse" triple junction between the Nubian, Somali, and Antarctic plates has been estimated to between 26°E and 32°E or just west of the Andrew Bain transform fault . This diffuse triple junction forms the southern end of the East African Rift system. 180 Ma-old rocks, dated from zircons in diorite and gabbro , were dredged from

2992-719: The deformation associated with their tectonic mode of emplacement. Peridotites have two primary modes of origin: as mantle rocks formed during the accretion and differentiation of the Earth, or as cumulate rocks formed by precipitation of olivine ± pyroxenes from basaltic or ultramafic magmas. These magmas are ultimately derived from the upper mantle by partial melting of mantle peridotites. Mantle peridotites are sampled as ophiolites in collisional mountain ranges, as xenoliths in basalt or kimberlite, or as abyssal peridotites (sampled from ocean floor). These rocks represent either fertile mantle (lherzolite) or partially depleted mantle (harzburgite, dunite). Alpine peridotites may be either of

3060-401: The depth of formation of the peridotite. For example, the aluminium in lhertzolite is present as plagioclase at depths shallower than about 20 kilometers (12 mi), while it is present as spinel between 20 km and 60 kilometers (37 mi) and as garnet below 60 km. Peridotite is the dominant rock of the Earth's mantle above a depth of about 400 km; below that depth, olivine

3128-509: The direction of the magnetic field reverses . Thus, the pattern of stripes is a global phenomenon and can be used to calculate the velocity of seafloor spreading . In the Space Odyssey series by Arthur C. Clarke , a series of monoliths are left by extraterrestrials for humans to find. One near the crater Tycho is found by its unnaturally powerful magnetic field and named Tycho Magnetic Anomaly 1 (TMA-1). One orbiting Jupiter

3196-528: The fraction of pyroxenes exceeds 60% are classified as pyroxenites rather than peridotites. Pyroxenes are typically dark in color. Hornblende is an amphibole , a group of minerals resembling pyroxenes but with a double chain structure incorporating water. Hornblende itself has a highly variable composition, ranging from tschermakite ( Ca 2 (Mg,Fe) 3 Al 2 Si 6 Al 2 O 22 (OH) 2 ) to pargasite ( NaCa 2 (Mg,Fe) 4 AlSi 6 Al 2 O 22 (OH) 2 ) with many other variations in composition. It

3264-606: The mantle in subduction zones . However, pieces can be emplaced into or overthrust on continental crust by a process called obduction , rather than carried down into the mantle. The emplacement may occur during orogenies , as during collisions of one continent with another or with an island arc . The pieces of oceanic plates emplaced within continental crust are referred to as ophiolites . Typical ophiolites consist mostly of peridotite plus associated rocks such as gabbro , pillow basalt , diabase sill-and-dike complexes, and red chert. Alpine peridotite or orogenic peridotite massif

3332-749: The motion of Africa and Antarctica since break-up of Gondwana in the Late Cretaceous. The SWIR opened during the break-up of Gondwana when Antarctica broke off from Africa during the Permian-Triassic Karoo large igneous province c. 185–180 Ma in what is now the Mozambique Basin and the Riiser-Larsen Sea . The spreading direction between the continents started to change around 74 Ma and 69–64 Ma spreading slowed (c. 1 cm/yr) then changed orientation to NE-SW. Fracture zones near Prince Edward FZ are from

3400-424: The nodules preserve isotope ratios of osmium and other elements that record processes that occurred when Earth was formed, and so they are of special interest to paleogeologists because they provide clues to the early composition of Earth's mantle and the complexities of the processes that occurred. The word peridotite comes from the gemstone peridot , which consists of pale green olivine. Classic peridotite

3468-415: The oceans have revealed a characteristic pattern of anomalies around mid-ocean ridges. They involve a series of positive and negative anomalies in the intensity of the magnetic field, forming stripes running parallel to each ridge. They are often symmetric about the axis of the ridge. The stripes are generally tens of kilometers wide, and the anomalies are a few hundred nanoteslas. The source of these anomalies

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3536-451: The ophiolite association and representing the uppermost mantle below ocean basins, or masses of subcontinental mantle emplaced along thrust faults in mountain belts. Layered peridotites are igneous sediments and form by mechanical accumulation of dense olivine crystals. They form from mantle-derived magmas, such as those of basalt composition. Peridotites associated with Alaskan-type ultramafic complexes are cumulates that probably formed in

3604-407: The original cumulus crystals (called chadrocrysts ). Another texture is a well-annealed texture of equal sized anhedral crystals with straight grain boundaries intersecting at 120°. This may result when slow cooling allowed recrystallization to minimize surface energy. Cataclastic texture, showing irregular fractures and deformation twinning of olivine grains, is common in peridotites because of

3672-422: The relative proportions of pyroxenes , chromite , plagioclase , and amphibole . Peridotite is the dominant rock of the upper part of Earth's mantle . The compositions of peridotite nodules found in certain basalts are of special interest along with diamond pipes ( kimberlite ), because they provide samples of Earth's mantle brought up from depths ranging from about 30 km to 200 km or more. Some of

3740-519: The remains of which project on either side of the ridge, and fills the rift valley in between, resulting in a double-peaked volcano sitting on the SWIR. East of the seamount (11°30'-10°24'E) there is a 180 km-long and 4,200 m-deep amagmatic segment. Reaching a maximum depth of 4,700 m, its deepest part has a rough floor void of signs of recent volcanism but filled with irregular horst blocks partially made of serpentinised peridotite. The "orthogonal supersegment" (16 to 25°E), in contrast,

3808-613: The ridge axis and produced almost all the divergence along this section of the SWIR. In the easternmost SWIR, east of Melville FZ (60°45' E), the mantle is unusually cold and the crust thin (3.7 km in average) resulting in only partial melting in the mantle and a decrease in melt supply to the SWIR in this region. This shortage in magma supply has resulted in fewer but taller seamounts east of Melville; there are more than 100 seamounts per 10 km about 50 m tall west of Melville whereas east of Melville there are fewer than 10 seamounts per 10 km more than 100 m tall. The SWIR

3876-471: The rock as a peridotitic chromitite .) Other common accessory minerals include spinel , garnet , biotite , or magnetite . A peridotite containing significant amounts of one of these minerals may have its classification refined accordingly; for example, if a lhertzolite contains up to 5% spinel, it is a spinel-bearing lhertzolite , while for amounts up to 50%, it would be classified as a spinel lhertzolite . The accessory minerals can be useful for estimating

3944-598: The root zones of volcanoes. Cumulate peridotites are also formed in komatiite lava flows. Komatiites are high temperature partial melts of peridotite characterized by a high degree of partial melting deep below the surface. Eclogite , a rock similar to basalt in composition, is composed primarily of omphacite (sodic clinopyroxene) and pyrope -rich garnet. Eclogite is associated with peridotite in some xenolith occurrences; it also occurs with peridotite in rocks metamorphosed at high pressures during processes related to subduction. Peridotite may potentially be used in

4012-428: The serpentine group, chrysotile , is a type of asbestos. Most peridotite is green in color due to its high olivine content. However, peridotites can range in color from greenish-gray to nearly black to pale yellowish-green. Peridotite weathers to form a distinctive brown crust in subaerial exposures and to a deep orange color in submarine exposures. Peridotites can take on a massive form or may be in layers on

4080-687: The spring of 1980. It had a caesium vapor scalar magnetometer and a fluxgate vector magnetometer. CHAMP , a German satellite, made precise gravity and magnetic measurements from 2001 to 2010. A Danish satellite, Ørsted , was launched in 1999 and is still in operation, while the Swarm mission of the European Space Agency involves a "constellation" of three satellites that were launched in November, 2013. There are two main corrections that are needed for magnetic measurements. The first

4148-793: The subsurface peridotite formation. Peridotite is named for the gemstone peridot , a glassy green gem originally mined on St. John's Island in the Red Sea and now mined on the San Carlos Apache Indian Reservation in Arizona. Peridotite that has been hydrated at low temperatures is the protolith for serpentinite , which may include chrysotile asbestos (a form of serpentine) and talc . Layered intrusions with cumulate peridotite are typically associated with sulfide or chromite ores. Sulfides associated with peridotites form nickel ores and platinoid metals; most of

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4216-440: The total volume of these four mineral families in the rock. Peridotites are further classified as follows: Mantle peridotite is highly enriched in magnesium, with a typical magnesium number of 89. In other words, of the total content of iron plus magnesium, 89 mol% is magnesium. This is reflected in the composition of the mafic minerals making up the peridotite. Olivine is the essential mineral found in all peridotites. It

4284-403: The ultra-slow sections lack such transforms and have magmatic segments linked by amagmatic troughs. Spreading in the SWIR is slow, but the plate boundary is intersected by the much slower but more diffuse Nubian – Somalian boundary. The variation in spreading rates indicate the SWIR is not a spreading centre between two rigid plates, but that the previously assumed single African plate north of

4352-556: The vertical gradient of the magnetic field is 0.03 nT/m or less, so an elevation correction is generally not needed. The magnetization in the surveyed rock is the vector sum of induced and remanent magnetization : The induced magnetization of many minerals is the product of the ambient magnetic field and their magnetic susceptibility χ : Some susceptibilities are given in the table. Minerals that are diamagnetic or paramagnetic only have an induced magnetization. Ferromagnetic minerals such as magnetite also can carry

4420-533: The vicinity to the BTJ. Between 9 and 25°E, the SWIR trends E-W and lacks transforms. This section is composed of orthogonal magmatic accretionary segments linked by oblique amagmatic accretionary segments. The oblique portion of this area (9 to 16°E), the "oblique supersegment" is highly variable in axial orientation, ranging from orthogonal to 56°, and its series of magmatic and amagmatic segments results in abruptly fluctuating magmatism and ultra-slow spreading. West of

4488-431: The volume of the rock are classified as ultramafic rocks . Such rocks typically contain less than 45% silica. Ultramafic rocks are further classified by their relative proportions of olivine , orthopyroxene , clinopyroxene , and hornblende , which are the most abundant families of mafic minerals in most ultramafic rocks. Peridotite is then defined as coarse-grained ultramafic rock in which olivine makes up 40% or more of

4556-833: The widest (120 km). Its extension extends south from the Mozambique Escarpment (between the Mozambique Ridge and Basin) to the Astrid Ridge off Antarctica. East of the Andrew Bain TF is the "Marion Swell", the geoid high of the Southern Ocean, between 35°E and 50.5°E, and the Madagascar Plateau and the Del Cano Rise. The SWIR crosses the flank of the swell before reaching the Marion hotspot at 36°E. Marion Island, where

4624-457: Was important evidence for seafloor spreading , a concept central to the theory of plate tectonics . Magnetic anomalies are generally a small fraction of the magnetic field. The total field ranges from 25,000 to 65,000  nanoteslas (nT). To measure anomalies, magnetometers need a sensitivity of 10 nT or less. There are three main types of magnetometer used to measure magnetic anomalies: In ground-based surveys, measurements are made at

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