dans une komatiite canadienne. Implicit in the definition of komatiite is the notion, difficult to prove, that komatiites crystallise from liquids that contained > 18% MgO. Un article de Wikipédia, l'encyclopédie libre. In a typical komatiite lava, the textural profile of the lava from top to bottom (Figure 49.3; Arndt et al., 1977) consists of the following: Figure 49.3. The komatiite cycles are confined within extrusive and intrusive Mount Edwards tholeiitic basalts. La plupart sont vieilles de plus de deux milliards d'années et on ne connaît en raison de leur âge que des komatiites métamorphisées (en toute rigueur des métakomatiites). Année: 2008. Some deposits represent a mixture of types II, I and/or IV. 5 and 6). … The overall mineralized system remains open to the north and south, and is subject to ongoing exploration. (1977). Thicker lava units resulted from ponding in seafloor depressions or inflation under a chilled crust, as happens for modern basalt lavas. Al-depleted komatiites (ADKs) are prevalent in older Archean terranes, decline sharply in proportion to other komatiite types after about 2.7 Ga, and disappear altogether in the Proterozoic. For komatiite to reach the surface it must flow rapidly and continuously, without pausing on its way. Komatiites were produced from a relatively wet mantle. From magma temperatures (Fig. This would be the case when komatiite erupts in an oceanic setting where solidified basalt near the surface has a density similar to that of the komatiite liquid, and the cumulates or other intrusive rocks at deeper levels have higher densities. They can have comb-like shapes in detail and have a subhorizontal orientation. Evidence of this is from their association with felsics, occurrences of komatiitic tuffs, Nb anomalies and by S- and H2O-bor… Ce magmatisme très spécifique produit des granitoïdes calco-alcalins très riche en magnésium, présentant souvent des phénocristaux de feldspath potassique appelé sanukitoïdes[2]. Slow cooling of stagnant lava in the tube allowed crystallization, forming the ortho- and mesocumulate textural zones. The orebodies extend in arcuate shape for 3.5 km and occur as discontinuous lenses with steep dips. Jetzt Konto anlegen Erweiterte. Pyroxene spinifex forms in the upper parts of komatiitic basalt flows. Komatiite magmas had extreme characteristics, being very hot (up to 1650°C) and very fluid (viscosities on the range 1-10 Pa s, close to that of olive oil; Arndt et al., 2008). Evidence of this is from their association with felsics, occurrences of komatiitic tuffs, Niobium anomalies and by S- and H2O-borne rich mineralizations. Composition and physical properties Komatiites are ultrabasic igneous rocks with low SiO2 content (<45 wt%), high MgO contents (>18% wt%), low calculated viscosities at eruption (0.1–10 Pa s), high densities (2.8 g cm –3) and high temperatures at eruption (1400–1650°C) (Arndt & Nisbet 1982, Huppert et al. The viscosity of komatiite magma is very low, one to two orders of magnitude less than that of basalt. No single T melt or T. crystallization All affect viscosity, explosivity. Elle tient son nom de la rivière Komati, en Afrique du Sud, où elle est particulièrement visible. These are interpreted as channels through which the lavas passed during their passage from vent to flow front. By continuing you agree to the use of cookies. Figure 7.6. The texture forms as a result of constrained, in-situ crystallization of olivine or pyroxene during moderately rapid cooling of low-viscosity ultramafic liquid. The left-hand drill core is 5 cm wide. Komatiite by Nicholas Arndt, 9781108460514, available at Book Depository with free delivery worldwide. Thick, massive, olivine-rich units are present in most regions where komatiite lavas are abundant. Envoyer vers Kindle ou courriel . Faure et al. km (174 sq. Polyhedral olivine grains, which were present before eruption or crystallized during flowage, settle to the base of the flow or crystallize in situ to form the lower cumulate layer (Figure 6). Both komatiite sequences are exposed all around the margins of the Widgiemooltha dome as concentric rims centering on the large massive granitoid core. David A Williams, Ross C Kerr, C.Michael Lesher, Stephen J Barnes, Analytical/numerical modeling of komatiite lava emplacement and thermal erosion at Perseverance, Western Australia, Journal of Volcanology and Geothermal Research, 10.1016/S0377-0273(01)00206-2, 110, 1-2, (27-55), (2001). However, when komatiite traverses or erupts onto a granitic substrate, as in the Kambalda area in Western Australia, the high density of mantle rocks lower in the liquid column must counterbalance the low density of the granites. These physical properties and the deep marine setting influenced their flow behavior and morphology. Cette différence abouti à une enfoncement de l'unité plus dense à état solide (komatiites) dans une unité moins dense, fonctionnement inverse du phénomène de diapirisme[1]. of komatiite-associated Ni—Cu—PGE deposits (Chapter 10). The Widgiemooltha dome structure is a large regional anticline dome with surface coverage of 450 sq. On this basis, we can predict that komatiites probably erupted initially along fissures, as a series of lava fountains. These physical properties and the deep marine setting influenced their flow behavior and morphology. Diagram illustrating the crystallization of a layered spinifex-textured komatiite flow. This will be followed by a description of two notable Paleoproterozoic deposits, namely the Hotinvaara and Lomalampi deposits in the Central Lapland Greenstone Belt. About Cookies, including instructions on how to turn off cookies if you wish to do so. However, only radionuclide emission and accretionary heat, after Kelvin’s contracting stars, were considered, and the view has been that the whole Earth cools to space, both of which are in error (Chapter 2). The Dachine komatiite of French Guiana contains single diamonds of over 500 μm in size, which are predominantly resorbed octahedrons and cubes with typical surface characteristics indicative of a deep origin. Anhydrous komatiite, containing 28% MgO, has a density of about 2800 kg m− 3, significantly higher than that of many rocks in the upper crust. Komatiites often show pillow lava structure, autobrecciated upper margins consistent with underwater eruption forming a rigid upper skin to the lava flows. Le témoin le plus jeune de komatiite est d'âge phanérozoïque et se trouve sur le plateau océanique des Caraïbes sur l'île Gorgona en Colombie. Faciès spinifex. Volcanic rocks contain high MgO and display olivine–chromite cumulate texture. Sets of blades increase in size downward. Other flows grade along strike from layered spinifex-textured portions to massive olivine-phyric units. Their unusual chemical compositions and exceptionally high formation temperatures produced highly fluid lava that crystallized as spectacular layered flows. Elle est caractérisée par une grande richesse en magnésium MgO (18 à 35 % soit trois à quatre fois plus qu'un basalte classique, qui en contient moins de 10 %), qui est un élément compatible (c'est-à-dire qui se concentre préférentiellement dans une phase solide à la suite d'une fusion partielle) donc signe également un taux de fusion important. Komatiite is also a relatively dense magma. Komatiite magmas were very high temperature magmas (1400–1650 °C), and had ultralow viscosities (0.1–10.0 Pa s) and very high density (2800 kg m-3) (Huppert and Sparks, 1985). mi). Melts are a three-phase system of crystals, liquid, and gas/volatiles. (B) Outcrop of thin komatiite lava showing upper blade spinifex and lower orthocumulate textural zones. Komatiites are almost invariably associated with basalts in Archean greenstone sequences. After Arndt et al. A2—Random spinifex texture: Gradationally under A1, there is a layer of apparent needle-like, but actually blade-like, skeletal crystals of olivine with a random orientation (Figure 49.4(A)). The granitoid, rhyolitic porphyry dikes, pegmatites, and Proterozoic dolerite dikes crosscut the sequence. Haldar, in Platinum-Nickel-Chromium Deposits, 2017. I. The nickel mine includes: Mount Edward, Miitel, Marines, and Redross mines in the eastern flank, and the Wannaway mine in the western flank. When it flows rapidly past crustal rocks, it is capable of thermally eroding and assimilating them; if it ponds in a crustal magma chamber, it will fractionally crystallise and lose its ultramafic character. The width varies between 2 and 10 m. The total postproduction reserves and resources at Miitel Mine stand at 629,000 tonnes (∼693,000 tons) garaging 3.1% Ni [19,500 tonnes (∼21,500 tons) of nickel metal] as of June 2015. Each core rod is 5 cm wide. J.W. Lesher, in Reference Module in Earth Systems and Environmental Sciences, 2016. The underlying cumulate textural zone is interpreted to represent a lava tube insulated from contact with the water by the spinifex-textured lava crust. There is a continuous spectrum, present in most greenstone belts through the Archean, from these depleted plume-derived basalts to strongly LREE-enriched basalts, with no evident change in their relative proportions through the Archean. Grid Metals Corp. is pleased to provide shareholders with an update on corporate developments and activities. We have developed a mathematical model to evaluate the flow and erosional potential of submarine, channelized komatiite lavas at Kambalda, Western Australia. They analysed the magnetic, mass and electrical properties of 104 samples and noted in particular the extreme conductivities and high mag-netic susceptibilities and densities of the sulfide ores. The base of this textural zone is sharp, and may be planar to gently undulating. Profile through a complete komatiite lava, showing the variations in the upper spinifex (A) and lower cumulate (B) textural zones. J. Konnunaho, ... T. Törmänen, in Mineral Deposits of Finland, 2015. In order to understand the planetary interior better, phase relations and mineral physics properties of compositions other than pyrolite may also be important. Komatiites have low silicon , potassium and aluminium , and high to extremely high magnesium content. The characteristic combination of high Nb/Yb and moderate Th/Yb of Phanerozoic ocean island basalts is also lacking in Archean basalts, reflecting falling mantle temperatures and an increase with time in the proportion of low-degree melts, and perhaps an increase in the amount of subducted crustal material recycled into plume sources. This change is attributed to a decline in the temperature of komatiite sources through time; ADKs formed at depths of greater than 300 km in mantle plumes with temperatures probably more than 400°C greater than ambient mantle, which itself was between 150 and 200°C hotter than present-day ambient mantle. Thick dunitic sills rarely have spinifex zones, but may have marginal ortho- to mesocumulate zones. Affleurements et répartitions dans le temps, Les komatiites à l'origine de la sagduction, Roches volcaniques (classification chimique), https://fr.wikipedia.org/w/index.php?title=Komatiite&oldid=176664978, Portail:Sciences de la Terre et de l'Univers/Articles liés, licence Creative Commons attribution, partage dans les mêmes conditions, comment citer les auteurs et mentionner la licence. Eriksson, ... O. Catuneanu, in Developments in Precambrian Geology, 2004. Hill, RET, Barnes, SJ, Gole, MJ and Dowling, SE 1995, The volcanology of komatiites as deduced from field relationships in the Norseman- Strongly MgO-enriched cumulate-dominated komatiite sequences, associated with magmatic nickel sulfide mineralization, are found close to the margins of older cratonic nuclei in greenstone belts 2.9 Ga or younger. Mt Keith mine, Western Australia. Elle tient son nom de la rivière Komati, en Afrique du Sud, où elle est particulièrement visible. Lesher and S.J. Their unusual chemical compositions and exceptionally high formation temperatures produced highly fluid lava that crystallized as spectacular layered flows. Ce phénomène explique la tectonique verticale au milieu de plaque tectonique à l'Archéen et l'Hadéen ou il n'existe pas d'épaississant crustal comme on peut l'observer aujourd'hui conduisant à une tectonique verticale[1]. Magma = liquid + crystals + volatiles +Xenos. Texture spinifex d'une komatiite au microscope. (2008), Ashwal (1993), Weinberg and Hasalová (2014). The massive nickel sulfide bodies are located exclusively at the contact between the Mount Edwards tholeiitic basalt and the overlying Widgiemooltha komatiites in the upper cycle. Active lavas had very thin flow fronts, perhaps only cm thick, and spread as very thin sheets and lobes (Cas et al., 1999). Komatiite avec sa patine à l'affleurement. They are also of great importance in unravelling the tectonic history of greenstone belts. A3—Blade spinifex texture: This layer gradationally underlies A2, and consists of sets of blade-like crystals that are stacked at slightly different angles against each other and have a subvertical orientation (Figure 49.4(A) and (B)). An example of the first type is a phenocryst-charged basaltic liquid (a picrite according to some definitions); an example of the second is meimechite (Arndt et al., 1995), a rare alkaline lava with unusual major and trace element composition. La komatiite est une roche volcanique ultramafique à olivine et pyroxène, riche en magnésium, très fréquente à l'Hadéen et l'Archéen. Proximal volcanic facies are thinner and interleaved with sulfidic sediments, black shales, cherts and tholeiitic basalts. Rocks with hardness 1-3 are soft rocks from 3-6 are medium hardness rocks and 6-10 are hard rocks. There is no single melting temperature or crystallization temperature. The enriched category may contain a component of calc-alkaline subduction-related basalt, but basalts with low Nb/La and low La/Sm, and some other characteristics of modern subduction-related basalts, are absent. Their carbon isotope values have a mode of − 24‰ and whilst most of the diamonds are Type II, (see color for a definition), a mode of − 28‰ has been determined for nitrogen isotopes (δ15N). Anne M. Hofmeister, in Heat Transport and Energetics of the Earth and Rocky Planets, 2020. komatiites, called “Al-undepleted” or Munro-type, has near chondritic ratios of Al2O3/TiO2. Pages: 488 / 489. Importantly, the surface temperature of the Earth is controlled by the Sun to just above the freezing point of water (Chapter 10). In addition, jigsaw-fit to clast-rotated monomictic breccia may be present at the tops and bases of lavas. However, lavas with hyaloclastite at the top imply that steam film collapse occurred and resulted in quench fragmentation. serpentine) because of low-temperature alteration. Komatiite lavas are famous for their unique olivine spinifex and cumulate crystal textures and internal textural zonation. The Archean was indeed warmer than the Proterozoic, but this due not to a hot initial Earth, but largely to the dissipation of spin in the Hadean to the outer layers, with peak radioactive decay being simultaneous. Other articles where Komatiite is discussed: Africa: General considerations: The rock type komatiite is particularly diagnostic of those volcanic sequences and is almost exclusively restricted to the Archean Eon. Olivine phenocrysts are separated by a fine aphyric groundmass (Figure 49.4(C)). and Gd/Yb. The major deposits are situated on the eastern flank of the Widgiemooltha dome within the fault-bounded Coolgardie domain of the Kalgoorlie terrane (Vaillant et al., 2015b). The primary liquid is assumed to have had 28% MgO, and at this value, Barberton‐type komatiite has about 25 ppm Zr, and Munro‐type komatiite has about 15 ppm Zr. Thin syn-depositional komatiite sills that were intruded into unconsolidated sediment may have all of the above zones as well as sediment-matrix komatiite breccia (peperite) along the contacts. From the ignition weight loss, the … Preservation of substantial volumes of gneisses by ~2.9 Ga, shortly after the bulk of komatiite production, cooroborates the timeing of stabilization and radial cooling of the outer layers. These small pulses of lava may never have moved far from the feeder channel before ponding and crystallising beneath a thin elastic crust. MINCOR Resources NL, Australia, discovered the North Miitel orebody in 2002 and the South Miitel orebody in 2005, and has continued to extend these discoveries ever since. The pyroxene needles range in length from a few mm to several cm and their orientation is either random or perpendicular to the flow top (see Fleet and MacRae, 1975; Arndt and Fleet, 1979). Copyright © 2021 Elsevier B.V. or its licensors or contributors. Figure 49.4. (C) Adcumulate texture in drill core, represented by serpentine pseudomorphing coarse olivine. A third type, Al-enriched komatiite, is common in komatiites from Gorgona. The simple description is that “komatiite” is an ultramafic volcanic rock (Arndt and Nisbet, 1982b) with a lower limit of 18% MgO separating komatiites from picrites, ankaramites or magnesian basalts. We have very little idea how a komatiite behaves during eruption. Komatiite is the signature rock type of Archean greenstone belts and is found across almost the entire age span of Archean crust. The underlying layer of platy olivine spinifex has an organised structure wherein arrays of large bladed olivine crystals, possibly metres long, are oriented roughly perpendicular to the flow top (A3), forming sheaf-like structures that fan out from flow tops and serve as top indicators. ISBN 13: 9780511424038. l'évolution de la chimie du manteau conduisant à la disparition de mise en place de komatiites vers -2,5 Ga, et l'épaississement des plaques lithosphériques qui a augmenté durant l’Archéen. Komatiite, mainly of Archean and Proterozoic age, derived from high degrees of partially melted mantle at extremely high temperatures and very low viscosity, are significant targets for magmatic sulfide mineralization. In this texture, needle-like crystals, commonly with pigeonite cores and augite margins, lie in a matrix of augite, altered glass and/or plagioclase and oxides. Given that many komatiite lavas are entirely coherent from base to top, an insulating steam layer must have been created at the interface between lava and seawater, preventing quench fragmentation. Rocks are rated on the on the Moh's Hardness Scale which rates the rocks on the scale from 1 to 10. On the other hand, meimechites, picrites and other rock types that contain no spinifex are excluded. Complications arise from the existence of other volcanic rocks with more than 18% that either formed through the accumulation of olivine from less magnesian liquids, or crystallised from magmas with chemical characteristics quite unlike those of most komatiites. They can be a cm to tens of cm in length, and in some cases, m in length. Investigation of the extreme conditions in which komatiites formed provides important evidence about the thermal and chemical evolution of the planet, and the nature of the Precambrian mantle… Adcumulate texture is diagnostic of very thick sills that have cooled slowly and crystallized fully. We use cookies to help provide and enhance our service and tailor content and ads. The cooling rate for the upper mantle at ~0.2 K Ma−1 is not that much slower than the early warming rate at ~1.3 K Ma−1 (Fig. Komatiites erupted billions of years ago as pulsating streams of white-hot lava. The heterogeneous nature of komatiite magmatism at ∼2.9 and 2.7 Ga is consistent with the impingement of a mantle plume onto lithosphere of variable thickness (Figs. The Miitel orebody is elongated, near vertical, with gently plunging configuration, making it ideal for underground mining. N.T. Suche nach Autor, Titel, Verlag, ISBN, ... Herzlich willkommen Mein Konto Neuer Kunde? A1—This uppermost layer is aphyric or extremely fine grained and only a few mm to a few cm thick. Komatiites erupted billions of years ago as pulsating streams of white-hot lava. Active lavas had very thin flow fronts, perhaps only cm thick, and spread as very thin sheets and lobes (Cas et al., 1999). The PGE-enriched deposits also tend to be enriched in Cu compared to the PGE-poor deposits (e.g., Sarvisoaivi, Ruossakero, and Hotinvaara). Pyke Hill, Ontario, Canada. WorldCat Home About WorldCat Help. Island, Colombia (Echeverria 1980), and a fourth type, Ti-rich komatiite occurs in the Baltic. Many of these enriched compositions are the result of crustal contamination of plume-derived tholeiites and/or komatiites, but conditions in subduction zones may also have changed with time. Cookies if you wish to do so Neuer Kunde komatiites contiennent des olivines spinifex ( en. Fissures, as a result of constrained, in-situ crystallization of a hot early.. Overall mineralized system remains open to the lava flows texture in drill core, by. Layering of komatiite lavas are famous for their unique olivine spinifex and orthocumulate... In length, and hydrothermal processes Depository with free delivery worldwide their unusual chemical compositions and exceptionally formation. Age span of Archean greenstone belts and is found across almost the entire age span of Archean greenstone belts is. Proved well continuous containing this texture ” size downward from a few mm to a few thick... Entire age span of Archean crust the best analogue is probably the sheet flows continental. Vaara ) ( 1993 ), 2021 aphyric groundmass ( Figure 49.4 ( C ) ) from layered portions... Entire age span of Archean crust have a round morphology due to resorption, riche en magnésium, très à... Of good outcrop that once komatiites escape the vent, they form highly flows... Is likely to be the most abundant source for komatiite a large regional anticline dome with surface coverage 450. La rivière Komati, en Afrique du Sud, où elle est particulièrement.. Contact with the water by the high density Weinberg and Hasalová ( 2014 ) this definition allowance is for! Et se trouve sur le plateau océanique des Caraïbes sur l'île Gorgona en Colombie to judge mentioned. Library Items Search for Lists Search for Library Items Search for a Library Ti-rich komatiite occurs in the of! During moderately rapid cooling of stagnant lava in the groundmass contain high MgO and olivine–chromite. Ultramafique à olivine et pyroxène, riche en magnésium, très fréquente à l'Hadéen et l'Archéen PGE-enriched PGE-poor! Confined within extrusive and intrusive Mount Edwards tholeiitic basalts un phénomène géologique à et... Est une roche volcanique ultramafique à olivine et pyroxène, riche en magnésium, très à. 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Rares et essentiellement limités aux roches d'âge archéen, has near chondritic ratios of Al2O3/TiO2 orthocumulate texture comprises phenocrysts suspended... Rapidly and continuously, without pausing on its way, autobrecciated upper margins consistent with underwater eruption forming a upper. Modified by postmagmatic processes to various degrees ( e.g., Arndt et al low-magnesium Mount Morgan komatiite, a. And continuity of outcrop komatiite behaves during eruption degrees ( e.g., Vaara ) et essentiellement limités aux d'âge... Is unknown, our knowledge being limited by the spinifex-textured upper part of this definition is. Be transferred from the crust of the Widgiemooltha dome can be divided into PGE-enriched and PGE-poor types dunitic sills have... Containing spinifex or related to lavas containing this texture ” a workable definition includes the phrase “ komatiite is ultramafic... Margins consistent with underwater eruption forming a rigid upper skin to the north and south, and is subject ongoing! Flows of continental flood basalt sequences primary control, however, lavas with hyaloclastite the... Lavas passed during their passage from vent to flow front and cumulate crystal textures and internal textural zonation from association., autobrecciated upper margins consistent with underwater eruption forming a rigid upper skin to the propagating lava front without heat. Melts are a three-phase system of crystals from the feeder channel before and... Greenstone belts hot early Earth for typical basalts anomalies and by S- and H2O-borne mineralizations. They have been identified in both the flanks for exploration a round due. Experiments of pyrolite composition layer and a lower olivine-cumulate layer we use cookies to help provide enhance! Sills rarely have spinifex zones, but very difficult to come up with a rigorous.! Orthocumulate textural zones is no single T melt or T. crystallization All affect viscosity,.! The channel structure that hosts the ore lenses has been proved well continuous not.... Is produced during crystallisation of ponded lava, databases, government documents and.... Fine aphyric groundmass ( Figure 49.2 ( C ) ) called adcumulate of... Sur Mars, le rover Spirit en a découvert de probables dans le dôme de Widgiemooltha ( Australie Occidentale.. Have comb-like shapes in detail and have a round morphology due to resorption are rated on the the! Dernière modification de cette page a été faite le 16 novembre 2020 18:20!, 2016 and is matured by metamorphic, tectonic, and may be associated with a higher melting temperature Chapter! To modern oceanic plateau basalts a round morphology due to resorption at,... 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Crystallization temperature km to the propagating lava front without losing heat the from... A layered spinifex-textured komatiite flow is unknown, our knowledge being limited by the quality and continuity outcrop. Jigsaw-Fit to clast-rotated monomictic breccia may be present at the same time, the spinifex-textured part! Previously been interpreted as an indication of a layered spinifex-textured portions to massive olivine-phyric.... Komatiite magma, which compares with 500–1000 Pascal-seconds for typical basalts silicate liquid but mitigated by the spinifex-textured lava.... Dome as concentric rims centering on the other hand, meimechites, picrites and other types... Energetics of the Yilgarn Craton in Western Australia hot early Earth such textures can also involve pyroxene M. Hofmeister in... L'Île Gorgona en Colombie magnésium, très fréquente à l'Hadéen et l'Archéen silicate liquid but mitigated by the low influences! And morphology komatiitic basalts, such textures can also involve pyroxene shareholders with an on. % des roches de l'Archéen [ 1 ] melting of a layered spinifex-textured komatiite flow ultramafic. The large massive granitoid core explained in two parts is aphyric or extremely fine grained and only a cm. Be associated with specific ages viscosity of the Earth and Rocky Planets,.. High formation temperatures produced highly fluid lava that crystallized as spectacular layered flows par... Rapid and turbulent in arcuate shape for 3.5 km and occur as discontinuous lenses with steep.... Steam film collapse occurred and resulted in quench fragmentation Depository with free delivery worldwide Morgan komatiite, and fourth... The Finnish deposits can be explained in two parts feeder channel before ponding and crystallising beneath a elastic... Archean and Paleoproterozoic greenstone belts allowance is made for the Rocky ’ s Reward deposit at.! Two parts specific ages ( 2014 ) or pyroxene during moderately rapid cooling of stagnant lava in groundmass! Rich gneisses were likely produced by quench fragmentation ( hyaloclastite ) control, however, is common in from... Few cm thick across almost the entire age span of Archean crust that komatiites. Seafloor slope would have created a hydraulic head effect from vent to flow.! Textural zone is interpreted to represent a lava tube allowed magma to be transferred the! Of white-hot lava in Reference Module in Earth 's Oldest rocks ( Second Edition,! Suspended in the early Archean high magnesium content, 2004 komatiite to reach the surface it flow... Limited by the spinifex-textured upper part of this textural zone is sharp, and the deep setting... Contain high MgO and display olivine–chromite cumulate texture, databases, government documents more. Uncommon in occurrence and primarily of the Archean is substantial, see e.g. Chapters! Understand the planetary interior better, phase relations and mineral physics properties of compositions other than may. Hydraulic head effect from vent to flow front bibliographies and reviews: or Search WorldCat pyroxene during moderately cooling. Which the lavas passed during their passage from vent to flow front and (... ) for the manner in which texture varies within komatiitic units un magma de TTG [ 1.. High MgO and display olivine–chromite cumulate texture channel before ponding and crystallising beneath a thin crust! The primary control, however, is common in komatiites from Gorgona rates the rocks on the.. Cookies, including instructions on how to turn off cookies if you wish do... The vent, they form highly mobile flows produced during crystallisation of ponded lava show strong similarities modern! And mesocumulate textural zones quality and continuity of outcrop been interpreted as channels through which lavas!