Their unusual chemical compositions and exceptionally high formation temperatures produced highly fluid lava that crystallized as spectacular layered flows. Orthocumulate texture comprises phenocrysts wholly suspended in the groundmass. A workable definition includes the phrase “komatiite is an ultramafic volcanic rock containing spinifex or related to lavas containing this texture”. Hill, RET, Barnes, SJ, Gole, MJ and Dowling, SE 1995, The volcanology of komatiites as deduced from field relationships in the Norseman- Komatiites were produced from a relatively wet mantle. Sets of blades increase in size downward. We have developed a mathematical model to evaluate the flow and erosional potential of submarine, channelized komatiite lavas at Kambalda, Western Australia. A viscosity of 1–2 Pascal-seconds was estimated for dry komatiite containing 28% MgO, which compares with 500–1000 Pascal-seconds for typical basalts. Komatiite is a fragile magma. Thick dunitic sills rarely have spinifex zones, but may have marginal ortho- to mesocumulate zones. Texture spinifex d'une komatiite au microscope. After Arndt et al. of komatiite-associated Ni—Cu—PGE deposits (Chapter 10). Although these diamonds are not polycrystalline, their isotopic characteristics are very similar to those found for carbonados. Pyke Hill, Ontario, Canada. Arndt, C.M. From magma temperatures (Fig. The texture forms as a result of constrained, in-situ crystallization of olivine or pyroxene during moderately rapid cooling of low-viscosity ultramafic liquid. Les masses basaltiques ayant une densité trop faible pour initier une sagduction, seules les komatiites, avec une densité de 3,3 permettent d'initier le phénomène[1]. The orebodies extend in arcuate shape for 3.5 km and occur as discontinuous lenses with steep dips. 9.10), peak temperatures occurred in the early Archean. On the other hand, meimechites, picrites and other rock types that contain no spinifex are excluded. 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… Importantly, the surface temperature of the Earth is controlled by the Sun to just above the freezing point of water (Chapter 10). 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. On this basis, we can predict that komatiites probably erupted initially along fissures, as a series of lava fountains. Un article de Wikipédia, l'encyclopédie libre. I. Jetzt Konto anlegen Erweiterte. Ces granites constituent 5 à 10 % des roches de l'Archéen[1]. Ce magmatisme très spécifique produit des granitoïdes calco-alcalins très riche en magnésium, présentant souvent des phénocristaux de feldspath potassique appelé sanukitoïdes[2]. Figure 9.10. These breccias are autoclastic and commonly produced by quench fragmentation (hyaloclastite). Melts are a three-phase system of crystals, liquid, and gas/volatiles. This will be followed by a description of two notable Paleoproterozoic deposits, namely the Hotinvaara and Lomalampi deposits in the Central Lapland Greenstone Belt. Komatiite de la localité type de Komati River (Afrique du Sud). Komatiite lavas would have been turbulent mostly near the vent, becoming laminar farther away (Cas et al., 1999), similar to carbonatite lavas at Oldoinyo Lengai volcano in Tanzania. 0,00 EUR. All eight main deposits are associated with the MgO-rich and Cr-poor cumulate portion of komatiitic or komatiitic basalt units and they mostly consist of disseminated sulfides (e.g., Vaara, Lomalampi, and Ruossakero) and minor massive to semimassive sulfides (e.g., Tainiovaara, Hietaharju, and Peura-aho). 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)). Figure 7.6. Thicker lava units resulted from ponding in seafloor depressions or inflation under a chilled crust, as happens for modern basalt lavas. (1977). 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. For example, many komatiite flows have an upper spinifex-textured layer and a lower olivine-cumulate layer. Barnes, 2006; Lesher and Barnes, 2009). physical properties (i.e., high-temperature, low-viscosity) of komatiitic liquids. Thick, massive, olivine-rich units are present in most regions where komatiite lavas are abundant. They can have comb-like shapes in detail and have a subhorizontal orientation. 9.9). The characteristic layering of komatiite flows is produced during crystallisation of ponded lava. 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. Figure 49.4. If it is assumed that the source contained 50% olivine, 25% opx, 15% cpx, and 10% garnet and adopting literature values for partition coefficients [e.g., Green , 1994 ], the degree of melting is about 30%. The viscosity of komatiite magma is very low, one to two orders of magnitude less than that of basalt. Active lavas had very thin flow fronts, perhaps only cm thick, and spread as very thin sheets and lobes (Cas et al., 1999). Create lists, bibliographies and reviews: or Search WorldCat. Their unusual chemical compositions and exceptionally high formation temperatures produced highly fluid lava that crystallized as spectacular layered flows. WorldCat Home About WorldCat Help. 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 spinifex textural zone has been interpreted as a chilled lava crust; the various skeletal spinifex zones reflect extremely rapid chilling of this crust due to the subaqueous environment and the very high temperature gradient between the lava surface and the water. At the same time, the spinifex-textured upper part of the flow crystallizes through downward growth of crystals from the crust of the flow. Prévisualiser. The nickel mine includes: Mount Edward, Miitel, Marines, and Redross mines in the eastern flank, and the Wannaway mine in the western flank. Sur Terre, les komatiites sont très rares et essentiellement limités aux roches d'âge archéen. In komatiitic basalts, such textures can also involve pyroxene. Evidence of this is from their association with felsics, occurrences of komatiitic tuffs, Nb anomalies and by S- and H2O-bor… However, lavas with hyaloclastite at the top imply that steam film collapse occurred and resulted in quench fragmentation. 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. The base of this textural zone is sharp, and may be planar to gently undulating. Some of the deposits were modified by postmagmatic processes to various degrees (e.g., Vaara). They have been classified as Ni-(Cu-PGE) and Ni-(Cu) deposits in this work, respectively. With the last part of this definition allowance is made for the manner in which texture varies within komatiitic units. The PGE-enriched deposits are found in both age groups and are either S-poor (e.g., Lomalampi) or S-rich (e.g., Hietaharju and Peura-aho). McCloy deposit, Kambalda, Western Australia. Most of the komatiite flows preserved in greenstone belts represent lateral facies – small sheets or lobes of lava that spread out from a central feeder. Implicit in the definition of komatiite is the notion, difficult to prove, that komatiites crystallise from liquids that contained > 18% MgO. Evidence of this is from their association with felsics, occurrences of komatiitic tuffs, Niobium anomalies and by S- and H2O-borne rich mineralizations. The cooling rate indicates an average loss of heat from the upper mantle of ~3.8 TW, which is compatible with our estimate of today's radioactive power as 1.5 TW (Section 9.2.5). Komatiite is a type of mantle-derived ultramafic volcanic rock, uncommon in occurrence and primarily of the Archean greenstone family. 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). Komatiite lavas are famous for their unique olivine spinifex and cumulate crystal textures and internal textural zonation. Komatiites were produced from a relatively wet mantle. Role of Solids, Liquids and Gas in Melts. Stanford Libraries' official online search tool for books, media, journals, databases, government documents and more. In order to understand the planetary interior better, phase relations and mineral physics properties of compositions other than pyrolite may also be important. Adcumulate texture is diagnostic of very thick sills that have cooled slowly and crystallized fully. 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). Elle tient son nom de la rivière Komati, en Afrique du Sud, où elle est particulièrement visible. mi). Lesher ; Komatiite-associated Ni-Cu-PGE deposites / by C.M. Pyroxene spinifex forms in the upper parts of komatiitic basalt flows. Island, Colombia (Echeverria 1980), and a fourth type, Ti-rich komatiite occurs in the Baltic. We have seen in Chapter 8 that komatiite liquids had remarkable physical properties that distinguished them from the nearest modern counterparts, basalts. The left-hand drill core is 5 cm wide. 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. The silica rich gneisses were likely produced by flux melting of a wet upper mantle. We know from work in areas of good outcrop that once komatiites escape the vent, they form highly mobile flows. Mesocumulate texture comprises crystals that are just touching each other. 4.3.3-2d). Stratigraphic column of Widgiemooltha structure showing the ultramafic–mafic and felsic sequence from base to top and characteristic localization of nickel–copper sulfide orebodies at the contact between the Mount Edwards tholeiitic basalt and the magnesium-rich upper komatiite cycle. Barnes and C.M. Mt Keith mine, Western Australia. Figure 6. It will be enhanced by the low viscosity of the silicate liquid but mitigated by the high density. 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. Langue: english. The violence of this fountaining is difficult to judge. Thus carbonados may be associated with a terrestrial origin that is not kimberlitic. Fichier: PDF, 16,57 MB. $70.00 (P) Authors: Nicholas Arndt, Université Joseph Fourier, Grenoble; C. Michael Lesher, Laurentian University, Ontario; Steve J. Barnes, Division of Exploration and Mining, CSIRO, Australia; Date Published: March 2018; availability: Available ; format: Paperback; isbn: 9781108460514; Rate & review $ 70.00 (P) Paperback . 7.6). 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. For komatiite to reach the surface and erupt, it must fill a continuous liquid column within rocks whose average density is greater than that of the komatiite itself. 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. km (174 sq. serpentine) because of low-temperature alteration. 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 komatiite-hosted Kambalda nickel sulfide deposit is ∼50 km to the north. 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. 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. 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). In terms of their PGE content, the Finnish deposits can be divided into PGE-enriched and PGE-poor types. Komatiite by Nicholas Arndt, 9781108460514, available at Book Depository with free delivery worldwide. Copyright © 2021 Elsevier B.V. or its licensors or contributors. 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. (C) Adcumulate texture in drill core, represented by serpentine pseudomorphing coarse olivine. Other flows grade along strike from layered spinifex-textured portions to massive olivine-phyric units. 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. showed in 2002 that the presence of a thermal gradient, such as exists at the margin of every flow and high-level intrusion, is instrumental in the formation of spinifex texture. 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. Rocks with hardness 1-3 are soft rocks from 3-6 are medium hardness rocks and 6-10 are hard rocks. These rocks show strong similarities to modern oceanic plateau basalts. Haldar, in Platinum-Nickel-Chromium Deposits, 2017. Komatiites often show pillow lava structure, autobrecciated upper margins consistent with underwater eruption forming a rigid upper skin to the lava flows, under which considerable lava tubes and pools accumulate. Komatiite-hosted nickel sulfide deposits represent some of the world’s largest nickel deposits, and are commonly higher grade than intrusion-related or lateritic nickel deposits (Hoatson et al., 2006).Western Australia hosts several world-class examples of komatiite-hosted nickel sulfide deposits, including Kambalda, Mt Keith and Perseverance (Hoatson et al., 2006). La sagduction définit un phénomène géologique à l'Archéen et l'Hadéen, lié à une instabilité de roches de différentes densités[1]. A1—This uppermost layer is aphyric or extremely fine grained and only a few mm to a few cm thick. Melts are chemically complex. Knowledge of the mineralogical constituents of the Earth's mantle depends heavily on high- P experiments of pyrolite composition. 3.2.1). 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. The main dunitic textural zone is holocrystalline and mostly consists of polygonal olivine without groundmass (Figure 49.2(C)) called adcumulate. These physical properties and the deep marine setting influenced their flow behavior and morphology. 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. Année: 2008. La dernière modification de cette page a été faite le 16 novembre 2020 à 18:20. In order to examine the amount of volatile elements in the starting material, the komatiite was heated at 1000°C for about 12 h at the Ni–NiO buffer. 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. Sulfur saturation occurs when silicate magma exceeds its capacity to dissolve S and/or contaminated in excess by a large quantity of S to the magma from country rocks. Komatiites are almost invariably associated with basalts in Archean greenstone sequences. 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. B1—Harrisitic texture: This zone is a cm to tens of cm thick and also consists of skeletal crystals, which may be elongate to equant in form. Jocelyn McPhie, Ray Cas, in The Encyclopedia of Volcanoes (Second Edition), 2015. Natural komatiite contains some hydrous minerals (e.g. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780128020418000109, URL: https://www.sciencedirect.com/science/article/pii/B978044463901100006X, URL: https://www.sciencedirect.com/science/article/pii/B9780123859389000493, URL: https://www.sciencedirect.com/science/article/pii/B9780124104389000042, URL: https://www.sciencedirect.com/science/article/pii/B9780124095489095798, URL: https://www.sciencedirect.com/science/article/pii/B9780128020418000079, URL: https://www.sciencedirect.com/science/article/pii/B9780124095489120834, URL: https://www.sciencedirect.com/science/article/pii/B9780128184301000094, URL: https://www.sciencedirect.com/science/article/pii/S0166263504800069, Distribution and Geochemistry of Komatiites and Basalts Through the Archean, Volcanic Successions Associated with Ore Deposits, The Encyclopedia of Volcanoes (Second Edition), Komatiite-Hosted Ni-Cu-PGE Deposits in Finland, Reference Module in Earth Systems and Environmental Sciences, Heat Transport and Energetics of the Earth and Rocky Planets. Stephen J. Barnes, Nicholas T. Arndt, in Earth's Oldest Rocks (Second Edition), 2019. Komatiites erupted billions of years ago as pulsating streams of white-hot lava. They can be a cm to tens of cm in length, and in some cases, m in length. Slow cooling of stagnant lava in the tube allowed crystallization, forming the ortho- and mesocumulate textural zones. 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. Komatiites erupted billions of years ago as pulsating streams of white-hot lava. Pages: 488 / 489. Elle tient son nom de la rivière Komati, en Afrique du Sud, où elle est particulièrement visible. La komatiite est une roche volcanique ultramafique à olivine et pyroxène, riche en magnésium, très fréquente à l'Hadéen et l'Archéen. The different varieties of komatiite—Al-depleted, Al-undepleted, Al-enriched, and Ti-enriched—are found at all ages through the Archean but decline rapidly in abundance in post-Archean terranes. 4.3.1-1), the type of spinifex texture varies systematically (Fig. … These are interpreted as channels through which the lavas passed during their passage from vent to flow front. Evidence for Earth’s outer layers cooling since the Archean is substantial, see e.g., Chapters 1 and 2Chapter 1Chapter 2. Eriksson, ... O. Catuneanu, in Developments in Precambrian Geology, 2004. We have very little idea how a komatiite behaves during eruption. Within the upper parts of komatiite flows (Fig. Some deposits represent a mixture of types II, I and/or IV. B2—Cumulate texture: Crystals are equant to slightly elongate, and have a round morphology due to resorption. The primary control, however, is the volatile content in the komatiite magma, which probably is low in most komatiites. It is far hotter than surrounding rocks, especially when it passes through the lithosphere, and it has a strong capacity to interact with them. The nickel Reserve base for the Belt has been estimated at 3.276 Mt at 3.6% Ni, equivalent to 117,100 tonnes (∼129,000 tons) of nickel metal (Source: MINCOR Resources NL, Australia). Is diagnostic of very thick sills that have cooled slowly and crystallized fully MgO and display olivine–chromite texture!, is common in komatiites from Gorgona in both the flanks for exploration,,. Ago as pulsating streams of white-hot lava few mm to several cm in arcuate for! Kambalda nickel sulfide deposit is ∼50 km to the propagating lava front without heat. Of a wet upper mantle B ) outcrop of thin komatiite lava upper! Minéraux en forme de baguettes ), Weinberg and Hasalová ( 2014.. Along strike from layered spinifex-textured komatiite flow is unknown, our knowledge being limited by the viscosity! 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