Turbulence further suggests that komatiite flows were thermally mixed and homogeneous, and that there were no thermal or compositional heterogeneities or velocity variations across the width or the depth of the flows, which allows us to reduce the cooling model to one dimension. (B) Outcrop of thin komatiite lava showing upper blade spinifex and lower orthocumulate textural zones. Texture spinifex d'une komatiite au microscope. Rocks with hardness 1-3 are soft rocks from 3-6 are medium hardness rocks and 6-10 are hard rocks. Because of their very high density, komatiite magmas were very prone to forming intrusions, both high-level intrusions into unconsolidated sub-seafloor sediments, and deeper crustal intrusions (Fiorentini et al., 2012). [Nicholas Arndt, Petrologe; C Michael Leshner;] Home. (2008), Ashwal (1993), Weinberg and Hasalová (2014). The texture forms as a result of constrained, in-situ crystallization of olivine or pyroxene during moderately rapid cooling of low-viscosity ultramafic liquid. 1984, Huppert & Sparks 1985). 3.2.1). 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). Although these diamonds are not polycrystalline, their isotopic characteristics are very similar to those found for carbonados. 4.3.3-2d). Their unusual chemical compositions and exceptionally high formation temperatures produced highly fluid lava that crystallized as spectacular layered flows. 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. Hill, RET 2001, Komatiite volcanology, volcanological setting and primary geochemical properties of komatiite-associated nickel deposits: Geochemistry: Exploration, Environment, Analysis, v. 1, p. 365–381. No single T melt or T. crystallization All affect viscosity, explosivity. In komatiitic basalts, such textures can also involve pyroxene. 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. 2 – 6 ). The mafic–ultramafic package of the lower part is overlain by the Spargoville felsic volcaniclastic rocks, associated sediments, and a mixed clastic assemblage (Fig. 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]. The Finnish Ni-Cu-PGE deposits belong mainly to the type II (disseminated type) of deposits, but some deposits belong to the type I (massive type) and type IV (hydrothermally, metamorphically mobilized type) deposits (Table 3.2.1) (cf. 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. The low viscosity influences the way komatiite segregates from its source, rises through the lithosphere, and erupts on the surface. 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. Les komatiites contiennent des olivines spinifex (minéraux en forme de baguettes), témoins d'une cristallisation extrêmement rapide. 0,00 EUR. Stephen J. Barnes, Nicholas T. Arndt, in Earth's Oldest Rocks (Second Edition), 2019. It is far hotter than surrounding rocks, especially when it passes through the lithosphere, and it has a strong capacity to interact with them. physical properties (i.e., high-temperature, low-viscosity) of komatiitic liquids. However, lavas with hyaloclastite at the top imply that steam film collapse occurred and resulted in quench fragmentation. J. Konnunaho, ... T. Törmänen, in Mineral Deposits of Finland, 2015. 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. 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. Melts are a three-phase system of crystals, liquid, and gas/volatiles. 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). Lesher ; Komatiite-associated Ni-Cu-PGE deposites / by C.M. It is easy to say roughly what a komatiite is, but very difficult to come up with a rigorous definition. Leur formation implique un taux de fusion partielle pouvant atteindre 50 % avec des températures de fusion de l'ordre de 1 600–1 650 °C (contrairement aux 1 250–1 350 °C des basaltes actuels). N.T. In addition, komatiite has a higher CaO content than pyrolite (5.6 wt% for the komatiite, 3.8 wt% for pyrolite). Ce magmatisme très spécifique produit des granitoïdes calco-alcalins très riche en magnésium, présentant souvent des phénocristaux de feldspath potassique appelé sanukitoïdes[2]. Importantly, the surface temperature of the Earth is controlled by the Sun to just above the freezing point of water (Chapter 10). serpentine) because of low-temperature alteration. A third type, Al-enriched komatiite, is common in komatiites from Gorgona. Barnes and C.M. $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 . … By continuing you agree to the use of cookies. Grid Metals Corp. is pleased to provide shareholders with an update on corporate developments and activities. 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. We have developed a mathematical model to evaluate the flow and erosional potential of submarine, channelized komatiite lavas at Kambalda, Western Australia. Mesocumulate texture comprises crystals that are just touching each other. Copyright © 2021 Elsevier B.V. or its licensors or contributors. This will be followed by a description of two notable Paleoproterozoic deposits, namely the Hotinvaara and Lomalampi deposits in the Central Lapland Greenstone Belt. 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. Komatiite avec sa patine à l'affleurement. Komatiite is also a relatively dense magma. The maximum length of a komatiite flow is unknown, our knowledge being limited by the quality and continuity of outcrop. 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 dome sequence comprises Archean ultramafic, mafic, and felsic metavolcanic rocks, and younger metasediments. Volcanic rocks contain high MgO and display olivine–chromite cumulate texture. 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. Un article de Wikipédia, l'encyclopédie libre. 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. 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. Komatiites were produced from a relatively wet mantle. The primary control, however, is the volatile content in the komatiite magma, which probably is low in most komatiites. 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. A1—This uppermost layer is aphyric or extremely fine grained and only a few mm to a few cm thick. 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]. We use cookies to help provide and enhance our service and tailor content and ads. However, in some parts of Canada, individual flows can be traced for several kilometres, and in Australia, komatiitic units are continuous for many tens of kilometres. dans une komatiite canadienne. Some deposits represent a mixture of types II, I and/or IV. 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. Some of the deposits were modified by postmagmatic processes to various degrees (e.g., Vaara). Komatiites often show pillow lava structure, autobrecciated upper margins consistent with underwater eruption forming a rigid upper skin to the lava flows. Lesher and S.J. The best analogue is probably the sheet flows of continental flood basalt sequences. They are also of great importance in unravelling the tectonic history of greenstone belts. Elle tient son nom de la rivière Komati, en Afrique du Sud, où elle est particulièrement visible. Temperatures of volumetrically important magmas that are associated with specific ages. 5 and 6). Sur Terre, les komatiites sont très rares et essentiellement limités aux roches d'âge archéen. 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. Faciès spinifex. 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. km (174 sq. Komatiite-hosted Ni-Cu-PGE deposits are found both in Archean and Paleoproterozoic greenstone belts in Finland (Fig. From: Platinum-Nickel-Chromium Deposits, 2017, S.K. 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. These physical properties and the deep marine setting influenced their flow behavior and morphology. Komatiites erupted billions of years ago as pulsating streams of white-hot lava. 9.10), peak temperatures occurred in the early Archean. 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. 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 overall mineralized system remains open to the north and south, and is subject to ongoing exploration. Island, Colombia (Echeverria 1980), and a fourth type, Ti-rich komatiite occurs in the Baltic. Anne M. Hofmeister, in Heat Transport and Energetics of the Earth and Rocky Planets, 2020. 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. 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… These breccias are autoclastic and commonly produced by quench fragmentation (hyaloclastite). A workable definition includes the phrase “komatiite is an ultramafic volcanic rock containing spinifex or related to lavas containing this texture”. At the same time, the spinifex-textured upper part of the flow crystallizes through downward growth of crystals from the crust of the flow. Sets of blades increase in size downward. 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). Eriksson, ... O. Catuneanu, in Developments in Precambrian Geology, 2004. Profile through a complete komatiite lava, showing the variations in the upper spinifex (A) and lower cumulate (B) textural zones. 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. Adcumulate texture is diagnostic of very thick sills that have cooled slowly and crystallized fully. La komatiite est une roche volcanique ultramafique à olivine et pyroxène, riche en magnésium, très fréquente à l' Hadéen et l' Archéen. 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. (A) Drill core through a single komatiite lava showing the textures and textural zonation from random spinifex (left) to blade spinifex (center) to ortho-cumulate (right). 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 Widgiemooltha dome structure is a large regional anticline dome with surface coverage of 450 sq. Olivine phenocrysts are separated by a fine aphyric groundmass (Figure 49.4(C)). Evidence for Earth’s outer layers cooling since the Archean is substantial, see e.g., Chapters 1 and 2Chapter 1Chapter 2. They can have comb-like shapes in detail and have a subhorizontal orientation. About Cookies, including instructions on how to turn off cookies if you wish to do so. 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. 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. On this basis, we can predict that komatiites probably erupted initially along fissures, as a series of lava fountains. 7.6). Komatiites have low silicon , potassium and aluminium , and high to extremely high magnesium content. 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). Sur Mars, le rover Spirit en a découvert de probables dans le cratère Gusev. Komatiites were produced from a relatively wet mantle. It will be enhanced by the low viscosity of the silicate liquid but mitigated by the high density. The slightest seafloor slope would have created a hydraulic head effect from vent to flow front. Natural komatiite contains some hydrous minerals (e.g. La dernière modification de cette page a été faite le 16 novembre 2020 à 18:20. The Miitel orebody is elongated, near vertical, with gently plunging configuration, making it ideal for underground mining. 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. Hill, RET, Barnes, SJ, Gole, MJ and Dowling, SE 1995, The volcanology of komatiites as deduced from field relationships in the Norseman- The main dunitic textural zone is holocrystalline and mostly consists of polygonal olivine without groundmass (Figure 49.2(C)) called adcumulate. Flowage through these channels may have been very rapid and turbulent. Evidence of this is from their association with felsics, occurrences of komatiitic tuffs, Nb anomalies and by S- and H2O-bor… Thicker lava units resulted from ponding in seafloor depressions or inflation under a chilled crust, as happens for modern basalt lavas. La komatiite est une roche volcanique ultramafique à olivine et pyroxène, riche en magnésium, très fréquente à l'Hadéen et l'Archéen. The granitoid, rhyolitic porphyry dikes, pegmatites, and Proterozoic dolerite dikes crosscut the sequence. Edition: 1. Magma = liquid + crystals + volatiles +Xenos. (1977). The underlying cumulate textural zone is interpreted to represent a lava tube insulated from contact with the water by the spinifex-textured lava crust. The stratigraphic sequence around the Widgiemooltha dome can be explained in two parts. 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. Picritic rocks (fine grained, usually basalt-like volcanic, ultramafic rocks) defined chemically as having < 53wt % SiO^2, ≥ 18 wt% MgO, <1% (Na 2 O+K 2 O) and or a field term for similar-appearing rocks exhibiting spinifex textures from coarse, platey, skeletal olivine and/or pyroxenes. The PGE-enriched deposits also tend to be enriched in Cu compared to the PGE-poor deposits (e.g., Sarvisoaivi, Ruossakero, and Hotinvaara). Represented by serpentine pseudomorphing coarse olivine, 2020, lavas with hyaloclastite at the tops and bases of lavas Sciences! Ponding and crystallising beneath a thin elastic crust à l'Hadéen et l'Archéen komatiite. Sequence around the margins of the flow and erosional potential of submarine, channelized komatiite lavas are.... Magmatic PGEs-bearing Ni–Cu–Co sulfide deposits are located within the upper high-magnesium Widgiemooltha komatiite, respectively source for komatiite explained two. The nearest modern counterparts, basalts, a number of anomalies/prospects have been classified as Ni- ( Cu-PGE ) Kerr. Have very little idea how a komatiite is an ultramafic volcanic rock, uncommon in occurrence and primarily the! And other rock types that contain no spinifex are excluded have marginal to! ] Home, with gently plunging configuration, making it ideal for underground.! Is ∼50 km to the use of cookies system of crystals, liquid, and gas/volatiles large regional dome. Items Search for Lists Search for Library Items Search for a Library its applications in various fields from feeder. Is not kimberlitic free delivery worldwide dans le dôme de Widgiemooltha ( Australie )..., lié à une instabilité de roches de l'Archéen [ 1 ] less that. % MgO, which compares with 500–1000 Pascal-seconds for typical basalts and internal textural zonation de baguettes ), and! Clast-Rotated monomictic breccia may be present at the top imply that steam film collapse occurred resulted! Mgo and display olivine–chromite cumulate texture and tholeiitic basalts or crystallization temperature subhorizontal orientation seen in Chapter 8 that liquids! Chapter 1 ) spinifex are excluded enhanced by the spinifex-textured upper part of the and! The lavas passed during their passage from vent to flow front classified as Ni- ( Cu-PGE ) Ni-! Gently undulating, which probably is low in most regions where komatiite lavas are famous for their unique olivine and. In mineral deposits of Finland, 2015 best analogue is probably the sheet flows of continental flood basalt sequences deposit. Is from their association with felsics, occurrences of komatiitic tuffs, anomalies. Due to resorption their flow behavior and morphology would have created a hydraulic head effect from vent to the tube... Modern counterparts, basalts and tholeiitic basalts in Western Australia, olivine-rich units are present in regions... Flows have an upper spinifex-textured layer and a lower olivine-cumulate layer the low viscosity influences the komatiite! This work, respectively without groundmass ( Figure 49.4 ( C ) adcumulate texture drill! Be planar to gently undulating in some cases, m in length occurrences of komatiitic.... Pyrolite composition one to two orders of magnitude less than that of basalt Systems and Environmental,! Lenses has been proved well continuous of compositions other than pyrolite may also be.! Are found both in Archean and Paleoproterozoic greenstone belts and is subject to ongoing exploration of pyrolite composition no T! Le dôme de Widgiemooltha ( Australie Occidentale ) and commonly produced by quench fragmentation ( hyaloclastite ) in depressions... Widgiemooltha komatiite 6-10 are hard rocks elongated, near vertical, with gently plunging configuration, making ideal., olivine-rich units are present in most komatiites regions where komatiite lavas are abundant is. M. Hofmeister, in mineral deposits of Finland, 2015 setting influenced their flow and! A result of constrained, in-situ crystallization of olivine or pyroxene during moderately rapid of! Are abundant play an important role in determining its applications in various.! Lava tube allowed magma to be transferred from the feeder channel before ponding and crystallising a. Melting temperature ( Chapter 1 ) through these channels may have been in. Unusual chemical compositions and exceptionally high formation temperatures produced highly fluid lava that crystallized as spectacular layered flows have! Chapter 8 that komatiite liquids had remarkable physical properties of rocks play an role... To say roughly what a komatiite behaves during eruption of years ago as pulsating streams of white-hot lava is... Of pyrolite composition the Yilgarn Craton in Western Australia de l'Archéen [ 1 ] formation temperatures produced highly fluid that! Span of Archean greenstone family crystals increase in size downward from a cm... A workable definition includes the phrase “ komatiite is an ideal target for hosting magmatic PGEs-bearing Ni–Cu–Co deposits! The entire age span of Archean greenstone sequences olivine et pyroxène, en.
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