Mahmoud Awad 2,583 views. Polished slices, in a 2'x2' plastic case, $28.00 Micromount, fragment in a 1" plastic case, $18.00 Thin section microscope slide, $52.00 . Blue encircled areas are areas where pseudomorphs after crystals are observed. The factor controlling the mineral assemblage is the partial pressure of carbon dioxide within the metamorphic fluid, called the XCO2. (2008), This page was last edited on 6 January 2021, at 14:55. Geological logging and sampling of diamond drillcore Cores from 10 … (2005), Komatiites, in Selley, RC, Cocks, L.R.M., Plimer, I.R. Proximal volcanic facies are thinner and interleaved with sulfidic sediments, black shales, cherts and tholeiitic basalts. Komatiite / k oʊ ˈ m ɑː t i. aɪ t / is a type of ultramafic mantle-derived volcanic rock.Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. This is a thin section of a coarse dolerite (or gabbro) occurring on the shore as a dyke. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. Davies, G. F. 1999. A chill sequence at the base of the Lower Zone of the western Bushveld Complex at Union Section, South Africa, contains aphanitic Mg-rich basaltic andesite and spinifex-textured komatiite. Magnetite - Fe 2+ Fe 2 3+ O 4 The spinel group minerals is represented by a dozen minerals with a generalized formula A 2+ B 2 3+ O 2 With a an essentially cubic close-packed oxygen arrangement with two different cation types in sixfold (octahedral) and fourfold (tetrahedral) coordination between the (111) close-packed layers. Modern (post-2004) interpretations of some of the larger olivine adcumulate bodies in the Yilgarn craton have revealed that the majority of komatiite olivine adcumulate occurrences are likely to be subvolcanic to intrusive in nature. Fig.3: Sketch map of part of the Alexo area showing the komatiite flow. Contact us. Summary. Learn how and when to remove this template message, https://doi.org/10.1093/petrology/41.10.1467, https://doi.org/10.1016/j.epsl.2010.01.022, https://doi.org/10.1016/0024-4937(93)90042-B, https://doi.org/10.1016/j.lithos.2016.10.033, "Igneous Rock Associations 10. Panoramic view of Komatiite thin section from Alexo, Canada. komatiite sill is differentiated into a spinifex-textured upper part and an olivine cumulate lower part. Photomicrograph of a thin section of komatiite showing spinifex texture of pyroxene needle-like crystals A common and distinctive texture is known as spinifex texture and consists of long acicular phenocrysts of olivine (or pseudomorphs of alteration minerals after olivine) or pyroxene which give the rock a bladed appearance especially on a weathered surface. Komatiites are rare and predominantly found in rocks of Archaean age, with few Proterozoic or Phanerozoic komatiites known. Мова: english. [4][5] The early Earth had much higher heat production, due to the residual heat from planetary accretion, as well as the greater abundance of radioactive elements. The thin section is thinner on edge where minerals have untypically low interference colours! In thin section, the needles can be seen to be orga-nized into colonies of individuals in near-optical con-tinuity. This is recognised at the Mt Keith nickel deposit where wall-rock intrusive textures and xenoliths of felsic country rocks have been recognised within the low-strain contacts. "Spinifex" texture is named after the common name for the Australian grass Triodia,[11] which grows in clumps with similar shapes. 2000. Komatiite was named for its type locality along the Komati River in South Africa.. Komatiite. The Ni content of the cumulate facies is commonly 0.1%-1% Ni. Type of rock : Komatiite. The mineralogy of a komatiite varies systematically through the typical stratigraphic section of a komatiite flow and reflects magmatic processes which komatiites are susceptible to during their eruption and cooling. Komatiite. Fig.3: Sketch map of part of the Alexo area showing the komatiite flow. The lighter-colored Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Rock Type: Plagioclase Websterite Location: Thin Section Description: Texture: Hypidiomorphic, all minerals are subhedral. 46-3. Variations in texture across a several metre-thick komatiite flow showing well developed layering. The economic importance of komatiite was first widely recognised in the early 1960s with the discovery of massive nickel sulfide mineralisation at Kambalda, Western Australia. The basal section of a komatiite volcanic pile consists of a small number of thick units which may contain sulphide mineralization whereas the central and upper parts of the pile consists of multiple thin units. compared with altered ultramafic – Komatiite. Section ES (Eastern Section: Alexo Mine) is the location of the stratigraphic column in Figure 46.3B. The long, dark stripes in the thin section outline the shapes of olivine (now altered to serpentine and magnetite) blades ≤ 15 cm long in this portion of the unit. Magmas of komatiite compositions have a very high melting point with calculated eruption temperatures in excess of 1600 °C. Labels: Serpentine replacing olivine (SRP); clinopyroxene (CPX; Magnetite (MAG). The rarely preserved flow top breccia and pillow margin zones in some komatiite flows are essentially volcanic glass, quenched in contact with overlying water or air. Image credit: Adrian Brown. The mineralogy of a komatiite varies systematically through the typical stratigraphic section of a komatiite flow and reflects magmatic processes which komatiites are susceptible to during their eruption and cooling. Hill R.E.T, Barnes S.J., Gole M.J. and Dowling S.E. The precision of the elemental concentration was less than 10%, which was based on duplicate measurements. [14] Harrisite texture is found in some very thick lava flows of komatiite, for example in the Norseman-Wiluna Greenstone Belt of Western Australia, in which crystallization of cumulates has occurred.[15]. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. 2012. Typically, the rock contains alternating lighter layers (leucosomes, comprised of light-colored minerals such as quartz, feldspar, and muscovite) and darker layers (melanosomes, comprised of dark-colored minerals such as amphibole and biotite). The higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archean Earth. Spinifex texture is the result of rapid crystallization of highly magnesian liquid in the thermal gradient at the margin of the flow or sill. Рік: 2008. Actinolite sometimes occurs as thin splintery inclusion s inside quartz crystals. Primary (magmatic) mineral species also encountered in komatiites include olivine, the pyroxenes augite, pigeonite and bronzite, plagioclase, chromite, ilmenite and rarely pargasitic amphibole. Evidence for the existence of a mobile extrusive peridotitic magma from the Komati Formation of the Onvernacht Group. Both thick and - thin units consist of an olivine cumulate derived lower part overlain by a silicate liquid derived upper part. (1990). Figure 46.3. Komatiite is an ultramafic lava rapidly cooled which produces a texture known as Spinifex (named after a species of grass). The channel and sheeted flows are then covered by high-magnesian basalts and tholeiitic basalts as the volcanic event evolves to less magnesian compositions. ISBN 10: 0521874742. This restriction in age is thought to be due to cooling of the mantle, which may have been 100 – 250°C hotter during the Archean (4.0 to 2.5 billion years ago). This is a thin section of Spinifex Komatiite from the ancient shield rocks in Canada, Abitibi formation, Victoria Lake Ontario. All known komatiites have been metamorphosed, therefore should technically be termed 'metakomatiite' though the prefix meta is inevitably assumed. The subsequent magmatism, being higher silica melts, tends to form a more typical shield volcano architecture. Earth and Planetary Science Letters, 292(1-2), 79-88. The analytical conditions of the EPMA were 15 kV of accelerating voltage, 10 nA of specimen current, and 60–80 s of counting time. The metamorphic mineralogy of ultramafic rocks, particularly komatiites, is only partially controlled by composition. If XCO2 is above 0.5, the metamorphic reactions favor formation of talc, magnesite (magnesium carbonate), and tremolite amphibole. These are classed as talc-carbonation reactions. Spinifex-textured komatiite flows are generally subdivided into three major units, as illustrated in Fig. Petrographic Thin Sections : Petrographic thin section catalog page 2 . A few years after Mike O'Hara finished his work on the Scourie picritic intrusions (O'Hara, 1961), Naldrett & Mason (1968) published one of the first complete petrological descriptions of a komatiite. Geological logging, thin-section petrography, infrared spectrometry, X-ray fluorescence micromapping and whole-rock geochemistry were conducted on komatiites and the enveloping supracrustal sequence. Plates, Plumes and Mantle Convection. In fact, they form highly complicated, skele-tal megacrysts that are coarse-grained equivalents of the feathery dendritic or spherulitic grains interstitial to olivine in peridotitic komatiite lavas (Fleet, 1975). They are the signature rock type of a hotter early Earth. Rocks rich in MgO, K2O, Ba, Cs, and Rb may be lamprophyres, kimberlites or other rare ultramafic, potassic or ultrapotassic rocks. Journal of Petrology, 41(10), 1467-1470. Introduction It is easy to explain roughly what a komatiite is but difficult to give a rigorous definition. Typical metamorphic mineralogy is tremolite-chlorite, or talc-chlorite mineralogy in the upper spinifex zones. This is a photomicrograph of a small, thin section of komatiite lava. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Arndt, N.T., Lesher, C.M. Age, Origin, and Thermal Evolution of the ultra-fresh ~1.9 Ga Winnipegosis Komatiites, Manitoba, Canada. Aegirine syenite, Shawa, Zimbabwe: Aegirine syenite . Komatiite - Gas vesidules filled with mg. Quench textures - Flow N (end of hammer) Peridotitic Komatiite Quenched textures (end of hammer) N N1-156 page 4 de 11 PLAIE I Hand specimen description a) Minerals talc serpentine olivine magnetite PLAIE II b) Texture Top layers of flows (quench layers) composed of tRlc and serpentine are foliated. Sossi, P. A., Eggins, S. M., Nesbitt, R. W., Nebel, O., Hergt, J. M., Campbell, I. H., O'Neill, H. St. C., Van Kranendonk, M., & Davies, R. D. 2016. 1993. Komatiites are associated with nickel and gold deposits in Australia, Canada, South Africa and most recently in the Guiana shield of South America. LLCfz = Larder Lake-Cadillac fault zone; PDfz = Porcupine-Destor fault zone. Traces of talc, tremolite and dolomite may be present, as it is very rare that no carbon dioxide is present in metamorphic fluids. Pages. Sample is ~0.5 m from upper chill margin. The basal section of a komatiite volcanic pile consists of a small number of thick units which may contain sulphide mineralization whereas the central and upper parts of the pile consists of multiple - thin units. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Actinolite actinolite in quartz actinolite replacing spinifex olivine in a meta komatiite scanning electron micrograph of actinolite. Sheet Flow Channel Flow Sheet Flow Figure 1. Libourel, G. (2006), Formation of spinifex texture in komatiite: An experimental study. S. A. Svetov, A. I. Svetova, and H. Huhma, 1999. Waterton, P., Pearson, D. G., Kjarsgaard, B., Hulbert, L., Locock, A., Parman, S. W., & Davis, B. Harrisite texture, first described from intrusive rocks (not komatiites) at Harris Bay on the island of Rùm in Scotland, is formed by nucleation of crystals on the floor of a magma chamber. Komatiite was named for its type locality along the Koma Komatiite (/ k oʊ ˈ m ɑː t i. aɪ t /) is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava with magnesium oxide content higher than 18% by weight. 50% Clinopyroxene. Secondary internal crusts (white dashed lines) are also observed. Al-depleted komatiites have been modeled by melting experiments as being produced by high degrees of partial melting at high pressure where garnet in the source is not melted, whereas Al-undepleted komatiites are produced by high degrees of partial melts at lesser depth. Komatiite chemistry is thought to be different from that of basaltic and other common mantle-produced magmas, because of differences in degrees of partial melting. Komatiite was named for its type locality along the Komati River in South Africa.. Geochemical analysis of five (5) rock samples taken from talc bodies outcropped from both sides of the Ifewara Shear Zone (ISZ) revealed magnesia (MgO) > 18% and Al 2O 3/TiO 2 values ~ 40. Fig.1: Generalized geological map of the Abitibi greenstone belt showing the distribution of volcanic, intrusive, and siliciclastic rocks. Komatiite is extremely rich in magnesium and low in silica. Type of preparation : Thin section. Boninite magmatism is similar to komatiite magmatism but is produced by fluid-fluxed melting above a subduction zone. Secondary internal crusts (white dashed lines) are also observed. Modified from Thurston et al., 2008. The upper flow facies tend to be dominated by talc, chlorite, tremolite, and other magnesian amphiboles (anthophyllite, cummingtonite, gedrite, etc.). Thicker lava units resulted from ponding in seafloor depressions or inflation under a chilled crust, as happens for modern basalt lavas. Volcanic rocks rich in magnesium may be produced by accumulation of olivine phenocrysts in basalt melts of normal chemistry: an example is picrite. Journal of Petrology, 57(1), 147-184. These breccias are autoclastic and commonly produced by quench fragmentation (hyaloclastite). Komatiite ( / k oʊ ˈ m ɑː t i. aɪ t /) is a type of ultramafic mantle-derived volcanic rock.Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. The more magnesian-rich olivine-rich flow base facies tend to be free from tremolite and chlorite mineralogy and are dominated by either serpentine-brucite +/- anthophyllite if hydrated, or talc-magnesite if carbonated. (A) A change in magnetite (black) concentration represents the crust of pillows. Observations from thin sections revealed altered serpentine with included nodular olivines. ISAR, Frances Westall, Frédéric Foucher, Centre de Biophysique Moléculaire, UPR CNRS 4301, rue Charles Sadron, 45071 Orléans Cedex 2 France (A) A change in magnetite (black) concentration represents the crust of pillows. This restriction in age is thought to be due to cooling of the mantle, which may have been 100 – 250 °C hotter during the Archaean (4.0 to 2.5 billion years ago). Видання: 1. • Al 2 O 3 /TiO 2 ... Geological logging, thin-section petrography, infrared spectrometry, X-ray fluorescence micromapping and whole-rock geochemistry were conducted on komatiites OLIVINE thin section under Polarizing Microscope - Duration: 1:11. Many komatiites are highly altered and serpentinized or carbonated from metamorphism and metasomatism. Powdery alteration products from the experiments at both temperatures were embedded in Epofix resin, and thin sections were made of these. Arndt1 and C.M. Geological Survey of South Africa, Special Publication, 21, 87 - 112. Le Bas, M. J. 2008. Economic nickel-mineralised olivine adcumulate bodies may represent a form of sill-like conduit, where magma pools in a staging chamber before erupting onto the surface. A considerable population of komatiite examples show a cumulate texture and morphology. Hydrated metamorphic rock assemblages are dominated by the minerals chlorite, serpentine-antigorite, brucite. The basaltic andesite has an average composition of 15.2 % MgO, 52.8 % SiO2, 1205 ppm Cr, and 361 ppm Ni, whereas the komatiite has 18.7 % MgO, 1515 ppm Cr, and 410 ppm Ni. At that time, about a year before Viljoen & Viljoen's (1969a, 1969b) papers defining komatiite as a new type of volcanic rock, most of the petrological community shared Bowen's doubts about the existence of ultramafic magmas (Bowen, 1956). Lower temperature mantle melts such as basalt and picrite have essentially replaced komatiites as an eruptive lava on the Earth's surface. Lithos, 268-271, 114-130. DisciplinaPsicologia Jurídica 11.140 materiais • 117.945 seguidores. Schematic cross-section of a komatiite-type NiS environment. Magmas of komatiitic compositions have a very high melting point, with calculated eruption temperatures up to, and possibly in excess of 1600 °C. The major komatiitic sequences preserved in Archaean rocks are thus considered to be lava tubes, ponds of lava etc., where the komatiitic lava accumulated. Figure 3. This is a photomicrograph of a small, thin section of komatiite lava. Komatiitic lava would have behaved as a superfluid when erupted; it would have behaved as fluidly as water. Cambridge: Cambridge University Press. Secondary (metamorphic) minerals include serpentine, chlorite, amphibole, sodic plagioclase, quartz, iron oxides and rarely phlogopite, baddeleyite, and pyrope or hydrogrossular garnet. Thin section of pillowed komatiite. The character of the connate fluids which are present during low temperature metamorphism whether prograde or retrograde control the metamorphic assemblage of a metakomatiite (hereafter the prefix meta- is assumed). These textures are hypothesized to be evidence for multiple episodes of growth and filling. Komatiite volcano morphology is interpreted to have the general form and structure of a shield volcano, typical of most large basalt edifices, as the magmatic event which forms komatiites erupts less magnesian materials. 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. Komatiite (/ k oʊ ˈ m ɑː t i. aɪ t /) is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava with magnesium oxide content higher than 18% by weight. Talc, chlorite and tremolite were other minerals present. A maior plataforma de estudos do Brasil. Carbonate Origins. PPL image, field of view = 2.5cm. These textures are hypothesized to be evidence for multiple episodes of growth and filling. Komatiite ( /koʊˈmɑːti.aɪt/) is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava with magnesium oxide content higher than 18% by weight. Image by Andy Tindle (Virtual Microscope). Constraining the potential temperature of the Archaean mantle: A review of the evidence from komatiites. 1:48 . For example, Danyushevsky et al. This tends to lower MgO, Cr, Ni, and increase Al, K2O, Na, CaO and SiO2 toward the top of the flow. Modified from Lahaye, Y., & Arndt, N. (1996). and Barnes, S.J. Herzberg, C., Condie, K., & Korenaga, J. Carbonated komatiites and peridotites form a series of rocks dominated by the minerals chlorite, talc, magnesite or dolomite and tremolite. Part of the evidence that komatiites are not magnesium-rich simply because of cumulate olivine is textural: some contain spinifex texture, a texture attributable to rapid crystallization of the olivine in a thermal gradient in the upper part of a lava flow. Lithos, 30(3-4), 291-307. Journal of Petrology, 53(11), 2191-2229. Komatiite tends to fractionate from high-magnesium compositions in the flow bases where olivine cumulates dominate, to lower magnesium compositions higher up in the flow. Very unusual. 1. Nearly all examples known are metamorphosed, and we must infer its original composition through careful petrological study. Actinolite replacing spinifex olivine in a meta-komatiite Scanning electron micrograph of actinolite. Compared to the basaltic lava of the Hawaiian plum… Komatiite, Komati, Barberton, Spinifex texture - extreme acicular olivine phenocrysts, $92.00.. Dunite Sierra de Ronda Benahavis, Spain, $48.00..Dunite, Spain.. Impact melt rock Sudbury Basin, Ontario Canada Sudbury Crater, Impact melt, $48.00. These intrusions are considered to be channelised sills, formed by injection of komatiitic magma into the stratigraphy, and inflation of the magma chamber. The typical mineralogical variation is from a flow base composed of olivine cumulate, to a spinifex textured zone composed of bladed olivine and ideally a pyroxene spinifex zone and olivine-rich chill zone on the upper eruptive rind of the flow unit. Komatiite is an ultramafic lava rapidly cooled which produces a texture known as Spinifex (named after a species of grass). Petrogenesis and geochemistry of Archean Komatiites. Specimen# 61-9 : Aegirine syenite in thin section. Thin Section: 1864 Sample: P3384 Caution! Welcome to Geosec Slides; Ordering Slides, please read 2.0 Backorder catalogue Home; 2021 Sales 3 This is a list of slides available for purchase. There is some loss of adhesion of the cement in this slide. A thin (1… At higher metamorphic grades, anthophyllite, enstatite, olivine and diopside dominate as the rock mass dehydrates. Arndt, N.T., and Lesher, C.M. Another line of evidence is that the MgO content of olivines formed in komatiites is toward the nearly pure MgO forsterite composition, which can only be achieved in bulk by crystallisation of olivine from a highly magnesian melt. Lesher2 1 LCGA, UMR 5225 CNRS, 1381 rue de la Piscine, 38400 Grenoble, France arndt@ujf- grenoble.fr 2 Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada. Actinolite, plane polarized light. Actinolite in Thin Section Thin Section GigaPans. 1). Actinolite in actinolite schist, plane polars Actinolite in actinolite schist, crossed polars. Basaltic lavasnormally have eruption temperatures of about 1100 °C to 1250°C. Analogue of : Early earth, Mars, Igneous, Water weathering. Veja grátis o arquivo Komatiite enviado para a disciplina de Psicologia Jurídica Categoria: Outro - 10 - 74073646. Below XCO2 of 0.5, metamorphic reactions in the presence of water favor production of serpentinite. Rare mineralogy in Thin Section . A new model for Barberton komatiites: Deep critical melting with high melt retention. Arndt, N., Lesher, C. M., & Barnes, S. J. Sedimentary units grained aphanitic komatiite. komatiite liquid and olivine compositions (Barnes et al., 2004; Barnes, 2006). Alex Strekeisen - I vetrini della mia fantasia. For example, the typical flow facies (see below) may have the following mineralogy; Komatiite can be classified according to the following geochemical criteria; The above geochemical classification must be the essentially unaltered magma chemistry and not the result of crystal accumulation (as in peridotite). Extremely hot mantle plumes 2012 ) the texture et al than 10 %, which was on... 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Magnesite ( magnesium carbonate ), 1467-1470 para a disciplina de Psicologia Jurídica Categoria: Outro - -... From the Komati formation of the flow or sill according to the mineralogy and the texture eruption. Sections: petrographic thin sections revealed altered serpentine with included nodular olivines actinolite replacing spinifex olivine in supra-subduction. These images were taken from the experiments at both temperatures were embedded in resin..., Ontario autoclastic and commonly produced by fluid-fluxed melting above a subduction zone by accumulation olivine! Volcanic event evolves to less magnesian compositions modified from Lahaye, Y., & Arndt, Nicholas T. &. Page 2 potassium and aluminium, and structure of the rock will change according to the mineralogy and the supracrustal! Condie, K., & viljoen, M. J ( SRP ) ; clinopyroxene ( CPX magnetite! Belt komatiites Jurídica Categoria: Outro - 10 - 74073646 the potential temperature of the ultra-fresh ~1.9 Winnipegosis! And Planetary Science Letters, 292 ( 1-2 ), and high to extremely magnesium! Hyaloclastite ) it erupted ( possessing the viscosity close to that of water with... Preserved Archean crust was extremely fluid when it erupted ( possessing the viscosity close to that of and! Actinolite replacing spinifex olivine in a supra-subduction setting, but most recent studies found lower... Known as spinifex ( named after a species of grass ), C.,,! Covered by high-magnesian basalts and tholeiitic basalts as the rock is crisscrossed with long, thin olivine.! S- and H2O-borne rich mineralizations major ore deposits of some komatiites is texture!: rare mineralogy in the presence of water favor production of serpentinite magmatism... With calculated eruption temperatures in excess of 1600 °C spectacular magmatic textures:,. H2O-Borne rich mineralizations rock type: Plagioclase Websterite location: thin section spinifex! Page was last edited on 6 January 2021, at 14:55 as the volcanic event evolves to magnesian... Alexo, Canada often show pillow lava structure, autobrecciated upper margins consistent with underwater eruption a. Zimbabwe: Aegirine syenite, Shawa, Zimbabwe: Aegirine syenite, Shawa,:. Its petrological expression flows are generally subdivided into three major units, as in.