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. Secondary (metamorphic) minerals include serpentine, chlorite, amphibole, sodic plagioclase, quartz, iron oxides and rarely phlogopite, baddeleyite, and pyrope or hydrogrossular garnet. These textures are hypothesized to be evidence for multiple episodes of growth and filling. Spinifex-textured komatiite flows are generally subdivided into three major units, as illustrated in Fig. Variations in texture across a several metre-thick komatiite flow showing well developed layering. Modified from Lahaye, Y., & Arndt, N. (1996). Typical metamorphic mineralogy is tremolite-chlorite, or talc-chlorite mineralogy in the upper spinifex zones. This tends to lower MgO, Cr, Ni, and increase Al, K2O, Na, CaO and SiO2 toward the top of the flow. Сторінки: 488 / 489. This is a thin section of Spinifex Komatiite from the ancient shield rocks in Canada, Abitibi formation, Pyke Hill, Ontario. 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 . 410 Likes, 7 Comments - TravelingGeologist (@travelinggeologist) on Instagram: “Here’s a lovely thin section of #spinifex texture of a #komatiite. Talc, chlorite and tremolite were other minerals present. Nisbet, E. G., Cheadle, M. J., Arndt, Nicholas T., & Bickle, M. J. Herzberg, C., Condie, K., & Korenaga, J. In addition, jigsaw-fit to clast-rotated monomictic breccia may be present at the tops and bases of lavas. Volcanic rocks rich in magnesium may be produced by accumulation of olivine phenocrysts in basalt melts of normal chemistry: an example is picrite. 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. The precision of the elemental concentration was less than 10%, which was based on duplicate measurements. and Barnes, S.J. Komatiite N.T. 46-3. The higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archean Earth. This then flows outwards from the vent fissure, concentrating into topographical lows, and forming channel environments composed of high MgO olivine adcumulate flanked by a 'sheeted flow facies' aprons of lower MgO olivine and pyroxene thin-flow spinifex sheets. 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. 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). Panoramic view of Komatiite thin section from Alexo, Canada. These breccias are autoclastic and commonly produced by quench fragmentation (hyaloclastite). please bear this in mind when purchasing. The pristine volcanic mineralogy of komatiites is composed of forsteritic olivine (Fo90 and upwards), calcic and often chromian pyroxene, anorthite (An85 and upwards) and chromite. Komatiites are associated with nickel and gold deposits in Australia, Canada, South Africa and most recently in the Guiana shield of South America. Primary (magmatic) mineral species also encountered in komatiites include olivine, the pyroxenes augite, pigeonite and bronzite, plagioclase, chromite, ilmenite and rarely pargasitic amphibole. Komatiites were first recognized in the late 1960s in the Barberton Mountainland greenstone belt in South Africa (Viljoen & Viljoen, 1969a,b).They have extremely high MgO contents; 18-30 wt.% compared to 10-15 wt.% for the most mafic mid-ocean ridge basalts (MORB) or … Видання: 1. (Editors). r/geology: The scientific study of the origin, history, and structure of the Earth. 2012. Nearly all examples known are metamorphosed, and we must infer its original composition through careful petrological study. Entrar; Criar perfil grátis; 489 pág. 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). Evidence of this is from their association with felsics, occurrences of komatiitic tuffs, Niobium anomalies and by S- and H2O-borne rich mineralizations. Thin section (left) in transmitted light of serpentinized komatiite rock (right) from the Mount Ada Basalt in Australia's Pilbara region (hammer shown for scale). Komatiite was named for its type locality along the Komati River in South Africa, and ... leaving extremely thin lava flows (down to 10 mm thick). Komatiite is an ultramafic lava rapidly cooled which produces a texture known as Spinifex (named after a species of grass). Le Bas, M. J. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. [6][7][8][9] Basaltic lavas normally have eruption temperatures of about 1100 to 1250 °C. Active lavas had very thin flow fronts, perhaps only cm thick, and spread as very thin sheets and lobes (Cas et al., 1999). B zones are poorly developed to absent, as not enough through-flowing liquid existed to grow the adcumulate. Secondary internal crusts (white dashed lines) are also observed. Komatiite is extremely rich in magnesium and low in silica. 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. Thin Section: 1864 Sample: P3384 Caution! IUGS reclassification of the high-Mg and picritic volcanic rocks. 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. In the distal thin flow facies, Thin Spinifex Textured Flows Mafic Footwall Generalized Cross Section of a Mineralized Flow Hangingwall Ore After Hill, & Cowden & Roberts. These are classed as talc-carbonation reactions. 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. Section ES (Eastern Section: Alexo Mine) is the location of the stratigraphic column in Figure 46.3B. Komatiites often show pillow lava structure, autobrecciated upper margins consistent with underwater eruption forming a rigid upper skin to the lava flows. Arndt, N.T., Lesher, C.M. LLCfz = Larder Lake-Cadillac fault zone; PDfz = Porcupine-Destor fault zone. The lighter-colored Constraining the potential temperature of the Archaean mantle: A review of the evidence from komatiites. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. 2008. 2010. A maior plataforma de estudos do Brasil. These images were taken from the thin section being offered. Geographically, komatiites are predominantly restricted in distribution to the Archaean shield areas, and occur with other ultramafic and high-magnesian mafic volcanic rocks in Archaean greenstone belts. 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. These intrusions are considered to be channelised sills, formed by injection of komatiitic magma into the stratigraphy, and inflation of the magma chamber. compared with altered ultramafic – Komatiite. of the samples (1/3 of the thin sections contain fresh olivine relicts) enhanced the quality and the confidence over the actual and future results on the chromite chemistry. komatiite magma that originated in a supra-subduction setting, but most recent studies found far lower water contents. Observations from thin sections revealed altered serpentine with included nodular olivines. Faure, F., Arndt, N.T. A considerable population of komatiite examples show a cumulate texture and morphology. Evidence for the existence of a mobile extrusive peridotitic magma from the Komati Formation of the Onvernacht Group. Journal of Petrology, 57(1), 147-184. 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. In thin section jig- Bavinton (1981) de¢ned three sedimentary rock saw-¢t textures are still apparent even in matrix- types at Kambalda : (1) pale siliceous sedimentary supported domains. A total of 47 samples have been . However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. 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. 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). Alex Strekeisen - I vetrini della mia fantasia. 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). [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. 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. Variations in texture across a several metre-thick komatiite flow showing well developed layering. Sheet Flow Channel Flow Sheet Flow Figure 1. Spinifex is one of the most spectacular magmatic textures. The slightest seafloor slope would have created a hydraulic head effect from vent to flow front. Actinolite in Thin Section Thin Section GigaPans. 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. Contact us. Image by Andy Tindle (Virtual Microscope). Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallized from a lava with ≥ 18 wt.% MgO. Cambridge: Cambridge University Press. Fig.3: Sketch map of part of the Alexo area showing the komatiite flow. Actinolite, plane polarized light. 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. Komatiites. 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