Panoramic view of Komatiite thin section from Alexo, Canada. Boninites with 10–18% MgO tend to have higher large-ion lithophile elements (LILE: Ba, Rb, Sr) than komatiites. Komatiite was named for its type locality along the Komati River in South Africa,[2] and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.[3]. 1. Robin-Popieul, C. C. M., Arndt, N. T., Chauvel, C., Byerly, G. R., Sobolev, A. V., & Wilson, A. 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. The analytical conditions of the EPMA were 15 kV of accelerating voltage, 10 nA of specimen current, and 60–80 s of counting time. 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. Fig.3: Sketch map of part of the Alexo area showing the komatiite flow. Geological logging, thin-section petrography, infrared spectrometry, X-ray fluorescence micromapping and whole-rock geochemistry were conducted on komatiites and the enveloping supracrustal sequence. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. 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. Komatiite is extremely rich in magnesium and low in silica. Komatiites are rare, ultra-high-temperature (∼1,600 °C) lavas that were erupted in large volumes 3.5–1.5 bya but only very rarely since. The factor controlling the mineral assemblage is the partial pressure of carbon dioxide within the metamorphic fluid, called the XCO2. The previous interpretations of these large komatiite bodies was that they were "super channels" or reactivated channels, which grew to over 500 m in stratigraphic thickness during prolonged volcanism. 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. 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 is an ultramafic lava rapidly cooled which produces a texture known as Spinifex (named after a species of grass). Komatiites often show pillow lava structure, autobrecciated upper margins consistent with underwater eruption forming a rigid upper skin to the lava flows. DisciplinaPsicologia Jurídica 11.140 materiais • 117.945 seguidores. 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. There are two geochemical classes of komatiite; aluminium undepleted komatiite (AUDK) (also known as Group I komatiites) and aluminium depleted komatiite (ADK) (also known as Group II komatiites), defined by their Al2O3/TiO2 ratios. The lighter-colored Schematic cross-section of a komatiite-type NiS environment. 1). The youngest komatiites are from the island of Gorgona on the Caribbean oceanic plateau off the Pacific coast of Colombia, and a rare example of Proterozoic komatiite is found in the Winnipegosis komatiite belt, Manitoba, Canada. OLIVINE thin section under Polarizing Microscope - Duration: 1:11. Thicker lava units resulted from ponding in seafloor depressions or inflation under a chilled crust, as happens for modern basalt lavas. Actinolite sometimes occurs as thin splintery inclusion s inside quartz crystals. Both thick and - thin units consist of an olivine cumulate derived lower part overlain by a silicate liquid derived upper part. The precision of the elemental concentration was less than 10%, which was based on duplicate measurements. The typical komatiite lava flow has six stratigraphically related elements; Individual flow units may not be entirely preserved, as subsequent flow units may thermally erode the A zone spinifex flows. 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. Very unusual. For example, Danyushevsky et al. These images were taken from the thin section being offered. Cambridge: Cambridge University Press. (A) A change in magnetite (black) concentration represents the crust of pillows. The channel and sheeted flows are then covered by high-magnesian basalts and tholeiitic basalts as the volcanic event evolves to less magnesian compositions. 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 liquid and olivine compositions (Barnes et al., 2004; Barnes, 2006). 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). 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. Naldrett & Mason (1968)presented detailed descriptions of skeletal olivine mor… 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. Journal of Petrology, 57(1), 147-184. Komatiite-hosted nickel-copper sulfide mineralisation today accounts for about 14% of the world's nickel production, mostly from Australia, Canada and South Africa. 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. This is a thin section of a melaseyenite nown as Shonkinite, from the type locality, the Shonkin Sag laccolith, Montana, USA. 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. Lower temperature mantle melts such as basalt and picrite have essentially replaced komatiites as an eruptive lava on the Earth's surface. and Barnes, S.J. There are thus two main classes of metamorphic komatiite; carbonated and hydrated. In the distal thin flow facies, ISBN 10: 0521874742. 2008. Figure 46.3. Geological logging and sampling of diamond drillcore Cores from 10 … In thin section, the needles can be seen to be orga-nized into colonies of individuals in near-optical con-tinuity. 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. Thin Spinifex Textured Flows Mafic Footwall Generalized Cross Section of a Mineralized Flow Hangingwall Ore After Hill, & Cowden & Roberts. Arndt, N.T., Lesher, C.M. Compared to the basaltic lava of the Hawaiian plume basalts at ~1200 °C, which flows the way treacle or honey does, the komatiitic lava would have flowed swiftly across the surface, leaving extremely thin lava flows (down to 10 mm thick). Basaltic lavasnormally have eruption temperatures of about 1100 °C to 1250°C. The subsequent magmatism, being higher silica melts, tends to form a more typical shield volcano architecture. Magmas of komatiite compositions have a very high lavas ... the komatiitic lava would have been incredibly swift in travelling across the surface, leaving extremely thin lava flows (down to 10 mm thick). 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. 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). Sheet Flow Channel Flow Sheet Flow Figure 1. Komatiite ( /koʊˈmɑːti.aɪt/) is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava with ≥ 18 wt% MgO. please bear this in mind when purchasing. The thin section is thinner on edge where minerals have untypically low interference colours! A considerable population of komatiite examples show a cumulate texture and morphology. Earth and Planetary Science Letters, 292(1-2), 79-88. 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. Waterton, P., Pearson, D. G., Kjarsgaard, B., Hulbert, L., Locock, A., Parman, S. W., & Davis, B. r/geology: The scientific study of the origin, history, and structure of the Earth. Komatiite magma is extremely dense and unlikely to reach the surface, being more likely to pool lower within the crust. 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. The matrix is ¢ner-comminuted ¢ne- 4.5.1. Rare mineralogy in Thin Section . 2000. Plates, Plumes and Mantle Convection. Main Menu: Thin Section page 1: Rare mineralogy in Thin Section . 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. Мова: english. Image credit: Adrian Brown. This is a thin section of Spinifex Komatiite from the ancient shield rocks in Canada, Abitibi formation, Pyke Hill, Ontario. 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. Thin section of pillowed komatiite. 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). (2002) estimated 0.2-0.3 wt.% of H 2 O in the melt inclusions in olivine from Belingwe Greenstone Belt komatiites. The Ni content of the cumulate facies is commonly 0.1%-1% Ni. Panoramic view of Komatiite thin section … Volcanic rocks:Subvolcanic rocks:Plutonic rocks: An ultramafic mantle-derived volcanic rock, Mineralogic variations in komatiite flow facies. However, the initial flux of the most magnesian magmas is interpreted to form a channelised flow facie, which is envisioned as a fissure vent releasing highly fluid komatiitic lava onto the surface. Actinolite actinolite in quartz actinolite replacing spinifex olivine in a meta komatiite scanning electron micrograph of actinolite. Thin section of pillowed komatiite. Komatiite chemistry is different from that of basaltic and other common mantle-produced magmas, because of differences in degrees of partial melting. [1] Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Modified from Thurston et al., 2008. Primary (magmatic) mineral species also encountered in komatiites include olivine, the pyroxenes augite, pigeonite and bronzite, plagioclase, chromite, ilmenite and rarely pargasitic amphibole. 46-3. 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). [12][13] Harrisites are known to form megacrystal aggregates of pyroxene and olivine up to 1 metre in length. [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]. Arndt, N.T., and Lesher, C.M. Thermal history of the Earth and its petrological expression. This is a thin section of a coarse dolerite (or gabbro) occurring on the shore as a dyke. Thus, the current metamorphic mineralogy of a komatiite will reflect the chemistry, which in turn represents an inference as to its volcanological facies and stratigraphic position. In addition, jigsaw-fit to clast-rotated monomictic breccia may be present at the tops and bases of lavas. Modified from Thurston et al., 2008. Journal of Petrology, 41(10), 1467-1470. Komatiite. Komatiite is a key reference for researchers and advanced students interested in petrology, Archaean geology, economic geology, and broader questions about the evolution of the Earth's crust and mantle. Thin Section: 1864 Sample: P3384 Caution! There is some loss of adhesion of the cement in this slide. Cambridge: Cambridge University Press. Geology thin section microscope slides for sale. Panoramic view of Komatiite thin section from Alexo, Canada. Compared to the basaltic lava of the Hawaiian plum… Contact us. 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. 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. Analogue of : Early earth, Mars, Igneous, Water weathering. Komatiites are associated with nickel and gold deposits in Australia, Canada, South Africa and most recently in the Guiana shield of South America. If XCO2 is above 0.5, the metamorphic reactions favor formation of talc, magnesite (magnesium carbonate), and tremolite amphibole. 1:48 . Veja grátis o arquivo Komatiite enviado para a disciplina de Psicologia Jurídica Categoria: Outro - 10 - 74073646. Because they are rapidly cooled, they represent the liquid composition of the komatiites, and thus record an anhydrous MgO content of up to 32% MgO. A new model for Barberton komatiites: Deep critical melting with high melt retention. The metamorphic mineralogy of ultramafic rocks, particularly komatiites, is only partially controlled by composition. Faure, F., Arndt, N.T. Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallized from a lava with ≥ 18 wt.% MgO. 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. Section ES (Eastern Section: Alexo Mine) is the location of the stratigraphic column in Figure 46.3B. 50% Clinopyroxene. They are the signature rock type of a hotter early Earth. Geological Survey of South Africa, Special Publication, 21, 87 - 112. 2012. Aegirine syenite. 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. Viljoen, M. J., & Viljoen, R. P. 1969a. These lavas have the highest…” Sedimentary units grained aphanitic komatiite. The white box outlines close-up area … Rock Type: Plagioclase Websterite Location: Thin Section Description: Texture: Hypidiomorphic, all minerals are subhedral. "Spinifex" texture is named after the common name for the Australian grass Triodia,[11] which grows in clumps with similar shapes. These are classed as talc-carbonation reactions. Migmatite: A heterogeneous silicate rock with properties of both igneous and metamorphic rocks. The higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archaean Earth. 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. Spinifex texture is the result of rapid crystallization of highly magnesian liquid in the thermal gradient at the margin of the flow or sill. Blue encircled areas are areas where pseudomorphs after crystals are observed. PPL image, field of view = 2.5cm. 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. These textures are hypothesized to be evidence for multiple episodes of growth and filling. Davies, G. F. 1999. BIOTITE thin section under Polarizing Microscope - Duration: 1:48. 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. 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 . 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 … Рік: 2008. Constraining the potential temperature of the Archaean mantle: A review of the evidence from komatiites. When robotic exploration of Mars began, many scientists expected to see large sedimentary deposits … Actinolite, plane polarized light. Actinolite in thin section thin section gigapans. 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. 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). Traces of talc, tremolite and dolomite may be present, as it is very rare that no carbon dioxide is present in metamorphic fluids. 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. 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). The usual cumulate mineralogy is highly magnesium rich forsterite olivine, though chromian pyroxene cumulates are also possible (though rarer). Image by Andy Tindle (Virtual Microscope). XPL image, field of view = 2.5cm. 410 Likes, 7 Comments - TravelingGeologist (@travelinggeologist) on Instagram: “Here’s a lovely thin section of #spinifex texture of a #komatiite. Plutonic rocks: Plutonic rocks: Plutonic rocks: an ultramafic lava rapidly cooled which produces a texture known spinifex., the metamorphic fluid, called the XCO2 1100 to 1250 °C Mason ( 1968 ) detailed... Form megacrystal aggregates of pyroxene and olivine up to 1 metre in length ( though rarer ) volumes. The cumulate facies is commonly 0.1 % -1 % Ni of both igneous and metamorphic rocks of pyroxene and up. Its original composition through careful petrological study is the location of the Onvernacht Group 87... Of ultramafic rocks, particularly komatiites, in which the rock is crisscrossed with,. Explain roughly what a komatiite is an ultramafic mantle-derived volcanic rock, Mineralogic variations in komatiite showing... 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