These two classes of komatiite are often assumed to represent a real petrological source difference between the two types related to depth of melt generation. Petrogenesis and geochemistry of Archean Komatiites. Published 10 Jan 2021. 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 . 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. Journal of Petrology, 53(11), 2191-2229. The matrix is ¢ner-comminuted ¢ne- 4.5.1. They are the signature rock type of a hotter early Earth. The channel and sheeted flows are then covered by high-magnesian basalts and tholeiitic basalts as the volcanic event evolves to less magnesian compositions. 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 is a thin section of Spinifex Komatiite from the ancient shield rocks in Canada, Abitibi formation, Pyke Hill, Ontario. 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. There are thus two main classes of metamorphic komatiite; carbonated and hydrated. 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. 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. Secondary internal crusts (white dashed lines) are also observed. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. Komatiite was named for its type locality along the Komati River in South Africa.. (2005), Komatiites, in Selley, RC, Cocks, L.R.M., Plimer, I.R. The most striking examples comprise large olivine plates that form parts of giant, decimeter-sized dendritic crystals that are oriented perpendicular to the roof of the komatiitic unit (Fig. 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. The higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archaean Earth. Thin section of pillowed komatiite. Type of rock : Komatiite. 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. Hydrated metamorphic rock assemblages are dominated by the minerals chlorite, serpentine-antigorite, brucite. Actinolite in thin section thin section gigapans. There is some loss of adhesion of the cement in this slide. komatiite magma that originated in a supra-subduction setting, but most recent studies found far lower water contents. 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). [12][13] Harrisites are known to form megacrystal aggregates of pyroxene and olivine up to 1 metre in length. Image by Andy Tindle (Virtual Microscope). 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). Aegirine syenite, Shawa, Zimbabwe: Aegirine syenite . 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. 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 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 ( /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. These intrusions are considered to be channelised sills, formed by injection of komatiitic magma into the stratigraphy, and inflation of the magma chamber. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. 1. 2000. The morphology of these large, preferentially oriented crystals has not previously been reproduced experimentally (Donaldson, 1982; Ginibre et al., 1997). 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. 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). A total of 47 samples have been . Talc, chlorite and tremolite were other minerals present. 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. Below XCO2 of 0.5, metamorphic reactions in the presence of water favor production of serpentinite. These breccias are autoclastic and commonly produced by quench fragmentation (hyaloclastite). Komatiites are rare and predominantly found in rocks of Archaean age, with few Proterozoic or Phanerozoic komatiites known. 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. This is a photomicrograph of a small, thin section of komatiite lava. 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 … 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. r/geology: The scientific study of the origin, history, and structure of the Earth. Type of preparation : Thin section. The heterogeneous nature of the … 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). 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 subsequent magmatism, being higher silica melts, tends to form a more typical shield volcano architecture. compared with altered ultramafic – Komatiite. [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) A change in magnetite (black) concentration represents the crust of pillows. Many komatiites are highly altered and serpentinized or carbonated from metamorphism and metasomatism. 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. Secondary internal crusts (white dashed lines) are also observed. The higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archean Earth. Analogue of : Early earth, Mars, Igneous, Water weathering. Fig.2: Geological map of the southern Abitibi greenstone. Cambridge: Cambridge University Press. The lighter-colored 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. Active lavas had very thin flow fronts, perhaps only cm thick, and spread as very thin sheets and lobes (Cas et al., 1999). Primary (magmatic) mineral species also encountered in komatiites include olivine, the pyroxenes augite, pigeonite and bronzite, plagioclase, chromite, ilmenite and rarely pargasitic amphibole. The thin section is thinner on edge where minerals have untypically low interference colours! 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. Herzberg, C., Condie, K., & Korenaga, J. 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]. (2002) estimated 0.2-0.3 wt.% of H 2 O in the melt inclusions in olivine from Belingwe Greenstone Belt komatiites. Мова: english. Magmas of komatiitic compositions have a very high melting point, with calculated eruption temperatures up to, and possibly in excess of 1600 °C. Journal of Petrology, 57(1), 147-184. PPL image, field of view = 2.5cm. Figure 46.3. Subhedral grains pleochroic from grey to light yellow. Thin section of pillowed komatiite. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Panoramic view of Komatiite thin section from Alexo, Canada. This is a photomicrograph of a small, thin section of komatiite lava. Faure, F., Arndt, N.T. Thin Section: 1864 Sample: P3384 Caution! (A) A change in magnetite (black) concentration represents the crust of pillows. (1990). Naldrett & Mason (1968)presented detailed descriptions of skeletal olivine mor… Komatiites often show pillow lava structure, autobrecciated upper margins consistent with underwater eruption forming a rigid upper skin to the lava flows. 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. Geochemistry of Chromite from the Alexo Komatiite, Dundonald Township: Preliminary Results from ... Houlé, Lesher and Davis 2012). Komatiites are considered to have been formed by high degrees of partial melting, usually greater than 50%, and hence have high MgO with low K2O and other incompatible elements. Berry et al. The Ni content of the cumulate facies is commonly 0.1%-1% Ni. 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. Komatiites are associated with nickel and gold deposits in Australia, Canada, South Africa and most recently in the Guiana shield of South America. 410 Likes, 7 Comments - TravelingGeologist (@travelinggeologist) on Instagram: “Here’s a lovely thin section of #spinifex texture of a #komatiite. 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. This is a thin section of Spinifex Komatiite from the ancient shield rocks in Canada, Abitibi formation, Victoria Lake Ontario. Panoramic view of Komatiite thin section from Alexo, Canada. 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. Traces of talc, tremolite and dolomite may be present, as it is very rare that no carbon dioxide is present in metamorphic fluids. 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. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Actinolite in actinolite schist, plane polars Actinolite in actinolite schist, crossed polars. One distinctive feature of some komatiites is spinifex texture, in which the rock is crisscrossed with long, thin olivine crystals. Thermal history of the Earth and its petrological expression. Komatiite is an ultramafic lava rapidly cooled which produces a texture known as Spinifex (named after a species of grass). The precision of the elemental concentration was less than 10%, which was based on duplicate measurements. (Editors). This results in significant changes to the mineralogy and the texture. Volcanic rocks rich in magnesium may be produced by accumulation of olivine phenocrysts in basalt melts of normal chemistry: an example is picrite. Both thick and - thin units consist of an olivine cumulate derived lower part overlain by a silicate liquid derived upper part. Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallized from a lava with ≥ 18 wt.% MgO. 1:48 . A new model for Barberton komatiites: Deep critical melting with high melt retention. Alex Strekeisen - I vetrini della mia fantasia. The slightest seafloor slope would have created a hydraulic head effect from vent to flow front. Basaltic lavasnormally have eruption temperatures of about 1100 °C to 1250°C. 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. 46-3. Komatiite chemistry is different from that of basaltic and other common mantle-produced magmas, because of differences in degrees of partial melting. If XCO2 is above 0.5, the metamorphic reactions favor formation of talc, magnesite (magnesium carbonate), and tremolite amphibole. [1] Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Rare mineralogy in Thin Section . Boninites with 10–18% MgO tend to have higher large-ion lithophile elements (LILE: Ba, Rb, Sr) than komatiites. Very unusual. The factor controlling the mineral assemblage is the partial pressure of carbon dioxide within the metamorphic fluid, called the XCO2. Rock Type: Plagioclase Websterite Location: Thin Section Description: Texture: Hypidiomorphic, all minerals are subhedral. Journal of Petrology, 41(10), 1467-1470. B) Schematic They are the signature rock type of a hotter early Earth. 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. ISAR, Frances Westall, Frédéric Foucher, Centre de Biophysique Moléculaire, UPR CNRS 4301, rue Charles Sadron, 45071 Orléans Cedex 2 France Komatiite is an ultramafic lava rapidly cooled which produces a texture known as Spinifex (named after a species of grass). Section ES (Eastern Section: Alexo Mine) is the location of the stratigraphic column in Figure 46.3B. 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. 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. Sedimentary units grained aphanitic komatiite. Main Menu: Thin Section page 1: Rare mineralogy in Thin Section . 2008). These textures are hypothesized to be evidence for multiple episodes of growth and filling. [6][7][8][9] Basaltic lavas normally have eruption temperatures of about 1100 to 1250 °C. PPL image, field of view = 2.5cm. Viljoen, M. J., & Viljoen, R. P. 1969a. This is a thin section of Spinifex Komatiite from the ancient shield rocks in Canada, Abitibi formation, Pyke Hill, Ontario. Komatiites. In thin section, the needles can be seen to be orga-nized into colonies of individuals in near-optical con-tinuity. Actinolite in Thin Section Thin Section GigaPans. Age, Origin, and Thermal Evolution of the ultra-fresh ~1.9 Ga Winnipegosis Komatiites, Manitoba, Canada. This is a thin section of a melaseyenite nown as Shonkinite, from the type locality, the Shonkin Sag laccolith, Montana, USA. 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. Thicker lava units resulted from ponding in seafloor depressions or inflation under a chilled crust, as happens for modern basalt lavas. 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. Ultra-High-Temperature ( ∼1,600 °C ) lavas that were erupted in large volumes bya. 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