komatiite thin section

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). Rock Type: Plagioclase Websterite Location: Thin Section Description: Texture: Hypidiomorphic, all minerals are subhedral. 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. Specimen# 61-9 : Aegirine syenite in thin section. 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. 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. 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. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. The map also highlights the location of major ore deposits. 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. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. Komatiite N.T. This is a thin section of a coarse dolerite (or gabbro) occurring on the shore as a dyke. 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). Herzberg, C., Condie, K., & Korenaga, J. Volcanic rocks:Subvolcanic rocks:Plutonic rocks: An ultramafic mantle-derived volcanic rock, Mineralogic variations in komatiite flow facies. Thin Spinifex Textured Flows Mafic Footwall Generalized Cross Section of a Mineralized Flow Hangingwall Ore After Hill, & Cowden & Roberts. IUGS reclassification of the high-Mg and picritic volcanic rocks. 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. 46-3. Komatiite was named for its type locality along the Koma Modified from Lahaye, Y., & Arndt, N. (1996). PPL image, field of view = 2.5cm. Komatiite was named for its type locality along the Komati River in South Africa.. 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 … (2008), This page was last edited on 6 January 2021, at 14:55. Geochemistry of Chromite from the Alexo Komatiite, Dundonald Township: Preliminary Results from ... Houlé, Lesher and Davis 2012). Actinolite in thin section thin section gigapans. Variations in texture across a several metre-thick komatiite flow showing well developed layering. 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 subsequent magmatism, being higher silica melts, tends to form a more typical shield volcano architecture. (A) A change in magnetite (black) concentration represents the crust of pillows. 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. 1. Hydrated metamorphic rock assemblages are dominated by the minerals chlorite, serpentine-antigorite, brucite. All known komatiites have been metamorphosed, therefore should technically be termed 'metakomatiite' though the prefix meta is inevitably assumed. 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. The upper flow facies tend to be dominated by talc, chlorite, tremolite, and other magnesian amphiboles (anthophyllite, cummingtonite, gedrite, etc.). Spinifex-textured komatiite flows are generally subdivided into three major units, as illustrated in Fig. Hill R.E.T, Barnes S.J., Gole M.J. and Dowling S.E. Basaltic lavasnormally have eruption temperatures of about 1100 °C to 1250°C. Journal of Petrology, 57(1), 147-184. These images were taken from the thin section being offered. 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 . 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. Fig.3: Sketch map of part of the Alexo area showing the komatiite flow. Sample is ~0.5 m from upper chill margin. 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 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. 2012. Below XCO2 of 0.5, metamorphic reactions in the presence of water favor production of serpentinite. The morphology of these large, preferentially oriented crystals has not previously been reproduced experimentally (Donaldson, 1982; Ginibre et al., 1997). 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. 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. 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). The higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archaean Earth. 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. B) Schematic 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. 50% Clinopyroxene. Carbonate Origins. 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. Faure, F., Arndt, N.T. r/geology: The scientific study of the origin, history, and structure of the Earth. This results in significant changes to the mineralogy and the texture. 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. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. komatiite liquid and olivine compositions (Barnes et al., 2004; Barnes, 2006). Carbonated komatiites and peridotites form a series of rocks dominated by the minerals chlorite, talc, magnesite or dolomite and tremolite. Komatiite is extremely rich in magnesium and low in silica. 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. Image by Andy Tindle (Virtual Microscope). Thin section through the coarse-bladed olivine spinifex zone in the slowly cooled interior of komatiite unit. 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. Komatiite was named for its type locality along the Komati River in South Africa.. 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. Cambridge: Cambridge University Press. The metamorphic mineralogy of ultramafic rocks, particularly komatiites, is only partially controlled by composition. 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. Magmas of komatiitic compositions have a very high melting point, with calculated eruption temperatures up to, and possibly in excess of 1600 °C. 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. 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. A. Svetov, A. I. 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