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. Panoramic view of Komatiite thin section from Alexo, Canada. When robotic exploration of Mars began, many scientists expected to see large sedimentary deposits … please bear this in mind when purchasing. Below XCO2 of 0.5, metamorphic reactions in the presence of water favor production of serpentinite. Sheet Flow Channel Flow Sheet Flow Figure 1. Komatiite was named for its type locality along the Komati River in South Africa.. The precision of the elemental concentration was less than 10%, which was based on duplicate measurements. Thin Spinifex Textured Flows Mafic Footwall Generalized Cross Section of a Mineralized Flow Hangingwall Ore After Hill, & Cowden & Roberts. Geological logging and sampling of diamond drillcore Cores from 10 … Sedimentary units grained aphanitic komatiite. Secondary (metamorphic) minerals include serpentine, chlorite, amphibole, sodic plagioclase, quartz, iron oxides and rarely phlogopite, baddeleyite, and pyrope or hydrogrossular garnet. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. This is a photomicrograph of a small, thin section of komatiite lava. Komatiitic lava was extremely fluid when it erupted (possessing the viscosity close to that of water but with the density of rock). The morphology of these large, preferentially oriented crystals has not previously been reproduced experimentally (Donaldson, 1982; Ginibre et al., 1997). Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Panoramic view of Komatiite thin section from Alexo, Canada. 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 … 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. 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 ( / 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. All known komatiites have been metamorphosed, therefore should technically be termed 'metakomatiite' though the prefix meta is inevitably assumed. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Hill R.E.T, Barnes S.J., Gole M.J. and Dowling S.E. 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 (/ 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. These images were taken from the thin section being offered. These lavas have the highest…” IUGS reclassification of the high-Mg and picritic volcanic rocks. 46-3. Lower temperature mantle melts such as basalt and picrite have essentially replaced komatiites as an eruptive lava on the Earth's surface. Spinifex texture is the result of rapid crystallization of highly magnesian liquid in the thermal gradient at the margin of the flow or sill. 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. The metamorphic mineralogy of ultramafic rocks, particularly komatiites, is only partially controlled by composition. Analogue of : Early earth, Mars, Igneous, Water weathering. def. This is a thin section of a melaseyenite nown as Shonkinite, from the type locality, the Shonkin Sag laccolith, Montana, USA. Traces of talc, tremolite and dolomite may be present, as it is very rare that no carbon dioxide is present in metamorphic fluids. 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. 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. The higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archean Earth. Talc, chlorite and tremolite were other minerals present. 1. Hydrated metamorphic rock assemblages are dominated by the minerals chlorite, serpentine-antigorite, brucite. 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. Modified from Thurston et al., 2008. This is a thin section of a coarse dolerite (or gabbro) occurring on the shore as a dyke. Komatiite ( /koʊˈmɑːti.aɪt/) is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava with ≥ 18 wt% MgO. 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. Compared to the basaltic lava of the Hawaiian plum… compared with altered ultramafic – Komatiite. 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. Komatiite was named for its type locality along the Komati River in South Africa.. Sample is ~0.5 m from upper chill margin. Main Menu: Thin Section page 1: Rare mineralogy in Thin Section . Proximal volcanic facies are thinner and interleaved with sulfidic sediments, black shales, cherts and tholeiitic basalts. The white box outlines close-up area … Modified from Thurston et al., 2008. Pages. 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. 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. 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. Rare mineralogy in Thin Section . Volcanic rocks rich in magnesium may be produced by accumulation of olivine phenocrysts in basalt melts of normal chemistry: an example is picrite. Variations in texture across a several metre-thick komatiite flow showing well developed layering. [12][13] Harrisites are known to form megacrystal aggregates of pyroxene and olivine up to 1 metre in length. A thin (1… 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. Evidence of this is from their association with felsics, occurrences of komatiitic tuffs, Niobium anomalies and by S- and H2O-borne rich mineralizations. Petrol 47, 1591–1610. 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. Analyses carried out : Status : 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 has been heavily altered and contains secondary prehnite and garnets. 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). 50% Clinopyroxene. 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. Schematic cross-section of a komatiite-type NiS environment. Actinolite, PPL Actinolite, Plane Polars. The upper flow facies tend to be dominated by talc, chlorite, tremolite, and other magnesian amphiboles (anthophyllite, cummingtonite, gedrite, etc.). 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). Komatiite magma is extremely dense and unlikely to reach the surface, being more likely to pool lower within the crust. This tends to lower MgO, Cr, Ni, and increase Al, K2O, Na, CaO and SiO2 toward the top of the flow. Earth and Planetary Science Letters, 292(1-2), 79-88. 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, and ... leaving extremely thin lava flows (down to 10 mm thick). Published 10 Jan 2021. 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. 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. Journal of Petrology, 53(11), 2191-2229. Secondary internal crusts (white dashed lines) are also observed. 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. 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 major komatiitic sequences preserved in Archaean rocks are thus considered to be lava tubes, ponds of lava etc., where the komatiitic lava accumulated. 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. Actinolite actinolite in quartz actinolite replacing spinifex olivine in a meta komatiite scanning electron micrograph of actinolite. The Ni content of the cumulate facies is commonly 0.1%-1% Ni. Komatiites", "An Origin for Harrisitic and Granular Olivine in the Rum Layered Suite, NW Scotland: a Crystal Size Distribution Study", "Fault-Controlled Magma Ascent Recorded in the Central Series of the Rum Layered Intrusion, NW Scotland", https://en.wikipedia.org/w/index.php?title=Komatiite&oldid=998679848, Short description is different from Wikidata, Articles needing additional references from November 2020, All articles needing additional references, Creative Commons Attribution-ShareAlike License, A1 – pillowed and variolitic chilled flow top, often grading and transitional with sediment, A2 – Zone of quickly chilled, feathery acicular olivine-clinopyroxene-glass representing a, A3 – Olivine spinifex sequence composed of sheaf and book-like olivine spinifex, representing a downward-growing crystal accumulation on the flow top, B1 – Olivine mesocumulate to orthocumulate, representing a harrisite grown in flowing liquid melt, B2 – Olivine adcumulate composed of >93% interlocking equant olivine crystals. 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