Introduction It is easy to explain roughly what a komatiite is but difficult to give a rigorous definition. ISBN 10: 0521874742. def. The matrix is ¢ner-comminuted ¢ne- 4.5.1. Thin section through the coarse-bladed olivine spinifex zone in the slowly cooled interior of komatiite unit. Schematic cross-section of a komatiite-type NiS environment. 1). A considerable population of komatiite examples show a cumulate texture and morphology. Boninite magmatism is similar to komatiite magmatism but is produced by fluid-fluxed melting above a subduction zone. BIOTITE thin section under Polarizing Microscope - Duration: 1:48. 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. In the distal thin flow facies, Magmas of komatiite compositions have a very high melting point with calculated eruption temperatures in excess of 1600 °C. 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. and Barnes, S.J. Actinolite, plane polarized light. Secondary internal crusts (white dashed lines) are also observed. In transmitted light, thin sections reveal the texture and crystal shape of minerals and other identifying characteristics while through polarized light, the mineralogy is awash in color allowing for more complete mineral identification. Geochemistry of Chromite from the Alexo Komatiite, Dundonald Township: Preliminary Results from ... Houlé, Lesher and Davis 2012). Thin Section: 1864 Sample: P3384 Caution! 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. 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). Volcanic rocks rich in magnesium may be produced by accumulation of olivine phenocrysts in basalt melts of normal chemistry: an example is picrite. 410 Likes, 7 Comments - TravelingGeologist (@travelinggeologist) on Instagram: “Here’s a lovely thin section of #spinifex texture of a #komatiite. Mahmoud Awad 10,666 views. In thin section, the needles can be seen to be orga-nized into colonies of individuals in near-optical con-tinuity. komatiite liquid and olivine compositions (Barnes et al., 2004; Barnes, 2006). Figure 3. All known komatiites have been metamorphosed, therefore should technically be termed 'metakomatiite' though the prefix meta is inevitably assumed. Panoramic view of Komatiite thin section … 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. A new model for Barberton komatiites: Deep critical melting with high melt retention. For example, Danyushevsky et al. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. A) General schematic stratigraphic column of Dundonald Township (after Houlé et al. 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). 50% Clinopyroxene. Komatiite magma is extremely dense and unlikely to reach the surface, being more likely to pool lower within the crust. 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. 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. Aegirine syenite, Shawa, Zimbabwe: Aegirine syenite . Fig.3: Sketch map of part of the Alexo area showing the komatiite flow. Specimen# 61-9 : Aegirine syenite in thin section. Nearly all examples known are metamorphosed, and we must infer its original composition through careful petrological study. Pages. Variations in texture across a several metre-thick komatiite flow showing well developed layering. Arndt, N., Lesher, C. M., & Barnes, S. J. 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. Herzberg, C., Condie, K., & Korenaga, J. Both thick and thin units consist of an olivine cumulate derived lower part overlain by a silicate liquid derived upper part. 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. Labels: Serpentine replacing olivine (SRP); clinopyroxene (CPX; Magnetite (MAG). 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 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. Viljoen, M. J., & Viljoen, R. P. 1969a. 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. Geological logging and sampling of diamond drillcore Cores from 10 … 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. 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). Evidence of this is from their association with felsics, occurrences of komatiitic tuffs, Niobium anomalies and by S- and H2O-borne rich mineralizations. Actinolite actinolite in quartz actinolite replacing spinifex olivine in a meta komatiite scanning electron micrograph of actinolite. There are thus two main classes of metamorphic komatiite; carbonated and hydrated. (A) A change in magnetite (black) concentration represents the crust of pillows. Volcanic rocks:Subvolcanic rocks:Plutonic rocks: An ultramafic mantle-derived volcanic rock, Mineralogic variations in komatiite flow facies. Thicker lava units resulted from ponding in seafloor depressions or inflation under a chilled crust, as happens for modern basalt lavas. 2008. Nisbet, E. G., Cheadle, M. J., Arndt, Nicholas T., & Bickle, M. J. 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. 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. They are the signature rock type of a hotter early Earth. 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. 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. Modified from Thurston et al., 2008. 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. 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. Thin Spinifex Textured Flows Mafic Footwall Generalized Cross Section of a Mineralized Flow Hangingwall Ore After Hill, & Cowden & Roberts. 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. 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. The higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archean Earth. 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. Komatiites often show pillow lava structure, autobrecciated upper margins consistent with underwater eruption forming a rigid upper skin to the lava flows. Komatiite chemistry is different from that of basaltic and other common mantle-produced magmas, because of differences in degrees of partial melting. Cambridge: Cambridge University Press. Sample is ~0.5 m from upper chill margin. Petrogenesis and geochemistry of Archean Komatiites. The subsequent magmatism, being higher silica melts, tends to form a more typical shield volcano architecture. 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. Naldrett & Mason (1968)presented detailed descriptions of skeletal olivine mor… Secondary (metamorphic) minerals include serpentine, chlorite, amphibole, sodic plagioclase, quartz, iron oxides and rarely phlogopite, baddeleyite, and pyrope or hydrogrossular garnet. Arndt, N.T., Lesher, C.M. The white box outlines close-up area … Below XCO2 of 0.5, metamorphic reactions in the presence of water favor production of serpentinite. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Arndt1 and C.M. Komatiite is extremely rich in magnesium and low in silica. 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. Actinolite in actinolite schist, plane polars Actinolite in actinolite schist, crossed polars. The major komatiitic sequences preserved in Archaean rocks are thus considered to be lava tubes, ponds of lava etc., where the komatiitic lava accumulated. This is a thin section of Spinifex Komatiite from the ancient shield rocks in Canada, Abitibi formation, Victoria Lake Ontario. [12][13] Harrisites are known to form megacrystal aggregates of pyroxene and olivine up to 1 metre in length. Evidence for the existence of a mobile extrusive peridotitic magma from the Komati Formation of the Onvernacht Group. 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. Thermal history of the Earth and its petrological expression. B) Schematic (2002) estimated 0.2-0.3 wt.% of H 2 O in the melt inclusions in olivine from Belingwe Greenstone Belt komatiites. Petrol 47, 1591–1610. 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