Komatiit
Komatiites are ultramafic , from the Earth's mantle -derived volcanic rocks . However, some deposits are also considered intrusive rocks ( subvolcanic and plutonic ). They were recognized as a special type of rock in 1962 and named after the type locality on the Komati River in South Africa .
True komatiites are very rare and essentially restricted to rocks from the Archean . Only a few Proterozoic or Phanerozoic komatiites are known, even if similarly magnesium-rich lamprophyres occur in the Mesozoic Era. This age restriction is attributed to the fact that the Earth's mantle is slowly cooling and that it was up to 500 ° C hotter than it is today due to the higher abundance of radioactive elements in the early Earth's mantle during the Middle Archean (4.5 to 2.6 billion years). Smaller reservoirs of the archaic mantle possibly persist to this day.
From a geographical point of view, komatiites are found mainly in the archaic shields . They occur along with other ultramafic and magnesium-rich mafic volcanic rocks in archaic greenstone belts . In Canada, amphibolites with ultramafic komatiitic sills from the so-called Nuvvuagittuq greenstone belt near Hudson Bay in northern Quebec have been dated to an age of about 4.280 billion years - currently the oldest known rocks on earth. The youngest komatiites formed before 87 Ma on the Pacific island of Gorgona and geologically belong to the Caribbean ocean plateau.
Komatiites have low SiO 2 , K 2 O and Al 2 O 3 contents, but a high to extremely high proportion of MgO .
Petrology
Magmas of comatose composition have a very high melting point , the temperature during eruptions has been calculated to be more than 1600 ° C ("dry" - i.e. without fluids). Basaltic lava usually have eruption temperatures of about 1100 ° C to 1250 ° C. The high temperatures, which are necessary for the formation of komatiites, are attributed to the presumably higher geothermal gradient in the archaic earth, or to very old reservoirs in the earth's mantle. Under which exact circumstances (high temperature of over 1600 ° C or presence of fluids) komatiite is part of the scientific discussion; different statements may also apply to different komatiite.
When it erupted, komatitic lava had the properties of a supercritical fluid , namely the viscosity of a gas , but the density of a rock . Compared to the basalt lavas of Hawaii , which emerge at a temperature of ~ 1200 ° C with the viscosity of syrup or honey, they have flowed over the surface at great speed and left extremely thin layers of lava (up to 10 mm thick). The large komatiite sequences in archaic rocks are therefore viewed as lava tubes , lava lakes or other collections of komatiitic lava.
According to current knowledge, the geochemistry of the komatiites differs from that of basaltic and other frequently occurring mantle magmas due to differences in the degree of partial melt . In the case of komatiites, the degree of melting was probably more than 50%, so they have a high proportion of magnesium and, compared to other rocks formed from partial melting, only a small proportion of K 2 O and other incompatible elements . For example, kimberlite , which is also derived from the Earth's mantle , is another rock rich in magnesium, comparatively rich in potassium and other incompatible elements; its formation is attributed to the partial melting of less than one percent of the original rock, promoted by water and carbon dioxide.
The komatiites can be divided geochemically into two groups. The komatiites of group I are not aluminum- undepleted komatiite (AUDK), while those of group II are deficient in aluminum (ADK, aluminum depleted komatiite ). The differences between the two groups are due to melts occurring at different depths. Petrological experiments on komatiites have shown that the partial melt of water-rich shell material at low pressure does not lead to the melting of aluminum-rich pyroxenes (ADK), while higher pressure, as prevails at greater depths, leads to the melting of the pyroxene and the melt becomes aluminum-rich (AUDK).
Boninitic magmatism is similar to komatiitic magmatism, but it does not arise from melting as a result of a decrease in pressure, but is driven by volatile phases over subduction zones . Boninites with 10–18% MgO normally have higher proportions of lithophilic elements with large ions ( LILE ) ( barium , rubidium , strontium ) than komatiites. The komatiites differ from the chemically very similar Meimechites in that they have a TiO 2 content of less than 1%.
Komatiitic magmas are considered in a work on the Karelian greenstone belt in northwest Russia as a source of tholeiitic basalts spatially associated with komatiites .
According to temperature measurements by American planetologists using the Galileo space probe, komatiitic lavas may be formed today on Jupiter's moon Io . The volcanic eruptions there produce lavas with temperatures of up to 1700 ° C.
mineralogy
Unchanged volcanic komatiites consist of forsteritic olivine (Fo90 and more), calcium- and often chromium-containing pyroxene , anorthite (An85 and more) and chromite .
A larger part of the komatiites shows a Kumulat - texture and structure . The normal mineralogy of the cumulates is a magnesium-rich forsterite olivin, even if pyroxene cumulates containing chromium are possible.
Volcanic rocks with a high content of magnesium can arise from the accumulation of olivine phenocrystals in basalt melts of normal chemistry: one example is picrit . Part of the evidence that komatiites are not only rich in magnesium because of accumulated olivine is based on their texture: some show the Spinifex texture, named after an Australian grass , which goes back to the rapid crystallization of olivine from a magnesium-rich melt. Another part of the evidence is that the magnesium content of the komatiitic olivines is close to the content of pure forsterite, which can only arise from the crystallization of olivine from a high magnesium content melt.
The mostly seldom preserved surface of komatiite effusions is formed as a breccia , which, like the edge zones of lava pillows in some komatiites, consists essentially of volcanic glass , quenched by the contact of hot melt with overlying water or in the air. Because of this deterrent, they represent the composition of liquid komatiites, namely an anhydrous MgO content of up to 32%. Magnesium-rich komatiites with preserved textures that allow conclusions to be drawn about the original composition after the previous considerations are known, for example, from the Weltevreden formation of the Barberton greenstone belt near Barberton in South Africa , from which a composition of up to 34% MgO from total rock analyzes and Olivine chemistry can be derived.
The mineralogy of komatiites changes systematically via the typical stratigraphic profile of a komatiite effusion and reflects magmatic processes to which they are subject during eruption and cooling. This change ranges from a base composed of olivine cumulate to a zone with a Spinifex texture made from blade-like olivine to an ideally olivine-rich cooling zone on the surface of the effusion.
Primary igneous minerals, which are also found in komatiites, are, in addition to olivine, the pyroxene augite , pigeonite and bronzite , plus plagioclase , chromite, ilmenite and, rarely, pargasitic amphibole . Secondary (metamorphic) minerals are serpentine , chlorite , amphibole, high-potassium plagioclase, quartz , iron oxides and rarely phlogopite , baddeleyite and pyrope or water-rich, grossular-bearing garnet .
metamorphosis
At the present time, there are no unmetamorphic komatiites in the earth's crust, so that technically correct meta komatiites should be used. Because of this ubiquitous metamorphosis , the mineralogy of the komatiites shows not only the primary igneous, but also the chemistry of the rock, which has been changed by metamorphic fluids. Komatiites are usually greatly changed and serpentinized or carbonated by metamorphosis and metasomatosis . This leads to a significant change in mineralogy and the original texture is seldom preserved.
Hydration and carbonation
The metamorphic mineralogy of ultramafic rocks, especially that of komatiites, is only partially determined by the original composition. The modal inventory , especially of the metamorphic minerals newly formed during low-temperature metamorphosis, be it progressive or retrograde , depends primarily on the type of metamorphic fluids.
The determining factor for the mineralogical composition is the partial pressure of carbon dioxide in the metamorphic fluids, also known as XCO 2 . If XCO 2 is higher than 0.5, the formation of talc , magnesite (magnesium carbonate) and tremolite amphibole is preferred in the metamorphic reactions . These are known as talc-carbonate reactions . Below an XCO 2 value of 0.5, the metamorphic reaction with the participation of water prefer the formation of serpentinite.
There are two classes of metamorphic komatiites, namely carbonated and hydrated. Carbonated komatiites and also peridotites form a group of rocks that are dominated by the minerals chlorite, talc, magnesite or dolomite and tremolite. Hydrated metamorphic rock groups mainly show the minerals chlorite, serpentine antigorite and brucite . Traces of talc, tremolite, and dolomite may be present as it is rare that carbon dioxide is not found in metamorphic fluids. With a higher degree of metamorphosis, anthophyllite , enstatite , olivine and diopside are the predominant minerals due to increasing dehydration .
Mineralogical variations in komatiite effusions
In massive komatiites there is a tendency towards fractional crystallization , from a magnesium-rich composition at the base, where olivine cumulates dominate, to a magnesium-poor composition higher up. This is also followed by the mineralogy of the metamorphic formations, which reflects the chemism, and thus also gives indications of the volcanological facies and the stratigraphic position.
The typical metamorphic minerals are tremolite chlorite, or talc chlorite in the upper Spinifex zone. The magnesium- and olivine-rich base of an effusion is largely free of tremolite and chlorite, it is either dominated by serpentine brucite +/- anthophyllite (if hydrated) or talc magnesite (if carbonated). The facies of the upper area are dominated by talc, chlorite, tremolite and other magnesium amphiboles (anthophyllite, cummingtonite, gedrit etc.).
For example, the typical flow facies (see below) have the following composition:
| Facies: | Hydrated | Carbonated |
|---|---|---|
| A1 | Chlorite tremolite | Talc-chlorite tremolite |
| A2 | Serpentine tremolite chlorite | Talc tremolite chlorite |
| A3 | Serpentine chlorite | Talc-magnesite-tremolite-chlorite |
| B1 | Serpentine Chlorite Anthophyllite | Talc magnesite |
| B2 | Massive serpentine brucite | Solid talc magnesite |
| B3 | Serpentine brucite chlorite | Talc-magnesite-tremolite-chlorite |
geochemistry
Komatiites can be classified according to the following scheme:
- SiO 2 ; typically 40 - 45%
- MgO more than 18%
- Low K 2 O content (<0.5%)
- Low CaO and Na 2 O content (together <2%)
- Low barium content, accumulation of cesium and rubidium (incompatible trace elements); ΣLILE <1,000 ppm
- High content of nickel (> 400 ppm), chromium (> 800 ppm), cobalt (> 150 ppm)
The above geochemical classification relates only to the chemistry of the unchanged magma, and not to a composition changed by the accumulation of crystals (as in peridotites). However, it must be taken into account that even a typical komatiite can change its composition as a result of fractionation occurring during the eruption. This fractionation leads to a depletion of magnesium, chromium and nickel in the upper ranges and a simultaneous accumulation of aluminum, potassium, sodium, calcium and silicate.
Other rocks with a high content of magnesium, potassium and LILE are lamprophyres, kimberlites or other rare ultramafic, potassium-containing and high-potassium rocks.
Morphology and occurrence
Komatiites often show pillow lava structures, the upper edges of which indicate a submarine eruption, with the deterred, rigid upper skin breaking as the lava advances (auto-contraction), as large lava tubes and lava lakes accumulated. Proximal - that is, deposited closer to the location of the eruption - komatiite layers are much thinner and alternate with sulphurous sediments , black schists , cherts and tholeiitic basalts. Along with the komatiites , Felsic magmas , komatiitic tuffs , niobium anomalies and a mineralization carried by sulphides and water occur, all of which are signs of the formation of komatiites from relatively water-rich mantle material.
Textural features
A common and special texture is the Spinifex texture . It consists of long, leaf-shaped phenocrystals of olivine (or pseudomorphoses of conversion minerals to olivine), which give the rock the aspect of a collection of blades, especially on weathered surfaces. This texture is the result of a rapid solidification of a supercooled melt . If the prerequisites for solidification are given, then the crystallization proceeds quickly and the melt solidifies in a very short time.
Harrisite texture , first described from Harris Bay on the Isle of Rum , Scotland , is created by the formation of crystals at the base of the lava flow. These can form megacrystal aggregates of pyroxene and olivine, which can be up to one meter long.
volcanology
The general structure and shape of a Komatiite volcano is interpreted as that of a shield volcano , which is typical for most large basalt volcanic buildings, since a Komatiite eruption also emits less magnesium-containing material.
Nonetheless, the initial phase of outflow from most magnesium-rich lavas is viewed as a form of canalized flow that occurs when fissure eruptions of highly motile komatiitic lava hit the surface. The lava then flows away from the exit fissure and concentrates in terrain depressions. In this case, a channel facies forms ( channel facies ), in the magnesium-rich cumulates forming layers of a Marginal Facies ( sheeted flow facies ) is lined with less magnesium rich olivine and pyroxene, form textures Spinifex in the thin layers.
A typical komatiitic lava flow has six stratigraphically connected elements:
- A1 - surface cooled in a pillow-shaped and variolitic manner (in small spheres), often merging into sediments
- A2 - Zone with rapidly cooled, feather-like olivine-pyroxene glass: the quenched edge of the effusion
- A3 - Olivine-Spinifex-Sequence, from sheaf-like and book-like olivine-Spinifex: downward crystal growth on the surface of the effusion
- B1 - Mixed cumulate containing olivine to cumulate: harrisite texture grown in flowing melt
- B2 - Olivine enrichment zone (accumulate) made up of> 93% entangled olivine crystals of the same size
- B3 - Lower cooling edge: olivine accumulate to mixed accumulation, with smaller grain size
Individual lava flows can be incomplete if subsequent flows melt or erode the Spinifex structures of zone A again. In the distal facies of the thin-layer effusions, i.e. remote from the eruption point, the horizons of zone B are poorly formed or missing, since there is not enough melt to form the accumulation structures.
Lava channels and stratified effusions are covered in the course of further eruption activity by lavas that are increasingly poor in magnesium, initially by magnesium-rich basalts, then by tholeiitic basalts. The increasingly silicate melts form a volcanic building that is typical of shield volcanoes.
Intrusive komatiite
Komatiite magma is very dense, it only reaches the surface in a few cases and mostly forms magma chambers in deeper horizons of the earth's crust. More recent research (after 2004) has been able to demonstrate this, for example, on the komatiites of the Yilgarn craton in Western Australia, which are now regarded as komatiites of sub-volcanic to intrusive nature.
The same applies to the nickel deposit of Mt. Keith near Leinster (Western Australia) , in which textures were discovered at the only slightly deformed contact between komatiite and host rock, which suggest an intrusion of the melt into the host rock, as well as xenolites of the rocky rock.
The previous interpretation of these large komatiite bodies saw them as “superchannels” or as reactivated supply channels that grew to a stratigraphic thickness of more than 500 m during extensive volcanic episodes . Today they are viewed as storage corridors formed by the penetration of komatiites into the stratification , and as increasingly extensive magma chambers . Economically minable deposits of nickel mineralization in olivine accumulates could represent a storage corridor in which the magma collected like a magma chamber before it rose to the surface.
Economical meaning
The economic importance of komatiites became apparent in the early 1960s when massive nickel sulfide mineralization was discovered in Kambalda , Western Australia . Nickel-copper sulfide mineralization today accounts for 14% of world nickel production, mostly with ores from Australia , Canada and South Africa .
Associated with komatiites are nickel and gold deposits in Australia, Canada, South Africa and - only recently discovered - in the Guyana shield in South America .
literature
- NT Arndt, EG Nisbet: Komatiites . Unwin Hyman, 1982, ISBN 0-04-552019-4 .
- Harvey Blatt and Robert Tracy: Petrology . 2nd Edition. Freeman, 1996, ISBN 0-7167-2438-3 , pp. 196-197 .
- Myron G. Best: Igneous and Metamorphic Petrology . WH Freemann & Company, San Francisco 1982, ISBN 0-7167-1335-7 , pp. 539 ff .
- PC Hess: Origins of Igneous Rocks . President and Fellows of Harvard College, 1989, ISBN 0-674-64481-6 , pp. 276-285 .
- RET Hill, SJ Barnes, MJ Gole and SE Dowling: Physical volcanology of komatiites; A field guide to the komatiites of the Norseman-Wiluna Greenstone Belt, Eastern Goldfields Province, Yilgarn Block, Western Australia . Geological Society of Australia, 1990, ISBN 0-909869-55-3 .
- RH Vernon: A Practical Guide to Rock Microstructure . Cambridge University Press, 2004, ISBN 0-521-81443-X , pp. 43-69, 150-152 .
Web links
- Unusual lava types: Komatiites. Department of Geological Sciences, San Diego State University
- Delphine Nna-Mvondo, Jesus Martinez-Frias: Komatiites: From Earth's Geological Settings to Planetary and Astrobiological Contexts.
- Stephen Parman: Komatiites and the Plume Debate. The Komatiite and Their Role in Studying the Early Earth Times
Individual evidence
- ↑ Jarek Trela, Esteban Gazel, Alexander V. Sobolev, Lowell Moore, Michael Bizimis: The hottest lavas of the Phanerozoic and the survival of deep Archaean reservoirs . In: Nature Geoscience . advance online publication, May 22, 2017, ISSN 1752-0908 , doi : 10.1038 / ngeo2954 ( nature.com [accessed May 25, 2017]).
- ↑ Trela, J., Gazel, E., Sobolev, A. et al. The hottest lavas of the Phanerozoic and the survival of deep Archaean reservoirs. Nature Geosci 10, 451-456 (2017) doi: 10.1038 / ngeo2954
- ↑ Jonathan O'Neil, Richard W. Carlson, Don Francis, Ross K. Stevenson: Neodymium-142 Evidence for Hadean Mafic Crust . In: Science . tape 321 , no. 5897 , September 26, 2008, p. 1828–1831 , doi : 10.1126 / science.1161925 .
- ↑ Science News The oldest primary rock on earth found in Canada
- Jump up ↑ The petrogenesis of Gorgona komatiites, picrites and basalts: new field, petrographic and geochemical constraints - ScienceDirect. Retrieved May 25, 2017 (English).
- ^ A Web Browser Flow Chart for the Classification of Igneous Rocks. ( Memento of the original from May 10, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. IUGS Subcommission on the Systematics of Igneous Rocks
- ↑ Alexander V. Sobolev, Evgeny V. Asafov, Andrey A. Gurenko, Nicholas T. Arndt, Valentina G. Batanova: Komatiites reveal a hydrous Archaean deep-mantle reservoir . In: Nature . tape 531 , no. 7596 , March 31, 2016, ISSN 0028-0836 , p. 628–632 , doi : 10.1038 / nature17152 ( nature.com [accessed May 25, 2017]).
- ^ MJ Le Bas: IUGS Reclassification of the High-Mg and Picritic Volcanic Rocks. Journal of Petrology 41 (10), 2000, pp. 1467-1470
- ↑ SA Svetov, AI Svetova and H. Huhma: Geochemistry of the Komatiite – Tholeiite Rock Association in the Vedlozero – Segozero Archean Greenstone Belt, Central Karelia Archived from the original on July 24, 2004. Info: The archive link was automatically inserted and not yet checked . Please check the original and archive link according to the instructions and then remove this notice. In: Geochemistry International . 39, Suppl. 1, 1999, 2001, pp. S24-S38. Retrieved July 26, 2005.
- ↑ Komatiite on Io? ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. Bavarian State Collection for Paleontology and Geology
