Carbonatite

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Lava of the Ol Doinyo Lengai

In geology, carbonatite is a rare igneous rock that is heavily undersaturated in silicon dioxide and, by definition, contains more than 50 percent by volume of carbonate minerals .

The carbonatites, which belong to the ultramafic rocks , occur predominantly intrusively , effusive equivalents are very rare.

Geochemically they are by a very strong enrichment of incompatible elements such as strontium , barium , cesium , and rubidium , and the element phosphorus and the light rare earths ( engl. LREE) characterized. However , they show a clear depletion of HFSE elements such as hafnium , zirconium and titanium .

First description and type locality

Carbonatites were first scientifically described by Waldemar Christofer Brøgger (German: Brögger) in 1920 in the publication series Videnskaps Skrifter of the Norske Videnskaps-Akademi . Its processing is based on the outcrops of the Fen area in Telemark in southern Norway ( type locality ).

Definition and carbonatite varieties

Magnesiocarbonatite from the Verity Paradise Carbonatite Complex in British Columbia, Canada. The handpiece is 75 millimeters wide

The definition of carbonatites has as a necessary condition the modal presence of more than 50 percent by volume of carbonate minerals.

On the basis of the predominant modus mineral, the following varieties can be distinguished in the rock group of carbonatites:

In parallel to this, a classification based on the predominant geochemical element is often used :

  • Calcium carbonatite
  • Magnesium carbonatite
  • Iron carbonatite
  • Rare earth carbonatite

In a process-oriented manner, carbonatites can be divided into two groups:

  • Primary carbonatites
  • Carbothermal residuals

Primary carbonatites are of magmatic origin and are associated with:

However, carbonatites resulting from carbothermal residues come from liquids that are relatively low-temperature and enriched in carbon dioxide , water and fluorine .

In general, the carbonate content of carbonatites is over 50 percent by volume and in rare cases can reach up to 90 percent by volume. If the carbonate content is below 50 percent by volume up to and including 10 percent by volume, the rocks in question are referred to as carbonatitic . Carbonatites in which the SiO 2 content is greater than 20 percent by weight and at the same time exceeds the carbonate content are silicon carbonatites .

Ternary classification diagram of carbonatites (in percent by weight) with less than 20 percent by weight SiO 2 , according to Woolley (1989)

Carbonatites with an SiO 2 content <20 percent by weight are subdivided as follows (see adjacent figure):

  • Calciocarbonatite : fulfills the condition CaO / (CaO + MgO + FeO + Fe 2 O 3 + MnO) <0.8.

This means that CaO ≥ 80 percent by weight.

Thus, MgO: 100 to 10 percent by weight, FeO + Fe 2 O 3 + MnO: 0 to 50 percent by weight and CaO: 0 to 80 percent by weight.

Consequently FeO + Fe 2 O 3 + MnO: 100 to 10 percent by weight, MgO: 0 to 50 percent by weight and CaO: 0 to 80 percent by weight.

Mineralogical composition

In general, the proportion of carbonate minerals, which are mainly represented as calcium and CO 2 in the empirical formula, is 70 to 90 percent by volume.

The main minerals are the carbonates:

In addition, there may be:

Chemical composition

The following table of various chemical analyzes is intended to clarify the variability of carbonatitic magmas (listing according to Le Bas, 1981):

Weight% Average carbonatite Carbonatitic lava Aegirin-Sövit Average alvikite Dolomite carbonatite
beforsit		 Natrocarbonatite Ferrocarbonatite
SiO 2 5.67 12.99 4.86 1.50 6.12 0.58 1.49
TiO 2 0.50 1.74 0.17 0.07 0.68 0.10 0.22
Al 2 O 3 1.77 3.03 0.30 0.18 1.31 0.10 1.11
Fe 2 O 3 8.00 12.81 1.71 2.70 7.55 0.29 6.97
MnO 0.76 0.40 0.27 0.57 0.75 0.14 1.58
MgO 6.10 8.55 0.31 0.58 12.75 1.17 1.62
CaO 37.06 35.97 50.45 49.40 29.03 15.54 46.29
Na 2 O 1.09 0.73 1.09 0.13 0.14 29.56 0.34
K 2 O 0.87 0.20 0.17 0.08 0.79 7.14 0.22
P 2 O 5 1.73 3.32 1.00 0.66 2.66 0.95 1.63
CO 2 32.16 14.79 34.84 39.85 37.03 31.72 33.97
SO 3 0.91 0.88 0.40 0.65 0.89 2.48 0.57
F. 0.38 0.30 0.09 2.26 0.53
Cl 0.31 2.90
BaO 0.45 0.15 0.13 0.54 0.11 1.04 1.93
SrO 0.89 0.63 0.26 0.26 0.01 2.09 0.12

The chemistry of carbonatites is generally dominated by CaO (up to 50 percent by weight), with sodium carbonatites also by Na 2 O (up to 30 percent by weight) and CO 2 (up to 40 percent by weight - a maximum of 44 percent by weight is possible). They are very clearly undersaturated in SiO 2 .

Physical Properties

Compared to associated alkaline silicate melts, carbonatite melts have remarkable physical properties. For example, their solubilities for elements that are seldom present in silicate melts are very high. Their absorption capacity for water and other volatile substances such as halogens are the highest among all melts at the pressures prevailing in the earth's crust. Carbonatite melts are also very efficient in transporting carbon from the earth's mantle to the overlying crust, as they remain liquid over wide temperature ranges.

Carbonatitic magmas are characterized by a very low viscosity . This should contribute significantly to maintaining the interconnectivity between the grain boundaries in the mantle rock and thus metasomatic accumulations in the upper mantle can take place. Carbonatite melts are ionized liquids which, in contrast to silicate liquids, have little or no tendency to polymerize .

Another important physical property of carbonate melts is their high electrical conductivity , which exceeds the conductivity of silicate melts by up to three orders of magnitude and the conductivity of hydrogenated shell material by up to five orders of magnitude. Consequently, carbonate melts can be used to explain increased conductivity anomalies in the deep asthenospheric region. Melts with a carbonate content of only 0.1 percent by volume are completely sufficient. In the past, these anomalies were associated with molten silicate or hydrous olivine.

Leaked carbonatitic lava flows are unstable on the earth's surface and react by absorbing water in contact with the atmosphere.

Petrology

Appearance

The Tororo carbonatite stock in southeast Uganda

The external appearance of carbonatites is subject to great fluctuations due to their chemical and structural variability. Their grain sizes range from fine-grained to giant-grained ( pegmatite-like ), and their hue varies from light to dark, depending on the proportion of mafic minerals . When alteration begins, carbonates are broken down. In particular, iron-rich carbonates can give the rock a beige, red to dark brown color when they decay. The weathering of carbonatites in the subtropical and tropical climates leads to lateritization and an enrichment of non-carbonatic minerals, which can occasionally represent minable deposits. In rare cases, carbonatites are more resistant than the silicate rocks surrounding them; a good example of this is the carbonatite stock from Tororo in Uganda, which towers over its surroundings by almost 300 meters.

Karst phenomena can also occur in carbonatites under humid climatic conditions .

Socialization

Typical of carbonatites is their association with SiO 2 -undersaturated magmatites. In total, around 80% of the carbonatites are associated with alkaline silicate rocks, the chemical composition of which can span a wide spectrum from ultramafic (e.g. dunite ) to acidic igneous rocks (such as syenite ). Their close connection to melilite rocks , kimberlites , meimechites and related rocks, all of which are characterized by very low SiO 2 contents, is also of great importance . In particular, there are all transitions to the carbonatitic kimberlites.

Only 20% of the carbonatites occur completely independently (such as the carbonatite from Mount Weld in Australia).

The following associations can be distinguished:

Associated magmatites include: ijolite , melteigite , teschenite , lamprophyre , phonolite , nephelinite , foyaite , shonkinite , pyroxenite ( essexite ) which is undersaturated in silica and foid-containing, and nepheline syenite .

Occur

Profile section through a typically cylindrical carbonatite intrusion below an alkali rock volcano

With a few exceptions, carbonatites are generally associated with alkaline magmatism and occur mostly as sub-volcanic or flat-bed plutonic complexes such as lopolites . Carbonatites are subordinate to nephelinitic lavas and pyroclastics . In zoned Alkaliintrusivkomplexen (ring complexes with ring structures such as Phalaborwa in South Africa), they form small sticks ( Engl. Plugs ), sills , transitions , veins and breccias . The sticks are often designed as a cylindrical, central intrusive body, which can be intruded in several phases, as in the carbonatites of Mud Tank and Mount Weld in Western Australia. Often, carbonatite breccias or carbonatite-silicate mixed breccias are found above the intrusion, documenting the explosive penetration and subsequent movements of the carbonatite. Furthermore, carbonatite dikes can be sent from the upper edge of the stock as cone sheets , radial or ring dikes in one or several generations into the overlying neighboring rock (see figure opposite).

In the Proterozoic mobile belts of Australia, carbonatites appear as ducts and discontinuous lenses.

A characteristic of carbonatite intrusions is the hydrothermal alteration or metasomatosis of the neighboring rock caused by them , which is referred to as fenitization . The hot solutions penetrating into the neighboring rock can originate from both the carbonatite melt and / or the associated silicate magma. There are two types of rock transformations:

  • Sodium fenitization
  • Potassium fenitization

whereby the sodium fenitization usually predominates. As a result, sodium-rich silicates such as arfvedsonite , barkevikite and glaucophane are formed in the adjacent rock . Additional new formations are hematite and other iron and titanium oxides as well as phosphates. The conversion products are called fenites .

Carbonatites appear extrusive (as carbonatite lavas) only rarely. Tuffs are a little more common - as effusive carbonate tuffs or Schlottuffe with a melilithic-carbonatite composition. The only active carbonatite volcano is the Ol Doinyo Lengai in Tanzania, which is surrounded by some Pleistocene carbonatite volcanoes that were not extinct so long ago - this includes Mount Homa , which is already in Kenya .

structure

In the case of carbonatites, the following structures are considered to be of primarily magmatic origin:

  • Porphyry structures
  • Fluid structures
  • Comb-shaped structures
  • Granoblastic structures

In the case of porphyry structures, larger calcites or dolomites appear as sprinkles (porphyroblasts) in a finer-grained matrix. The flat rhombohedral Einsprenglinge are mostly idiomorphic, but can also be rounded. Very seldom an antiporphyric structure is also encountered, in which the inserts are smaller than the non-idiomorphic basic mass grains . The fluid structures show a trachytoid adjustment of the calcite tablets that were first crystallized. Comb- shaped structures or comb textures ( comb layering ) are created by the preferred growth of long prismatic or thin- sheet carbonate minerals in preferred directions. They are mainly found in carbonatite dikes (such as in Oberbergen in the Kaiserstuhl), where they are arranged perpendicular to the Salband. The granoblastic structures occur predominantly in massive intrusions, whereas the first three structures mentioned are bound to carbonatite dykes. The individual grain boundary outlines of the granoblasts are either simple with individual grains arranged in a mosaic or pavement-like manner, but more often bulky or even lobed.

Due to their variable chemistry and diverse geological occurrences, carbonatites have very different texture forms :

  • Directionless massive textures
  • Layered or streaky textures
  • Parallel textures
  • Comb textures
  • Bubble textures
  • Globulitic textures
  • Brecciated textures
  • Clastic textures

Layered textures arise either from differences in grain size or from a preferred distribution of the mineral components , especially the non-carbonate minerals. Parallel textures can be designed as gneiss-like or as fluid textures. In them, lenticular or plate-shaped minerals such as carbonates or mica are regulated. Bubble or vesicular and also globular textures are rare, whereby the cavities can be filled with minerals. The brecciated textures are either monomictic or polymictic. Clastic textures consist of irregularly shaped fragments. They are found together with lapilli, bead or drop textures in carbonate tuffs.

Overall, it should be remembered that the mineral stocks and structures found in carbonatites are often not of a primary nature. After the intrusion, tectonic deformations can cause twin lamination in calcite and lattice distortions and generally lead to recrystallization . Liquid and gas inclusions in minerals indicate that the original carbonatite magmas were richer in alkalis, which they lost to the neighboring rock as a result of the alkali metasomatosis (fenitization) that started from them. Carbonatites therefore correspond to the non-alkaline part of the original magma and also show the characteristics of later recrystallizations.

Petrogenesis

The formation process of carbonatites (petrogenesis) is explained using three models:

  • Primarily in the earth's mantle :
    • direct generation by partial melting (at a low melting rate) of a carbonate- containing mantle peridotite with subsequent melt differentiation.
  • Secondary from silicate melts:
    • Separation of the carbonatite melt due to its immiscibility with silicate melts.
    • Separation of the carbonatite melt due to unusual and extreme crystal fractionation processes .

All three model concepts can be substantiated by examples; combinations with one another are also conceivable.

In the past, the view was still held that invading magma pulses had melted marbles and limestone and thus created the carbonatite magmas. However, detailed mineralogical and geochemical investigations contradict this . For example, the strontium isotope ratio of 87 Sr / 86 Sr for carbonatites is between 0.702 and 0.705 with an average value of 0.7035, whereas for sedimentary limestone and marble, this ratio usually exceeds 0.706. In addition, carbonatites differ from sedimentary calcite rocks solely in their clear enrichment of strontium, barium and rare earths in the calcite.

Place of origin

There are still very different opinions about the place of origin of magmas rich in carbonates. As already indicated above, however, it is becoming apparent that a large number of carbonatites are either directly or indirectly connected with upwelling areas of the earth's mantle such as hotspots or plumes . The further assumption that most carbonate melts should have their ultimate place of origin in the mantle is supported by the isotope ratios of strontium, neodymium and lead , analyzes of the isotope composition of noble gases and stable elements such as carbon and oxygen as well as experimentally determined phase equilibria. In general, these geochemical parameters indicate the relatively contamination-free ascent of carbonatitic magmas through the earth's crust.

Isotope ratios δ 13 C and δ 18 O stretch in primary casing rocks the so-called Mantelbox on which δ 13 C-values from -5 to -7 ‰ and their δ 18 O values between +5 and +8 ‰ SMOW move . Primary igneous carbonatites show a practically congruent and only slightly expanded range of –4 to –8 ‰ for the δ 13 C values ​​and +5 to +10 for the δ 18 O values. Fractional crystallization extends this range to –1 to –9 ‰ for the δ 13 C values ​​and to +5 to +15 for the δ 18 O values. Hydrothermally overprinted carbonatites can even reach the δ 13 C values ​​of +30, which are characteristic of carbonates of sedimentary origin .

Isotope studies on carbonatites come to the following results in this context:

  • The mantle components HIMU , EM I and FOZO found in oceanic island basalts are also present in carbonatites that are less than 200 million years old.
  • Noble gas isotope analyzes carried out on some carbonatites suggest that they came from a relatively primitive mantle source.
  • The depleted mantle component DMM is obviously not involved in the genesis of carbonatites, which in turn rules out the inclusion of oceanic lithosphere in the source region.
  • Some carbonatites are characterized by non-radiogenic hafnium isotopic compositions; this suggests an old, unenriched, deep-seated mantle reservoir as the source region.

The depth of magma formation is still up for debate. Even formation in the lower mantle has recently been considered likely - if this assumption were to be true, this would result in a fundamental control function over the mobility and long-term stay of carbon in deep mantle areas. The depths at which carbonate melts can still exist ultimately depends on the stability range of the carbonate minerals in question, which in turn is critically linked to the oxidation state of the shell.

Petrological laboratory experiments also suggest that carbonate melts can be found in the oceanic Low Velocity Zone (LVZ) and in the deep mantle. However, the behavior of these melts under extremely high pressures is still new research territory. Possible clues in this area are carbonate inclusions in diamonds or carbonates in impact craters modified by shock wave metamorphosis .

meaning

There is no doubt that carbonatites play a very important role in the earth's carbon cycle. Due to their presumed formation in deep mantle areas, they are likely to be of great importance for the transport of carbon from the interior of the earth to the earth's surface. It is assumed that the carbon content of the metallic earth's core is up to 5 percent by weight - corresponding to 10 20 kilograms of solid C. A part of this carbon reservoir is finally also shifted into the earth's mantle.

Carbonatitic volcanism is ultimately responsible for light elements and carbon from the earth's mantle reaching the earth's surface. Carbon is either released directly into the earth's atmosphere in the form of carbon dioxide via crater chimneys , maars and geysers or infiltrates into the groundwater cycle , where it can be deposited as travertine . For example, a daily flow rate of 800 tons of CO 2 of non-volcanic origin was measured in central Italian carbonatite vents.

Associated deposits

A characteristic property of carbonatites is their extraordinary enrichment in rare earths (especially the light rare earths - LREE ), phosphorus , niobium , radioactive elements such as uranium and thorium as well as copper , iron , titanium , barium, fluorine , zirconium and other rare and incompatible items . Some carbonatite complexes are very rich in vanadium , zinc , molybdenum and lead.

Carbonatites are a source of economically important minerals such as fluorite , barite , apatite , vermiculite , magnetite and pyrochlore , an important niobium and uranium mineral.

The enrichments are found in mineralized veins that run through either the carbonatite stock itself or its metasomatized aureole.

Rare earth

Satellite image of the Bayan Obo Mine (with English labeling)

The largest known enrichment of rare earth minerals in the world is the Bayan Obo Mine in Inner Mongolia, China , associated with dolomite marble and carbonatite veins. Their total deposits are estimated at 2.22 million tons of pure rare earth oxide. The stocks in Phalaborwa in South Africa are only marginally lower at 2.16 million tons of pure rare earth oxide. Another huge deposits forms the Sulphide Queen deposit in Mountain Pass district in California with 1.78 million tons of supplies of pure rare earth oxide. It is bound to carbonatite dikes and potash rich intrusions . The name of the deposit is a bit misleading, by the way, as it is not named after any sulphide minerals, but after the Sulphide Queen Mountains. Their most important ore minerals are bastnasite ( cerium , lanthanum and yttrium ) and parisite (cerium, lanthanum and neodymium ). Oka’s supplies in Canada are 0.12 million tons of pure rare earth oxide.

Apatite or phosphorus

The open pit mine at the paleoproterozoic carbonatite complex of Phalaborwa in South Africa is unusual . A serpentinite-magnetite-apatite rock, which bears the local and commercial name phoscorite, is mined from a carbonatite core . By-products include magnetite, apatite, gold , silver , platinum metals and uranium . The world's largest igneous phosphate deposit is also located within the same alkali rock complex. Apatite-rich pyroxenite is also mined there. The stocks of pure P 2 O 5 in Phalaborwa are estimated at 42 million tons.

Similar carbonatite-alkali rock complexes can be found on the Kola Peninsula (for example in Kovdor ). There, too, apatite is the most important phosphate mineral. In Brazil, apatite is mined in the carbonatites of Araxá and Jacupiranga.

Pyrochlore or niobium

Uranium-rich pyrochlore from Oka, Québec

Most of the world production of niobium comes from the pyrochlore of the carbonatites. Examples of niobium extraction are Oka in Canada with 0.5 million tons of reserves of Nb 2 O 5 , Panda Hill in Tanzania with reserves of 0.34 million tons of pure niobium oxide, the Bayan Obo Mine in China and Araxá in Brazil. Pyrochlore can sometimes be very rich in uranium, which is why carbonatites often also have high levels of radioactivity .

Fluorite

The Amba Dongar carbonatite complex in India has significant fluorite deposits with reserves of 3.48 million tons .

Iron ore

Occasionally, because of their magnetite and / or hematite content , carbonatite complexes also form minable iron ores , as is the case in the Bayan Obo mine, in the Fen complex, in Kovdor and in Phalaborwa. Copper sulphides are also used in Phalaborwa.

copper

The Phalaborwa mine in South Africa has rich reserves of copper with 1.97 million tons of pure copper oxide . The mineralization took place mainly in the copper sulfide minerals chalcopyrite , bornite and chalcosine . The extremely rich deposit also contains apatite, vermiculite , magnetite (subordinate), the cobalt sulfide Linneit and the zirconium-hafnium oxide baddeleyite . Gold, silver, nickel and platinum are produced as by-products.

Diamonds

In melanocratic carbonatites of Uzbekistan - alvikite veins in a diatreme consisting of the trio of mantle fragments, melilithite and calcite carbonatite - even diamonds were discovered.

Occurrence

In terms of volume, carbonatites are one of the least common types of rock; up to 2008, only 527 carbonatite complexes were documented worldwide, only 49 of which are extrusive in nature. Among the extrusive carbonatites, calciocarbonatites (41) and dolomitic carbonatites (7) predominate, of natrocarbonatite there is only one occurrence.

The age of carbonatites extends from the late Archean to the present. The oldest carbonatite to date is the Sövite of Siilinjärvi in Finland , which was dated to a maximum of 2,600 million years BP . The next oldest carbonatite from Phalaborwa in South Africa returned paleoproterozoic ages between 2063 and 2013 million years BP. The frequency of carbonatite occurrences seems to increase with decreasing age in recent geological history , but this is possibly only an artifact that is based on the easier erodibility of carbonatites compared to silicate rocks.

Carbonatites usually occur in a strictly localized manner in areas only a few square kilometers in size. They are present on every continent, including Antarctica . Your preferred geotectonic milieu is the stable interior of the plate, but also the edges of the plate. A third of the deposits are concentrated in Africa . So far, only three occurrences are known on oceanic islands, namely on the Kerguelen , Cape Verde and the Canary Islands .

The occurrence of carbonatites is primarily associated with hot-spot volcanism and the associated continental rift systems. With its East African rift valley , Africa is the continent with the world's only active carbonatite volcano - the Ol Doinyo Lengai - the lava consisting of natro carbonatite erupts directly at an extremely low temperature of 540 to 593 ° C.

In addition, Ernst and Bell (2010) note a close connection between carbonatites and igneous large provinces (LIP) such as the flood basalts of the Deccan or the Paraná Basin . This is not surprising, since igneous large provinces in turn are directly connected to rifts and hotspots and their ultimate cause is an asthenospheric surge ( mantle plume ) of the sublithospheric mantle.

About 50% of Carbonatites are connected under elongation property crust areas such as grave fractures ( engl. Rift Valley ) associated with many are also significant tectonic anisotropies such as deep faults , lineaments and shear zones in context. Often they are also linked to structural reactivations and large-scale dome-like lithospheric bulges. In the production of CO 2 -rich melts, thickened areas of the lithosphere, such as those found under the ancient cratons of the Archaic era, seem to play a not insignificant role.

Up to now, almost no relations at all were known to subduction zones . Carbonatite deposits are also rare above the oceanic lithosphere. Nevertheless, spaces filled with carbonatite melts in dunite- xenolites of the Kerguelen Archipelago suggest a further spread in the oceanic area. Recent discoveries of carbonatite metasomatosis under ocean islands, of carbonatites within ophiolites, and even in deep subduction zones seem to support this suggestion.

listing

The following is a list of carbonatite deposits by country:

Carbonatite from Jacupiranga, Brazil
Carbonatite from Oka (Okaite), Québec
Carbonatite from Lake Chilwa in Malawi
Carbonatite from the Palabora complex in South Africa
Sövit from Magnet Cove in Arkansas
Carbonatite from Dreamer's Hope in Colorado

literature

  • Roland Vinx: Rock determination in the field. 2nd Edition. Springer publishing house. Berlin / Heidelberg 2008, ISBN 978-3-8274-1925-5 , p. 213 f.

Web links

Individual evidence

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