کربناتیت - Carbonatite

Carbonatite

 

An igneous rock in which carbonate minerals make up at least half the volume. Individual occurrences of carbonatite are not numerous (about 330 have been recognized) and generally are small, but they are widely distributed. Carbonatites are scientifically important because they reveal clues concerning the composition and thermal history of the Earth's mantle. Economically, carbonatites provide important mines for some mineral commodities and are virtually the only sources of a few.  See also: Carbonate minerals

Mineralogy

 

The carbonate minerals that dominate the carbonatites are, in order of decreasing abundance, calcite, dolomite, ankerite, and rarely siderite and magnesite. Sodium- and potassium-rich carbonate minerals have been confirmed in igneous rocks at only one locality, the active volcano Oldoinyo Lengai in Tanzania (Fig. 1). Noncarbonate minerals that typify carbonatites are apatite, magnetite, phlogopite or biotite, clinopyroxene, amphibole, monticellite, perovskite, and rarely olivine or melilite. Secondary minerals, produced by alteration of primary magmatic minerals, include barite, alkali feldspar, quartz, fluorite, hematite, rutile, pyrite, and chlorite. Minerals that are important in some carbonatites because they carry niobium, rare-earth elements, and other metals in concentrations high enough for profitable extraction are pyrochlore, bastnaesite, monazite, baddeleyite, and bornite.

 

 

Fig. 1  Carbonatite lava from Oldoinyo Lengai volcano, Tanzania; thin section between crossed polarizers. Longest dimension of the field of view is 2.5 mm.

 

 

 

 

 

Styles of occurrence

 

Carbonatites occur in a variety of forms, both intrusive and volcanic. Lava flows are small and rare, but pyroclastic carbonatites are numerous as thick near-vent accumulations (tuff cones) and thin but widespread airfall and surge deposits. Some pyroclastic carbonatites are especially significant, because they preserve the forms of quenched droplets of carbonate-rich liquid and also record the mineral assemblages that are nearly always erased in intrusive rocks by low-temperature alteration. Carbonatite airfall tuffs are paleontologically important, because they tend to be fine grained and rapidly become lithified, preserving delicate organic structures. The hominid footprints at Laetoli, Tanzania, and remarkably detailed molds of plants and caterpillars are examples.  See also: Tuff

Most intrusive carbonatites form arrays of dikes that may be parallel, radiating, or concentric and arcuate. More irregular intrusive bodies are also known, but all are small compared to those of silicate-rich igneous rocks; the largest known carbonatite body, at Sokli in Finland, has a surface area of only 20 km2 (8 mi2). Carbonate-rich magmas appear to have had low viscosities, but some were emplaced with explosive violence because of their high gas content. A few carbonatites extend for several kilometers along fault surfaces; they may have been older igneous bodies that were caught up in the fault displacement and acted as cold but weak ductile lubricant between the rock masses; or they may have been injected as magma. Except for the absence of large homogeneous plutons, carbonatites form the full array of igneous rock styles that are shown by silicate-rich igneous rocks.  See also: Magma; Pluton

 

Associated rocks

 

Carbonatites usually have been found with low-silica, high-alkali igneous rocks, typically melilitites, nephelinites, tephrites, and phonolites. The silicate and carbonate rocks are intimately mixed in many intrusive complexes, and the carbonatites are among the youngest and generally least abundant of rock types in any complex. Carbonatites that are unaccompanied by other igneous rocks have been recognized. Examples are at Fort Portal, Uganda; Kaluwe, Zaire; and Sarfartoq, Greenland.

 

Geographic distribution

 

Carbonatites are widely scattered (Fig. 2). About half of all carbonatite occurrences so far recognized are in Africa. The only occurrences known on oceanic crust are in the Canary and Cape Verde islands; however, carbonatite associated with deep-water marine sediments occurs under the Semail ophiolite in the United Arab Emirates. These occurrences suggest that carbonatites might actually be common on and within oceanic crust but are usually buried or destroyed by subduction.  See also: Ophiolite

 

 

Fig. 2  Generalized distribution of carbonatites. Individual spots represent clusters of occurrences.

 

 

 

 

 

Chemical compositions

 

Carbonatites, compared to the inferred composition of the Earth's mantle and to other igneous rocks, are greatly enriched in niobium, rare-earth elements, barium, strontium, phosphorus, and fluorine, and they are relatively depleted in silicon, aluminum, iron, magnesium, nickel, titanium, sodium, potassium, and chlorine. These extreme differences are attributed to strong fractionation between carbonate liquid on the one hand and silicate and oxide solid phases on the other during separation of the carbonate liquid from its source. Strontium and neodymium isotope ratios indicate that the sources of carbonatites are geologically old, inhomogeneous, and variably depleted in the radioactive parent elements rubidium and samarium.

 

Genesis of carbonate-rich magma

 

For more than two centuries, it has been known that calcite decomposes to lime (calcium oxide; CaO) and carbon dioxide (CO2) when exposed to high temperature at atmospheric pressure. Furthermore, at high pressure calcium carbonate (CaCO3) liquid is stable only at temperatures far higher than are reasonably expected in the Earth's crust. It was therefore difficult for geologists to imagine liquid carbonate being stable at or near the Earth's surface. In 1960 it was demonstrated experimentally that the addition of water (H2O) to the system CaO-CO2 at high pressure will stabilize a carbonate liquid at temperatures expectable in the Earth's crust. Also in 1960, the volcano Oldoinyo Lengai in Tanzania erupted carbonate-rich ash and subsequently carbonatite lava. Geologic mapping has confirmed that most carbonatites occur in small volumes associated with mantle-derived, high-alkali, low-silica igneous rocks.  See also: Calcite

There are three possible origins (not mutually exclusive) for carbonatite magmas: (1) they are “primary” liquids, arriving directly from their mantle sources without change during ascent; (2) they are products of fractional crystallization from carbonate-bearing silicate magmas (if silicate minerals crystallize early from the magma, carbonate concentration should build up in the remaining liquid); or (3) they are products of immiscible separation of carbonate-rich and silicate-rich liquids (the properties of highly ionic carbonate liquid are so different from those of silicate liquid that, during cooling, carbonate-bearing silicate magma separates into two liquids, one containing nearly all the carbonate, the other nearly all the silicate).

The first origin is favored by evidence from altered mantle xenoliths (rock fragments enclosed by an unrelated igneous rock) carried to the surface by basalts, nephelinites, and kimberlites; these altered fragments demonstrate that carbonatite liquid has percolated through the mantle. A primary liquid origin is also supported by the presence in carbonatites, rarely, of magnesium- and chromium-rich spinels typical of mantle rocks, and by the absence, in some carbonatite occurrences, of associated silicate rocks that could represent parents or siblings of the carbonatites. The lack of mantle xenoliths in carbonatites is evidence against primary liquid, but the low density and low viscosity of carbonatite liquid may permit such dense fragments to settle out quickly.  See also: Spinel; Xenolith

The second and third origins are favored by the small volume of carbonatite relative to silicate rocks in most occurrences, and by the lateness of carbonatites in intrusive and eruptive sequences; but they are opposed by differences in isotopic ratios between carbonatite and silicate rocks in some occurrences (neither fractional crystallization nor immiscible liquid separation should change the isotopic ratios). The second origin is also opposed by experimental results indicating that silicates and carbonates crystallize simultaneously through a large temperature interval, preventing buildup of carbonate in the remaining liquid. The third origin is supported by experiments demonstrating that during cooling a carbonate-bearing silicate liquid can split into carbonate-rich and silicate-rich liquids, and by the natural occurrence of “rock emulsions” consisting of blobs of one composition enclosed in the other. On balance, the evidence is still inconclusive, and possibly any of the three mechanisms of origin can operate at different times and places.

 

Criteria for recognition

 

It is not easy to discriminate carbonatites from thoroughly recrystallized and metamorphosed marbles or from vein fillings deposited by dilute, low-temperature aqueous solutions. Because calcite has very low strength and easily deforms by plastic flow, carbonate-rich rocks can intrude their more brittle surroundings and thus imitate magmatic rocks. Furthermore, because marbles and recrystallized carbonatites converge toward the same textures and mineral assemblages, carbonatites can be misidentified as metamorphosed sedimentary rocks. The strongest indications of igneous origin for a carbonate-rich rock are considered to be quenched carbonate-rich liquid in the form of droplets or bubble walls, association with nepheline- or melilite- bearing rocks, presence of strontium-rich calcite, presence of pyrochlore and other minerals that are rich in niobium or rare-earth elements, and presence of apatite that has higher concentrations of rare-earth elements and silicon than apatite from other rock types.  See also: Apatite; Niobium; Rare-earth elements

 

Economic significance

 

Carbonatites yield a variety of mineral commodities, including phosphate, lime, niobium, rare-earth elements, anatase, fluorite, and copper. Agricultural phosphate for fertilizer is the most valuable single product from carbonatites; most is obtained from apatite in lateritic soils that have developed by tropical weathering of carbonatites, dissolving the carbonates and thereby concentrating the less soluble apatite. Lime for agriculture and for cement manufacture is obtained from carbonatites in regions where limestones are lacking.

The carbonatites at Bayan Obo, China, and Mountain Pass, California, dominate the world suppliers of rare-earth elements, but many other carbonatites contain unexploited reserves. Tropical weathering at several carbonatites in Brazil has produced economically important concentrations of anatase (TiO2) from decomposition of perovskite (CaTiO3). Other carbonatites are locally important sources for some commodities; for example, Amba Dongar, India, provides fluorite, and Phalaborwa, South Africa, is a major producer of copper, with zirconium, gold, silver, platinum group elements, and uranium as by-products.

 

Tectonic significance

 

Ultramafic xenoliths from lithospheric mantle commonly show textures and mineral assemblages that indicate modification of the original rock. This alteration typically results in strong enrichment in light rare-earth elements, uranium, thorium, and lead, but much less enrichment in titanium, zirconium, niobium, and strontium. These changes are commonly attributed to interaction of lithospheric mantle with an invading carbonate-rich magma. The wide geographic dispersal of these altered xenoliths suggests that carbonate-rich liquid has been more common in the upper mantle than the low abundance of carbonatites in the upper crust would suggest. According to the testimony of these samples, carbonatite magma, ascending through lithospheric mantle, commonly is trapped before it can invade the crust. In addition to the factors that can stop the rise of any magma (heat loss, increase of solidus temperature with decrease in pressure, decrease in density and increase in strength of wall rock), carbonatite magma can be halted by reaction with wall rock to form calcium and magnesium silicates plus CO2, and by less oxidizing conditions to reduce carbonate to elemental carbon (graphite or diamond) or to methane. Both of these changes subtract dissolved CO2 from the magma, causing crystallization.  See also: Diamond; Earth interior; Graphite

In some regions carbonatite magmatism has recurred after intervals of 100 million years, suggesting that carbonate-rich liquids can be regenerated by a small degree of partial fusion of locally and repeatedly carbonated mantle. This possibility, in turn, suggests that carbonate in the mantle is replenished by recycling of sedimentary carbonate.  See also: Subduction zones

Carbonatites are not restricted to a single tectonic regime. They occur in oceanic and continental crust and have formed in compressional fold belts and stable cratons as well as regions of crustal extension. Rather than indicating the stress field in the shallow crust in which they were emplaced, carbonatites are useful in modeling the long-term thermal and chemical development of the mantle.  See also: Igneous rocks

Daniel S. Barker

 

Bibliography

 

•  
• K. Bell (ed.), Carbonatites: Genesis and Evolution, Unwin-Hyman, Boston, 1989
• Ali Fazeli = egeology.blogfa.com
• R. H. Mitchell (ed.), Undersaturated Alkaline Rocks: Mineralogy, Petrogenesis, and
• Economic Potential, Short Course Vol. 24, Mineralogical Association of Canada, Winnipeg, 1996
• Ali Fazeli = egeology.blogfa.com
• A. R. Woolley, Alkaline Rocks and Carbonatites of the World, pt. 1: North and South
• Ali Fazeli = egeology.blogfa.com
• America, British Museum (Natural History), London, 1987
• Ali Fazeli = egeology.blogfa.com

 

Additional Readings

 

 

• Carbonatites in General
•  Ali Fazeli = egeology.blogfa.com