carbonatite

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carbonatites Volcanism is dominated by the eruption of silicate melts, as might be expected from the abundance of silicon in the Earth's mantle and crust (which contain about 66 per cent of silicon by weight). Carbon dioxide (CO₂) is one of the important gases emitted by volcanoes, but the amount of CO₂ is tiny compared with the melts and ashes, and molten carbonate might not therefore be expected to occur. Carbonate eruptions are, however, the only important exception to silicate magmatism. This unexpected composition, and the nature of the activity, although minute in amount, afford special insights into Earth processes.

Igneous rocks containing over 50 per cent of carbonate are defined as carbonatites. More than 350 carbonatite intrusions are known at present; most of them consist of calcium carbonate. Less common are magnesio-carbonatites (with calcium and magnesium) and ferro-carbonatites (with calcium, magnesium, and iron). All three types may occur in the same complexes, most of which are sub-volcanic intrusions. Surface eruptions of ashes and lavas (rare) are also preserved; the list of discoveries is growing rapidly with new research interest. In northern Tanzania, a volcano built largely of alkali silicates is currently erupting sodium, calcium, and potassium carbonates as lava and ash.

Many carbonatites form parts of alkali-rich silicate magmatic complexes. The first discoveries of carbonatites were in such complexes, and this led to a fierce controversy in the early half of the twentieth century as to whether or not carbonatites were merely sedimentary limestones that had been mobilized by the heat from silicate magmas moving through the Earth's crust. It is now clear from their chemistry, however, that the ultimate source of carbonatites must lie below the crust, in the Earth's mantle. Most of the known carbonatite bodies are small, with areas in the region of 1 km²; none has yet been recorded that is larger than 20 km². In age they range from mid-Precambrian (2000 Ma) to the present, but most are younger than 150 million years (150 Ma). Nearly all are located in the continental plates, very commonly in the rift zones of the stable interiors; about half the recorded occurrences are in Africa.

An outstanding feature of carbonatites is that they contain relatively large amounts of elements that are otherwise rare in the outer layers of the Earth. Elements such as phosphorus, normally found in trace amounts, reach percentage levels. This exotic aspect of carbonatite chemistry (as well as a similar global distribution) is shared by kimberlites, and both are widely regarded as small-volume partial melt extracts from the mantle—a process in which uncommon elements would be concentrated. Kimberlites also contain carbonate, and with increasing amounts of carbonates grade into carbonatite. A feature of carbonatite intrusions is that the surrounding rocks show intense chemical alteration, marked by new sodium and potassium minerals replacing the primary ones. This distinctive alkali metasomatism is known as fenitization. (It was first described from the intrusion at Fen, southern Norway.)

A further aspect is gradually emerging as more data reveal patterns in the ages of carbonatite eruption. Not only is the activity repeated in the same areas, over periods extending back into the Precambrian, but repeated episodes take place at the same time at widely separated locations. These episodes, occurring within plates, coincide with plate-collision events elsewhere and with other major igneous activity worldwide. To explain carbonatite activity may thus call for radical re-thinking about mantle processes and geodynamics.

D. K. Bailey

carbonatite Unusual igneous rock, rich in calcite and other carbonate minerals (including dolomite and ankerite), which is considered to be mantle-derived. Carbonatites occur as intruded masses, dykes, and as cone sheets, and rarely as lavas and tephra; and are found in association with alkali-rich igneous rocks, notably those erupted by the volcanoes of the East African Rift. Rare elements, including the REEs (rare earth elements), barium, niobium, thorium, and phosphorus, are often enriched in comparison with many crustal and mantle-derived igneous rocks.