Harrow and Hillingdon Geological Society

Carbonate Volcanism

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Calcium carbonate, found in some cliffs in the UK, constitutes chalk. It is comprised of calcite microfossils, a sedimentary rock laid down under the ocean. Metamorphic CaCO3, or marble, is limestone which is pushed beneath the earth where, under heat and pressure, it re-crystallises to form marble, but this is not carbonatite.

Basalt molten larva at 12000C is an igneous rock, which is molten and then crystallises. This is the most common and all contain silicate, but this is not carbonatite.

Larval flows of molten igneous rock, which contain >50% of CaCO3 are carbonatite. These are very rare rocks, and only about 50 sources are known in the world; and of these only one has been seen to erupt. This is Oldoinyo Lengai in Tanzania, East Africa, which is a classical dome-shaped volcano of 2900m high and in the 1960s was discovered as a source of carbonatite. A further eruption was witnessed in 1988 and this is the only place where erupting volcanic carbonate material can be seen. The fresh larva is black and about 5500C when it emerges but on weathering becomes white, a process which begins within 24 hours. Most carbonatic volcanism is explosive and most are at >10000 C, therefore Oldoinyo Lengai is atypical.

Maar Lake in Italy is an extinct volcanic crater and shows evidence of a pyroclastic wave in the carbonatic ash. The south of Italy is a subduction zone where the African plate goes under Italy, but carbonatites should not be found in subduction zones and are normally associated intra-continentally especially in rift environments. However, carbonatites were discovered in the Vulture region of Italy in 1990 in a mantle xenolith 50km under Vulture.

Carbonatite and silicate will not mix, and may never be mixed, therefore how do carbonatites get up from the mantle? Calatrava is a volcanic area of central Spain with quartzite hills around the edge of depressions but no lakes. These are explosive craters and there are carbonatite deposits. Sources of CO2 coming up from the mantle may indicate other sources of carbonatites.

As with all talks by Frances Wall, this was an extremely interesting, well-presented lecture. It benefited from the use of high quality slides and a video filmed on the side of Oldoinyo Lengai. We also had a demonstration of immiscibility using oil and water to explain carbonatite and silicate chemistry, and Frances brought samples of preserved carbonatite for us to inspect.

Oldoinyo Lengai
Oldoinyo Lengai, Tanzania is the only volcano ever seen to erupt molten carbonate (carbonatite)
Fresh and weathered lavas
O. Lengai lavas are different to all other ‘normal’ lavas because they are much less viscous and lower temperature (about 550°C). The carbonatite lava flows are very thin. The lava starts to alter as soon as it comes into contact with the air. Fresh lava is black but within a few days it starts to turn white.
Explosive eruption

In contrast to Oldoinyo Lengai it is now known that most carbonatite volcanism is explosive Eruptions were similar to kimberlites. No one has ever seen one
No one has yet found an equivalent of Oldoinyo Lengai in the geological record

(Impression of kimberlite eruption by Nick Russell, from the Diamonds exhibition at the Natural History Museum )

Calcite lapilli
Calcite carbonatite lapilli, Kaiserstuhl, Germany
Surge wave deposits
Explosive surge wave deposits, including a bomb, at Vulture, Italy

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