If you had to pick the most unique volcano on Earth, you'd be hard pressed to find a better candidate than Tanzania's Ol Doinyo Lengai. Not only does it look like a volcano designed by HR Giger (below), but it is the only place on the planet that is currently erupting carbonatite lava, some of the strangest stuff you will ever see (see the excellent video above). These lavas are like no other lava, chock full of calcium, sodium and carbon dioxide, leading to some of the odd properties of these eruptions. However, the ultimate source of these carbonatite lavas is still hotly debated – and to make it more complicated, Ol Doinyo Lengai doesn't even erupt the usual carbonatite (if you can call any carbonatite "usual") lava. Not only that, but carbonatites might be a good source for mining rare earth elements, so understanding how they form is going to become increasingly important.

Carbonatites are magma that are full of alkali elements – calcium, sodium, sometimes potassium – along with abundant carbon dioxide. Why is that odd? Most terrestrial magma is silicate, that is to say that much of the magma is made from bonded chains of silicon and oxygen. Even what we'd call "low silica" magma like basalt has 45 weight percent silica (SiO 2 ) and "high silica" magma like rhyolite can be over 70 weight percent silica. Now, these carbonatite magma (made mainly of CaCO 3 – calcium carbonate) are so saturated in alkali elements that they only have a few to less than a quarter weight percent silica! Instead, the bulk of the mass of the magma is composed on mainly calcium, CO 2 (and in the case of Ol Doinyo Lengai, sodium).

This has real consequences for the behavior of the magma. Those chains of silica in silicate magma are what give it some of its strength, where even the runniest basaltic lava is, in fact, quite viscous – remember the sampling a basaltic lava flow at Tolbachik to see how sticky basalt can be. However, without the chains of silica to give the magma structure, carbonatite magma can have much lower viscosity, allowing for the strange "garden hose" eruptions that exemplify activity in the crater of Oldoinyo Lengai. The lack of structure and its composition also allows for carbonatite magma to erupt at much cooler temperatures than silicate magma. Your run-of-the-mill basalt might erupt at 1100-1200ºC, but carbonatite lava erupts at ~480-590ºC. That is likely a few hundred degree cooler than even the coolest silicate magmas (rhyolite).

Carbonatite lavas even weather differently than silicate lavas. They are composed on carbonate minerals like calcite (or even strangest minerals like nyerereite and gregoryite), so when exposed to water or even humid atmosphere, they break down quickly. This gives Oldoinyo Lengai its unique coloration, where dark carbonatite lavas erupt black to grey but after cooling at weather, appear stark white (see above).

Now, if you know a bit about the behavior of volcanoes, you'd know that a low viscosity magma isn't likely to erupt explosively. That's because gas bubbles can escape from the lava without becoming trapped, which would then lead to fragmentation. So, you'd expect that Ol Doinyo Lengai only erupts as lava flows with such low viscosity lava. However, the volcano has had both explosive and effusive eruptions over the past decade – this is likely due to how much carbon dioxide can be dissolved in the magma. The more carbon dioxide (or any gas) you can pack into the magma, the more likely it is to erupt explosively, regardless of its viscosity. Ol Doinyo Lengai had an impressive explosive eruption in 2008 (see below) that produced an ash plume and throughout its recent history, carbonatite ash falls and tephra are found. In the video of the eruption (above), you can see that some of the lava looks silvery and this color betrays the large amount of bubbles even in these relatively passive eruptions.

The March 2008 explosive eruption at Ol Doinyo Lengai. Image: Cessna 206 / Flickr.

So, from where does this weird carbonatite magma arise? That is a tricky question and their are two models for the ultimate source of carbonatite magma: (1) direct from the mantle and (2) liquid separation from an alkaline silicate magma (powerpoint link). I mentioned that Ol Doinyo Lengai is the only place on the planet where carbonatite magma is currently erupting. However, it isn't the only place we can find evidence for carbonatite volcanism – there are dozens of locations around the world where such deposits are found (although over a third are in Africa, many associated with the East African Rift). There are also dozens of locations where we find carbonatite magma that solidified underground (plutonic), but determining which model can describe the occurrence of carbonatites is difficult.

The first model, where carbonatite magmas rise straight from a mantle source, is favored at places called kimberlites. These are craters formed by violent explosive eruptions that bring material up from depths of 100-200 km (lower crust and upper mantle), including diamonds! The inclusion of diamonds in kimberlites betrays a carbon-rich source in the lower crust and upper mantle. These seem to be their own class of explosive carbonatities where the extreme volatile (CO 2 ) content of the magma drives their violent eruption.

At Ol Doinyo Lengai, it seems that separation of a carbonatite liquid from alkaline silicate magma is the likely scenario. Alkaline silicate magmas (like basanite or phonolite) are enriched in the alkali elements, so there can be a situation where they become so enriched in alkali elements and carbon dioxide that they separate from the silicate magma – something called immiscibility. Think of this like mixing oil and vinegar. You can make a solution of the two, but if you let them sit, the oil and vinegar will separate, with the oil floating on top.

This is loosely what might occur when a body of alkaline silicate magma ponds in the crust and crystallizes, enriching the remaining magma in alkali elements and carbon dioxide. The fact that in some carbonatite lavas erupted at Ol Doinyo Lengai you can find blebs of silicate magma suggests that this separation might be occurring. This separation also allows for some passive degassing of the carbonatite, allowing for the abundant lava flows at the volcano. In both models, however, the carbonatite magma doesn't seem to interact with the continental crust even if it formed from ponding of alkaline silicate melts, at least based on looking at the trace element and isotopic composition of carbonatites.

Spattering natrocarbonatite lava from a vent on Ol Doinyo Lengai, seen in August 2003. Image: Tom Pfeiffer / Volcano Discovery, used by permission.

One of the most interesting things about Ol Doinyo Lengai is that it hasn't always been erupting carbonatites (or natrocarbonatites, due to the enrichment in sodium). Its earlier history was one of alkaline silicate volcanism, producing phonolite tuffs, followed by "normal" carbonatites (that is, calcium-enriched), then natrocarbonatites (Ca and Na enriched). In fact, the most recent natrocarbonatite volcanism at Ol Doinyo Lengai has only been occurring for the past few millennia. To me, this suggests that we're having continued separation of carbonatite liquids from whatever source is feeding the volcano.

Carbonatites are some of the strangest magma on Earth. Even their ultimate source isn't well understood – one of the great mysteries in petrology. What makes carbonatites especially important to understand these days is they are one of the best sources of rare earth elements (REE), a key component for many modern electronics. In fact, the only working REE mine in the United States is in a carbonatite deposit – as is the world's largest REE deposit in China. So, not only are they a geologic oddity, but they may be an increasingly valuable resource.

References

Video: Photovolcanica / Richard Roscoe, used by permission.