Episode 397 Carbonatites

Carbonatites are strange igneous rocks made up mostly of carbonates – common minerals like calcite, calcium carbonate. Igneous rocks that solidify from molten magma usually are high-temperature rocks containing lots of silicon which results in lots of quartz, feldspars, micas, and ferro-magnesian minerals in rocks like granite and basalt. Carbonatites crystallize from essentially molten calcite, and that’s really unusual.

Most carbonatites are intrusive, meaning they solidified within the earth, and it wasn’t until 1960 that the first carbonatite volcano erupted in historic times, proving that they form from cooling magma. The eruption at Ol Doinyo Lengai in Tanzania occurred on a branch of the East African Rift System, and most carbonatites are associated with these breaks in continental crust where eventually a new ocean may form.

Mt Lengai, Tanzania, photo by Clem23 (Creative Commons License - source)

Eruptions at Lengai, whose name means “mountain of god” in the Maasai language, are the lowest-temperature magmas known because calcite melts at a much lower temperature than silica-rich compounds, around 510 degrees C versus 1000 degrees or more for most magmas. It isn’t even red-hot like most lava flows.

A simple and early interpretation of carbonatites was that they represented melting of limestone, but geochemical data indicate that they really do come from primary igneous material that probably originated in the mantle. Exactly how they form is debated, in part because they are so rare, but one idea is that they result from special cases of differentiation within more common magmas, or maybe an example of certain chemicals – the carbonates – separating out in an unusual way.

Another unusual aspect of carbonatites is the minerals associated with the dominant calcite. It’s common to get rare-earth compounds, tantalum, thorium, titanium, and many other minerals that are unusual in high concentrations in other settings. The Mountain Pass rare-earth deposit in California, once the largest producer of rare earths in the world, is in a Precambrian carbonatite. Rare earths are used in lots of modern technologies, including turbines for wind energy, batteries in electric car motors, cell phones, solar cells, and eyeglasses.

Rare earths are also produced from the Mt. Weld carbonatite in Western Australia, but it’s more famous for its tantalum, an element that’s vital in capacitors