Secrets buried within ancient crystal reveal origin of African continent

26 Mar 2018

Image: Willyam Bradberry/Shutterstock

The violent birth of the nucleus of the African continent has been revealed, thanks to new research into an ancient and well-preserved crystal.

Our understanding of tectonic movement and a quick look at a map of the globe shows us that our continents can never be called truly stable, and now new research reveals the violent origin of the continent of Africa.

An international team led by Trinity College Dublin (TCD) professor Emma Tomlinson has unearthed evidence that suggests the major craton – which makes up the nucleus of the continent of Africa – was formed billions of years ago by an ascending plume of extremely hot rock from the earth’s core-mantle boundary.

All of the seven continents have one of these craton points of origin, and their growth approximately 3bn years ago was critical to the emergence of land masses on Earth.

Until now, little was known about how these cratons and their supporting mantle keels formed. However, clues can be found in samples of mantle brought to Earth’s surface – known as peridotite xenoliths – in magma.

So, it proved to be a remarkable discovery when the TCD-led team revealed in the journal Geology that it had discovered an exceptionally well-preserved crystal from the Kaapvaal craton in southern Africa.

This crystal supports the idea that Earth’s cratons were formed by mantle plumes billions of years ago. When formed, it would have been one single mineral, which subsequently unmixed into garnet and orthopyroxene.

This unmixing process occurs when the crystal experiences a significant change in its surroundings, typically through intense cooling or a major increase in pressure.

Birth from a mantle plume

Most importantly to the craton origin, the chemical composition of the garnet and orthopyroxene in this large crystal is identical to the garnet and orthopyroxene in the host peridotite xenolith.

This connection between the three implies that garnet and orthopyroxene in normal cratonic peridotite also formed by the unmixing of a single parental mineral.

“The chemical composition of the parent mineral suggests that it was formed at high temperature of about 1,750C, and pressure of about six gigapascals, which equates to what you would find at 180km depth,” Tomlinson said.

“This implies the crystal – and, by extension, the peridotitic mantle below the Kaapvaal craton – formed at very high temperature and significant depth, which supports models that suggest the cratonic mantle formed from a mantle plume. The cratonic mantle then cooled to around 950C, which caused the garnet and orthopyoxene to unmix.”

With this new knowledge, researchers can focus on the origin and condition of formation of the parent mineral, using samples from the Kaapvaal and also the North Atlantic craton.

Colm Gorey was a senior journalist with Silicon Republic