The ‘Kimberlite Bloom’ During Pangea’s End: Caused by Mantle Cooling Or Deep Carbon Effects?

People have many uses for diamonds, ranging from engagement rings to industrial saws, and all of those uses were made possible by ancient eruptions of melted kimberlite. This carbon-rich magma forms more than 200 kilometers deep in the mantle and blasts through Earth’s continental lithosphere, including the continental crust, carrying along diamonds and other deep minerals. Nowadays, these eruptions are rare, but between 250 and 50 million years ago, Earth experienced a “kimberlite bloom,” which brought diamonds to the surface in South Africa and many other parts of the world that sit on top of ancient and thick lithosphere.

To understand the conditions that created this bloom, DCO members Sebastian Tappe, Malcolm Massuyeau (both at the University of Johannesburg, South Africa), and Katie Smart (University of the Witwatersrand, South Africa), combined and analyzed several existing geoscientific data sets to explain how these kimberlite eruptions evolved. In a new paper in Earth and Planetary Science Letters [1], the researchers propose that after 2 billion years ago, a cooling Earth created just the right conditions for kimberlites to become more prominent within the mantle.

Contrary to previous models, the researchers from South Africa suggest that in the last 2 billion years, carbon-bearing kimberlite melts were always present in the mantle beneath thick, old continents, but that the assembly and breakup of the supercontinents helped kimberlites drain to the surface. When Pangaea, the youngest supercontinent, began to break apart, moving tectonic plates created weak spots in the lithosphere that allowed kimberlite magmas to erupt at the surface, resulting in the bloom.

To perform this analysis, Tappe and colleagues compiled several databases containing geologic information for more than 1,000 kimberlite clusters including locations, eruption ages, and volcanic crater sizes. The researchers also compared the kimberlite locations with reconstructions of Earth’s tectonic plate configurations for the last 300 million years, which covers the life cycle of Pangaea.

The first known kimberlite eruptions occurred 2.85 billion years ago, but eruptions were highly unusual until about 1.2 billion years ago. Some geologists have argued that older eruptions have simply disappeared from the geologic record due to erosion. “When you look at the age distributions for these sorts of rocks, there’s a lot of stuff happening in the second half of Earth’s history and not much at the beginning,” said Tappe. “People tend to conveniently brush this off as a preservation issue – the old stuff is eroded away and we don’t see it anymore.”

Tappe’s research, however, points to a different main cause for this skewed age distribution. During the first 2 billion years of Earth’s history, the mantle may have been too hot to make kimberlite magma. At that time, the mantle likely reached about 1600° Celsius and frequently produced eruptions of komatiite, a magnesium-rich silicate lava that requires higher melting temperatures. Around 2 billion years ago, Earth’s mantle likely cooled to below 1400° Celsius, the temperature around which the mantle beneath thick continental plates begins to melt and produces carbonate-bearing silicate melts such as kimberlites.

The researchers also considered the sizes of the kimberlite deposits, called emplacements, to see if older ones had shrunk over time due to erosion. They saw no connection between the ages of emplacement and the sizes of the kimberlite volcanic bodies, suggesting that poor preservation has not erased older eruptions from the geologic record as had been surmised.

In the latter half of Earth’s history, several spikes of kimberlite activity occurred, which corresponded to the demise of various supercontinents. “When Earth cooled, conditions were ripe for these melts, but they could only come to the surface when the plates started to move faster. And that typically happens when supercontinents break apart,” said Tappe. About 60% of the known kimberlite clusters occurred between 250 and 50 million years ago, a tiny fraction of Earth’s history, which coincides with Pangaea’s breakup.

These findings may have important implications for the deep carbon cycle. Previously, some researchers have interpreted the high carbon content of kimberlites to be evidence that the mantle has been richer in carbon since about 1.2 billion years ago, when modern plate tectonics began efficient recycling of surface carbon deep into the mantle. But the new findings suggest that the global evolution of kimberlite eruptions is more closely related to mantle temperature than carbon content. “Just because these rocks have a lot of carbon doesn’t mean that the mantle needs a lot of carbon to produce them,” said Tappe.

Tappe hopes that the newly compiled database will be useful for future studies, for example, to ground truth geophysical models of how and at what rate Earth cooled.

In modern times, kimberlite eruptions rarely happen because Earth’s tectonic plates are moving so slowly. The “youngest” kimberlites erupted about 30 million years ago in relation to active continental rifting in eastern Africa. As Earth continues to cool, kimberlite eruptions may cease entirely. “I think we’ll probably still be fine for the next couple hundred million years,” said Tappe, “but at some point it will just be too cold to make kimberlites.”

 

More than 60% of global kimberlite magma eruptions occurred during the “kimberlite bloom” that occurred on every continent between 250-50 million years ago. Credit: Sebastian Tappe, adapted from Tappe et al. 2018.
 

Kimberlite magma eruptions, depicted here as red dots, peaked at around 90 million years ago, in association with the Pangea supercontinent breakup. Rare ‘young’ kimberlites that erupted around 30 million years ago in eastern Africa show a clear link to continental rifts. Credit: Sebastian Tappe, adapted from Tappe et al. 2018.

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