Witnesses to Earth’s early history

Determining the composition of rock in the deepest layer of the Earth’s mantle is impossible to do directly. But thanks to isotope measurements of volcanic rocks, ETH researchers are now able to show that the mantle is still home to material from the planet’s earliest days.

Microscopic image of a kimberlite
Microscopic image of a kimberlite from Somerset Island (Canada). Like other kimberlites, this rock originates from the deepest layers of the Earth’s mantle. (Image: A. Giuliani / ETH Zurich)

What exactly are the deepest parts of the Earth made of? Geoscientists apply highly sophisticated techniques in pursuit of this question. Seismic waves, for example, help them map the structures in the Earth’s interior. The scientists can then draw conclusions regarding the composition of these structures and then make hypotheses about their formation.

In addition, some locations on the Earth’s surface have “witnesses” that provide direct information about the lower mantle. The seismic data can reveal, say, that lava from certain volcanic islands such as Hawaii must originate at a tremendous depth. Over a long period of time, the hot material raised up from the deepest regions of the mantle, close to the core, until it reached the surface of the Earth.

More help from a combination of data

Chemical analysis of these volcanic rocks allows for conclusions about the composition of the very bottom of Earth’s mantle. But there’s just one thing: the oldest of these volcanic islands are just about 150 million years old, which is quite young in the Earth history. Using these rocks alone to trace the 4.5 billion years of mantle evolution does not go very far.

But fortunately another source sheds light on this subject. Most kimberlites also originate in the bottommost layer of the mantle and they are much older than volcanic islands in the oceans. The oldest kimberlites are more than 2 billion years old, found primarily in the ancient parts of the continents, called cratons.

Very long half-lives

Andrea Giuliani, Swiss National Science Foundation Ambizione Fellow, and postdoctoral scholar Angus Fitzpayne, both in the Department of Earth Sciences at ETH Zurich, joined with colleagues from the US and Australia in collecting data on kimberlites, adding their own measurements to the mix. They focused on the radiogenic isotopes of three elements: strontium, neodymium and hafnium. These make it possible to reconstruct the composition of the source material of these rocks throughout the Earth history, since their radiogenic isotopes have very long half-lives.

With these data, Giuliani and coworkers demonstrate that kimberlites and oceanic islands share the same source material. Now they have an answer to a question that geologists have been arguing about for a long, long time, what is the origin of this deep source? Some geologists maintain that this must be ancient material from the very beginnings of the Earth; others believe that it was formed later by upheaval in the mantle.

Similarity to chondritic meteorites

From the kimberlite data, Giuliani and colleagues have extrapolated what the isotopic composition of the tested rocks must have been 4.5 billion years ago. Their work has led them to the realisation that the rocks’ source material must be similar in composition to the chondritic meteorites that formed the Earth. In other words, the deepest layers of the mantle contain material effectively unchanged since the beginnings of the Earth’s history.

Giuliani acknowledges that their results, recently published in the journal PNAS, are still somewhat speculative. “The oldest rocks we looked at are about 2 billion years old,” he explains. “That means our extrapolations back in time are still a bit uncertain.” As a next step, Giuliani hopes to incorporate other, older kimberlites as well as other rock types into the analysis to further refine our understanding of the mantle evolution.

Reference

Giuliani A et.al.: Remnants of early Earth differentiation in the deepest mantle-derived lavas. PNAS January 5, 2021. doi: 10.1073/pnas.2015211118

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