Amazing discovery of rocks from Earth's mantle helps explain life on Earth

Scientists have extracted a massive amount of rock from the Earth's mantle, the layer beneath the crust. A unique discovery that helps us learn more about the mantle's role in the origins of life on Earth.

It will also teach us more about the volcanic activity that occurs when the Earth's mantle melts and how this drives global cycles of important elements like carbon and hydrogen.

This is a nearly continuous piece of rock 1,268 metres long. It was extracted from the Earth's mantle from what is called a tectonic window, a region on the seafloor where rocks rise from the mantle to the surface along the Mid-Atlantic Ridge, a mountain range that is largely submerged in the Atlantic Ocean.

Researchers from twenty countries
Scientists have been trying to find this rock since the 1960s. The fact that they finally succeeded is a major achievement. An international marine research team from more than 20 countries worked on it, extracting cylindrical samples of sedimentary rock from the ocean floor.

Lead researcher Professor Johan Lissenberg from Cardiff University “When we got the rock back last year, it was a major achievement in the history of Earth science, but its real value lies in what mantle cores can tell us about the formation and evolution of our planet,” he explains. “We examine the minerals in the rock as well as its chemical composition.

less pyroxene
The researchers immediately saw something they hadn’t expected. “There’s much less pyroxene in the rocks and much higher concentrations of magnesium in the rocks than we thought. Both are the result of more molten material than we expected. This melting happened when the mantle rose to the surface from deeper parts of the Earth.”

in saintias.nl “Overall, the amount of orthopyroxene is lower than expected, and its sister mineral clinopyroxene is almost absent,” Lissenberg explains in more detail. “We think this is because the part of the mantle we drilled into has melted to a greater extent than we expected. The interesting question is when this melting occurred: it may have happened recently in the last few million years, but some melting may have also occurred very early in Earth’s history. If so, that could explain why the melting is stronger than we expected. We can now test this with laboratory measurements of the samples.”

Volcano feeding
The researchers claim that further research into this process could have major implications for understanding how magma forms and causes volcanoes. “We also found channels through which melt was transported through the mantle. This way we can follow the path of magma after it forms and rises to the Earth’s surface,” says the professor.

“This is important because it tells us how the mantle melts and fuels volcanoes, especially those at the bottom of the oceans, which are responsible for the majority of volcanoes on Earth. The rocks found allow us to make the connection between the volcanoes and the ultimate source of the magma.

Olivine and sea water
It also sheds light on how olivine, a common mineral in mantle rocks, interacts with seawater, triggering a series of chemical reactions that produce hydrogen and other molecules that could lead to life. Scientists believe this was one of the key processes in the origin of life on Earth. “One hypothesis about how life began is that biochemical pathways began by mimicking geochemical pathways. In other words, the series of chemical reactions that cells undergo to obtain energy (their metabolic pathways) or transmit information (proteins) were the same series of chemical reactions that occurred without any biology, just through a mixture of rocks, water, and salts. Warmth,” Lissenberg explains.

“When olivine and other minerals in mantle rocks are exposed to water and heat, they transform into other minerals and hydrogen is formed in the process,” he continues. “Hydrogen is a great source of energy that can react with carbon dioxide to form small organic molecules without the intervention of biology. Scientists have previously observed a wide variety of small organic molecules in many different similar environments where these rocks interact with water, and have suggested that some of them form without biology.”

oldest life forms
“Theoretically, some of these organic molecules, formed from rocks alone plus water plus heat, could be the same set of organic molecules needed for biochemical processes, such as acetate, or for information transfer, such as amino acids,” concludes the professor. “Identifying the set of organic molecules that are made in this type of natural system without the intervention of biology is a goal for future research. Because we were able to dig so deeply, we were able to reach beyond the surface layers where biology plays a role. This gives us the opportunity to look for chemical signals where the biological influence is low or absent.”

“The rocks on early Earth are more similar to those we discovered on this mission than the more common rocks that make up our continents today,” summarizes researcher Susan Q. Lang. “By analyzing them, we gain crucial insights into the chemical and physical environment of early Earth that provided a steady source of fuel and favorable conditions over geologically long timescales to host the earliest forms of life.”

volcanoes in hawaii
The international team of more than thirty scientists from the JOIDESsolution mission is therefore continuing its research on the recovered drill cores to address a wide range of issues. Think about the chemical exchange between mantle rocks and seawater, how these interactions can support microbes, and the limits and extent of life in the ocean interior. “In Cardiff we are analysing the abundance of trace elements and isotopes in core samples to determine the composition of the mantle and relate this to the history of melting. The mantle has different compositions, some of which give rise to large volcanoes such as in Hawaii or the Canary Islands, and others to long volcanic chains on the ocean floor.

“One of the most important questions is how many different compositions exist and at what scale. We have necessarily used volcanic rocks as a proxy, but now we can look directly into the mantle and check it at different scales,” the researchers said.

The whole puzzle
The Earth scientists speak in superlatives about their findings. Lissenberg: “The fact that we found the rocks is actually amazing. We only planned to drill a 200-meter-deep hole, based on previous drilling experience in mantle rocks: those wells are usually very unstable and the actual recovery of the core is low. That’s why we haven’t been able to find a long section of mantle rocks before. As a result, we’ve been largely limited to samples fished out from the seafloor. We’ve learned a lot from these recovered monsters, but the downside is that they don’t provide spatial context and continuity, so they’re just individual pieces of the puzzle but we have no idea how to put them together. Our long, largely continuous section allows us to see the mantle in context, and therefore the whole picture.”

There is still a lot to discover.
But it’s not just the discovery itself, the first results are also impressive. “So far I would say the most exceptional detailed documentation of the mineral composition of the mantle. This is the first time we’ve been able to look at the spatial diversity of mantle minerals, and the variations we found were much larger than we expected. The mineral orthopyroxene in particular was present in varying amounts, from centimetres to hundreds of metres,” he concludes. “We associate the variations in orthopyroxene with the flow of magma through the upper mantle: as the upper mantle rises, it melts, and this molten material then rises to the surface to fuel volcanoes.”

Researchers are still at the beginning of their search for all the knowledge and ideas hidden in the rock, but the first results are already promising.