A sample of rock from Earth’s mantle viewed under a microscope
Johan Lissenberg
In the middle of the North Atlantic Ocean, geologists have burrowed 1268 metres below the seafloor – the deepest hole drilled into Earth’s mantle yet. Analysis of the resulting rock core offers fresh clues about the evolution of our planet’s outermost layers, and perhaps even the origins of life.
Earth is broadly made up of a few different layers, including a solid outer crust, an upper and lower mantle and a core. The upper mantle, which sits just below the crust, is composed primarily of a magnesium-rich rock called peridotite. This layer drives key planetary processes such as earthquakes, the water cycle and the formation of volcanoes and mountains.
“To date, we’ve only had access to fragments of the mantle,” says Johan Lissenberg at Cardiff University, UK. “But there are a number of places where the mantle is exposed on the seafloor.”
One of these areas is an underwater mountain located called Atlantis Massif, located near a volcanically active region of the mid-Atlantic ridge. Continuously surfacing and melting parts of the mantle give rise to many of the volcanoes in the area. Meanwhile, as seawater seeps deeper into the mantle, the hotter temperatures heat it up and produce chemical compounds such as methane, which bubble back up through hydrothermal vents and provide fuel for microbial life.
“There’s a kind of chemical kitchen in the subsurface of Atlantis Massif,” says Lissenberg.
To learn more about this dynamic region, he and his colleagues initially planned to bore 200 metres into the mantle with the drilling ship JOIDES Resolution, deeper than researchers had ever managed so far.
“Then we started drilling and things went amazingly well,” says team member Andrew McCaig at the University of Leeds, UK. “We recovered really long sections of continuous rocks and decided to stick with it and go as deep as we could.”
Eventually, the team managed to dig 1268 metres down into the mantle.
Upon analysing the drill core sample, the researchers found that it had much lower levels of a mineral called pyroxene compared with other mantle samples collected from around the world. That suggests this particular section of the mantle has undergone significant melting in the past, which has depleted the pyroxene, says Lissenberg.
In the future, he hopes to reconstruct this melting process, which could help us understand how the mantle melts and how that molten rock migrates to the surface to feed oceanic volcanoes.
Some scientists think life on Earth began in the depths of the ocean near hydrothermal vents. So, by examining the chemicals that appear along the cylindrical rock core, microbiologists are hoping to determine the conditions that may have led to life and how deep beneath the ocean floor they occurred.
“It’s a very important drill hole because it’s going to be a reference section for scientists from many branches of science,” says McCaig.
“A one-dimensional sample of the Earth cannot provide full information on the three-dimensional migration pathways of melt and water, but is nevertheless a major achievement,” says John Wheeler at the University of Liverpool, UK.
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