Rocks that originated in Earth’s mantle could reveal secrets of planet’s history

Scientists have recovered the longest and most complete core of mantle rocks ever sampled from the ocean floor.

The mantle forms a 2,900km layer beneath the Earth’s crust which is not normally accessible from the surface. To counter that, the nearly continuous 1,268-metre core was drilled at a “tectonic window,” a section of the seabed where rocks from the deep mantle are uplifted and exposed along the Mid-Atlantic Ridge.

It is hoped these samples from deep beneath the Atlantic Ocean will help unravel the mantle’s role in the origins of life on Earth, the volcanic formation of Earth’s crust, and its contribution to global cycles of chemicals including carbon and hydrogen.

The samples were collected by an international team of scientists in the Spring of 2023, during Expedition 399 (Building Blocks of Life, Atlantis Massif) of the International Ocean Discovery Program. The international marine research consortium of more than 20 countries retrieves cores – cylindrical samples of sediment and rock – from the ocean floor.

Since the conclusion of the expedition aboard the ocean drilling vessel JOIDES Resolution, the team of more than 30 scientists – including Dr Andrew Parsons, from the University of Plymouth – has been compiling an inventory of the recovered mantle rocks to understand their composition, structure and context.

Their initial findings, presented in the journal Science, reveal that the rocks’ record of mantle melting, which provides the source of volcanism at the seafloor, is more extensive than expected.

Dr Parsons, a Postdoctoral Research Fellow in Marine Geoscience and recently appointed Lecturer, joined the nine-week expedition as part of his ongoing work to investigate the processes that facilitate and control the tectonic evolution of Earth.

This melting occurred as the mantle rose from the deeper parts of the Earth towards the surface. Results from further analysis of this process could have major implications for the understanding of how magma is formed and leads to volcanism, the researchers claim.

The study also provides initial results on how olivine, an abundant mineral in mantle rocks, reacts with seawater, leading to a series of chemical reactions that produce hydrogen and other molecules that can fuel life. Scientists believe this might have been one of the underpinning processes in the origins of life on Earth.

Dr Susan Q Lang, an associate scientist in Geology and Geophysics at the Woods Hole Oceanographic Institution, was a co-chief scientist on the expedition. She said:

“The rocks that were present on early Earth bear a closer resemblance to those we retrieved during this expedition than the more common rocks that make up our continents today. Analysing them gives us a critical view into the chemical and physical environments that would have been present early in Earth’s history, and that could have provided a consistent source of fuel and favourable conditions over geologically long timeframes to have hosted the earliest forms of life.”

Joining a research expedition to the middle of the Atlantic Ocean

When IODP Expedition 399 set off from the Azores in early 2023, it marked the start of Dr Andy Parsons’ first research cruise. Having completed his PhD looking into how deformation deep beneath the Earth’s surface controlled the formation of mountain belts such as the Himalaya, he has since focused increasingly on its impact within the world’s ocean.

His ongoing role in the project team is to understand the deformation processes which brought the deep mantle rocks up to the seafloor of the North Atlantic Ocean. To do that, this cruise focused on an underwater mountain – the Atlantis Massif – which rises around 4,000m from the seafloor, and was created by the shifting of tectonic plates underneath the ocean floor.

After three days of sailing to the site, drilling 1,200 metres into the mountain took between four and five weeks. Then, as samples were brought onto the ship, Andy spent hours at a time, along with other members of the team, examining and cataloguing parts of the core on the JOIDES resolution vessel. The results of that analysis formed the basis of the new paper in Science.