Diamonds that formed deep in the Earth’s mantle contain evidence of deep earth recycling processes

This cartoon shows a subducting oceanic plate moving like a conveyor belt from the surface to the lower mantle. The white arrows show the relatively well established shallow recycle pathway in the upper layer of the plate (crust and sediment), which feeds the arc volcanoes. New findings from the diamond study research team reveal a deeper recycling path, shown in light blue. Fractures infiltrating water into the sea floor hydrate the rocks inside the plate, forming “serpentinite”, and these hydrated rocks can sometimes be carried up the lower mantle. It is a major pathway that transfers water, carbon and other surface elements deep into the mantle. Credit: Illustration by Wenjia Fan, W. Design Studio

The results allow us to trace how minerals from the surface are sucked into the mantle.

Diamonds that formed deep in the Earth’s mantle contain evidence of chemical reactions that took place on the seabed. Exploring these gems can help geoscientists understand how matter is exchanged between the planet’s surface and its depths.

New work published in Scientific progress confirms that serpentinite – a rock that forms from peridotite, the main type of rock in the Earth’s mantle, when water enters cracks in the ocean floor – can carry surface water up to 700 kilometers deep by plate tectonic processes.

“Almost all of the tectonic plates that make up the seabed end up bending and sliding into the mantle – a process called subduction, which has the potential to recycle surface materials, such as water, back into the Earth,” explained Peng Ni of Carnegie, who co-led the research effort with Evan Smith of the Gemological Institute of America.

An illustration showing how diamonds can provide researchers with insight into the processes that take place inside our planet, including the recycling of surface materials in deep lands. Credit: Illustration by Katherine Cain, courtesy of the Carnegie Institution for Science

Serpentinite residing inside sub-conduction plates may be one of the most important, but lesser-known, geochemical pathways through which surface materials are captured and transported to the depths of the Earth. The presence of deeply subducted serpentinites was previously suspected – due to Carnegie and GIA research into the origin of blue diamonds and the chemical makeup of the erupting mantle material that makes up mid-ocean ridges, seamounts and oceanic islands. But the evidence demonstrating this path has not been fully confirmed until now.

The research team – which also included Steven Shirey of Carnegie and Anat Shahar, as well as Wuyi Wang and Stephen Richardson of the University of Cape Town’s GIA – found physical evidence to confirm this suspicion by studying a type of large diamonds that come from deep within the planet.

“Some of the most famous diamonds in the world fall into this special category of relatively large and pure cut diamonds, like the world famous Cullinan,” Smith said. “They form between 360 and 750 kilometers deep, at least as deep as the transition zone between the upper and lower mantle.”

Examples of CLIPPIR rough diamonds from the Letseng mine, Lesotho. These are the same types of diamonds as those analyzed in this study. The largest stone measures 91.07 carats. Credit: photo by Robert Weldon; © GIA; courtesy of Gem Diamonds Ltd.

Sometimes they contain inclusions of tiny minerals trapped during diamond crystallization that provide a glimpse of what is happening at these extreme depths.

“Studying small samples of minerals formed during deep crystallization of diamond can tell us a lot about the composition and dynamics of the mantle, as the diamond protects the minerals from further changes on their way to the surface,” explained Shirey.

In this case, the researchers were able to analyze the isotopic composition of iron in the metal inclusions. Like other elements, iron can have different numbers of neutrons in its nucleus, resulting in iron atoms of slightly different mass or different “isotopes” of iron. Measuring the ratios of “heavy” and “light” iron isotopes gives scientists a kind of fingerprint of iron.

The diamond inclusions studied by the team exhibited a higher heavy to light iron isotope ratio than that typically found in most mantle minerals. This indicates that they probably did not originate from deep Earth geochemical processes. Instead, it points to magnetite and other iron-rich minerals formed when peridotite in oceanic plates turned into serpentinite on the seabed. This hydrated rock was eventually submerged hundreds of kilometers into the mantle transition zone, where these particular diamonds crystallized.

“Our results confirm a long-suspected route for deep earth recycling, allowing us to trace how surface minerals are sucked into the mantle and create variability in its composition,” Shahar concluded.

Reference: March 31, 2021, Scientific progress.