Superdeep Diamonds Hint at Primordial Helium Reservoir

Results from the first ever analysis of helium isotopes from fluids trapped in superdeep diamonds support the existence of a reservoir of ancient helium that has existed in Earth’s interior since its formation.

Microscopic pockets of fluid trapped in diamonds from deep below Earth’s surface are providing new insight into the makeup of the rock deep beneath our feet. These fluids provide evidence for a reservoir of helium deep in the mantle, leftover from Earth’s earliest days.

DCO Reservoirs and Fluxes Community members Suzette Timmerman (formerly at Australian National University, now at University of Alberta, Canada), Graham Pearson (University of Alberta, Canada), and an international team of colleagues analyzed the content of the fluid pockets preserved by superdeep diamonds during the trip to the surface. The teams reports the findings in a new paper [1] in Science.

“For the first time ever, we’ve been able to examine helium isotopes trapped in fluid inclusions in superdeep diamonds,” said Timmerman. “These isotopes can tell us new information about the structure of the Earth, advancing our understanding of the evolution of our planet.” Timmerman is a DCO early career scientist and participated in the second DCO Summer School, held July 2016, at Yellowstone National Park, USA.

Juina superdeep diamonds
Many of these diamonds from the Juína area are superdeep diamonds. Credit: Graham Pearson

Helium isotopes, which are atoms of helium with a different number of neutrons in the nucleus, are an important tool in determining the chemical structure of our planet, and in tracking the movement and potential long-term isolation of mantle material. As Timmerman explains, at the moment, our view of deep Earth is almost solely based on data from basalt magma from the mantle that makes its way to the surface. These rocks don’t tell the full story, as they undergo significant modification on their geological journey.

“Superdeep diamonds are very special,” said Timmerman. “They’re our only direct window into the deeper parts of Earth.”

Timmerman conducted the isotope analysis portion of the research at the Australian National University, using superdeep diamonds collected from the town of Juína in Brazil – the same location that produced the discovery of hydrous ringwoodite in Earth’s mantle. The researchers sampled material from the diamonds through a tool that performs laser ablation, using lasers to carve tiny pieces off these incredibly durable diamonds for study.

“The trace element analysis of our research was performed using this unique setup at the University of Alberta,” explained Timmerman. “This is the only way we can get enough material to measure, and even those amounts are still incredibly small, including just a few trillionths of a gram of lead, for example.”

Suzette Timmerman
Timmerman examines a diamond by eye. Credit: Gareth Davies

One result from these analyses confirms the existence of a theorized ancient “primordial” reservoir of rock, one that has been largely isolated from the rest of Earth since our planet’s mantle formed almost 4.5 billion years ago.

“Previous studies have argued about whether such a reservoir exists and whether it would be found in the lower mantle or upper mantle,” said Timmerman. “Our superdeep diamonds confirm this reservoir exists and narrows down that it must be located somewhere below a depth of 410 kilometers in a region known as the transition zone or lower mantle, giving us a clearer picture of the structure of Earth.”

Interestingly, even though the researchers see evidence of deep, primordial helium, the carbon isotopes from the fluids suggest that much of the carbon derives from carbonates in the ocean crust formed from living matter, which were recycled into the mantle.

Timmerman will continue her research as a Banting Fellow at the University of Alberta, focusing her studies on superdeep diamonds by exploring the age of these unique rocks and what they can tell us about our planet’s carbon cycle.

Article partially adapted from materials provided by the University of Alberta.

Main image: Juína diamonds viewed through an electron microscope. Credit: Courtesy of Suzette Timmerman

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