Jessai Flores, Illustrations Editor

Oceanic island hotspots — Hawaii and Iceland — are the final destinations of elemental isotopes from the beginnings of Earth, in concentrations found nowhere else on Earth’s crust. 

Until now, it was an open question of how these concentrations were achieved, but Yale scientists have quantitatively modeled the journey of elemental isotopes from the Earth’s core, up to its mantle and finally to its crust, accurately predicting the geochemistry of the islands. On Jan. 17, the Yale team of Jun Korenaga, professor of earth and planetary sciences, and doctoral student Amy Ferrick published a paper in The Proceedings of the National Academy of Sciences, which is the first paper to quantitatively illustrate how both tungsten and helium isotopes can travel from the core to the crust of the earth via diffusion, forming oceanic island basalt.

“We showed that the core-mantle diffusion quantitatively transports tungsten and helium isotopes to almost exactly match the geochemistry of the hotspot islands,” Korenaga told the News.

In rocks, some elemental isotopes, including helium-3 and tungsten-184 are archaic signatures, only formed through stars and fission. Their counterparts, helium-4 and tungsten-182, are instead formed through radioactive decay and are found in much greater quantities on the planet’s surface. Oceanic island basalt, found in Hawaii and Iceland, has significantly more primordial isotopes in its crust than in other parts of the surface. 

It was previously theorized that the tungsten and helium isotope signatures were achieved by slow convection within the mantle, allowing for helium-3 and tungsten-184 to congregate near the bottom and emerge from the depths via large magma plumes. However, according to Korenaga this does not accurately represent the convection of the mantle, and requires numerous factors to be aligned in order to achieve the proper ratio of isotopes. 

The research findings outline a diffusion process from the core to the mantle that occurs at the necessary rate for the concentration of archaic signatures observed in oceanic islands to be achieved. Furthermore, this is the only theory that provides a single solution to how high levels of both primordial tungsten and primordial helium are found.

“I modeled the diffusion process at the core-mantle boundary, tracking the isotopic ratios of tungsten and helium over Earth’s entire history of 4 billion years,” Ferrick said in describing her research process 

Through her computer model, she determined the isotopic composition of the deep-mantle reservoir from Earth’s formation to the present day. 

Then, Ferrick modeled a magma plume, which is when a hot spot rises through the entirety of the mantle to the crust above. The plume, formed of background mantle, pulls along magma from the deep-reservoir. This results in the plume having a much greater concentration of primordial isotopes than the average magma of the mantle.

The composition of rocks on Hawaii and Iceland matched her model’s predictions for the isotopic ratios of both helium and tungsten in the magma plume.

Ferrick began the model for a class project, but after discovering the remarkable result — that all ocean island primordial isotope signatures could be explained through diffusion — she continued her research with Korenga. 

“It’s particularly exciting that this publication developed from a project that Amy carried out early in her Ph.D. program,” wrote Maureen Long, chair of earth and planetary sciences, in an email to the News. “We encourage our graduate students to dive into cutting-edge research from Day 1 in our program — one of the thrilling things about doing a Ph.D. is the creation of new knowledge, and our students do that right off the bat. I love seeing the amazing science that our grad students produce here in EPS.”

Amy Ferrick presented her research at the 2022 American Geophysical Union conference.