Science

Primordial Helium May Be Leaking from Earth’s Core


A new analysis of ancient lava flows in the Canadian Arctic suggests helium trapped in Earth’s core could be slowly “leaking” into the mantle and then reaching the surface—an idea that challenges the scientific understanding of our planet’s inner workings.

It’s the latest evidence supporting the hypothesis that primordial “reservoirs” of helium and other elements were trapped in Earth’s core when the young sun and protoplanets coalesced from a cloud of gas and dust more than 4.5 billion years ago.

The findings “suggest that somewhere in the deep portions of our planet, gases are preserved from Earth’s formation,” says the new study’s lead author Forrest Horton, a geochemist at the Woods Hole Oceanographic Institution.

Scientists can get some idea of where an atom of helium originated by looking at the number of neutrons in its nucleus—a figure that distinguishes different species, or isotopes, of the element. For example, the isotope helium 3, which has two protons and one neutron, was made in stars and during the big bang. This isotope is extremely rare on Earth.

Meanwhile helium 4, which makes up most of the gas that fills party balloons and helps cool down magnetic resonance imaging machines, has two protons and two neutrons in each nucleus. This isotope is relatively common on Earth, where it forms from the natural radioactive decay of uranium and thorium in our planet’s interior.

For the new study, which was published in Nature, Horton and his colleagues analyzed samples of 62-million-year-old lava flows in the east of Baffin Island, an Arctic island in Canada’s far north that is covered in rock, snow, and ice and inhabited by polar bears. Geologists have been studying the lavas for decades to try to learn more about how Earth’s mantle works. For instance, in a study published in 2003, researchers first found anomalously high levels of helium 3, compared with helium 4, in the lavas—the highest ever recorded in rocks from Earth’s interior and up to 50 times the ratio in the atmosphere. In line with the prevailing geological theories, they reasoned that the helium 3 probably came from a primordial helium reservoir within the mantle, the layer of Earth’s interior below the crust.

In the summer of 2018 Horton’s team set out to replicate these results with a two-week expedition to Baffin Island to collect samples of lava. In laboratories at Woods Hole and the California Institute of Technology, the researchers analyzed a mineral called olivine in the samples that contained microscopic pockets of helium gas. This trapped gas had an even higher ratio of helium 3 to helium 4 that was at least 65 and up to 69 times the atmospheric ratio.

Elevated isotopic helium ratios are also found in volcanic rocks from other hotspots around the world, such as Hawaii and the Galápagos Islands, Horton says. The ratios in the Baffin Island lavas are about twice as high as those found anywhere else, however.

These unprecedented findings suggested to Horton’s team that the helium came not from the mantle but from an even deeper source: Earth’s core. The lavas contained other elements, such as neon, with isotopic ratios that suggest they may have come from the core, he says. This possibility has implications for the formation of Earth and other planets, including exoplanets around other stars.

Yet how would this primordial gas have reached Earth’s surface? Horton proposes the helium could have first leaked from the outer parts of the planet’s core into the neighboring mantle. Then the helium could have risen in a buoyant plume of rock within the mantle that melted as it ascended so that the resulting magma eventually erupted on the surface as lava.

If so, Horton says, the findings give geochemists a rare glimpse of the processes happening at the boundary of Earth’s core and mantle, almost 3,000 kilometers beneath our feet.

The findings could also influence how scientist think about the evolution of our planet. During the early stages of Earth’s formation, helium and other gases may have been abundant in the rocky mantle. But Horton says the hypothesis that helium leaks from the core suggests that nearly all the initial helium was lost from the rocky portions of our planet during later stages of “convective mixing” within the mantle, so the mantle may be more thoroughly mixed than previously supposed.

Horton warns, however, that this is not yet a definitive answer to a debate within geochemistry about the origins of Earth’s helium and its other “noble,” or unreactive, gases, which include neon and argon. Geochemists have long questioned whether these gases came from primordial reservoirs or were added after our planet formed from irradiation by the solar wind or on helium-bearing meteorites.

And while the new evidence suggests the gases escape the core, Horton notes that this hasn’t been proved absolutely. “I would say there’s still a good deal of uncertainty about whether the helium is coming from the core,” he says.

Experts are divided on what they can conclude from the study. Cornelia Class, a geochemist at the Lamont-Doherty Earth Observatory at Columbia University, who wasn’t involved in the study, thinks Horton may be overly cautious. In fact, she says, the latest study is “very good evidence” for the argument that helium is leaking from the core.

But geochemist Manuel Moreira  of the Observatory of Sciences of the Universe at the University of Orléans in France, who also wasn’t involved in the study, is more equivocal. “The recurring proposition that helium is stored and subsequently leaks from the core remains speculative,” he says. “This study nonetheless contributes further insights into the origins of noble gases on Earth.”


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