Each year, about 2 kg of helium-3, a rare isotope of helium gas, escapes from Earth’s interior, mostly along the mid-ocean ridge system. Helium-3 is primordial, created shortly after the Big Bang and acquired from the Solar Nebula as the Earth formed. Geochemical evidence indicates the Earth has deep reservoirs of helium-3, but their locations and abundances remain uncertain. New models of volatile exchange during Earth’s formation and evolution implicate the planet’s metallic core as a leaky reservoir that supplies the rest of the Earth with helium-3.
Sketches illustrating core-mantle helium exchange processes: (a) 3He acquisition during Earth’s accretion by ingassing from the nebular atmosphere and transport through the magma ocean to the proto-core; and (b) 3He transport from the core to the mantle and from the mantle to the ocean after accretion. Image credit: Olson & Sharp, doi: 10.1029/2021GC009985.
Earth’s inventory of helium consists of two stable isotopes, the more abundant helium-4 (4He) and the far more rare helium-3 (3He).
Unlike terrestrial 4He, which is mainly produced by decay of uranium and thorium, terrestrial 3He is largely of primordial origin, synthesized in the aftermath of the Big Bang and incorporated into the Earth primarily during its formation.
In spite of its primordial status and 4.56 billion years of planet evolution, 3He continues to leak from Earth’s interior.
“About 2 kg of 3He leak out of the Earth every year, about enough to fill a balloon the size of your desk,” said Dr. Peter Olson, a geophysicist in the Department of Earth and Planetary Sciences at the University of New Mexico.
“It’s a wonder of nature, and a clue for the history of the Earth, that there’s still a significant amount of this isotope in the interior of the Earth.”
In their research, Dr. Olson and his colleague, Professor Zachary Sharp, modeled helium during two key stages of Earth’s history: early formation, when the planet was accumulating helium, and following the formation of the Moon, after which helium was lost.
Evidence suggests an object one-third the size of the Earth hit our planet early in its history, around 4 billion years ago and that impact would have re-melted the Earth’s crust, allowing much of the helium to escape.
“Using the modern 3He leak rate along with models of helium isotope behavior, we estimated there are between 1013 and 1015 grams of 3He in the core — a vast quantity points to Earth’s formation inside the Solar Nebula, where high concentrations of the gas would have allowed it to build up deep in the planet,” Dr. Olson said.
“However, future work looking for other nebula-created gases, such as hydrogen, leaking in similar rates and locations as 3He could be a smoking gun for the core as the source.”
“There are many more mysteries than certainties,” he added.
The findings were published in the journal Geochemistry, Geophysics, Geosystems.
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Peter L. Olson & Zachary D. Sharp. 2022. Primordial Helium-3 Exchange Between Earth’s Core and Mantle. Geochemistry, Geophysics, Geosystems 23 (3): e2021GC009985; doi: 10.1029/2021GC009985
