A helium-bearing compound deep inside Earth is found computationally
Although helium is the second most abundant element in the universe, it is rare on Earth. Most of the gas is thought to reside deep underground. Plumes of magma at volcanic hot spots host helium with isotopic ratios of primordial 3He to radiogenic 4He that are high compared with what’s found in shallower rock. But the source of such high concentrations of primordial helium in the mantle is a long-standing puzzle. The element’s stable electron configuration makes it extremely unreactive. Only recently have scientists identified the high pressure and temperature conditions at which it can react with such compounds as water, sodium, and iron oxide.
Hanyu Liu (Jilin University in China), Changfeng Chen (University of Nevada, Las Vegas), Yanming Ma (also at Jilin University), and their colleagues have now used a computational algorithm to search for materials whose free energies are lower when helium is included in their crystal structures. Of the roughly 1000 structures the researchers generated, they identified an FeO2He compound as the only plausible candidate in the putative, deep-Earth reservoir. The compound quickly stabilizes at around 135 GPa and 3000 K, conditions at which it adopts a cubic structure. (As shown here, gold, red, and white spheres in the lattice represent Fe, O, and He atoms, respectively.)
Those conditions are typical at Earth’s core–mantle boundary. To assess the likelihood of the compound residing there, the researchers also performed molecular dynamics simulations, particularly of the velocity of sound traveling through the compound. Reassuringly, the calculated values were consistent with seismic wave speeds in “ultralow velocity zones"—regions tens of kilometers thick and hundreds of kilometers wide, directly above the core–mantle boundary. (J. Zhang et al., Phys. Rev. Lett. 121, 255703, 2018 .)