Centre of the Earth could hold large reservoir of iron–helium compounds

Iron can form compounds with helium at pressures as low as 5GPa – about 50,000 atmospheres – researchers in Japan report. This result could provide novel insight into the chemistry of the most abundant noble gas in the universe and suggests that the Earth’s core could hold a large reservoir of primordial helium-3.

According to astrophysical models, the Earth and other solar planets condensed from a stellar nebula that predominantly comprised hydrogen and helium-3. No known natural process exists on Earth today to produce helium-3, so any detected must be primordial. ‘What is interesting is that if you look at, for example, Hawaiian volcanoes, they are enriched in helium-3,’ says high pressure geophysicist Kei Hirose at the University of Tokyo. ‘People have long believed that such helium-3 derives from the deep, rocky mantle.’ The problem with this explanation, says Hirose, is that, as the Earth cooled and solidified, free gases would have escaped the mantle.

In the new work, Hirose and colleagues at other institutions in Japan and Taiwan propose that the iron core of the Earth formed bonds with helium, allowing the core to act as a reservoir. Heavier noble gases can form compounds at experimentally accessible pressures, but the reactivity of helium – especially below the 350GPa found at the centre of the Earth – was thought to be almost nonexistent.

The researchers tested this by compressing iron with helium gas in laser-heated diamond-anvil cells. They carried out five experimental runs at pressures ranging from five to 54GPa and temperatures from 750–2550°C. Depending on the conditions, x-ray diffraction showed that the iron either had a face-centred cubic or a hexagonal close packed crystal structure, as they expected. Crucially, however, there were some crystallographic distortions, which suggested expansions in the unit cell to accommodate helium.

Perhaps most remarkably, the researchers were able to release the pressure and recover some of the samples – such as a hexagonal close packed sample of FeHe_0.32 – at ambient conditions. This allowed them to conduct more direct measurements such as secondary ion mass spectrometry to confirm the presence of helium. ‘Once you detect helium in a secondary ion mass spectrum there’s no question about it,’ says Hirose. ‘The helium should be in the iron lattice.’ The researchers constructed first principles simulations indicating their compounds were dynamically stable.

The researchers believe that, having shown helium can form compounds with iron, they now need to investigate whether these compounds form inside the Earth by establishing whether helium would have migrated to the core. ‘Our next step should be to look at the partitioning between the silicate melt and the molten iron, or in other words between the mantle and the core.’

‘It’s very exciting,’ says theoretical chemist Maosheng Miao at California State University Northridge in the US. ‘They’ve suddenly found something we weren’t looking for.’ He cautions that more comprehensive computational backing for the researchers’ claims is needed. ‘Systematic support has to give you a good prediction of the structures and good thermodynamic calculations that show that the compound is stable or, if it’s metastable, that it’s not too high in energy relative to the compounds that it would decompose into…[The researchers] observe a structure and run a calculation, but they don’t run a large-scale calculation to compare all possibilities.’ Nevertheless, he concludes, ‘this is the first round, I’m sure more study will follow and if it’s true it’s very exciting’.