Mystery iron-rich magma entombed in dead volcanoes could be rich source of rare earth elements

Volcanoes that are rich in iron might be prime locations to find high concentrations of rare earth elements. Recent lab experiments have demonstrated that when iron-rich magmas experience volcanic pressures and temperatures, the resulting iron oxide-apatite (IOA) deposit separates into two unmixable melts, one of which is highly enriched in rare earth elements (REEs).

"The rare earth element contents can be close to 200 times higher than in the silicate-rich melts," said Shengchao Yan, a doctoral student at the Chinese Academy of Sciences' Institute of Geology and Geophysics and lead researcher on the new experiments.

The research, which was published in Geochemical Perspectives Letters, supports the idea that deposits of iron oxide and apatite, an iron-phosphate mineral mined globally for its iron, could be rich targets for REE exploration.

Not rare, but hard to mine

Rare earth elements, the lanthanide series as well as yttrium and scandium, are key to a green energy transition because they are required for producing electric vehicle and wind turbine magnets, solar panels, and storage batteries. With the growing need to address the climate crisis, economies around the world face an increasing demand for REEs.

But supply is hard to come by. Despite the name, REEs are not rare. These metals exist around the world but are often found in small concentrations or are locked in other minerals. This makes REE extraction economically and environmentally unsustainable for most countries. Currently, 63% of the world's REE mining occurs in China.

However, rocks enriched in REEs have been found unexpectedly at iron mines in Kiruna, Sweden; El Laco, Chile; and elsewhere. The enrichment makes those REEs easier to extract.

These mines are sited on extinct iron-rich volcanoes that have large IOA deposits.

"In many cases when we find rare earth elements or metals in general, we find them by accident," explained Michael Anenburg, an experimental petrologist at Australian National University in Canberra and a coauthor of the new research. "Those mines are mining iron oxide. They're mining magnetite. They never looked [to see] if they even have any rare earth elements."

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The discovery of concentrated REEs inside IOA deposits has prompted mining experts and geologists to ask, "Is that just by accident, or is there something about those magmas that make them like that?" Anenburg said.

To explore the possible conditions under which the REEs became separated out and concentrated in the IOA deposits, the researchers subjected magmatic mixtures to volcanic pressures and temperatures in the laboratory. They observed that under those conditions, the magmas separated into two unmixable, or immiscible, components: an iron phosphate (FeP) melt and a silicate melt.

The REEs concentrated more strongly in the iron phosphate melt than in the silicate-rich melt, Yan said, and lighter REEs concentrated more strongly than the heavier ones. The FeP melt was enriched in lanthanum, the lightest of the lanthanide series, about 200 times more than the silicate was, and lutetium, the heaviest lanthanide, was enriched about 100 times more. (Yttrium and scandium, the lightest nonlanthanide REEs, inexplicably did not follow this trend.)

Untapped potential

Although these experiments are not the first to show that IOAs are rich in rare earth elements, they can help geologists understand one mechanism by which these melts become enriched.

"Overall, I think it's a fantastically useful contribution and sheds important light on the debated process of IOA genesis and in particular the process and extent of REE enrichment in this enigmatic class of deposits," Tobias Keller, a computational geochemist at the University of Glasgow in Scotland who was not involved with the research, wrote in an email.

These experiments add weight to the hypothesis of a volcanic origin for IOA deposits, Keller explained, and provide "important confirmation that REE partitioning between such immiscible liquid pairs strongly favors the FeP-rich melt." This research helps explain the occurrence of REE-enriched apatite in Kiruna, Sweden, Keller added. But just how these distinct melts form separate bodies of iron-rich magnetite and REE-rich apatite is still a mystery, he wrote.

The iron-rich volcanoes upon which IOA deposits are found are now extinct, Yan noted. By modeling iron-rich volcanoes throughout their evolution, he said he hopes to explore how REE enrichment may have changed over Earth's history.

"We can try to find the optimal formation conditions of the deposits, so people can reduce or narrow down the exploration location of these deposits," Yan said.

"Rare earth elements are critical metals," Anenburg said. A country might not need a large supply now, but global demand will only continue to grow. Knowing whether an active iron mine might also be an untapped source of REEs could pay dividends in the future.

"It's a win-win," he said, "because the company gets more value out of the stuff they're mining anyway. And then the environment wins, because we don't need to put a new hole in the ground."

This article was originally published on Eos.org. Read the original article.