Where the young hot Earth cached its gold

DEEP DOWN  Gold and other related "iron-loving" metals were trapped deep in the planet by ancient chemical reactions involving sulfur, a new study suggests.

There’s a new twist to the story of how Earth’s most precious metals, including gold and platinum, got to where they are in the planet.

Some 4.6 billion years ago, space rocks pummeling the infant Earth kept it hot and molten. As the nascent planet grew bigger, a new study suggests, the heat and pressure kept precious metals trapped within its upper layers rather than allowing them to sink into the newly forming core. Later, chemical reactions involving sulfur pulled the metals down deep.

The work illuminates not only what happened during the Earth’s early years, but also why gold, platinum and related metals are so scarce in its upper layers today (SN: 8/6/16, p. 22).

“It’s giving us a lot of new insights into how the planets formed,” says David Rubie, a geochemist at the University of Bayreuth in Germany whose team reports the discovery in the Sept. 9 Science.

Gold and platinum belong to a class of chemical elements known as the highly siderophile, or “iron-loving,” elements. When molten, they tend to form alloys with iron. Today, roughly 98 percent of Earth’s highly siderophile elements are tucked away in its iron-rich core.

Most researchers think the highly siderophile elements joined with iron and sank into the core as it formed, in the first tens of millions of years in Earth’s history. Rubie and his colleagues say the story is a little more complex. They have been studying how Earth glommed together from fragments orbiting the newborn sun. Each time a space rock smashed into Earth, heating it up and increasing its size, the pressures inside the planet went up.

At these higher pressures and temperatures, the iron-loving elements become less iron-loving. Rubie’s team calculated that in the first 100 million years or so of Earth’s existence, once the planet grew to about 60 percent of its modern-day mass, the conditions kept the highly siderophile elements in the planet’s middle layer, the mantle, rather than dropping into the core.

“This is an attractive idea, as it realistically paints a picture of a dynamic system where conditions are constantly changing,” says Raúl Fonseca, a geochemist at the University of Bonn.

But then the question becomes how the iron-loving elements eventually made it out of the mantle and into the core, where most of them are today. The answer, Rubie says, is sulfur. High-pressure experiments involving molten iron and sulfur showed that the presence of sulfur triggered the highly siderophile elements to eventually crystallize out, closer to the core.

By this point, Rubie’s scenario had explained how most of the iron-lovers got into the core. But the team still needed to account for the traces of gold and platinum that remain sprinkled throughout the mantle and crust. For that the researchers invoke a final step popular with other scientists: the idea that meteorites later ferried a fresh dusting of iron-loving elements to Earth’s surface. That final cosmic delivery is the source of the gold, platinum and other rare metals that miners dig up today.

Richard Walker, a geochemist at the University of Maryland in College Park, says the new study is interesting but not all that compelling. “It’s a model that may not really be necessary,” he says. He agrees that higher pressures made the iron-loving elements less iron-loving but thinks they could have still separated out into the core without the extra steps proposed by Rubie’s team.

Rubie and colleagues are now working to expand the results beyond Earth. “We might even be able to extend this sort of approach to exoplanets,” he says, “and make predictions about their early histories.”