Iron-loving elements tell stories of Earth’s history

Four and a half billion years ago, after Earth’s fiery birth, the infant planet began to radically reshape itself, separating into distinct layers. Metals — mostly iron with a bit of nickel — fell toward the center to form a core. The growing core also vacuumed up other metallic elements, such as platinum, iridium and gold.

By the time the core finished forming, about 30 million years later, it had sequestered more than 98 percent of these precious elements. The outer layers of the planet — the mantle and the crust — had barely any platinum and gold left. That’s why these metals are so rare today.
Battles have been fought, and wars won, over the pull of shiny precious metals, which have long symbolized power and influence. But for scientists, the rare metals’ lure is less about their shimmering beauty than about the powerful stories they can tell about how the Earth, the moon and other planetary bodies formed and evolved.

By analyzing rare primordial materials, researchers are uncovering geochemical fingerprints that have survived essentially unchanged over billions of years. These fingerprints allow scientists to compare Earth rocks with moon rocks and test ideas about whether giant meteorites once dusted the inner solar system with extraterrestrial platinum and gold. Such research can help scientists learn how volatiles such as water may have spread, leaving some worlds water-rich and others bone-dry.

These explorations, motivated by a growing appreciation of what such rare metals reveal about Earth’s history, are now possible thanks to new analytical techniques. “They give us a window into all kinds of processes that we want to understand,” says Richard Walker, a geochemist at the University of Maryland in College Park.

Geochemical memory
Platinum and gold are among eight occupants of the periodic table belonging to the category known as the highly siderophile elements. That name dates back to the 1920s, when Victor Goldschmidt, a mineralogist at the University of Oslo, divided the elements into groups depending on what they liked to combine with in nature. His four classifications are still used today: the lithophiles (rock-lovers), the chalcophiles (ore- or sulfur-lovers), the atmophiles (gas-lovers) and siderophiles, the iron-lovers.

The siderophile elements tend not to ally themselves with the oxygen- and silicon-based compounds that form the bulk of Earth’s crust. They form dense alloys with iron instead. One such element, tungsten (symbolized by W in the periodic table), is an iron-lover that has been important in recent scientific studies of Earth’s geologic history. A step beyond tungsten are those highly siderophile elements, which are even bigger fans of iron. They are ruthenium, rhodium, palladium, rhenium, osmium and iridium along with platinum and gold.

Because highly siderophile elements are relatively abundant in the core and scarce in the mantle and crust, they help scientists trace how Earth’s insides have evolved over time. Dig up a rock from deep within a mine, or pick up one from a freshly erupted volcano, and you can measure the siderophile elements within. The measurements might show whether a radioactive version of one such element has decayed into another, or whether the rock has higher amounts of one particular variety of siderophile. In turn, that information can reveal how material has shifted around and been chemically processed deep within the planet.
By analyzing the iron-lovers within each rock, scientists can probe what the rock has been doing for billions of years. “We can trace the entire evolutionary process of how a planet formed,” says James Day, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif. “That’s why someone like me is interested.”

For instance, Walker and his colleagues have explored siderophile elements in some of the oldest rocks on Earth. Through the process of plate tectonics, in which plates of Earth’s crust grind against, pull apart from and occasionally dive beneath one another, most ancient rocks have been dragged back into the planet and destroyed by melting. But in southwestern Greenland, in a place called Isua, a chunk of ancient crust never got pulled down by plate tectonics (SN: 3/24/07, p. 179). Walker and colleagues, led by Hanika Rizo of the University of Quebec in Montreal, recently studied siderophile elements in these 3.3-billion- to 3.8-billion-year-old rocks.

The scientists looked at the abundance of highly siderophile elements in the Greenland rocks but found that, in this case, the biggest clues came from the slightly less iron-loving tungsten. The rocks contain more of one variety of tungsten, known as tungsten-182, than expected. That isotope forms from the radioactive decay of hafnium-182, which existed only during Earth’s first 50 million years. The Greenland rocks thus serve as a sort of time capsule that helps reveal the history of the early solar system, Rizo, Walker and colleagues wrote in February in Geochimica et Cosmochimica Acta.

“We believe we are accessing parts of Earth’s mantle that formed and took on some of their chemical characteristics while the Earth was still growing,” Walker says. “You can call it accessing a building block of the Earth.”

Studies of these remnants of the ancient planet suggest that Earth’s mantle has remained chemically patchy. Like lumps of flour in poorly mixed cake batter, clumps of primordial material, with higher amounts of tungsten-182, are studded throughout a smoother, more evenly mixed matrix. That’s surprising because researchers thought that the hot, churning insides of the Earth would have stirred everything around over the course of billions of years. Somehow portions of the mantle resisted the planet’s best blending efforts, Walker reported in June at the Goldschmidt geochemistry meeting in Yokohama, Japan.

By studying where those patches are and what they are made of, researchers can investigate such questions as how much convection there was inside the early Earth, and whether any volcanoes today tap into this primordial material. In May, for instance, Walker’s team reported that it had used siderophile elements to identify geochemically primitive lavas in Canada’s Baffin Bay and in the South Pacific (SN: 6/11/16, p. 13).
Like the ancient Greenland crust, these rocks also had an overabundance of tungsten-182. Apparently the Canadian and Pacific volcanoes tapped into a deep reservoir of primordial material, which flowed up through the throat of a volcano and out onto the surface. Studying the iron-loving elements in those rocks is like taking a siderophile time machine into the past and seeing what the Earth was like 4.5 billion years ago.

“It never ceases to amaze me what the rocks can tell,” says Amy Riches, a geochemist at Durham University in England.

A dusting from space
Highly siderophile elements can teach about more than just the planet Earth. They can reveal secrets about the history of the moon, Mars and other nearby planetary bodies. That’s because all the worlds in the inner solar system apparently got a dusting of gold, platinum and other highly siderophile elements during meteorite bombardments around 4 billion years ago.

The early solar system was something of a cosmic shooting gallery. After the planets coalesced, there were still a lot of leftover space rocks careening around. One enormous impact is thought to have smashed the Earth and spalled off enough debris to form the moon. Other, smaller impacts continued to pummel the inner planets for the first half-billion years or so of their existence. Each collision would have brought a little more fresh material to each world.

On Earth, meteorite impacts could have delivered half a percent to 1 percent of the planet’s total mass. Many meteorites that fall to Earth and are analyzed contain relatively high amounts of highly siderophile elements, which suggests that meteorites hitting the early Earth would have carried a lot of them, too. If so, then the cosmic smashups regularly showered Earth with a fresh coating of gold, platinum and other precious elements. By this time, Earth had already finished forming its core, so the highly siderophile elements remained sprinkled throughout its upper layers rather than being vacuumed into its depths.

This “late accretion” of fresh material could help explain a long-standing puzzle. The amounts of highly siderophile elements in Earth’s mantle are higher than predicted, according to laboratory experiments that try to mimic how molten metal separated from rock as Earth was forming. But a shower of meteorites hitting soon after core formation stopped could have done the trick, a process that Day, Walker and Alan Brandon of the University of Houston discuss in the January Reviews in Mineralogy & Geochemistry.
Not everyone accepts the late accretion idea. Some scientists, including Kevin Righter at NASA’s Johnson Space Center in Houston, note that siderophile elements become less iron-loving when squeezed at high pressures and temperatures. That could mean fewer of them dived deep into Earth’s core, and more of them would be left behind for the mantle and the crust. No need for an express meteorite delivery.

Debate probably won’t end anytime soon, as various laboratory experiments seem to support both conclusions. “People are still hacking away at trying to understand this,” says James Brenan, a geochemist at Dalhousie University in Halifax, Canada. Clarity is important for getting to the heart of what the highly siderophile elements can tell scientists — where they came from, how they separated out within the primordial Earth, and what they have been doing since then.

Less precious moon
Another big unanswered question is why the Earth and the moon seem to be so different from each other when it comes to highly siderophile elements.

Researchers have a very limited sample of moon rocks to study — just those brought back by the Apollo astronauts, and a few lunar meteorites that happened to fall on Earth and were picked up. None of these samples come from the moon’s deep interior. But by extrapolating from the chemistry of the rocks they have in hand, researchers have calculated that the moon’s mantle has surprisingly lower amounts of the highly siderophile elements than Earth’s mantle — just about 2 percent as much.

If the late-accretion idea is right, then both Earth and the moon should have been dusted by the same meteoritic bombardment of gold, platinum and other elements, and they should have similar amounts of highly siderophile elements in their mantles. That doesn’t seem to be the case. The explanation may lie partly in the fact that the moon is a lot smaller than the Earth, Day and Walker noted last year in Earth and Planetary Science Letters.

Think of the meteorite bombardment as someone throwing snowballs at a pair of very different-sized dogs, Day says. “The statistical chance of these snowballs colliding with a Rottweiler are much higher than with a Chihuahua,” he says. In other words, Earth acquired more platinum and gold simply because it is so much larger than the moon. Both went through the same snowball bombardment, but the bigger object collected more snow coating.

As with most things geochemical, there is another possible explanation. The moon does not have a core that would have sucked highly siderophile elements into its interior. But it’s possible that something else could be holding the gold and platinum deep within the moon, Brenan says. That something is sulfur.

The iron-lovers are also sulfur-likers. In the absence of metal, highly siderophile elements tend to clump with sulfur instead. By studying the interplay between the two, geochemists can start to tease out how the various elements behave as rocks are squeezed, melted and otherwise altered over billions of years of geologic history.

In recent laboratory experiments, Brenan mixed up a recipe of rock meant to simulate the lunar mantle. Earlier work had suggested that there was simply not enough sulfur deep in the moon for iron sulfide to be present. But his work, which used a more realistic representation of the lunar mantle, suggests that iron sulfide can indeed exist and be stable there. That iron sulfide would have kept the iron-lovers deep inside the moon — trapping the highly siderophile elements out of sight.
The sulfur work may have even broader implications for understanding how the Earth, moon and other worlds in the inner solar system got their water. Both sulfur and water are relatively volatile compounds that often appear together. Researchers thought both had been lost from the moon long ago. After all, the lunar surface today is dry and barren. But in recent years, scientists have been analyzing droplets of melt in lunar rocks and have found surprisingly high amounts of sulfur and water. That indicates that the moon may once have been wetter than thought. “That has really changed our thinking,” Brenan says.

By looking at the concentration of these elements in lunar rocks, geochemists can cross-check their measurements of sulfur and water — and begin to understand the differences between Earth and the moon.

Still searching
At the University of Münster in Germany, geochemist Mario Fischer-Gödde has been working to pull together the various threads of what highly siderophile elements can reveal. Many researchers have suggested that Earth may have gotten much of its water and other volatile elements during the meteorite bombardment of the late accretion. So Fischer-Gödde is systematically analyzing different types of meteorites found on Earth to see if they could have actually delivered these volatiles.

He focuses on the element ruthenium. Like the other highly siderophile elements, it probably arrived on Earth aboard meteorites during the late accretion. Weirdly, though, none of the dozens of meteorites Fischer-Gödde has analyzed contain ruthenium isotopes that match those found in the mantle. He concludes that none of the meteorite types found on Earth so far could be the source of the late accretion materials. Some other source — maybe other rocky bits that were flying around the inner solar system — must have brought ruthenium and other siderophiles to Earth, he reported at the Durham workshop.

And that means the highly siderophile elements still have many mysteries to reveal, and there’s plenty of work to be done. With new ever-more-sensitive techniques under development — such as scans that reveal individual atoms of highly siderophile elements within small grains of metal — researchers are pushing forward in their efforts to analyze the siderophile elements, hoping to squeeze more stories of Earth’s beginning from the discreet iron-lovers.