A meteorite maelstrom that pummeled Earth and pocked the moon with craters billions of years ago may have had a silver lining — and linings made of other metals, too.
New evidence from old rocks suggests that many of the precious metals mined today were delivered to the planet by a bombardment of stony meteorites called chondrites that lasted for hundreds of millions of years.
“Adding a tiny amount of chondritic material could explain where many present-day metals came from,” says Matthias Willbold, a geochemist at the University of Bristol in England whose team reports the new findings in the Sept. 8 Nature.
The abundance of heavy metals near Earth’s surface has long puzzled scientists. Within 50 million years of the planet’s formation, much of its iron sank to the middle to form the core. Silver, platinum and some other precious metals followed, according to laboratory experiments that recreate this partitioning. These elements moved beyond the reach of human tools, leaving the mantle outside the core barren.
Searching for a source that could have later replenished these metals, Willbold and colleagues turned to some of the oldest known rocks on the planet’s surface. The 3.8-billion-year-old Isua greenstone belt, found in West Greenland, probably formed from melted bits of mantle that date to just after the creation of the Earth’s core but before the massive influx of meteorites that geologists call the late heavy bombardment.
The researchers focused on different forms, known as isotopes, of the metal tungsten in the rocks. When the core formed, it swallowed up most of this metal. But a short-lived element called hafnium left behind in the mantle quickly broke down into a lighter isotope, tungsten-182.
Compared with the Greenland specimens, newer rocks from Hawaii, Iceland and the Canary Islands that formed after the bombardment contain slightly lower ratios of light tungsten to standard tungsten, the team found. Detecting this tiny difference, a mere 13 parts per million, was so difficult that a previous attempt in another laboratory got it wrong.
“They have done a hell of a job,” says Ronny Schönberg, an isotope geochemist at the University of Tübingen in Germany. “This measurement pushes the boundaries for isotope testing.”
Chondritic meteorites don’t have much tungsten-182 compared with the Earth’s mantle, so a veneer of primitive meteorites mixed into the mantle over geologic timescales could have diluted the younger rocks. The amount of incoming material needed to explain the tungsten discrepancy — about 0.5 percent of the Earth’s mass — would also bring enough metals like silver and platinum to the planet to account for the quantities observed today, says Willbold.
But other recently discovered isotope anomalies may prove difficult to explain with the meteorite interpretation. A second team of scientists found that rocks from the Baltic Sea that formed after the bombardment contain isotope ratios similar to those in the Isua belt. And 3.5 billion-year-old rocks from South Africa that should be enriched with the lighter isotope are not.
“The scenario that the Bristol group is referring to isn’t the only scenario that could generate tungsten anomalies,” says Mathieu Touboul, an isotope geochemist at the University of Maryland in College Park, who presented the Baltic Sea results August 18 at the Goldschmidt Conference in Prague.
Touboul and other researchers suggest that processes hidden deep in the Earth may be stirring the pot. Pockets of material leftover from Earth’s infancy, he says, could be slowly remixing, feeding material into the mantle and perhaps explaining the patchiness of the anomalies now rising the surface.
