Twinkling stars may look like diamonds in the sky, but iron meteorites seeded the world with gold.
That's the fascinating conclusion that a new study, "The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment," published in the journal Nature reveals. Although geologists have speculated for many years that at least some of Earth's gold came from meteorites, it was impossible to prove.
Many suspected meteorites delivered gold to the planet because it's known that as the Earth cooled during its creation iron fell towards the center forming the molten core. It seemed reasonable to assume that all heavy elements would have also found their way to the core, but that's not where gold is found.
"Iron meteorites typically consist of approximately 90 to 95 percent iron, with the remainder comprised of nickel and trace amounts of heavy metals including iridium, gallium and sometimes gold. [Geoffrey Notkin, Geology.com]
The mystery of gold's origin, solved
Instead, gold, and several other types of heavier elements—including iron—are found in the silicate mantle. The amount in the mantle is about 1,000 times more than the model for the formation of the Earth can account for, creating a mystery.
The mystery of the gold can be solved if meteorites seeded the mantle with prodigious amounts of the heavier elements, and this is what a team of scientists proved in their study conducted at the university of Bristol.
Their research revealed that elements such as gold and platinum appeared in the mantle long after the formation of the planet. A 600 million years after the Earth had cooled—about 3.9 billion years ago—a "terminal bombardment" began. Showers of space debris rained on the surface of the world.
Scientists estimate that the bombardment delivered as much as 20 billion billion tons of debris composed of meteoric and asteroid matter.
Lead researcher Dr. Matthias Willbold told the BBC News, "It is not clear whether this would have come in the form of many small impacts, or just two or three mega-impacts."
Tungsten analysis key to finding proof
The meteoric origins of Earth's gold was proven using a new analysis technique. Previous approaches were not exacting enough and failed.
"The proportions of gold and other precious metals are difficult to measure because they concentrate into nuggets, and we need to analyze a lot of rocks to get meaningful data," explained Willbold referring to the research.
According to geologists, tungsten has similar properties to the heavier metals like gold and has a variety of atomic isotope signatures.
Using rock samples obtained from Greenland, the team focused on tungsten as a marker for the meteoric origin of gold. Any significant difference in the isotopes found in modern versus ancient rocks would prove gold came from space rocks.
Their analysis found a small, but significant, difference between the samples that proved the more modern rocks had formed with tungsten—and therefore gold too—from falling meteors.
The ancient Greenland rocks, 3.9 billion years old, had no gold or tungsten enrichment. The enrichment process came later when the terminal bombardment occurred.
There's gold in them there meteorites!
"It has been estimated that all the gold mined by the end of 2009 totaled 165,000 tonnes. At a price of US$1900/oz., reached in September 2011, one ton of gold has a value of approximately US$60.8 million. The total value of all gold ever mined would exceed US$9.2 trillion at that valuation."
Geologists believe the total amount of gold still buried in the Earth is about 100,000 tons. Start digging!