Researchers at the University of Bristol have found evidence that Earth’s minable precious metals, like gold, came from billions of tons of meteorites that hit Earth more than 200 million years after our planet formed – likely the same meteorites that left craters on the moon. Results of the research appear in the September 7, 2011, issue of Nature.
As Earth formed, molten iron sank to the center, making the core. This attracted the vast majority of Earth’s precious metals, such as gold and platinum, which migrated to the core with iron. There are enough precious metals in the core to cover the entire surface of the Earth with a layer four meters thick (over 12 feet).
The concentration of gold in the core should have left the outer portion of the Earth without any. But precious metals are abundant in the Earth’s silicate mantle. Some scientists think this over-abundance resulted from a cataclysmic meteorite shower that hit Earth after the core formed. The full load of meteorite gold was thus added to the mantle alone and not lost to the deep interior.
To test this theory, Matthias Willbold and Tim Elliott analyzed rocks from Greenland that are 3.8 billion years old – some of the oldest rocks on Earth – and got a glimpse into the composition of our planet shortly after the formation of the core but before the proposed meteorite bombardment.
They measured tungsten isotopes in the ancient rocks and compared that amount with tungsten isotopes found in our present-day mantle. The addition of meteorites to the Earth would have left a definite mark on its tungsten isotope composition, and that’s exactly what the researchers found.
According to Willbold and Elliott, the accessible gold on Earth is the fortunate by-product of meteorite bombardment. Gradually, the gold-laden meteorites were stirred into the Earth’s mantle by convection. After that, geological processes formed the continents and concentrated the precious metals – including tungsten – in ore deposits that are mined today.
Our work shows that most of the precious metals on which our economies and many key industrial processes are based have been added to our planet by lucky coincidence when the Earth was hit by about 20 billion billion tons of asteroidal material.
Bottom line: Researchers Matthias Willbold and Tim Elliott of the University of Bristol compared tungsten isotopes from rocks 3.8 billion years old with tungsten isotopes in younger rocks. The ratios support the theory that meteorites bombarded Earth, leaving precious metals that mixed into Earth’s mantle. Results of their research appear in the September 7, 2011, issue of Nature.
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