Ultra high precision analyses of some of the oldest rock samples on Earth by researchers at the University of Bristol provides clear evidence that the planet’s accessible reserves of precious metals are the result of a bombardment of meteorites more than 200 million years after the Earth was formed. The research is published in Nature.

“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 tonnes of asteroidal material.” Dr Matthias Willbold.

During the formation of the Earth, molten iron sank to its centre to make the core.  This took with it the vast majority of the planet’s precious metals – such as gold and platinum.  In fact, there are enough precious metals in the core to cover the entire surface of the Earth with a four metre thick layer.

The removal of gold to the core should leave the outer portion of the Earth bereft of bling.  However, precious metals are tens to thousands of times more abundant in the Earth’s silicate mantle than anticipated.  It has previously been argued that this serendipitous over-abundance results from a cataclysmic meteorite shower that hit the 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, Dr Matthias Willbold and Professor Tim Elliott of the Bristol Isotope Group in the School of Earth Sciences analysed rocks from Greenland that are nearly four billion years old, collected by Professor Stephen Moorbath of the University of Oxford.  These ancient rocks provide a unique window into the composition of our planet shortly after the formation of the core but before the proposed meteorite bombardment.

Image: NASA

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