Where does gold come from?
Have you ever wondered where the gold found on Earth comes from? Well if you haven't perhaps you should. Most people assume that gold and other heavy metals were formed inside our planet way back when the Earth was still a mass of molten rock. But here's the problem. Gold is heavy. So if gold, platinum, tungsten or any of the other ‘heavy metals’ were around before the Earth’s crust solidified, these metals would have sunk through the liquid rock and have vanished deep into the Earth’s core. Instead they are (relatively) abundant in the Earth's crust.
Now there's a new theory. A paper recently published in the journal Nature suggests that the heavy metals mined today fell to Earth approximately 3.8 million years ago. That's when the Earth was bombarded with literally billions of tons of material in the course of a massive asteroid shower. There was gold in those asteroids, and they ended up embedded in the ground, just ready for us to mine it.
A research team led by Matthew Willbold from the University of Bristol (UK) sampled rocks from Isua in south-west Greenland. Although these rocks erupted to the surface around 3.9 billion years ago, the mantle reservoir that the rocks sprang from was formed 4.3 billion years ago. This allowed the scientists to study a rock composition from very early in the history of the Earth, well before the proposed asteroid showers. As Willbold pointed out: ‘these rocks provide a sort of time capsule that gave us the possibility to calculate how much material had to be added to the Earth to satisfy the tungsten isotopic composition that we find in the Earth today.’ In other words, later rocks can be checked against these pre-asteroid shower rocks for added heavy metal content.
The Willbold group used high-precision analysis of different tungsten isotopes (isotopic tungsten ratio 182W/184W ) to measure the composition of the Isua rocks. The results were compared with those from the same tests on later rocks, and also with the results of tests on meteorite samples.
These results showed that Isua rocks are different both from later rock samples and from samples obtained from meteorites. The difference suggests that around one-half of one percent of today's mantle arrived in the form of meteors. Or to put it more graphically, once the Earth’s crust had solidified, about a further 300 billion billion tons of material landed on it from space. This extra-terrestial material included precious heavy metals.
Not everyone agrees with the interpretation put forward by the Willbold group. The New Scientist in a review article argues that if the hypothetical asteroid bombardment were a complete explanation there would be more heavy metals in sedimentary rocks laid at the time.
It is clear we have not heard the last of this story.
Matthias Willbold, Tim Elliott & Stephen Moorbath. The tungsten isotopic composition of the Earth's mantle before the terminal bombardment. Nature, 477, 195–198
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