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January 28, 2022

Gold on Earth originated from meteorites

By Mo-Yu Zhou | September 21, 2011

Researchers at the University of Bristol have analyzed some of Earth's oldest rock samples and found that our planet's accessible precious metals were added to it by chance. If not for a cataclysmic meteorite shower that struck the earth over 200 million years after its formation, we would not be able to access today much of the precious metals that make up many of our thriving industries.

The outer portion of the Earth should be lacking in precious metals such as platinum and gold. Molten iron sinking down and creating the core during the formation of the Earth brought along with it nearly all of the planet's precious metals. There are so many precious metals in the Earth's core that if we were to cover the surface of the Earth with those metals, the resulting layer would be four meters thick.

However, there is tens of thousands times more precious metal in the Earth's silicate mantle than there should be, according to this story. One theory that has been raised in the past, but which had not been tested due to insufficient technology, is that the abundance of precious metals results from a bombardment of meteorites that struck the Earth after the core was fully formed. This means that the metals from the meteorites were added to the Earth's mantle through a convection process (the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise) and did not sink down as they would have during the Earth's creation.

This theory was tested by research published in Nature on Sept. 8 by Dr. Matthias Willbold and Professor Tim Elliot of the Bristol Isotope Group in the School of Earth Sciences and Professor Stephen Moorbath of the University of Oxford. Their study involved ultra-high precision analysis of nearly four billion year-old rocks from Greenland. These rocks existed shortly after the formation of the Earth's core but also before the meteorite bombardment's estimated occurrence.  

The procedure determined the tungsten (W) isotopic composition of the rocks. Like gold and other precious metals, tungsten should have entered the Earth's core during its formation. It is still currently a very rare element, with one gram of rock containing about one-ten-millionth of a gram of tungsten.

Because of the rarity of tungsten, its usefulness has been unharnessed in the past because the technology had not been available to measure such tiny quantities. However, these researchers at the Bristol Isotope Group have become the first in the world to make such accurate measurements.

In order to find out more about the origin of the tungsten, the researchers decided to look at the isotopes of the element manifested in the rocks; Isotopes are atoms with the same number of protons and properties but different numbers of neutrons or atomic weights. These isotopes are significant because meteorites that struck the earth would affect tungsten's isotope composition.

The researchers discovered that there was a 15 parts per million decrease in the relative abundance of a certain isotope of tungsten, 182W, between the rocks from Greenland and modern-day rocks. While small, this change is also significant and indicates that meteorite bombardment was responsible for the abundance of accessible precious metals on Earth.

We also now know that the meteorites were mixed into the Earth's mantle through a convection process and then concentrated, although the speed at which this process occurred is unknown, and a possible area for future research.

In fact, the same geological processes that formed the continents concentrated the precious metals and tungsten into the ore deposits that are now mined. Thus, this new research serves to support a compelling theory about the metals that are an important part of the way we interact with the Earth today.

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