In a study, researchers at the University of Bristol conduct ultra high precision analyses of some of the oldest rock samples on Earth.
They find that the planet’s accessible reserves of precious metals are the result of a bombardment of meteorites more than 200 million years after Earth was formed.
The research is published in Nature.
During the formation of Earth, molten iron sank to its center 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 Earth with a four-meter thick layer.
The removal of gold to the core should leave the outer portion of Earth bereft of bling. However, precious metals are tens to thousands of times more abundant in Earth’s silicate mantle than anticipated.
It has previously been argued that this serendipitous over-abundance results 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, the researchers analysed rocks from Greenland that are nearly four billion years old.
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.
The researchers determined the tungsten isotopic composition of these rocks.
Tungsten (W) is a very rare element (one gram of rock contains only about one ten-millionth of a gram of tungsten) and, like gold and other precious elements, it should have entered the core when it formed.
Like most elements, tungsten is composed of several isotopes, atoms with the same chemical characteristics but slightly different masses.
Isotopes provide robust fingerprints of the origin of material and the addition of meteorites to Earth would leave a diagnostic mark on its W isotope composition.
The researchers observed a 15 parts per million decrease in the relative abundance of the isotope 182W between the Greenland and modern day rocks.
This small but significant change is in excellent agreement with that required to explain the excess of accessible gold on Earth as the fortunate by-product of meteorite bombardment.
One author said: “Extracting tungsten from the rock samples and analysing its isotopic composition to the precision required was extremely demanding given the small amount of tungsten available in rocks.”
“In fact, we are the first laboratory world-wide that has successfully made such high-quality measurements.”
The impacting meteorites were stirred into Earth’s mantle by gigantic convection processes.
A tantalising target for future work is to study how long this process took.
Subsequently, geological processes formed the continents and concentrated the precious metals (and tungsten) in ore deposits which are mined today.
This research was funded by the Natural Environment Research Council (NERC), the Science and Technology Facilities Council (STFC) and the Deutsche Forschungsgemeinschaft (DFG).
Citation: Willbold M. (2011). The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment. Nature, 2011; 477 (7363): 195. DOI: 10.1038/nature10399.
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