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.
During the
formation of the 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 the Earth with a 4-m 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, Matthias Willbold and Professor Tim Elliott of the Bristol Isotope
Group in the School of Earth Sciences analyzed 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.
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 comprised 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 the Earth would leave a diagnostic
mark on its W isotope composition.
Willbold observed
a 15 ppm 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.
Willbold says:
“Extracting tungsten from the rock samples and analyzing 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 the Earth’s mantle by gigantic convection
processes. A tantalizing 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.
Willbold
continues: “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.”
