14 September 2011, by Tom Marshall
Precious metals are surprisingly abundant in the upper parts of the Earth because a huge meteor shower rained them down out of space, scientists have shown.
The authors of a paper published in Nature analysed different isotopes of tungsten in four billion-year-old rocks taken from Greenland to work out when and how these metals were added to the upper reaches of the Earth.
The findings support the 'terminal bombardment' hypothesis - that the same meteor shower that left the moon scarred with craters also added back rare metals to the upper Earth after the mantle had separated from the core.
'The first three or four hundred million years of the Earth's history are fascinating, but there is no direct geological record so it's very hard to know exactly what was going on,' says Dr Matthias Willbold, a geochemist at the University of Bristol and lead author of the paper. 'Our work is a major contribution to our understanding of the early Earth.'
The presence of many metals in the crust has long puzzled scientists. When the planet formed, almost all of its original stock of the metals that are now rare, including gold, platinum and tungsten, were drawn into the planet's core and have been locked there beyond our reach ever since.
This is because they're what geologists call 'siderophile' – they have a chemical affinity with iron, which is the core's main ingredient. When the iron core divided from the silica-rich mantle, iron-loving metals disappeared into it.
So why are they still found at all in the mantle and crust? Something must have happened to restore the siderophile elements to the planet's upper regions. One popular theory is that a period of meteorite bombardment between around 4.3 and 3.9 billion years ago was responsible, but until now it hadn't been confirmed by experiment.
Scientists could test this idea by analysing the samples from Greenland because these rocks were created before this late veneer of metals was added. They are the oldest rocks we know. They still bear the chemical signature of the mantle environment they formed in 4.3 billion years ago, even though they subsequently melted and reformed a mere 3.8 billion years ago.
The team analysed the ratio between two different kinds of tungsten, known as 'isotopes', one slightly heavier than the other. These isotopes exist in small concentrations, so they had to develop new techniques to let them analyse much larger quantities of rock than normal in order to end up with enough tungsten to work with.
The researchers managed to show that these ancient rocks have a ratio of two different tungsten isotopes, 182W and 184W, that's significantly higher than in modern rocks. This closely fits what you'd expect if the meteor bombardment theory was true.
Tungsten isotopes were a good way to tackle the problem because the 182W isotope is created by the radioactive decay of another element, hafnium. This has a comparatively short half-life of around nine million years, and it isn't siderophile. So when the core formed, all the tungsten disappeared, but the hafnium stayed in the mantle and quickly - in geological terms, at least - began to break down and supply more tungsten, albeit only in one of its isotopic forms, 182W. So 182W is relatively much more abundant in the Earth's mantle after the core formed than it was beforehand.
Meteorites have never undergone core formation like the Earth has, so their tungsten isotopic composition is that of the primordial material of the solar system. If the present-day isotopic signature of the Earth's mantle is considered as zero, that of meteorites is lower by 2 parts per ten thousand, whereas the rocks from Greenland have a slightly but significantly positive signature of 13 parts per million.
'We think this is the isotopic composition of the original Earth's mantle, after core formation but before terminal bombardment' says Willbold. The idea is that the meteorites' content brought this primitive signature down to present-day levels.
The researchers say their results fit well with the terminal bombardment hypothesis, but much less well with alternative theories. One of those was that some material from the core - which also has a negative isotopic signature - might have been returned to the surface regions by a mantle plume - a column of hot rock rising from the outer core to near the surface, accounting for the discrepancy in isotope ratios. But looking at rocks from Hawaii and other areas known to have formed above mantle plumes doesn't support this; their isotope ratio is similar to other modern rocks.
Another possible explanation is that a reservoir of older mantle material could have become isolated from the rest of the mantle, preserving a negative isotopic signature, and then later mixed back in to bring down the mantle's overall signature. But Willbold says the numbers don't add up; this explanation would imply a vastly higher tungsten isotope signature for the modern Earth's mantle than what experiments show.
As well as changing the mantle's chemical composition, the researchers say the meteors may also have changed its dynamics, perhaps by forming huge pools of molten rock on the surface, kilometres across, that separated into layers according to relative density. If the densest layers at the bottom eventually sank back into the mantle, this could have set up the convection currents that are still mixing up this region of the Earth to this day. Willbold emphasises that this can't yet be tested, but says it may merit investigation by specialists in modelling mantle dynamics.