Like the glaze on a pastry, the Earth, after its inner structure was established, received a final icing of gold to it's crust. The abundance of gold ore make it an easily accessible economic resource. However, this incredibly heavy element, and other iron-loving elements like it, should have sunk 3 000 km deeper, into the Earth's core.
New research, published this month in Nature, lends support to the theory that without a late bombardment of meteorites being stirred in, there would be hardly any siderophile (iron-loving) elements in Earth’s mantle.
Willbold et al, from the Bristol Isotope Group, measured the variations of tungsten in rocks from the Isua Greenstone belt against an average found in modern day mantle. The Isua rocks are unique as they were unaffected by the late bombardment 500 million years ago, but were still formed after the Earth possessed it’s core. In theory, they should have slightly more tungsten compared to rocks that formed whilst the Earth was still accumulating material.
Other theories suggest that, when the Earth collided with large objects, such as the Mars-sized planet now widely accepted to have resulted in our Moon, it caused huge areas of melting. During this melting, iron and other metals seperated out and sank to the core. At the pressure and temperatures found beneath the huge magma ocean, it was thought that siderophiles might lose their affinity to iron and rise up through the mantle. Although, this would not have worked for all of the elements.
Experiments predicted that, to have the current proportion of gold in the crust, we would need 0.5-1% of primitive meteoritic material to have mixed into the mantle. Willbold et al measured tungsten more precisely than ever before, to show that the samples from the Isua Greenstone belt are slightly enriched, compared to average mantle. The magnitude of this small enrichment, 13 parts per million, is exactly that predicted if there had been 0.5-1% of primitive meteoritic material.
Dr Willbold commented that his "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 tonnes of asteroidal material.”
It is possible that the late mixing of material by convection occurred only in large patches of crust. Some areas of the Earth's mantle may not have mixed with late accreted material and so would have remained, and in fact still remain, isolated. The late bombardment might have given us more than just ore deposits, it may have given us the mantle dynamics we still see today.
