Early Earth was showered in meteorites containing precious metals left over from the beginnings of the solar system, claims a group of researchers in the UK. This explains why Earth's surface contains far more precious metals than is predicted from our understanding of how the Earth formed. These meteorite impacts may also have triggered the onset of large-scale convection cells in the Earth's mantle – the driving force behind plate tectonics.

As Earth began some 4.56 billion years ago to emerge from the protoplanetary disc surrounding the Sun, its first 100 million years were marked by violence and turmoil. Collisions between Moon- to Mars-sized objects and the Earth caused widespread melting in the Earth's molten oceans, leading iron to become separated from surrounding liquid silicate. The iron began to sink into the Earth's interior, dragging with it metals such as gold and platinum-group elements, which have a high affinity for iron. Geologists believe that between five and ten million years into Earth's history, the vast majority of these metals should have accumulated at the planet's centre, forming the Earth's core. Indeed, it has been estimated that there are enough precious metals in the core to cover the entire surface of the Earth with a 4 m-thick layer.

But researchers have been puzzled to discover that these "iron-loving" elements exist at the Earth's surface, and inside its molten mantle, in much higher quantities than predicted by models and experiments. One proposed explanation is that these extra precious metals could have been delivered via meteorites that continued to crash into the Earth for a long time after the core had become established. Proponents of this theory suggest that an initial period of large impacts – one of which may have created the Moon – was followed by a sustained spell of smaller impacts lasting for 500 million years. This period terminated with a final bombardment around 4–3.8 billion years ago that could have enriched the mantle with a "late veneer" of precious metals.

'Late veneer'

"We still do not have a clear picture of how the Earth formed and what processes took place early on in its history." Matthias Willbold, University of Bristol

In a new study, a group of researchers led by Matthias Willbold at the University of Bristol has added significant weight to this theory after analysing some of the oldest known rocks on Earth – the 3.8-billion-year-old Isua belt in Greenland. These rocks have survived so long without being recycled by Earth's tectonic processes because they were deposited into the heart of a young continent, away from continental subduction zones. Willbold and colleagues realized that because some of these rocks predate the end of the final bombardment of meteorites, they contain a geological record of the mantle at this time. By comparing the chemical composition of these rocks with younger rocks, the researchers set out to discover whether the Greenland rocks contained evidence to support the late-veneer theory.

"We still do not have a clear picture of how the Earth formed and what processes took place early on in its history," Willbold tells physicsworld.com. We don’t have a well preserved geological record from this time so we have to use isotopic and chemical traces like in this study".

In their analysis, the researchers looked at the relative abundance of a particular tungsten isotope, 182-tungsten, which is generated by the decay of the isotope 182-hafnium. When the core formed, tungsten gradually sunk towards the core due to its affinity for iron – similar to all of the precious metals – but the hafnium isotopes continued to decay, enriching the mantle with tungsten. Willbold and colleagues showed that the ancient Greenland rocks from this time are, indeed, enriched in 182-tungsten by 13 parts in 1,000,000 when compared with younger rocks.

Primitive space rocks

The researchers attribute this finding to the fact that younger mantle rocks contain a small contribution from the space rocks that hit Earth in the final meteorite bombardment. Some of these meteorites known as chondrites consist of primitive material that contains relatively lower quantities of 182-tungsten and also higher amounts of precious metals. This terminal bombardment lowered the relative 182-tungsten content of the ancient Earth down to its low present-day level, and replenished the mantle with precious metals.

Thorsten Kleine, a geoscientist at the University of Münster in Germany who specialises in the formation of terrestrial planets, believes that the new findings strongly support the late-veneer hypothesis. "The new data prove that there have been late additions of meteoritic material to the Earth's mantle, because we don't know of any other way to produce the observed tungsten isotope variations," he says.

In addition to explaining the distribution of precious metals within the Earth, these meteorite impacts could also explain how large-scale convection cells became established within the mantle. These rising and falling columns of molten magma are the driving force behind tectonics, causing crust to be created where magma upwells at mid-ocean ridges, and to be destroyed at subduction zones where colder magma sinks deep into the Earth's interior.

Willbold and colleagues speculate that the high energies released by those meteorite impacts would have melted the crust, creating kilometre-wide melt ponds. "At the bottom of these magma lakes, dense and heavy rocks would have formed when these magma lakes crystallized," explains Willbold. Eventually, these heavy bottom layers would have sunk into the mantle and could have provided the thermodynamic disturbance to initiate a convection cell.

This research is described in a paper published in this week's Nature.