Early Earth’s ‘golden shower’

Rocks from Isua in south-west Greenland have been hailed as providing evidence for what geologists believe is the source of complex and heavy elements on Earth: an asteroid shower that endowed our young planet with gold (as well as platinum, iridium, nickel and tungsten).

The research to be published in Nature suggests that the different makeup of rocks from Earth’s mantle of 4.8 billion years ago and 3.8 billion years ago suggests a protracted meteor bombardment sometime around 3.9 billion years ago laced the crust with billions of tons of dead-star residues that we now mine.

The researchers, led by Matthew Willbold of the University of Bristol, sampled rocks from the earliest crust – formed when Earth cooled enough to actually have a solid rather than a liquid surface – and compared these to newer rocks.

In particular, they have identified different concentrations of tungsten in the different ages of rock, which serve as markers of the heavy metal arriving in the long window from 4.8 to 3.8 billion years ago.

If the metals had arrived on a molten planet, they would not have survived to be distributed in the mantle, instead sinking to the core.

Discovery quotes Willbold as saying the rocks provide “a sort of time capsule that gave us the possibility to calculate how much material had to be added to the Earth to satisfy the tungsten isotopic composition that we find in the Earth today.”

Around one-half of one percent of today’s mantle arrived in the form of meteors, he said, which equals about 300 billion billion tons of material.

Damping the excitement with a contrary view, New Scientist notes that there appears to be too little precious metals for the hypothetical bombardment to be a complete explanation. ®