Rock and a hard place: X-ray tomography digs deep

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  • Published: Oct 15, 2013
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Rock and a hard place: X-ray tomography digs deep

Percolation, the name of the game?

In a rock and metal sample created by Stanford scientists to mimic the make up of the early Earth mantle, drops of molten iron merge to form a network. In this X-ray tomography image of the sample, the channels labeled in blue are interconnected. Scientists think such a network played an important role in helping grow the planet’s core billions of years ago. (Photo: Crystal Shi / Stanford School of Earth Sciences)

X-ray tomography carried out on simulated rock and metal samples could get deep beneath the surface in helping US scientists to understand the way in which iron percolates into the Earth's mantle and how droplets of iron merge under such conditions to form interconnected networks of material.

Research by Wendy Mao, of Stanford University and the SLAC National Accelerator Laboratory offers new evidence that "percolation" is a plausible mechanism to explain the formation of the Earth's core. Writing in the journal Nature Geoscience, the team explains how a theory proposed about fifty years ago that posited that the high iron content of the core overlaid by a silicate-rich mantle as well as its layered internal structure might have resulted in many steps over the course of millions of years as temperatures and pressures fluctuated.

"We know that Earth today has a core and a mantle that are differentiated," explains Mao. "With improving technology, we can look at different mechanisms of how this came to be in a new light," she adds. Most geologists think that the Earth's origins were messy and chaotic and so explaining how the orderly arrangement of crust, mantle and core arose was a 4.5 billion year old mystery.

Beads or networks?

Of course, one explanation lies in the heat of frequent impacts from interplanetary bodies and the heat they generate as well as the energy released by radioactive decay processes within the Earth. If the planet was heated sufficiently by these processes then the rock and metal within would be sufficiently molten for a "magma ocean” to exist that would separate into distinct layers as a result of their different densities - iron would sink to the bottom while less dense silicates would stay floating on the top. However, other scientists have suggested that even if the early Earth’s temperature were not sufficiently hot to melt silicates, the molten iron matter might still have separated out by percolating through the solid silicate layer. The success of this theory hinged on whether or not pockets of molten iron trapped in the mantle layer could tunnel through the surrounding rock to create channels, or capillaries. This network of tunnels could have helped funnel molten iron towards the planet’s centre to be assimilated into the growing core. Unfortunately, the “percolation” theory was almost disregarded when researchers showed that iron forms, not web-like networks but simply isolated spheres in the upper mantle just as water droplets form "beads" on a waxy surface.

Lower mantle

Now, Mao and colleagues have recreated a speck of the molten silicate and iron material that scientists believe existed deep inside the early Earth. They used a diamond anvil apparatus blasted with laser light to aggregate tiny quantities of iron and silicate rock in a metal chamber. After cooling the resulting product, the team used X-ray-computed tomography to recreate a three-dimensional image of the structure of this object from a sequence of two-dimensional scans. The SLAC X-ray microscope allowed them to obtain nanometre-scale details. The results confirmed that molten iron in the upper mantle tended to form isolated blobs, which would have prevented percolation from happening.

However, the team also found that at the higher pressures and temperatures that would have been present in the early Earth’s lower mantle, the structure of the silicates was changed so that connections could form between pockets of molten iron, making percolation possible. "Scientists had said this theory wasn’t possible, but now we’re saying, under certain conditions that we know exist in the planet, it could happen," Mao explains. "So this brings back another possibility for how the core might have formed."

Related Links

Nature Geosci,  2013, online: "Formation of an interconnected network of iron melt at Earth’s lower mantle conditions"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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