Nanoscale 3-D images shed light on Earth’s core

Physicists can study the Earth’s inner history, thanks to a technique that provides a picture of minerals interacting at ultrahigh temperatures and pressures.

The new method, high-pressure nanoscale x-ray computed tomography, is being developed at SLAC National Accelerator Laboratory and has enabled Wendy Mao, a mineral physicist at Stanford University, to look at the separation of Earth’s mantle from its iron-rich core approximately 4.5 billion years ago.

Mao has obtained unprecedented 3-D detail of changes in the shape and texture of molten iron and solid silicate minerals under the same intense pressures and temperatures they would encounter deep underground. She presented the results of the first experiments with the technique at the American Geophysical Union’s annual meeting in December 2010.

Combining a diamond anvil cell, which compresses tiny samples between the tips of two diamonds, with nanoscale x-ray CT has allowed SLAC researchers to capture images of material at high pressure. At millions of times atmospheric pressure, only diamond can exert the necessary pressure without breaking under the force.

“It is pretty exciting, being able to measure the interactions of iron and silicate materials at very high pressures and temperatures, which you could not do before,” Mao said.

Physicists have been investigating the elements that make up Earth, trying to determine how the mantle and the core separated from and squeezed past each other. If the planet got hot enough to melt both elements, the difference in density could have sent iron to the bottom and silicates to the top. If the planet did not heat up enough for that, one theory holds that molten iron could have been able to move along the boundaries between grains of the solid silicates. But previous experimental work has shown that at low pressure, iron forms isolated spheres, Mao said, and spheres could not percolate through solid silicate material.

She said the results of her experiments using the CT technique suggest that, at high pressure, the silicate transforms into a different structure, and the iron takes on a more elongated, plateletlike form, spreading out on the surface of the silicate, where it connects and forms channels instead of isolated spheres.