Posts Tagged ‘Iron Carbide Fe7C3’

Most of earth’s carbon could be in the inner core as iron carbide Fe7C3

December 2, 2014

The composition of the earth’s inner core is inferred from the speed of passage of seismic waves through the earth. It has generally been taken to be crystalline iron with some small amounts of nickel and lighter elements. But it has been necessary to assume that some part of the inner core is liquid to be able to explain why the S-wave travels at only about half the speed it should.

Now a new paper suggests that some of the core could well be iron carbide Fe7C3. The amounts of iron carbide needed would imply that our understanding of the carbon cycle is still in its infancy. Fully two-thirds of the earth’s carbon could be tied up in the inner core.

Bin Chen et al, Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe7C3 , PNAS, doi: 10.1073/pnas.1411154111

The interior of the Earth (wikipedia)

 

Abstract: Earth’s inner core is known to consist of crystalline iron alloyed with a small amount of nickel and lighter elements, but the shear wave (S wave) travels through the inner core at about half the speed expected for most iron-rich alloys under relevant pressures. The anomalously low S-wave velocity (vS) has been attributed to the presence of liquid, hence questioning the solidity of the inner core. Here we report new experimental data up to core pressures on iron carbide Fe7C3, a candidate component of the inner core, showing that its sound velocities dropped significantly near the end of a pressure-induced spin-pairing transition, which took place gradually between 10 GPa and 53 GPa. Following the transition, the sound velocities increased with density at an exceptionally low rate. Extrapolating the data to the inner core pressure and accounting for the temperature effect, we found that low-spin Fe7C3 can reproduce the observed vS of the inner core, thus eliminating the need to invoke partial melting or a postulated large temperature effect. The model of a carbon-rich inner core may be consistent with existing constraints on the Earth’s carbon budget and would imply that as much as two thirds of the planet’s carbon is hidden in its center sphere.

From the press release:

“The model of a carbide inner core is compatible with existing cosmochemical, geochemical and petrological constraints, but this provocative and speculative hypothesis still requires further testing,” Li said. ” Should it hold up to various tests, the model would imply that as much as two-thirds of the planet’s carbon is hidden in its center sphere, making it the largest reservoir of carbon on Earth.”

It is now widely accepted that Earth’s inner core consists of crystalline iron alloyed with a small amount of nickel and some lighter elements. However, seismic waves called S waves travel through the inner core at about half the speed expected for most iron-rich alloys under relevant pressures.

Some researchers have attributed the S-wave velocities to the presence of liquid, calling into question the solidity of the inner core. In recent years, the presence of various light elements—including sulfur, carbon, silicon, oxygen and hydrogen—has been proposed to account for the density deficit of Earth’s core.

Iron carbide has recently emerged as a leading candidate component of the inner core. In the PNAS paper, the researchers conclude that the presence of iron carbide could explain the anomalously slow S waves, thus eliminating the need to invoke partial melting.

“This model challenges the conventional view that the Earth is highly depleted in carbon, and therefore bears on our understanding of Earth’s accretion and early differentiation,” the PNAS authors wrote. In their study, the researchers used a variety of experimental techniques to obtain sound velocities for iron carbide up to core pressures. In addition, they detected the anomalous effect of spin transition of iron on sound velocities. They used diamond-anvil cell techniques in combination with a suite of advanced synchrotron methods including nuclear resonant inelastic X-ray scattering, synchrotron Mössbauser spectroscopy and X-ray emission spectroscopy.

 

Advertisements

%d bloggers like this: