Carbon at Earth's Core

December 7, 2012

I once joked that a physicist is the type of scientist who tries to find steel in the Periodic Table. Since most physicists deal with the immaterial, such as fields, they can be excused. It's only their solid state brethren who work with actual materials. Steel, of course, is an alloy of carbon and iron, and this useful material has been with us for several centuries.

The Iron Age, the last of the three ages, began at about 1000 BC. The Iron Age was preceded by the Stone Age and the Bronze Age. There is no precise dating for the Iron Age, since iron foundry started at different times in different regions of the world. This first iron contained little carbon, so none of it was steel in the modern sense. Steel is essentially a product of the Industrial Revolution.

Metallurgy is so important to civilization that the ancients identified the Ages of Man with the succession of metals in common use; principally, gold->silver->bronze->iron. Hesiod, in his Works and Days, saw a parallel between the progression from noble to base metal and the apparent debasing of human existence (see figure).

"For now truly is a race of iron, and men never rest from labor and sorrow by day, and from perishing by night; and the gods shall lay sore trouble upon them." [Hesiod, Works and Days, ll. 176-178, refs. 1-2]. (Via Project Perseus).

Carbon atoms (atomic radius = 70 pm) are much smaller than iron atoms (atomic radius = 140 pm). Since their atomic radii are in a 1:2 ratio, the atomic volumes differ by a factor of eight. This size difference allows carbon atoms to populate the small interstices of the iron body-centered cubic crystal lattice (ferrite), but not to too great an extent. The maximum solid solubility of carbon in ferrite is just 0.02 weight-%, as the phase diagram shows. Carbon is still very miscible in liquid iron.

The iron-rich portion of the iron-carbon phase diagram. The maximum solid solubility of carbon in iron is just 0.02 weight-%. (Figure by Christophe Dang Ngoc Chan, slightly modified, via Wikimedia Commons).

Such phase diagrams represent alloys at standard pressure (about 1 atmosphere); and, since most solid components of alloys are quite incompressible, they're a good representation for even higher pressures. The center of the Earth is another environment altogether, with a temperature of about 5430 °C and a pressure of about 350 gigapascals (3,450,000 atmospheres).

Because of the high pressure, the inner core of the Earth is solid. It's surrounded by an outer core of liquid. The inner and outer cores are composed primarily of iron with about 4% nickel. These two metals together don't quite account for the core's density, so it's presumed that there are some lighter elements dissolved. There are chemical reasons to suppose that oxygen and sulfur are the lighter elements. It's very difficult to remove oxygen and sulfur from iron.

A cutaway diagram of the Earth, showing the core and other components. This diagram, except for the surface features, is to scale. (Modified Wikimedia Commons image).

Various studies have been done in an attempt to estimate the carbon concentration of Earth's core. Concentration estimates for carbon in iron at relevant pressures and temperatures have ranged from about 7.5 wt.-% (2410 °C, 2 GPa) for the liquid, outer core,[3] to 2.6-3.7 wt.-% for the solid, inner core.[4] This open question has prompted a recent first-principles molecular dynamics study that gives a far lower estimate of 0.1–0.7 wt-% for carbon concentration in the inner core.[5-6]

This study, published in a recent issue of the Proceedings of the National Academy of Sciences by Yigang Zhang of the Chinese Academy of Sciences (Beijing, China) and Qing-Zhu Yin of the Department of Geology, University of California (Davis, California), looks particularly how easily carbon will dissolve in silicate, as found in Earth's crust, as opposed to the iron of Earth's core.[5]

The quantity of core carbon is important to the composition of Earth's crust, since the presence of carbon would have determined how much of other light elements would have dissolved in the core. What was dissolved in the core would not have been present for crustal formation.[5]

References:

1. Greek text from Project Perseus; source, Hesiod, "Works and Days," from The Homeric Hymns and Homerica with an English Translation by Hugh G. Evelyn-White (Harvard University Press, Cambridge, MA., and William Heinemann Ltd., London), 1914.
2. English translation of ref. 1.
3. Rajdeep Dasgupta and David Walker, "Carbon solubility in core melts in a shallow magma ocean environment and distribution of carbon between the Earth's core and the mantle," Geochimica et Cosmochimica Acta, vol. 72 (2008), pp. 4627-4641 (PDF file).
4. Zulfiya G. Bazhanova, Artem R. Oganov and Omar Gianola, "Fe-C and Fe-H systems at pressures of the Earth's inner core," arXiv Preprint Server, June 3, 2012.
5. Yigang Zhang and Qing-Zhu Yin, "Carbon and other light element contents in the Earth's core based on first-principles molecular dynamics," Proc. Natl. Acad. Sci., vol. 109 no. 48 (November 27, 2012), pp. 19579-19583
6. Earth's Core & Carbon: Planetary Center Holds Largest Store Of Common Element, Computer Simulation Suggests, Our Amazing Planet, via Huffington Post, November 23, 2012.