Numerous seismic data have long puzzled scientists: Earth's core appears too light to be solely made up of pure iron and nickel. Various hypotheses postulate that the core contains a number of lighter elements such as carbon, oxygen, silicon or sulfur. Since the core lies at a depth of 2,900 km beneath the Earth's surface, it is impossible to have access to physical samples. So how can its composition be determined? One solution is to analyze the signature left by various compounds in the chemistry of Earth's mantle when they migrated to the center of the planet, forming the core.
During the Earth's formation, high-energy impacts melted the Earth's mantle, creating a magma ocean. In this ocean, metals, which are heavier, separated from the silicates due to their difference in density, and migrated under the effect of gravity towards the center of the Earth, forming the core.
Sulfur is too volatile to leave this type of signature, since it readily enters the gaseous state. So the researchers used copper, a chemical element that is chalcophile (in other words it has a strong affinity with sulfur) in order to track the fate of sulfur as it moved into the Earth's core.
The researchers used a large number of samples of mantle-derived lavas and crustal rocks in order to determine their copper isotopic composition and hence that of Earth's mantle. They then compared these results with the isotopic composition of primitive meteorites. Such meteorites are representative of the overall composition of the Earth, as if it had not differentiated into core and mantle. The researchers discovered that the mantle is depleted in isotopically light copper in comparison with the meteorites (which thus represent the isotopic composition of the mantle and core). They interpret these results as being due to the separation of a sulfur-rich liquid (rich in isotopically light copper) from the other chemical compounds at the end of crystallization of Earth's mantle. This sulfur-rich liquid, denser than the rest of the mantle, is thought to have descended to the base of the mantle and have subsequently become incorporated into the core. Based on the difference in isotopic composition between the mantle and the core, the researchers estimate that the quantity of sulfur-rich liquid that segregated from the mantle accounts for 0.5% of the core.
To confirm their results, the researchers carried out a series of laboratory experiments. They reconstructed the chemical composition of the Earth and subjected it to the same temperature and pressure conditions as those that existed during the separation of the core and mantle. Analysis of the isotopic composition of the materials (sulfides and silicates) produced in these experiments supports the hypothesis that a sulfur-rich liquid did segregate from the components of the early Earth's mantle.
These findings, which provide evidence of the presence of sulfur in Earth's core, were obtained using this novel method based on copper isotope analysis. The researchers hope to adapt it to all sorts of environments, including that of other planets such as Mars, once samples of materials from other planetary mantles are available for comparison.
Citation: Copper isotope evidence for large-scale sulphide fractionation during Earth's differentiation. P.S. Savage, F. Moynier, H. Chen, J. Siebert, J. Badro, I.S. Puchtel, G. Shofner. Geochemical Perspective Letters, 17 June 2015.