Geophys. Res. Lett. https://doi.org/jzg (2012)

The Earth's outer core should be composed largely of molten iron and nickel, but density estimates indicate the presence of at least one lighter element. High-pressure experiments suggest that early in Earth's history, silicon and oxygen could have dissolved into the metallic liquid that formed the outer core from an overlying silicate layer.

Kyusei Tsuno and colleagues at the University of Bayreuth, Germany, performed laboratory experiments at temperatures and pressures similar to those expected at depth in the early Earth. They assessed the partitioning of silicon and oxygen between molten silicate and a molten iron alloy, representative of Earth's mantle and core respectively. In the experiments — which reached up to 25 GPa and 3,080 K — both silicon and oxygen partitioned into the iron pool in similar amounts. Using thermodynamic modelling to extrapolate the results to higher temperatures and pressures, the researchers showed that temperatures of 3,300 to 3,500 K are required for silicon and oxygen to partition into molten iron at levels that can explain the observed density of Earth's outer core.

The release of silicon and oxygen from the outer core as it cools could result in iron oxide and silica enrichments at the base of the mantle.