ScienceDaily (Nov. 23, 2011) — The composition of Earth's core remains a mystery. Scientists know that the liquid outer core consists mainly of iron, but it is believed that small amounts of some other elements are present as well. Oxygen is the most abundant element in the planet, so it is not unreasonable to expect oxygen might be one of the dominant "light elements" in the core. However, new research from a team including Carnegie's Yingwei Fei shows that oxygen does not have a major presence in the outer core. This has major implications for our understanding of the period when Earth formed through the accretion of dust and clumps of matter.
http://www.sciencedaily.com/releases/2011/11/111123133137.htm