Earth's magnetic field is due to convection of its liquid iron-nickel core, which also contains an unknown amount of lighter elements such as silicon, oxygen, and sulfur. Hirose et al. performed experiments that show that silica unexpectedly crystallizes out of a liquid iron alloy at high pressures and temperatures. This discovery identifies a source of compositional buoyancy that would have driven the convection needed for a magnetic field very far back in Earth's history. It also sets a limit on how much silicon and oxygen remain in the outer core today.
Nature 10.1038/nature21367 (2017).