Lying between 650 and 2800 km beneath the Earth’s surface, the lower mantle makes up over half of the Earth’s volume and is made mostly from compounds containing the elements oxygen, magnesium and silicon. Roughly 5%, however, is iron, contained in compounds such as ferropericlase (an iron-magnesium oxide) and silicate perovskite (an iron-magnesium silicate).

Scientists know that the spin states of iron’s electrons help govern physical properties of the mantle such as its density and the speed with which sound passes through it. Recently theorists have shown that the spins should change gradually from “high” to “low” states as the pressure and temperature increases with depth up to 135 GPa and 4000 K at the bottom of the lower mantle. But now Jung-Fu Lin from Lawrence Livermore National Laboratory and colleagues from other US institutions and the European Synchrotron Radiation Facility in France have shown that the transition is a little more sudden.

Lin’s group subjected ferropericlase to varying pressures in a diamond anvil cell while heating it up with a laser, and charted the spin states using X-ray emission spectroscopy. They found that there is a definite transition region where the spin states swap over, between 1000 and 2200 km in depth. “Our results indicate the occurrence of a new layer that is defined by the spin transition, called the spin transition zone,” said Lin. The researchers think that silicate perovskite — the more prevalent source of iron in the lower mantle — should also exhibit the spin transition zone, although they have not tested it yet.