X-ray spectroscopy at high pressure and temperature helps in locating mantle's spin transition zone

Generally, the structure, composition, and dynamics of the Earth's lower mantle are influenced by the electronic state of iron within minerals at high pressure and temperature. For instance, if this electronic state leads to a change in the density of minerals, it would alter the travelling velocity of sound waves. Dr. Jung-Fu Lin (Lawrence Livermore National Laboratory, California, USA) and his colleagues recently determined the spin state of iron in ferropericlase [(Mg_0.75,Fe_0.25)O] at lower-mantle pressures and temperatures using an X-ray emission spectrometer with in-situ synchrotron X-ray diffraction in a laser-heated diamond cell. They found that the transition between high and low spin happens at temperatures ranging from 1,900 to 2,300 K and pressures of up to 95 GPa: conditions found between 1,000 and 2,200 km below the surface. Through integrated absolute difference (IAD) analysis of FeKb spectra, they obtained the ratio of the high-spin to low-spin states in the sample. An energy shift of 1.6 eV in the main emission peak was interpreted as evidence of the spin transition. X-ray spectroscopists might find further significant spectral changes, e.g., Kb' satellite, in their data. For more information, see the paper, "Spin Transition Zone in Earth's Lower Mantle", Jung-Fu Lin et al., Science, 317, 1740-1743 (2007).