High-temperature compression of ferropericlase and the effect of temperature on iron spin transition

High-temperature compression experiments with in situ X-ray diffraction of ferropericlase (Fp) with a composition of (Mg0.81Fe0.19)O were made in a laser-heated diamond anvil cell to pressures (P) of 116 GPa at a constant temperature (T) of 1600–1900 K. Room-temperature experiments with a laser annealing technique were also carried out on the same material. Anomalous unit-cell volume reductions that can be explained by the spin transition of ferrous iron were observed at P = 63–96 GPa and 45–63 GPa at T = 1600–1900 K and 300 K, respectively, indicating that the spin transition pressure interval expands with increasing temperature. The observed density changes across this spin transition at T = 1600–1900 K and 300 K are about 1.6% and 1.0%, respectively, indicating that the spin transition pressure interval expands with increasing temperature. The thermal expansivity of Fp is large in the mid-lower mantle due to the effect of the spin transition. In a peridotitic composition, the spin transition in Fp increases the rock density by 0.35% at the lowermost mantle conditions. Calculated densities show that both perovskitic and peridotitic mantle models may explain the PREM lower mantle density. However, the peridotitic lower mantle model requires less assumption to satisfy the PREM density and is more self-consistent.