Low-spin Fe 2+ in silicate perovskite and a possible layer at the base of the lower mantle

Abstract

We investigated the spin state of iron in Mg 0.82Fe 0.18SiO 3 silicate perovskite using Mössbauer spectroscopy and nuclear forward scattering (NFS) at pressures up to 130 GPa and temperatures up to 1000 K. Majorite starting material was loaded into diamond anvil cells in three separate experiments, and transformed to silicate perovskite through laser heating. We found, in agreement with previous work, the predominance of a component with high isomer shift (∼1 mm/s relative to α-Fe) and high-quadrupole splitting (QS) (>4 mm/s) in Mössbauer and NFS spectra up to 115 GPa at room temperature, and in accordance with previous work this component was assigned to intermediate-spin Fe 2+. At higher pressures, the intensity of the high QS component in the silicate perovskite spectrum decreased, while the intensity of a new component with low isomer shift (∼0 mm/s relative to α-Fe) and low quadrupole splitting (<0.5 mm/s) increased. This new component was assigned to low-spin Fe 2+, and its intensity increased with both increasing pressure and increasing temperature: at 120 GPa and 1000 K all Fe 2+ was in the low-spin state. X-ray diffraction data showed well crystallised perovskite in all runs, and although the stable phase above ∼110 GPa is expected to be post-perovskite, sluggish transition kinetics likely preserved the perovskite phase in a metastable state. Our results combined with data in the literature and thermodynamic and topological considerations suggest that there may be a region where silicate perovskite containing low-spin Fe 2+ is stable, which coincides with predicted pressure–temperature conditions near the D″ layer.