Recoil-free fractions of iron in aluminous bridgmanite from temperature-dependent Mössbauer spectra

Abstract

Aluminous bridgmanite (Al-Bm) is the dominant phase in the Earth’s lower mantle. In this study, the Mossbauer spectra of an Al-Bm sample Mg0.868Fe0.087Si0.944Al0.101O2.994 were recorded from 65 to 300 K at 1 bar. The temperature dependence of the center shift was fitted by the Debye model and yielded the Debye temperatures of 305 ± 3 K for Fe2+ and 361 ± 22 K for Fe3+. These values are lower than those of Al-free bridgmanite by 17 and 24%, respectively, indicating that the presence of Fe and Al increases the average Fe-O bond length and weakens the bond strength. At 300 K, the calculated recoil-free fractions of Fe2+ (0.637 ± 0.006) and Fe3+ (0.72 ± 0.02) are similar and therefore the molar fractions of Fe2+ and Fe3+ are nearly the same as the area fractions of the corresponding Mossbauer doublets. At 900 K, the calculated recoil-free fractions of Fe3+ is 46% higher than that of Fe2+, implying that the molar fraction of Fe3+ is only 41% for a measured spectral area fraction of 50%, and that the area fractions of iron sites may change with temperature without any changes in the valence state or spin state of iron. We infer that Fe3+ accounts for 46 ± 2% of the iron in the Al-Bm and it enters the A site along with Al3+ in the B site through the coupled-substitution mechanism. An Fe2+ component with large quadrupole splitting (~4.0 mm/s) was observed at cryogenic conditions and interpreted as a high-spin distorted iron site.