A synchrotron Mössbauer spectroscopy study of (Mg,Fe)SiO3 perovskite up to 120 GPa

The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe0.05Mg0.95SiO3 (Pv05) and Fe0.1Mg0.9SiO3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mössbauer spectroscopy (SMS). Such investigations of extremely small and dilute Fe-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the “three-doublet” model, which assumes two Fe-like sites and one Fe-like site that are well distinguishable by the hyperÞ ne Þ elds at the location of the Fe nuclei. At low pressures, Fe/ΣFe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fe remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO3 perovskite. The quadrupole splittings of all Fe sites Þ rst increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe from a highspin to a low-spin state. JACKSON ET AL.: (Mg,Fe)SiO3 PEROVSKITE UP TO 120 GPA USING SMS 200 Mössbauer spectroscopy, because the signals are too low. Recent developments in synchrotron Mössbauer spectroscopy (SMS) have led to an increase in photon ß ux density by many orders of magnitude (Gerdau and Waard 1999/2000; Sturhahn 2004). Agreement between SMS and MBS has been explicitly demonstrated with powdered samples (e.g., Alp et al. 1995). SMS was used to characterize heterogeneous electron-transfer kinetics, and analysis of reaction end-member specimens by both SMS and MBS yielded comparable Mössbauer parameters, such as Fe/Fe area ratios (Amonette et al. 2003). SMS was also used to monitor the electronic state of Fe in hedenbergite (CaFeSi2O6) at pressures up to 68 GPa in a diamond anvil cell (Zhang et al. 1999). The goal of this study is to observe the quadrupole splitting and weight fraction at the sites of the Fe nuclei to monitor the valence states of iron in (Mg,Fe)SiO3 perovskite under the pressure conditions where it is considered to be stable in the Earthʼs interior. In the present study, we extended the sensitivity limits of SMS and measured the electronic state of iron in (Mg,Fe)SiO3 perovskite between ambient pressure and 120 GPa using diamond anvil cells.