High-pressure X-ray diffraction and X-ray emission studies on iron-bearing silicate perovskite under high pressures

Abstract

Iron-bearing silicate perovskite is believed to be the most abundant mineral of the Earth's lower mantle. Recent studies have shown that Fe 2+ exists predominantly in the intermediate-spin state with a total spin number of 1 in silicate perovskite in the lower part of the lower mantle. Here we have measured the spin states of iron and the pressure-volume relation in silicate perovskite [(Mg 0.6 ,Fe 0.4 )SiO 3 ] at pressure conditions relevant to the lowermost mantle using in situ X-ray emission and X-ray diffraction in a diamond cell. Our results showed that the intermediate-spin Fe 2+ is stable in the silicate perovskite up to ∼125 GPa but starts to transition to the low-spin state at approximately 135 GPa. Concurrent X-ray diffraction measurements showed a decrease of approximately 1% in the unit cell volume in the silicate perovskite [(Mg 0.6 ,Fe 0.4 )SiO 3 ], which is attributed to the intermediate-spin to the low-spin transition. The transition pressure coincides with the pressure conditions of the lowermost mantle, raising the possibility of the existence of the silicate perovskite phase with the low-spin Fe 2+ across the transition from the post-perovskite to the perovskite phases in the bottom of the D layer.

Composition dependence of spin transition in (Mg,Fe)SiO3bridgmanite

“…In contrast to these observations, previous XES studies (Table 1) measured spin transitions in less Fe-rich bridgmanite over this pressure range, indicated by sharper changes in Kβ position and Kβ′ intensity (Badro et al 2004;Catalli et al 2010Catalli et al , 2011Lin et al 2010a;Fujino et al 2012) or much lower Kβ′ intensity (Li et al 2004(Li et al , 2006Lin et al 2008;Mao et al 2011). These studies examined compositions ranging from 5-50% FeSiO 3 and estimated or measured Fe 3+ /ΣFe from 0-100%.…”

“…Any Fe 3+ in the B site would be expected to undergo a spin transition, but a few percent low-spin Fe 3+ would not be detectable by either XES or Mössbauer spectroscopy in this work. In a similar experiment, Lin et al (2010a) suggested a transition in Fe#40 bridgmanite to a low-or intermediate-spin state based on analysis using the IAD method. Their calculated values for the total spin moment are in excellent agreement with our values for Fe#38 bridgmanite using this method (Fig.…”

“…Differences between references from Lin et al (2010b) and secondary references are used to estimate uncertainty. The integrated absolute difference (IAD) method has been used in multiple previous studies of bridgmanite at mantle pressures (e.g., Lin et al 2008Lin et al , 2010aGrocholski et al 2009;Catalli et al 2010Catalli et al , 2011Mao et al 2011) but suffers from underestimation of spin state due to pressure-induced broadening . Broadening effects can be corrected by using the integrated relative difference (IRD) method, which offsets decrease in Kβ′ with increase in intensity in the trough between Kβ′ and the main Kβ peak .…”

“…Pressure-induced spin transitions in bridgmanite were first identified at 70 and 120 GPa by X-ray emission spectroscopy (XES) (Badro et al 2004) and time-domain synchrotron Mössbauer spectroscopy (T-SMS) (Jackson et al 2005). These transitions were suggested to represent changes of high-spin iron to low spin (all electrons paired) (e.g., Badro et al 2004;Lin et al 2010a;McCammon et al 2010), mixed spin (for different Fe atoms, all or no electrons paired) (e.g., Badro et al 2004;Catalli et al 2010), or intermediate spin (some electrons paired) (Lin et al 2008;McCammon et al 2008). Studies have since reported a wide range of sharp or gradual transitions in bridgmanite for different compositions, experimental conditions, and techniques (Table 1).…”

“…Black triangles represent (Mg 0.25 Fe 0.74 Ca 0.01 )SiO 3 -composition bridgmanite while gray triangles represent (Mg 0.61 Fe 0.38 Ca 0.01 )SiO 3 -composition bridgmanite. Gray circles represent measurements of (Mg 0.60 Fe 0.40 )SiO 3 -composition bridgmanite byLin et al (2010a). Direction of triangle points indicates compression (APS 16-ID-D) or decompression (Soleil GALAXIES).…”