Earth's lower mantle is mainly composed of iron-bearing magnesium silicate perovskite (Mg,Fe)SiO3, which is the most abundant phase (about 80% by volume), and magnesiowüstite (Mg,Fe)O. Iron in magnesiowüstite undergoes a spin-pairing transition between 60 and 70 GPa. We measured the spin state of iron in magnesium silicate perovskite (Mg0.9Fe0.1)SiO3 at high pressure and found two electronic transitions occurring at 70 and 120 GPa, corresponding to partial and full electron pairing in iron, respectively. The measurements were performed on beamline ID16, studying the Kb emission line.

One of the main characteristics of low-spin (or spin-paired) iron-bearing minerals resides in the blue-shift of iron absorption bands (the absorption bands initially in the infrared and red region shift to the green-blue region). The proportion of iron in the low spin state thus grows with depth, increasing the transparency of the mantle in the infrared region, with a maximum at pressures consistent with the D" layer (the lowermost 300 kilometres of the mantle, a structurally and chemically complex layer sitting just above the core-mantle boundary). The resulting increase in radiative thermal conductivity suggests the existence of nonconvecting layers in the lowermost mantle.

Our experimental study of its major constituents suggests that the Earth's lower mantle is in a fundamentally different thermochemical state above 70 to 90 GPa, corresponding to depths greater than 1700 to 2000 km. This is due to a change in the electronic properties of iron in these phases at such pressures. The changes can have profound implications on the chemistry and dynamics of the lowermost mantle. They indicate that the lower mantle could be separated into three distinct regions in different thermochemical states (Figures 14 and 15):

  • above 1700 km (below 70 GPa), the "normal" state, where iron is in the high-spin state in both lower-mantle compounds
  • between 1700 and 2600 km (between 70 and 120 GPa), the "transitional" state, where iron can be in the low-spin state in magnesiowüstite, and partially in the high-spin state in perovskite
  • below 2600 and to the core-mantle boundary (between 120 and 135 GPa), the "profound" state, where iron is in the low-spin state in both lower-mantle compounds


Fig. 14: X-ray emission spectra collected on magnesium silicate perovskite (Mg0.9Fe0.1)SiO3 between 20 and 145 GPa. The spin state of iron transforms twice at 70 and 120 GPa, as indicated by the changes in Kß' line intensity. Moreover, the position of the Kß1,3 line shifts at each transition, and by a total of -0.75 eV between 20 and 145 GPa, which is in agreement with a high-spin to low-spin (HS­LS) transition in iron. The spectra have been vertically shifted (each group separately) for clarity; the first (bottom) group is characteristic of the HS state, the second (middle) is characteristic of the mixed state (mixture of HS and LS iron), and the third (top) is characteristic of the LS state. The solid lines are models constructed from reference molecular compounds, and are not fitted to the data.




Fig. 15: Average spin number on the iron atom as a function of pressure, derived from the intensity of the Kß' line (left) and the position of the Kß1,3 line (right). The blue and red lines are tentative curves (within the error bars) in the intermediate state plateau.


Interestingly, these distinct regions have geophysical signatures that have been previously reported. The "transitional" region corresponds to depths where chemical heterogeneities have been observed by seismic tomography. Thermodynamic modelling of the partitioning of iron in this state indicates that the spin transitions at 70 GPa could promote large scale chemical heterogeneities due to lateral temperature heterogeneities.

The depths and pressures of the "profound" region are in concordance with that of the D" layer. The transition pressure (120 GPa) is also in accord with a recently reported crystallographic transition in perovskite [1], and so is our estimated Clapeyron slope of 120 K/GPa [2]. The latter value is also in agreement with the seismically constrained [3] value of the Clapeyron slope at the D" layer of 170 K/GPa. All these concordances strengthen the suggestion that the present observation could provide a mineral-physics basis for the Earth's D" layer. In the "profound" state, the lower-mantle mineralogical assemblage has an increased radiative conductivity, and should be associated with a strong dynamical signature since the increase in thermal conductivity may hinder convection and favour layering in the lowermost mantle.

[1] Murakami et al., Science 304, 855 (2004).
[2] Iitaka et al., Nature 430, 442 (2004).
[3] Sidorin et al., Science 286, 1326 (1999).

J. Badro (a), J.-P. Rueff (b), G. Vankó (c), G. Monaco (c), G. Fiquet (a), F. Guyot (a), Science 305, 383 (2004); Science 300, 789 (2003).
(a) Laboratoire de Minéralogie Cristallographie, Université Paris VI, Institut de Physique du Globe, Paris (France)
(b) Laboratoire de Chimie Physique­Matière et Rayonnement, Université Paris VI, Paris (France)
(c) ESRF