Supplementary Information

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1 Supplementary Information Materials and Experiments 57 Fe-enriched enstatite sample [(Mg 0.6,Fe 0.4 )SiO 3 ] was synthesized by mixing powders of the oxides SiO 2, MgO, and 57 Fe 2 O 3 (90% enrichment or better) in the appropriate ratios. The mixed oxides were ground in an agate mortar and pestle under ethanol for twenty minutes, and then cold pressed into pellets measuring about 6 mm in diameter and 5 mm in height. The pellets were placed in a platinum wire basket that was designed to have only a few points of contact with the sample to minimize iron loss into the sample holder. The samples were then heated in a gas-mixing furnace with CO and CO 2 gases at 1295 o C and an oxygen fugacity 1.6 log units below the nickel-nickel oxide buffer (S1). The pellets were held at these conditions for 20 hours, at which point they were quenched by extracting them from the furnace, re-ground as described above, pressed into pellets, and placed back in the furnace. To synthesize a compositional homogeneous enstatite sample with negligible Fe 3+, this cycle was repeated four times. The samples were finally re-ground into a powder in preparation for the high-pressure experiment. Angle-dispersive x-ray diffraction pattern was taken from the powder sample to ensure its crystal structure. The 57 Fe-enriched sample also allowed us to measure its Mössbauer spectra and to determine its Fe 2+ and Fe 3+ contents (see further discussions below). A Be gasket was used to contain the sample and also to allow the Kβ fluorescence signals of approximately 7 kev to transmit to the analyzer and detector. However, the Be gasket alone is not strong enough to maintain a sufficiently thick sample for the laser- 1

2 heating XES experiments at pressures above 100 GPa. In addition, the boron gasket insert previously described (S2-3) appears to be unstable at such conditions. To overcome these technical problems, we have used polycrystalline cubic BN as the gasket insert material. This BN insert with high strength allowed us to prepare samples with sufficient thickness for the laser-heating XES experiments at unprecedented pressure-temperature conditions (SFig. 1). Since long-term laser-heating stability was required in order to collect high-quality XES spectra at the pressure-temperature conditions relevant to the lowermost mantle over an extended period of time (i.e., half a day to a day for a spectrum), two diode-pumped ytterbium fiber lasers with single-mode continuous wave and wavelength 1064nm were used to laser heat the sample from both sides of the DAC. The fiber lasers with the feedback enforced power control, pointing stability, a nearly ideal beam shape, and small divergence were combined with the beam shaping optics to focus the laser beams to flattop shapes of 25 µm in diameter, which provided homogeneous heating, at the sample position in the DAC. During the laser-heating experiments, temperatures were measured spectroradiometrically in approximately every minute; graybody temperatures were determined by fitting the thermal radiation spectrum between 670 and 830 nm to the Planck radiation function. The laser power and the thermal radiation from the laserheated sample were used to stabilize the temperatures in most of the experiments. The temperature uncertainty (1σ), averaged from multiple temperature measurements and temperatures from both sides of the sample across the laser-heated spots, was 50 to 150 K in the experiments. Scanning the samples using the intensity of the Fe Kβ fluorescence peak did not reveal any compositional variation of Fe before and after laser heating. 2

3 Mössbauer Analyses of the Enstatite Starting Sample Polycrystalline enstatite sample of 15 μm thick was mounted on a mylar sheet using clear nail varnish. A Ta foil of 25 μm thick, which absorbed 99% of 14.4 kev gamma rays, with a drilled hole of 150 μm was placed over the sample while looking under a microscope. Based on the 57 Fe-enrichment of 90% in enstatite [(Mg 0.6,Fe 0.4 )SiO 3 ] and the sample thickness of 15 μm, the estimated dimensionless Mössbauer effective thickness was calculated to be 19 for the sample, which corresponds to 47 mg unenriched Fe/cm 2. Mössbauer spectra were recorded at room temperature of 293 K in transmission mode using a constant acceleration Mössbauer spectrometer with a nominal 370 MBq 57 Co high specific activity source in a 12 μm thick Rh matrix at the Bayerisches Geoinstitut, Universität Bayreuth. The velocity scale was calibrated relative to 25 μm thick α-fe foil using the positions certified for (former) National Bureau of Standards standard reference material no. 1541; line widths of 0.36 mm/s for the outer lines of α-fe were obtained at room temperature. The spectrum took 3 days to collect and was fitted using the commercially available fitting program NORMOS written by R.A. Brand (distributed by Wissenschaftliche Elektronik GmbH, Germany). Mössbauer spectrum of the enstatite sample under ambient conditions is similar to those in the literature (S4) and was fitted to Lorentzian doublets using conventional constraints (i.e., equal areas and widths of components) (SFig. 2). The Mössbauer spectrum shows two doublets which can be assigned to the high-spin Fe 2+ in the M1 and M2 sites of the enstatite structure, and hyperfine parameters are consistent with those 3

4 reported in the literature (S2). The M1 site has a chemical shift of 1.16 (+0.02) mm/s (relative to alpha-fe) and quadrupole splitting of 2.63 (+0.05) mm/s with an area ratio of 38%, while the M2 site exhibits a chemical shift of 1.17 (+0.02) mm/s (relative to alpha- Fe) and quadrupole splitting of 1.99 (+0.05) mm/s with an area ratio of 62%. There was no evidence for preferred orientation in the spectrum (seen as unequal areas of doublet components) (SFig. 2) and no evidence for Fe 3+, but trial fits were made incorporating an additional doublet which enabled a detection limit of approximately 3% Fe 3+ /ΣFe to be estimated. Reference X-ray Emission Spectra: Proper references of the X-ray emission spectra for the high-spin and low-spin Fe 2+ are essential in deriving reliable local magnetic moments of iron from the experimental X-ray emission spectra at high pressures and temperatures. To test the reliability of the reference spectra used in this study, we have compared three different reference spectra for the high-spin and low-spin Fe 2+ (SFig. 3) (S3, S5-6). These spectra are essentially identical to each other for the high-spin and low-spin Fe 2+, respectively, insuring the reliability of our derived local 3d magnetic moment of Fe 2+ from the X-ray emission spectra of perovskite and post-perovskite at high pressures and temperatures. We also use the peak position of the Kβ emission line, which is independent of the reference spectra, to evaluate the total spin number independently. 4

5 SFigure 1 Image of the post-perovskite (PPV) sample at 134 GPa and 300 K. The image was taken in transmitted light after the sample had been continuously laser-heated up to 3200 K for more than two days. Cubic boron nitride (BN) was used as the gasket insert with the Be gasket in order to maintain sufficient sample thickness for laser-heating XES experiments at high pressure-temperature conditions over an extended period of time, i.e., days. The sample was sandwiched between two thin NaCl layers of ~5 um in the DAC, which served as the pressure calibrant and thermal insulator. 5

6 SFigure 2 Mössbauer spectrum of the starting enstatite sample [(Mg 0.6,Fe 0.4 )SiO 3 ] at room temperature. The two doublets fitted to the spectrum can be assigned to Fe 2+ in the M1 and M2 sites of the enstatite structure (S4). Solid circles, experimental data; thin lines, model Lorentzian doublets of Fe 2+ in the M1 and M2 sites; thick line, model spectrum. 6

7 SFigure 3 Representative synchrotron Mössbauer spectra (red dots) and modeled spectra (black lines) of iron in enstatite at ambient conditions (a), silicate perovskite at 110 GPa (b), and silicate post-perovskite at 134 GPa (c) with stainless steel as the center shift reference. The thickness of the stainless steel was µm. Corresponding absorption spectra calculated from the fits are shown in the left panels. The pressure of the perovskite had slightly increased to 110 GPa from 108 GPa for the Mössbauer experiments. Both perovskite and post-perovskite display extremely high QS and relatively high CS. The spectra were evaluated using the CONUSS program (S7). QS and IS values of these samples are tabulated in STable 1. 7

8 SFigure 4 Reference X-ray emission spectra of Fe 2+ in (A) Enstatite [(Mg 0.6,Fe 0.4 )SiO 3 ], (B) Ferropericlase [(Mg 0.75,Fe 0.25 )O (S3), (C) Molecular Fe 2+ [Fe(phen) 2 (NCS) 2 ] (S5-6). Red lines, high-spin state; blue lines: low-spin state. The high-spin spectra were collected at ambient conditions. The low-spin spectrum of enstatite was collected at 108 GPa, while the low-spin spectrum of ferropericlase was collected at 79 GPa. These spectra were used as references for deriving the average spin number of Fe 2+ in post-perovskite and perovskite at high pressures and temperatures. Since similar instrumental conditions between the references and the PV and PPV spectra are essential for deriving reliable spin number values, we used enstatite spectra in (A), which were collected under the same conditions as the sample spectra, as our main references. 8

9 SFigure 5 Schematic crystal-field splitting diagrams of the electronic structures of the 3d orbitals of Fe 2+ in the dodecahedral site (A site; 8-12 site) of the silicate perovskite (S8, S9). The figure depicts a simplified, non-distorted environment of the dodecahedral site in which the five 3d energy levels are split into three upper t 2g and two lower e g levels, separated by the crystal-field splitting energy (Δ C ). The energy difference between the opposite spins, as shown by red (spin up) and blue (spin down), on each orbital is the spin-pairing energy (П). Depending on the relative energies of Δ C, П, and the splitting energies between the individual t 2g and e g orbitals, the six 3d electrons can occupy three different three states, the high-spin Fe 2+ with four unpaired electrons (S=2), the intermediate-spin Fe 2+ with two unpaired electrons (S=1), or the low-spin Fe 2+ with all six electrons paired (S=0). For simplicity, the energies of the t 2g and e g levels are not drawn to scale in the diagrams. The electronic transition of Fe 2+ from S=2 to S=1 is observed in perovskite in this study, while post-perovskite and perovskite exhibit S=1 under lowermost-mantle pressure-temperature conditions. The S=1 state is called the intermediate-spin state here for historical reasons (S8-12). Fe 2+ site distortion is required to further split the energy levels to possibly reach S=0 in Fe 2+. 9

10 SFigure 6 Spin states of Fe 2+ in post-perovskite and perovskite under relevant lowermantle pressure-temperature conditions. Fe 2+ is known to exist in the high-spin state in perovskite at ambient conditions (S8-11), and a high-spin to intermediate-spin transition occurs at pressures of approximately 30 GPa and room temperature (S12). The intermediate-spin post-perovskite and perrovskite exist at pressure-temperature conditions of the lowermost mantle. The composition of perovskite and post-perosvkite in the experiments was (Mg 0.6,Fe 0.4 )SiO 3. The iron content in our samples is likely below the percolation threshold where the effect of the iron-iron interaction is generally negligible to have any effect on the spin transition pressure. Thus, Fe 2+ in silicate perovskite and post-perovskite with iron content close to the modeled lower-mantle composition such as (Mg 0.9,Fe 0.1 )SiO 3 would be in the intermediate-spin state under lowermost-mantle conditions. Black line, a model lower-mantle geotherm (S13); blue triangles, intermediate-spin post-perovskite; blue circles, intermediate-spin perovskite; red circle, high-spin perovskite at ambient conditions (S12). 10

11 STable 1 Derived quadrupole splitting (QS) and center shift (CS) of iron in the starting enstatite, perovskite at 110 GPa, and post-perovskite at 134 GPa and 300 K. The CS of the iron sites were measured with respect to the stainless steel, and have been converted to values with respect with iron foil in the table. The pressure of the perovskite had slightly increased to 110 GPa for the Mössbauer experiments. Uncertainty is approximately mm/s for the QS and mm/s for the CS. The QS and IS values of the enstatite sample derived from the traditional Mössbauer and synchrotron Mössbauer spectra are consistent with each other within experimental uncertainties. Previously reported QS and CS values of Fe, Fe 2+, and Fe 3+ in enstatite, perovskite, and other minerals, together with our XES and SMS results, are used to assign the spin and valence states of iron in these samples (S4,S8,S10-12,S14-15). HS, high-spin state; IS, intermediate-spin state. Specific assignment of the third, minor site in post-perovskite at 134 GPa remains to be understood. Sample % amount (Assignment) QS (mm/sec) IS (mm/sec) Enstatite at ambient 24 (HS Fe 2+ ) (HS Fe 2+ ) PV at 110 GPa 92 (IS Fe 2+ ) (IS Fe 2+ ) PPV at 134 GPa 70 (IS Fe 2+ ) (IS Fe 2+ ) (?)

12 References: S1. Holzapfel, C., Rubie, D.C., Mackwell, S., & Frost, D.J. Effect of pressure on Fe Mg interdiffusion in (Fe x Mg 1 x )O, ferropericlase. Phys. Earth Planet. Int. 139, (2003). S2. Lin, J.F., Shu, J., Mao, H.K., Hemley, R.J., & Shen, G. Amorphous boron gasket in diamond anvil cell research. Rev. Sci. Instrum. 74, (2003). S3. Lin, J.F. et al. Spin transition zone in Earth s lower mantle. Science 317, (2007). S4. Van Alboom, A., De Grave, E., & Vandenberghe, R.E. Study of the temperature dependence of the hyperfine parameters in two orthopyroxenes by 57 Fe Mössbauer spectroscopy. Phys. Chem. Minerals 20, (1993). S5. J. Badro et al. Electronic transitions in perovskite: possible nonconvecting layers in the lower mantle. Science 305, (2004). S6. Vankó et al. G. Probing the 3d spin momentum with X-ray emission spectroscopy: the case of molecular-spin transitions. J. Phys. Chem. B 110, (2006). S7. Sturhahn, W. CONUSS and PHOENIX: Evaluation of nuclear resonant scattering data. J. Phys.: Condens. Matter 16, (2000). S8. Burns, R.G. Mineralogical Applications of Crystal Field Theory, Cambridge Univ. Press, Cambridge, U.K. (1993). S9. Badro, J. et al. Electronic transitions in perovskite: possible nonconvecting layers in the lower mantle. Science 305, (2004). 12

13 S10. Li, J. et al. Electronic spin state of iron in lower mantle perovskite. Proc. Natl. Acad. Sci. U.S.A. 101, (2004). S11. Jackson, J.M. et al. A synchrotron Mössbauer spectroscopy study of (Mg,Fe)SiO 3 perovskite up to 120 GPa. Amer. Miner. 90, (2005). S12. McCammon, C. et al. Intermediate-spin ferrous iron in lower mantle perovskite. submitted Nature Geosciences (2008). S13. Boehler, R. High-pressure experiments and the phase diagram of lower mantle and core materials. Rev. Geophys. 38, (2000). S14. Fei, Y., Virgo, D., Mysen, B.O., Wang, Y. & Mao, H.K. Temperature dependent electron delocalization in (Mg,Fe)SiO3 perovskite. Am. Miner. 79, (1994). S15. Lauterbach, S., McCammon, C.A., van Aken, P., Langenhorst, F. & Seifert, F. Mössbauer and ELNES spectroscopy of (Mg,Fe)(Si,Al)O 3 perovskite: A highly oxidised component of the lower mantle. Contrib. Mineral. Petrol. 138, (2000). 13