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1 doi: /nature Effect of Thermal Diffusion It is known that extensive chemical segregation could occur in laser-heated DAC samples due to thermal diffusion, often called Soret diffusion, under large temperature gradients 1-3. In order to examine the effect of such thermal diffusion, we have carried out a couple of experiments, one at sub-solidus and one at super-solidus temperature, in which the sample was heated for relatively long duration. The first experiment was performed at sub-solidus condition of 77 GPa and 3500 K. The sample was heated for 5 sec. Even in such a short period, the recovered sample exhibited a significant chemical segregation (Supplementary Fig. 3a); Si was enriched at the center, whereas both Mg and Fe migrated to the edge of the laser-heated spot. As a consequence, the central part of the sample consisted of single-phase perovskite. Metallic iron was also present at the margin, where the temperature gradient should have been strong 4. The second sample was heated to above melting temperature for 10 sec at 25 GPa. The x-ray maps demonstrated that extensive chemical segregation occurred within the melt pocket (Supplementary Fig. 3b); high Si/Mg melt at the center and low Si/Mg melt on both sides. Furthermore, Fe was enriched at the margin of the sample, which is in strong contrast to that observed in separate experiment with <1 sec heating at 32 GPa (Fig. 1a). These results indicate that relatively long heating duration causes extensive chemical segregation due to thermal diffusion. Nevertheless, it could be avoided when the sample was heated only for <1 sec. In such a case, (1) the quenched melt pocket was chemically homogeneous, and (2) iron was always more enriched in the melt pool (see the x-ray maps in Fig. 1). 1

2 2. Attainment of Chemical Equilibrium Since the sample was heated only for <1 sec above the melting temperature in the present experiments, it is important to verify the attainment of chemical equilibrium. The diffusion coefficient of Si, the slowest species in liquid Mg 2 SiO 4, has been calculated to be m 2 /sec at 80 GPa and 4000 K, similar to conditions in our experiments (ref. 5). This implies a diffusion length of 43-µm in 1 sec, which is longer than the radius of the melt pocket (typically 5 to 10-µm). In addition, the Fe-Mg interdiffusion coefficient in perovskite is m 2 /sec at 80 GPa and 4000 K according to Holzapfel et al. 6, suggesting a diffusion length of ~95-nm in 1 sec, which is comparable to the typical grain size of perovskite in the DAC experiments. Thus the conditions for equilibrating melt pockets and crystals having the dimensions found in our experiments are satisfied even with only 1 sec heating duration, while still being short enough to avoid extensive compositional segregation that would accompany excessive Soret diffusion, as discussed above. We performed an additional melting experiment at 1 bar using the same starting material: (Mg 0.89 Fe 0.11 ) 2 SiO 4 olivine (run #48). The heating duration inside the DAC was <1 sec, the same as our other experiments at high pressure. The recovered sample showed that the large quenched melt pocket (>30-µm) with (Mg,Fe) 2 SiO 4 composition (Mg# = 86.0±0.5) was surrounded by olivine with Mg# = 96.8±0.4 (Supplementary Fig. 4). The result is indeed in good agreement with the well-known liquid-solid two-phase loop in Mg 2 SiO 4 -Fe 2 SiO 4 binary system. Furthermore, the Fe-Mg distribution coefficients K D between coexisting ferropericlase and melt were determined to be 0.85 and 0.60 at 20 and 32 GPa, respectively (Fig. 1a and Supplementary Table 1). They are consistent with the previous experiments using multi-anvil apparatus performed by Ohtani et al. 7, which reported K D values of 0.75 and 0.69 at 18 and 25 GPa, respectively. The K D between perovskite and melt at 36 GPa was found to be 0.251±0.046, in reasonably good agreement with the 2

3 previous report by Corgne et al. 8 of K D = 0.304±0.035 obtained in Al-free peridotite bulk composition at 25 GPa using the multi-anvil apparatus. These suggest that the present melting experiments should have attained chemical equilibrium between melt and coexisting solid phase for a 1 sec heating duration. 3. Valence State of Iron in Melt A certain amount of metallic iron was found in the quenched melt pocket formed above 36 GPa (Fig. 1b, c), where perovskite/post-perovskite appeared as the liquidus phase. At the same time, the high-resolution TEM images showed contrasting textures depending on the pressure of synthesis; a melt pool formed at 20 GPa (run #21) was quenched to glass, whereas those produced at 46 and 73 GPa (runs #18 and #17) were composed of ferropericlase microcrystals, amorphous (Mg,Fe)SiO 3 (likely converted from perovskite during Ar-ion milling), and metallic iron. The origin of such metallic iron is important to determine the Fe partitioning in this study. In previous melting experiments on chondritic material, molten iron accumulated to form a small number of large spheres (e.g., ref. 9). In the present experiments, on the contrary, very small particles of metallic iron (<500-nm size) were homogeneously distributed in the melt pocket. In order to check the effect of heating duration, we performed additional experiments, in which the sample was heated for 30 sec above melting temperature at 61 GPa. The average grain size of iron metal in the recovered sample, however, did not change. These results suggest that metallic iron was not present at high temperature but formed during temperature-quenching. Next, metal was absent at lower pressures where silicate melt was quenched to glass (Fig. 1a), suggesting that the formation of metallic iron was related to the formation of quench crystals. It is possible that quench crystals of perovskite formed upon rapid temperature-quenching may have incorporated iron partly as Fe 3+, which can 3

4 induce the disproportionation reaction; 3Fe 2+ Fe 0 + 2Fe 3+. Indeed, quenched melt now contains large amount of Fe 3+ ; we determined the Fe 3+ /(Fe 2+ + Fe 3+ ) ratio of the melt pocket to be 0.48±0.08 at 46 GPa (run #18) on the basis of electron energy-loss near-edge structure (ELNES) spectroscopy measurements (Supplementary Fig. 5). We also analyzed the Fe 2+ + Fe 3+ content to be 1.7±0.4 mol% for areas free of metallic iron under the TEM using the energy dispersive x-ray spectrometer (EDS). From the FE- EPMA analysis of total Fe content (2.3±0.2 mol%), we obtain 0.6±0.4 mol% metallic Fe 0, 0.9±0.2 mol% Fe 2+, and 0.8±0.2 mol% Fe 3+ in this melt pool. These analyses support the scenario that (1) all iron was present as Fe 2+ in partial melt and (2) Fe 2+ was then partially disproportioned into Fe 0 and Fe 3+ upon growth of quench crystals when temperature was quenched to 300 K. We thus consider all Fe to be Fe 2+ at high temperatures in this study. 1. Heinz, D. L. & Jeanloz, R. Measurement of the melting curve of Mg 0.9 Fe 0.1 SiO 3 at lower mantle conditions and its geophysical implications. J. Geophys. Res. 92, (1987). 2. Andrault, D. & Fiquet, G. Synchrotron radiation and laser heating in a diamond anvil cell. Rev. Sci. Instrum. 72, (2001). 3. Sinmyo, R. & Hirose, K. The Soret diffusion in laser-heated diamond anvil cell. Phys. Earth Planet. Inter. 180, (2010). 4. Fialin, M., Catillon, G. & Andrault, D. Disproportionation of Fe 2+ in Al-free silicate perovskite in the laser heated diamond anvil cell as recorded by electron probe microanalysis of oxygen. Phys. Chem. Miner. 36, (2009). 5. de Koker, N. P., Stixrude, L. & Karki, B. B. Thermodynamics, structure, dynamics, and freezing of Mg 2 SiO 4 liquid at high pressure. Geochim. Cosmochim. Acta 72, (2009). 4

5 6. Holzapfel, C., Rubie, D. C., Frost, D. J. & Langenhorst, F. Fe-Mg interdiffusion in (Mg,Fe)SiO 3 perovskite and lower mantle reequilibration. Science 309, (2005). 7. Ohtani, E., Moriwaki, K., Kato, T. & Onuma, K. Melting and crystal-liquid partitioning in the system Mg 2 SiO 4 -Fe 2 SiO 4 to 25 GPa. Phys. Earth Planet. Inter. 107, (1998). 8. Corgne, A. et al. Silicate perovskite-melt partitioning of trace elements and geochemical signature of a deep perovskitic reservoir. Geochim. Cosmochim. Acta 69, (2005). 9. Asahara, Y., Kubo, T. & Kondo, T. Phase relations of a carbonaceous chondrite at lower mantle conditions. Phys. Earth Planet. Inter , (2004). 10. Fiquet, G. et al. Melting of peridotite to140 gigapascals. Science 329, (2010). 11. Presnall, D. C., Weng, Y.-H., Milholland, C. S. & Walter, M. J. Liquidus phase relations in the system MgO MgSiO 3 at pressures up to 25 GPa constraints on crystallization of a molten Hadean mantle. Phys. Earth Planet. Inter. 107, (1998). 12. Mosenfelder, J. L., Asimov, P. D. & Ahrens, T. J. Thermodynamic properties of Mg 2 SiO 4 liquid at ultra-high pressures from shock measurements to 200 GPa on forsterite and wadsleyite. J. Geophys. Res. 112, B06208 (2007). 5

6 Supplementary Table 1 Experimental results Run# P (GPa) T (K)* Phases SiO 2 (wt%) MgO (wt%) FeO (wt%) Mg# K D (Solid/Liq) Liq 41.4(5) 45.4(9) 13.2(5) (11) Ol 40.9(6) 55.8(6) 3.3(1) (100) Liq 47.2(10) 42.9(16) 9.9(8) (319) Fp 8.2(34) 76.5(43) 15.3(39) 89.9 Fp 83.6(47) 16.4(42) (300) Liq 47.7(4) 42.4(4) 9.9(3) (104) Fp 13.2(60) 75.2(56) 11.7(9) 92.0 Fp 87.7(65) 12.3(9) (300) Liq 45.7(11) 41.4(8) 12.9(9) (46) Pv 57.8(10) 39.2(9) 3.1(3) (300) Liq 45.3(13) 46.6(13) 8.1(7) (91) Pv 58.4(9) 39.8(6) 1.7(5) (300) Liq 41.7(104) 50.9(65) 7.4(45) (139) Pv 58.0(11) 40.5(9) 1.5(2) (350) Liq 40.6(9) 48.6(10) 10.8(8) (91) Pv 58.6(13) 38.9(11) 2.5(6) (250) Liq 40.8(4) 49.4(5) 9.8(7) (61) Pv 60.2(6) 38.1(6) 1.7(3) (250) Liq 37.1(10) 47.8(9) 15.1(18) (20) Pv 59.3(6) 39.8(5) 0.9(2) (250) Liq 38.2(49) 45.3(61) 16.5(18) (26) Pv 58.5(18) 40.6(16) 0.9(2) (400) Liq 37.4(8) 48.0(6) 14.6(11) (26) PPv 60.3(5) 38.7(3) 1.0(2) (450) Liq 33.7(9) 46.9(7) 19.5(14) (16) PPv 59.1(10) 39.7(10) 1.2(2) 98.3 Numbers in parenthesis indicate uncertainties; one standard deviations in the last digits for K D values. *Inferred from the melting curves of Mg 2 SiO 4 and natural peridotite. See Methods for details. Liq, liquid; Ol, olivine; Fp, ferropericlase; Pv, perovskite; PPv, post-perovskite Mg# = 100 Mg/(Mg+Fe) molar ratio Estimated by mass-balance calculations 6

7 5000 Temperature (K) Mg 2 SiO 4 liquidus Peridotite solidus Pressure (GPa) Supplementary Figure 1 Experimental pressure and temperature conditions. The upper and lower bounds of the temperature in the present experiment are given by the liquidus temperature of Mg 2 SiO 4 and the solidus temperature of natural peridotite 10. Liquidus curve of Mg 2 SiO 4 was drawn on the basis of Simon s equation using the previous multi-anvil press data to 23 GPa 11 and shock-wave data at GPa 12. The uncertainties of these melting curves are shown by envelopes. The value at 20 GPa was estimated from the partial melt composition based on the previous multi-anvil studies

8 (Mg+Fe) / Si Pressure (GPa) Supplementary Figure 2 Variations in (Mg+Fe) / Si molar ratio in partial melt as a function of pressure. 8

doi:10.1038/nature09940 Supplementary Figure 3 Backscattered electron images and the x-ray maps for Si, Mg, and Fe for (a) unmelted and (b) melted samples with relatively long heating duration.

9 doi: /nature09940 Supplementary Figure 3 Backscattered electron images and the x-ray maps for Si, Mg, and Fe for (a) unmelted and (b) melted samples with relatively long heating duration. a, The sample was heated for 5 sec at subsolidus condition of 77 GPa and 3500 K. Note that perovskite and ferropericlase segregated from each other. b, The sample was heated to above melting temperature for 10 sec at 25 GPa. Chemical segregation is observed within the melt pocket. 9

doi:10.1038/nature09940 Supplementary Figure 4 Backscattered electron image and the x-ray maps for Si, Mg, and Fe for experiment at 1 bar. (Mg0.89Fe0.

10 doi: /nature09940 Supplementary Figure 4 Backscattered electron image and the x-ray maps for Si, Mg, and Fe for experiment at 1 bar. (Mg0.89Fe0.11)2SiO4 olivine was melted in the DAC similarly to other high-pressure experiments. Chemical compositions of melt and coexisting olivine were consistent with the known liquidsolid two-phase loop in Mg2SiO4 Fe2SiO4 binary system, indicating that chemical equilibrium was attained after a short heating duration (<1 sec). 10

11 Fe L 3 Fe 2+ Fe 3+ Intensity Fe L Energy loss (ev) Supplementary Figure 5 Typical ELNES spectrum of quenched melt pocket. Fe L 2,3 -edge spectrum for run #18 at 46 GPa, showing the Fe 3+ /(Fe 2+ + Fe 3+ ) ratio of 0.48±

Redox-induced lower mantle density contrast and effect on mantle structure and primitive oxygen

SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO2772 Redox-induced lower mantle density contrast and effect on mantle structure and primitive oxygen Tingting Gu, Mingming Li, Catherine McCammon and Kanani K.