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Hirofumi Ishii 6 & Nozomu Hiraoka 6 Ryuichi Nomura Tokyo Institute of Technology A melt has greater volume than a silicate solid of the same composition.

interior of the Earth (and other terrestrial bodies) has long been a source of considerable speculation 1,2.

explaining a variety of observed features in the core mantle boundary region 3.

1 LETTER doi: /nature09940 Spin crossover and iron-rich silicate melt in the Earth s deep mantle Ryuichi Nomura 1,2, Haruka Ozawa 1,3, Shigehiko Tateno 1, Kei Hirose 1,3, John Hernlund 4, Shunsuke Muto 5, Hirofumi Ishii 6 & Nozomu Hiraoka 6 Ryuichi Nomura Tokyo Institute of Technology A melt has greater volume than a silicate solid of the same composition. But this difference diminishes at high pressure, and the possibility that a melt sufficiently enriched in the heavy element iron might then become more dense than solids at the pressures in the interior of the Earth (and other terrestrial bodies) has long been a source of considerable speculation 1,2. The occurrence of such dense silicate melts in the Earth s lowermost mantle would carry important consequences for its physical and chemical evolution and could provide a unifying model for explaining a variety of observed features in the core mantle boundary region 3. Recent theoretical calculations 4 combined with estimates of iron partitioning between (Mg,Fe)SiO 3 perovskite and melt at shallower mantle conditions 5 7 suggest that melt is more dense than solids at pressures in the Earth s deepest mantle, consistent with analysis of shockwave experiments 8. Here we extend measurements of iron partitioning over the entire mantle pressure range, and find a precipitous change at pressures greater than 76 GPa, resulting in strong iron enrichment in melts. Additional X-ray emission spectroscopy measurements on (Mg 0.95 Fe 0.05 )SiO 3 glass indicate a spin collapse around 70 GPa, suggesting that the observed change in iron partitioning could be explained by a spin crossover of iron (from high-spin to low-spin) in silicate melt. These results imply that (Mg,Fe)SiO 3 liquid becomes more dense than coexisting solid at 1,800 km depth in the lower mantle. Soon after the Earth s formation, the heat dissipated by accretion and internal differentiation could have produced a dense melt layer up to 1,000 km in a Laser heating Fp melt 5 μm 32 GPa Si b Re 5 μm Pv thickness underneath the solid mantle. We also infer that (Mg,Fe)SiO 3 perovskite is on the liquidus at deep mantle conditions, and predict that fractional crystallization of dense magma would have evolved towards an iron-rich and silicon-poor composition, consistent with seismic inferences of structures in the core mantle boundary region. Our melting experiments were performed on samples with bulk composition (Mg 0.89 Fe 0.11 ) 2 SiO 4 at pressures from 20 to 159 GPa in a laser-heated diamond-anvil cell (DAC; Supplementary Table 1). The heating duration was short in order to avoid anomalous thermal diffusion (Supplementary Information), but this prevented us from measuring the melting temperature. Nevertheless, the upper and lower bounds of the temperature in our experiment are given by the liquidus temperature of Mg 2 SiO 4 and the solidus temperature of natural peridotite, respectively (see Methods and Supplementary Fig. 1). Samples were recovered from the DAC and examined with a high-resolution fieldemission-type electron probe micro-analyser (FE-EPMA). Recovered specimens exhibited a concentric texture that reflected the temperature distribution during heating (Fig. 1), which is similar to that observed in conventional multi-anvil experiments 5 7. We consistently found a pocket at the hottest part of the sample that possessed nonstoichiometric composition, which we interpret as quenched partial melt. The (Mg1Fe)/Si molar ratio of this quenched melt increased with pressure, from 1.50 at 36 GPa to 2.56 at 159 GPa (Supplementary Fig. 2). The melt pocket was surrounded by a single-phase solid layer (ferropericlase or perovskite, depending on pressure), which we interpret to be 76 GPa melt Si c melt PPv 159 GPa 55μm Si Haruka Ozawa, Shigehiko Tateno, Kei Hirose, Tokyo Institute of Technology John Hernlund, University of California Shunsuke Muto, Nagoya University Hirofumi Ishii, & Nozomu Hiraoka NSRRC Mg Fe Mg Fe Mg Fe Fe (Nomura et al., 2011 Nature, 473, 199) 1

2 How do we know about the deep Earth interior? Meteoritics and theory of Solar system genesis Terrestrial rock samples Seismology and other observations (C) NASA chemical composition Physical Properties Mineral Physics 2

3 Structure of deep Earth interior crust: 5-50 km SiO2 rich rock silicate mantle: to ~2900 km SiO2 poor rock silicate core: to ~6400 km Fe-Ni alloy (C) Calvin J. Hamilton Layered structure of the Earth s interior 3

4 Differentiation and evolution of the Earth Core-mantle segregation -core formation Mantle Differentiation -crust formation (Stevenson, 2001 Nature) melting and density contrast cause the layered structure of the earth 4

5 eutectic phase relation with incongruent melting Liquid Temperature Liq. + A Liquid composition Liq. + B -slow diffusion process in solid phase B -density contrast between Liq. and B A + B ->chemical differentiation A Bulk Composition -What is A and B -Which direction the eutectic composition move at different P-T condition B 5

6 Elemental compositions of the Earth s mantle etc (Al2O3, CaO,...) FeO 8.1wt% 9.1wt% SiO2 45wt% MgO 37.8wt% (McDonough and Sun, 1995 Chem. Geol.) Melting phase relations at SiO2-MgO-FeO systems are important 6

7 Mineral composition of the Earth s mantle Magnesium-perovskite (Mg,Fe)SiO3 Magnesiowüstite (Mg,Fe)O (Hirose, 2006 RG) Mw and MgPv is dominant phases in SiO2-MgO-FeO systems 7

8 Our knowledge on deep Earth melting relations? ~1000km, 30GPa Multi-anvil apparatus (C) Calvin J. Hamilton (Ito, 2007 Treatise on Geophysics) No melting experiments at >~30 GPa 8

9 Our knowledge on deep Earth melting ~30GPa Phase diagram Liquid? Temperature L + Mw L+MgPv 31 GPa (C) Calvin J. Hamilton Mg/Si MgO MgSiO3 (Ito et al., 2004 PEPI) Eutectic comp. evolved toward Mg-rich 9

10 Our knowledge on deep Earth melting ~30GPa Iron partitioning into melt 1 K D = (Fe/Mg) solid /(Fe/Mg) liquid ] (C) Calvin J. Hamilton? Fe-rich in melt KD(solid/ melt) 0.1 0? Pressure (GPa) 135 No information of Fe partitioning behaviors >~30 GPa 10

11 Our goal of this study is...? to clarify -phase relations -iron partitioning behaviors into melt in SiO2-MgO-FeO system (C) Calvin J. Hamilton Melting experiments at entire mantle conditions 11

diamonds gasket diamond sample φ10-100 d10-80 (μm) Need for

12 How we generate high P-T conditions of deep Earth? Diamond anvil cell sample chamber thermal insulator diamonds gasket diamond sample φ d10-80 (μm) Need for -Brilliant Synchrotron Radiation -Nano technologies for chemical analysis 12

13 Experimental conditions High P-T generation Nd:YAG Laser-heated DAC Starting material (Mg0.89Fe0.11)2SiO4 olivine Laser heating Ar Sample Diamond Re gasket Pressure measurement Raman peak shift of Chemical analysis FE-EPMA 13

14 32GPa (Mg,Fe)O laser heating 159GPa Compression axis Melt Melt (Mg,Fe)SiO3 10µm 5µm Liquidus phase Temperature Mw (Mg,Fe)O Liq. + Mw Liquid High-Pressure Bulk Composition Liq. + MgPv MgPv [(Mg,Fe)SiO3] 14

15 32GPa (Mg,Fe)O laser heating 159GPa Compression axis Melt Melt (Mg,Fe)SiO3 10µm 5µm Fe mapping iron in melt Strong iron partition into melt at high Pressure 15

16 Mg/Si ratio in melt 3 Mw (Mg,Fe)O (Mg+Fe) / Si 2 Starting material 1 0 MgPv (Mg,Fe)SiO Pressure (GPa) liquidus phase from Fp to GPa -eutectic composition of high Mg/Si at high P 16

17 Fe partition coefficient (MgPv/melt) 1 1 (Corgne et al, 2005 GCA) K D = (Fe/Mg) solid /(Fe/Mg) liquid ] Fe-rich in melt KD(solid/ melt) 0.11 High-spin melt Low-spin melt Pressure (GPa) bottom of mantle 180 abrupt change of KD value from 0.25 to 17

18 solid-melt density crossover in SiO2-MgO-FeO Magma: Sink MgPv with pressre Mantle Magma: Float (Funamori and Sato, 2010 EPSL) -density crossover at 76 GPa (~1800 km depth) 18

19 solid-melt density crossover b CMB 6 Liq (Mg,Fe)SiO3 liq Fp CaPv MgPv Density (g cm 3 ) PREM 4,000 K Pressure (GPa) Intensity (arbitrary units) density crossover at 76 GPa (~1800 km depth) 7,

20 Existence of basal magma ocean a a cooling Magma Ocean Core Initial fully molten Earth (Labrosse et al., 2007 Nature) Dense melt form magma ocean at the bottom of the mantle 20

21 Crystallization of the basal magma ocean LETTER RESEARCH magma ascend 1800 km Fe-rich Mg-rich cumulates magma sink Wustite crystallization Figure 4 Evolution and crystallization of dense melts in the d, 1,800 belowkm d KD value for a, During Earth s early history, any melts that form sink and accumulate at the base of the mantle, while any cry owing to cooling of this dense magma will rise upward into th 2-poor composition (that b, Fe-poor perovskite crystallization leaves a residual liquid e and depleted in SiO 2, and crystals forming from this evolved liquid dense enough to form thermo-chemical piles at the base of Evolution through fractional crystallization as described above c, The ﬁnal stage of crystallization involves a composition stit clo leaving behind a very dense thin layer that is consistent with would also have aﬀected the composition of cumulates that formed properties from the BMO. In particular, cumulates should become more Fe-rich inferred inside ULVZs. White arrows indicate sche 21 with time, and presumably more dense, as they crystallizepatterns from anin the convecting solid mantle. Chemical heterogeneity at the bottom of the mantle

Seismic structure of the deep Earth Mantle Upper mantle 0 660 1500 Seismic

Lower mantle Sharp side ULVZ D" Core Pv ppv Low T, +dvs (Garnero et al.

22 Seismic structure of the deep Earth Mantle Upper mantle Seismic reflections STZ LLSVP High T, high, dvs CMB reactions ppv High T, +dvs Lower mantle Sharp side ULVZ D" Core Pv ppv Low T, +dvs (Garnero et al., 2008 Science) Seismic observation(llsvp, ULVZ) can be explained by our cooling Earth model 22

23 4,18 Implications for evolution and structure of the deep LETTER RESE Earth Fe % at the Less dense liquids rise upward high-(mg 1 KD at KD 5 culations km 3 4,18 Basal Magma Ocean Less dense liquids at the ascend magma sink 19 rise upward -gravitational stability is3confirmed high-(mg Seismic Observation (LLVSPs, ULVZs) -iron-rich cumulates from BMO crystallization -gravitationally stable partial melt (Mao et al., 2006 PNAS) 3 1 Fe-rich cumulates 1 Wustite crystallization 23

24 Abrupt change of Iiron partition behavior 1 1 (Corgne et al, 2005 GCA) K D = (Fe/Mg) solid /(Fe/Mg) liquid ] Fe-rich in melt KD(solid/ melt) 0.11 High-spin Low-spin melt melt Pressure (GPa) 180 What cause this abrupt change of iron behavior? 24

25 spin transition in iron HS-LS transition induce large volume decrease of ferric ion and iron partitioning into melt (Badro et al., 2003 Science) 25

26 X-ray emission spectroscopy SPring8 NSRRC (Mg0.95Fe0.05)SiO3 glass Temperature: 300K Pressure: up to 85 GPa 26

X-ray emission spectroscopy (XES) @BL12XU SPring8

27 X-ray emission spectroscopy SPring8 NSRRC Analyzing crystal X-ray Detector Diamond Anvil Cell 27

28 X-ray emission spectroscopy (XES) measurements Intensity (arbitrary units) 8 GPa 22 GPa 36 GPa 48 GPa 59 GPa 77 GPa 85 GPa Kβ Kβ 7,020 7,030 7,040 7,050 7,060 7,070 7,080 Energy (ev) Figure 3 Evolution of X-ray emission spectra of (Mg 0.95 Fe 0.05 )SiO 3 glass with increasing pressure. Measurements were conducted at 300 K. All spectra are normalized to transmitted intensity, and shifted so that the weighted average of main (Kb) plus satellite (Kb9) emission lines is set to 7,058 ev. The spin crossover in (Mg0.95Fe0.05)SiO3 glass at 59-77GPa 28

29 Interpretation of abrupt iron partition behavior Melting experiments on (Mg0.89Fe0.11)2SiO4 up to 159 GPa -abrupt change of KD from ~0.25 to X-ray emission spectroscopy on (Mg0.95Fe0.05)SiO3 glass -High spin-low spin transition occur at 59-77GPa room temperature -spin crossover of liquid silicate cause strong iron partitioning into melt 29

30 Summary Melting experiments on (Mg0.89Fe0.11)2SiO4 up to 159 GPa -Liquidus phase changes from Fp to GPa -eutectic composition evolved toward high Mg/Si ratio at high pressures -abrupt change of KD from ~0.25 to X-ray emission spectroscopy on (Mg0.95Fe0.05)SiO3 glass -High spin-low spin transition occur at 59-77GPa suggesting that spin crossover in silicate melt cause the strong iron partitioning into melt Implications for Mantle melting and Earth s evolution -BMO is gravitationally stable -fractional crystallization may explain seismic observations 30

Chemical Composition of the Lower Mantle: Constraints from Elasticity. Motohiko Murakami Tohoku University

Chemical Composition of the Lower Mantle: Constraints from Elasticity Motohiko Murakami Tohoku University Acknowledgements Jay D. Bass (University of Illinois) Stanislav V. Sinogeikin (University of Illinois)