1. Fe-Mg partitioning in the lower mantle: in-situ XRD and quantitative analysis Li Zhang a, Yue Meng b, Vitali Prakapenka c, and Wendy L. Mao d,e a Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA b High Pressure Collaborative Access Team, Carnegie Institution of Washington, IL 60439, USA c GeoSoilEnviroCARS, University of Chicago, IL 60439, USA d Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA e Photon Science and Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 1 2011 COMPRES ANNUAL MEETING Acknowledgements: we thank Yingwei Fei, Scott Price, John Armstrong and Jinfu Shu for their assistance. This work is supported by NSF - Geophysics Grants EAR-0738873 and EAR-0911492. The XRD experiments were performed at HPCAT and GSECARS. 2011 Annual Meeting

2. Introduction Geophysical studies indicates that seismic heterogeneities exist in the middle of the lower mantle (e.g., Dziewonski and Anderson, 1981; Kellogg et al., 1999; van der Hilst and Kárason, 1999; Trampert et al., 2004). The distribution of iron (Fe) and magnesium (Mg) in mineral phases as well as electronic transitions of Fe have been proposed to interpret the seismic features in the mid-lower mantle. From the mineral physics point of view, precise measurements of structural transitions, electronic transitions and element partitioning in lower mantle mineral phases at the high pressure (P) and temperature (T) conditions are required. 22011 COMPRES ANNUAL MEETING

3. 3 In this study, we combined in-situ X-ray diffraction (XRD) measurements at high P-T with quantitative analysis on quenched samples to study crystal structure and chemical composition in bulk composition (Mg 0.6 Fe 0.4 )SiO 3 at high P- T conditions corresponding to the mid-lower mantle.

4. Experimental procedure 42011 COMPRES ANNUAL MEETING 1.Load synthetic (Mg 0.6 Fe 0.4 )SiO 3 orthopyroxene in Neon media in a diamond anvil cell (DAC). 2. Bring the sample to a pressure of interest and perform laser heating. 2. Collect in-situ XRD at high PT as well as during decompression. 3. Perform high resolution scanning electron microscopy (SEM) as well as energy dispersive X-ray spectroscopy (EDS) on recovered samples.

5. In-situ XRD measurements XRD pattern after decompression from 63 GPa&2000 K in Neon (Ne) medium to ambient condition: (Mg,Fe)SiO3-Pv; (Mg,Fe)O-Mw; SiO2-St; Platinum-Pt; Neon-Ne 52011 COMPRES ANNUAL MEETING

6. Quenched product from 63 GPa&2000 K Examination of quenched product is important for us to better understand in-situ XRD data and a powerful tool to map the phase distribution across the heated spot… 62011 COMPRES ANNUAL MEETING Most Mw exists in the outer margin of the laser heated spot. Sinmyo et al. (2008) suggested significant Fe variation in Fp from gel (Mg 0.9 Fe 0.1 ) 2 SiO 4 due to temperature gradient. Consistently, We did not observe Mw in the in-situ XRD pattern collected during laser heating at 63 GPa. Our conclusion: Pv and Mw phases were not in equilibrium in (Mg 0.6 Fe 0.4 )SiO 3 at 63 GPa and 2000 K for partitioning!

7. Composition of Pv: Unit-cell volumes of quenched (Mg,Fe)SiO 3 -Pv at ambient conditions 72011 COMPRES ANNUAL MEETING x is the molar fraction of Fe/ (Mg+Fe) in Pv.

8. Composition of Pv: EDS analysis (63 GPa&2000 K) 2011 COMPRES ANNUAL MEETING Pv: Fe#=30.98  0.76 Mw: Fe#=96.07  0.20 Pv Mw

9. Conclusions We investigated phase distribution, crystal structure and chemical composition in bulk composition (Mg 0.6 Fe 0.4 )SiO 3 from 63 and 54 GPa heated at 2000 K. Most Mw was found in the outer margin of the heated spot from 63 GPa and 2000 K, due to Fe migration from hot to cold region. Thus, Pv and Mw were not in equilibrium for partitioning. We analyzed the composition of Pv from both XRD (x=0.24) and EDS (x=0.31) synthesized at 63 GPa and 2000 K. 9 2011 COMPRES ANNUAL MEETING

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13. Unit-cell Refinement of Pv Orthorhombic unit cell (Pbnm): a=4.8006(18); b=4.9395(59); c=6.9086(30) V 0 =163.82(22) Orthorhombic unit cell (Pbnm): a=4.8028(17); b=4.9341(26); c=6.9036(27) V 0 =163.60(12) Consistency of chosen diffraction lines for unit-cell refinement at all pressures. Example: Pv decompressed to ambient condition, from 63 GPa&2000 K in (Mg 0.6,Fe 0.4 )SiO3. 132011 COMPRES ANNUAL MEETING

14. Evaluation of deviatoric stress 142011 COMPRES ANNUAL MEETING 4-5 diffraction lines of Pt were used for the calculation of pressure. The standard deviation of pressure calculated from each line of Pt is within 1 GPa, indicating very low deviatoric stress in our experiments. Pressures calculated from Fei et al.(2007).