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Fe-Mg partitioning in the lower mantle: in-situ XRD and quantitative analysis

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

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Fe-Mg partitioning in the lower mantle: in-situ XRD and quantitative analysis

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  1. Fe-Mg partitioning in the lower mantle: in-situ XRD and quantitative analysis Li Zhanga, Yue Mengb, Vitali Prakapenkac, and Wendy L. Maod,e aGeophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA bHigh Pressure Collaborative Access Team, Carnegie Institution of Washington, IL 60439, USA cGeoSoilEnviroCARS, University of Chicago, IL 60439, USA dGeological and Environmental Sciences, Stanford University, Stanford, California 94305, USA ePhoton Science and Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 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 COMPRES ANNUAL MEETING 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. 2011 COMPRES ANNUAL MEETING

  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 (Mg0.6Fe0.4)SiO3 at high P-T conditions corresponding to the mid-lower mantle. 2011 COMPRES ANNUAL MEETING

  4. Experimental procedure 1.Load synthetic (Mg0.6Fe0.4)SiO3 orthopyroxenein 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. 2011 COMPRES ANNUAL MEETING

  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 2011 COMPRES ANNUAL MEETING

  6. Quenched product from 63 GPa&2000 K Most Mw exists in the outer margin of the laser heated spot. Sinmyo et al. (2008) suggested significant Fe variation in Fp from gel (Mg0.9Fe0.1)2SiO4 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 (Mg0.6Fe0.4)SiO3 at 63 GPa and 2000 K for partitioning! 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… 2011 COMPRES ANNUAL MEETING

  7. Composition of Pv: Unit-cell volumes of quenched (Mg,Fe)SiO3-Pv at ambient conditions x is the molar fraction of Fe/ (Mg+Fe) in Pv. 2011 COMPRES ANNUAL MEETING

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

  9. Conclusions • We investigated phase distribution, crystal structure and chemical composition in bulk composition (Mg0.6Fe0.4)SiO3 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. 2011 COMPRES ANNUAL MEETING

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  13. Unit-cell Refinement of Pv Example: Pv decompressed to ambient condition, from 63 GPa&2000 K in (Mg0.6,Fe0.4)SiO3. Orthorhombic unit cell (Pbnm): a=4.8006(18); b=4.9395(59); c=6.9086(30) V0=163.82(22) Orthorhombic unit cell (Pbnm): a=4.8028(17); b=4.9341(26); c=6.9036(27) V0=163.60(12) Consistency of chosen diffraction lines for unit-cell refinement at all pressures. 2011 COMPRES ANNUAL MEETING

  14. Evaluation of deviatoric stress Pressures calculated from Fei et al.(2007). 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. 2011 COMPRES ANNUAL MEETING

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