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Seismological observations Earth’s deep interior, and their geodynamical and mineral physical interpretation

Seismological observations Earth’s deep interior, and their geodynamical and mineral physical interpretation. Arwen Deuss, Jennifer Andrews University of Cambridge, UK John Woodhouse University of Oxford, UK. Global tomography. Velocity heterogeneity in the Earth: * thermal in origin?

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Seismological observations Earth’s deep interior, and their geodynamical and mineral physical interpretation

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  1. Seismological observations Earth’s deep interior, and their geodynamical and mineral physical interpretation Arwen Deuss, Jennifer Andrews University of Cambridge, UK John Woodhouse University of Oxford, UK

  2. Global tomography Velocity heterogeneity in the Earth: * thermal in origin? * also chemical/compositional heterogeneity? * lithosphere/asthenosphere boundary? * what happens in the transition zone? * where do slabs go? Ritsema, van Heijst & Woodhouse (1999)

  3. Mantle discontinuities mineral physics seismology Seismology (Deuss & Woodhouse, GRL, 2002)

  4. Two different data types … * reflected waves * both continents and oceans * converted waves * only beneath stations

  5. Transition zone Precursors SS precursors: * 410 and 660km visible in all PP precursors: * 410km always visible * 660km visible in some regions

  6. 660-km discontinuity Precursors Clear reflections from 660 km depth in PP precursors (Deuss et al., Science, 2006)

  7. 660-km discontinuity Precursors Long period: single peaks Short period: double peaks

  8. Transition zone Receiver functions Single peak at 660 Double peaks at 660 * Receiver functions also show complex structure of 660km, while 410km discontinuity is simple * No 520 km discontinuity

  9. Mineral physics: 660 km discontinuity For pyrolite mantle composition (after Hirose, 2001)

  10. Application: mantle plumes Modified from http://www.mantleplumes.org

  11. Application: mantle plumes Using SS precursors in plume locations from Courtillot et al, 2003 (Deuss, P4, in press, 2007) Mantle plumes are characterised by deep 410, in combination with both deep or shallow 660 (dependent on temperature)

  12. 520-km discontinuity Precursors Splitting of 520-km discontinuity * more complicated than just olivine * garnet phase change? trace elements? (Deuss & Woodhouse, Science, 2001)

  13. Splitting observations 520 km discontinuity * no correlation with tectonic features

  14. Mineral physics: 520 km discontinuity Pyrolite phase diagram b a g * high Fe-content: no b-g transition * wet conditions: b-g much sharper * low Ca-content: no gt-CaPv transition

  15. But: there is more … SS precursors In addition to transition zone: * Reflectors at 220, 260 and 320 km in the upper mantle * Continuous range of scatterers in the lower mantle Receiver functions

  16. Upper mantle Precursors

  17. Upper mantle Clapeyron slopes Lehmann discontinuity: mainly negative Clapeyron slopes (Deuss & Woodhouse, EPSL (2004))

  18. Upper mantle Mineral physics Phase transitions: * Coesite –Stishovite, 250-300 km depth, dP/dT=2.5-3.1 * Orthoenstatite – High clinoenstatite, 250-300 km depth, dP/dT=1.4 Change in deformation mechanism: * Dislocation-diffusion creep dry: 340-380 km depth, dP/dT=-2.4 wet: 240-280 km depth, dP/dT=-2.4 Karato (1993)

  19. Lower mantle Precursors Stack for North America 220 410 520 660 800 1050 1150 (Deuss & Woodhouse, GRL, 2002)

  20. Lower mantle Precursors Stack for Indonesia 220 410 520 660 1050 1150 (Deuss & Woodhouse, GRL, 2002)

  21. Lower mantle 800-900km * in different regions, both continental and oceanic

  22. Lower mantle 1000-1200 km * mainly in subduction zone areas related to slabs?

  23. Lower mantle – Mineral physics Phase transitions * stishovite -> CaCl2-type (in SiO2) free silica? * (Mg,Fe)SiO3 perovskite, orthorhombic -> cubic phase unlikely! Others * change in chemical composition? * change in deformation mechanism? * MORB heterogeneity, mechanical mixture?

  24. Conclusions * to explain the seismic observations of transition zone discontinuities, we need phase transitions in garnet in addition to the olivine phase transitions (consistent with a pyrolite mantle model ) * lateral variations in minor elements are also required, which will influence slab penetration and upwelling of mantle plumes differently from region to region * significant amount of seismic scatterers in upper and lower mantle, without a mineral physical explanation in the lower mantle * focus research towards discoveries in mineral physics, i.e. discontinuities in attenuation, free silica lower mantle, mechanical mixture vs. equilibrium

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