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Probing Earth’s deep interior using mantle discontinuities

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  1. Probing Earth’s deep interior using mantle discontinuities Arwen Deuss University of Cambridge, UK also: Jennifer Andrews, Kit Chambers, Simon Redfern, John Woodhouse

  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. History Number of papers per depth, since 1959

  5. Questions (1) what is the nature of the Lehmann discontinuity at 220 km depth? (2) what happens in the transition zone and how much thermal vs. chemical heterogeneity? (3) are there discontinuities in the lower mantle?

  6. Data Data: 7018 traces * 6.0 < Mw < 7.0 * 100 < distance < 160 * depth < 75 km

  7. Global data coverage SS-wave bounce points

  8. Complete data set * synthetics for PREM: discontinuities at 220, 400 and 670 km depth *complete data set: discontinuities at 410, 520 and 660 km depth

  9. Robustness of reflections Stack for North America 220 410 520 660 800 1050 1150 (Deuss & Woodhouse, GRL, 2002)

  10. Robustness of reflections Stack for Indonesia 220 410 520 660 1050 1150 (Deuss & Woodhouse, GRL, 2002)

  11. Reflections per depth * clear reflections from transition zone discontinuities at 410 and 660 km depth * additional discontinuities in upper and lower mantle at 220, 260, 310 and 800 km depth (Deuss & Woodhouse, GRL 2002)

  12. Questions (1) what is the nature of the Lehmann discontinuity at 220 km depth? (2) what happens in the transition zone and how much thermal vs. chemical heterogeneity? (3) are there discontinuities in the lower mantle?

  13. Upper mantle reflectors (Deuss & Woodhouse, GRL, 2002)

  14. Mantle discontinuities - Mineral physics Seismological observations Clapeyron Slopes (Deuss & Woodhouse, EPSL, 2004)

  15. Mineral physical mechanisms 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)

  16. Questions (1) what is the nature of the Lehmann discontinuity at 220 km depth? (2) what happens in the transition zone and how much thermal vs. chemical heterogeneity? (3) are there discontinuities in the lower mantle?

  17. Transition zone structure

  18. 410km topography (Chambers, Deuss & Woodhouse, EPSL, 2005)

  19. 520-km discontinuity Observations Splitting of 520-km discontinuity * more complicated than just olivine * garnet phase change? trace elements? (Deuss & Woodhouse, Science, 2001)

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

  21. Phase transitions: 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

  22. WKBJ synthetics Two reflectors can indeed be observed!

  23. Regional stacks Transition zone SS precursors: * 410 and 660km visible in all PP precursors: * 410km always visible * 660km visible in some regions

  24. 660-km discontinuity Observations Clear reflections from 660 km depth in PP precursors (Deuss et al., Science, 2006)

  25. 660-km discontinuity Observations Long period: single peaks Short period: double peaks

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

  27. Seismic amplitudes Variations in amplitudes are consistent with the pyrolite model (using Weidner & Wang, 1998)

  28. Questions (1) what is the nature of the Lehmann discontinuity at 220 km depth? (2) what happens in the transition zone and how much thermal vs. chemical heterogeneity? (3) are there discontinuities in the lower mantle?

  29. Lower mantle Data * reflections around 800km and 1000-1200km

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

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

  32. 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?

  33. What next? Fresnel zones Dahlen, 2003

  34. Conclusions * comparison of seismic observations of mantle discontinuities with mineral physics implies significant amount of chemical heterogeneity * important implications for mantle flow * we need to expand to other data types and implement new techniques (such as finite frequency kernels) to further understand the level of heterogeneity