Constraints on the observation of mantle plumes using global seismology

1. Constraints on the observation of mantle plumes using global seismology Arwen Deuss University of Cambridge, UK

2. Mantle plumes…? * tomography: narrow plumes? super plumes? flattening at thermal boundary layers? density? temperature, melting? * discontinuities: plumes in the transition zone? structure at the CMB? Seismologists study: velocity, anisotropy, attenuation

3. Body wave data * sensitive to velocity * small scale structure * limited global coverage * tomography and discontinuities temperature and composition?

4. Normal mode data Surface patterns Radial patterns * sensitive to velocity and density * large scale structure * good global coverage temperature and composition!

5. Tomographic models * mostly: velocity models using P or S body waves plumes or not? resolution? which kernels to use? * few: velocity and density models using normal modes are plumes bouyant and hot? full model space search?

6. Global tomography Shear wave velocity model S20RTS: * body waves * surface waves * normal mode splitting functions Ritsema, van Heijst & Woodhouse (1999)

7. Tomography S20RTS S waves and modes Ritsema et al. 1999 * complex low shear wave velocity structure beneath Africa extending from CMB

8. Tomography S20RTS * low velocity anomaly below Iceland is confined to the upper mantle BUT: are these hot and bouyant? Ritsema et al. 1999

9. Tomography modes Density models Trampert et al. 2004 670-1200 km 1200-2000 km 2000-2891 km * super plumes are dense and compositional in origin …

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

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

12. Transition zone discontinuities * discontinuities are caused by olivine phase changes thin transition zone in hotspot/plume regions

13. Discontinuities Deuss et al. 2005 * using SS precursors, only large scale structure * thin transition zone, correlation with hotspots/plumes?

14. Discontinuities Hotspots * hotspots with deep or CMB origin (Courtillot et al. 2003, Montelli et al. 2004) * only Afar, Ascension and Tristan have thinner TZ than average * Azores, Hawaii and Reunion have thicker TZ than average Deuss, 2005

15. Discontinuities Mineral Physics BUT: * at high temperature, not olivine but garnet has the dominant phase transition at 660 km… (Weidner & Wang, 1998) (Hirose, 2002)

16. Conclusions * global seismology is very useful in searching for mantle plumes * seismology only provides tomographic velocity models, and (?) large scale density models * mineral physics is very important for interpreting the results * at the moment, there is not enough evidence (yet) to interpret if mantle plumes are observed in seismology

17. Multi-disciplinary approach …. van der Hilst, 2004