1. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.04 1 Modeling Gravity Anomalies Caused by Mantle Plumes Gabriele Marquart Mantle Plumes in Observations (seismic tomography & gravity) Mantle Plumes in numerical Simulation Effect of Lithosphere Rheology Global Gravity Effect of Mantle Plumes

2. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.042 Indications for Mantle Plumes Coffin, 2000 Plumes have been proposed to explain the surface observation of volcanic hotspots and are responsible for ~10% of the Earth mass exchange Number of hotspots : 30 - 100 Plumes have been proposed to explain the surface observation of volcanic hotspots and are responsible for ~10% of the Earth mass exchange Number of hotspots : 30 - 100

3. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.043 Observation of plumes High resolution Seismic tomography – Whole mantle plumes Montelli et al., Science, 2004

4. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.044 Montelli et al., Science, 2004 High resolution Seismic tomography – upper mantle plumes

5. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.045 The Gravity Signal of Mantle Plumes

6. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.046 The Gravity Signal of a Mantle Plume Tahiti Marquesas MacDonald

7. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.047 The Gravity Spectrum of a Mantle Plume

8. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.048 The Gravity Signal of Mantle Plumes Short wavelengths ( ~5 - 200 km) strong central anomaly ~ >200 mgal, negative side lobes Medium wavelengths (~200-1000 km) Geoid height anomalies ~ 6-8 m, gravity anomalies ~ 55 mgal Weak positive lateral anomalies (?) Diameter of the plume anomaly ~ 400-800 km

9. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.049 The Gravity Signal of Mantle Plumes – Density Anomalies related to Plumes Isostatic gravity anomaly and related isostatic topography: - Thickening of the crust due to volcanic eruptiva and intrusions - Thermal thinning of the lithosphere - Positive small wavelength central gravity anomaly - Bending of the lithosphere under the volcanic chain Dynamic gravity anomaly and related dynamic topography: - reduced density of the hot mantle rocks - Dynamic topography due to the flow pressure of the rising material - Dynamic topography of the Core-Mantle Boundary - Density anomalies related to position changes of phase boundaries -> wider gravity anomaly and sidelobes Additional Effects: e.g. Melting related gravity anomalies - Density changes due to petrological changes during melting and Compaction.

10. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0410 Modeling the gravity field of a single plume

11. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0411 Heat transport: Mass transport: Rayleigh-Number: Modeling the Gravity Signal of Mantle Plumes 1. Modeling the Fluid Dynamic

12. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0412 Mantle viscosity Upper mantle Lower mantle Temperature Gradient Core Mantle Mineral Phase Changes Perovskite Spinel Olivine Plume T- Gradient Mantle T- Gradient  ~200 kg/m 3 Primary Constraints for a Simulation of Mantle Plumes

13. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0413 Gravity anomaly: Dynamic topography: Potential equations: Modeling the Gravity Signal of Mantle Plumes 2. Modeling Gravity dynamic topography thermal anomaly phase anomaly

14. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0414 3D Numerical Model of a Mantle Plume Principal Layout Length: x,y 4000 km, z=3000 km periodic in horizontal,  T : 2100°C 500°C at the CMB and a central “plume seed”, resolution 128x128x100 primitive variables, hybrid spectral-FD, wave number dep. iteration for µ(T(x,y,z)), semi-lagrangian Length: x,y 4000 km, z=3000 km periodic in horizontal,  T : 2100°C 500°C at the CMB and a central “plume seed”, resolution 128x128x100 primitive variables, hybrid spectral-FD, wave number dep. iteration for µ(T(x,y,z)), semi-lagrangian 1.“Reference Model “ 2.visco-plastic rheology 3. Strong phase boundary

15. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0415 Plume Gravity Signal: “Reference case”

16. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0416 Plume Gravity Signal: Plastic Yielding of the Lithosphere

17. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0417 Plume Gravity and Topography Spectra Reference CaseVisco-plastic Case

18. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0418 Plume Gravity Signal: Strong Phase Boundaries

19. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0419 Modeling the global gravity field plumes

20. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0420 Global Plume locations and Mass Fluxes

21. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0421 Mercator Projection of Cartesian Plumes on a Spherical Earth z x y  z  x0x0 y0y0 Oblique Mercator Projection:  r = 2 arctg exp{ (x-x 0 )/r a } –  /2  r = y-y 0 /r a  = arcsin ( cos  0 sin  r + sin  0 cos  r cos r ) = arcsin (cos  r sin r / cos  ) Oblique Mercator Projection:  r = 2 arctg exp{ (x-x 0 )/r a } –  /2  r = y-y 0 /r a  = arcsin ( cos  0 sin  r + sin  0 cos  r cos r ) = arcsin (cos  r sin r / cos  )

22. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0422 Plume related global densities 3D Cartesian Plume Model - Temperature  Densities with  (p,T) Using plume flux as a weight - Mercator Projection at 44 plume locations - Regriding on 1°x1° at 20 depth levels 3D Cartesian Plume Model - Temperature  Densities with  (p,T) Using plume flux as a weight - Mercator Projection at 44 plume locations - Regriding on 1°x1° at 20 depth levels

23. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0423 Gravity anomalies due to plumes Kernels for GravityKernels for Geoid - Expand density field in spherical harmonics l=2-18 at 20 depth levels - Convolve with the geoid (gravity) kernels - Expand density field in spherical harmonics l=2-18 at 20 depth levels - Convolve with the geoid (gravity) kernels

24. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0424 Global effects of plumes

25. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0425 Comparison to observation and a slab geoid model CHAMP hydrostatic geoid Geoid based on a slab model Residual geoid Plume geoid model

26. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0426 The power spectrum for the plume model Plumes are important! Observed Geoid Modeled Plume Geoid

27. G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 27.-29.9.0427 Conclusions Plumes in a rising stage below the phase boundary have gravity signal below 5 mgal and are unlikely to be detected by GOCE since separation of the weak signal is hardly possible Above the phase boundary plumes rise rapidly and cause gravity signals ~ 30-50 mgal and dynamic topography of ~400m. They might be detectable even before they cause surface volcanism The detailed gravity signal and dynamic topography produced by mantle plumes depend on stage of plume rise, rheology, phase boundary strength (etc.) A high resolution gravity field in the wavelength rage between 200-600 km as provided by GOCE may help to decide about the different scenarios Plumes are a primary source for global gravity anomalies for l>20. New gravity data will help to better constrain their influence.