G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 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

G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 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 : 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 :

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

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

G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, The Gravity Signal of Mantle Plumes

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

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

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

G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 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.

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

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

G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 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

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

G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, D 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

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

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

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

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

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

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

G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 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  )

G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 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

G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 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

G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, Global effects of plumes

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

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

G. Marquart Gravity Effect of Plumes Geodynamik Workshop, Hamburg, 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 ~ 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 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.