Geological data, geophysics and modelling of the mantle Yanick Ricard & Joerg Schmalzl " Geophysical observations; Introduction " Geochemical measurements " Physics of convection " Modeling of the mantle; mixing properties " Connecting chemical and physical observation

Introduction & Motivation " Dynamic Earth - most prominent observations: -Earth quakes -Volcanos " Very hard to get any data

" Reason --> moving plates

How can we learn more about the Earth's interior ?? " Drilling --> e.g. KTB, Kola

Geochemical/geological observations - collect samples at mid-ocean ridges & volcanos --> compare with: - composition of chondrites - each other -> strength is the temporal resolution -> weakness in the spatial domain Being treated in separate lecture.

Geophysical Observations - Seismic waves - Heat flow measurments - Post glacial rebound - Gravimetrie => Problem: Snapshot of present situation But.. spatial information Mixing is THE link between the two fields !

How to proceed: " Gather information about the convective regime i.e. material properties, boundary conditions, e.t.c. => have a model of convection in the Earth mantle and understand the mixing properties => feed information derived from the geochemical investigations back into the model and try to refine it until it fullfills geochemical constrains => the SUPER-MODEL

But.... " parameters are only poorly constrained " convection & convective mixing are very non-linear processes Therefore a fundamental understanding of the physical processes is required to isolate robust features of convective mixing. (seperate lecture)

Geophysical Observations Seismics: - 1 st order spherical - lower BC - material Prop.

Deviation from the spherical shape: - different wave forms - non uniform distribution of sender/receiver - agreements on hot anomaly under Africa and Pacific - herterogeneity near CMB

On a smaller scale:

Hotspots:

seem too: - exist over long time - being stationary - independent of the plate motion - are geochemically distinced

Seismic evidence for a mantle plume From: VanDecar et al. 1995

Wavefront healing:

What todo ?? - use ocean bottom seismometers - European plume project - Iceland or La Reunion

origin unclear, very hard to see using seismic, not in fluid-dynamical models ???

Concluding from seismology: " Solid mantle and liquid outer core " velocity change at 670km defining upper and lower mantle " Global tomography showing large warm regions under Pacific & Africa " Large variations in seismic velocity near core mantle boundary " Subducting plates are penetrating into lower mantle " Origin of Hotspots still unclear (670km ??, D´´,..)

Paleomagnetism:

Estimating the parameters for convection: The vigor of convection is determined by the Rayleigh number Ra:

Estimates of the viscosity:

Surface heatflux : " Heat flux at the core-mantle boundary " Rate of internal heating due to radioactive decay (20-80%)

Material parameters from experiments

Concluding " Plate motion indicates that there is convection in the Earth mantle " Estimates of the parameters reveals that convection in the Earth mantle is vigorous " But material parameters are poorly constrained and also functions of P,T " By understanding the connection to geochemical observations the parameters could be much better constrained