Geological data, geophysics and modelling of the mantle Yanick Ricard & Joerg Schmalzl " Geophysical observations; Introduction " Geochemical measurements.

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:- 1st 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:

Hotspots:

seem too:

- exist over long time- being stationary- independent of the

plate motion- are geochemically

distinced

Seismic evidence for a mantle plume

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l. 19

95

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