Composition Mantle and Core

7/31/2019 Composition Mantle and Core

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Composition Mantle and Core


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Overview

Introduction

Density within the earth

Temperature in the earth

Composition of the mantle

Composition of the D

Composition of the Core

Seismology and planetary evolution


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Introduction

Seismology

geology, geodesy, geomagnetism,

cosmochemistry, and physics chemistry of materials

composition

of the earth


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Density within the earth

The density is an important constraint on

the nature of the material.The basic constraint on the earth's density is

that its average is given by the earth's mass.

A second constraint on the density, whichalso indicates a dense core.

Vm


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Seismic velocities and density for the Preliminary

Reference Earth Model (PREM)


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Density, gravity,

pressure, and mass as

functions of depth forthe PREM model


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Seismology gives insight into the geotherm.

The geotherm depends on source of heath

and modes the heat is transferred upward inthe earth.


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A sample mantelgeotherm with steeptemperaturegradients in the

thermal boundarylayer at the top andbottom of themantle and a nearadiabatic gradient inthe lower mantle.

The melting curve,or solidus is alsoshown


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Composition of Mantle

Models of composition of the mantle are derived

by comparing the velocity and density (and

therefore pressure) profiles derived from seismic

data to temperature profiles and results for earthmaterial at high pressure and temperature.

Result from experiments is that the bulk soundspeed and the density for a material are

approximately linearly.


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Composition of Mantle

Bulk soundspeed as a

function of

density for

various

materials.


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Predicted

mineral

assemblages

as a functionof depth for a

mantle of

pyrolite

composition


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A model for the relative proportions of major mineralphase as a function of depth in upper mantle.


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Schematic phasediagrams showingthe relationsbetween the natureof a phase change

and thecorrespondingvelocitydiscontinuity.


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Composition of D

(a). General thermal convection (b). The interaction of

subducted slabs with a chemicalboundary layer consisting ofdense mantle dregs

(c). A chemical boundary layerformed from delaminated post-eclogitic ocean crust broughtdown with the slabs

(d). A mineralogical phasechange


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Core has a composition similar to that of

iron, but with a less dense element of lower

number added.

Cosmochemistry : meteorites are roughly

divided into stony meteorites (mantle) and

iron meteorites (composed of an iron-nickel

alloy).


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Seismology gives a snapshot of the present

stage of earths thermal and chemical

evolution.

Seismology shows the present thickness of

the lithosphere, which may have increased

with time, and provides much of our

information about plate tectonic processes

and mantle convection.


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Evolution of the

core geothermal


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Seismology shows the present sizes of inner

core and outer core that reflect the

progressive freezing of the solid inner core

from the liquid outer core.

Hence the approaches used to study the

earth interior can be applied to other

planets.


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Refraction of Waves


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The basic constraint

Acceleration of gravity at the surface

Mis the volume integral of density, so

if density varies only with depth

Where o is average density

2aGMg M

drrrMa

2

0

4 3

34 aMo

Th d i


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The second constraint

The ratio of the moment inertia to the mass gives a scalar

that depend on the density distribution.

If the earth homogeneous (r) = o

A more realistic case is a two-shell planet

a

drrrC

0

4

3

8

438 aC s2

4 aMs

67,02 MaC

55

0

4

0

4

15

8

3

8cmcm

a

m

r

cradrrdrrC

c

333

4cmcm raM

V i i f d i i h d h


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Variation of density with depth

Region of uniform material density increases with

depth as the material is self-compressedby its own weight.

Gradient of the hydrostatic pressure :

Modulus Bulk :

Adams-Williams Equation

gdr

dP

2rGmg 2

r

Gm

dr

dP

d

dP

d

d

d

dP

d

dPK

2

2

Kr

Gm

dr

dP

dP

d

dr

d

K22 34 r rgrrr rmGrrdrd 2


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Why is Interior Hot?

Pressure- Compressed material heats upadiabatically.

Other Sources of Heat

Sunlight-Not important for Interior, onlypenetrates few kilometers

Radioactive isotopes

Primordial or Original heat, from Earth'saccretion [kinetic energy released by meteoriteimpacts]


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Why is Interior Hot?


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Transport of Heat

Documents

Composition Mantle and Core