Transcript Overheads for mantle composition

Geological background
• 88 elements found in the Earth's crust -- of these, only 8
make up 98%: oxygen, silicon, aluminum, iron, calcium,
magnesium, potassium and sodium
• In the whole earth, only 4 elements dominate: iron, oxygen,
silicon and magnesium
• These elements go up to make minerals. A mineral is a
naturally occurring, inorganic solid with a characteristic
chemical composition and a crystalline structure
• Even though there are more than 2500 minerals knows, only
nine minerals make up most of the rocks of the Earth's crust - these are the "rock-forming minerals"
Normal igneous rock composition:
Major element
> 1.0 wt. % of the rock or mineral
Minor element
0.1 - 1.0 wt. %
Trace element
<0.1 wt. % (<1,000 ppm)
Earth composition continued…..
 lithophile elements (oxygen, oxides, silicate minerals,
Greek lithos - stone)
 chalcophile (sulphides, Greek khalkos=copper)
 siderophile (metallic, Greek sideros=iron)
The rock-forming minerals
• Minerals containing silicon and oxygen are called silicates.
These make up more than 95% of the crust. The seven
most abundant silicates in the crust are feldspar, quartz,
pyroxene, amphibole, mica, clay minerals, and olivine.
Olivine and pyroxene are the main constituents of the
uppermost mantle.
• Most silicates are formed from SiO4 tetrahedra (a silicon
atom surrounded by four oxygens) arranged in a variety of
ways. Exceptions are quartz and feldspar which are socalled framework silicates. The silicate tetrahedron is
exceptionally stable and allows close-packed structures to
be formed.
The rock-forming minerals (continued)
•
Olivine is made up of individual tetrahedra, pyroxene and
amphibole are made up of chains of tetrahedra, mica and
clay minerals are made up of sheets of tetrahedra (giving
mica its platey character)
• Two other rock-forming minerals are carbonates: calcite
(calcium carbonate) and dolomite (calcium-magnesium
carbonate)
• As low pressure minerals are squeezed, they may suddenly
transform to a denser high-pressure phase.
Spinel structure (ringwoodite)
Undistorted (cubic) perovskite structure
Post-perovskite
Rocks
• Under certain conditions, rocks of the upper mantle and
lower crust melt, forming a molten or semi-molten material
called magma. Igneous rocks form when this magma cools
-- sometimes in surface volcanic eruptions (volcanic rocks)
though more often on continents, magma cools and
solidifies below the surface forming so-called plutonic
rocks
• Igneous rock makes up half of the Earth's crust -- the most
common igneous rocks are granite and basalt. Rock
samples from the mantle are "peridotites" which are
dominantly olivine and pyroxene.
The Earth’s mantle:
How do we know the composition & mineralogy of the mantle?
• Cosmochemical constraints
• Geophysical constraints
• Experimental & theoretical constraints
• Direct samples of the mantle
- basalts
- crystalline samples
* alpine/orogenic
peridotite
* abyssal peridotite
* ophiolite
* nodules/xenoliths
* xenoliths in/&
kimberlite/lamproite
DIRECT SAMPLES (Peridotite)
Common mantle minerals:
• Olivine
• Orthopyroxene
• Clinopyroxene
• Al-phase
 Spinel
 Garnet
Mineralogy of the mantle
Lherzolite is probably fertile unaltered mantle
Dunite and harzburgite are refractory residuum after basalt
has been extracted by partial melting
Tholeiitic basalt
15
10
Figure 10-1 Brown and Mussett, A.
E. (1993), The Inaccessible Earth: An
Integrated View of Its Structure and
Composition. Chapman &
Hall/Kluwer.
5
Lherzolite
Harzburgite
Dunite
0
0.0
0.2
Residuum
0.4
Wt.% TiO2
0.6
0.8
Ophiolite emplacement will lead
to (pressure-release) melting –
this also happens at mid-ocean
ridges
Figure 1.9 Diagrammatic
cross-section through the
upper 200-300 km of the
Earth showing geothermal
gradients reflecting more
efficient adiabatic
(constant heat content)
convection of heat in the
mobile asthenosphere
(steeper gradient in blue) )
and less efficient
conductive heat transfer
through the more rigid
lithosphere (shallower
gradient in red). The
boundary layer is a zone
across which the transition
in rheology and heat
transfer mechanism occurs
(in green). The thickness
of the boundary layer is
exaggerated here for
clarity: it is probably less
than half the thickness of
the lithosphere.
The Geothermal Gradient
Phase diagram for aluminous 4-phase
lherzolite:
Al-phase =

Plagioclase


Spinel


50-80 km
Garnet


shallow (< 50 km)
80-400 km
Si ® VI coord.

> 400 km
Figure 10.2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus
reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.
How does the mantle melt??
1) Lower the pressure
– Adiabatic rise of mantle with no conductive heat loss
– Decompression partial melting could melt up to 30%
Figure 10.4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus
and traverses the shaded melting interval. Dashed lines represent approximate % melting.
• MORB = produced through adiabatic decompression of the
upper mantle along the 60,000 km long ocean ridge system
Typical Ophiolite
Fig. 8-2 (BT)
Ophiolite model
Have to put the melt back in!
• REE (Ringwood)-- pyrolite
• Compare major element ratios with
meteorites (Jagoutz, Zindler, Hart) -losimag
(Can get other elements by using chondritic ratios)
Note:
All peridotites are metamorphic rocks that have had complex
subsolidus history after melt extraction ceased - strain, crystal
segregation, deformation, metasomatism, etc. Thus peridotites
show compositional variations, particularly in their trace
element contents. Nevertheless, they show definite and coherent
trends - the least-depleted peridotites (lowest MgO, but highest
CaO, Al2O3 and other incompatible trace elements that partition
into the liquid phase during partial melting (i.e., fertile) plot
closest to the composition of the primitive mantle (PM).
Trace element content of the PM has also been estimated basically
following similar assumptions and arguments used for the majors.
HSE (Os, Ir, Pt, Ru, Rh, Pd, Re, Au) are low in the Earth’s mantle,
but not low enough as expected - hence the “late veneer” hypothesis..
Mantle samples
Composition of the mantle of the Earth assuming average
solar system element ratios for the whole Earth versus PM
mantle compositions
Ref. solar model
MgO 35.8
Al2O3 3.7
SiO2
51.2
CaO
3.0
FeOt
6.3
Total
100
(1)
36.77
4.49
45.40
3.65
8.10
98.41
(2)
38.1
3.3
45.1
3.1
8.0
97.6
(3)
38.3
4.0
45.1
3.5
7.8
98.7
(4)
36.8
4.1
45.6
3.5
7.5
97.5
(5)
35.5
4.8
46.2
4.4
7.7
98.6
(6)
37.8
4.06
46.0
3.27
(7)
37.8
4.4
45.0
3.5
8.1
98.8
(8)
37.77
4.09
46.12
3.23
7.49
98.7
Mg#, molar Mg/(Mg+Fe); FeOt, all Fe as FeO; (RLE/Mg)N, refractory
lithophile elements normalized to Mg- and CI-chondrites. References:
(1) Palme & ONeil’04 (2) Ringwood’79 = “pyrolite” model (3) Jagoutz
et al.’79 (4) Wa¨nke et al.’84 (5) Palme & Nickel’85 (6) Hart &
Zindler’86 (7) McDonough & Sun’95 (8) Alle`gre et al.’95
Element abundance in Earth’s mantle – normalized to CI chondrite
Proposed structure for oceanic lithosphere
Earth composition continued…..
Post-accretional chemical planetary processes
• shift from low-P to high-P processes on planets
• element segregation - grouping of elements, from
cosmochemical to geochemical