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SIO 224
Models for bulk Earth, crust,
mantle, and core composition
Composition of Earth cannot be understood in isolation
• Earth formation closely linked to sun and meteorite formation
• Nucleosynthesis in stars, from mainly H + He to other elements
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
Earth composition continued…..
 lithophile elements (oxygen, oxides, silicate minerals,
Greek lithos - stone)
 chacophile (sulphides, Greek khalkos=copper)
 siderophile (metallic, Greek sideros=iron)
Earth composition composition…..
Goldschmidt’s classification is based on distribution in
meteorites and Earth’s major geochemical reservoirs, but
elements can still be further grouped based on their relative
behavior in the Earth’s silicate portion, mantle and crust
Relative abundances:
• of the >100 known elements, only 90 occur naturally on Earth
• only 14 elements make up > 99% of the naturally occurring
inorganic chemical compounds (minerals)
 H, C, O, Na, Mg, Al, Si, P, S, K, Ca, Ti, Mn, and Fe
 O, Mg, Si, Fe, Al, and Ca make up > 99% of the BSE
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)
Core composition:
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
COSMOCHEMICAL
PREVIOUSLY…..
The six most abundant, nonvolatile rock-forming elements in the Sun are
Si (100), Mg (104), Fe (86), S (43), Al (8.4), and Ca (6.2). As the mantle
of the Earth contains neither metal nor significant amounts of Fe3+, the
sum of all oxides (by weight) must add up to 100%:
MgO + SiO2 + Al2O3 + CaO + FeO = 100% (1)
By inserting into Equation (1) average solar system abundance ratios,
e.g., Si/Mg, Ca/Mg, and Al/Mg, one obtains
2:62 x MgO + FeO = 100% (2)
Considering that iron is distributed between mantle and core, the mass
balance for iron can be written as
Fecore x 0.325 + Femantle x 0.675 = Fetotal (3)
and similarly for magnesium, assuming a magnesium-free core,
Mgmantle x 0.675 = Mgtotal (4)
By assuming that sulfur is quantitatively contained in the core and
accounting for nickel in the core (Fetotal/Ni = 17), the amount of iron in
the core, Fecore, is calculated to be 75%. From Equations (2) to (4)
and by using the solar abundance ratio for Fetotal/Mgtotal, the
hypothetical composition of the Earth’s mantle is obtained as:
Earth’s mantle solar model
MgO
SiO2
FeO
Al2O3
CaO
35.8
51.2
6.3
3.7
3.0
DIRECT SAMPLES (Peridotite)
Common mantle minerals:
• Olivine
• Orthoppyroxene
• Clinopyroxene
• Spinel
• Garnet
Mineralogy of the source
Pyrolite:
hypothetical
mixture of of
residual
mantle material
(xenolith) +
primitive
basaltic magma
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
GEOPHYSICAL & EXPERIMENTAL CONSTRAINTS
Pressure increases with depth:
P = gh; for the upper few hundred km,  = 3.3 g/cc
= 0.33 kbar h, where h is in km.
Is the mantle compositionally layered or not………
Oceanic basalts as probes of the upper mantle
Adiabatic decompression partial melting of the mantle:
Intraplate magmatism: linear island chains & LIPs
Partial melting of the mantle (intraplate setting):
Typical
normalized
element
patterns of
terrestrial
igneous rocks
Convergent margin magmatism
Partial Melting of the mantle (subduction zone setting):
Major elements of island
arc volcanic rocks &
magma series
IAB : MORB:
MgO <
(Mg#)<
K2O >
Al2O3 >
(although variable &
some C-A basalts
have 17-20% (high-Al
basalts), some believe
that these are parental
to C-A series rocks)
Trace elements of island arc volcanic rocks & magma series
(continued)
Note enrichment in LIL and depletion in HFS
Several potential source components for island arc magmas
Continental crust
From Rudnick & Gao, 2005
Continents (early studies):
•an average intermediate or andesitic composition
•only 0.6% by mass of BSE, but 20-70% of incomp. elements
•contains the oldest rocks (4.0 Ga Acasta gneiss) & minerals
(4.4 Ga detrital zircon) = rich geological history
•seismically divided into
•upper- [granodiorite]
•deep•middle•lower-crust [high-grade metamorphic rocks &
granulite xenoliths; increasing metamorphic grade &
mafic rx = more mafic?]
•vertically stratified and laterally heterogeneous
Upper
crust
composition:
- weighted
averages
of exposed
rocks
- averages
of finegrained
sediments
or glacial
deposits
Deep crust composition:
(1) samples from deep crust
(2) seismic velocities
(3) heat flow measurements
Comparison of (a) REE &
(b) additional trace
element compositions of
the upper, middle and
lower crust recommended
by Rudnick & Gao ‘05.
Middle crust metasedimentary rocks,
but dominated by DTTG
Deep crust mafic
Comparison of (a) REE &
(b) additional trace element
compositions of the bulk
crust - this study = Rudnick
& Gao, ‘05.
Bulk cc composition:
•intermediate
•high Mg#
•up to 50% of BSE’s inc. el.
•depleted in Nb relative to La
•enriched in Pb
•subchondritic Nb/Ta
If the crust grows ultimately by igneous processes,
then the disparity between crust and primary mantle
melt compositions requires additional process(es) such as:
1) Recycling of mafic/ultramafic lower crust and upper
mantle (density foundering or delamination)
2) Mixing silicic melts from subducted slab and mafic melt
from mantle peridotite (e.g., Archean DTTG)
3) Weathering of the crust, with preferential recycling of Mg
& Ca into the mantle via subduction (not supported
by observation)
4) Ultramafic cumulates complementary to andesitic crust
are present in the upper mantle
But Nb depletion suggests that ~80% of the crust was
generated in a convergent margin
Major elements of island
arc volcanic rocks &
magma series
IAB : MORB:
MgO <
(Mg#)<
K2O >
Al2O3 >
(although variable &
some C-A basalts
have 17-20% (high-Al
basalts), some believe
that these are parental
to C-A series rocks)
Selected references:
Ringwood, A.E., 1979, Origin of the Earth and Moon,
Springer-Verlag, N.Y., 295 p.
Holland, H.D. and Turekian, K.K., 2003, Treatise on
Geochemistry, vol. 1, Meteorites, Comets and Planets.
Stevenson, D.J., 1981, Models of the Earth’s Core,
Science 214, #4521, 611-619.