Transcript Composition of the Earth

Composition of the Earth
GLY 4200
Fall, 2015
1
Interior of the Earth
• Earth’s interior is
divided into zones,
with differing
properties and
compositions
• Since we live on the
crust, it is the most
studied
• The core and mantle
are very important in
understanding the
behavior of the earth
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Composition of the Crust –
Major Elements
• Earth’s crust is
composed
predominantly of eight
elements
• Figure for Si here is
correct – figure 5.2 in
text has a misprint
• Numbers are in weight
percent
3
Abundances Measurements
• We can specify abundances using differ
methods
• The most common are:
 Weight per cent
 Atom per cent
 Volume percent
4
Comparison of Methods
Element
O
Si
Al
Fe
Ca
Na
K
Mg
Weight %
46.60
27.72
8.13
5.00
3.63
2.83
2.59
2.09
Atom %
62.55
21.22
6.47
1.92
1.94
2.64
1.42
1.82
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Minor and Trace Element Definition
• Minor elements have abundances between 0.1
to 1.0 weight percent
• Elements with abundances less than 0.1% are
called trace elements
• Their abundance is usually given in parts per
million (ppm) or parts per billion (ppb)
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Minor and Trace Elements in Crust
• Only 17 elements occur with abundances of at least
200 parts per million (ppm) – in addition to those on
the major element slide, these include:
Element
Weight %
Element
Weight ppm
Ti
0.44%
F
625
H
0.14%
Sr
375
P
0.10%
S
260
Mn
0.09%
C
200
Ba
0.04%
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Ores
• Many valuable elements are in the trace
element range, including the gold group (Au,
Ag, and Cu) and the platinum group (Pt, Pd, Ir,
Os), mercury, lead, and others
• Useage does not always reflect abundance –
copper (55 ppm) is used more than zirconium
(165 ppm) or cerium (60 ppm)
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Effect of Pressure
• As pressure increases, minerals transform to
denser structures, with atoms packed more
closely together
• This is seen in the mantle
• The upper mantle is dominated by the mineral
olivine, Mg2SiO4
• Magnesium is in VI, and Si in IV
9
Transition Zone
• In the transition zone, from about 410 to 660
kilometers below the surface, olivine
transforms to denser structures
 olivine (ρ = 3.22 gm/cm3) →
wadsleyite (ρ = 3.47 gm/cm3) →
ringwoodite (ρ = 3.55 gm/cm3)
• Wadsleyite is β- Mg2SiO4 and Ringwoodite is
γ-Mg2SiO4, polymorphs of olivine
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Hydrous Transition Zone
• The transition zone has been investigated as a hydrous zone
within the earth’s mantle
• Experiments by Ye et al. (2012) on synthetic ringwoodite
indicate it can hold about 2.5% water as hydroyxl groups
• Wadsleyite is also reported to hold up to 3% water
• This has significant implications for the physical properties
and rheology of the transition zone
• Pearson et al. (2014) found ringwoodite in diamonds from
the Juína district of Mato Grosso, Brazil and concluded that
this provided direct evidence that, at least locally, the
transition zone is hydrous, to about 1 weight per cent water
• This was the first evidence for the terrestrial occurrence of any
higher-pressure polymorph of olivine
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Lower Mantle
• Pressures are so great that silicon becomes six
coordinated (CN = VI), and some magnesium
becomes eight-coordinated (perovskite
structure)
 Ringwoodite (ρ = 3.55 gm/cm3) → MgSiO3
(perovskite structure) and (Mg, Fe)O
(magnesiowűstite - halite structure)
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Core
• The core is divided into two regions, the liquid outer
core and the solid inner core
• There is a definite chemical discontinuity between the
lower mantle and the outer core
• The main elements in the core are an iron and nickel
alloy
• Increasing temperature first melts the alloy to make
the outer core
• Increasing pressure freezes the alloy to produce the
inner core
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Outer Core
• Ranges from 2900 to 5100 kilometers below
the earth
• Composition is iron with about 2% nickel
• Density of 9.9 gm/cm3 is too low to be pure
metal
• Best estimates are that silica makes up 9-12%
of the outer core
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Inner Core
• From 5100 to 6371 kilometers below surface
• 80% iron, 20% nickel alloy
• Pressures reach about 3 megabars, or 300,000
megapascals
• Temperature at the center is about 7600ºC
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