Geoscience Reference
In-Depth Information
Table 3.4
Cosmic abundances of the elements (Atoms/10
6
Si)
10
10
10
4
1
H
2.72
×
24
Cr
1.34
×
48
Cd
1.69
72
Hf
0.176
10
9
2
He
2.18
×
25
Mn
9510
49
In
0.184
73
Ta
0.0226
3
Li
59.7
26
Fe
9.00
×
10
5
50
Sn
3.82
74
W
0.137
4
Be
0.78
27
Co
2250
51
Sb
0.352
75
Re
0.0507
5
B
24
28
Ni
4.93
×
10
4
52
Te
4.91
76
Os
0.717
6
C
1.21
×
10
7
29
Cu
514
53
I
0.90
77
Ir
0.660
7
N
2.48
×
10
6
30
Zn
1260
54
Xe
4.35
78
Pt
1.37
8
O
2.01
×
10
7
31
Ga
37.8
55
Cs
0.372
79
Au
0.186
9
F
843
32
Ge
118
56
Ba
4.36
80
HG
0.52
10
Ne
3.76
×
10
6
33
As
6.79
57
La
0.448
81
TI
0.184
5.70
×
10
4
11
Na
34
Se
62.1
58
Ce
1.16
82
Pb
3.15
10
6
12
Mg
1.075
35
Br
11.8
59
Pr
0.174
83
Bi
0.144
×
10
4
13
Al
8.49
36
Kr
45.3
60
Nd
0.836
90
Th
0.0335
×
10
6
14
Si
1.00
37
Rb
7.09
62
Sm
0.261
92
U
0.0090
×
10
4
15
P
1.04
38
Sr
23.8
63
Eu
0.0972
×
10
5
16
S
5.15
39
Y
4.64
64
Gd
0.331
×
17
Cl
5240
40
Zr
10.7
65
Tb
0.0589
10
5
18
Ar
1.04
41
Nb
0.71
66
Dy
0.398
×
19
K
3770
42
Mo
2.52
67
Ho
0.0875
10
4
20
Ca
6.11
44
Ru
1.86
68
Er
0.253
×
21
Sc
33.8
45
Rh
0.344
69
Tm
0.0386
22
Ti
2400
46
Pd
1.39
70
Yb
0.243
23
V
295
47
AG
0.529
71
Lu
0.0369
Anders and Ebihara (1982).
abundant
volatile
elements,
solar
abundance
chondritic in major-element chemistry, then the
deeper mantle must be rich in pyroxene and
garnet and their high-pressure phases. Figure 3.3
is a schematic illustration of how the original
accreting silicate material of a planet (
primitive
mantle
) may fractionate into a melt (
magma ocean
)
and dense refractory crystals. Crystallization of
the magma ocean creates the materials that we
sample from the upper mantle. In a large planet,
the original differentiation may be irreversible
because of the effects of pressure on material
properties,
values are used.
The very light and volatile elements (H, He,
C, N) are extremely depleted in the Earth rel-
ative to the Sun or carbonaceous chondrites.
Moderately volatile elements (such as K, Na, Rb,
Cs and S) are moderately depleted in the Earth.
Refractory elements (such as Ca, Al, Sr, Ti, Ba,
U and Th) are generally assumed to be retained
bytheplanetsintheircosmicratios.Itisalso
likely that magnesium and silicon occur in a
planet in chondritic or cosmic ratios with the
more refractory elements. The Mg/Si ratio, how-
ever, varies somewhat among meteorite classes.
Sometimes it is assumed that magnesium, iron
and silicon may be fractionated by accretional
or pre-accretional processes, but these effects, if
they exist, are slight.
The upper mantle of the Earth is olivine-rich
and has a high Mg/Si ratio compared with the
cosmic ratio (Figures 3.1 to 3.3). If the Earth is
such
as
the
thermal
expansion
coefficient.
Composition of the terrestrial
planets
The mean uncompressed densities of the ter-
restrial planets decreases in the order Mercury,
Earth,
Venus,
Mars,
Moon
(Figure
3.4).
Some
