Geoscience Reference
In-Depth Information
Table 14.1
Representative compositions of basalts and andesites.
Tholeiite
Oceanic Cont. Rift
Continental
Alkali
Alkali
Andesite
Ridge
Arc
Rift
Island
High-Al
Basalt
Basalt
Arc
Low-K
High-K
SiO
2
49.8
51.1
50.3
49.4
51.7
47.4
47.8
57.3
59.5
60.8
TiO
2
1.5
0.83
2.2
2.5
1.0
2.9
2.2
0.58
0.70
0.77
Al
2
O
3
16.0
16.1
14.3
13.9
16.9
18.0
15.3
17.4
17.2
16.8
Fe
2
O
3
∗
10.0
11.8
13.5
12.4
11.6
10.6
12.4
8.1
6.8
5.7
MgO
7.5
5.1
5.9
8.4
6.5
4.8
7.0
3.5
3.4
2.2
CaO
11.2
10.8
9.7
10.3
11.0
8.7
9.0
8.7
7.0
5.6
Na
2
O
2.75
1.96
2.50
2.13
3.10
3.99
2.85
2.63
3.68
4.10
K
2
O
0.14
0.40
0.66
0.38
0.40
1.66
1.31
0.70
1.6
3.25
Cr
300
50
160
250
40
67
400
44
56
3
Ni
100
25
85
150
25
50
100
15
18
3
Co
32
20
38
30
50
25
60
20
24
13
Rb
1
5
31
5
10
33
200
10
30
90
Cs
0.02
0.05
0.2
0.1
0.3
2
>
3
∼
0.1
0.7
1.5
Sr
135
225
350
350
330
800
1500
215
385
620
Ba
11
50
170
100
115
500
700
100
270
400
Zr
85
60
200
125
100
330
800
90
110
170
La
3.9
3.3
33
7.2
10
17
54
3.0
12
13
Ce
12
6.7
98
26
19
50
95
7.0
24
23
Sm
3.9
2.2
8.2
4.6
4.0
5.5
9.7
2.6
2.9
4.5
Eu
1.4
0.76
2.3
1.6
1.3
1.9
3.0
1.0
1.0
1.4
Gd
5.8
4.0
8.1
5.0
4.0
6.0
8.2
4.0
3.3
4.9
Tb
1.2
0.40
1.1
0.82
0.80
0.81
2.3
1.0
0.68
1.1
Yb
4.0
1.9
4.4
1.7
2.7
1.5
1.7
2.7
1.9
3.2
U
0.10
0.15
0.4
0.18
0.2
0.75
0.5
0.4
0.7
2.2
Th
0.18
0.5
1.5
0.67
1.1
4.5
4.0
1.3
2.2
5.5
Th/U
1.8
3.3
3.8
3.7
5.9
6.0
8.0
3.2
3.1
2.5
K/Ba
105
66
32
32
12
28
16
58
49
68
K/Rb
1160
660
176
630
344
420
55
580
440
300
Rb/Sr
0.007
0.022
0.089
0.014
0.029
0.045
0.13
0.046
0.078
0.145
La/Yb
1.0
1.7
10
4.2
3.7
11
32
1.1
6.3
4.0
∗
TotalFeasFe
2
O
3
. (After Candie, 1982.)
(LIL) elements (such as K, Rb, Cs, Sr, Ba, Zr, U,
Th and REE) and depleted isotopic ratios (i.e.
low integrated Rb/Sr, U/Pb etc. ratios). Tholeiitic
basalts have low viscosity and flow for long dis-
tances, constructing volcanic forms of large area
and small slope. The major- and trace-element dif-
ferences between the tholeiitic and calc-alkaline
suites can be explained by varying proportions
of olivine, plagioclase and pyroxene crystallizing
from a basaltic parent melt.
The world-encircling midocean-ridge system
accounts for more than 90% of the material flow-
ing out of the Earth's interior. The whole ocean
floor has been renewed by this activity in less
than 200 Ma. Hotspots represent less than 10% of
the material and the heat flow. The presence of a
crust and core indicates that the Earth is a differ-
entiated body. Major differentiation was contem-
poraneous with the accretion of the Earth. The
high concentrations of incompatible elements in
