Geoscience Reference
In-Depth Information
Table 14.4
Composition of marginal basin basalts compared with other basalts
North
Mariana
Scotia
Lau
Bonin
Fiji
Arc
Island
Oxide
MORB
MORB
Trough
Sea
Basin
Rift
Plateau
Tholeiites
Tholeiite
SiO
2
49.21
50.47
50.56
51.69
51.33
51.11
49.5
48.7
51.1
49.4
TiO
2
1.39
1.58
1.21
1.41
1.67
1.10
1.2
0.63
0.83
2.5
Al
2
O
3
15.81
15.31
16.53
16.23
17.22
18.20
15.5
16.5
16.1
13.9
FeO
7.19
10.42
8.26
8.28
5.22
8.18
6.2
8.4
—
—
Fe
2
O
3
2.21
—
—
—
2.70
—
3.9
3.4
11.8
12.4
MnO
0.16
—
0.10
0.17
0.15
0.17
0.1
0.29
0.2
—
MgO
8.53
7.46
7.25
6.98
5.23
6.25
6.7
8.2
5.1
8.4
CaO
11.14
11.48
11.59
11.23
9.95
11.18
11.3
12.2
10.8
10.3
Na
2
O
2.71
2.64
2.86
3.09
3.16
3.12
2.7
1.2
1.96
2.13
K
2
O
0.26
0.16
0.23
0.27
1.07
0.38
0.3
0.23
0.40
0.38
P
2
O
5
0.15
0.13
0.15
0.18
0.39
0.18
0.1
0.10
0.14
—
H
2
O
—
1.6
—
1.18
—
1.4
—
—
—
87
Sr
86
Sr
0.7023
0.7028
0.7028
0.7030
0.7036
—
—
—
—
—
/
Gill (1976), Hawkins (1977).
Table 14.5
Representative properties of basalts and kimberlites
Midocean
Alkali-
Ridge
Back-Arc Basin
Island-Arc
Ocean-Island
Olivine
Ratio
Basalt
Basalt
Basalt
Basalt
Basalt
Kimberlite
Rb/Sr
0.007
0.019--0.043
0.01--0.04
0.04
0.05
0.08
K/Rb
800--2000
550--860
300--690
250--750
320--430
130
Sm/Nd
0.34--0.70
0.31
0.28--0.34
0.15--0.35
0.37
0.18
U/Th
0.11--0.71
0.21
0.19--0.77
0.15--0.35
0.27
0.24
K/Ba
110--120
33--97
7.4--36
28--30
13.4
7.5
Ba/Rb
12--24
9--25
10--20
14--20
15
15
La/Yb
0.6
2.5
2--9
7
14
125
Zr/Hf
30--44
43
38--56
44
41
36
Zr/Nb
35--40
4.8
20--30
14.7
2.7
2.3
87
Sr
/
86
Sr
0.7023
0.7027--0.705
0.703--0.706
0.7032--0.706
—
0.71
206
Pb
/
204
Pb
17.5--18.5
18.2--19.1
18.3--18.8
18.0--20.0
—
17.6--20.0
perhaps by contamination of MORB rising from
a deeper layer. The high 3He/4He of Lau Basin
basalts also suggests a shallow origin for 'primi-
tive' gases.
Both back-arc basins and midocean ridges
have shallow seafloor, high heat flow and thin
sedimentary cover. The upper mantle in both
environments has low seismic velocity and high
attenuation. The crusts are typically oceanic and
basement rocks are tholeiitic. The spreading cen-
ter in the back-arc basins, however, is much
less well defined. Magnetic anomalies are less
coherent, seismicity is diffuse and the ridge
crests appear to jump around. Although exten-
sion is undoubtedly occurring, it may be oblique
to the arc and diffuse and have a large shear
