Petrology Lecture 7
Mid-Ocean Ridge Volcanism
GLY 4310 - Spring, 2015
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MOR System
2
Table 13-1. Spreading Rates of Some Mid-Ocean
Ridge Segments
Category
Ridge
Fast
East Pacific Rise
Slow
Indian Ocean
Mid-Atlantic Ridge
Latitude
21-23oN
13oN
11oN
8-9oN
2 oN
20-21oS
33oS
54oS
56oS
SW
SE
Central
85oN
45oN
36oN
23oN
48oS
Rate (cm/a)*
3
5.3
5.6
6
6.3
8
5.5
4
4.6
1
3-3.7
0.9
0.6
1-3
2.2
1.3
1.8
MOR
Spreading
Rates
From Wilson (1989). Data from Hekinian (1982), Sclater et al .
(1976), Jackson and Reid (1983).
*half spreading
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Oceanic Crust Cross-Section
Figure 13-5 Modified
after Brown and
Mussett (1993) The
Inaccessible Earth: An
Integrated View of Its
Structure and
Composition.
Chapman & Hall.
London.
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Oceanic Crust & Upper Mantle Structure
Layer 1
A thin layer
of pelagic
sediment
Oceanic Crust & Upper Mantle Structure
Layer 2 is basaltic
Subdivided into
two sub-layers
Layer 2A & B =
pillow basalts
Layer 2C = vertical
sheeted dikes
Layer 3 more complex and controversial
Believed to be mostly gabbros, crystallized from a shallow axial
magma chamber (feeds the dikes and basalts)
Layer 3A = upper
isotropic and
lower, somewhat
foliated
(“transitional”)
gabbros
Layer 3B is more
layered, & may
exhibit cumulate
textures
Oceanic Crust &
Upper Mantle
Structure
Discontinuous diorite
and tonalite
(“plagiogranite”)
bodies = late
differentiated liquids
Figure 13.4. Lithology and thickness of
a typical ophiolite sequence, based on
the Samial Ophiolite in Oman. After
Boudier and Nicolas (1985) Earth
Planet. Sci. Lett., 76, 84-92.
Layer 4 = ultramafic rocks
Ophiolites: base of 3B
grades into layered
cumulate wehrlite &
gabbro
Wehrlite intruded into
layered gabbros
Below  cumulate dunite
with harzburgite xenoliths
Below this is a tectonite
harzburgite and dunite
(unmelted residuum of the
original mantle)
Table 13-2. Average Analyses and CIPW Norms of MORBs
(BVTP Table 1.2.5.2)
Oxide (wt%)
SiO2
TiO2
Al2O3
FeO*
MgO
CaO
Na2O
K2O
P2O5
Total
All
50.5
1.56
15.3
10.5
7.47
11.5
2.62
0.16
0.13
99.74
MAR
50.7
1.49
15.6
9.85
7.69
11.4
2.66
0.17
0.12
99.68
EPR
50.2
1.77
14.9
11.3
7.10
11.4
2.66
0.16
0.14
99.63
IOR
50.9
1.19
15.2
10.3
7.69
11.8
2.32
0.14
0.10
99.64
Norm
q
or
ab
an
di
hy
ol
mt
il
ap
0.94
0.95
22.17
29.44
21.62
17.19
0.0
4.44
2.96
0.30
0.76
1.0
22.51
30.13
20.84
17.32
0.0
4.34
2.83
0.28
0.93
0.95
22.51
28.14
22.5
16.53
0.0
4.74
3.36
0.32
1.60
0.83
19.64
30.53
22.38
18.62
0.0
3.90
2.26
0.23
Chemical
Analyses of
MORB
All: Ave of glasses from Atlantic, Pacific and Indian Ocean ridges.
MAR: Ave. of MAR glasses. EPR: Ave. of EPR glasses.
IOR: Ave. of Indian Ocean ridge glasses.
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Fenner
Diagrams for
MORB
Figure 13-6. “Fenner-type”
variation diagrams for
basaltic glasses from the
Amar region of the MAR.
Note different ordinate
scales. From Stakes et al.
(1984) J. Geophys. Res.,
89, 6995-7028.
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CaO/Al2O3
vs. Mg.
Figure 13-7. From Stakes et al. (1984) J. Geophys.
Res., 89, 6995-7028.
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MORB
Variation
Diagrams
Figure 13-8. Data
from Schilling et al.
(1983) Amer. J. Sci.,
283, 510-586.
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Glass
Composition:
Slow vs. Fast
Spreading
Ridges
Figure 13-9. Histograms of over
1600 glass compositions from
slow and fast mid-ocean ridges.
After Sinton and Detrick (1992)
J. Geophys. Res., 97, 197-216.
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K2O vs. Mg for MAR MORB
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REE
Patterns for
MAR
MORBS
Figure 13-11. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.
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LREE vs. Mg#
• Blue = E-Morb
• Red = N-Morb
• Green = T-Morb
Figure 13-12. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.
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143Nd/ 144Nd
vs.
87Sr/ 86Sr
Figure 13-13. Data from Ito et al. (1987) Chemical
Geology, 62, 157-176; and LeRoex et al. (1983) J.
Petrol., 24, 267-318.
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Generation
of N-MORB
and EMORB
Figure 13-14.
After Zindler et al.
(1984) Earth Planet.
Sci. Lett., 70, 175
-195. and Wilson
(1989) Igneous
Petrogenesis,
19
Kluwer.
The Axial Magma Chamber
Original Model
• Semi-permanent
• Fractional crystallization
 derivative MORB
magmas
• Periodic reinjection of
fresh, primitive MORB
• Dikes upward through
extending/faulting roof
Figure 13.16. From Byran and Moore (1977)
Geol. Soc. Amer. Bull., 88, 556-570.
Hekinian et al. (1976)
Contr. Min. Pet. 58, 107.
Semi-Permanent
Axial Magma
Chamber
• Infinite onion model, since it
resembled an infinite number of
onion shells created from within
and added to the walls
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Axial Magma Chamber,
Fast-Spreading Ridge
Figure 1317. After
Perfit et al.
(1994)
Geology, 22,
375-379.
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Crystal Mush
Zone
The crystal mush zone
contains perhaps 30%
melt and constitutes
an excellent boundary
layer for the in situ
crystallization process
proposed by Langmuir
Figure 11.12 From Winter
(2001) An Introduction to
Igneous and Metamorphic
Petrology. Prentice Hall
Discontinuous Axial Magma
Chamber
Figure 13-21 After Sinton and Detrick
(1992) J. Geophys. Res., 97, 197-216.
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Axial Magma Chamber,
Slow-Spreading Ridge
Depth (km)
2
Rift Valley
4
Gabbro
6
Moho
Transition
zone
Mush
8
10
5
0
Distance (km)
5
10
Figure 13.22 After Sinton and Detrick
(1992) J. Geophys. Res., 97, 197-216
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Oceanic Basalt
• Figure 10-16 (a) Initial 143Nd/144Nd vs. 87Sr/86Sr for oceanic
basalts. From Wilson (1989). Igneous Petrogenesis. Unwin
Hyman/Kluwer. Data from Zindler et al. (1982) and Menzies
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(1983).
Ultramafic
Xenoliths
• Figure 10-16 (b) Initial 143Nd/144Nd vs. 87Sr/86Sr for
mantle xenoliths. From Wilson (1989). Igneous
Petrogenesis. Unwin Hyman/Kluwer. Data from Zindler et
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al. (1982) and Menzies (1983).