Download Figure 13-5.

Chapter 13: Mid-Ocean Rifts
The Mid-Ocean Ridge System
Figure 13-1. After Minster et al.
(1974) Geophys. J. Roy. Astr.
Soc., 36, 541-576.
Ridge Segments and Spreading Rates
Table 13-1. Spreading Rates of Some Mid-Ocean
Ridge Segments
• Slow-spreading ridges:
< 3 cm/a
• Fast-spreading ridges:
> 4 cm/a are considered
• Temporal variations are
also known
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
From Wilson (1989). Data from Hekinian (1982), Sclater et al .
(1976), Jackson and Reid (1983).
*half spreading
Oceanic Crust and Upper Mantle Structure




4 layers distinguished via seismic velocities
Deep Sea Drilling Program
Dredging of fracture zone scarps
Ophiolites
Oceanic Crust and
Upper Mantle Structure
Typical Ophiolite
Figure 13-3. 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.
Oceanic Crust and Upper Mantle Structure
Layer 1
A thin layer
of pelagic
sediment
Figure 13-4. Modified after
Brown and Mussett (1993) The
Inaccessible Earth: An
Integrated View of Its Structure
and Composition. Chapman &
Hall. London.
Oceanic Crust and Upper Mantle Structure
Layer 2 is basaltic
Subdivided into
two sub-layers
Layer 2A & B =
pillow basalts
Layer 2C = vertical
sheeted dikes
Figure 13-4. Modified after
Brown and Mussett (1993) The
Inaccessible Earth: An
Integrated View of Its Structure
and Composition. Chapman &
Hall. London.
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 and
Upper Mantle
Structure
Discontinuous diorite
and tonalite
(“plagiogranite”)
bodies = late
differentiated liquids
Figure 13-3. 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)
Petrography and Major Element Chemistry


A “typical” MORB is an olivine tholeiite with
low K2O (< 0.2%) and low TiO2 (< 2.0%)
Only glass is certain to represent liquid
compositions

The common crystallization sequence is: olivine (
Mg-Cr spinel), olivine + plagioclase ( Mg-Cr
spinel), olivine + plagioclase + clinopyroxene
Figure 7-2. After Bowen
(1915), A. J. Sci., and
Morse (1994), Basalts and
Phase Diagrams. Krieger
Publishers.

Fe-Ti oxides are restricted to the groundmass, and
thus form late in the MORB sequence
Figure 8-2. AFM diagram for
Crater Lake volcanics,
Oregon Cascades. Data
compiled by Rick Conrey
(personal communication).
The major element chemistry of MORBs


Originally considered to be extremely
uniform, interpreted as a simple petrogenesis
More extensive sampling has shown that they
display a (restricted) range of compositions
Table 13-2. Average Analyses and CIPW Norms of MORBs
(BVTP Table 1.2.5.2)
The major element
chemistry of MORBs
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
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.

MgO and FeO

Al2O3 and CaO

SiO2

Na2O, K2O, TiO2,
P2O5
Figure 13-5. “Fenner-type” variation
diagrams for basaltic glasses from the
Afar region of the MAR. Note different
ordinate scales. From Stakes et al.
(1984) J. Geophys. Res., 89, 6995-7028.
Conclusions about MORBs, and the processes
beneath mid-ocean ridges
 MORBs are not the completely uniform
magmas that they were once considered to
be
 They show chemical trends consistent
with fractional crystallization of olivine,
plagioclase, and perhaps clinopyroxene
 MORBs cannot be primary magmas, but
are derivative magmas resulting from
fractional crystallization (~ 60%)


Fast ridge segments
(EPR)  a broader range
of compositions and a
larger proportion of
evolved liquids
(magmas erupted slightly
off the axis of ridges are
more evolved than those
at the axis itself)
Figure 13-8. Histograms of over 1600 glass
compositions from slow and fast midocean ridges. After Sinton and Detrick
(1992) J. Geophys. Res., 97, 197-216.

For constant Mg# considerable variation is still apparent.
Figure 13-9. Data from Schilling et
al. (1983) Amer. J. Sci., 283, 510-586.
Incompatible-rich and incompatible-poor mantle source
regions for MORB magmas
 N-MORB (normal MORB) taps the depleted upper
mantle source
 Mg# > 65: K2O < 0.10 TiO2 < 1.0
 E-MORB (enriched MORB, also called P-MORB for
plume) taps the (deeper) fertile mantle
 Mg# > 65: K2O > 0.10 TiO2 > 1.0
Trace Element and Isotope Chemistry

REE diagram for MORBs
Figure 13-10.
Data from
Schilling et al.
(1983) Amer. J.
Sci., 283, 510-586.
E-MORBs (squares) enriched over N-MORBs (red
triangles): regardless of Mg#
 Lack of distinct break suggests three MORB types
 E-MORBs La/Sm > 1.8
 N-MORBs La/Sm < 0.7
 T-MORBs (transitional) intermediate values
Figure 13-11. Data from
Schilling et al. (1983) Amer.
J. Sci., 283, 510-586.


N-MORBs: 87Sr/86Sr < 0.7035 and 143Nd/144Nd >
0.5030,  depleted mantle source
E-MORBs extend to more enriched values 
stronger support distinct mantle reservoirs for Ntype and E-type MORBs
Figure 13-12. Data from Ito
et al. (1987) Chemical
Geology, 62, 157-176; and
LeRoex et al. (1983) J.
Petrol., 24, 267-318.
Conclusions:

MORBs have > 1 source region

The mantle beneath the ocean basins is not
homogeneous
N-MORBs tap an upper, depleted mantle
 E-MORBs tap a deeper enriched source
 T-MORBs = mixing of N- and E- magmas
during ascent and/or in shallow chambers

Experimental data: parent was multiply saturated with
olivine, cpx, and opx  P range = 0.8 - 1.2 GPa (25-35 km)
Figure 13-10.
Data from
Schilling et al.
(1983) Amer. J.
Sci., 283, 510-586.
Implications of shallow P range from major element data:
 MORB magmas = product of partial melting of mantle
lherzolite in a rising solid diapir
 Melting must take place over a range of pressures
 The pressure of multiple saturation represents the point at
which the melt was last in equilibrium with the solid
mantle phases
Trace element and isotopic characteristics of the melt reflect the
equilibrium distribution of those elements between the melt and
the source reservoir (deeper for E-MORB)
The major element (and hence mineralogical) character is
controlled by the equilibrium maintained between the melt and
the residual mantle phases during its rise until the melt separates
as a system with its own distinct character (shallow)
MORB Petrogenesis
Generation




Separation of the plates
Upward motion of mantle
material into extended zone
Decompression partial
melting associated with
near-adiabatic rise
N-MORB melting initiated
~ 60-80 km depth in upper
depleted mantle where it
inherits depleted trace
element and isotopic char.
Figure 13-13. After Zindler et al. (1984) Earth
Planet. Sci. Lett., 70, 175-195. and Wilson (1989)
Igneous Petrogenesis, Kluwer.
Generation


Region of melting
Melt blobs separate at about
25-35 km
Figure 13-13. After Zindler et al. (1984) Earth
Planet. Sci. Lett., 70, 175-195. and Wilson (1989)
Igneous Petrogenesis, Kluwer.

Lower enriched
mantle reservoir
may also be
drawn upward and
an E-MORB
plume initiated
Figure 13-13. After Zindler et al.
(1984) Earth Planet. Sci. Lett., 70,
175-195. and Wilson (1989) Igneous
Petrogenesis, Kluwer.
The Axial Magma Chamber
Original Model




Semi-permanent
Fractional crystallization
 derivative MORB
magmas
Periodic reinjection of
fresh, primitive MORB
from below
Dikes upward through the
extending and faulting
roof
Figure 13-14. From Byran and Moore (1977)
Geol. Soc. Amer. Bull., 88, 556-570.



Crystallization near top and
along the sides  successive
layers of gabbro (layer 3)
Dense olivine and pyroxene
crystals  ultramafic
cumulates (layer 4)
Layering in lower gabbros
(layer 3B) from density
currents flowing down the
sloping walls and floor?
Figure 13-14. From Byran and Moore (1977)
Geol. Soc. Amer. Bull., 88, 556-570.
A modern concept of the axial
magma chamber beneath a fastspreading ridge
Figure 13-15. After Perfit et al.
(1994) Geology, 22, 375-379.
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


Melt body  continuous reflector up to several
kilometers along the ridge crest, with gaps at fracture
zones, devals and OSCs
Large-scale chemical variations indicate poor mixing
along axis, and/or intermittent liquid magma lenses,
each fed by a source conduit
Figure 13-16 After Sinton
and Detrick (1992) J.
Geophys. Res., 97, 197-216.
Model for magma chamber beneath a slow-spreading
ridge, such as the Mid-Atlantic Ridge


Dike-like mush zone and a smaller transition zone beneath
well-developed rift valley
Most of body well below the liquidus temperature, so
convection and mixing is far less likely than at fast ridges
Depth (km)
2
Rift Valley
4
6
Moho
Figure 13-16 After
Sinton and
Detrick (1992) J.
Geophys. Res., 97,
197-216.
Transition
zone
Gabbro
Mush
8
10
5
0
Distance (km)
5
10


Nisbit and Fowler (1978) suggested that numerous, small,
ephemeral magma bodies occur at slow ridges (“infinite leek”)
Slow ridges are generally less differentiated than fast ridges
 No continuous liquid lenses, so magmas entering the axial
area are more likely to erupt directly to the surface (hence
more primitive), with some mixing of mush
Depth (km)
2
Rift Valley
4
6
Moho
Transition
zone
Gabbro
Mush
8
10
5
0
Distance (km)
5
Figure 13-16 After Sinton and Detrick (1992)
J. Geophys. Res., 97, 197-216.
10
Figures I don’t use in class
Figure 13-6. From Stakes
et al. (1984) J. Geophys.
Res., 89, 6995-7028.
Figures I don’t
use in class
Figure 13-7. Data from Schilling et
al. (1983) Amer. J. Sci., 283, 510-586.