Download Ophiolites

Mid-Ocean Ridge Basalts (MORB),
oceanic crust and ophiolites
The Mid-Ocean Ridge System
Figure 13-1. After Minster et al.
(1974) Geophys. J. Roy. Astr.
Soc., 36, 541-576.
Rifting of continental crust to form a new ocean basin
Subducting oceanic
lithosphere deforms
sediment at edge of
continental plate
Collision – welding
together of continental
crust
Post-collision: two
continental plates are
welded together, mountain
stands where once was
ocean
Ophiolites in Himalaya
Zanskar
Indian crust
MHT
Distribution of European Ophiolites


European ophiolites are related to the collision of
Europe with Africa.
They represent remnants of the Jurassic Tethyan
Ocean
Oman (Semail) Ophiolite
Greenschist facies
shear zones
Layered … massive
gabbros
Dykes
Pillows
Obduction
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.
Pillow lavas in the Semail Ophiolite
Basaltic pillows
Pillow Lavas in the Josephine Ophiolite
Submarine eruptions and pillows
Sheeted Dyke / Lava Transition
The vertical slabs of rock are dikes intruding into lavas that erupted
on the seafloor. This section represents the transition from lavas to
sheeted dikes and is thought to correspond to seismic Layer 2B
Sheeted Dykes in Semail Ophiolite
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
Layered Gabbros and Moho Semail
Gabbros
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.
Plagiogranites
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)
Serpentinites
(weathered peridotites)
Evidence for melting in serpentinites
16
14
Na2O+K2O
12
10
8
6
F = FeO total
4
2
0
38
48
58
68
78
SiO2
A = Na2O + K2O
M = MgO
Petrography and Major Element Chemistry
Q
Quartzolite
90

A “typical”
MORB is an
olivine tholeiite
with low K2O (<
0.2%) and low
TiO2 (< 2.0%)
90
Quartz-rich
Granitoid
60
60
Granite
Alkali Fs. 20
Quartz Syenite
Alkali Fs. 5
Syenite
A
10
Granodiorite
20
Quartz
Monzodiorite
Syenite35 Monzonite 65 Monzodiorite90
10
(Foid)-bearing (Foid)-bearing (Foid)-bearing
Syenite
Monzonite
Monzodiorite
Quartz
Syenite
Quartz
Monzonite
Qtz. Diorite/
Qtz. Gabbro
5 Diorite/Gabbro/
Anorthosite
P
10 (Foid)-bearing
Diorite/Gabbro
(Foid)-bearing
Alkali Fs.Syenite
(Foid)
(Foid)
Monzosyenite Monzodiorite
60
60
(Foid)olites
F
The major
element
chemistry of
MORBs
Table 13-2. Average analys es and Norm s of MORB
glas s es (BVTP Table 1.2.5.2).
Oxide W t%
All
MAR
EPR
IOR
SiO 2
50.53
50.68
50.19
50.93
TiO2
Al 2O3
FeO*
MgO
CaO
Na 2O
K 2O
1.56
15.27
10.46
7.47
11.49
2.62
0.16
1.49
15.60
9.85
7.69
11.44
2.66
0.17
1.77
14.86
11.33
7.10
11.44
2.66
0.16
1.19
15.15
10.32
7.69
11.84
2.32
0.14
P 2O5
Total
Mg# (m ol) a
Norm
Q
or
ab
an
di
hy
mt
il
ap
0.13
99.69
56
0.12
99.7
58
0.14
99.65
53
0.10
99.68
57
0.94
0.95
22.17
29.44
21.62
17.19
4.44
2.96
0.30
0.76
1.00
22.51
30.13
20.84
17.32
4.34
2.83
0.28
0.93
0.95
22.51
28.14
22.50
16.53
4.74
3.36
0.32
1.60
0.83
19.64
30.53
22.38
18.62
3.90
2.26
0.23
A ll: A ve f rom A tlantic, Pacif ic and Indian Ocean ridges.
MA R: A ve. f rom Mid-A tlantic Ridge. EPR: A ve. f rom EPR.
IOR: A ve. Indian Ocean.
a
Min. value since all Fe as FeO
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

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.

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.
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.
Some complications


N-MORBs and E-MORBs
Fast and slow spreading ridges, Harzburgite
and Lherzolite ophiolites
There must be incompatible-rich and incompatible-poor
source regions for MORB magmas in the mantle beneath
the ridges
 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.

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.
Fast and slow spreading ridges
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
Latitude Rate (cm/a)*
o
21-23 N
3
o
13 N
5.3
o
11 N
5.6
o
8-9 N
6
o
2N
6.3
o
20-21 S
8
o
33 S
5.5
o
54 S
4
o
56 S
4.6
Indian Ocean
SW
1
SE
3-3.7
Central
0.9
o
Mid-Atlantic Ridge 85 N
0.6
o
45 N
1-3
o
36 N
2.2
o
23 N
1.3
o
48 S
1.8
From Wilson (1989). Data from Hekinian (1982), Sclater
(1976), Jackson and Reid (1983).
*half spreading
et al .
Two extension models on ridges

High magma flux,
magmatism >
tectonic

Lower magma
influx, tectonic >
magmatism
The Futuna Ridge (W.
Pacific), a fast-spreading ridge
OSC = Overlaping Spreading Center
Schematic view of a fast ridge
Oceanic crust of a fast ridge
The Vema Fracture Zone (N. Atlantic)
A slow ridge
The “FAMOUS” area,
N. Atlantic
Model of a slow ridge
Oceanic crust in a slow ridge
Pillow-lavas:
ophiolitic pillows in the French alps
Moho
Fast vs. slow ridges



No axial valley
Important magmatism
“complete” sequence
(peridotite-gabbrosbasalts)



Deep axial valley
Moderate magmatism
Incomplete sequence
“HOT” vs. “LOT”


Abundant basalts => thick crust => fast
ridge = HOT
Moderate amounts of basalts => finer crust
=> slow ridge = LOT
Thermal modelling: melt fraction
under fast and slow ridges
K2O
MgO
CaO
MORB
0.16
7.5
11.5
DM
0.1
31
5
Residues for successive F values:
Restite composition
F=
0.10
31.24
4.93
0.02
0.10
31.48
4.87
0.05
0.10
32.24
4.66
0.1
0.09
33.61
4.28
0.2
0.09
36.88
3.38
0.25
0.08
38.83
2.83
0.3
0.07
41.07
2.21
0.4
0.06
46.67
0.67
0.43
0.05
48.73
0.10
14.00
12.00
MORB
10.00
CaO
0.01
8.00
6.00
DM
4.00
Residues for
increasing F
2.00
0.00
0.00
10.00
20.00
30.00
MgO
40.00
50.00
60.00


Melt abundant = fast
ridge = thick crust =
depleted mantle, HOT
Melt moderate = slow
ridge = fine crust = less
depleted mantle, LOT