Title: Mid-Ocean Ridge Basalts (MORB), oceanic crust and ophiolites
1Mid-Ocean Ridge Basalts (MORB), oceanic crust and
ophiolites
2- The Mid-Ocean Ridge System
Figure 13-1. After Minster et al. (1974) Geophys.
J. Roy. Astr. Soc., 36, 541-576.
3Rifting of continental crust to form a new ocean
basin
4Subducting 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
5Ophiolites in Himalaya
6Worlds distribution of ophiolites
7Distribution of European Ophiolites
- European ophiolites are related to the collision
of Europe with Africa. - They represent remnants of the Jurassic Tethyan
Ocean
8Oman (Semail) Ophiolite
Greenschist facies shear zones
Layered massive gabbros
Pillows
Dykes
9Obduction
10Oceanic Crust and Upper Mantle Structure
- 4 layers distinguished via seismic velocities
- Deep Sea Drilling Program
- Dredging of fracture zone scarps
- Ophiolites
11Oceanic Crust and Upper Mantle Structure
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.
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13Oceanic 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.
14Oceanic 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.
15Pillow lavas in the Semail Ophiolite
16Basaltic pillows
17Pillow Lavas in the Josephine Ophiolite
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19Submarine eruptions and pillows
20Sheeted 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
21Sheeted Dykes in Semail Ophiolite
22Layer 3 more complex and controversialBelieved
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
23Layered Gabbros and Moho Semail
24Gabbros
25Oceanic 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.
26Plagiogranites
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28Layer 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)
29Serpentinites
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31Evidence for melting in serpentinites
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3365 Ma
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35Petrography and Major Element Chemistry
- A typical MORB is an olivine tholeiite with low
K2O (lt 0.2) and low TiO2 (lt 2.0)
36- The major element chemistry of MORBs
37- 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
38- 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.
39- 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.
40Figure 13-15. After Perfit et al. (1994) Geology,
22, 375-379.
41The 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
42- 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.
43Some complications
- N-MORBs and E-MORBs
- Fast and slow spreading ridges, Harzburgite and
Lherzolite ophiolites
44- 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 gt 65 K2O lt 0.10 TiO2 lt 1.0
- E-MORB (enriched MORB, also called P-MORB for
plume) taps the deeper fertile mantle - Mg gt 65 K2O gt 0.10 TiO2 gt 1.0
45Trace Element and Isotope Chemistry
Figure 13-10. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.
46- E-MORBs (squares) enriched over N-MORBs (red
triangles) regardless of Mg - Lack of distinct break suggests three MORB types
- E-MORBs La/Sm gt 1.8
- N-MORBs La/Sm lt 0.7
- T-MORBs (transitional) intermediate values
Figure 13-11. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.
47- N-MORBs 87Sr/86Sr lt 0.7035 and 143Nd/144Nd gt
0.5030, depleted mantle source - E-MORBs extend to more enriched values stronger
support distinct mantle reservoirs for N-type 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.
48- 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.
49Fast and slow spreading ridges
- Slow-spreading ridges
- lt 3 cm/a
- Fast-spreading ridges
- gt 4 cm/a are considered
- Temporal variations are also known
50Two extension models on ridges
- High magma flux, magmatism gt tectonic
- Lower magma influx, tectonic gt magmatism
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52The Futuna Ridge (W. Pacific), a fast-spreading
ridge
53OSC Overlaping Spreading Center
54Schematic view of a fast ridge
55Oceanic crust of a fast ridge
The Vema Fracture Zone (N. Atlantic)
56A slow ridge
The FAMOUS area, N. Atlantic
57Model of a slow ridge
58Oceanic crust in a slow ridge
59Pillow-lavasophiolitic pillows in the French
alps
Moho
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62Fast vs. slow ridges
- No axial valley
- Important magmatism
- complete sequence (peridotite-gabbros-basalts)
- Deep axial valley
- Moderate magmatism
- Incomplete sequence
63HOT vs. LOT
64- Abundant basalts gt thick crust gt fast ridge
HOT - Moderate amounts of basalts gt finer crust gt
slow ridge LOT
65Thermal modelling melt fraction under fast and
slow ridges
66Restite composition
 K2O MgO CaO
MORB 0.16 7.5 11.5
DM 0.1 31 5
Residues for successive F values Residues for successive F values Residues for successive F values Residues for successive F values
F Â Â Â
0.01 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
MORB
DM
Residues for increasing F
67- Melt abundant fast ridge thick crust
depleted mantle, HOT - Melt moderate slow ridge fine crust less
depleted mantle, LOT
68Fast-spreading ridge
Figure 13-15. After Perfit et al. (1994) Geology,
22, 375-379.
69- 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 the well-developed rift valley - The bulk of the body is well below the liquidus
temperature, so convection and mixing is far less
likely than at fast ridges
Figure 13-16 After Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216.