Mid-Ocean Ridge Basalts (MORB), oceanic crust and ophiolites

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Mid-Ocean Ridge Basalts (MORB), oceanic crust and
ophiolites
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• The Mid-Ocean Ridge System

Figure 13-1. After Minster et al. (1974) Geophys.
J. Roy. Astr. Soc., 36, 541-576.
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Rifting of continental crust to form a new ocean
basin
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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
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Ophiolites in Himalaya
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Worlds distribution of ophiolites
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Distribution of European Ophiolites

• European ophiolites are related to the collision
  of Europe with Africa.
• They represent remnants of the Jurassic Tethyan
  Ocean

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Oman (Semail) Ophiolite
Greenschist facies shear zones
Layered massive gabbros
Pillows
Dykes
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Obduction
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Oceanic Crust and Upper Mantle Structure

• 4 layers distinguished via seismic velocities
• Deep Sea Drilling Program
• Dredging of fracture zone scarps
• Ophiolites

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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.
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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.
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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.
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Pillow lavas in the Semail Ophiolite
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Basaltic pillows
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Pillow Lavas in the Josephine Ophiolite
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Submarine eruptions and pillows
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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
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Sheeted Dykes in Semail Ophiolite
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Layer 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
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Layered Gabbros and Moho Semail
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Gabbros
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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.
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Plagiogranites
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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)
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Serpentinites

• (weathered peridotites)

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Evidence for melting in serpentinites
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65 Ma
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Petrography and Major Element Chemistry

• A typical MORB is an olivine tholeiite with low
  K2O (lt 0.2) and low TiO2 (lt 2.0)

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• The major element chemistry of MORBs

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• 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

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• 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.
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• 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.
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Figure 13-15. After Perfit et al. (1994) Geology,
22, 375-379.
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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
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• 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.
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Some complications

• N-MORBs and E-MORBs
• Fast and slow spreading ridges, Harzburgite and
  Lherzolite ophiolites

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• 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

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Trace Element and Isotope Chemistry

• REE diagram for MORBs

Figure 13-10. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.
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• 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.
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• 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.
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• 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.
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Fast 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

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Two extension models on ridges

• High magma flux, magmatism gt tectonic
• Lower magma influx, tectonic gt magmatism

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The Futuna Ridge (W. Pacific), a fast-spreading
ridge
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OSC Overlaping Spreading Center
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Schematic view of a fast ridge
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Oceanic crust of a fast ridge
The Vema Fracture Zone (N. Atlantic)
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A slow ridge
The FAMOUS area, N. Atlantic
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Model of a slow ridge
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Oceanic crust in a slow ridge
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Pillow-lavasophiolitic pillows in the French
alps
Moho
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Fast vs. slow ridges

• No axial valley
• Important magmatism
• complete sequence (peridotite-gabbros-basalts)

• Deep axial valley
• Moderate magmatism
• Incomplete sequence

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HOT vs. LOT
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• Abundant basalts gt thick crust gt fast ridge
  HOT
• Moderate amounts of basalts gt finer crust gt
  slow ridge LOT

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Thermal modelling melt fraction under fast and
slow ridges
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Restite 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      
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
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• Melt abundant fast ridge thick crust
  depleted mantle, HOT
• Melt moderate slow ridge fine crust less
  depleted mantle, LOT

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Fast-spreading ridge
Figure 13-15. After Perfit et al. (1994) Geology,
22, 375-379.
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• 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.