Chapter 13: Mid-Ocean Rifts

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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.
2
Ridge Segments and Spreading Rates

• Slow-spreading ridges
• lt 3 cm/a
• Fast-spreading ridges
• gt 4 cm/a are considered
• Temporal variations are also known

3
Oceanic Crust and Upper Mantle Structure

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

4
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.
5
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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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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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.
9
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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Petrography and Major Element Chemistry

• A typical MORB is an olivine tholeiite with low
  K2O (lt 0.2) and low TiO2 (lt 2.0)
• Only glass is certain to represent liquid
  compositions

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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.
12

• 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).
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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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• The major element chemistry of MORBs

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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.
16

• 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)

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• 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 mid-ocean ridges.
After Sinton and Detrick (1992) J. Geophys. Res.,
97, 197-216.
18

• For constant Mg considerable variation is still
  apparent.

Figure 13-9. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.
19

• Incompatible-rich and incompatible-poor mantle
  source regions for MORB magmas
• 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.
21

• 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.
22

• 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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• Conclusions
• MORBs have gt 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

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• 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.
25

• 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)

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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.
27
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.
28

• 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.
29
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.
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• 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.
31
A modern concept of the axial magma chamber
beneath a fast-spreading ridge
Figure 13-15. After Perfit et al. (1994) Geology,
22, 375-379.
32
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
33

• 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.
34

• 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

Figure 13-16 After Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216.
35

• 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

Figure 13-16 After Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216.
36
Figures I dont use in class
Figure 13-6. From Stakes et al. (1984) J.
Geophys. Res., 89, 6995-7028.
37
Figures I dont use in class
Figure 13-7. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.