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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. Ridge Segments and Spreading ... – PowerPoint PPT presentation

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Title: Chapter 13: Mid-Ocean Rifts


1
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.
6
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.
7
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
8
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)
10
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

11
  • 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).
13
  • 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

14
  • The major element chemistry of MORBs

15
  • 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)

17
  • 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

20
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.
23
  • 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

24
  • 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)

26
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.
30
  • 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.
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