Title: Chapter 13: Mid-Ocean Rifts
1Chapter 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.
2Ridge 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
3Oceanic Crust and Upper Mantle Structure
- 4 layers distinguished via seismic velocities
- Deep Sea Drilling Program
- Dredging of fracture zone scarps
- Ophiolites
4Oceanic 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.
5Oceanic 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.
6Oceanic 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.
7Layer 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
8Oceanic 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.
9Layer 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)
10Petrography 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
20Trace Element and Isotope Chemistry
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)
26MORB 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.
27Generation
- 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.
29The 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.
31A modern concept of the axial magma chamber
beneath a fast-spreading ridge
Figure 13-15. After Perfit et al. (1994) Geology,
22, 375-379.
32The 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.
36Figures I dont use in class
Figure 13-6. From Stakes et al. (1984) J.
Geophys. Res., 89, 6995-7028.
37Figures I dont use in class
Figure 13-7. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.