Petrology Lecture 7

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Petrology Lecture 7

• Mid-Ocean Ridge Volcanism
• GLY 4310 - Spring, 2012

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MOR System
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MOR Spreading Rates
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Oceanic Crust Cross-Section
Figure 13-5 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 Upper Mantle Structure

• Layer 1 A thin layer of pelagic sediment

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Oceanic Crust Upper Mantle Structure
Layer 2 is basaltic Subdivided into two
sub-layers
Layer 2A B pillow basalts Layer 2C vertical
sheeted dikes
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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 Upper Mantle Structure
Discontinuous diorite and tonalite
(plagiogranite) bodies late differentiated
liquids
Figure 13.4. 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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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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Chemical Analyses of MORB
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Fenner Diagrams for MORB
Figure 13-6. Fenner-type variation diagrams for
basaltic glasses from the Amar region of the MAR.
Note different ordinate scales. From Stakes et
al. (1984) J. Geophys. Res., 89, 6995-7028.
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CaO/Al2O3vs. Mg.
Figure 13-7. From Stakes et al. (1984) J.
Geophys. Res., 89, 6995-7028.
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MORB Variation Diagrams
Figure 13-8. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.
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Glass CompositionSlow vs. Fast Spreading Ridges
Figure 13-9. Histograms of over 1600 glass
compositions from slow and fast mid-ocean ridges.
After Sinton and Detrick (1992) J. Geophys. Res.,
97, 197-216.
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K2O vs. Mg for MAR MORB
Fig. 13-10 shows the variation in K2O with Mg
for the MAR data set of Schilling et al. (1983)
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REE Patterns for MAR MORBS
Figure 13-11. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.
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LREE vs. Mg

• Blue E-Morb
• Red N-Morb
• Green T-Morb

Figure 13-12. Data from Schilling et al. (1983)
Amer. J. Sci., 283, 510-586.
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143Nd/ 144Nd vs. 87Sr/ 86Sr
Figure 13-13. 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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Generation of N-MORB and E-MORB

• Figure 13-14.
• After Zindler et al.
• (1984) Earth Planet.
• Sci. Lett., 70, 175
• -195. and Wilson
• (1989) Igneous
• Petrogenesis,
• Kluwer.

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The Axial Magma Chamber

• Original Model
• Semi-permanent
• Fractional crystallization derivative MORB
  magmas
• Periodic reinjection of fresh, primitive MORB
• Dikes upward through extending/faulting roof

Figure 13.16. From Byran and Moore (1977) Geol.
Soc. Amer. Bull., 88, 556-570.
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Semi-Permanent Axial Magma Chamber

• Infinite onion model, since it resembled an
  infinite number of onion shells created from
  within and added to the walls

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Axial Magma Chamber, Fast-Spreading Ridge
Figure 13-17. After Perfit et al. (1994) Geology,
22, 375-379.
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Crystal Mush Zone
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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Discontinuous Axial Magma Chamber
Figure 13-21 After Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216.
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Axial Magma Chamber,Slow-Spreading Ridge
Figure 13.22 After Sinton and Detrick (1992) J.
Geophys. Res., 97, 197-216
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Oceanic Basalt

• Figure 10-16 (a) Initial 143Nd/144Nd vs.
  87Sr/86Sr for oceanic basalts. From Wilson
  (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.
  Data from Zindler et al. (1982) and Menzies
  (1983).

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Ultramafic Xenoliths

• Figure 10-16 (b) Initial 143Nd/144Nd vs.
  87Sr/86Sr for mantle xenoliths. From Wilson
  (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.
  Data from Zindler et al. (1982) and Menzies
  (1983).