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Petrology Lecture 7

Petrology Lecture 7. Mid-Ocean Ridge Volcanism GLY 4310 - Spring, 2012. MOR System. MOR Spreading Rates. 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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Petrology Lecture 7

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  1. Petrology Lecture 7 Mid-Ocean Ridge Volcanism GLY 4310 - Spring, 2012

  2. MOR System

  3. MOR Spreading Rates

  4. 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.

  5. Oceanic Crust & Upper Mantle Structure Layer 1A thin layer of pelagic sediment

  6. Oceanic Crust & Upper Mantle Structure Layer 2 is basaltic Subdivided into two sub-layers Layer 2A & B = pillow basalts Layer 2C = vertical sheeted dikes

  7. Layer 3more 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 & 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.

  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. Chemical Analyses of MORB

  11. 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.

  12. CaO/Al2O3vs. Mg. Figure 13-7. From Stakes et al. (1984) J. Geophys. Res., 89, 6995-7028.

  13. MORB Variation Diagrams Figure 13-8. Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.

  14. Glass Composition:Slow 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.

  15. 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)

  16. REE Patterns for MAR MORBS Figure 13-11. Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.

  17. 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.

  18. 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.

  19. 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.

  20. Hekinian et al. (1976) Contr. Min. Pet. 58, 107. The Axial Magma Chamber Original Model • Semi-permanent • Fractional crystallization ® derivative MORB magmas • Periodic reinjectionof fresh, primitive MORB • Dikes upward through extending/faulting roof Figure 13.16.From Byran and Moore (1977) Geol. Soc. Amer. Bull., 88, 556-570.

  21. 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

  22. Axial Magma Chamber, Fast-Spreading Ridge Figure 13-17.After Perfit et al. (1994) Geology, 22, 375-379.

  23. 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

  24. Discontinuous Axial Magma Chamber Figure 13-21 After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.

  25. 2 Rift Valley 4 Depth (km) Gabbro 6 Transition zone Moho Mush 8 10 10 5 0 5 Distance (km) Axial Magma Chamber,Slow-Spreading Ridge Figure 13.22 After Sinton and Detrick (1992)J. Geophys. Res., 97, 197-216

  26. 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).

  27. 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).

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