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Chapter 13: Mid-Ocean Rifts

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 principal types of basalt in the ocean basins. Tholeiitic Basalt and Alkaline Basalt.

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Chapter 13: Mid-Ocean Rifts

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  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. 2 principal types of basalt in the ocean basins Tholeiitic Basalt andAlkalineBasalt Common petrographic differences between tholeiitic and alkaline basalts • Table 10-1 Tholeiitic Basalt Alkaline Basalt Usually fine-grained, intergranular Usually fairly coarse, intergranular to ophitic Groundmass No olivine Olivine common Clinopyroxene = augite (plus possibly pigeonite) Titaniferous augite (reddish) Orthopyroxene (hypersthene) common, may rim ol. Orthopyroxene absent No alkali feldspar Interstitial alkali feldspar or feldspathoid may occur Interstitial glass and/or quartz common Interstitial glass rare, and quartz absent Olivine rare, unzoned, and may be partially resorbed Olivine common and zoned Phenocrysts or show reaction rims of orthopyroxene Orthopyroxene uncommon Orthopyroxene absent Early plagioclase common Plagioclase less common, and later in sequence Clinopyroxene is pale brown augite Clinopyroxene is titaniferous augite, reddish rims after Hughes (1982) and McBirney (1993).

  3. Thin section textures of basalts

  4. Pressure Effects • Raises melting point • Shift eutectic position • (and thus X of first melt, etc.) Figure 6-15. The system Fo-SiO2 at atmospheric pressure and 1.2 GPa. After Bowen and Schairer (1935), Am. J. Sci., Chen and Presnall (1975) Am. Min.

  5. Ridge Segments and Spreading Rates • Slow-spreading ridges: • < 3 cm/a • Fast-spreading ridges: • > 4 cm/a are considered • Temporal variations are also known

  6. Oceanic Crust and Upper Mantle Structure 4 layers distinguished via seismic velocities Deep Sea Drilling Program Dredging Ophiolites

  7. 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.

  8. Oceanic Crust and Upper Mantle Structure Layer 1A 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.

  9. 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.

  10. Layer 3 more complex and controversialBelieved to be mostly gabbros, crystallized from a shallow axial magma chamber (feeds the dikes and basalts) Layer 3A = somewhat foliated (“transitional”) gabbros Layer 3B is more layered, & may exhibit cumulate textures

  11. Layer 4 = ultramafic rocks layered cumulate wehrlite, dunite, harzburgite

  12. Oceanic lithosphere • Note seds get thicker away from ridge

  13. Heat flow drops away from ridge • How? • Convecting seawater through fractured basalt • Process decreases away from ridge

  14. Seafloor gets deeper away from ridge • Why? • Crust cools, contracts, more dense, “sinks”

  15. Petrography • A “typical” MORB is an olivine tholeiite • Only glass is certain to represent liquid compositions (found in rinds of pillows) • Pillow rinds with forsterite, little plagioclase and rare cpx

  16. 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.

  17. Major element chemistry • Uniform • SiO2 47-51% • Low concentration of incompatibles (Ti, P) • Phenocrysts: olivine, plag, cpx • Anything taking up Fe? • Trend on AFM diagram?

  18. Fe-Ti oxides are restricted to the groundmass, and thus form late in the MORB sequence Plag, cpx, Fe-Ti oxides, no olivine in groundmass Figure 8-2. AFM diagram for Crater Lake volcanics, Oregon Cascades. Data compiled by Rick Conrey (personal communication).

  19. The major element chemistry of MORBs

  20. 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.

  21. 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%) (too low MgO)

  22. 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

  23. Trace Element and Isotope Chemistry • REE diagram for MORBs Figure 13-10. Data from Schilling et al. (1983) Amer. J. Sci., 283, 510-586.

  24. Incompatible-rich and incompatible-poor mantle source regions for MORB magmas • N-MORB (normal MORB) taps the depleted upper mantle source • E-MORB (enriched MORB) taps the (deeper) fertile mantle

  25. Evidence of extraction of magmas from upper mantle peridotite • More melting episodes, more depleted mantle gets

  26. Conclusions: • MORBs have > 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

  27. Mid Atlantic Ridge • Slow spreading ridge • Small magma production • Lower T • More narrow zone of melt

  28. 2 Rift Valley 4 Depth (km) Gabbro 6 Transition zone Moho Mush 8 10 10 5 0 5 Distance (km) • 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 13-16After Sinton and Detrick (1992)J. Geophys. Res., 97, 197-216.

  29. 2 Rift Valley 4 Depth (km) Gabbro 6 Transition zone Moho Mush 8 10 10 5 0 5 Distance (km) • 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 13-16After Sinton and Detrick (1992)J. Geophys. Res., 97, 197-216.

  30. East Pacific Rise • 1-2% melt in dotted region below ridge • Fast spreading ridge • “shallow” zone of melt • Broad zone of melt • 100 km’s wide • 150 km deep

  31. Iceland • On Mid Atlantic Ridge • Surface version of submarine ridge • Lots more magma production than most rifts • Unusual hot mantle below

  32. Iceland plume • 300 km wide • 410 km deep • Very deep source • Ascending plume begins melting deeper • Generates more melt • Builds thicker crust

  33. Iceland mantle melting • Hotter rising mantle • Hits solidus earlier • More melt generated

  34. 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.

  35. 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.

  36. Lower enriched mantle reservoir may also be drawn upward and an E-MORBplume initiated Figure 13-13. After Zindler et al. (1984) Earth Planet. Sci. Lett., 70, 175-195. and Wilson (1989) Igneous Petrogenesis, Kluwer.

  37. Rising magma fills chamber • Phenocrysts form • Episodically replenished with new magma, mixes with more evolved magma

  38. Source of mid-ocean ridge basalts

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