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Week 7: Feb. 21 - Feb. 25

The mantle and generation of basalt (cont'd)
  • Sources of information about the mantle (cont'd)
    • ophiolites
      • seismic layers 1, 2 and 3
      • sediments, pillow lava, sheeted dikes, isotropic gabbro, layered gabbro, layered peridotite, tectonized peridotite, metamorphic sole
      • seismic Moho vs. petrologic Moho
    • alpine peridotites
    • conclusion about the upper mantle - mostly harzburgite and lherzolite
  • Melting experiments with dry lherzolite
    • the dry solidus
    • plagioclase, spinel, and garnet peridotites
    • "typical" geotherms
  • Causes of melting of the mantle
    • "pressure release melting" (adiabatic decompression)
      • At mid-ocean ridges
      • In plumes beneath hot spots
    • lowering of solidus by addition of H20 and CO2
      • at subduction zones
  • Brief mention of the origins of granitic magma (to be covered more fully later). Two "end-member" origins:
    • fractional crystallization of basalt
    • crustal melting
  • Brief introduction to the causes of variety in igneous rocks (to be covered more fully as we go along)
Igneous rocks of the oceanic lithosphere
  • Mid ocean ridges - MORBs
    • primarily tholeiitic
    • origin
  • Ocean islands / hot spots - OIBs
    • tholeiitic and alkaline
    • possible origins of alkaline magmas
      • more alkaline rich mantle source
      • smaller degree of partial melting
        • essential (major) elements vs trace elements
        • behaviour of incompatible trace elements during melting
    • A closer look at Hawaii and Pacific hot spots
      • hot spots and seamount chains
      • motion of Pacific plate
      • abundances of tholeiitic and alkaline rocks in Hawaii
      • broad changes in composition with age of each island
      • possible explanation of tholeiitic and alkaline magmatism in relation to Pacific plate motion and a "fixed" heat source

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