Ocean Crust/Lithosphere by mPg36k5q

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									         Structure of the Ocean
              Lithosphere
• How do we know the structure of the
  lithosphere?
   – Geophysical data
      • Seismic reflection and refraction, magnetics,
        gravity, heat flow
   – Dredges and cores (fracture zones)
   – Seafloor mapping
      • Side scan sonar, Gloria (Geological Long
        Range Inclined Asdic), swath mapping, deep
        tow mapping, direct observation (Alvin)         Drilling at Hess Deep
   – Ophiolites
     Structure of the Ocean
          Lithosphere




OB Fig. 4.1
            Ocean Lithosphere
• Layer 1
  – Deep sea sediments, often include radiolarian
    cherts
                                 Ribbon Chert

                                            Braincon, France




    Bridge River Complex, BC


http://radpage.univ-lyon1.fr/field.html
     Structure of the Ocean
          Lithosphere




OB Fig. 4.1
            Ocean Lithosphere
• Layer 2
  – 2A, 2B and 2C- change in seismic velocity
  – 2A = pillow lavas and sheet flows (extrusive)
    with voids
  – 2B = pillow lavas and sheet flows (extrusive)
    with void filling clays and minerals
  – 2C = sheeted dikes- injected into fractures in
    the ocean crust (intrusive), ~ 1 m wide
     Structure of the Ocean
          Lithosphere




OB Fig. 4.1
            Ocean Lithosphere
• Layer 3
  – Massive gabbro
• Layer 4
  – Layered peridotite – ultramafic cumulates
  – Massive peridotite
       Ocean Lithosphere-
       Seismic Velocities




OB Fig. 4.1
Ocean Lithosphere-
    The Moho
     Seismic Moho = seismic discontinuity at
     transition from mafic to ultramafic rocks

      Petrologic Moho = transition from base
      of magma chamber in the crust to true
      mantle material

Seismic Moho

Petrographic Moho
Ocean Lithosphere Models
         2A
              2B      Fast-spreading
                  3

              4




                      Slow-spreading




                      OB Fig. 4.5
          Ocean Lithosphere
• Data indicate large magma chambers are
  short-lived or non existent
• Instead injection of crystal mush- starts to
  crystallize during ascent
  – Fast-spreading- lens of magma over the xl
    mush
  – Slow-spreading- narrower mush zone (less
    supply), no lens of magma
       Ocean Lithosphere-
     Geochemistry (abridged)
• MORB = tholeiitic basalts
• Produced by partial melt of depleted mantle
  material

• Depleted Mantle- subject to previous melt
  event which removed already removed
  incompatible elements
       Ocean Lithosphere-
     Geochemistry (abridged)
• Incompatible Elements (partitioned into melt)
  – Elements that that have difficulty in entering
    cation sites of the minerals
  – are concentrated in the melt phase of magma
  – HFS (High Field Strength)- high charge, highly
    insoluble in water dominated fluids, (Nb, Ta, Ti.
    Zr, Hf)
  – LIL (Large Ion Lithophile)- 1+ and 2+ large ion
    elements that tend to be concentrated in silica
    melts, high radius/charge (K, Rb, Cs, Sr, Pb, Ba)
Ocean Lithosphere-Geochemistry
Ocean Lithosphere-Geochemistry
• Magma Series- evolution of mafic magma
• Tholeiites- lower Na, produced from reduced
  magmas. First xlz Mg-rich olivines and pyroxenes
  (Bowen’s Rxn Series), Mg/Fe decreases
  – Rifting environments- partial melt of depleted mantle
• Calc-Alkaline- higher Na, produced from
  oxidized magmas. Fe oxidized  magnetite ,
  Mg/Fe more constant
  – Subduction zones- partial melt of depleted mantle plus
    subducted material (hydrated)
Ocean Lithosphere-Geochemistry
• Magma Series
• Alkaline- higher Na and K relative to SiO2.
  More enriched in incompatibles.
  – Ocean island-hotspot environments- partial
    melt of more primitive mantle
Ocean Lithosphere-Geochemistry
     Mg                            Bowen’s Reaction Series
          Fe




                                            alkaline




                      tholeiitic                             Calc-alkaline
Perfit and Davidson
          Ocean Lithosphere
• What happens to the crust after it forms?

• Aging process
  – Contracts, cools, deepens (density)
  – Increase in seismic velocity of the upper crust
  – Decrease in conductive and convective heat
    flow
  – Decrease in remnant magnetism
        Aging of Ocean Crust
• Age-Depth relationship
• Young, hot, buoyant  older, cooler, more
  dense
  – Depth to the ocean crust increases
    systematically with age
               Aging of Ocean Crust
                      Fast-spreading
                       Slow-spreading



                         D = 2500 + 350t(in my)1/2

                                          Best fit for data
                                          Theoretical curve
                                          assuming thermal
                                          contraction


OB Fig. 2.13
        Aging of Ocean Crust
 D = 2500 + 350t(in my)1/2
• Limitations
  – Only applicable to ~80 Ma, after that
    lithosphere mostly cooled, nearing equilibrium
  – Not all ridges start at 2500 m
        Aging of Ocean Crust
• Increased seismic velocity
  – Rocks become more dense with age due to:
     • Cooling and contracting
     • Infilling of pore spaces and fractures (calcite
       and zeolite cements- usually related to fluid
       flow
 Aging of Crust
Seismic Velocity
        Aging of Ocean Crust
• Heat Flow
  – Lithosphere cools through
     • Conduction- diffusion of heat from hot lithosphere
       to cold seawater or sediment interface
     • Convection- transfer of heat by mass movement


  – Heat flow decreases with age
         Aging of Ocean Crust

                              Theoretical heat flow
                              assuming conduction only

                                Observed heat flow




                Difference = heat lost through
OB Fig. 5.6     convection (hydrothermal circulation)
        Aging of Ocean Crust
• Decreased remnant magnetization
  – Remnant = permanent magnetization induced
    by an applied field

  – Low temperature alteration of the crust includes
    oxidation of titanomagnetites  decreased
    magnetization
  – Greatest change over the first 20 Ma
     Formation of Ocean Crust

• Where is ocean crust forming?
                                      Formation of Ocean Crust
                                      • Spreading Centers
Versions of hotspot (alkali basalt)



                                        – Mid ocean ridges (tholeiitic basalt)
                                      • Hotspots tracks/Aseismic Ridges
                                        – Hawaii, Line Islands
                                        – Walvis Ridge, Rio Grande Rise, Ninety-east Ridge
                                      • Large igneous provinces- voluminous outpourings
                                        of mafic material
                                        – Ontong Java Plateau, Kerguelen, Deccan
OJP= Ontong Java
Plateau
MP = Manahiki
Plateau
HP = Hikurangi
Plateau

MP formed at a triple
jct

MP-HP separated by
spreading center
      Formation of Ocean Crust
• Today ~90% of new ocean crust is formed at the
  mid ocean ridge
   – Crustal production rate ~1.8 x 106 km3/my


• During Cretaceous ~70% of new ocean crust
  formed at the mid ocean ridge, 30% formed at hot
  spots (large igneous provinces)
   – Crustal production rate ~3.3 x 106 km3/my
   – Hotspot activity may exceed ridge processes for short
     intervals
Formation
of Ocean
  Crust




                      Cret normal

   Fig. From Larson
      Formation of Ocean Crust
• Implications of LIPs
  – Thickening of ocean crust
       • Ontong Java ~40 km; Kerguelen ~25 km
  –   Reheat and uplift surrounding lithosphere
  –   Resist subduction? Nucleus for continent?
  –   Sea level
  –   Greenhouse gases
     Formation of Ocean Crust
• Suggests 2 modes of heat and mass transfer
  from the mantle
  – Prevalence of each mode varies through time
  – Related to activity at the core/mantle boundary?
  – Heat flux from plumes ~ cooling of the core
• Present day
  – 60% of plume flux in the Pacific
                 Hot Spots
• What are their source depths?
  – CMB (D”), 670 discontinuity, both…
• What is the link to climate?
  – Greenhouse gases, sea level, circulation…
• What determines the location of a hotspot?
  – Distance from ridge, random…
• What triggers hotspot activity?
  – Continental breakup, plate reorganization, core
    processes…
                                    FUMAGES
                                               Crust
                                               Granite and Basalt
400 km
   670 km             Mantle                   <1% Earth’s mass
discontinuities
                     peridotite
                   70% Earth’s mass

                                      D” layer- lowermost ~200 km of mantle
        2900 km
                      Liquid Outer Core
                                                      Fe (+ Ni + S)
                                                      30% of Earth’s mass
            5200 km
                            Solid Inner Core

                  6370 km

								
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