We have been talking about minerals primarily in the earth s crust What About the rest Mantle composition rheology dicontinuities Harzburgite 80 olv 20 OPX lithosphereLherzolite 60 olv 40 OPX grt MOHO asthenosphere Pyrolite lherzolite like Upper mantle Olivine spinel 410 Pyroxene spinel stishovite 660 Spinel Lower mantle perovskite periclase Upper Mantle Minerals Olivine 60 Pyroxene 30 Garnet 10 Rest is compositionally homogeneous discontinuities What s different Harzburgite 80 olv 20 OPX lithosphere Lherzolite 60 olv 40 OPX grt MOHO asthenosphere Pyrolite lherzolite like Upper mantle Olivine spinel 410 Pyroxene spinel stishovite Lower mantle Spinel perovskite periclase 660 Upper Mantle Olivine Olivine Thought to be about 10 12 Fe in upper mantle At pressures around the 410 km discontinuity Fo rich olivine transforms to a ccp structure called wadsleyite Iron rich olivines do not undergo this transformation At higher pressures both the Fa rich olivine and wadsleyite transform to a spinel structure Mg Fe 2SiO4 called ringwoodite This occurs when the pressure forces the structure to become as closest packed as it can get in order to become more dense it must transform to a new phase Garnet Pyroxene As pressure increases Pyroxene Garnet primarily pyrope Increases from 50 to 520 km Past 520 km Garnet Ca perovskite Past 720 km more Mg rich Garnets begin to form Mg perovskite Lower Mantle Minerals Perovskite Mg Fe SiO3 Magnesiow stite Mg Fe O and Stishovite SiO 2 80 Perovskite 20 Magnesiowustite minor stishovite which doesn t form if Mg or Fe are around At these high pressures all Si is 6coordinate SiO6 subunits Octahedral coordination Perovskite Mg Fe SiO3 As the major mineral in the lower mantle possibly the most abundant earth material Do we have any Lower Mantle Minerals NO How do we know they are there METEORITES P S waves tell us something about composition Nuclear chemistry also tells us something about composition EXPERIMENTS simulate P T see what you get Core Core Outer Core Liquid made of Iron Fe and Nickel Ni about 4 and some S Si and O enough to make the density less than Fe and Ni alone Movement of this liquid is responsible for earth s magnetic field Inner Core Solid Hexagonally closest packed Fe and Ni Geodynamo The inner core spins what happens when a solid is spun inside a liquid containing ions Generate an electromagnetic field The polarity of that field has flopped many times in earth s history Model of a magnetic reversal taking 1 000 years http www es ucsc edu glatz geodynamo html The electromagnetic field also protects the planet from solar sourced ionic particles solar wind The geodynamo is additionally responsible for the position of magnetic north
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