We have been talking about minerals primarily in the earth’s crust What About the rest??
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Transcript of We have been talking about minerals primarily in the earth’s crust What About the rest??
• We have been talking about minerals primarily in the earth’s crust
• What About the rest??
Mantle composition (rheology)
Lherzolite (60%olv 40%OPX, grt)Harzburgite (80%olv 20%OPX)
Pyrolite (lherzolite-like)
Olivine -spinel
Pyroxene -spinel + stishovite
Spinel perovskite + periclase
lithosphere
asthenosphere
Uppermantle
410
660
MOHO
dicontinuities
Lower mantle
Upper Mantle Minerals• Olivine (60%), Pyroxene (30%), Garnet (10%)• Rest is compositionally homogeneous• What’s different??
Lherzolite (60%olv 40%OPX, grt)Harzburgite (80%olv 20%OPX)
Pyrolite (lherzolite-like)
Olivine -spinel
Pyroxene -spinel + stishovite
Spinel perovskite + periclase
lithosphere
asthenosphere
Uppermantle
410
660
MOHO
discontinuities
Lower mantle
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, Magnesio-wüstite: ((Mg,Fe)O), and Stishovite (SiO2)
• ~80% Perovskite, ~20% Magnesio-wustite, minor stishovite (which doesn’t form if Mg or Fe are around)
• At these high pressures, all Si is 6-coordinate (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
http://www.es.ucsc.edu/~glatz/geodynamo.html
Model of a magnetic reversal taking ~1,000 years…
• The geodynamo is additionally responsible for the position of magnetic north
• The electromagnetic field also protects the planet from solar sourced ionic particles (solar wind)