Download Mantle Materials

• 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  b-spinel
410
Pyroxene 
b-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  b-spinel
410
Pyroxene 
b-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, Magnesiowüstite: ((Mg,Fe)O), and Stishovite (SiO2)
• ~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