Download Lecture 34 - Mantle Materials

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Transcript
• 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)
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…
Meteorite types
• Stony, stony-iron, and iron
• Stony: Chondritic or Achondritic
– Chondrules – spherical bits of olive or pyroxene
condensed from the hot soar nebula over 4 ½
billion years ago
• Stony-iron: Composed of nickel-iron metal
and stony components (mafic minerals)
• Iron: Nickel-iron alloys  Kamosite and
Taenite
Perovskite
• (Mg, Fe)SiO3
• As the major mineral in the
lower mantle, possibly the
most abundant earth material!
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
Earth
• Image taken by
Voyager I in
1990
• Photos taken 4
billion miles
away from earth
• A mosaic image,
filtered and
magnified
•
•
•
We succeeded in taking that picture and, if you look at it, you see a dot. That's here. That's
home. That's us. On it, everyone you ever heard of, every human being who ever lived, lived
out their lives. The aggregate of all our joys and sufferings, thousands of confident religions,
ideologies and economic doctrines, every hunter and forager, every hero and coward, every
creator and destroyer of civilizations, every king and peasant, every young couple in love,
every hopeful child, every mother and father, every inventor and explorer, every teacher of
morals, every corrupt politician, every superstar, every supreme leader, every saint and sinner
in the history of our species, lived there on a mote of dust, suspended in a sunbeam.
The earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by
all those generals and emperors so that in glory and in triumph they could become the
momentary masters of a fraction of a dot. Think of the endless cruelties visited by the
inhabitants of one corner of the dot on scarcely distinguishable inhabitants of some other
corner of the dot. How frequent their misunderstandings, how eager they are to kill one
another, how fervent their hatreds. Our posturings, our imagined self-importance, the delusion
that we have some privileged position in the universe, are challenged by this point of pale
light.
Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity -- in all this
vastness -- there is no hint that help will come from elsewhere to save us from ourselves. It is
up to us. It's been said that astronomy is a humbling, and I might add, a character-building
experience. To my mind, there is perhaps no better demonstration of the folly of human
conceits than this distant image of our tiny world. To me, it underscores our responsibility to
deal more kindly and compassionately with one another and to preserve and cherish that pale
blue dot, the only home we've ever known.
Formation of the Universe
•
Courtesy JPL http://cougar.jpl.nasa.gov/HR4796/anim.html
Formation of the Universe
•
Courtesy JPL http://cougar.jpl.nasa.gov/HR4796/anim.html
Planetary Geology
• How can we know anything about the
geology of another planet?
Venus
•
•
•
•
Clouds
Magnetic field
Geology?
Have we ever
been there?
• Data shows that most of the rocks are
mafic – basalts with seemingly little soil
development… make sense??