• 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)