We have been talking about minerals primarily in

• 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) lithosphere. Lherzolite (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)2 Si. O 4, 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)Si. O 3, Magnesiowüstite: ((Mg, Fe)O), and Stishovite (Si. O 2) • ~80% Perovskite, ~20% Magnesio-wustite, minor stishovite (which doesn’t form if Mg or Fe around) • At these high pressures, all Si is 6 coordinate (Si. O 6 subunits; Octahedral coordination)

Perovskite • (Mg, Fe)Si. O 3 • 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