Unveiling Earth's Layers: Upper and Lower Mantle Mineral Transformations

1. We have been talking about minerals primarily in the earth’s crust • What About the rest??

4. 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  perovskite + periclase Lower mantle Mantle composition (rheology)

5. discontinuities 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  perovskite + periclase Lower mantle Upper Mantle Minerals • Olivine (60%), Pyroxene (30%), Garnet (10%) • Rest is compositionally homogeneous • What’s different??

6. 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.

7. 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

8. 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)

9. Perovskite • (Mg, Fe)SiO3 • As the major mineral in the lower mantle, possibly the most abundant earth material!

10. 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…

11. Core

12. 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

13. 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

14. 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