week 2 and 3- Plate tectonics: Geochem and rock- forming minerals

Evidence for continental drift

• Glacial: permian glaciers found 4 continents

• fossils

• Matching structures and rocktypes

• Earths magemtoc pole: magnetic offset 11-5 degrees difference: declination

• pole is relatively sationary: instead continents move

• sea floor bathymetry - take soundings (1940)

Paleomagnetism

the study of Earth's ancient magnetic field recorded in rocks, sediments, and archaeological materials

magnetic signals achieved by Fe minerals

isostatic equilibrium

the state where Earth's rigid crust "floats" on the denser, flowing mantle (asthenosphere) at an elevation determined by its thickness and density, like an iceberg, balancing gravitational forces

Divergent boundary

plates move apart

lithosphere thickens away from ridge axis

aka spreading boundary, mid ocean ridge

Convergent boundary

plates move together

process of plate consumption is called subduction

aka subduction zone, tench

transform boundary

plates slide sideways

plate material is not created / destroyed

transform fault/ transform

Divergent Boundaries: phase 1

Rifting

rifting: what happens

• lithosphere strectched

• doming and faulting

• crust thins- mantle upwells

• pressure release causes melting

• volcanism develops

Divergent boundaries Phase 2:

Drifting

Drifting- What happens

• ocean crust formation

• development of MOR

• initation of sea floor spreading

• development of new continental margins

Subduction

• old oceanic lithosphere is more dense than mantle - doesnt subduct easily

• once bent downward - leading edge sinks downward 45 degrees

subduction features

• accretionary prisms

• volcanic arcs

• back-arc basins

triple junctions

• point where three plate boundaries intersect

• multiple boundary combinations occur

• triple junctions migrate and change across time

Hot spots

• plumes of deep mantle material independent of plates

• burn through plates and add lines of volanoes to them

• hotspot seamounts age away from originating hot spot

Plate velocities

• mapped by:

  • plotting plate motion relative to fixed spot in mantle

  • measuring volcano ages/ distance along hot spot track

Driving mechanisms for plate motion

• ridge push- gravity acts on young elevated lithosphere

• slab pull- gravity pulls a subducting plate downward

Chemical bonding is driven by

thermodynamics (2nd law) :

• chemical compounds are stable when the total energy of the combination is lower than the separated atoms

Victor goldschmidt

recognized a partitioning of elements according to their prefered host stages

Atmophile

• volatile

• form gases

• mainly in atmosphere

lithophile

• silicates, oxides

• rock forming, dominate crust and mantle

Chalcophile

• have an affinity for a sulpide liquid phase

Siderophile

• have an affinity for a metallic liquid phase

• rarest in crust

oxygen content determines what

Size of planets lithophile and silicate layer

sulphur content determines what

location of chalophiles

excess metal determines what

size of siderophile layer

most important mineral class

silicates

Silicates

• rock- forming minerals

• constitute almost the entire crust and mantle on earth

• most common minerals

• ie quartz

Oxides

• metal cations bonded to oxygen

• ie magetite (Fe3 O4), hematite (Fe2 O3), Rutile (TiO2)

Sulfides (S-)

• Metal cations bonded to a sulfide anion

• ie: Pyrite (FeS2)

• Galena (PbS)

• Sphalerite(ZnS)

• Chalcosite(Cu2S)

Sulfates (SO4 ²-)

Metal cation bonded to a sulfate anionic group

• e.g.

  • Gypsum

  • Anhydrite

Halides (Cl- or F-)

ie Halite (NaCl)

many sulfates and halides form by

evaporation of sea water in evaporite deposits - rock salt

Carbonates

• Calcite (CaC)3)

• Dolomite (Ca2Mg(CO3)2)

Native elements

• pure masses of a single metal

  • copper (Cu)

  • Gold (Au)

  • Silver (Ag)

which minerals take up 74.3% of crustal minerals

• oxygen and silicon

percentage of crustal mass from 8 elements

98%

how many minerals abundant

50

Si:O ratio is a important control on

• melting temp

• mineral structure and cations present

• susceptibility to chemical weathering

Independent Tetrahedra

Si:O 1:4

silica tetrahedra share no oxygens

linked by cations

• ie Olivine and Garnet

Single chains

Si:O 1;3

• silica tetrahedra link to share two oxygens

• pyroxenes

• dark long crystals

• two cleavages near 90degrees

Framework silicas

Si:O 1:2

all four oxygens in each silica tetrahedra are shared

feldspars- plagioclase

• orthoclase

• silica (Quartz) group contains only Si and o

Sheet silcates

Si:O 2:5

• silica tetrahedra share 3 oxygens

• create 2D flat sheets of linked tetrahedra

• characterised by one direction of perfect cleavage

• ie Micas

  • Biotite (dark)

  • Muscovite (light)

  • clays - Kaolinite