1. Petrology Igneous and Metamorphic Convection demo “Lava Lamp”
Density displacement demo: oil and water are immiscible.
Marble Demo :Fractionation

2. The Texts An Introduction to Igneous and
Metamorphic Petrology 1st ed by J. D. Winter
or Principles of Igneous and Metamorphic Petrology J.D. Winter 2nd ed.

3. The Earth’s Interior Crust: Oceanic crust Continental Crust
Usually < 10 km
ophiolite suite: list
Ophiolites are usually listed from top to bottom:
At the top a cover of sediment, below that:
Basaltic Pillow lava with WET sediment between the pillows, then
Basaltic Sheeted Gabbro dikes, flat because they fill extension crack
Ultramafic Peridotite from the uppermost mantle. When Peridotite is exposed to low pressure by extensional cracks, it partially melts
Continental Crust
Thicker: km average ~35 km
Variable composition but average a granodiorite

4. The Silicate Tetrahedron O2 -
2_25
O2 -
Si4+
O2 -
O2 -
The basis of most rock-forming minerals, charge - 4
O2 -

5. The Mantle is mostly Silicates

7. The Earth’s Interior Mantle: Peridotite (ultramafic)
Upper Mantle to 410 km
olivine, pyroxenes, spinel - structure minerals, and garnet
Low Velocity Layer km Aesthenosphere
Transition Zone as velocity increases km , olivine not stable, replaced by high P polymorphs with ~ same composition: wadsleyite (beta-spinel structure), and ringwoodite (gamma-spinel structure)
Lower Mantle 660 Upper minerals unstable, ® perovskite-type structure SiIV ® SiVI
Mantle rocks shallower than about 410 km depth consist mostly of olivine, pyroxenes, spinel-structure minerals, and garnet;[12] typical rock types are thought to be peridotite,[12] dunite (olivine-rich peridotite), and eclogite. Between about 400 km and 650 km depth, olivine is not stable and is replaced by high pressure polymorphs with approximately the same composition: one polymorph is wadsleyite (also called beta-spinel type), and the other is ringwoodite (a mineral with the gamma-spinel structure). Below about 650 km, all of the minerals of the upper mantle begin to become unstable. The most abundant minerals present, the silicate perovskites, have structures (but not compositions) like that of the mineral perovskite followed by the magnesium/iron oxide ferropericlase.[15] The changes in mineralogy at about 400 and 650 km yield distinctive signatures in seismic records of the Earth's interior, and like the moho, are readily detected using seismic waves. These

8. Seismic Tomography

9. The Earth’s Interior Core: Fe-Ni metallic alloy Sulfur
Outer Core is liquid
No S-waves
Inner Core is solid
Discussions: Differentiation
Iron Meteorites, Impactor
Density and Buoyancy

10. Note how S-wave velocities drop to zero in the Liquid outer core
LVZ
Note how S-wave velocities drop to zero in the Liquid outer core
Note how S-wave velocities drop to zero in the Liquid outer core
Source: Recommended Text Kearey and Vine (1990), Global Tectonics.

11. Upper Mantle Samples
Samples of the upper mantle occasionally appear where faulting has exposed it in oceanic fracture zones, thrust it up in collision zones, or where brought up in diatreme and basalt eruptions. The rock revealed is usually Peridotite, which is three-quarters Dunite (pure olivine) and one-quarter basalt. The Basalt forms by the partial melting of this peridotite, which drives off the basaltic melt, leaving behind the solid “depleted “ dunite (basaltic components removed).
The original (fertile) mantle has more Al, Ca, Ti, Na, and K and lower Mg# = Mg/(Mg +Fe) than Dunite
So some of the above go into the basalt.

12. Molten- VERY Hot Molten- Not so hot 100% Solid 1553 oC 3-D 3-D 3-D 3-D
Fo Mg C Fa Fe C
Molten- VERY Hot
No solids
First mineral to crystallize out
1900 oC
Independent Tetrahedra
1553 oC
3-D
Single
chains
Double
chains
“Basaltic”
sheets
“Andesitic”
3-D
3-D
Molten- Not so hot
sheets
3-D
“Granitic”
100% Solid

14. Dark Green
Gray
Gray
Pink to Salmon
Fine crystals
Need a microscope
Low silica, HOT, fluid
Intermediate
High silica, warm, viscous
Course crystals
Easily seen

15. If crystals are left in contact with melt …
Ultramafic to Basaltic
Gray needles are Plagioclase (Plag) Feldspar, Yellow-brown crystals are Pyroxene (Py), brightly colored crystals are Olivine (Ol). At lower Temps, the Olivine xtals have been partially resorbed by the melt, their atoms reused to make Py & Plag.
Plagioclase Feldspar

16. Stable composition varies with Temperature
If the first formed crystals of Calcium-rich (Ca) Plagioclase Feldspar are left in contact with the melt ,
as the melt cools more stable sodium-rich layers will be deposited on their outer rims
Zoned feldspar (plagioclase) showing change in
composition with time in magma chamber
(calcium-rich in core to sodium-rich at rim)

17. Isolated Olivine crystals
Early formed Olivine crystals can sink to the bottom of a magma chamber, so they are isolated from the very reactive ions in the melt.

18. If early crystals are removed (isolated), the melt becomes richer in Silica
Fe, Mg, Ca
Some Si
Left with
K and Al
Most of Si
You can start with a
Mafic (silica-poor) magma
and end up with some
Felsic (silica-rich)
Granites.
Marble Demo
A melt will crystallize its mafic components first, and the remaining melt may be granitic

19. Pressure Gradient P increases = rgDh » 1 GPa at base of crust
We need to be able to estimate pressures
Pressure Gradient
P increases = rgDh
» 1 GPa at base of crust
Linear increase mantle
~ 30 MPa/km
Core: r increases more rapidly since Fe-Ni alloy more dense

20. Pressure Calcs
To calculate pressures at the base of a stack of layers with different densities, start from the top layer, calculate the pressure at the base as
P0-1 = r0-1gDh0-1
For the second layer,
P2 = P0-1 + r1-2gDh1-2
Etc.

21. Multi-layer Pressure Calc Example
Upper crust 25 km thick, density 2.75 Mg/m3
r0-1 = 2.75 Mg/m3 x 1000 kg/1Mg = 2.75 x 103 kg/m3
P1 = r0-1gDh0-1
= 2.75 x 103kg/m3 2 x 9.81 m/s2 x 25 x 103 m
= x 108 kg . m/s2 x 1/m2 (aka “Pascals”)
Next layer down, 10 km basalt r1-2 = 3 x 103 kg/m3
P2 = P1 + r1-2gDh1-2
Etc. See the handout, after the lecture

22. Mg3Al2(SiO4)3

23. Olivine Example
At high TP, the a olivine structure is no longer stable.
Below depths of about 410 km olivine undergoes an exothermic phase transition to the sorosilicate, wadsleyite , the b Olivine
At about 520 km depth, wadsleyite transforms exothermically into ringwoodite, the g Olivine, which has the spinel structure.
At a depth of about 670 – 700 km ringwoodite decomposes into silicate perovskite ((Mg,Fe)SiO3) and ferropericlase ((Mg,Fe)O) in an endothermic reaction.
These phase transitions lead to a discontinuous increase in the density of the Earth's mantle that can be observed by seismic methods. They are also thought to influence the dynamics of mantle convection in that the exothermic transitions reinforce flow across the phase boundary, whereas the endothermic reaction hampers it.
This leads some workers to believe that the 700 km boundary blocks convection from the core mantle boundary, and upper mantle convection cells are distinct.
Exothermic materials heat, expand, more buoyant

24. Phase diagram for aluminous 4-phase Lherzolite:
Notice the mantle will not melt under normal ocean geotherm!
Al-phase =
Ca++ Plagioclase
shallow (< 50 km)
Spinel Lherzolite Spinel is MgAl2O4
50-80 km
Garnet Lherzolite km
Si[4] ® Si[6] coord.
> 400 km
Note: the mantle will not melt under normal ocean geotherm!
Si [4] => Si [6]
Figure Phase diagram of aluminous Lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,

25. Heat Sources in the Earth
Impact heat from the early accretion and differentiation of the Earth
Convection cells redistribute heat to cold surface

26. Heat Sources in the Earth
1. Heat from the early accretion and differentiation of the Earth
still slowly reaching surface
2. Heat released by the radioactive breakdown of unstable nuclides

27. Heat Transfer 1. Radiation 2. Conduction 3. Convection 1. Radiation
Requires transparent medium
Rocks aren’t (except perhaps at great depth)
2. Conduction
Rocks are poor conductors
Very slow
3. Convection
Material movement (requires ductility)
Heat-induced expansion and buoyancy
Much more efficient than conduction
1. Radiation
Requires transparent medium
Rocks aren’t (except perhaps at great depth)
2. Conduction
Rocks are poor conductors
Very slow
3. Convection
Material movement (requires ductility)
Heat-induced expansion and buoyancy
Much more efficient than conduction

28. Geothermal Gradient Hot Cool
Silica-rich rocks (with Quartz, K-feldspar) melt at cooler temperatures.
Melts are viscous
Silica-poor rocks (with Olivine, Pyroxene,
Ca-feldspar) melt at higher temperatures
Melts are very fluid
Hot

29. Lithosphere Buoyancy

30. Ocean and Continental Lithosphere Thermal Gradients

31. Melting depths vary w\ volcanic province Most within upper few hundred kilometers

32. Heat highest at MOR, suggests rising convection cells there

33. Origin of Basaltic Magma - MOR
Harry Hess’ Seafloor Spreading
Role of Pressure in divergent margin
Reducing the pressure lowers the melting temperature – the mantle partially melts
Mid-ocean ridge and rift valley: called decompression melting

34. Mantle loses heat at surface, becomes denser
Mantle loses heat at surface, becomes denser. Pulls lithosphere down into “Subduction Zone”

35. Origin of Basaltic Magma 2 Subduction Zone
Role of volatiles - WATER
INITIALLY BASALTIC

36. Origin of Basaltic Magma 3 Plumes, also basaltic

37. Assimilation and magmatic differentiation
Why are the continents so silica rich? Weathering dissolves high-temp. minerals, but also:
Fractionation: if early crystals settle out, remaining melt is relatively richer in silica
Show Samples

38. Origin of Andesite & Diorite: intermediate silica content
Basaltic here
Good diagram for the Andes Mountains
Small blobs, not much heat in them
Assimilate some crust, fractionate

39. Origin of Granitic Rocks
Magma rises further distance, more fractionation. Passes through thicker crust, more assimilation.
Huge blobs w/ low temps but lots of magma, fractionation & assimilation => Granite Batholiths
Can also get small amounts of granites from deep felsic rock passed by ascending magma

40. Plate Tectonic - Igneous Genesis
1. Mid-ocean Ridges
2. Intracontinental Rifts
3. Island Arcs
4. Active Continental
Margins
5. Back-arc Basins
6. Ocean Island Basalts
7. Miscellaneous Intra- Continental Activity
kimberlites, carbonatites, anorthosites...
Provinces we will consider this semester.

41. Or, for Kimberlites (7)
Many workers think plumes from the core-mantle boundary can punch through the endothermic km transition. Diamonds formed from subducted organic carbon are lifted by rising plumes that happen to hit a subducted slab of ocean lithosphere.

42. Isotope Signatures
Plate tectonic provinces have a characteristic stable isotope signature