Transcript Ch 01w Intro Earth`s Interior

Igneous and Metamorphic
Petrology
Convection demo “Lava Lamp”
Density displacement demo: oil and water are immiscible.
Marble Demo :Fractionation
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.
The Earth’s Interior
Crust:
Oceanic crust
Usually < 10 km
ophiolite suite: list
Continental Crust
Thicker: 20-90 km average ~35 km
Variable composition but average a granodiorite
O2 -
The Silicate Tetrahedron
2_25
Si4+
O2 -
O2 The basis of most rock-forming
minerals, charge - 4
O2 -
The Mantle is mostly Silicates
The Earth’s Interior
Mantle:
Peridotite (ultramafic)
Upper Mantle to 410 km
olivine, pyroxenes, spinel - structure minerals,
and garnet
 Low Velocity Layer 60-220 km Aesthenosphere
Transition Zone as velocity increases 410 660 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
Seismic
Tomography
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
LVZ
Note how S-wave
velocities drop to zero
in the Liquid outer
core
Source: Recommended Text Kearey and Vine (1990), Global Tectonics.
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.
Fo Mg++ 1900C Fa Fe++ 1500C
Molten- VERY Hot
No solids
1900 oC
First mineral to crystallize out
Independent Tetrahedra
1553 oC
3-D
Single
chains
Double
chains
“Basaltic”
sheets
“Andesitic”
3-D
3-D
Molten- Not so hot
sheets
3-D
100% Solid
“Granitic”
Dark Green
Gray
Gray
Pink to Salmon
Fine crystals
Need a microscope
Low silica, HOT, fluid
Course crystals
Easily seen
Intermediate
High silica, warm, viscous
If crystals are left in contact with melt …
http://www4.nau.edu/meteori
te/Meteorite/Eucrite.html
• 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
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)
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.
If early crystals are removed (isolated), the
melt becomes richer in Silica
Remove
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
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
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.
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
• = 6.744 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
Mg3Al2(SiO4)3
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
Phase diagram for aluminous
Notice the mantle will
4-phase Lherzolite: not melt under normal
Al-phase =

ocean geotherm!
Ca++ Plagioclase
 shallow

(< 50 km)
Spinel Lherzolite
Spinel is MgAl2O4
 50-80

km
Garnet Lherzolite
 80-400

Si [4] => Si [6]
km
Si[4]  Si[6] coord.

> 400 km
Figure 10-2 Phase diagram of aluminous Lherzolite with melting interval (gray), sub-solidus
reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.
Heat Sources
in the Earth
• Impact heat from the early accretion
and differentiation of the Earth
– Convection cells redistribute heat to
cold surface
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
Heat Transfer
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

Geothermal
Gradient
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
Lithosphere Buoyancy
Ocean and Continental
Lithosphere Thermal Gradients
Melting depths vary w\ volcanic province
Most within upper few hundred kilometers
Heat highest at MOR,
suggests rising convection
cells there
Highest at MORs
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
http://volcanoes.usgs.g
ov/about/edu/dynamicpl
anet/nutshell.php
Mantle loses heat at
surface, becomes
denser. Pulls
lithosphere down into
“Subduction Zone”
Origin of Basaltic Magma 2
Subduction Zone
• Role of volatiles - WATER
INITIALLY BASALTIC
Origin of Basaltic Magma 3
Plumes, also basaltic
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
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
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
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 IntraContinental Activity

kimberlites, carbonatites,
anorthosites...
Or, for Kimberlites (7)
Many workers think plumes from the core-mantle boundary
can punch through the endothermic 670-700 km
transition. Diamonds formed from subducted organic
carbon are lifted by rising plumes that happen to hit a
subducted slab of ocean lithosphere.
Isotope Signatures
• Plate tectonic provinces have a
characteristic stable isotope signature