Download - INTRODUCTION TO PETROLOGYlibvolume3.xyz/.../petrology/petrologytutorial2.pdf · WHAT IS PETROLOGY??? Study of rocks (“petros”) igneous & metamorphic chieﬂy in the lithosphere

INTRODUCTION TO PETROLOGY

WHAT IS PETROLOGY???


Study of rocks (“petros”)

igneous & metamorphic

chiefly in the lithosphere





We will be dealing with hot rocks

tell us about composition & history of lithosphere

origin of rocks involves:

transfer of heat (energy)

movement of material





We will be dealing with hot rocks

tell us about composition & history of lithosphere

origin of rocks involves:

transfer of heat (energy)

movement of material

LITHOSPHERE

THINK LIKE A PETROLOGIST

what criteria do we use to distinguish rocks?

what do we want to know?

how do we answer these questions?

WHAT DO WE WANT TO KNOW?

how do we make melts?

what is melted, and where? what is the role of water?

how do melts behave during solidification?

what causes metamorphism?

how are metamorphism & deformation related?

how to rocks flow in the interior of mountain belts?

how do tectonic rates compare to heat conduction rates?

in what tectonic settings do these rocks form?

BASIS FOR UNDERSTANDING

field methods & sample study (observation)

theory, experiment & modeling (analytical)

THINGS TO CONSIDER

THINGS TO CONSIDER

materials of earth

THINGS TO CONSIDER

materials of earth

physical conditions

energy

pressure

temperature & heat

THINGS TO CONSIDER

materials of earth

physical conditions

energy

pressure

temperature & heat

relationship to tectonics

THINGS TO CONSIDER

materials of earth

physical conditions

energy

pressure

temperature & heat

relationship to tectonics

EgyPT

EARTH INTERIOR

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Structure of Earth:

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Structure of Earth:

chemical divisions

core, mantle & crust

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Structure of Earth:

chemical divisions

core, mantle & crust

mechanical divisions

mesosphere, asthenosphere & lithosphere

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Core:

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Core:

Fe-Ni metallic alloy

outer core is liquid (no S-waves)

inner core is solid

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Core:

Fe-Ni metallic alloy

outer core is liquid (no S-waves)

inner core is solid

differentiation at work!

compositional separation within the planet (fractionation)

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Mantle:

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Mantle:

peridotite (ultramafic)

greatest V, m & E (moves & carries heat)

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Mantle:



upper layer to 410 km (olivine to spinel)

contains low velocity layer 60-220 km

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Mantle:





transition zone between 410-660 km (spinel to perovskite)

SiIV to SiVI

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Mantle:





transition zone between 410-660 km (spinel to perovskite)

SiIV to SiVI

lower mantle has more gradual velocity increase

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Crust:

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Crust:

mafic (magnesium + ferric)to felsic (feldspar + silica)

rich in Si, Al, K, Na, Ca

Si-r

ich

Fe-r

ich

EARTH INTERIOR

Crust:

mafic (magnesium + ferric)to felsic (feldspar + silica)

rich in Si, Al, K, Na, Ca

two main types:

oceanic

continental

+ “transitional”

Si-r

ich

Fe-r

ich

EARTH INTERIOR

EARTH INTERIOR

Oceanic crust:

EARTH INTERIOR

Oceanic crust:

thin: ~10 km on average

EARTH INTERIOR

Oceanic crust:


dense: ρavg = 3.0 g/cm3

EARTH INTERIOR

Oceanic crust:



relatively uniform stratigraphy(= ophiolite suite)

EARTH INTERIOR

Oceanic crust:




sediments

EARTH INTERIOR

Oceanic crust:




sediments

pillow basalt

EARTH INTERIOR

Oceanic crust:




sediments

pillow basalt

sheeted dikes

EARTH INTERIOR

Oceanic crust:




sediments

pillow basalt

sheeted dikes

massive gabbro

EARTH INTERIOR

Oceanic crust:




sediments

pillow basalt

sheeted dikes

massive gabbro

ultramafic rocks (mantle)

EARTH INTERIOR

mafic rocks

Oceanic crust:




sediments

pillow basalt

sheeted dikes

massive gabbro

ultramafic rocks (mantle)

CONTINENTAL OCEANIC

EARTH INTERIOR

EARTH INTERIOR

Continental crust:

thicker: 20-90 km (avg = 35 km)

less dense: ρavg = 2.7 g/cm3

highly variable composition

average = granodiorite

CHEMICAL DIVISIONS

divisions separate Earth into Si-rich and Fe-rich spheres, largely a result of early chemical differentiation based on a redistribution of matter, prior to major solidification, by density

CHEMICAL DIVISIONS


later, continued differentiation was (and is) mostly a result of melting and igneous process

CHEMICAL DIVISIONS



how do we know these things???

CHEMICAL DIVISIONS




seismic velocity structure

CHEMICAL DIVISIONS





meteorites

CHEMICAL DIVISIONS





meteorites

xenoliths in volcanics

CHEMICAL DIVISIONS





meteorites

xenoliths in volcanics

experimental petrology

CHEMICAL DIVISIONS

MECHANICAL DIVISIONS

Figure 1-3. Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left, rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford.


Velocity structure (v)




v increases with density (ρ)

v = f (ρ)





v = f (ρ)

ρ dependent on physical properties & compositions

ρ = f (X, T)





v = f (ρ)


ρ = f (X, T)

v increases with depth (z) — mostly!





v = f (ρ)


ρ = f (X, T)

v increases with depth (z) — mostly!

v discontinuities indicate a change in material composition ± properties



composition property

LVZ

CMB

OC-IC


Velocity boundaries


LVZ

CMB

OC-IC


Velocity boundaries

source of LVZ?


LVZ

CMB

OC-IC


Velocity boundaries

source of LVZ?

warmer? liquid?


LVZ

CMB

OC-IC


Velocity boundaries

source of LVZ?

warmer? liquid?

source of CMB?


LVZ

CMB

OC-IC


Velocity boundaries

source of LVZ?

warmer? liquid?

source of CMB?

change in composition


LVZ

CMB

OC-IC


Velocity boundaries

source of LVZ?

warmer? liquid?

source of CMB?


source of OC-IC?


LVZ

CMB

OC-IC


Velocity boundaries

source of LVZ?

warmer? liquid?

source of CMB?


source of OC-IC?

phase change (S to L)


LVZ

CMB

OC-IC


Velocity boundaries

source of LVZ?

warmer? liquid?

source of CMB?


source of OC-IC?

phase change (S to L)


LVZ

CMB

OC-IC

LET’S LOOK IN MORE DETAIL

PETROLOGY & TECTONICS

major mineral transformations occur at ~410 and ~660 km

result from isochemical phase changes due to increased P

marked by seismic velocity discontinuities

Mineral Structure Depth Density

olivine tetrahedral low

spinel dense tetrahedral

perovskite octahedral high

~410 km

~660 km

VARIATION IN SEISMIC VELOCITY, DENSITY & MINERAL STRUCTURE


HI

LO


HI

LO

HI

LO


MG-SILICATES

HI

LO

HI

LO


MG-SILICATES

HI

LO

HI

LO

ASTHENOSPH.

MESOSPHERE

T.Z.


Resulting divisions

lithosphere

asthenosphere

mesosphere


Resulting divisions

lithosphere

asthenosphere

mesosphere

properties?


Resulting divisions

lithosphere

asthenosphere

mesosphere

properties?

connections?


Resulting divisions

lithosphere

asthenosphere

mesosphere

properties?

connections?

is the mantle solid everywhere?

MANTLE GEOTHERM

MANTLE GEOTHERM

compare continental, oceanic & ridge

MANTLE GEOTHERM


typical continental geotherm = 25 °C/km

MANTLE GEOTHERM



T @ 100 km = 1000 °C(enough to melt rocks!)

MANTLE GEOTHERM



T @ 100 km = 1000 °C(enough to melt rocks!)

are they molten? P too high? where?

compare geotherm to petrologic solidus for mantle rocks (peridotite)

if “dry” conditions, no melting possible (geotherm below solidus)

solid lherzolite is stable at T above geotherm

CAN MELTING OCCUR?

under “wet” conditions (with H2O or CO2), solidus shifts to lower T

melting can occur where T > solidus

low seismic velocities indicate partial melting between 100-250 km (the LVZ)

the LVZ marks the base of “plates” formed by rigid lithosphere

CAN MELTING OCCUR?

LITHOSPHERE

LITHOSPHERE

lithosphere includes crust + upper mantle

LITHOSPHERE


can be oceanic, continental, or both

LITHOSPHERE



typical lithosphere is 100 km(70-125 km)

LITHOSPHERE




largely solid material (silicates)

LITHOSPHERE





lowest densities (2.7-3.0 g/cm3)

LITHOSPHERE






slowest seismic velocities (6-8 km/sec)

LITHOSPHERE







internal boundary is the Moho (density boundary)

LITHOSPHERE








base of lithosphere is the low-velocity zone (LVZ)

LITHOSPHERE








base of lithosphere is the low-velocity zone (LVZ)

lithosphere = plate

EgyPT

Physical conditions of Earth necessary to understand petrologic process:

1. pressure

2. temperature

3. energy & heat

PRESSURE GRADIENT

Figure 1-8. Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science.

PRESSURE GRADIENT

P = ρgh


PRESSURE GRADIENT

P = ρgh

P increases with depth


PRESSURE GRADIENT

P = ρgh


Mantle: nearly linear through mantle

~ 30 MPa/km

≈ 1 GPa at base of avg crust


PRESSURE GRADIENT

P = ρgh



~ 30 MPa/km


slope (ΔP/Δz) depends on density (composition & compressibility) of material


PRESSURE GRADIENT

P = ρgh



~ 30 MPa/km


slope (ΔP/Δz) depends on density (composition & compressibility) of material


LO

W D

EN

SIT

Y

HIG

H DEN

SITY

PRESSURE GRADIENT

P = ρgh


Core: ρ increases more rapidly since alloy is more dense

smaller increase in P with depth suggests inner core is more uniform, solid, and has decreasing compressibility


PRESSURE IN THE CRUST

h

ρgh


in the crust, rocks “feel” a lithostatic pressure

like a submarine feels hydrostatic pressure on its hullimplies equal pressure in all directions when materials can flow h

ρgh



like a submarine feels hydrostatic pressure on its hullimplies equal pressure in all directions when materials can flow

for shallow rocks, can have differential pressure (where horizontal stress ≠ vertical stress), causing rocks to deform

h

ρgh



like a submarine feels hydrostatic pressure on its hullimplies equal pressure in all directions when materials can flow

for shallow rocks, can have differential pressure (where horizontal stress ≠ vertical stress), causing rocks to deform

based on normal crustal densities, 1 kbar = 3.3 km

h

ρgh

ENERGY

ENERGY

Energy is the capacity to do work (subatomic, to mountain, to mantle scale)

ENERGY


1. kinetic energy: EK = 1/2mv2

• motion of a body

ENERGY




2. potential energy: EP = mgz

• energy of position; can be converted to Ek

ENERGY




2. potential energy: EP = mgz

• energy of position; can be converted to Ek

3. thermal energy: ET = EK + EP

• motions & attractions in a body (subatomic and larger)

• ET ≠ heat (transferred energy)

HEAT SOURCES IN THE EARTH


1. Heat from the early accretion and differentiation of the Earth

• “original heat” of early core separation (PV work of compression)

• still slowly reaching surface as geotherm decays





2. Heat released by the radioactive breakdown of unstable nuclides

• heat production (A) from decay of U, Th & K

• mostly in crustal rocks





2. Heat released by the radioactive breakdown of unstable nuclides

• heat production (A) from decay of U, Th & K

• mostly in crustal rocks

3. Latent heat associated with outer core crystallization

• continues today!

HEAT PRODUCTION

Rock Abundance of radioactive elementHeat

produced(joules/kg/yr)

U Th K A

Granite 4 13 4 0.03

Basalt 0.5 2 1.5 0.005

Peridotite 0.02 0.06 0.02 0.0001

from Decker & Decker (1981)

HEAT PRODUCTION

Rock Abundance of radioactive elementHeat

produced(joules/kg/yr)

U Th K A

Granite 4 13 4 0.03

Basalt 0.5 2 1.5 0.005

Peridotite 0.02 0.06 0.02 0.0001

from Decker & Decker (1981)

If more heat-producing elements in continental crust, why are T’s lower??

HEAT PRODUCTION

Crustal T’s are lower because of:

1. less of it compared to mantle

2. continental crust is a good insulator

3. it’s thicker

4. it suffers from surface cooling

HEAT TRANSFER

1. radiation

• conversion of IR energy from hot body; travels as a wave

• efficient in a vacuum or transparent material; not efficient in rocks!

HEAT TRANSFER

2. conduction

• transfer of EK by vibration & contact, one molecule to another

• does not occur in a vacuum

• greatest transfer with greatest ΔT (thermal gradient)

• conduction increases with increasing surface area

• depends on thermal conductivity (k) of material, given by:

q = kΔT/Δz

HEAT TRANSFER

3. convection

• movement of material with contrasting T, which changes density

• gravity acts on Δρ, in which less dense material (ie, hotter) rises

• movement of solid can occur in viscous mantle rocks, including rise of plumes

HEAT TRANSFER

4. advection

• heat “carried” by flowing liquids or viscous bodies (e.g., water, magma) to cooler surroundings

MAGMA

LAVA FLOW

WHAT FORMS OF HEAT TRANSFER?

GEOTHERMAL GRADIENT

Figure 1-9. Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After

Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311.

GEOTHERMAL GRADIENT

ΔT/Δz is the slope of T variation with depth



GEOTHERMAL GRADIENT


gradient drives conductive cooling toward the surface



GEOTHERMAL GRADIENT



why steeper curve at depth?



GEOTHERMAL GRADIENT






more heat production in crust

GEOTHERMAL GRADIENT






more heat production in crust

more efficient convective mixing of heat in the mantle

IGNEOUS TECTONIC SETTINGS

1. mid-ocean ridge

2. continental rift

3. oceanic island arc

4. continental-margin arc

5. back-arc basin

6. oceanic hotspot

7. continental hotspot

IGNEOUS TECTONIC SETTINGS

1. mid-ocean ridge

2. continental rift

3. oceanic island arc

4. continental-margin arc

5. back-arc basin

6. oceanic hotspot

7. continental hotspot

WHAT’S MELTING? HOW? WHERE DOES IT GO?

IGNEOUS ROCKS

next week we’ll begin igneous rocks!

Download - INTRODUCTION TO PETROLOGYlibvolume3.xyz/.../petrology/petrologytutorial2.pdf · WHAT IS PETROLOGY??? Study of rocks (“petros”) igneous & metamorphic chieﬂy in the lithosphere

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