Download IGNEOUS and METAMORPHIC PETROLOGY

Lecture 1 Introduction and the Earth’s Interior
Wednesday, January 26th, 2005
IGNEOUS
and
METAMORPHIC
PETROLOGY
GEO - 321
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PETROLOGY – comes from petros for rock – hence the study
of rocks
Sedimentary – deposition of material from water or air
Igneous – formed through the solidification of molten material
Metamorphic – formed from a previously existing rock
(usually at high temperatures and pressures)
PETROLOGY encompasses:1. Description of rocks
2. Their classification
3. Generation and interpretation of data
4. Theories on how these rocks formed
Tools of the trade include:1. Field relationships
2. Hammer and hand lens
3. Thin sections and petrological microscope
4. Mineralogy and electron microprobe
5. Major element data
6. Trace element data
7. Isotopic data
8. High pressure and temperature experiments
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The Earth’s Interior
Crust:
Oceanic crust
Thin: 5-10 km
Relatively uniform stratigraphy
= ophiolite suite:
Sediments
pillow basalt
sheeted dikes
more massive gabbro
ultramafic (mantle)
)
)
)
)
)
Continental Crust
Thicker: 20-90 km average ~35 km
Highly variable composition
‹ Average ~ granodiorite
The Earth’s Interior
Mantle:
Peridotite (ultramafic)
Upper to 410 km (olivine → spinel)
‹ Low Velocity Layer 60-220 km
Transition Zone as velocity increases ~ rapidly
‹ 660 spinel → perovskite-type
)
SiIV → SiVI
Lower Mantle has more gradual
velocity increase
Figure 1-2. Major subdivisions of the Earth.
Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
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The Earth’s Interior
Core:
Fe-Ni metallic alloy
Outer Core is liquid
‹
No S-waves
Inner Core is solid
Figure 1-2. Major subdivisions of the Earth.
Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
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.
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Figure 1-5. Relative atomic abundances of the seven most common elements that comprise 97% of the Earth's mass. An
Introduction to Igneous and Metamorphic Petrology, by John Winter , Prentice Hall.
The Pressure Gradient
z
z
P increases = ρgh
Nearly linear through mantle
‹
‹
z
~ 30 MPa/km
≈ 1 GPa at base of ave crust
Core: P incr. more rapidly
since alloy more dense
Figure 1-8. Pressure variation with depth. From Dziewonski and
Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier
Science.
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Some useful pressure conversions:
1 bar = 102 * 103 Pa
1 kbar = 102 * 106 Pa = 102 MPa = 0.1 Gpa
10 kbar = 1.02 * 109 Pa = 1 GPa
Approximate pressure gradients in the crust and mantle
Crust:
30 MPa/km or 0.29 kb/km
Mantle:
35 MPa/km or 0.35 Kb/km
What will the gradients be in GPa?
Calculate pressure at depth
Pressure = Density x Acceleration due to gravity x Depth
Garnet peridotite is thought to start melting at a depth of
about 130 km in the mantle to produce Hawaiian basalts.
Assuming the density of mantle peridotite is 3.3 gm/cc,
Calculate the pressure of melting
IMPORTANT for Pa, units need to be in kg and m
P (Pa) = 3300 kg x 9.8 m x 130,000 m
m3
s2
= 4.2 x 109
= 4.2 GPa
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Heat Sources
in the Earth
1. Heat from the early accretion and
differentiation of the Earth
‹
still slowly reaching 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
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The 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.
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...
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