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Lecture 1 Introduction and the Earth’s Interior

Wednesday, January 26th, 2005

IGNEOUSand

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 rocks2. Their classification3. Generation and interpretation of data4. Theories on how these rocks formed

Tools of the trade include:-1. Field relationships2. Hammer and hand lens3. Thin sections and petrological microscope4. Mineralogy and electron microprobe5. Major element data6. Trace element data7. Isotopic data8. High pressure and temperature experiments

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The Earth’s InteriorCrust:Oceanic crust

Thin: 5-10 kmRelatively uniform stratigraphy

= ophiolite suite:Sedimentspillow basaltsheeted dikesmore massive gabbroultramafic (mantle)

Continental CrustThicker: 20-90 km average ~35 kmHighly 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 ~ rapidly660 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 liquidNo 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

P increases = ρghNearly linear through mantle

~ 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 Pa1 kbar = 102 * 106 Pa = 102 MPa = 0.1 Gpa10 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 depthPressure = Density x Acceleration due to gravity x Depth

Garnet peridotite is thought to start melting at a depth ofabout 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 mm3 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 Earthstill slowly reaching surface

Heat Sources in the Earth

1. Heat from the early accretion and differentiation of the Earthstill 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 Ridges2. Intracontinental Rifts3. Island Arcs4. Active Continental

Margins

5. Back-arc Basins6. Ocean Island Basalts7. Miscellaneous Intra-

Continental Activitykimberlites, carbonatites, anorthosites...