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Petrology Lecture 1

Fundamental Concepts

GLY 4310

Spring, 2015

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Major Subdivisions of

the Earth

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Seismic Wave Velocities versus Depth

• 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.

Origin of the Solar System

• Solar Nebula

• Rotational Flattening

• Gravitational Collapse

• Initiation of nuclear reactions

• Planetesimal formation

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Processes within the Disc

• Strong gradients

• Escape of volatiles

• Retention of refractory compounds

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Composition of the Earth• ElementWeight Percent Atom percent Volume percent

• O 46.60 62.55 ≈ 94• Si 26.72 21.22┐• Al 8.13 6.47│• Fe 5.00 1.92│• Ca 3.63 1.94├ ≈ 6• Na 2.83 2.64│• K 2.59 1.42│

• Mg 2.09 1.84┘• Ti 0.44• H 0.14• P 0.10

Goldschmidt Classification

• Lithophile: Literally, "stone-loving". Elements which incorporate into silicate phases, generally of low density.

• Chalcophile: Literally, "copper-loving". However, since copper often forms sulfide phases, this really means elements which form sulfide phases, typically of intermediate density.

• Siderophile: Literally, "Iron-loving". Elements, typically iron and alloying elements, which form dense phases. Often these are metallic, although bonding to sulfur or carbon is possible.

• Atomphile: Light, gaseous elements. Some may have been retained during initial accretion, but most were lost to space. These substances, which form the atmosphere and oceans, probably accumulated slowly later in the earth's history.

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Density Calculations

• Whole earth density = 5.52 g/cm3

• Crustal rocks is around 3.0 g/cm3

• Infer that there is a region of much higher density within the earth

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Abundance of Elements

Additional Constraints

• Laboratory studies of seismic wave velocities

• Natural samples of the mantle

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Meteorites

• Pieces of extra-terrestrial solid material that survive the plunge through the earth's atmosphere

• Geological concentration of meteorites

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Meteorite Categories

• Irons

• Stones

• Stony-irons

• Collection problems

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Gradients

• Both temperature and pressure increase with increasing depth below the surface Geothermal gradient Geobarometric gradient

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Heat Loss

• Radiation

• Conduction

• Convection

• Advection

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Importance of Heat Loss

• Processes controlled by heat loss: Metamorphism Melting Crystaliization

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GeothermsFigure 1.11 Estimates of oceanic (blue curves) and continental shield (red curves) geotherms to a depth of 300 km. The thickness of mature (> 100Ma) oceanic lithosphere is hatched and that of continental shield lithosphere is yellow. Data from Green and Falloon ((1998), Green & Ringwood (1963), Jaupart and Mareschal (1999), McKenzie et al. (2005 and personal communication), Ringwood (1966), Rudnick and Nyblade (1999), Turcotte and Schubert (2002).

Pressure at the Base of the Crust

• Putting units into the equation, we get:

• ~ 30 MPa/km• » 1 GPa at base of average crust

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P kg m . m s m

kg m s P a G P a

2 8 0 0 9 8 3 5 0 0 0

9 6 1 0 9 6 1 0 1

3 2

8 2 8

,

. .

Units of Pressure

• Traditionally, pressure was expressed in units of bars or kilobars

• 1 bar = 105 Pa (0.1MPa), so this is about 300 bars or 0.3 kbars/km

• For the upper mantle, ρ ≈3.35 g/cm3. This gives a pressure gradient of about 35 Mpa/km

• Remember that these numbers are good only near the earth’s surface

• Core: ρ increases more rapidly since alloy more dense

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Geobarometric Gradient

• P increases = ρgh• Nearly linear through

mantle• Figure shows the

PREM (Preliminary Reference Earth Model) of Dziewonski and Anderson, which is a better reference to consult for pressures at depth within the earth

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Tectonics and Magma Generation

• 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...