Petrology

Petrology: the study of rocks, especially aspects such as physical, chemical, spatial and chronoligic.p y , , p g

Associated fields include:Associated fields include:– Petrography: study of description and classification of rocks– Petrogenesis: study of the histories and origins of rocks

Classification:– Igneous: crystallized from a melt or magma– Metamorphic: changed in response to heat, pressure, directed

stress or chemically active gases or liquidsy g q– Sedimentary: formed at Earth’s surface, largely observable

What do you want to learn from Petrology?How did a specific rock originate?

– What is it? Descriptive (textural, minerals) e.g., graniteWhere did it come from? E g mantle or crust?– Where did it come from? E.g., mantle or crust?

– What processes were involved? Partial melting? Fractional crystallization?

– Under what conditions did it form? Pressure, temperature? – When did it form? Radiogenic isotopes (4.567 Ga, Earth)

What does the rock tell us about Earth/planetary history?– How does the planet work? E.g., plate tectonics

H d h l f d l ? E M– How does the planet form and evolve? E.g., Moon

There are still many controversies explaining the origins and compositions of different types rocks! A lot of work yet to be done!

This course will be divided into Igneous and Metamorphic petrologyand Metamorphic petrology

• Igneous Rocks: formed by the cooling and solidification ofIgneous Rocks: formed by the cooling and solidification of magma, defined as mobile molten rock whose temperature is generally in the range of 700-1200°C (1300-2200°F). Most magmas are dominated by silicate melts on EarthMost magmas are dominated by silicate melts on Earth.

• Metamorphic Rocks: formed by the reconstitution of pre-i i k l d ll b h hexisting rocks at elevated temperatures well beneath the

surface of the Earth. Lower bound of temperature range is poorly defined, but usually > 200°C. Upper range b d d b l i ( 700°C) b hi h i hbounded by melting (~700°C), above which we are in the igneous realm.

Review of Earth Basics

Is the Earth homogenous?

No, very heterogeneous! – HorizontallyHorizontally– Vertically– Petrologically– Mineralogically– Chemically

Isotopically– Isotopically

The Earth: Horizontally

Crust: obvious from space that Earth has two fundamentally differentphysiographic features: oceans (71%) and continents (29%)

global topography

from: http://www.personal.umich.edu/~vdpluijm/gs205.html

The Earth: Vertically

AtmosphereBiosphereHydrosphereySolid Earth• Crust• Mantle• CoreCore

Figure 1.2 Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

The Solid EarthC tCrust:Oceanic crust

Thin: 10 kmRelatively uniform stratigraphy

hi lit it= ophiolite suite:• sediments• pillow basalt• sheeted dikes• more massive gabbro• ultramafic (mantle)ultramafic (mantle)

Continental CrustThicker: 20-90 km average ~40 kmHighly variable composition

Average ~ granodiorite

The Solid Earth

Mantle:Peridotite (ultramafic)

Upper to 410 km (olivine → spinel) Low Velocity Layer 60-220 km (asthenosphere

Transition Zone as velocity increases ~ rapidly

660 spinel → perovskite-type660 spinel → perovskite type SiIV → SiVI

Lower Mantleo e a t e

Figure 1.2 Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

The Solid Earth

Core: Fe-Ni metallic alloyy

Outer Core is liquidNo S wavesNo 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.

Can you calculate the volume/mass percentageCan you calculate the volume/mass percentage of the crust, mantle and core in the bulk Earth?

By volume:C t 0 6%• Crust: 0.6%

• Mantle: 83%• Core: 16.4%

V = 4/3 x (Pi) x r^3 where is the radius ofthe ball

How do we know the Earth’s interior?

Seismic data

Crust: low density2.8 to 3.3 g/ccgP-wave: 6.1 to 6.5 km/s

M l hi h d iMantle: higher density3-5 g/ccP-wave 5-13 km/sP wave 5 13 km/sLow velocity zone:Asthenosphere

Core: high densityFe Ni S 10-13 g/ccFe, Ni, S, 10-13 g/ccP-wave 8-10 km/s

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.

Distribution of pressure within the Earth

Rocks under high pressure do not have high shear strength g p g gand tend to flow like viscous liquid. Thus pressures within the Earth can normally be calculated based on lithostatic pressure i e the load pressure from abovelithostatic pressure i.e., the load pressure from above. This is similar to calculating hydrostatic pressure:

Relation is: P = ρgh where P is pressure in Pascal, ρ is density in kg/m3, g is the acceleration of gravity at the d h id d i / 2 ( h diffdepth considered, in m/s2 (not the same at different depths, or for different planets), and h is the depth in m.

Distribution of pressure within the Earth

Example: what is the pressure at the base of 40-km hi k i i i h d i f 2800 k / 3?thick granitic crust with a density of 2800 kg/m3?

Answer: P = 2800 kg/m3 x 9.80 m/s2 x 35,000 m = 0.96 x 109 kg/m2/s2 or Pascal (Pa)= 0.96 GPa= 9.6 kbar

For the Earth’s crust, the relation between pressure and depth is roughly 1 GPa or 10 kbar per 35-40 km.

The Pressure Gradientin the Earth: P vs. depthin the Earth: P vs. depth

• P = ρgh• Nearly linear through mantle

~ 30 MPa/km~ 1 GPa at base of ave crust

• Core: r incr. more rapidly p ysince alloy more dense

• Density (ρ) increases with depth y (ρ) pbut pressure gradients decreases, due to decreasing of g.

Figure 1.8 Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356.

Distribution of temperature within the EarthIn comparison to pressure, temperature calculations for a

given depth are not so easy! E.g., on Earth’s surface

Heat sources in the Earth• Heat from the early accretion and differentiation of the Earth

– Accretion: conversion of kinetic energy to thermal– Core formation: conversion of gravitational potential energy to heatg p gy

• Heat released by radioactive decay– Long-live radioactive elements (40K, 235U, 238U, 232Th)

Short live radioactive elements (26Al)– Short-live radioactive elements (26Al)• Solar energy (minor)

Earth is cooling as a consequence of mantle convection

In the Earth’s crust, heat is generated mostly from radioactive decay This can be calculatedradioactive decay. This can be calculated.

The rate of heat production per unit volume of a rock AThe rate of heat production per unit volume of a rock, A, is the sum of the products of the decay energies of each radioactive isotope present ei, and the concentration of p p i,the isotope in the rock, ci (ppm), and the density of the rock (ρ) such that: A = ρ Σei ci in µW/m3

Heat Transfer from Regions of High-Temperature to Regions of Low-Temperature (the surface)

• Radiation: involves emission of EM energy from the surface of hot body into the transparent cooler surroundings. Only important at T’s >1200°C, e.g., deep mantle. Vacuum OK.

• Advection: involves flow of a liquid through openings in a rock whose T is different from the fluid (mass flux) Important nearwhose T is different from the fluid (mass flux). Important near Earth’s surface due to fractured nature of crust.

• Conduction: transfer of kinetic energy by atomic vibration. Cannot occur in a vacuum. For a given volume, heat is conducted away faster if the enclosing surface area is larger.

• Convection: movement of material having contrasting T’s from• Convection: movement of material having contrasting T s from one place to another. T differences give rise to density differences. In a gravitational field, higher density (generally colder) materials sink.

Geothermal gradient in the Earth: T vs. depth

To obtain thermal gradient must account for:

• Heat production from radioactive element• Convective cooling at depth• Radiative cooling

The Geothermal GradientFigure 1.9 DiagrammaticFigure 1.9 Diagrammatic cross-section through the upper 200-300 km of the Earth showing geothermal gradients reflecting more

ffi i t di b ti ( t tefficient adiabatic (constant heat content) convection of heat in the mobile asthenosphere (steeper gradient in blue) ) and less g ) )efficient conductive heat transfer through the more rigid lithosphere (shallower gradient in red). The boundary layer is a zoneboundary layer is a zone across which the transition in rheology and heat transfer mechanism occurs (in green). The thickness of the boundary layer is exaggerated here for clarity: it is probably less than half the thickness of the lithospherelithosphere.

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

The Geothermal GradientFigure 1.9 A similarFigure 1.9 A similar example for thick(continental) lithosphere.

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

The Geothermal GradientFigure 1.9 Notice thatFigure 1.9 Notice that thinner lithosphere allows convective heat transfer to shallower depths, resulting in a higher geothermal

di t thgradient across the boundary layer and lithosphere.

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

The GeothermalGeothermal

Gradient

Figure 1.11 Estimates of oceanic (blue curves) and continental shield (red curves)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 andMcKenzie et al. (2005 and personal communication), Ringwood (1966), Rudnick and Nyblade (1999), Turcotte and Schubert (2002). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

How to calculate temperature within the Earth with a given geothermal gradient?Earth with a given geothermal gradient?

Example: what is the temperature at the base of 40-km thick granitic crust with a geothermal gradient 15 oC/km?

Answer: T = 40 km x 15 oC/km= 600 oC

This temperature is not high enough to melt crustal rocks.

Very important figure!y p g

Oceanic and continental thermal gradients withthermal gradients, with the melting curves for peridotite and granite i l d d N t th tincluded. Note that melting should not occur within the continental crust given these gradients. Somehow, these gradients must bethese gradients must be perturbed. i.e., plate tectonics

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Summary

• Three types of rocks• The interiors of the Earth (by seismic waves)• The interiors of the Earth (by seismic waves)

– Crust-mantle-core– Know how to calculate vol/mass percentage of ow ow o c cu e vo / ss pe ce ge o

crust/mantle/core in Earth

• Pressure distribution in the Earth – Know how to calculate pressure in the Earth: P = ρgh

• Temperature distribution in the Earth – Heat sources– Know how to read/interpret thermal gradient figures (TGFs)

h l l i h h i h G– Know how to calculate temperature in the Earth with TGFs