Petrology Petrology: the study of rocks, especially aspects such as physical, chemical, spatial and chronoligic. 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 liquids 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., granite 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 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!
Classification of crystalline rocks Igneous Rocks: formed by the cooling and solidification of magma, defined as mobile molten rock whose temperature is generally in the range of 700-1200 o C (1300-2200 F). Most magmas are dominated by silicate melts on Earth. Metamorphic Rocks: formed by the reconstitution of preexisting 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 bounded by melting (~700 C), above which we are in the igneous realm.
This course will be divided into two parts: Lecture and Laboratory
Review of Earth Basics Is the Earth homogenous? No, very heterogeneous! Horizontally Vertically Petrologically Mineralogically Chemically Isotopically
The Earth: Horizontally Crust: obvious from space that Earth has two fundamentally different physiographic features: oceans (71%) and continents (29%) from: http://www.personal.umich.edu/~vdpluijm/gs205.html global topography
The Earth: Vertically Atmosphere Biosphere Hydrosphere Solid Earth Crust Mantle Core Figure 1.2 Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The Solid Earth Crust: Oceanic crust Thin: 10 km Relatively uniform stratigraphy = ophiolite suite: sediments pillow basalt sheeted dikes more massive gabbro ultramafic (mantle) Continental Crust Thicker: 20-90 km average ~40 km Highly variable composition Average ~ granodiorite
Figure 1.2 Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 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-type Si IV Si VI Lower Mantle
The Solid Earth 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.
Can you calculate the volume/mass percentage of the crust, mantle and core in the bulk Earth? By volume: Crust: 0.6% Mantle: 83% Core: 16.4% V = 4/3 x (Pi) x r^3 where is the radius of the ball What is the percentage of Mg in the mantle?
How do we know the Earth s interior? Seismic wave data
Crust: low density 2.8 to 3.3 g/cc P-wave: 6.1 to 6.5 km/s Mantle: higher density 3-5 g/cc P-wave 5-13 km/s Low velocity zone: Asthenosphere Core: high density Fe, Ni, S, 10-13 g/cc P-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.
Summary Crust-mantle-core Rocks are different Minerals are different Seismic waves are different Other properties?
Distribution of pressure within the Earth Rocks under high pressure do not have high shear strength and 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 above. This is similar to calculating hydrostatic pressure: Relation is: P = ρgh where P is pressure in Pascal, ρ is density in kg/m 3, g is the acceleration of gravity at the depth considered, in m/s 2 (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 thick granitic crust with a density of 2800 kg/m 3? Answer: P = 2800 kg/m 3 x 9.80 m/s 2 x 40,000 m = 1.01 x 10 9 kg/m 2 /s 2 or Pascal (Pa) = 1.01 GPa = 10.1 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 Gradient in 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 Density (ρ) increases with depth but 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 Earth In 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 heat Heat released by radioactive decay Long-live radioactive elements ( 40 K, 235 U, 238 U, 232 Th) Short-live radioactive elements ( 26 Al) 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 calculated. The 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 e i, and the concentration of the isotope in the rock, c i (ppm), and the density of the rock (ρ) such that: A = ρ Σe i c i 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 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 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 Gradient Figure 1.9 Diagrammatic cross-section through the upper 200-300 km of the Earth showing geothermal gradients reflecting more efficient adiabatic (constant heat content) convection of heat in the mobile asthenosphere (steeper gradient in blue) ) and less efficient conductive heat transfer through the more rigid lithosphere (shallower gradient in red). The boundary 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 lithosphere. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The Geothermal Gradient Figure 1.9 Notice that thinner lithosphere allows convective heat transfer to shallower depths, resulting in a higher geothermal gradient across the boundary layer and lithosphere. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The Geothermal Gradient Figure 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). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
How to calculate temperature within the 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 o C/km? Answer: T = 40 km x 15 o C/km = 600 o C This temperature is not high enough to melt crustal rocks.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Very important figure! Oceanic and continental thermal gradients, with the melting curves for peridotite and granite included. Note that melting should not occur within the continental crust given these gradients. Somehow, these gradients must be perturbed. i.e., plate tectonics