1. Fundamental Concepts GLY 4310 Spring, 2016
Petrology Lecture 1
Fundamental Concepts
GLY 4310
Spring, 2016

2. Major Subdivisions of the Earth
Basic divisions of the earth have been discussed in earlier courses, and are based on composition. They include the crust, the mantle, and the core. The crust is usually divided into continental crust and oceanic crust.
Continental crust: Averages about 36 kilometers, but may reach as deep as 90 kilometers. Composed of igneous, metamorphic, and sedimentary rock. Continental crust is about 1% of the volume of the earth. Its most important tectonic property is its low density. Continental crust is light and buoyant - it is never subducted. Continental crust grows with time, principally by the addition of rock from mantle-derived melts. Some continental crust has survived from the earliest rocks on earth, nearly four billion years old.
Oceanic crust: Oceanic crust is much thinner than continental crust, averaging about 10 kilometers in thickness. It is much more uniform, being composed primarily of basalt or its intrusive equivalent, gabbro. Oceanic crust is created at spreading centers and consumed in subduction zones, so it is constantly recycled. The oldest oceanic rocks are less than 200,000,000 million years.
Mantle: The largest region by volume of the earth is the mantle, about 83% of the total volume. The mantle is divided into three parts, the upper, transition, and lower zones. The upper mantle begins just below the Mohorovičić Discontinuity (often called the Moho). There is a sharp change in seismic wave velocity at the Moho. P wave velocity increases from 7 to over 8 km/sec, making it an excellent seismic reflector. The upper mantle is composed primarily of iron and magnesium silicate minerals. Within the upper mantle is the low-velocity zone, where seismic waves suffer a substantial reduction in velocity (about 0.5 km/sec). Current thinking is that the seismic low-velocity zone is partially molten, probably in the 1-10% range. The melt coats mineral surfaces, and retards the progress of seismic waves. The liquid also makes this layer less rigid and more ductile. The low-velocity zone varies in thickness, but is approximately between the depths of 60 and 220 kms. below the surface.
Another discontinuity occurs at 410 kms. The mineral olivine is transformed to a cubic-closed packed equivalent, or spinel type structure. At 660 km, the tetrahedral coordination of Si4+ changes to octahedral (VI) coordination in a perovskite type structure. At both depths, the increased density causes an abrupt increase in seismic wave velocity. The region between the discontinuities is called the Transition Zone. From 660 km to 2900 km, seismic waves increase uniformly with increasing depth. Increasing pressure compresses the perovskite structure, making the minerals denser. But at 2900 km, the composition changes abruptly.
Core: The core of the earth extends from 2900 km to about 6370 km, the center of the earth. It is divided into two zones, the outer core ( km) and the inner core (5145 to 6370 km). The outer core is liquid, the inner core solid. In both regions, the composition is that of an iron-rich alloy, with nickel as an important accessory element, and sulfur, silicon, oxygen, and perhaps other elements present in small amounts. The reason for the liquid-solid transition is a battle between P and T. Both increase with depth. Initially, increasing temperatures cause melting.
Increasing pressure tends to cause freezing. Finally, pressure prevails. We know the outer core is liquid because seismic S waves cannot travel through liquids. Since they don't traverse the outer core, the region is known to be liquid.

3. 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.
We may also subdivide the earth on the basis of rheological properties. Rheology is the study of the deformation and flow of matter. Under this scheme, the entire crust and the uppermost mantle above the low-velocity zone are called the "lithosphere". (Gk lithos = stony) The lithosphere behaves like a hard, rigid solid. The lithosphere is about kms. below the oceans, and kms. below the continents. The next layer is the asthenosphere (Gk asthenes = without strength). This layer is ductile, and provides a zone of dislocation upon which rigid lithospheric plates move. It extends from the bottom of the lithosphere to the beginning of a rigid layer. The exact boundary is not well defined. Originally many geologists placed the bottom of the asthenosphere, rater arbitrarily, at 300 km below the surface. Today, it is generally said to be about 700 km deep. The region below the asthenosphere is the mesosphere, which extends to the bottom of the mantle. The remaining layers are simply said to be liquid (outer core) and solid (inner core).

4. Origin of the Solar System
Solar Nebula
Rotational Flattening
Gravitational Collapse
Initiation of nuclear reactions
Planetesimal formation
The solar system is thought to have originated from a solar nebula, composed of molecular H2, He, some Li and Be (produced by the Big Bang), and about 2% heavier elements produced by subsequent nuclear synthesis in other star systems. The nebula was rotating. Centrifugal "force" caused flattening of the nebula into a disk shape. Perhaps one to ten percent of the nebular mass was concentrated into the center of the disk. Gravity became important, pulling material toward the center. Centrifugal forces and the necessity to conserve angular momentum counteracted gravity. Gravitational collapse generated heat. Eventually, the temperatures became hot enough for the nuclear conversion of hydrogen to helium became possible. At that point, a star was born. The sun became very luminous, radiating a lot of energy while collapse continued for about 100,000 years. As collapse slowed and stopped, the sun became less luminous, entering a stage called the T-Tauri stage, for about 10,000,000 years. The solar wind, radiating out from the sun, blew about half the mass of the nebula back into inter-stellar space. Most of the remaining mass (99.9%) collapsed into the sun. The remaining 0.1%, with high angular momentum, remained around the sun, forming meter to kilometer sized planetesimals. The planetesimals were concentrated in the flat part of the disk, eventually coagulating to form proto-planets

5. Processes within the Disc
Strong gradients
Escape of volatiles
Retention of refractory compounds
Within the disk, there was an extremely strong temperature and pressure gradient radiating out from the sun. The volatile particles escaped the inner planets and were blown outward by the intense solar wind, to regions where T was low enough to allow condensation. The inner proto-planets were composed of only the most refractory compounds. Compounds like alumina (Al2O3), CaO, and TiO2 either did not evaporate at all or condensed quickly. More volatile substances were distributed outward in order of their condensation temperatures. These include Fe-Ni alloys, Fe-Mg-Ni silicates, alkali metals and their silicates, sulfides, hydrous silicates, and water were included. The least refractory substances, such as ammonia, methane, and remaining hydrogen and helium were condensed only in the outer part of the solar system. The innermost, so-called terrestrial, planets (Mercury to Mars) were largely refractory, while the outermost gaseous planets were largely non-refractory.

6. Composition of the Earth
Element Weight Percent Atom percent Volume percent
O ≈ 94
Si ┐
Al │
Fe │
Ca ├ ≈ 6
Na │
K │
Mg ┘ Ti H P
As the planetesimals collided, they gradually accreted into larger and larger masses. The impact velocity was partially converted to heat, and the mass became warmer and larger. Some speculate that the heavier iron-nickel planetesimals may have collided first, due to higher gravitational attraction, forming a dense "core" for a proto-planet. In any case, as heating progressed, melting began, and the denser iron-nickel would have sunk toward the center. Computer models suggest that the heat released was sufficient to melt the entire earth (possible excluding the cold, outermost crust). The earth, and probably the other terrestrial planets, became differentiated.

7. 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.
In 1937, Victor Goldschmidt suggested the earth's elements tended to separate into phases which depended on their geochemical affinities.
Goldschmidt's ideas are useful, but certainly do not give a completely accurate picture of the earth. There are many reasons for this. Many atoms are ionized, and each ion has its own individual preferences. In addition, ionization requires that we maintain electrical neutrality. For example, iron is siderophile, but is also found in the lithophile and chalcophile phases as well. Elements which are chalcophile prefer to be in sulfide phases, but there isn't enough sulfur to combine with all chalcophile cations. Oxygen is found primarily in the lithophile phase, combined with silicon. There is insufficient silicon to combine with all the oxygen, so it is necessary for other cations to be present. In addition, the differentiation processes are never perfectly efficient. Gold is siderophile, but some is retained in the crust. Later processes, such as alteration by hydrothermal solution, also play a role in the distribution we see today.
Hypotheses offer explanations of observed facts, and make predictions that can be tested. The most useful hypothesis advances to become a theory. The more observed facts a theory can explain, in the simplest manner, without violating known physical laws, the more confidence we have in it. As an example, Plate Tectonics advanced from the Continental Drift hypothesis to become a theory in the last century.

8. 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
The mass of the earth has been calculated from measurements of the gravitational constant and the moment of inertia. Knowing the size of the earth allows us to calculate its volume. From the mass and volume, we calculate the average density of the earth, which turns out to be 5.52 g/cm3. Since the density of the crustal rocks is around 3.0 g/cm3, we infer that there is a region of much higher density within the earth.

9. Abundance of Elements
Measurement of spectra from the sun, planets, asteroids, comets and other stars gives us an idea of their composition. Together with data from meteorites, this allows us to estimate elemental abundances in the solar nebula. Figure 1-6 in the textbook gives one such estimate. Note that the vertical scale is a logarithmic scale. The sawtooth distribution is the result of the Oddo-Harkins rule, which relates to pairing of nucleons in the nucleus. Isotopes with an even number of protons and an even number of neutrons are the most stable. Stable isotopes will be the most abundant. Thus 12C and 16O are quite abundant. Elements with an odd atomic number have an odd number of protons, and are thus less stable and less abundant. "Odd-odd" isotopes are the least abundant.
Knowledge of element abundances in the solar nebula provides constraints for the composition of the interior of the earth. It must be composed largely of the most abundant elements, with the exception of hydrogen and helium, which are largely in the sun. Elements like iron, nickel, and magnesium, which are more abundant in the solar nebula than in the earth's crust, are assumed to be present in the deeper regions of the earth.

10. Additional Constraints
Laboratory studies of seismic wave velocities
Natural samples of the mantle
Laboratory studies of seismic wave velocities (both P and S) in various phases at elevated T and P, corresponding to conditions within the earth, also place constraints on the materials comprising the earth. Reflection and refraction from layers of different density within the earth provide evidence for the internal structure of the earth.
Natural processes in the earth are believed to bring samples of the mantle to the surface. Careful studies of these rocks helps to confirm the models of the earth's interior derived from composition and seismic data.

11. Meteorites
Pieces of extra-terrestrial solid material that survive the plunge through the earth's atmosphere
Geological concentration of meteorites
Meteorites are pieces of extra-terrestrial solid material that survive the plunge through the earth's atmosphere. In the past, finds of meteorites have often been by chance. If not found immediately, terrestrial weathering can alter or destroy meteorites. About twenty years ago, it was discovered that meteorites hitting the Antarctic continent were often transported by glacial movement, and concentrated in glacial deposits. One such glacial dumping ground is the Allen Hills region of Antarctica. The cold temperatures and low humidity on the Antarctic continent contribute to fine preservation of meteorites. Meteorites are believed to represent early to intermediate stages in the development of the solar nebula, which have not undergone further alteration before reaching earth. In addition, it has recently been discovered that a number of meteorites come from other planets, particularly Mars. When a large meteorite hits the Martian surface at a low angle, much debris is shot towards space. Some of this debris, consisting largely of Martian surficial rocks, has escape velocity. It enters space in the region between Earth and Mars, where the large gravity field of the earth captures much of it.

12. Meteorite Categories Irons Stones Stony-irons Collection problems
Irons - Fe-Ni alloys. They are very heavy and thus relatively easily discovered. Typical phases are kamactite and taenite, often exsolved from a single, homogeneous phase as the meteorite material cooled. The resulting pattern is known as the Widmanstätten texture. In addition, a chalcophile phase called troilite (FeS) is often present.
Stones - Silicate minerals. They look like terrestrial rocks, and are thus infrequently discovered. Typically they are subdivided into those that contain chondrules (chondrites) and those that do not (achondritic). Chondrules are spherical inclusions, typically 0. to 3 mm in diameter, which originated as glass but have subsequently crystallized. Chondrites are thought to be undifferentiated meteorites, since the temperatures necessary to produce differentiation would have melted and destroyed the chondrules. The tiny chondrules would have cooled in under an hour, and their lack of crystallization indicates formation in a cooler part of the nebula. Most likely, they formed after condensation of nebular material, but before planetesimal formation. This makes them the most "primitive" of the meteorite classes. They are heavily used in providing the data for solar system elemental abundance for this reason. Achondrites, like the irons and stony-irons, are considered to be differentiated meteorites.
Stony-Irons - Subequal mixtures of the above types. The phases present include those in the irons, plus a substantial amount of lithophile phases.
Collection problems - Museum collections have traditionally been dominated by irons. They are believed to be the core samples from differentiated terrestrial type planets. When meteorites are collected at observed fall sites, stones comprise about 94% of the observed material.

13. Gradients
Both temperature and pressure increase with increasing depth below the surface
Geothermal gradient
Geobarometric gradient
Geothermal gradient
The geothermal gradient varies from place to place within the earth. Heat comes from two sources:
1. Heat from the original accretion and gravitational differentiation of the planet. This heat has been escaping since the planet formed, so it diminishes with time. However, there is evidence that solidification of iron at the inner-outer core boundary continues to contribute some heat.
2. Heat from the radioactive decay of long-lived radioactive elements (U, Th, K). These tend to be concentrated in the continental crust (felsic rock). Thus, heating from radioactive decay is uneven, producing from 30 to 50% of the heat reaching the surface in any given area.
Heat generated by these mechanisms is then transferred from the hotter interior regions to the cooler exterior regions of the planet, creating the geothermal gradient.

14. Heat Loss Radiation Conduction Convection Advection
1. Radiation: Heat is lost from the surface by IR radiation into space, and gained by absorption of some UV and visible at the surface. Radiation is impossible within the earth, although there is speculation and great depth and pressure that some silicate minerals may be transparent to IR radiation.
2. Conduction: Rigid, opaque substances transfer kinetic energy as vibrational excitation of bonds. It is much more efficient for metals than for silicates.
3. Convection: Ductile substances may move from hot regions to cooler regions, carrying heat with them. Hot substances expand, becoming more buoyant, and rise. Thought to be important in the asthenosphere, and probably in the outer core. Likely source of heating at the mid-ocean ridges. It is also extremely important in hydrothermal systems.
4. Advection: Transfer of heat by solid substances that are in motion. This may result from tectonic uplift, or by erosion followed by isostatic rebound, for example.

15. Importance of Heat Loss
Processes controlled by heat loss:
Metamorphism
Melting
Crystaliization
Heat transfer controls several important processes within the earth, including metamorphism, melting, and crystallization. Lava flows, pluton emplacement, and explosive volcanism, depend on the type and extent of heat transfer. Heat flow is higher in younger rocks, particularly oceanic crust. With time, heat flow approximates a steady state. This happens in oceanic crust after about 180 million years, and in continental crust that is older than 800 my. Typical values of steady state heat flow are 21 to 34 mW/m2 beneath the continents, and 25 to 38 mW/m2 beneath the ocean.

16. Geotherms
Figure 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).
Figure 1-11 illustrates the oceanic and continental geotherms. There is an overlap region. Both curves steepen with increasing depth. The near-surface geotherm is much too high at depth, and can only be used in the upper few kilometers. The oceanic and continental geotherms converge at depth, and become equal about 150 to 200 kilometers below the surface. Below this range, the geotherm is nearly adiabatic (meaning having constant heat content), with a gradient of about 0.3̊C/km, or 10̊C/GPa. In the core, the gradient is near zero, because of conduction in the metal and convection in the liquid outer core

17. Pressure at the Base of the Crust
Putting units into the equation, we get:
~ 30 MPa/km
» 1 GPa at base of average crust
Continental crust has a density of about 2.8 g/cm3. To calculate pressure 35 km down, we can substitute appropriate units, we get,
Translating units, this corresponds to a pressure gradient of 1GPa/35 km, or 30MPa/km.

18. 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

19. 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
It is much easier to describe the geobarometric gradient than the geothermal gradient in near surface environments. Pressure exerted in fluids or ductile solids comes from the weight of the overlying material, and is give by equation above.
P is pressure, ρ = density, g = acceleration due to gravity, and h = the height of the column of material above the object. Pressure such as this, which is equal in all directions, is called hydrostatic (in water) or lithostatic (in rock).

20. Tectonics and Magma Generation
5. Back-arc Basins
6. Ocean Island Basalts
7. Miscellaneous Intra Continental Activity
Kimberlites, Carbonatites, Anorthosites...
1. Mid-ocean Ridges
2. Intracontinental Rifts
3. Island Arcs
4. Active Continental Margins
Most magmas originate in the mantle. As they pass through the crust, they often partially melt and assimilate crustal materials. Plate tectonics is responsible for the generation of many, but not all, magmas. Some magmas originate at depths below those influenced by tectonic forces. Figure 1-10 shows a representative summary of magmas types and geologic settings:
Divergent plate boundaries are the most volume metrically significant source of magmas. Mid-ocean ridges represent divergent ocean-ocean boundaries, where melting in the shallow mantle produces a basaltic magma that rises and solidifies on the earth’s surface to produce MORB (Mid-ocean ridge basalt)(location 1). A similar process can occur on land, for example in the East African Rift Zone (location 2). Continental rifts typically produce a much more alkaline magma due to contamination of the original magma while passing through thick continental crust.
Moving oceanic plates are eventually subducted at convergent plate boundaries. Melting occurs in subduction zones. Subducted material includes components of the mantle, subducted oceanic crust, subducted continental crust, and subducted sediments. The resulting magmas show much more diversity than divergent boundary magmas. At an ocean-ocean plate boundary (location 3), a volcanic island arc forms (The Aleutians are a good example). Subduction beneath a continent (Location 4) produces a continental arc. The continental arcs are more siliceous than the oceanic arcs (again, contamination by continental crustal material). Plutons are more commonly associated with continental arcs (The Sierra Nevada batholiths).
“Back-arc” (location 5) extensional tectonics is commonly associated with both types of volcanic arcs. Back-arc magma is similar to MORB. A ridge can sometimes form in the ocean at back-arc sites. However, extension is a slow process, so volcanism is irregular and much less voluminous than at MOR sites. Japan probably separated from the Asian mainland because of this type of process.
Volcanism can also occur away from plate boundaries, both in the ocean and on continents. An example of such volcanism in the ocean is the Hawaiian - Emperor volcanic chain (location 6). The magma is basaltic, but is more alkaline then MORB. Such volcanism is attributed to “Hot spots”, regions below the moving tectonic plates, either in the deep asthenosphere, lower mantle, or possibly the core-mantle boundary, which are geographically fixed and which produce a continuous stream of magma. The Yellowstone Caldera
(location 7) is an example of the same phenomenon below a continent.