Yaoling Flashcards
Zeroth law
If two thermodynamic systems are separately in thermal equilibrium with a third, they are also in thermal equilibrium with each other.
That is, if T(A) = T(B), and if T(B) = T(C), then T(A) = T(C)
1-3 laws of thermodynamics
First law: Energy can neither be created nor destroyed, but can only be transformed from one form to another (e.g., mechanical, thermal, electrical, light etc.).
Second law: The total entropy of any isolated thermodynamic system tends to increase over time, approaching a maximum value.
Third law: As a system asymptotically approaches absolute zero of temperature all processes virtually cease, and the entropy of the system asymptotically approaches a minimum value
Note: Thermodynamics ONLY applies to equilibrium situations or processes towards equilibrium
However, thermodynamics still applies for small portions of the rock body. That is, you need to choose or define your system to work with
What is a system?
A region of thermodynamic interest, which is separated from its surroundings by a boundary. Mass, heat and work may be exchanged across the boundary of the system.
We may not choose the entire intrusion as a system, but a very small portion of it as a system: a piece of rock, a thin-section etc. We can be readily convinced that equilibrium has been reached on such a small scale – looking at the thin section under microscope etc.
A system can be:
Isolated - no exchange of mass, heat or work across the boundary;
Closed – exchange of heat and work, but no mass;
Open – exchange of heat and work and mass;
Adiabatic – a closed system with thermally insulated boundaries.
Variables or variables of state
- Variables - Measurable properties of matter which define the state of a system.
- Two types of variables – Extensive variables vs. Intensive variables:
- Extensive variables – depending on the amount of material in the system – the values are additive. For example, volume, mass, energies etc.
- Intensive variables – independent of the amount of material in the system; non-additive, e.g., temperature, pressure, density, composition etc
Other important concepts
- Species – a chemically identified substance (e.g., SiO2, H2O, FeO, K2O etc.).
- Component – the minimum number of species required to completely define a system.
- Phases – a physically distinct, “homogeneous” portion of a system (different minerals, liquids, gas etc.).
The familiar Phase Rule or Gibbs Phase Rule
f + p = C + 2
f = C + 2 – p
p = C + 2 - f
f = degree of freedom, i.e., the maximum number of intensive variables that YOU can change without affecting the equilibrium condition of the system C = number of components or minimum number of species required to fully describe the system p = number of phases (minerals) 2 = refers to the two intensive variables T and P
Gibbs Phase Rule for a unary system
For a one component system, or “unary” system, C = 1, p = 3 -f
If f = 0, p = 3 (invariant point)
If f = 1, p = 2 (univariant line) – reaction line
If f = 2, p = 1 (divariant plane or field)
P = number of minerals Centre point = where all coexist F = 0 = P = 3 = all minerals coexist F = 1 =P = 2 = reaction line between 2 minerals F = 2 =P= 3 = Singular mineral
Gibbs Phase Rule for a binary system
For two component system, or “binary” system, p = C + 2 –f, p = 4 - f
If f = 0, p = 4 (invariant point)
If f = 1, p = 3 (univariant line) – reaction line
If f = 2, p = 2 (divariant plane or field)
Gibbs Phase Rule for a ternary system
For three component system, or “ternary” system, p = C + 2 –f, p = 5 - f
If f = 0, p = 5 (invariant point)
If f = 1, p = 4 (univariant line) – reaction line
If f = 2, p = 2 (divariant plane or field)
First law of thermodynamics:
dU = dQ + dW
Change in total energy = the change in heat energy + work done
dU = dQ - PdV
Work done = constant pressure x change in volume (PdV)
Calculations of Enthalpy Changes:
Enthalpy of formation at standard state -
- delta Hf = enthalpy at formation
- Standard state: T = 298.15 K (25°C), P = 1 bar, one mole of pure substance
- At standard state enthalpy of elements (e.g., H, O, Si, Fe, Mg etc.) are arbitrarily assigned “zero”.
- Use the sum of the enthalpy formation change of the products minus the sum of the enthalpy formation of the reactants
If H > 0, endothermic – needs heat in order for the forward reaction to occur;
If H < 0, exothermic – gives off heat in order for the forward reaction to occur
2nd + 3rd law
Second Law of Thermodynamics:
Verbally - The spontaneous reaction within a system occurs only when the S of a system increases.
Third Law of Thermodynamics:
The entropy of a pure, perfectly crystalline substance is zero at absolute temperature of zero degree (0K) – verbal description of the third law
Pseudosections
- A type of phase diagram that shows the fields of stability of different equilibrium mineral assemblages for a single bulk-rock composition.
- Standard phase diagrams may include many reactions but, depending on its composition, a particular rock may only experience a few (or none) of them.
- Pseudosections only include those reactions that affect a particular composition. Fields on a pseudosection are labeled (specifying the equilibrium mineral assemblage) with the reaction lines unlabeled (although the specific reaction can be deduced).
Lever rule
graphic representation of the abundance of components in a binary mixture is analogous to the position of a fulcrum (pivot) on a lever to the weights at each end of the lever
Peritectic
- Reaction
- A peritectic reaction is a reaction where a solid phase and liquid phase will together form a second solid phase at a particular temperature and composition
Eutectic
- Relating to or denoting a mixture of substances (in fixed proportions) that melts at a single temperature
- Eutectic point is point where both substances melt
What is metamorphism
- Metamorphism is a process that changes an existing rock, in response to the changing P-T conditions, into a new rock with new mineral assemblages
- Metamorphism occurs under sub-solidus conditions, i.e., changes taking place at solid state without melting. However, a fluid phase may be involved, which function as a catalyst facilitating metamorphic reactions.
- Some metamorphism requires water, i.e., formation of chlorite schist and amphibolite from basalts: hydration (or carbonation) reactions
- Some other metamorphic processes remove water from the system, i.e., formation of granulite from amphibolite and formation of eclogite from blueschist etc: dehydration (decarbonation) reactions.
- Some metamorphic reactions are isochemical, i.e., no gain or loss of chemical elements, but simply a process of re-crystallization. Most metamorphic reactions involve chemical exchanges, i.e., it is inevitable when a fluid phase (H2O, CO2 etc.) is involved
- 1.6 GPa is the accepted maximum pressure in the crust – anything more is below the moho which is no longer in the crust (mantle) and therefore is not metamorphism
Metamorphic agents:
- Temperature
- Pressure
- Fluids
(geo)thermal gradient
\: T = dT/dz • Change in temperature T with depth z • = 5-10 K/km in cold subduction zones • = 20 K/km in stable cratons • = 50 K/km in rifts and magmatic arcs
Heat production in the Earth
• By its early accretion and continues to be produced by the crystallization of the inner core from the outer core
• By radioactive decay of radionuclides (most importantly: K, U, Th:
235U —> 207Pb, 238U —> 206Pb, 40K —> 40Ar, and 232Th —> 208Pb)
Types of metamorphism
Regional Contact Cataclastic Hydrothermal Burial
Texture terms
Porphyroblast: a grain that has grown larger than others (typical examples are garnet, staurolite, chloritoid, andalusite).
Poikiloblast: a grain with inclusions of other minerals–also called ‘sieve’ texture (typical examples are garnet, staurolite, cordierite, andalusite).
Porphyroclast: a relict grain that is larger than others surviving in a deformed rock. The key difference between porphyroblast and porphyroclast is that porphyroblasts are bigger than surrounding grains because of growth, whereas porphyroclasts are bigger because surrounding grains have undergone grainsize reduction during deformation.
Lineation: a linear fabric; typically defined by either linear minerals, such as hornblende, or stretched grains, such as quartz. Rocks with purely linear fabrics are called L tectonites and form by a constrictional deformation (prolate strain ellipsoid)
Foliation: a planar fabric; typically defined by platy minerals such as mica or flattened grains such as quartz. This is a more general term that encompasses cleavage and schistosity. Rocks with purely planar fabrics are called S tectonites and form by a flattening deformation (oblate strain ellipsoid). Most metamorphic rocks are L-S tectonites that form from a more general strain history.
Cleavage: a foliation formed by platy minerals such as mica. Most commonly used to describe low-grade micaceous (pelitic) rocks such as slate.
o Crenulation cleavage: a special kind of cleavage that forms as a result of shortening at a low angle to a pre-existing cleavage
Metamorphic changes
Textural Changes with increasing metamorphism, the protolith undergoes increasing recrystallization and fabric change
o Limestone marble
o Quartz arenite quartzite
o Shale —> slate —> phyllite —> schist —> gneiss —> melt.
o Shale is a sedimentary term. Slate refers to a fine-grained, low-grade metasedimentary rock in which bedding is typically still visible:
o Schist refers to a medium- to high-grade, medium- to coarse-grained metamorphic rock of any composition that has a fabric.
o Gneiss refers to a high-grade, coarse-grained metamorphic rock of any composition that has a fabric
Paragneiss: a metasedimentary gneiss.
Orthogneiss: a meta-igneous gneiss.
Schistosity: a foliation that is lumpier than cleavage; typically formed by a combination of platy minerals and more equant phases such as feldspar and quartz. Most commonly used to describe medium-grade rocks such as schists of nearly any bulk composition.
Texture changes
Schistosity: a foliation that is lumpier than cleavage; typically formed by a combination of platy minerals and more equant phases such as feldspar and quartz. Most commonly used to describe medium-grade rocks such as schists of nearly any bulk composition.
Gneissic: foliation of equant minerals such as feldspar and quartz. Most commonly used to describe high-grade metaplutonic rocks such as gneisses.
Pseudomorph: replacement of one phase by another, generally preserving shape
Idioblastic: sharp crystal form (euhedral in igneous terminology)
Hypidioblastic: intermediate form (subhedral in igneous terminology
Xenoblastic: no clear crystal form (anhedral in igneous terminology).
Barrovian Zones
- Chlorite Zone: Chlorite + Muscovite + Quartz + Albite
- Biotite Zone: Biotite + Chlorite + Muscovite + Albite + Quartz
- Garnet Zone: Almandine + Quartz + Biotite + Muscovite + Albite-rich Plagioclase
- Staurolite Zone: Staurolite + Almandine + Quartz + Muscovite + Biotite + Plagioclase
- Kyanite Zone: Kyanite + Almandine + Quartz + Muscovite + Biotite + Plagioclase
- Sillimanite Zone: Sillimanite + Almandine + Muscovite + Biotite + Quartz + Plagioclase + K feldspar
Buchan Zones
Near Buchan, the same protolith, i.e., mudstone (or pelite), yet the index mineral sequence goes:
Staurolite
Cordierite
Andalusite
Sillimanite
P Buchan < P Barrovian
Metamorphic facies definition
“Metamorphic facies is a set of metamorphic mineral assemblages (parageneses), each for a specific rock compositions, that form over a specific range of P and T”
Facies
- Zeolite Facies: zeolite
- Prehnite-pumpellyite Facies: prehnite and pumpellyite
- Blueschist Facies: glaucophane + lawsonite or episode (+ albite ± chlorite)
- Greenschist Facies: chlorite + albite + epidote (or zoisite) ± actinolite
- Epidote-Amphibolite Facies: plagioclase (albite-oligoclase) + hornblende + episode ± garnet
- Amphibolite Facies: plagioclase (oligoclase-andesine) + hornblende ± garnet
- Granulite Facies: orthopyroxene (+ clinopyroxene + plagioclase ± hornblende ± garnet)
- Eclogite Facies: omphacitic pyroxene + garnet
The boundary between any two facies is gradational
Isograd:
An isograd is a plane of constant metamorphic grade in the field; it separates metamorphic zones of different metamorphic index minerals.
Uplift Vs Exhumation:
Uplift: increase in elevation
Exhumation: decrease in vertical separation between a rock and the surface
Importance of Protolith – different rocks formed under similar P-T conditions:
Protolith Metamorphic Rock Sandstone Quartzite Limestone Marble Basalt Amphibolite Granite Granitic gneiss Shale Sillimanite gneiss Peridotite Olivine-tremolite schist
Tectonic regions and metamorphism
Stable cratons (green): cratons are stable and relatively cold, with ‘normal’ thermal gradients of ~20 K/km.
Magmatic arcs (red-orange): magmatic arcs are sites where heat is advected to shallow levels, producing low P/T metamorphism.
Crustal extension (orange): crustal extension via normal faulting leads to advection of heat to shallow levels, followed by cooling to a normal thermal gradient . Oceanic extension--mid-ocean ridges (red-orange): convection carries heat to very shallow levels, where 7-km thick oceanic crust forms; hydrothermal circulation produces low P/T metamorphism.
Ophiolite soles (red): are thrust zones beneath very hot oceanic lithosphere emplaced onto passive continental margins; in contrast to other low P/T metamorphism, inverted metamorphic gradients form because the emplacement rate is rapid compared to the rate at which the extreme heat is conducted away.
Subduction zones (blue): rapid subduction advects cold material into the mantle, producing high P/T metamorphism.
Metamorphic rocks in increasing grades of metamorphism
Slate Phylite Schist Blue schist Gneiss Magnetite
