Metamorphic Petrology

Question 1

What is heat and how is it measured?

Answer 1

Heat is a form of energy expressed by molecular and atomic vibrations; heat intensity is measured through temperature

Question 2

Which type of crust has a higher geothermal gradient and why?

Answer 2

Continental crust, due to radioactive decay

Question 3

Where are high and low T/P geotherms most commonly located?

Answer 3

High geotherm: areas of plutonic activity and rifting
Low geotherm: subduction zones

Question 4

How does increased temperature affect metamorphic texture?

Answer 4

Temeprature rise promotes recrystallisation and crystal growth

Question 5

What is pressure and how does it differ from stress?

Answer 5

Pressure is the external force exerted on a rock per unit area. Differential pressure can cause stress, acting on rocks through the lithostatic and hydrostatic gradients

Question 6

What are the 3 types of stress?

Answer 6

Tensile stress occurs from stretching
Shear stress occurs from perpendicular outwards forces working in opposite directions
Compressive stress occurs from compaction

Question 7

What is strain and what are its 2 main forms?

Answer 7

Strain is a rock’s reaction to differential stress
Foliated fabrics are composed of parallel bands of minerals in a rock, typically occurring under moderate to high pressure metamorphism
Lineated fabrics are composed of elongated or aligned grains along a particular direction, often occurring in tense pressure metamorphism

Question 8

What are metamorphic facies?

Answer 8

Groups displaying the progression along a gradient through the formation of new characterising minerals and changing assemblages

Question 9

What is the isobaric phase rule for metamorphism?

Answer 9

The number of phases in a system + degrees of freedom = the number of chemical components required to describe the system + 2

Question 10

When do violations of the phase rule occur?

Answer 10

When a system does not reach equilibrium

Question 11

What are discontinuous reactions? What are the 3 dominant types of discontinuous reaction?

Answer 11

Univariant reactions in which changes in crystal structure, net transfer between solid reactants and products, or volatile phases are released at high pressure occur in abrupt stepwise processes

Question 12

What are continuous reactions?

Answer 12

Reactions in which rocks undergo a continuum of changes over time. Most continuous reactions are simply P/T changes following the gradient, but some involve ion exchange

Question 13

What are the index minerals for Barrovian Zones?

Answer 13

Chlorite, Muscovite, Biotite, Garnet Staurolite, Kyanite, Sillimanite

Question 14

What are the index minerals for Buchan Zones?

Answer 14

Andalusite, Sillimanite, Kyanite, Staurolite

Question 15

What does Gibbs Free Energy describe?

Answer 15

The thermodynamic potential of a system, which is minimised at equilibrium with the lowest Free Energy indicating the most stable phase

Question 16

What does the Clapeyron equation calculate and what can it be used for?

Answer 16

The slope of a reaction line in P/T space. It is used to predict which phase transitions a material will undergo

Question 17

What 3 components are visualised on an AFM diagram? Which components are projected outwards from? Where is biotite displayed?

Answer 17

Top corner: Al2O3 (-K2O)
Left corner: FeO
Right corner: MgO
Quartz, garnet, and water are located outside the axis
Biotite is displayed in the negative zone

Question 18

What do a point, line/band, +, tie line, and triangle represent in an AFM diagram?

Answer 18

Point: fixed composition
+: mineral projection
line/band: phase with a fixed solid solution
Tie line: 2 compatible phases in equilibrium at a concentration
Triangle: 3 compatible phases

Question 19

How are discontinuous, included-phase, and continuous reactions displayed on an AFM diagram?

Answer 19

Discontinuous: 4 points and 2 tie lines create a quadrilateral, with a tie line being crossed
Included-phase: 1 phase (a point) breaks down into 3 (a triangle)
Continuous: triangles (3 components) migrate

Question 20

How are texture and fabric different?

Answer 20

Texture refers to grain scale arrangements and fabric to the spatial configuration of minerals

Question 21

How and where is foliation created?

Answer 21

New fabrics are generated through recrystallisation in response to differential stress, generating dislocations or lattice distortions in regional metamorphic rocks, lying parallel to the axis of a fold

Question 22

How do unfoliated rocks form and what texture do they have?

Answer 22

Unfoliated rocks form in shallow plutonic environments with low pressure and a high geothermal gradient, with alterations in fabric and texture appearing approaching an igneous contact. They typically have a granoblastic texture, approximately equal sized crystals, and isotropic texture

Question 23

What is the driving force behind nucleation? How does nucleation initiate?

Answer 23

Difference in Free Energy between 2 phases; the process is not often spontaneous and requires temperature overstep

Question 24

What are the 2 components of Gibbs Free Energy?

Answer 24

Body (volume) and surface (interfacial) energy

Question 25

What are the 2 competing factors in the process of nucleation?

Answer 25

Increase in surface energy due to formation of an interface and decrease in volume energy due to formation of a new phase

Question 26

What is nucleation rate and what does it depend upon?

Answer 26

The speed required to produce new grains, which depends on the degree of temperature overstep, as well as diffusion/source of transport

Question 27

What is Ostwald Ripening and what does it control?

Answer 27

Growth of grain boundaries to minimise surface energy, which controls grain coarsening

Question 28

What geometry do junctions take when formed at equilibrium stress?

Answer 28

3 grain junctions of the same size will meet around 120°

Question 29

How does recrystallisation change as grade increases?

Answer 29

It becomes more dominant, leading to fewer relict textures, increased grainsize, less evidence of strain, snd straighter grain contacts

Question 30

How are changes in pressure and temperature transmitted in forward modelling P/T paths?

Answer 30

Pressure changes are immediately transmitted and temperature changes are transmitted by conduction (tectonics) and advection (magma)

Question 31

What causes anticlockwise P-T-t paths?

Answer 31

Regional magmatic driven contact metamorphism

Question 32

What is thermobarometry?

Answer 32

The quantitative determination of temperature and pressure at which a metamorphic or igneous rock reached chemical equilibrium

Question 33

What is forward modelling?

Answer 33

The process of simulating rock evolution through various temperature and pressure changes over time