Metamorphic Petrology Flashcards
(33 cards)
What is heat and how is it measured?
Heat is a form of energy expressed by molecular and atomic vibrations; heat intensity is measured through temperature
Which type of crust has a higher geothermal gradient and why?
Continental crust, due to radioactive decay
Where are high and low T/P geotherms most commonly located?
High geotherm: areas of plutonic activity and rifting
Low geotherm: subduction zones
How does increased temperature affect metamorphic texture?
Temeprature rise promotes recrystallisation and crystal growth
What is pressure and how does it differ from stress?
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
What are the 3 types of stress?
Tensile stress occurs from stretching
Shear stress occurs from perpendicular outwards forces working in opposite directions
Compressive stress occurs from compaction
What is strain and what are its 2 main forms?
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
What are metamorphic facies?
Groups displaying the progression along a gradient through the formation of new characterising minerals and changing assemblages
What is the isobaric phase rule for metamorphism?
The number of phases in a system + degrees of freedom = the number of chemical components required to describe the system + 2
When do violations of the phase rule occur?
When a system does not reach equilibrium
What are discontinuous reactions? What are the 3 dominant types of discontinuous reaction?
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
What are continuous reactions?
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
What are the index minerals for Barrovian Zones?
Chlorite, Muscovite, Biotite, Garnet Staurolite, Kyanite, Sillimanite
What are the index minerals for Buchan Zones?
Andalusite, Sillimanite, Kyanite, Staurolite
What does Gibbs Free Energy describe?
The thermodynamic potential of a system, which is minimised at equilibrium with the lowest Free Energy indicating the most stable phase
What does the Clapeyron equation calculate and what can it be used for?
The slope of a reaction line in P/T space. It is used to predict which phase transitions a material will undergo
What 3 components are visualised on an AFM diagram? Which components are projected outwards from? Where is biotite displayed?
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
What do a point, line/band, +, tie line, and triangle represent in an AFM diagram?
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
How are discontinuous, included-phase, and continuous reactions displayed on an AFM diagram?
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
How are texture and fabric different?
Texture refers to grain scale arrangements and fabric to the spatial configuration of minerals
How and where is foliation created?
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
How do unfoliated rocks form and what texture do they have?
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
What is the driving force behind nucleation? How does nucleation initiate?
Difference in Free Energy between 2 phases; the process is not often spontaneous and requires temperature overstep
What are the 2 components of Gibbs Free Energy?
Body (volume) and surface (interfacial) energy
