Petrology Flashcards
What is the AFM diagram used for
to show the relative proportions of magnesium, iron, and alkaline oxides
What are the 2 trends on the AFM diagram
Tholeiitic
Calc-alkaline
Describe the path of the tholeiitic trend and where it is seen on Earth
initially moves upwards towards Fe as Mg end member crystallised first, then moves down to alkali
Seen at rift and plume volcanism
Describe the Calc-alkaline trend and where it is seen in magmas on earth
follows a straight path of increasing alkali
seen at subduction zone volcanism, depleted in Fe
Why are subduction zone magmas depleted in Fe
oxidised conditions due to melting involving water, magnetite readily crystallises out of the melt
What does the TAS diagram look like and what is it used for
graph of alkali content (TA) again silica (S), with
rift volcanism,
ocean island volcanism,
subduction volcanism
in order of decreasing gradient
Used to classify rocks based on major element chemistry
Where are flood basalts formed and what is special about their chemistry
A combination of rifting and hotspot volcanism causes extensive eutectic melting, resulting in high proportion of quartz
What is the granite problem
it is very difficult to produce large volumes of granitic magma from fractional crystallisation of basaltic melts
What is the solution to the granite problem
Add crustal material to basaltic melt
- plume moves up through continental, or at subduction zone
- rises though C.C. causing melting of silica rich material
- incorporated into melt
- becomes more viscous therefore gets stuck
= not flood rhyolites
another = P+T from mountain building melts crustal material
What are the 4 types of granite
S-type
I-type
A-type
M-type
How are S-type granites formed
Melting of sedimentary rocks in cores of mountain belts
clockwise P-T curve (pressure first)
How are I-type granites formed
melting igneous material
- subduction zones -> basaltic melt -> incorporate as move up
How are A-type granites formed
form in plume melting settings
anorogenic, anhydrous
How are M-type granites formed
extensive fractional crystallisation of basaltic melts - usually v.small volumes
why do granite plutons rise to the surface as regularly spaced heads and what does this tell us
due to high viscosity and low density. Distance between plutons depends on density contrast between granite and crust, so can be used to calculate viscosity
what are the 2 categories of metamorphism when P-T increases and decreases
Prograde metamorphism (increase)
Retrograde metamorphism (decrease)
Why can we see high P-T assemblages at the surface
retrograde metamorphism restricted due to
- chem reactions faster at high T
- fluids used up making hydrous minerals in prograde
- fluids unable to catalise reactions
What are the 4 types of metamorphism
Thermal
Dynamic
Dynamothermal
Chemical
Name 5 metamorphic environments
Contact (intrusion)
Regional (orogenesis)
Fault
Chemical (hydrothermal)
Shock (impact events)
Equation and conditions for contact metamorphism
- at shallow depths, large temperature gradient between magma and crust
Q = -k . dT/dz
(k=thermal conductivity)
What is Fourier’s Law of heat conduction
time for heat to dissipate:
dT/dt = k . ∂2T/∂x2
(k = thermal diffusivity)
Equation for thermal diffusivity
thermal diffusivity (kappa) = conductivity / (density . shc)
equation for characteristic time it takes for an intrusion to cool
t = L^2 / π^2 . kappa
(kappa = diffusivity)
what does the size of a contact aureole depend on
- size
- temperature
- country rock wet or dry
How does the wetness of the country rock impact a contact aureole
Impacts the method by which heat is lost and therefore the rate
wet = convection = more efficient = smaller aureole
dry = conduction = less efficient = wider aureole
What are the conditions in regional metamorphism
orogenic events have P and T increase due to burial and radioactive decay
equation for pressure a rock experiences by overlying rock column
P = rho . g . h
estimates for crustal density and thickness
felsic: 2700, 35km
ultramafic: 3300, 7km
how much does pressure increase with 10km crustal depth
0.3 GPa
Regional metamorphism estimates for pressure in subduction zones and orogenesis
subduction: P>3 GPa
base of mountains: P=2 GPa
Describe the process of orogenesis metamorphism
- isotherms flat
- isotherms pushed down as pressure increases
- burial rate decreases, temp catches up, isotherms flatten (radioactive decay)
- erosion of mountains = isotherms up
- reaches equilibrium, thicker crust
(clockwise PTt curve)
what are isograds and how are they identified
boundary between regions of different metamorphic grade, identified by new metamorphic phase appearing
examples of index minerals used to identify P-T conditions in metamorphism
- chlorite
- biotite
- garnet
- andalusite
- sillimanite
what do the different PTt curves show
clockwise = rocks exhumed during metamorphic event
(high P then T)
(orogenesis)
anticlockwise = very deep and exhumed after event
(high T then P)
(intrusion)
Example of how mineral inclusions can be used to identify PTt curves
decreasing P and increasing T = kyanite -> sillimanite
kyanite preserved in inclusions
Example of reaction textures revealing PTt paths
Andalusite being replaced by sillimanite = increasing T conditions
How is element zoning used to understand PTt paths
zoning = low T, or short time at high T
eg Mn rich core, Mn poor rim in Garnet
what is metasomatism (chemical metamorphism)
change in mineral assemblage by percolating fluid
olivine -> serpentinite at subduction zone
what is the reaction of olivine and water in subduction zones
olivine + water -> serpentine + H + methane + heat + magnetite
serpentine: percolates structure
magnetite: used in palaeomag
what setting other than subduction zones does metasomatism occur
at slow spreading ridges, extension -> faulting -> enhance hydration
serpentine = very smooth and makes rock tectonically weak
for mudstones, what is the sequence of 5 rocks with increasing grade
- slate
- phyllite
- schist
- gneiss
- migmatite
identifying characteristic of slate
slatey cleavage in defined orientation
thin layered cleavage, flakes away
identifying characteristics of phyllite
shiny lusture due to presence of micas
identifying characteristics of schist
growth of porphyroblasts
identifying characteristics of gneiss
segregation of mafic and felsic components into bands
identifying characteristics of migmatite
partial melting of segregated bands
what are metamorphic facies
refer to specific regions of P-T space
defined by concept that rocks of similar composition metamorphosed under same P-T conditions should generate assemblage of similar minerals in similar proportions
what is a metamorphic protolith
refers to the original unmetamorphosed rock
how does Gibbs phase rule relate to mineral assemblages in metamorphic rocks
varying P+T means f=2
therefore c = p
high number of chemical components = large mineral assemblage formed (eg mafic) (opposite eg calcareous)
What does each new zone in Barrow’s Zones represent
mineral with defining higher P-T conditions, therefore greater crustal burial depth
What texture is seen in most metamorphic rocks (regional not contact)
foliation
- cleavage
- schistosity
- gneissosity
(with increasing grade)
what metamorphic textures are seen in contact metamorphism
boundaries between minerals at 120º to minimise surface tension
what is crenulation cleavage and what does it indicate
when original foliation is deformed by later compression at an angle to original foliation. Indicates multiple stages of deformation
what do textures in porphyroblasts show about timing of deformation
comparing texture or porphyroblast and matrix:
same texture = p. growth post deformation
different texture = p. growth pre or during deformation
