Petrology Flashcards
(55 cards)
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)
