L6 - The Thermal Structure of the Lithosphere Flashcards
What are the three dominant effects that control the T in the lithosphere? + define them (3)
Diffusion of heat - E from hot to cool by molecular collisions, lowers T gradient
Advection of heat - motion of an object that takes heat with it
Radiogenic heating - heat from radioactive isotope decay
What is the diffusion equation in 1D? (1)
What is κ? (3)
∂T/∂t = κ * ∂2T/∂x2
κ = K/ρ*C_p
K is thermal conductivity
C_p is specific heat capacity
What is the advection equation? (1)
∂T/∂t = v * ∂T/∂x
What is the Peclet number? (4)
Ratio of heat transfer by advection to that by diffusion
Pe = vL/κ
If large, advection dominates
If Pe = 1, both effects are important
What are the dominant radiogenic elements? (3)
How are they distributed? (1)
Why? (1)
U, Th, K
Concentrated in continents, negligible in oceanic crust + mantle
Incompatible elements so concentrated in liquid during the formation of continental crust
How has the relationship between depth and plate age in the oceans come about? (3)
Mid-ocean ridges are close to mantle T at the surface
Contact with cool ocean leads to gradual cooling by vertical diffusion
Rocks become more dense so plate subsides
How does T on the base of the plate control rate and geometry of oceanic plate cooling? (3)
As rocks upwell, their T controls the degree of partial melting
Melt forms the oceanic crust
Oceanic crustal thickness ~7km = mantle potential T ~1300
How does plate thickness control rate and geometry of oceanic plate cooling? (2)
Controls the length of time over which the plate cools and subsides
Oceanic lithosphere ~90km thick
Where are EQs distributed in oceanic plates? (1)
Below the T=600 isotherm
What is the thermal structure of a subducting slab? (2)
Cool lithosphere advects into the hot underlying mantle
Diffusion through slab
What can we calculate using advection and diffusion models on a subducting slab? (1)
What can be done with this result? (1)
How is this done? (2)
What does this work tell us? (1)
T distribution in the slab
Work out the force causing the cold, dense slab to sink by gravity into the hotter, less dense mantle
Combining thermal models and metamorphic petrology
Estimate mineral assemblage and density with P and T of the slab
Most negative buoyancy is from the mantle part of the slab
How are P-T estimates obtained for xenoliths? (3)
Find source (kimberlites)
Measure cation distribution between mineral phases
Use experimental calibrations to estimate P-T
Why does the geotherm increase T with depth faster in the crust than in the mantle? (2)
Effect of radiogenic in the crust
Incompatibles concentrated in the crust
How can lithospheric thickness be estimated in the continents? (2)
Use mantle xenoliths to see when T = mantle potential = 1300
Seismic tomography = mapping seismically faster regions as rocks are cool
What are the implications of radiogenic heating having a larger effect than previously thought? (2)
Moho is hotter than thought, in cratons ~600 (+/- 100)
Explains how high-grade metamorphic rocks made and crust melted to form granites in mountain building as T=600 would be too low
