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Low Temperature Thermochronology (Applications (Dating topography, Link…
Low Temperature Thermochronology
Applications
Dating topography
Link process & form
Exhumation/denudation rates
- how quickly did things happen ?
Erosion rates
- detrital/cosmogenic record
Thermochronometer
= specific range of geochronometers sensitive to temperature
Temp of interest
= f (process being studied)
i.e. igneous intrusive --> high
Low Temp Thermochron Range
Broadly operate at < 400°C
Most sensitive @ 250°C or less
Examples
40Ar/39Ar Dating
must have potassium
Fission track dating
around for longer ∴ range of uses
zircon or apatite crystals
(U-Th)/He Dating
Titanite / Zircon / Apatite crystals
U-Th/He Dating
He = so sensitive to temp
∴ tells about
particle pathway
not formation ages
since He keeps reacting up until shallow depth
Requires U & Th (in minerals apatite or zircon)
Apatite & Zircon
Have lotsa fission tracks & U/Th
∴ Suited to both FT and U-Th/He dating
Integrating both techniques gives insight into
all shallow levels of the crust
Ar/Ar Dating
Requires potassium (parent)
which decays to argon (daughter)
(Need unstable elements for decay to daughter)
Uses low temp thermochron
Study processes affecting rock
as it travels through shallow depths
∴ Not age of geological formation/event
unless material cooled
extremely
quickly
Geothermal Gradient
great uncertainty
but fundamental to data interpretation
lower
thermochronometers will tell you about
deeper
higher
" shallower
Technique Outline
What to measure?
U-rich, K-rich minerals
How to measure?
see this lecture
What affects interpretation?
Reconstruct thermal histories/rock trajectories
not (necessarily) geological events
Convert temp sensitivity to Z
Use geothermal gradient
e.g. assuming 25°C/km
Apatite FT sensitive to 60 - 120°C ∴ Z ~ 2.4 - 4 km
Zircon FT = 6 - 8 km