Download U-Th/He

Quaternary Dating with Thermochronology
Tim Randol
November 3, 2015
Quaternary Geology
Thermochronology
Greek root “thermôs”— warm, heat.
Termo-Man
Greek root “chrónos”– time
Geothermal Gradient
• Increasing
temperature with
depth.
• Rate changes with
regions.
Thermochronology
Useful for Dating:
• Intrusion/eruption ages (cooling of magma/lava)
• Hydrothermal activity
• Metamorphism of rocks
• Exhumation (bringing rocks to the surface)
What Is Thermochronology?
Dating the temperature-time
(cooling) history of a rock.
Radiometric dating
• U-Th, Ar-Ar, U-Pb, etc.
Closure temperatures
• The last time the crystal reached high
enough temperatures for radioactive
isotopes to enter or leave the crystal
system.
Thermochronometers
• Minerals with known closure
temperatures.
(adapted from P. Fitzgerald,
S. Baldwin, G. Gehrels, P.
Reiners, and M. Ducea)
High temperature:
Thermochronometers
• U-Pb in zircon
• Th-Pb in Monazite
Useful for old rocks
• Unlikely for reset
temperature to occur
(David Schneider) Nunavut, Canada
Low temperature:
•
Fission tracks in apatite (AFT)
•
U-Th/He in apatite (Ahe)
Thermochronometers
In Zeitler et al., 1987
• Helium interpreted as lowtemperature cooling ages of
the sample.
Most powerful for investigating interactions between topogrphy and exhumation of the crust.
Apatite Fission Track
How Does It Work?
(after D’ALESSIO, M., BLYTHE, A., BURGMANN, R., AGU 2000)
• Fission tracks develop in a
crystal over time as uranium
(238U) spontaneously decays.
• Age calculated using ratio of
# of FT’s to amount of 238U
present.
• Partial anneal Zone (PAZ)
between ~80-110C
(from University of Leicester)
U-Th/He
How Does It Work?
• Once the rock cools, 4He starts accumulating as alpha particles.
• This 4He is measured and compared with the U and/or Th within the sample.
U-Th/He
How Does It Work?
U-Th/He
Apatite from granite in western Utah
Procedures
• 4-5 grains are picked for sampling.
• A lazer is used to extract the He that is trapped in
the crystal.
• An isotopic ratio is measured for the
extracted 4He and a known amount of
3He.
• With the measured amounts of U, Th,
and He, an age is calculated.
• Interpretation of the age can be the
most complicated part.
( from apparentdip.wordpress)
Interpretaton of the Age
Exhumation Rates
McQuarrie, N., 2014
Complications of (U-Th/He)
• Calculated (U-Th)/He ages of a crystal can be strongly dependent on variations in fractional
He loss caused by crystal size variations.