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Long-term denudation rates (Method (U-Th)/He (Big sample apatite /…
Long-term denudation rates
(U-Th)/He
Lower temp thermochronometer than fission track
Irrespective of mineral used
Closure concept
Temps at which mineral starts to retain age info
When does it close completely?
Partial annealing/
retention zone
= temperature range over which
some info retained & some info lost
(U-Th)/He Dating
Concerned w product of the U-Th decay chain
as they decay to daughter isotope, Pb
How many steps it takes to get from
parents (U & Th) to daughters (He)?
Diffusion
He = noble gas ∴ diffuses through crystals
Apatite (U-Th)/He
Temp ~ 40-80°C
Z ~ 0.8 - 2.2 km
Zircon = higher temps
Remember
Different minerals have different temp-sensitivities
Isotopes
= elements with
same no. protons
variable number of neutrons
He
He = α-particle / product of decay chain
α decay releases product with mass 4 (He)
U-decay series
Encompasses all steps from parent to daughter
e.g. 238 U (parent) --> 208 Pb (stable daughter)
Discern no. of α-decay steps
Subtracting 2 & divided by 4 until got to 208
∴ 8 decay steps/He produced
U isotopes
-> stable daughter
238 U --> 206 Pb
8 He produced
235 U --> 207 Pb
7 He produced
232 Th --> 208 Pb
6 He produced
Can determine time to accumulate 4He
If we know
1. Decay life of parent isotope
( λ)
2. He-4 produced
Calc Helium Production
Time = solved iteratively via He production
Measure
U & Th concentrations
He concentrations
Know decay life (λ) ∴ solve iteratively to find
time
(U-Th)/He Rqmts
Must use euhedral crystals
Crystal geometry
Need to know
3D size dimensions of crystal
= important for inferring He diffusion rate through crystal
Otherwise cannot estimate He loss via diffusion
Method
(U-Th)/He
Big sample apatite / zircon
Crush up, ρ separate
n.b.
Can use single crystal to get age (if sufficient conc)
Pack mineral in foil
Measure He gas
using noble gas spectrometer
Assumptions
He-4 =
only produced by U & Th
No significant initial 4He
No loss of intermediate daughter isotopes
in decay chain
No He loss to environment (on entering PAZ)
Sm conc = so low ∴ insignif He contribution