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

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)

as they decay to daughter isotope, Pb

How many steps it takes to get from
parents (U & Th) to daughters (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

  1. Big sample apatite / zircon
  1. Crush up, ρ separate

n.b. Can use single crystal to get age (if sufficient conc)

  1. Pack mineral in foil
  1. Measure He gas using noble gas spectrometer

Assumptions

  1. He-4 = only produced by U & Th
  1. No significant initial 4He
  1. No loss of intermediate daughter isotopes in decay chain
  1. No He loss to environment (on entering PAZ)
  1. Sm conc = so low ∴ insignif He contribution