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Applications U-Th/He dating (4He/3He method (He conc profiles within…
Applications U-Th/He dating
Cooling Histories
He age only informs formation timing if
if sample cooled v quickly (instantly)
i.e. volcanic tuff
Instead, He age usually indicates
Time-temp history
For period when sample within PRZ
i.e. neither losing/retaining all He
4He/3He method
He conc profiles within apatite crystals
= Thermal history indicator
He-3 artificially induced
(uniform distrib by radiation boombardment)
Temp ↑ progressively
He-4/He-3 measured
Temp paths down to 30°C = deduced
Helium Problem
No physical entity
like a fission track...
for long time could only get age info from technique i.e. no history
4He/3He yields cooling history
Study diffusion to get @ thermal history
Since He diffusion = temp-dependent
higher temp --> higher diffusion
Diffusion study
To overcome heterogeneity U/Th
Induce uniform distribution U-Th
Measure ratio of:
Uniformly distributed, induced He-3
Heterogeneously distributed He-4
Learn about thermal history from He-4 distribution
4He/3He method
Artificially produces He-3 in sample
via proton induced radiation bombardment
∴ uniform He-3 distribution
He-4 distribution = unknown
Heat up sample
Measure He-4 / He-4 ratio
How is gas released as ↑ temp
Infer thermal history
down to 30°C (upper km crust)
Range 4He/3He method
Temp up to 30°C
Depth up to 1 km
Observed He-4 Conc
4He/3He vs He release
F3He
= total amount He released as heated until finally degassed
Arises from competing effects
Radiogenic ingrowth
(α-loss)
Diffusive
integrated over time-temp history of sample
He-4 conc distribution signals cooling path
Volcanic grain
4He conc = uniform
except α-ejection effects
Intrusive grain
extremely rounded He conc profile
diffusion dominates
Nature of He release profile
Can indicate things about thermal history of a sample