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Helium (Diffusion rate = f ( (Temperature, Nature of lattice, Element…
Helium
He loss results from:
Damage to the crystal
Fission tracks joined together can form conduits for He loss
Very old sample
He will diffuse out of sample to some extent
Diffusion
= important
Need to know
how rapidly mineral will lose He naturally
Diffusion
= process by which element spreads out naturally through crystal
such that end up with homogeneous conc. distribution
i.e. moves toward homogeneous equilibrium profile wrt He conc in crystal
Strongly temperature dependent
Diffusion rate
= f (
Temperature
Nature of lattice
Element & conc distribution
How rapidly can element move through crystal?
Size matters
Crystal size influences diffusion rate
Too small = unsuitable
Most He lost as soon as produced
∴ cannot use for U-Th/He dating
He produced has sufficient energy to travel 20 μm
Bigger = better
More distance to travel ∴ slower diffusion
∴ will probable retain more daughter products
Min crystal size required
100 - 150 μm ∴ bigger = better
Durango apatite
Experimental data for standard apatite
Shows diffusion =
Thermally activated
Rate-dependent
Temp = important in dictating how fast He travels through apatite crystal
Geospeedometers
Use distribution of element in crystal
to infer rates of geological processes
i.e. 'diffusion-dependent timescales'
Most crystal
= zoned
i.e. Do not have equilibrium conc profile of elements
Must take into account when using U-Th/He for dating
Why size matters?
Smaller crystal
--> faster element can diffuse through ∴ be lost
U/Th distribution
dictates whether He will be lost/retained
Greater He loss potential
When decay occurs closer to margin
Why?
He produced has sufficient energy to travel ~20 μm
At places > 20 μm from margin
Greater confidence U-Th/He conc measurements
--> give accurate representation of age
Correcting for He loss
α-correction
Predict how much He might escape
If we
know crystal size/geometry
assume even U/Th distribution
∴ predict loss of x proportion of He
when calculating ages
Helium retention
At margin
~50% chance of He retention (0-20 μm from margin)
20 μm
from margin
All He = retained
with limits of how far He can move
