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Partial retention zone (Partial Closure (Closure is not instantaneous…
Partial retention zone
Partial Closure
Closure is not instantaneous
Rather, it occurs over range of temps
U-Th/He dating = Partial retention zone
Below PRZ
All He = retained
U-Th/He PRZ
Apatite ≈ 40 - 70 °C
Z ≈ 2-3.5 km
Zircon ≈ 160 - 200°C
Titanite ≈ 160-220°C
U-Th/He Drill Hole Data
Various minerals all display affect of PRZ
∴ 3 zones
Retain all He
Retain some He
Lose all He
Listed with increasing Z & decreasing age
Fault zone
Fault zones may disturb age distributions i.e. offset etc
PRZ Shape = f(
U conc
Lower U conc
PRZ extends shallower (lower temps)
Higher U conc
Closure interval = deeper i.e. starts @ higher temp/z
Closure temperature
Size
Position PRZ = f(
Parent isotope conc in grain
Lower U conc --> shallower PRZ
Grain size
Smaller grain --> shallower PRZ (different curvature)
∴ Temps @ which He retained = variable