Dating group team
Having made these predictions, the next step is to compare them with data from studies on zircons that are found at ambient temperatures and are not likely to have been at elevated temperature during the last 6000 years (RATE’s estimate of the age of the earth).
There are several studies in the literature that report such data.
presents diffusion rates for helium in the Fenton Hill zircons over a range of temperatures. The diffusion rates are plotted on semilog axes against the reciprocal of the temperature in degrees Kelvin.
The result is a line with a negative slope, which is typical of the Arrhenius Law behavior of a thermally activated process.
To answer this question, I will make some predictions based on RATE’s data and conclusions and then compare them with field observations.
The ratios of uranium and thorium to the corresponding daughter product lead isotopes can be used to date the time of formation of the crystal, based on the known half-lives of the original uranium and thorium isotopes.
In theory, the helium contents can also be used for dating the crystals, but generally are not because at elevated temperatures the helium will rapidly diffuse out. Kenneth Farley of the California Institute of Technology and Dr.
That radius is typical of the zircons studied by the RATE group and others and the diffusion rate is extrapolated from their data.
The figure shows that the helium concentration needs nearly 50,000 years to drop to approximately 0.1 times the original level, while nearly 100,000 years are needed for the residual level to reach 0.02 times the original.