RADIATION DAMAGE IN ZIRCON: A POSSIBLE AGE METHOD

The relationship has been studied between crystal structure damage and total alpha particle irradiation in zircon. The total irradiation was based on estimated or measured age times alpha activity, and the damage was determined by the change of the x-ray diffraction angle from the (112) plane as measured to .01° by x-ray spectrometer. The angle was found to be 35.635° (2θ) for completely undamaged material and it approached 35.1° asymptotically as the zircon had suffered increasing dosages of alpha irradiation and become metamict. The difference between the measured angle and 35.635° was found to equal 0.535 exp [− 2.31 × 10⁻¹⁶_α] degrees, where α = total irradiation in alphas/mg. The results have been interpreted to indicate that 4.5 × 10⁸ atoms are displaced per alpha disintegration and that the annealing rate is essentially zero at earth's surface temperature. The process appears to be usable for age measurement but generally limited to zircons of less than 500 alphas/mg·/hr· activity.

It was further found that the zircon became “half-metamict” after a radioactive energy release of 685,000 cal./gm., indicating that only a fraction of a per cent of the energy can be stored in the structure as disordered atoms, the remainder being lost as heat. Observations on the high annealing rate of common rock minerals shows that the amount of energy capable of being stored in a rock by alpha damage is negligible in questions relating to petrogenesis and volcanism.