% 39Ar released | 7 | 15 | 20 | 25 | 35 | 70 | 100 |
40Ar*∕39Ar | 2.27 | 4.97 | 6.68 | 9.58 | 10.25 | 10.10 | 10.26 |
The analysis was done using a co-irradiated 1.062 Ga biotite age
standard yielding a 40Ar*/39Ar-ratio of 27.64. Construct the
40Ar/39Ar age spectrum and use this to comment on the thermal
history of the host rock. t1∕2(40Ar) = 1.248 Gyr.
[Answer: 115, 243, 319, 442, 470, 463 and 470 Ma]
ρs | ρi | ρd |
(×105cm-2) | (×106cm-2) | (×105cm-2) |
36.56 | 6.282 | 2.829 |
38.97 | 7.413 | 3.313 |
56.53 | 7.878 | 2.457 |
41.05 | 8.578 | 3.485 |
45.87 | 6.985 | 2.482 |
Compute the average ζ-calibration factor and use this to calculate the zircon fission track ages of the following rocks:
ρs | ρi | ρd | |
(×105cm-2) | (×106cm-2) | (×105cm-2) | |
Tardree rhyolite | 60.49 | 2.66 | 1.519 |
Bishop tuff | 6.248 | 1.299 | 0.081 |
The half-life of 238U is t1∕2 = 4.47 Gyr.
[Answer: Tardree – 57 Ma; Bishop – 643 ka]