(source: Dickin, AP, 1995)
| Decay system | Decay constant | Half-life |
| Rubidium to Strontium (very widely used) | ||
| 87Rb - 87Sr | 1.42x 10-11 yr-1 | 48.8 Byr |
| Lutetium to Hafnium (not widely used.yet?) | ||
| 176Lu- 176Hf | 1.94±0.07 x 10-11 yr-1 | 35.7±1.2 Byr |
| Uranium to Lead (widely used) | ||
| 238U - 206Pb | 1.55 x 10-10 yr-1 | 4.47 Byr |
| Potassium to Argon (very widely used) | ||
| 40K - 40Ar | 0.581 x 10-10 yr-1 | 1.93 Byr |
all long enough to easily reach back over life of solar system. High resolution mass spectrometers allow decreasingly old rocks to be dated (but difficult to bring younger than 2 Myr - not enough daughters).
Short-lived systems:
Generally can be applied back over about 7 half-lives of their system
U-series
Decay of uranium produces a chain of intermediate isotopes with very short half-lives (so not important for U-Pb dating back beyond say 10 Ma). But the short-lived decay series is very useful for young material.
| 238U to 234U | half-life: 245 ka | (used for dating coral) |
| 238U to 230Th | half-life:75.4 ka | (date young volcanics, carbonates) |
Radio-carbon
14C - short half-life, parent created continuously by cosmic ray bombardment of 14N. 14C incorporated with other C isotopes (13C and 12C) into organic material. Fixed ratio to start then 14C decays back to 14N, so young material has more 14C than old.
| Basis of dating: 14C | half life = 5568±30 kyr. |