Three isotopes of uranium occur in nature: U-238 (99.3% of all U in natural systems), U-235 (0.7%), and U-234 (0.005%).
All of them are naturally-occuring radiogenic isotopes, and (as we have seen above) begin decay chains with geologically long half-lives.
For example, metamorphic processes can "reset" radiometric clocks.
If daughter products are noble gases - for example, Ar or Rn - loss of the daughter product can occur as the gases diffuse from minerals.
For example, all carbon atoms have six protons; isotopes of carbon can have 6, 7, or 8 neutrons (Table 1).