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(b) If the decay rate increased, atoms such as uranium, thorium, and samarium that decay by emitting alpha [helium nuclei] particles would have higher alpha emission energies.The alpha particles would then travel further in the mineral and damage a larger spherical halo in the mineral lattice.
– (a) Radiometric ages for some minerals may not give the time of emplacement of the volcanic or granitic rock because the minerals were formed earlier and the emplaced magma was not hot enough to melt them.
The resulting ages are based on several assumptions: (a) the decay rate has been constant; (b) the original number of parent and daughter atoms in a sample can be estimated; and (c) the system [bone, wood, mineral, rock, etc] has not lost or gained parent or daughter atoms after the time of the event being dated, except by radioactive decay.
In many cases these assumptions seem to be valid, and when they are not it is often obvious.
For example in a 1969 article, volcanic rock from Mt.
Rangitoto in New Zealand gave potassium-argon ages of hundreds of thousands of years, but trees destroyed by the lava gave carbon-14 ages of hundreds of years.