system
Given the following post, respond as if you're commenting on Reddit:
human
Age of Earth based on radioactive decay

The age of Earth has been calculated to be about 4.5b years, based on radioactive decay of long half life radioactive elements, such as uranium (the ratio between the uranium and the elements it decayed in found in the same ore).

This is all nice, but there is a problem: Earth is/was not capable of nuclear fusion, so that uranium had to come from a very big star, that exploded and threw its 'guts' all over the galaxy. I would assume that the uranium found in a local ore deposit (say an uranium mine) came from the same star.

So my problem is this:
Shouldn't we subtract from the age of Earth the period it took for that uranium to get to here? And we don't know from what star it came, so how can we know the age of Earth with enough precision?
gpt
No.

The uranium was indeed a product of a supernova. However, radiometric dating is carried out on individual mineral crystals. The reason for this is that they are closed systems; the atoms which were trapped in the lattice to start with should still be there however many billions of years later.  When we use radiometric dating we measure the ratio of daughter, parent and a stable isotope in the mineral, and are therefore able to work out how much of the daughter has been produced through decay, and therefore how long it has taken.

Although the daughter isotope is being produced throughout nature, the daughter and parent are different elements, and therefore behave differently chemically. This means they get partitioned from one another.  Because individual mineral crystals prevent the removal of the daughter isotope, we have a closed system and therefore a functional clock.