How can scientists be confident of isotope ratios in past eras and varied locations, used for radiometric dating?

Question

Radiometric dating relies on past isotope ratio predictions being very reliable.

However, this is not necessarily so. For example, in Uranium forensics, ²³⁵U and ²³⁸U ratios are about the same for all sources, but ²³⁴U varies and can sometimes be used to identify the source region. So, how can we be so sure that the isotope ratios used for radiometric dating are as reliable as we assume. What if the ratios of ²³⁸U and ²³⁵U we observe are a signature of the earth, like U234 is a signature of geographical regions?

Also, for meteorites, scientists say that the radiometric dating shows when the rock was last melted. Well, what if it was melted more than once? Does each melting reset the isotope ratios in the rock? If a rock of 4 billion years was fully melted, what happens to the isotope ratios in the rock? In pages that explain radiometric dating, that I have read, don't even mention this scenario - or the following one.

And what about the assumption that we can predict the isotopic ratios of the solar system 4 billion years ago? how is the assumption justified?

Answer 1

"Doctor! It hurts when I do this! «bonk»" "So don't do that then!"

One way around the issue of an unknown initial distribution of isotopes is to not do that. One of the most reliable mechanisms for dating the Earth is uranium-lead dating of zircons. Uranium can take the place of zirconium in the zircon crystal because of the chemical similarity of uranium is and zirconium. Lead on the other hand is very dissimilar chemically from zirconium and uranium. The crystal formation process strongly rejects lead. At the time a zirconium crystal forms it will contain no lead. All the lead in zircons comes from the decay of uranium. Two decay chains lead to lead, with ²³⁵U eventually decaying to ²⁰⁷Pb and ²³⁸U eventually decaying to ²⁰⁶Pb. Those two decay chains provide two independent mechanisms to date the same crystal.

Another approach is to find that initial distribution of isotopes. This is the approach Claire Patterson used in his estimate for the age of the Earth. See Claire Patterson, "Age of meteorites and the earth", Geochim Cosmochim Ac 10 (1956). He used a non-radioactive iron meteorites to establish the primordial distribution of isotopes of lead. With this as a baseline, he then used the disequilibrium ratios in meteorites to find the age of the Earth.

With regard to varying ²³⁴U/²³⁸U ratios, that's a red herring used by some who don't like the results of radioactive dating in a failed attempt to throw suspicion at those dates. The 256 thousand year half life of ²³⁴U means that there is no primordial ²³⁴U on the Earth. All of the ²³⁴U we see now came from the decay of ²³⁸U, via ²³⁸U→²³⁴Th→²³⁴Pa→²³⁴U. Those decays weaken rock at the site of the decays. When the rock weathers and is exposed to water, that weakened rock around a ²³⁴U atom means that ²³⁴U is preferentially leached from the rock.