(June 8, 2025 at 9:45 pm)SubtleVirtue Wrote:(June 7, 2025 at 11:52 pm)Paleophyte Wrote: Yes. I've obtained radiometric ages from rocks as old as 2.9 billion years old. I could easily get ages from older rocks but haven't worked with any yet.
The underlying principle is the same as for 14-C dating, you simply use a different radiometric system. No radiometric system can be used much beyond 10 half-lives of the radioactive isotope. There won't be enough of it left to date. Your rock keeps aging, but because there's no 14-C left, the clock stops "ticking". Instead, we use isotopes with much longer half-lives. 14-C has a half-life of just 5730 years, but 235-U has a half-life of 703.8 million years. 238-U, 40-K, and 87-Rb are even longer-lived.
The big difference is that living organisms don't typically contain any Uranium, and precious little potassium or rubidium. Occasionally, fossils are formed in ways that introduce U, K, or Rb, but that's rare and unreliable. Instead, we date volcanic rocks laid down in the same strata. By logical necessity, a rock can't be older than material that it includes and can't be younger than material that cuts it. So all you need to do is find some volcanic material immediately above and below your fossil that you can date. There you go, age bracketed to better than the error bars on the dating technique.
Zircon U-Pb ages are the gold standard because zircon is nigh on indestructible. When zircon forms, the chemistry that produces it excludes any "common lead" that may be lurking but incorporates abundant uranium, so you don't typically need to correct for common lead. You also get two independent clocks ticking away: 235-U decaying to 207-Pb and 238-U decaying to 206-Pb. You also typically get 232-Th decaying to 208-Pb for independent verification. You can get ages from a single zircon crystal that's too small to be seen with the naked eye by a variety of techniques, including wet chemical techniques and TIMS, in situ laser techniques coupled to ICP-MS, or in situ secondary ion sputtering.
how do you know it was that old; can you give a solid deduction axiom for axiom to deduce your claim?
Yes, but the TLDR is that we know the rate at which radioactive isotopes decay and what they decay into. That rate is constant. Measure the rate and measure the amount of decay within a sample, do some pretty rudimentary math, and you get the decay time. There are a few assumptions that you'll need to check for a robust result, but they probably aren't what you think.
