(June 9, 2025 at 6:35 pm)SubtleVirtue Wrote:(June 9, 2025 at 6:26 pm)Paleophyte Wrote: Stick a lump of it in a very well-shielded lab with a radiation detector and watch it tick. Amusingly, the shielding isn't to keep the radiation in, it's to keep it out. You want to measure relatively small amounts of radiation without getting confounding readings from every stray cosmic ray, weapons test, and RBMK, so you frequently see these facilities built deep underground. You'll see some creationist sites claiming that all geological isotopes are calibrated against one another, and that truly would be circular, but that simply isn't how it's done. If you want to know the decay rates for Uranium you ask the boys at Oak Ridge.
There are a few others like tritium, 10-Be, and 36-Cl that are also cosmogenic, similar to 14-C. Most of the rest are primordial, they were formed by the dying stars that fertilized the nebula that became our solar system. "We are all made of recycled nuclear waste" is so much less poetic than being made of stardust. The funny thing is you only find very long-lived isotopic systems, which tells you a lot about the age of the Earth right there.
if most of the tests are based on 'stardust' isotopes... how does that date the rocks in an eruption?
The radiometric "clocks" in a mineral don't start "ticking" until the mineral forms. And they don't actually start "ticking" until they cool below something called the "closure temperature", which can be a problem if the rock gets heated up again and ceases being "closed". Put simply, we're measuring two important quantities when we date any mineral: abundance of parent isotope and abundance of the daughter isotope. So how much 235-U and how much 207-Pb? Above the closure temperature the system behaves as if it were "open" and the daughter product diffuses out of the mineral, resetting the age to zero. So yes, the primordial isotopes do decay before the mineral ever forms, but until the mineral starts accumulating the daughter product the radiometric clock doesn't start ticking.
OK, that isn't quite true, you can use the decay of radiometric systems in an open system along with some interesting assumptions to produce something called a "model age", but that's a whole other can of worms. No point in going down the rabbit hole of Sm-Nd systematics and TDM just yet.
