So how do you get 4.5 billion years by dating isotopes that existed long before the formation of the earth?
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Edit: I’m not creationist trolling. I believe the #, just trying to learn about the sicence.
In: 180
Uranium decays into lead. Zircon is a crystal that incorporates uranium but excludes lead. So, when a zircon crystal is formed you can be sure that the amount of lead is 0%. The only lead in a zircon crystal comes from the uranium decay.
Edit: I should also say that the dating technique uses the ratio of lead to uranium to measure time. No lead corresponds to t = 0.
Certain crystals, when they are formed will concentrate certain materials from the environment. For example, when the gemstone zircon is formed, the crystal soaks up uranium from the earth and rock around it. At the same time, when zircon crystals form they expel any lead impurities.
This makes zircon crystals quite special – they contain a lot of uranium but absolutely no lead.
After the crystal forms, the uranium atoms decay and become lead. The lead becomes trapped in the crystal. You can analyse this type of crystal to measure how much lead and uranium are in it. Because when the crystal first formed it had no lead in it, any lead in the crystal must have come from the uranium. If you know how much lead and uranium are present, then you can calculate the time since the crystal first formed.
“Rock” is the short answer. “Rock and meteorites” is the longer answer. “Rock and meteorites and radioactive decay” is the longest short answer.
U-238 decays at a predictable rate. But you’re correct in that simply knowing that doesn’t tell you much – you have to know how much you started off with. So how do we know how much U-238 we started off with?
Well, U-238 will pretty much always decay into the same elements at the same rate. So what you do is you look at a sample that contains both U-238 and its decay elements (lead is the most reliable one) and look at the ratio of uranium to lead. That ratio lets you date the sample.
The way that you can use it to date a sample and not simply the age of the supernova is that you’re checking crystalline solid samples. When U-238 is created in a supernova, it’s a gaseous/dust form. So when it decays, the decay products are free to float off into the vacuum of space. When the U-238 is locked into a solid structure (such as a rock), however, the decay products can no longer escape, and so now you can start counting back until the creation of that crystalline solid.
You get 4.5 billion years by following a long, self-referencing, self-consistent chain of similar samples, some of which are young enough that you can use other evidence (such as the fossil record, ice-core samples or C-14 dating) to verify your math. Ultimately, you check your final math by comparing the decay ratios of your oldest Earth rocks with those of meteorites (which should be relatively close to each other). And you use as many samples as you can to correct for random drifts and sample corruption.
ETA: It’s a fun side note that the hunt for the age of the Earth is also what led to the eventual banning of leaded gasoline. See if you can figure out why without looking it up!
Sidenote: It is now widely thought that heavy elements like uranium were mostly produced in neutron-star collisions, rather than in supernova explosions.
https://en.wikipedia.org/wiki/Nucleosynthesis
A big part of the answer is that we do date those supernovae. We know when they happened, then how long from that until the earth formed, and on down.
We have to standardize our “earth numbers” to that of our solar systems standard by looking at very, very ancient mediorites. So we are effectively measuring from the age of those super nova for some systems, from the age of zircon formation for some others, from when things melted or got metamorphosed etc.
But all of this depends on having standards for these numbers. How much uranium? How much thorium or lead? That’s where we compare those levels to the solar system baseline.
For many systems, it’s less about the absolute amounts of an isotope, but about the ratios. So you set “this is how much there should be in the solar system” as zero, so the age of those super nova don’t matter, just how much is left over to now.
Again, this all depends on the system and there are a lot of different chemistries going on in geochemistry.
When radioactive elements decay, they change into a different element. If the material is already formed, then they just stay there. But if chemical reactions take place, then they’ll react differently, and so move away. If you find a bunch of radium and lead in a sample that, chemically, should just be uranium, then you know that sample crystallized a really long time ago.
Dating involves measuring parent and daughter isotopes in most situations (14C is the unusual exception). If you want to date using a decay series, there are two main methods: find a mineral that does not include any daughter isotope at time of formation (so any and all daughter isotope in that sample must have formed by decay of the parent), or measure many minerals and correlate the contents of parent and daughter (making a linear relationship) to establish the parent content via y intercept (the parent content for all minerals formed at that time if they had formed without the daughter).
There are loads of other ways to play with the data to define ages, too.
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