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It is said that the Earth and solar system are 4.6 billion years old. Presumably this date is achieved from radioactive decay. If this is the case, since most of the radioactive elements would have, to have been in existence prior to this time, how is this date established? I'm sure I am missing the point here, somewhere!!! Is the catastrophic period that the Earth went through, up until 3.9 billion years ago, proved, or just theory, and how was the 3.9 billion date, established?

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More on age of Earth and radioactive decay: physics.stackexchange.com/q/7172/2451 and physics.stackexchange.com/q/3833/2451 –  Qmechanic Apr 5 '12 at 15:18
    
Also of interest, though over on skeptics: skeptics.stackexchange.com/questions/5537/… where I discuss some measurements of the current radionuclide content of the deep Earth. –  dmckee Apr 5 '12 at 15:49
    
@Qmechanic Thank you both for info. Very interesting, especially, physics.stackexchange.com/q/3833/2451 relating back to pre Earth's existence. Partly answers my question but not totally. Not sure what the procedure is, if it had answered my question completely. Would I need to delete my question? I was not aware of the sKeptics, site so thank you.I suspect you could have a little bit of a battle their dmckee –  Clive Ballard Apr 5 '12 at 16:12
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Do you mean, how do you know the radioactive decay dates the formation of Earth, rather than the formation of those radioactive atoms before they become part of the Earth?

You look at the isotope ratios in rocks, assume that the daughter nucleides were create in situ and work out how many half-lives ago that was. You can't know the original composition of the rock and you can't work out how much of the uranium had already decayed before the rock formed, but if you find gas elements such as Argon/Krypton that are decay products you can be fairly certain that these were all produced by decay and didn't just drift into crystals.

ps. Your second question - the great remelt. When I was a student it was a 'fact' that the earth remelted 3500GY ago due to the decay heat of the initial isotopes. BUT I'm pretty sure I have since seen reports of rocks >3500GY old so either the dates have been changed a bit, the remelt wasn't quite as great or something else. I'm not really in that field so can't answer that.

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I sometimes asked questions as I think of them. This was one of those times. I think you've answered one part of the question. I wasn't sure how you'd distinguished from the time the decay originally started, and when it started on the Earth. With regards to Argon/Krypton, could not they have been associated with Uranium elsewhere? I suspect that there is not a method of establishing the 3.9 billion years date, other than if the colliding entity, brought other Uranium with it, which could cause confusion with any Earth dating. I.E. two Uranium sources 0.7 billion years apart. –  Clive Ballard Apr 5 '12 at 16:45
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Gas daughter nuclei are useful because they stay trapped inside the crystal along with the Uranium and it's unlikely that much gas would have been around in a molten rock forming the crystal. While lead may have been present alongside the U, so you aren't sure what the initial Pb/U ration in the crystal was. –  Martin Beckett Apr 5 '12 at 17:25
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