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Thread: Radiometric dating

  1. #1 Radiometric dating 
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    I am a biologist currently working in industry and I am also very much an agnostic. As such, I am constantly bombarded with so called "gotcha" questions pertaining to evolution, most of which I am able to readily answer and explain either through what I've learned and observed myself or from doing a little research but there are a few I have some trouble with as I am not a geologist (the things I study can usually study me back). So there are just a few points I would like to have clarified.

    1. Some of the radioactive particles could have been present during the formation of the rock being tested.

    2."Leaking out" of radioactive isotopes due to solubility of the isotope.

    For the second point to be true, wouldn't the "leaking" have to be selective? Otherwise the element that formed (i.e. lead from uranium) would also "leak out" right? Also, if someone could remind my how the radioactive isotopes get to be in the rock in the first place I'd appreciate it. I vaguely remember something about cosmic radiation but like I said my field of discipline is biology. Finally, I was curious as to why different radiometric methods are used instead of only, say, K-Ar testing. Obviously methods like carbon-14 dating can't be used for everything because of it's short shelf life.

    P.S. Like any good scientist I highly encourage citation and credentials (I'm not judging but one must do ones due diligence) just for my own use. Thank you.


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  3. #2  
    WYSIWYG Moderator marnixR's Avatar
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    funny enough, that's how the radio isotope dating question sticky started off as - i can only assume that the leakage issue is one of the standard creationist ploys to muddy the waters


    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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  4. #3  
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    Quote Originally Posted by JohnGlea View Post
    I am a biologist currently working in industry and I am also very much an agnostic. As such, I am constantly bombarded with so called "gotcha" questions pertaining to evolution, most of which I am able to readily answer and explain either through what I've learned and observed myself or from doing a little research but there are a few I have some trouble with as I am not a geologist (the things I study can usually study me back). So there are just a few points I would like to have clarified.

    1. Some of the radioactive particles could have been present during the formation of the rock being tested.

    2."Leaking out" of radioactive isotopes due to solubility of the isotope.

    For the second point to be true, wouldn't the "leaking" have to be selective? Otherwise the element that formed (i.e. lead from uranium) would also "leak out" right? Also, if someone could remind my how the radioactive isotopes get to be in the rock in the first place I'd appreciate it. I vaguely remember something about cosmic radiation but like I said my field of discipline is biology. Finally, I was curious as to why different radiometric methods are used instead of only, say, K-Ar testing. Obviously methods like carbon-14 dating can't be used for everything because of it's short shelf life.

    P.S. Like any good scientist I highly encourage citation and credentials (I'm not judging but one must do ones due diligence) just for my own use. Thank you.
    I'm not sure I fully understand your first point but on the 2nd, I thought one uses the ratio of the radioisotope to the stable isotope(s) of the same element - because any chemical process, such as leaching out, will affect all isotopes equally and thus cannot affect the ratio. Does this accord with what you have read?
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    You're right about the first point I made a mistake in writing it. The first point should read "1. Some daughter particles (i.e. lead) could have been present during the formation of the rock being tested."

    Just a quick side note. Notice that I readily admitted when there was a mistake with a point that I made. A notion I fear many creationists will never understand.
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  6. #5  
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    Quote Originally Posted by JohnGlea View Post
    You're right about the first point I made a mistake in writing it. The first point should read "1. Some daughter particles (i.e. lead) could have been present during the formation of the rock being tested."

    Just a quick side note. Notice that I readily admitted when there was a mistake with a point that I made. A notion I fear many creationists will never understand.
    Haha yes, how sadly true. One typo and your adversaries will be crowing for days.

    But I'm still not sure what your 1st point was about. Can you rephrase it in its entirety?
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  7. #6  
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    Sure. From my understanding the basic idea behind their argument is that there could be naturally occurring lead already present at the time that the rock was formed. As such the rock would appear artificially older (because the ratio between uranium and lead would incorrectly show more lead). I know there is a hole in this argument (perhaps lead formed from the decay of uranium would appear different than lead present at the time of rock formation? Or perhaps a different radiometric dating method involving an element that wouldn't ever be preset during the rocks formation would be used?)
    Last edited by JohnGlea; July 8th, 2013 at 02:30 AM.
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  8. #7  
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    Quote Originally Posted by JohnGlea View Post
    Sure. From my understanding the basic idea behind their argument is that there could be naturally occurring lead already present at the time that the rock was formed. As such the rock would appear artificially older (because the ratio between uranium and lead would incorrectly show more lead). I know there is a hole in this argument (perhaps lead formed from the decay of uranium would appear different than lead present at the time of rock formation? Or perhaps a different radiometric dating method involving an element that wouldn't ever be preset during the rocks formation would be used?)
    Right. I've now looked this up, here:https://en.wikipedia.org/wiki/Radiometric_dating

    and my take on it is as follows.

    First I was wrong to say the technique compares the ratio of a radioisotope to the stable isotopes of the same element. This is done in radiocarbon dating but apparently is not how geological dating is done. Mea culpa.

    OK, so, with the U/Pb method, it seems they look for a particular mineral called zircon, because this mineral has a strong affinity for uranium but none at all for lead. So one expects no lead at all to be present in crystals of this mineral, whereas significant amounts of uranium can be incorporated quite stably in the crystal lattice. Apparently U can readily take the place of Zr, presumably because ionic size and charge are very similar.

    The amount of Pb grows with time as U decays and, as there is no way to get it inside a crystal of this mineral except by radioactive decay, the amount present can all be attributed to this.

    The only chemical process that can alter the amounts of U and Pb within a crystal matrix is diffusion, i.e. the migration of atoms through the lattice. However, if the mineral is below its "closure temperature" this process is negligibly slow (not enough thermal energy to break and reform the bonds, as required to allow the atomic migration).

    This seems to be the core of it.

    Of course, the strength of the technique can then be judged by the agreement or otherwise it gives with other methods of dating rocks (.e.g from fossil types), in a wide variety of locations and conditions. If the agreement is good, which apparently it is, then one has legitimate reasons to trust it.
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