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Author | Topic: Quick radiometric dating question- misused techniques | |||||||||||||||||||||||||||
JonF Member (Idle past 198 days) Posts: 6174 Joined: |
In the particular cases of Austin and his Mt. St. Helens dacite, and Snelling and his lava from Mt. Ngauruhoe, they were absolutely sure of one thing; the samples they chose were a mixture of old and recent material, and therefore the K-Ar method would return a date that is older than the recent material is, and most likely would return a date much older than the minimum that the K-Ar method could measure. See Young-Earth Creationist 'Dating' of a Mt. St. Helens Dacite: The Failure of Austin and Swenson to Recognize Obviously Ancient Minerals. And, from Snelling himself in ANDESITE FLOWS AT MT NGAURUHOE, NEW ZEALAND, AND THE IMPLICATIONS FOR POTASSIUM-ARGON "DATING":
quote: {Emphasis addded} Whole-rock = crush the entire thing and test the result without separating anything. Xenolith (literally "foreign rock") = a piece of rock that is older than the lava flow that contains it. They may be easy or difficult to separate, but in this case they weren't separated. Smoking gun.
a.) How would you guess which radiometric method to try first? Well, if you have an agenda like Snelling or Austin does, you pick a method that can be fooled, even though it's been almost completely supplanted by Ar-Ar that's much more difficult to fool, and even though other methods are significantly more widely used. Then you pick samples that are guaranteed to fool the method. If you are really interested in finding the age of a sample with no prior information, you do it with several methods and you do them on individual minerals separated from the rocks. Then if the methods agree you almost certainly have a good date. In the real world, as you know, you essentially always do have prior information. The K-Ar method (and almost all radiometric methods) boils down to measuring a quantity of stuff. (The best methods actually measure the ratio between the quantities of two different stuffs, and then use other information about the quantity of one to calculate the quantity of the other.) When a rock is old, there's a lot of stuff to measure. When the rock is young, there's not much stuff to measure. A K-Ar lab will start by cleaning as much previous sample material and argon from the equipment as is feasible. (If you tell them you think your sample is relatively young, they will take extra care in the cleaning process. And charge more.) Then they will measure your sample and the readouts will tell them there's Y amount of argon. Every few runs they will run the equipment with nothing in the sample holder. The readouts will say there's a non-zero amount of argon, call it X, which they can calculate as being equivalent to an age of Z years. If the amount they measure in your sample, X, is pretty much equal to Y, then they will report the age as Z years or less. If X is greater than Y they will report an age older than Z with appropriate error bars. If X is less than Y they will try to figure out whats going on.
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JonF Member (Idle past 198 days) Posts: 6174 Joined: |
Expanding a little on Dr. Bertsche's explanation ...
I think of the Ar-Ar method as two methods. One, irradiating the sample, allows measuring the ratio of 39Ar to 40Ar instead of measuring the ratio of 40Ar to 40K. Measuring the ratio of two isotopes of the same element is easier and more accurate than measuring the ratio of two different elements. You could just vaporize the sample and calculate an age; but the other method, step-heating, allows compensating for various interfering phenomena as Dr. Bertsche explained. It's common to present the results of Ar-Ar analysis as step-heating diagrams. The horizontal axis is the heating step, from 0 to 100%, and the vertical axis is the age measured at that step. From Radiogenic Isotope Geology, the on-line version of a standard reference work (unfortunately the equations are unreadable), here's an great step-heating diagram for two Texas tektites in which there's no interfering phenomena and K-Ar would give the same result as Ar-Ar:
If there's excess argon, you get more argon out early on, initally showing an erroneously old age but flatttening out into a "plateau" after the excess argon is exhausted (the sstep-heating diagram is on the bottom):
If the material has been heated, leading to a loss of argon from places where it's loosely bound, you get an erroneously low age initially:
if the sample has been heated a lot, you may not get a plateau, but you can tgry to get an estimate by modeling the argon loss:
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JonF Member (Idle past 198 days) Posts: 6174 Joined: |
Well, you might ask him how old the Earth and life are if we are wrong about all decay rates decay rates by a factor of 100 …
Decay rates are known to within a few percent or better. One of the several reasons that U-Pb concordia-discordia dating is so often used is that the decay rte of uranium is known to much better than a percent. Bombs and reactors tend to attract lots of research money. The classic study on uranium is Jaffey A. H., Flynn K. F., Glendenin L. E., Bentley W. C., and Essling A. M. (1971). Precision measurement of half-lives and specific activities of 235U and 238U. Phys. Rev. C4, 1889—1906. Alas, this is not available online, but from Begemann, F., Ludwig, K. R., Lugmair, G. W., Min, K., Nyquist, L. E., Patchett, P. J., Renne, P. R. Shih, C.- Y., Villa, I. M. and Walker, R. J. (2001). Call for an improved set of decay constants for geochronological use. Geochim. Cosmochim. Acta 65, 111--21:
quote: There's been no studies since then that upset these findings. The bottom line is that, while we are improving our knowledge of decay rates all the time and some of our values may be off by as much as a few percent, there's no way that we are far enough off to make a YEC scenario conceivable, by thousands of orders of magnitude. And, indeed, the consilience of dates obtained with multiple methods using different isotopes with different decay modes is powerful evidence that our dates are correct to about the specified precision. Some useful links on consilience are Radiometeric Dating Does Work! (note that Table 2 includes the Hell Creek formation, from which the dinosaur bones with "blood cells" and "soft tissue" were extracted), Consistent Radiometric dates, Are Radioactive Dating Methods Consistent With Each Other? (one of my favorites), Are Radioactive Dates Consistent With The Deeper-Is-Older Rule?, Radiometric Dating, Radiometric Ages of Some Early Archean and Related Rocks of the North Atlantic Craton, and Radiometric Ages of Some Mare Basalts Dated by Two or More Methods.
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JonF Member (Idle past 198 days) Posts: 6174 Joined: |
Radioactive decay rates depend on some deep-down fundamental attributes of the universe, and if they changed traces would be left in a surprising number of places. The Constancy of Constants and The Constancy of Constants, Part 2 are good resources to start with the author is a well-knonw (in some circles) physicist.
Yep, what we see in stars is a good measure. The Oklo reactor is also a great one; Oklo natural nuclear reactor and Natural nuclear fission reactor Finally, one strong indicator that radioactive decay rates havfen't changed to anything near the extent required dby creationists is teh fact that all life was not wiped out by the radiation, nor was all life killed by the melting of the Earth due to the heat released. The RATE group, composed of those few YECs who appear to know how bad the problem is, alludes to this. In Helium Diffusion Rates Support Accelerated Nuclear Decay they write:
quote:{emphasis in original} And, in Helium Diffusion Age of 6,000 Years Supports Accelerated Nuclear Decay:
quote: Not yet fully solved, indeed. Needless to say no progress has been forthcoming in the five years since this paper appeared. Note that they implicitly acknowledge that there is no way that a natural process would accelerater defcay in the manner that they need; it rwequires a miracle. So it ain't science.
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JonF Member (Idle past 198 days) Posts: 6174 Joined: |
Well, I can't get at the second paper either. But measuring half-lives over a period of time much smaller than the half-life is fraught with peril. Stated uncertainties are best estimates, and are statistically based. They are not absolute boundaries. An uncertainly of ±x (2 σ ) means that their best estimate is that there is a 95% probability that the actual value lies between the quoted value + x and the quoted value - x. But there's a 5% chance it lies outside those bounds. And it's still an estimate and subject to its own errors. Again from Begemann, F., Ludwig, K. R., Lugmair, G. W., Min, K., Nyquist, L. E., Patchett, P. J., Renne, P. R. Shih, C.- Y., Villa, I. M. and Walker, R. J. (2001). Call for an improved set of decay constants for geochronological use. Geochim. Cosmochim. Acta 65, 111--21:
quote: Note especially the paragraph numbered 1. Judged from the fact that many of the counting experiments have yielded results that are not compatible with one another within the stated uncertainties, it would appear that not all the difficulties are always fully realized so that many of the given uncertainties are unrealistically small, and that many experiments are plagued by unrecognized systematic errors. But such incompatibilities are not necessarily evidence of change (and if they are, the changes increase half-life as often as they decrease half-life). If there were changes, even a percent or so, there would be other far-reaching effects that we would easily detect. (I'm not enough of a physicist to specify what they would be, but look back at Steve Carlip's posts on constants and reflect of how far-reaching go those effects are). No, they are evidence that we don't understand all the difficulties and complications involved in measuring half-lives well enough to measure them to better than a few percent or so (except for uranium). But we certainly know enough to measure them to a few percent. But the real key is not to get down in the mud and wrestle with the creo on infinitesimal details. Changes less than several orders of magnitude are insufficient to make the creationist position tenable, and changes would have to be correlated among many different and independent mechanisms (alpha decay, beta decay, and electron capture decay … and their many sub-types). Keep pounding away at consilience between wildly different radiometric methods and even with non-radiometric methods (I posted a couple of links showing the latter), and ask for a scientifically verifiable mechanism that accounts for those results. Pound on the heat and radiation and observations of stars problems acknowledged by the RATE group.
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JonF Member (Idle past 198 days) Posts: 6174 Joined: |
Thanks for the info. It clarifies things for me, but it would be tricky to think of how to phrase some of this to a YEC without them going away thinking, "They're admitting that there's a lot of uncertainty, so none of those radiometric dates are right! It's a method that can't be trusted!" I can imagine them assuming that the decay rate for potassium, for example, is in error, and so while K-Ar dates are in consilience, they're all wrong. Yes, I know all that. It's a losing proposition. That's why the key is not to go down in the mud, where the can use their lack of knowledge of what statistics, what error estimates are, and how utterly infinitesimal these differences are when compared to the changes proposed by YECs. They complain about extrapolating, then extrapolate over many orders of magnitude! You can't beat them at that game. Fundamentally they want a Biblical version of science; knowledge is graven in stone and known exactly. But I don't know anyone who's good at getting them to address the important points like consilience. If the decay rate for K is wrong, then so are all decay rates, and all by the same factor. To anyone with any knowledge of physics, that's ludicrous. mMybe this point can be gotten across to someone ignorant of physics. I don't know how to even get them to listen.
So, given the pitfalls described in the paper you quoted from, how is it that we can be confident that the decay rates of elements used in radiometric dating are known to within a few percent or less? How can we be sure that the decay rate of uranium is so precisely understood -- is it because, as you said a few posts back, there have simply been many studies because bombs are big business? How would they have avoided the problems with all the measuring techniques that you outlined? Much of the answer is in my previous quote from that paper:
quote: I'll see if I can dig up the Jaffey paper. The reason we can be sure that they are known to a few percent of less is that all of the numerous measurements without known systematic error lie within a few percent or less. Does that prove it? No. Nothing is ever proven in science. It's established far beyond reasonable doubt ... but not beyond unreasonable doubt.
Unfortunately this is a YEC who is very touchy about the subject (understandably, because it invalidates his belief system) and who is also a moderator. If I'm even allowed back onto the forum, I am going to have to take on the subject of the specific isotopes in those papers and why the new measurements are outside of the margins of error, while making sure I don't say so much about uncertainties that the creationist feels free to believe that no radiometric dating method is accurate. I can do the former thanks to you, but the latter could be challenging. Well,that's a situation in which you are near-certain to lose. Facts don't help much there. And I don't know how to teach somebody something that they refuse to learn.
Could you tell me where I can find those posts by Steve Carlip that you mentioned? Thanks. The Constancy of ConstantsThe Constancy of Constants, Part 2 Steve Carlip
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