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Author | Topic: A funny mistake by ICR and example of poor scholarship | |||||||||||||||||||||||
edge Member (Idle past 1736 days) Posts: 4696 From: Colorado, USA Joined: |
[QUOTE]Originally posted by Percipient:
I gave up too soon. I was closing browser windows, and when I came to the Humphreys reply I decided to give it a better go than I gave it the first time. It's not as complex as I thought. Basically he's saying that after a short period, less than thousands of years, the He concentration within the zircon would have reached equilibrium. I didn't try to check Humphreys math about the time period, but let's say he's correct. In that case the zircons are not evidence either way for either a young or old earth. So why does he claim otherwise in his impact article? --Percy[/B][/QUOTE] Correct. Since there is an He concentration gradient that forms during reopening, that means that there is always some He in the crystal! As long as He is being produced by U decay, the gradient is in equilibrium. This is not a clock.
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wehappyfew Inactive Member |
Percy,
you said:quote: That's not correct. IF Humphreys' math is correct, "excess" helium is evidence of accelerated decay in the very recent past. Your "equilibrium" assumes constant decay rates. I think what Humphreys now claims is that the helium levels are far above equilibrium - therefore decay rates were much higher a few thousand years ago. But let's check his math before throwing in the towel. The devil is in the details AND in the equations... I'm nearly hopeless in math, and it's even harder when we are not shown all the equations, but just looking at what Humphreys did give us, I see this:
quote: where a = radius of the zircons grains and D = diffusivity But "a" is a measure of length, and diffusivity has the units of 1/seconds, so if you look at the units of those terms, tci (closure interval) seems to have the units of length^2 * time I don't think we can express a time with those units. Something is missing (or added) to that equation to get to a "closure interval of dozens to thousands of years". This is where we desperately need some mathematical expertise. Maybe Joe can handle this stuff... I can't... especially with most of the equations missing or unexplained. As I nearly said in the very beginning, too bad Creationists never show (all) their work. We now have a little teaser from Humphreys, which is sufficient to raise serious questions about the units of the equations used (in my own nearly ignorant mind, at least), but we may have to wait until next summer to learn the rest. In the mean time, someone will have to pony up a little brain power, or call on Reiners again to explain diffusivity at a constant temp and rising He concentration. Notice that Humphreys' latest result clashes severely with Reiners' statement:
quote: This certainly seems to imply that a constant temp of 100-120 C will allow 90% retention for 1.5 byr. But Humphreys is basically accusing Reiners et al of ignoring Fick's Law (diffusion increases linearly with concentration). Clarification is required from Reiners on this point, IMHO. Any thoughts on the units of Humphreys' "closure interval", Joe? Am I missing something?
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Joe Meert Member (Idle past 5710 days) Posts: 913 From: Gainesville Joined: |
[QUOTE]
Any thoughts on the units of Humphreys' "closure interval", Joe? Am I missing something?[/B][/QUOTE]
JM: D/a^2 has units of 1/s (diffusivity is length^2/time) so everything is kosher with his units. The problem, as I see it, with Humphreys attempt to separate diffusion from closure temperature. Closure temperature is a function of diffusion and is defined as the temperature at which diffusion becomes negligible for the mineral. There is then some trickery in his math to come up with this tci thing. Look at it this way. Let's look, in a very simple way at how one would calculate an age in Humphreys world versus ours. Assume that retention is 100% at the closure temperature (but only for a short time). After that diffusion rate out equals production rate in (his tci concept). So, let's say that element A decays to element B with a characteristic half life of 1000 years. For simplicity, let's say that 100,000 A's were in the mineral at Tc. Decay then proceeds as follows (in the normal apostate decay world). 1st half life 50,000 parents and 50,000 daughters left and the age is 1000 years2nd half life 25,000 parents and 75,000 daughters left and the age is 2000 years 3rd half life 12,500 parents and 87,500 daughters left and the age is 3000 years etc.... Now let's do this with Humphreys definition of 'closure'1st half life (everything is 'normal') 50,000 parents and 50,000 daughters: Conventional interpretation of the age is 1000 years 2nd half life (now the crystal 'opens' so that flux of daughter in = flux of daughter out) 25,000 parents and 50,000 daughters (the 25,000 daughter products formed diffused out) 3rd half life 12,500 parents and 50,000 daughters (the 12,500 daughters formed diffused out) Our geochronologist does not know this and 'dates' the rock after two half lives and is unaware of Humphreys 1st law of diffusion. The age he/she gets is based upon the assumption that the system was closed to both parents and daughters. Therefore, the geochronologist assumes that at closure the mineral contained 75,000 parents (simply the sum of parents and daughters in the rock). The geochronologist figures that only 1 1/3 half lives have passed (this is an algebraic solution but useful to a first approximation) and therefore the rock is determined to be 1333 years old (it is actually 2000 years old) After the 3rd half life, the math is done again assuming that there were originally 62,500 parents in the rock. This time the age is 2.2 half lives and the geochronologist reports an age of 2200 years on a 3000 year old rock. Thus, the more time that the rocks obey Humphreys law, the younger the rocks appear relative to their 'true' age. Therefore, all our age determinations are too young! Unfortunately, he did not supply the relevant equations by which to check where this tci came from. I suspect trickery, but can't prove it without the paper. Cheers Joe Meert
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wehappyfew Inactive Member |
Thanks Joe, I suspected I was missing something simple like that on the units.
Can you use some real life numbers to illustrate this diffusion problem? I would like to see, in simple terms, how much helium is produced in a zircon with ~1000 ppm U, what the He concentration would be at various time intevals, and what the diffusion rate would be at those concentrations at various temps. In other words, show us mathematical dimwits how to use all those fancy equations that Reiners and Zeitler and Dodson throw around. Thanks
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Percy Member Posts: 22505 From: New Hampshire Joined: Member Rating: 5.4 |
I unfortunately can't give this the time it deserves this morning, but if He concentration within the zircon is a function of many factors including temperature history, geometry and size, diffusivity, ambient He concentration in material surrounding zircon, etc. and so forth, isn't this just another case of Creationists focusing on a process with sufficient complexity as to defy simply analysis, thereby making possible labyrinthine technical discussions where everyone gets lost at one time or another.
If the earth is young and most geology the result of a world-wide flood then the evidence would be everywhere, particularly in the easily accessible geologic layers. That the evidence isn't there is why Creationists tend to focus on obscure arguments like the diffusion of He from zircon buried at depth. At some point someone here will sufficiently understand the issues that it can be presented in a manner understandable by us all. In the meantime, I hope we conduct ourselves in the manner of objective investigation rather than as rabid evolutionists more intent on showing up Humphreys than understanding the issues. --Percy
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Minnemooseus Member Posts: 3945 From: Duluth, Minnesota, U.S. (West end of Lake Superior) Joined: Member Rating: 10.0 |
Terry has started a new topic, concerning this matter, at the Talk Origins board. It is here.
There, Terry has a link to a brand new (10/24) response from Humphreys. That is at News | The Institute for Creation Research I will post a link to this topic string, to the Talk Origins board. Moose
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Joe Meert Member (Idle past 5710 days) Posts: 913 From: Gainesville Joined: |
quote: JM: very astute observations Percy and quite accurate. Part of the problem with providing detailed criticism of these creationist stories is that they do not publish them in scientific journals. Instead, we have to try and understand what they are trying to say from things like Impact articles wirtten for the 'layperson'. I cannot provide detailed analysis of Humphreys article except to say that the tci concept appears to be a bit of smoke and mirrors to produce the desired result. I can also say that losing daughter product at the same rate it is formed would result in underestimates of the real age of the mineral in question. The whole point of trying to calculate closure temperatures is to help figure out the point at which daughter loss (via diffusion) is negligible in the mineral lattice. Humphreys is arguing (without basis, I believe) that non-negligible daughter loss occurs all the time. Unfortunately, he did not include that explanation in his reply to me. All that said, your original point stands on its own. Cheers Joe Meert
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Brad McFall Member (Idle past 5063 days) Posts: 3428 From: Ithaca,NY, USA Joined: |
Thanks for the link. As usual, ICR does not disappoint. I only wish I knew in which Choir nos### sang.
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Brad McFall Member (Idle past 5063 days) Posts: 3428 From: Ithaca,NY, USA Joined: |
The "process' that got me 'canned" from CU seems to my best guess to have been Provine's citing of "small diffusive effects" when it comes to Kimura etc and Stochasticizing that Wright in part id'd which was something Will composed when I was studying e-fish morphology in Africa so not only do I doubt this is simply a fillip on the creationists backbone but a major contribution to the minor issues that still seem to dissprotionately occupy out collective "mind". I am not Jungian either. I only have a "layman's" understanding of this work.
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edge Member (Idle past 1736 days) Posts: 4696 From: Colorado, USA Joined: |
Has anyone here figured out where Humphreys thinks we are on his [He] vs. time graph? It would seem to me that this graph (which has absolutely no data on it, by the way) cannot be used to tell any kind of age for anything. Helium concentration is clearly not a function of time after an unspecified interval. This looks like another one of Humphreys' graphs that we have puzzled over before... completely irrelevant.
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wj Inactive Member |
I have to wonder if Humphreys' extrapolation of the diffusion rate due to concentration gradient across the zircon crystal boundary is valid.
It seems to me that the decreasing rate of diffusion of radiogenic (or primordial) helium from zircon with a decrease in temperature is a consequence of thermodynamics. As the temperature of the crystal and helium decrease the fraction of helium atoms with sufficient kinetic energy to overcome the "barrier" of the zircon crystal boundary decreases. At or below the closure temperature effectively no helium atoms have enough kinetic energy to escape. Humphreys then seems to argue that if more radiogenic helium atoms are created then the increasing concentration will eventually cause the helium atoms to start leaking out again (reopening). But, is such a concept only applicable to a permeable or semipermeable barrier? If a radiogenic helium atom is created with insufficient kinetic energy to escape the crystal boundary immediately, how would that atom or another helium atom gain enough kinetic energy to escape irrespective of how many helium atoms are trapped in the zircon crystal? So, if the confined atoms do not have sufficient energy to cross a semipermeable barrier, does this mean that the bariier is now impermeable? Is Humphreys simply applying Flick's Law to the wrong situation?
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edge Member (Idle past 1736 days) Posts: 4696 From: Colorado, USA Joined: |
quote: Yes, many questions that I seriously doubt Humphreys and the RATE team have bothered to consider. I think one of your basic points is that there are so many variables to consider that this curve is absolutely meaningless, and probably contrived. And these same guys think that standard radiometric dating has too many assumptions!
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Sylas Member (Idle past 5290 days) Posts: 766 From: Newcastle, Australia Joined: |
quote: Hello, all. I am new to this forum, but am a talk.origins regular.I'm Chris Ho-Stuart; but am just using "cjhs" as my username here. I've been fielding some questions about this in talk.origins, and have found this forum to be a helpful reference. Just want to make some comments on the above. Closure temperature is not defined as a point ofnegligible diffusion. In fact, at the point where a mineral is at the closure temperature, there will still be substantial amounts of diffusion. I think it is worth understanding this. Closure temperature is not actually defined in terms of diffusionrates at all! The term can be misleading. Think of it as "The temperature at the apparent date of closure". If you have a crystal that cooled very slowly after its originalformation (10 C / Mya is a typical figure for cooling rates) then there will be a substantial loss of Helium during the cooling phase, between being effectively completely open and unable to retain any detectable amount of Helium and being completely closed with negligible loss to diffusion. If we subsequently find and date the crystal, and calculate an "age"based on the amount of Helium in the crystal, then this is called the "apparent age" of the crystal. This age will indicate a date somewhere older than the point at which diffusion became negligible (because at that point there was already some Helium that accumulated over the time of cooling) and somewhere younger than the point at which the crystal was very hot. That is, the apparent age actually gives the age to a point somewhere during the cooling phase of the crystal. The calculations and definition of "closure temperature" gives a way to describe the point at the "apparent age"; it is the temperature inferred for that point in time given by the apparent age. It is an odd definition, when you think about; but here is amotivation... if you take a collection of different crystals or minerals which have different closure temperatures, then a plot of apparent age against closure temperature gives an actual plot of the thermal history of the site being dated. (Think about it.) Where this is possible, it provides a cross check on the results,and also a confirmation of cooling rates that were assumed in the calculation of closure temperatures. (One can iterate this to a solution and actually measure the original cooling rates, rather than make plausible assumptions.) This means that the material has been dated, the thermal history of the site has been discovered, and some stringent double checks on the various assumptions have been made. Of course, in all of this we have the usual assumptions ofradiometric dating (constant decay rates, etc); and also an assumption that over the rest of the life of the crystal after the initial cooling, it was at temperatures were diffusion really is negligible -- which means well below the closure temperature. It is powerful evidence for those assumptions that "apparent age"/"closure temperature" plots tend to reveal sensible cooling curves. With respect to Humphreys "closure interval"; what he isactually calculating there seems to be the "apparent age at equilibrium", which is not the same thing at all! The formula used by Humphreys is "tci = a^2 / D / 15". This formula is derived and discussed by Wolf et al in: "Modeling of the temperature sensitivity of the Apatite (U-Th)/Hethermochronometer" by R.A. Wolf, K.A. Farley and D.M. Kass Chemical Geology 148 (1998) pp 105-114 If you have a crystal in a state of equlibrium between diffusionand production of helium, and apply a naive "apparent age" calculation based on this steady state amount of Helium, then what you get is the "equilibrium age". What Humphreys called the "closure interval" is actually the amount of time it take to get to equilbrium, which is not the same thing at all. Wolf et al discuss this as well, and indicate that Humphreys' "closure interval" is roughly one order of magnitude larger. If you think about it, this makes sense; the paper gives the mathematical justification. The above paper is on-line through ScienceDirect, but asubscription is needed to access it. Other references I have found useful are as follows: The original impact article by Humphreys Further comment by Humphreys in reply to Jor Meert The paper by Reiners, cited by Humphreys The 1982 paper by Gentry et al, describing the Jemez zircons Some lecture notes, which discuss closure temperature. Very useful short conference paper on thermochronology, by James Lee.
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Sylas Member (Idle past 5290 days) Posts: 766 From: Newcastle, Australia Joined: |
quote: His graph is a tolerably sensible representation of what would happen if a zircon cooled to a temperature where there is still some diffusion, and than eventually reached an equilibrium as Helium builds up in the crystal. This is quite plausible for hot zircons, and of course no one tries to apply the U-Th/He dating method to such hot zircons. Minor defects exist in the fine detail of shape of the curve, but to a first approximation it is not bad. The major problem is labelling of "closure temperature"; closure temperature really only makes sense for zircons that have passed through a cooling phase all the way to a point of negligible diffusion. A zircon which has only cooled to a point where there is enough diffusion to allow equilibrium to be reached within the age of the earth arguably does not have a sensible closure temperature. (See the definition of "closure temperature" I have given in another post.)
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wj Inactive Member |
Chris, it appears that Humphreys postualtes a "reopening" of zircon crystals to helium diffusion some time after closure temperature is reached.
Is it accurate to say that the term closure temperature is the temperature at which the rate of diffusion is equal to the rate of production by radioactive decay? You imply that there is a temperature at which diffusion becomes negligible, presumably at a lower temperature than the diffusion rate. What term could be used to identify this temperature? Humphreys asserts that the accumulation of radiogenic helium in the crystals would eventually cause the helium to "reopen". Is there any evidence that this occurs after crystals have cooled to the temperature at which diffusion becomes negligible? Could it be that Humphreys is using the confusion of the term "closure temperature" to model a process which happens just below the closure temperature of zircon but which does not occur when the zircon cools further to a temperature at whcih diffusion becomes negligible?
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