A funny mistake by ICR and example of poor scholarship

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Originally posted by wj:
Chris, it appears that Humphreys postualtes a "reopening" of zircon crystals to helium diffusion some time after closure temperature is reached.

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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?

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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?

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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?

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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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Reopening, as Humphreys describes it, is simply when a zircon reaches equilibrium. This can happen at any temperature; though for cool zircons this typically requires much more time than the age of the universe. For hot zircons, it can happen very quickly. But there is no one temperature that can be defined in this way.Reiners' zircons, from Fish canyon, were dated at roughly 21 million years old (raw apparent age). Closure temperatures were calculated at around 150 to 190 C (probably low estimates). At those temperatures, the equilibrium age is only about 3 to 4 million years. What that means is that if Fish Canyon zircons cooled to about 190C, and then remaining at that temperature, they would tend to be approaching equilibrium about now! Of course, they were actually found at much lower temperatures than this, which is why they could be dated.For the hot Jemez zircons, equilibrium is probably reached quite quickly. Humphreys' mention of a few dozen to a few thousand years sounds about right, for zircons at 313C. The last paragraph of his reply to Meert is as follows:

If the closure interval is long compared to the age of the zircon, then the zircon would indeed be a closed system. But would that be the case in the uniformitarian view of the Jemez zircons? Using the effective radius of the zircons, 30 m, and the measured values of D (Figure 7) in eq. (22) gives us tci values between a few dozen years and a few thousand years, depending on the temperature of the sample in the borehole. Those times are very small compared to the uniformitarian age of 1.5 billion years.

Well of course the equilibrium age is much less than the actual age for a hot zircon in equilibrium. There is no conflict here with any uniformitarian assumptions, since uniformitarians do not attempt to date hot zircons. This is misleading to the point of deliberate dishonesty. 1.5Gya is sufficiently long that it is plausible for zircons even at 150C (well below literature cited closure temperatures) to reach equilibrium.It really only makes sense to speak of a closure temperature for a zircon that has cooled from hot down to a point of negligible diffusion. You can't really get a reliable date from old zircons found at the published closure temperatures. That is too hot, and allows for too much diffusion. But if zircons drop down to (for example) 70C then the amount of diffusion is sufficiently small that it would take substantially more than the age of the Universe to get to equilibrium, and so dating can be applied quite sensibly.Closure temperature has nothing to do with balancing rates; it is rather the temperature at the time given by the crystal's apparent age. The descriptions of closure temperature as being a point of negligible diffusion (Meert), or as a point of balancing diffusion with production (Humphreys), are simply wrong. It is true to say that diffusion drops off very rapidly below closure temperature (it is an exponential function, after all).The functions involved here are nice smooth differentiable functions and so there is no well defined cut off point at which diffusion is negligible. What is negligible will depend on other sources of error in your calculations. But basically, if a zircon is sufficiently cool that only 1% of produced Helium has been lost by diffusion over the life of the crystal, then that should be negligible, and trying to compensate for it in calculations will make no difference.Remember, the function is exponential, so every twenty degrees less probably gives you are order of magnitude less diffusion! 70C would be as safe as houses for zircons, using Reiners' data.It is certainly possible for a zircon to cool a little bit, and then have sufficient Helium build up inside to bring it back to equilibrium again at the cooler temperature, without ever getting anywhere near published closure temperatures. This may well be the case for some of the Jemez zircons. They could also have been heated up at some point after having reached equilibrium at one temperature, after which they will again approach equilibrium but from a point of elevated Helium concentrations! The data is not sufficient to tell.

There certainly are more variables when one operates with both an evolution and a creation model. The problem for evolutionists is to afford this expanded network of variables on credit that it may provide tang even for their own pet projects while the creationist consensus is to find a sole issue for instance able use the broader analytic requirement to not only retain more in the mind (a la Maxwell) but to inform more such that sudents seeking relief can find it and be satiatied.

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Closure temperature has nothing to do with balancing rates; it is rather the temperature at the time given by the crystal's apparent age. The descriptions of closure temperature as being a point of negligible diffusion (Meert), or as a point of balancing diffusion with production (Humphreys), are simply wrong. It is true to say that diffusion drops off very rapidly below closure temperature (it is an exponential function, after all).

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JM: Well, of course I want to argue a bit on this point. Negligible diffusion does not mean NO diffusion. Diffusion can still take place at and below the closure temperature, but it such diffusion will not significantly affect the age of the zircons so much as to make them only 1000's of years old. What Humphreys wants is for the zircons to reflect a very young age for the earth, closure temperature does not help him in this regard.CheersJoe Meert

--Welcome back Joe Meert! I was beginning to worry that we lost a valuable participant here at evcforum. How was the vacation? --[Edit] - I'm still interested in your thoughts on my discrepancy illustrated in my e-mails from 11-2 & 11-4, if possible that would be fantastic.--[Edit 2] - Also, this is pretty simple I would guess but on Pg. 135 of Geodynamics - Second Edition (2002) they multiply 2 x 10⁸ km² by 65 mW m^-2 and get 1.30 x 10¹³ W. But I get the same quantity though with a power of 10. I have the feeling I will feel stupid later but I would like the correction on this point.-------------------[This message has been edited by TrueCreation, 12-22-2002]

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Originally posted by TrueCreation:
--Welcome back Joe Meert! I was beginning to worry that we lost a valuable participant here at evcforum. How was the vacation?

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--[Edit] - I'm still interested in your thoughts on my discrepancy illustrated in my e-mails from 11-2 & 11-4, if possible that would be fantastic.

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--[Edit 2] - Also, this is pretty simple I would guess but on Pg. 135 of Geodynamics - Second Edition (2002) they multiply 2 x 10⁸ km² by 65 mW m^-2 and get 1.30 x 10¹³ W. But I get the same quantity though with a power of 10. I have the feeling I will feel stupid later but I would like the correction on this point.

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JM: Did you convert km^2 to m^2 and convert mW to W?CheersJoe Meert

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Originally posted by TrueCreation:
--[Edit 2] - Also, this is pretty simple I would guess but on Pg. 135 of Geodynamics - Second Edition (2002) they multiply 2 x 10⁸ km² by 65 mW m^-2 and get 1.30 x 10¹³ W. But I get the same quantity though with a power of 10. I have the feeling I will feel stupid later but I would like the correction on this point.

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1.3 * 10¹³ is right,You should have:2 * 10⁸ * 10⁶ * 65 * 10^-3 = 1.3 * 10¹³[This message has been edited by joz, 12-23-2002]

2 x 10¹¹ m² X 65 x 10^-3W m^-2 = 1.30 x 10¹⁰ W--If you convert both don't you get still get a 10th power? I'm not challenging the book, I'm pretty sure I'm the fault here. Do I just convert the Area variable and leave the heat flow alone, or am I doing the conversion wrong?Joz: "2 * 108 * 106 * 65 * 10-3 = 1.3 * 1013" So I do the area conversion by ^6? Wouldn't it be ^3 to convert km to m?--Thanks both for the help.------------------[This message has been edited by TrueCreation, 12-23-2002]

Its 10⁶ because its Km² IOW (10³)² = 10⁶.......

Oh ok, I didn't think that the squared segment was relevant. Thanks------------------

Yeah, usually squared functions can be ignored in science.... Weird, eh?[This message has been edited by edge, 12-30-2002]

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Originally posted by Joe Meert:

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Closure temperature has nothing to do with balancing rates; it is rather the temperature at the time given by the crystal's apparent age. The descriptions of closure temperature as being a point of negligible diffusion (Meert), or as a point of balancing diffusion with production (Humphreys), are simply wrong. It is true to say that diffusion drops off very rapidly below closure temperature (it is an exponential function, after all).

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JM: Well, of course I want to argue a bit on this point. Negligible diffusion does not mean NO diffusion. Diffusion can still take place at and below the closure temperature, but it such diffusion will not significantly affect the age of the zircons so much as to make them only 1000's of years old. What Humphreys wants is for the zircons to reflect a very young age for the earth, closure temperature does not help him in this regard.

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Cheers

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Joe Meert

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Thanks for responding, Joe.I am concerned to give accurate information. The statement that diffusion at the closure temperature is negligible is wrong. It is, in fact, an error widely repeated even in technical literature; but it is still wrong.What is correct is that diffusion at the closure temperature is still significant. This is a necessary consequence of the definition of closure temperature.Closure temperature is defined as the temperature at the time given by the apparent age of the crystal. This is only really useful for a crystal which has cooled gradually from a point of high diffusion to a point of negligible diffusion, and which has remained with negligible diffusion since then. The apparent age will indicate a time somewhere between high diffusion and neglible diffusion; it therefore requires that diffusion at that point is NOT negligible.The temperature at that time is called a closure temperature, which is rather misleading for amateur readers. In this thread, people are getting into a bit of detail, and so the correct definitions should really be used.I have not read the RATE book. It looks a bit like Humphrey's problems include some technical errors, dependance on outdated results from the 70s on diffusion rates, and dependance on some figures by Gentry in 1982 which even at the time of original publication were identified as unreliable in various ways by the original authors. The model of Humphreys is not about diffusion at closure temperature being high enough to give ages in the thousands of years.For the technically minded, we can demonstrate just how significant diffusion is at the closure temperature. It is sufficiently high that zircons can reach an equilibrium in a few million years; and of course that is way too much diffusion to allow the zircons to be dated.Consider Reiner's data (which was cited by Humphreys). Reiner's paper is on-line atTable 1 gives the first sample (FCT1) with the following figures.

• Ea (Activation energy) 37.1 kcal/mol (up steps) and 44.1 kcal/mol (down steps)
• D0/a^2 9491 s^-1 (up steps) and 43670 s^-1 (down steps)
  

People can read the paper for more discussion of what this means. The paper uses the down step measurements to infer a closure temperature of 189C. I've done the up step case for myself, just for comparison, and obtained 149.4 C. In both cases I assumed a cooling rate of 10K/Mya. In both cases, the inferred time to reach equilibrium is about 3.5 Mya. All the other zircons in the table also show that between 3 and 4 Mya is enough to reach equilibrium at the closure temperature. Assuming greater cooling rates gives faster times to equilibrium.Reiner's zircons, however, were nowhere near equilibrium, and so they were able to be dated. They dated to around 28 Mya.I suspect Joe is aware of much of this. I look forward to his updated RATE page sometime soon. But please; when putting up a web page don't talk down to people too much. Try for something which is readable by a novice, but still acceptable to someone who is reading up on more technical stuff. This is not easy, I know.Closure temperature is not defined by its diffusion rate. It is not a point of negligible diffusion. By all means show that diffusion at the closure temperature is very low, if this is relevant. But when refuting Humphreys try to engage his written argument, which is not simply about the rate of diffusion at closure temperatures.Cheers -- Chris