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Author | Topic: New helium retention work suggests young earth and accelerated decay | |||||||||||||||||||||||
edge Member (Idle past 1733 days) Posts: 4696 From: Colorado, USA Joined: |
Double (triple!!?)posting deleted.
[This message has been edited by edge, 11-14-2002]
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Minnemooseus Member Posts: 3945 From: Duluth, Minnesota, U.S. (West end of Lake Superior) Joined: Member Rating: 10.0 |
From message 48:
quote: My bolds. and
quote: I am presuming that these difusion rates were measured at 1 atmosphere of pressure. Now, all this discussion is much to messy and complicated for a meer Moose brain to follow, but I will ask the question: Were presure considerations made, in studying the diffusion rates?Apparently at least some of the zircon samples were taken from a considerable depth (and higher much higher than atmospheric presure). Moose
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wehappyfew Inactive Member |
Actually, moose, diffusivity experiments are done under vacuum. That's the way the experimental apparatus must operate. Presumably, what really matters is the partial pressure of helium. Even at great depths, this is still pretty low (but not zero).
Mathematically, I think it's possible to work out the diffusivity at ambient helium partial pressures other than zero, based on the experiments. There's some differential calculus involved, so don't ask me to do it. Here's a few equations to chew on:Page not found - Canadian Society of Exploration Geophysicists A change in helium partial pressure (from inside the zircon to the surrounding medium) results in a change in diffusivity that is more linear, while changing temp has an exponential effect on the diffusivity curve.
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Randy Member (Idle past 6274 days) Posts: 420 From: Cincinnati OH USA Joined: |
quote: I couldn't get the pdf file to open. I have done a little work on diffusion in some very different contexts but the same principles should apply. The driving force for diffusion is the difference in themodynamic activity, usually expressed as chemical potential of helium inside and outside the crytal. It seems to me that the chemical potential of helium outside the crystal will be essentially zero no matter what the pressure but I am not sure about the effect of pressure on thermodynamic activity in this case where behavior will be far from ideal. However, it also seems to me that the diffusion constant of helium inside the crystal may have a dependance on pressure. If helium diffuses by moving through "free volume" in the crystal and free volume is reduced when the crystal is under high pressure then the diffusion constant will presumably be reduced. One caviat when extrapolating diffusion rates using activation energies is that the extrapolation is only valid if there is no phase change in the diffusion media. If the crystal undergoes any polymorphism it could be annealed at high temperature and then you could get a different down curve than up curve in a temperature study, which I think was seen. Of course I could be wrong about all of this because I have only studied diffusion in very different systems and that was some time ago.Randy
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edge Member (Idle past 1733 days) Posts: 4696 From: Colorado, USA Joined: |
quote: Why would you think this?
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Randy Member (Idle past 6274 days) Posts: 420 From: Cincinnati OH USA Joined: |
quote: LIke I said I could be wrong. I was considering diffusion from crystal to the atmosphere. As think about it you have crystals that are in contact with other solid material that the He is diffusing into so they may have an appreciable concentration of He. Ultimately it is going somewhere and diffusion rates will be determined by the chemical potential and diffusion constant in each material it diffuses through. I have only ever worked with diffusion through membranes where things like partition coefficients also come into play and I don't know if there is anything equivalent in this situation. I think the main point I was trying to make is that diffusion constant may be pressure dependant if the crystal structure is affect by the pressures involved and it not necessarily legitmate to extrapolate over huge temperature ranges using activation energies because of the possiblity of physical changes in the crytal which could effect diffusion. I don't know if that really makes any sense with the crystals being discussed here but it seems that way to me.Randy
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edge Member (Idle past 1733 days) Posts: 4696 From: Colorado, USA Joined: |
quote: Which would change the rate of diffusion. I cannot support this, but I have heard that there is a steady flux of He through the crust.
quote: I concur absolutely. The ability or inability to model actual, natural conditions has always bothered me.
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Coragyps Member (Idle past 762 days) Posts: 5553 From: Snyder, Texas, USA Joined: |
I'm not positive exactly how this factors in, but it's well established in the oilfield industry that pressure can have a very dramatic influence on "permeability" (approximately rate of diffusion) of gases in whole rocks. Measured perms in "tight gas sands" are frequently lower by a factor of 1000 at realistic native confining pressures than at atmospheric pressure. I would think that there should be some sort of analogy between this inter-grain permeability and the diffusivity along crystal defects that would control helium flow out of zircons. I'm not too sure I have the perseverance to look for documentation, though.
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wehappyfew Inactive Member |
quote: I agree. But I suspect that the effect is small enough at the shallow depths we are dealing with to safely ignored - at least for the relatively non-rigorous evaluation I am capable of. Any such effect would result in an overestimation of diffusivity from step-heating experiments conducted under vacuum. So this works in favor of Humphreys - my analysis will err in his favor.
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TrueCreation Inactive Member |
"And yet, we see on another thread that TC has written:
"Catastrophic tectonics, flood surges, helium retention and a creationist cosmology IMO is the answer to your problems with YEC."--Must've been another TC, whose the perpetrator? ------------------
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edge Member (Idle past 1733 days) Posts: 4696 From: Colorado, USA Joined: |
quote: Could be. These strained assertions are so abundant that it is hard to keep them straight. I think it's a creationist conspiracy.
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wehappyfew Inactive Member |
Time for the next installment:
Let's look at Humphreys' equation for "closure interval" which is identical to Wolf's(1998) "equilibrium age." Tci ~= a^2/(15*D) As Humphreys defines it, this "closure interval" means the time at which "the loss rate approaches the production rate, an event we call the reopening of the zircon." This is not strictly correct, and Humphreys admits this is an "estimate" using some simplistic assumptions as a "first approximation." But the equation itself is valid, and Humphreys use of it in this way is off by a factor of only 7.5, so this is not too bad for a Creationist. If we use this equation to evaluate Humphreys' results, we find some serious discrepancies between reality and Humphreys' ideas. From
Missing Link
| Answers in Genesis
...we have this quote: "After re-opening, [the zircons] would again be an open system, again losing helium as fast as nuclear decay generated it for most of the alleged 1.5 billion years. Today the zircons would have retained less than 0.0002% of the helium, instead of the (up to) 58% observed." This means the zircons would be unable to accumulate more helium because the "closure interval" was exceeded, diffusion equalling production, after only .0002% of the 1.5 billions year's worth of helium was produced. Thus the "closure interval" in this case is .0002% times 1.5Gya = 3000 years. Remember, this is the way Humphreys uses this equation, so it is not quite right, but even when applying it correctly, the results are the same (in this case). Thus we have 3000 years ~= a^2/(15*D) Solving for D/a^2, we get a value for diffusivity of about (7*10^-13)/sec at 378K. (378K = 105degC) Drawing again from
Missing Link
| Answers in Genesis
... Humphreys says:But the —196C I mentioned in the Impact article is what I will call here the retention temperature. That is the temperature at which the Jemez zircons would have small enough diffusion rates to retain the observed large amounts of helium for 1.5 billion years. That’s essentially what I said in the article: ‘For most of that alleged time [1.5 billion years], the zircons would have to have been as cold as liquid nitrogen (196C below zero) to retain the observed amount of helium.’ That's a very straightforward way of saying the closure interval is long enough to allow the observed level of helium only at the very low temperature of 77K. If we once more apply Humphreys' "closure interval" equation to approximate the "closure interval" at 77K, Tci = 1.5Gya = 1/(15*(D/a^2)) ...we get a value for diffusivity of (1.4*10^-18)/sec at 77K. Again, even if we correct Humphreys' usage of this equation, the results are essentially the same (in this case, a little worse for Humphreys). With 2 values of diffusivity vs temperature, we can now calculate the activation energy and infinite temperature intercept on an Arrhenius plot. I get 2.521 kcal/mol for the activation energy, and (2.0*10^-11)/sec for the infinite temperature intercept. While Joe and the other geochronologists are cleaning the spittle off their keyboards and monitors, we can use these numbers to illustrate for the rest of us how ridiculous Humphreys' fairy tale has become. If we use the diffusivity constants from above to calculate the closure temperature, we get the astonishing number of 120K !!! That's minus 153 deg C!!! Yet Humphreys states that he has calcualted a closure temp of +120 to +140 degC for these zircons. So we are right back to where we started. It looks like Humphreys has made some serious mistakes. What they are, we can't tell for sure without seeing all the data that went into his work. But from the results published so far, we can easily see that Humphreys' articles are not even internally consistent. In the next installment, we will see what realistic values for diffusivity can tell us about these zircons. .. . . Refs:Wolf R.A., Farley K.A. and Kass D.M., 1998. Modeling of the temperature sensitivity of the apatite (U-Th)/He thermochronometer. Chem. Geol., 148: 105-114. << edited to correct some minor rounding errors in the math, and a misread digit in one of the ln(D/a^2) results. 2.555 kcal/mol was changed to 2.521 kcal/mol, 2.5*10^-11 was changed to 2.0*10^-11, and (8*10^-13)/sec was changed to (7*10^-13)/sec. The net result was to change the final Tc number from 119K (rounded) to 119.8K (rounded to 120K). In Celsius, it remains at -153C. >> [This message has been edited by wehappyfew, 11-18-2002]
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Tranquility Base Inactive Member |
TB steps forward.
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wehappyfew Inactive Member |
Time for another update:
I must say that I am slightly disappointed at the lack of interest or even a response from the YECs on this board and other fora where I have tried to start a discussion of Humphreys' work. Is there something I am not explaining clearly enough, or maybe skipping some crucial concepts needed to understand this material? Or do they just close their eyes to the evidence that one of the most prominent YEC "scientists" has made some serious errors? Welcome to Chris Ho-Stuart, BTW, we're glad to have you. I was going to discuss the use of realistic diffusivity constants as measured by Reiners and apply them to Humphreys' zircons. But there's not really much to say. If we use Reiners' experimentally measured values for activation energy and D0 intercept we get the expected closure temp of 190degC; we find that diffusion drops off rapidly at lower temps; and the equilibrium age at 105degC is in the range of hundreds of millions of years or more. No surprises there. But what I find more interesting is how badly Humphreys' statements and numbers conflict with the physical reality of how diffusion works. If we examine more closely this paragraph from Humphreys' response to Joe's letter... "Closure temperature is irrelevant. You misunderstood my statement because you misunderstood closure temperature. You thought that zircons below that temperature would remain a closed system. Wrong! See the preprinted section below to understand why. Even if the Jemez zircons had gone below their closure temperature, according to the uniformitarian scenario they would have re-opened within a few dozen years to a few thousand years after closure. After re-opening, they would again be an open system, again losing helium as fast as nuclear decay generated it for most of the alleged 1.5 billion years. Today the zircons would have retained less than 0.0002% of the helium, instead of the (up to) 58% observed." In the previous installment (post 72) we've already covered the problems with the 0.0002% number. It requires a diffusion rate that is completely unrealistic given the known diffusivity constants. This time I'd like to point out the sentence in bold above... "Even if the Jemez zircons had gone below their closure temperature, according to the uniformitarian scenario they would have re-opened within a few dozen years to a few thousand years after closure." This is not correct. Due to the mathematics of the way closure temperature is calculated, at a temperature just below the closure temp (about 97.5%), the diffusivity (D/a^2) is approximately (1.7*10^-16)/sec. That translates into an equilibrium age of about 1.2 million years, requiring 7.5 times that interval to actually reach the equilibrium. This relationship holds true no matter what the medium, diffusivity constants or closure temp. For example, if we take the closure temperature calculated by Humphreys (120 to 140 degC), multiply by 97.5% (after converting to Kelvin), we get 110 to 130 degC. At that temp, we calculate the diffusivity (using any combination of diffusivity constants that give the stated closure temp), and find it to be about (1.7*10^-16)/sec. The equilibrium age at that diffusivity is about 1.2 million years. Humphreys cannot have the zircons "re-opening" after a few dozen to a few thousands years when the equilibrium age (he calls it closure interval) is 1.2 million years. Humphreys' claim in the paragraph cited above is mathematically impossible. All his claims seem to be based on a combination of incorrect data (which we knew right away from the numbers he gave), and now we can add mathematical error to his problems.
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wehappyfew Inactive Member |
quote:I've laid out several specific problems with Humphreys' paper and response to Meert, including the summary and calculations you asked for. Are you prepared to comment/check yet?
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