fter seven months of silence, an anti-creationist geochemist named Kevin Henke has recently replied to my rebuttal of his March 2005 rejection of scientific evidence that the world is only about 6,000 years old, the helium-leak age of zircons (radioactive crystals in granites). Most of his latest reply simply rehashes the issues to which I already replied, adding more words but less substance. However, he did bring up one new issue that is worth while to examine in detail — the possible effect of pressure (including zero pressure, a vacuum) on rates of helium diffusion (leakage) from the zircons while they were deep underground. It turns out that his case collapses under scrutiny. Here is his summary:
“Because Dr. Humphreys’ helium diffusion data were obtained under a vacuum rather than under pressures that realistically model the subsurface conditions at Fenton Hill, the actual helium diffusion results at Fenton Hill are probably orders of magnitude lower [than shown in his graphs].”
Henke’s “obtained under a vacuum” should not be a surprising revelation to anyone. First, I have mentioned it in most of my publications on this topic. More important, vacuum measurements are standard procedure for all zircon diffusion researchers. None of them have considered pressure an important enough variable to include in their experiments. Henke does not seem to have asked himself why the experts have done so. The answer, as I will show below, is that these pressures aren't important for zircons.
Diffusion Under Pressure
Back in 1996, when I first began to think about the helium-in-zircon data we had then, I considered Henke’s scenario: the possibility that the pressures deep underground might account for the extraordinary amounts of helium retained in the zircons. However, I gave up on that idea when I found that for hard materials, pressure has very little effect on diffusion rates. Hardness relates to incompressibility, which hinders pressure from diminishing the space between atoms and thereby slowing diffusion.
The pressure at 3 kilometers, the mid-range depth of our samples, is about 1 kilobar (about 1000 times atmospheric pressure). I found a paper showing that even in a relatively soft material like lead, one kilobar of pressure would reduce self-diffusion (lead atoms moving through lead) by less than 20%. On the geologists' Mohs scale, lead has a hardness of only 1.5. If you put it in a vise, it is not difficult to compress.
Large zircon crystal.
Zircon, on the other hand, is among the hardest of minerals, 7.5 on the Mohs scale. That is harder than the best steel (6.5), and even harder than quartz (7.0). That's why crushing granite to extract zircons is not a worry to researchers. If you put a ball bearing of the finest steel into a large vise and squeeze it as tightly as you can (producing kilobar pressures), the ball bearing will suffer little damage and little compression.
Zircon, being harder than steel, would be much less compressible than lead. So pressure should affect diffusion rates much less than in lead, which for kilobar pressures had a reduction of only 20% in the rates, according to the paper above. In 1996, those considerations made me think that the pressure effect on hard minerals is negligible. Below are even more reasons to think so.
As far as I know, nobody has measured the effect of pressure on helium diffusion in zircon. However I have at hand a paper that gives, among other data, the pressure effect on argon diffusion in glasses, such as rhyolite obsidian. At the highest temperature to which our helium-in-zircon experiment went, 500 degrees C, the pressure effect on the glasses was almost imperceptible, a few percent per kilobar. A few hundred degrees higher than our experiment, 600 to 700°C, the pressure effect was up to only a few dozen percent per kilobar.
Several factors combine to say that the pressure effect on helium diffusion in our zircon experiments was much less than the above few percent per kilobar:
- The cooler the mineral, the less the effect, and the critical part of our data was much cooler than the above, only 100 to 300 °C.
- Glasses should be more compressible than crystals of the same composition; glasses are generally not as hard because of weaker chemical bonds between parts. So our crystals of very hard zircon should suffer less from pressure than glasses that are softer than quartz.
- In a given mineral, helium diffusion is less affected by pressure than argon, because a helium atom is smaller than an argon atom. The smaller the atom, the less the effect on its diffusion for a given amount of pressure-induced reduction of the space between atoms.
All these factors strongly suggest that the diffusion rates in our zircons were influenced far less than one percent by removing them from underground pressures to a vacuum chamber.
Bait and Switch
Let’s see how big Henke alleges the effect would be:
“Numerous researchers have shown that the diffusion of helium or argon in silicate minerals may vary by many orders of magnitude at a given temperature depending on whether the studies were conducted in a vacuum or under pressure. For example, argon diffusion in phlogopite mica may be at least 3 to 6 orders of magnitude higher in a vacuum than under pressurized conditions (McDougall and Harrison, 1999, p. 154.)”
Henke’s “at least 3 to 6 orders of magnitude” would be a factor ranging from 1,000 to 1,000,000. That is enormously larger than the few percent effect the measurements on hard minerals I reported above. What would make such a huge difference?
One factor is the mineral, “phlogopite mica”. Micas are soft minerals. Their true hardness is low (2-3), but they appear even softer than that. The reason is that they consist of atom-thick sheets of silicates held together by very weak chemical bonds between the sheets. The gap between a pair of sheets is relatively big, several atom diameters wide. Most the helium or argon diffuses along the gaps between the sheets. The weak bonding between the sheets allows pressure to compress the gaps easily. So diffusion in micas is much more susceptible to pressure than hard minerals. Instead of a steel ball bearing, here we have a sponge!
A second factor is water. Water molecules can work their way into the gaps between the sheets and (lightly) bond chemically to them, thus hindering the diffusion of helium or argon. The book that Henke quotes (p. 154, Figure 5-13) compares two experiments on phlogopite mica, one in a vacuum with no water present, the other under high pressure with water in the mica.
So pressure was not the only variable, but wetness also. Try blowing air through a dry sponge and then through a wet sponge! The large difference between the two experiments is probably more due to the presence or absence of water than it was to pressure. Our samples, by the way, came from hot dry rock. Any water that may have been in the rock unit previously has probably been mostly cooked out of it.
The adjacent figure in the same book (p. 154, Figure 5-14) reviews an experiment on a similar mica, biotite, without having water as a variable. In that experiment the effect of a rather large change of pressure, 14 kilobars, was only two orders of magnitude. For a change of only 1 kilobar pressure, the change in diffusivity would probably be about one order of magnitude. This is far less than Henke's desired six orders of magnitude.
That one-order-of-magnitude number is useful to me because it suggests that in my analysis of the helium data, I was correct to use a diffusivity for the biotite surrounding the zircons about one order of magnitude less than the vacuum measurements. Our results are not very sensitive to the value of the biotite diffusivity, but it is comforting to know that my assumption was closer to reality than I thought.
The only other experiment on pressure that Henke reports is very similar:
“Argon diffusion in glauconite at 1,000 to 10,000 psi of water vapor is up to three orders of magnitude slower than under a vacuum (Dalrymple and Lanphere, 1969, p. 155).”
Glauconite is another soft mica, and again the experiments compare dry and low-pressure samples with wet and high-pressure samples.
Last, notice that the experiments were with argon, not helium. As I mentioned above, helium diffusion is less susceptible to pressure effects on the crystal than argon, because helium atoms are significantly smaller than argon atoms.
The upshot is that here Henke is playing the ancient merchant’s trick of “bait and switch”. Having lured the customer in with an implied promise about one item (helium, zircon, dry), he then tries to sell the customer an item (argon, mica, wet) which will cost him more and benefit him less. I hope you won't buy Henke's merchandise!
A Test of Henke’s Sincerity
If Henke wishes to make a real contribution to science, instead of to polemics, he could commission an experiment to determine the effect of pressure on the diffusion rates of helium in zircon. He could contact Ken Farley, a world expert on measuring helium diffusion in minerals (including zircons) at Cal Tech, and pay him to do such an experiment. Farley might even do it for free, since I know from personal experience that he has been searching for an alternative interpretation to our helium data for years. It is a bit suggestive that Farley, an expert, hasn’t resorted to the rather obvious “pressure” argument used by Henke, a non-expert. But if experiments were to yield a significant pressure effect in zircons, Farley would be overjoyed to publish the results in the prestigious journal Earth and Planetary Science Letters, of which he is editor-in-chief.
This is Henke’s chance for glory! He could become famous and respected in real scientific circles, instead of merely preaching to his fellow skeptics in a dark corner of the Internet. If he doesn’t seize this opportunity, you may suspect it is because he knows “pressure” is not a significant escape from his dilemma with helium.
 Henke, K. R., Young-earth creationist helium diffusion "dates", posted November 24, 2005 at
http://www.talkorigins.org/faqs/helium/zircons.html See November 24, 2005 copy archived here. It appears to be an extensive revision of Henke’s first article.
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 Humphreys, D. R. 2005. Helium evidence for a young world remains crystal-clear, posted April 27, 2005 at http://www.trueorigin.org/helium01.asp. [RETURN TO TEXT]
 Henke, K. R., Young-earth creationist helium diffusion "dates", posted March 17, 2005 at
http://www.talkorigins.org/faqs/helium/zircons.html See March 17, 2005 copy archived here.
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 Hudson, J. B. and Hoffman. 1961. The effect of hydrostatic pressure on self-diffusion in lead. Transactions of the Metallurgical Society of AIME 221:761-768, August. [RETURN TO TEXT]
 Dr. John Baumgarder, an ICR geophysicist on the RATE steering committee, estimates (from experimental “bulk modulus” data on minerals similar to zircon) that 1 kilobar of pressure would compress zircon by only about 0.1%. That means the pressure effect on the diffusion rates of helium in zircon would be less than a few tenths of one percent. [RETURN TO TEXT]
 Carroll, M. R. 1991. Diffusion of Ar in rhyolite, orthoclase, and albite composition glasses. Earth and Planetary Science Letters 103:156-168. See Table 2 on page 160 and compare changes in D for different pressures at a constant temperature. [RETURN TO TEXT]
 McDougall, I. and Harrison, T. M. 1999. Geochronology and Thermochronology by the 40Ar/39Ar Method, 2nd edition, Oxford University Press, New York. [RETURN TO TEXT]
 Harrison, T. M., Duncan, I., and McDougall, I. 1985. Diffusion of 40Ar in biotite: temperature, pressure and compositional effects. Geochimica et Cosmochimica Acta 49:2461-2468. [RETURN TO TEXT]
 Dalrymple, G. B. and Lanphere, M. A. 1969. Potassium-argon dating. W. H. Freeman and Company, San Francisco, p. 155, Figure 9-7. The original experiment Dalrymple and Lanphere report was: Evernden, J. F., Curtis, G. H., Kistler, R. W., and Obradovich, J. 1960. Argon diffusion in glauconite, microcline, sanidine, leucite, and phlogopite. American Journal of Science 258:563-604. [RETURN TO TEXT]