author: we_happyfew@...

from:
http://www.evcforum.net/ubb/Forum7/HTML/000051-4.html
http://www.evcforum.net/ubb/Forum7/HTML/000051-5.html

================================================================

[11-13-2002]

[snip]

The Story of Gentry and his Zircons

In the beginning, a man named Gentry got ahold of some zircons from
a geothermal energy test well in New Mexico. He measured the helium
and lead contents, found no evidence of significant lead diffusion,
and found helium concentrations like this (thanks to TC for the
table):

Depth (km) Temperature (oC) Helium (cc/kg) Retention (%)
---------- ---------------- -------------- -------------
0.95 105 86 58
1.17 151 36 27
2.90 197 28 17
3.50 239 0.72 1.2
3.93 277 ~0.2 ~0.1
4.31 313 ~0.2 ~0.1

From this data, he concluded that zircons would make a good place to
store nuclear waste, since plutonium diffuses even slower than lead
in zircon (which already diffuses incredibly slowly), and the lead
hasn't had time to leak out even after 1.5Gya, as evidenced by the
zircon data and calculated rates of lead diffusion.

But the helium was another matter. The only diffusion data Gentry
knew about was Magomedov(1970) whic gave an Activation Energy of 15
kcal/mol. Given a value this low, the helium should have quickly
diffused out of these zircons at these temperatures. Apparently
Gentry, working pretty far outside of his area of expertise, was
unaware of even older research by Hurley(1952), Hurley (1954), Damon
and Kulp(1957), and Damon and Green(1963) suggesting that zircon was
pretty good at retaining helium at moderate temperatures except when
heavily damaged by radiation.

Regardless, Gentry, presented with the opportunity to re-evaluate
his assumptions about zircon diffusivity, and thereby found a whole
new branch of geology we now call thermochronology... instead chose
to interpret this data as proof of his religious belief in a young
Earth. Only five years later, Zeitler(1987) DID lay the foundations
of thermochronology by studying apatite - another mineral that
retains helium (although not quite as well as zircon).

Fast forward a few years...
Gentry is still touting the "amazingly high" helium retention in his
zircons...
and creationists like the Drs. Walt Brown and Russ Humphreys are
citing Gentry's work as Young Earth evidence...
yet all the while, scientists like Zeitler, Wolf, Reiners, Farley,
et al are applying the concepts of thermochronology to a wider array
of new minerals and geologic settings and applications.

Now here's where the story gets a little murky...

Humphreys and his cohorts decide to study Gentry's zircons in more
depth. They promise to commission new diffusivity experiments,
construct elaborate mathematical models, make bold predictions based
on those models, and gather it all up in a book called RATE. But the
results of those promised experiments are yet to be published, the
models are quite puzzling in their peculiar logic and obtuse
language, and the predictions seem to bear little resemblance to
real-world conditions.

At this point it is difficult to discern what data Humphreys has
gathered, what new data he has, what comes from very old sources
like Magomedov(1970), and how he has combined it into a (presumably)
coherent explanation of helium behavior in zircon.

In the next installment, I will examine the few bits and pieces
revealed so far by Humphreys in an effort to shed some light on what
is going on behind the curtain at ICR.

Stay tuned...

Refs:
Damon, P.E., and Green, W.D., 1963, Investigations of the helium age
dating method by stable isotope dilution technique, Radioactive
Dating: Vienna, IAEA, p. 55-69.

Damon, P.E., and Kulp, J.L., 1957, Determination of Radiogenic
Helium In Zircon by Stable Isotope Dilution Technique: Transactions
of the Royal Society of Edinburgh, v. 38, p. 945-953.

Gentry, Robert V., Gush, Gary L., and McBay, Eddy R., Geophysical
Research Letters, Vol. 9, no. 10, p. 1129—1130, Oct 1982

Hurley, P., 1952, Alpha ionization damage as a cause of low He
ratios: Transactions of the American Geophysical Union, v. 33, p.
174-183.

Hurley, P.M., 1954, The helium age method and the distribution and
migration of helium in rocks, in Faul, H., ed., Nuclear Geology: New
York, John Wiley and Sons, p. 301-329.

Magomedov, S. A., Migration of radiogenic products in zircon,
Geokhimiya, 2, 263—267, 1970

Zeitler, P.K., Herczig, A.L., McDougall, I., and Honda, M., 1987, U-
Th-He dating of apatite: a potential thermochronometer: Geochimica
et Cosmochimica Acta, v. 51, p. 2865-2868.

================================================================

[11-17-2002]

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 http://www.answersingenesis.org/docs2002/1030meert.asp

...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
http://www.answersingenesis.org/docs2002/1030meert.asp ...

Humphreys says:
But the –196°C 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 (196°C 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.

================================================================

[11-21-2002]

[snip]

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.