Re: [asa] Kirk on C14 contamination

Kirk,

I read your RATE post on TheologyWeb
<http://www.theologyweb.com/campus/showthread.php?t=103916> and
Baumgardner's reply both on TheologyWeb
<http://www.theologyweb.com/campus/showpost.php?p=2140732&postcount=10>
and Answer in Genesis
<http://www.answersingenesis.org/articles/2007/11/30/feedback-rate-contamination>
websites.

First, thank you for your excellent postings and reasoned exchanges.
Congratulations for presenting an argument so cogent that Dr. Baumgardner
felt compelled to join your thread to defend his views.

Second, I wish to commend you on your civility in response to
Baumgardner's "less-than-professional" attacks on your character and
scientific qualifications. Your posts are an excellent example of how
scientists conduct a civil argument in publications. I highly recommend
this TheologyWeb thread to anyone interested in the RATE research.
(Unfortunately, to get to the good exchanges between Kirk & Baumgardner
you will have to wade through some petty bickering by a few other
less-well-behaved participants.)

As I was catching up on reading some of the past ASA posts, I just noticed
a question on this thread that was directed toward me.

>> "In regard to 14C production due to the presence of uranium in crustal
>> environments, I treat that topic in detail in section 7 of my chapter
and
>> show the maximum plausible 14C production rate, given measured
>> neutron fluxes in deep mines and measured reaction cross sections,
>> is more than four orders of magnitude too small to account even for
>> the small measured 14C levels in diamonds. This same analysis also
>> applies to coal. "

[Let me add a little bit more of the Baumgardner quote that was not
included in the previous post - SMS.]

"Uranium concentrations in coal are typically less than those measured for
granite, which is the setting for most of the diamonds we studied. (See
the USGS fact sheet on uranium concentrations in coal and granite in the
References.)"

> Maybe Steve Smith can answer this better, but I believe that his
> estimates came from granites, but that coals can have orders of
> magnitude higher levels of uranium and thorium. Steve--any comments?

I can't comment directly on Baumgardner's treatment of neutron fluxes and
his conclusion that this source of 14C production is too small to be
considered. I have not read his Chapter 7 and I am not inclined to
attempt a review of his math. I can, however, speak with some knowledge
on the concentrations of uranium and thorium in various rocks since this
pertains to my own specialization in mineral exploration geochemistry and
environmental geochemistry of metals. From this I will then make a few
general statements that may pertain to his treatment of neutron fluxes.

When I initially read this paragraph by Baumgardner, the first thing that
jumped out was the statement about their RATE diamonds from a granitic
setting. At best, Baumgardner is a little sloppy on his terminology here.
 Diamonds are not mined from or generally found in granitic settings.
Hardrock diamonds are mined from kimberlite, an ultramafic rock that is at
the opposite end of the spectrum from granites. Now, I am familiar with
the tendency of stone masons, geologic engineers, and geophysical
modellers to refer to any hard crystalline igneous as "granite" but
chemically, these two rock types are about as different as dark & light.
(Some geologists refer to ultramafic rocks as "primitive" and to granites
as "evolved" but that is another story.) So if you are going to talk
about the chemistry of diamond-bearing kimberlite, you would not look at
the chemistry of granite.

So what about uranium & thorium contents? The average crustal abundances
for uranium & thorium are estimated at about 2 and 8 parts-per-million
respectively (2 ppm = 0.0002%). Granites are generally enriched in
uranium & thorium with averages around 4 - 5 ppm U and 15 - 20 ppm Th.
Occasionally, some individual "highly evolved" granites have even higher
concentrations. Ultramafics, on the other hand, are depleted in uranium &
thorium with abundance averages around 0.001 ppm U and 0.004 ppm Th
(references available upon request). Thus if all of the observed neutron
flux in deep mines is generated from the decay of U & Th, then that flux
should be on the order of 1,000 times less in mine within a kimberlite
than one in a granite. Of course the flux would be much higher in an
uranium mine regardless of the source rock. Therefore, you should expect
several orders magnitude for neutron flux rates in different mines. Note:
 If Baumgardner truly measured or used neutron flux rates from deep mines
within granite then he could be overestimating the effects of U-Th
radiation on the production of 14C in diamonds. Despite his error in rock
identification, Baumgardner would see this as evidence in favor of his
argument. To truly evaluate the effects of U-Th neutron fluxes on 14C
contents in diamonds, one would need to measure the flux in the actual
mine that produced the diamond.

Unlike igneous rocks (granites or kimberlites) where the uranium content
is primarily derived from their initial magma chemistry, the plant
material that ultimately becomes coal is generally very low in uranium.
The bulk of uranium in coal is secondary; it came from somewhere else --
either early in the swamp, during the formation of the peat, during the
change to coal, or late after the coal has formed. The chemical behavior
of uranium gives us some clues. Uranium is very slightly water soluble in
oxidized (oxygen-rich) environments and insoluble in reduced (oxygen-poor)
environments. Many secondary uranium deposits of economic significance
are found near large quantities of weathered granite or volcanic ash of
granitic composition. The working model for these uranium deposits is
that oxygenated water (i.e. rainwater) seeps through these rocks
dissolving trace quantities of uranium (originally around 4 - 5 ppm U in
that granitic rock). The slightly uranium-enriched waters move out of the
granite or ash into more permeable rock layers (usually sandstones) and
move down gradient until the water reaches a reducing environment. At
this point the uranium precipitates out of solution and is left behind as
the water moves on. Eventually the uranium is enriched sufficiently at
this oxidation-reduction boundary (known in chemistry as a redox front) to
form a deposit, which may contain up to a few percent uranium and other
associated metals. As long as the groundwater flow regime remains
constant, the deposit actually migrates slowly through the sandstone as
oxygenated waters re-dissolve the uranium at the back of the deposit and
move it forward until it is again reduced. We call these occurrences
"Roll-Front Uranium" deposits or sometimes just "sandstone uranium
deposits."

I digressed into this discussion of uranium geochemistry and ore deposits
because the organic material preserved in rocks is one of these reducing
agents that causes uranium to precipitate out of solution. Sometimes
these sandstones contained coalified or partially petrified wood. Here in
Colorado, some of the sandstone uranium mines have actually processed
high-grade uranium-bearing petrified logs and even uranium-enriched
dinosaur bones as part of their ore. I've seen reports of coalified logs
containing >16% uranium (160,000 ppm U). (I have a specimen of hot
radioactive petrified wood that I store in a lead-lined ammo box. I also
commonly take a Geiger counter out with me when I take school kids, scout
groups, college groups, museum groups, church groups, and friends on local
field trips to see dinosaur bones in sandstones.) Thin layers of
interbedded black shale (compressed organic-rich mud) or coal in these
deposits may also form high-grade uranium zones. And at least one small
coal bed in Wyoming was mined in the early 1960's as an uranium deposit.

So can we say that coals are generally rich in uranium? (And should I be
worried about the amount of uranium pouring out a coal-fired power plant
near me?) The answer to both of these questions is ... with a few
exceptions, probably not. Before the screams of protest get too loud, let
me explain. The large seams of coal mined for power plants generally have
uranium concentrations that are indeed lower than granite. The average
abundance range of uranium in coals (collected from coal mines in the
United States) is generally between <1 to 4 ppm U; thorium concentrations
in coal also range from 1-4 ppm. According to the USGS Fact Sheet cited
by Baumgardner
<http://greenwood.cr.usgs.gov/energy/factshts/163-97/FS-163-97.html>,
coals with more than 20 ppm uranium are rare in the United States. (I
have met both authors of this USGS Fact Sheet and they don't appear any
more abnormal than the rest of us scientists <grin>.) According to a
report about uranium in coal for the National Uranium Resource Evaluation
(NURE) program (Facer, J.R., Jr, 1979, GJBX-56(79)) 250 samples of
"commercial grade coal and lignite" from the northern Great Plains of the
U.S. had uranium concentrations from 0.1 - 7.5 ppm averaging 0.9 ppm. A
subset of these coals from the Powder River Basin, a known uranium
producing region, ranged from 0.4 - 1.1 ppm and averaged 0.8 ppm. The few
coals in the U.S. that show higher uranium contents are (1) generally very
thin and have high ash contents (i.e., not good for power plants); or (2)
have a slightly enriched uranium zone near the surface of the coal bed
where it is in contact with a very permeable sandstone or conglomerate.
Apparently, the redox front does not move very far into a coal seam. I
know of one exception where high uranium coal is being used for power
plants and cooking fires. This one is in China. Unfortunately in this
locality, the people are suffering & dying of arsenic poisoning from the
coal emissions long before any of the other metals, like uranium, are
affecting them.

So now, let's get back to Baumgardner's argument. He is correct on this
point. Most coals have less uranium & thorium contents than granite.
Since his coal samples apparently come from a group of commercially mined
U.S. coals, in all probability his samples have low uranium & thorium
contents also. (Baumgardner could confirm this by submitting his samples
to a commercial geochemical laboratory for U & Th analyses. With costs
ranging between $10.00 and $60.00 per sample depending on the method and
the amount of sample prep work needed, the analyses would be cheap
compared to the radiocarbon work.)

Finally, what can we say about his argument that ...

>> "the maximum plausible 14C production rate, given measured neutron
>> fluxes in deep mines and measured reaction cross sections,
>> is more than four orders of magnitude too small to account even for
>> the small measured 14C levels in diamonds. This same analysis also
>> applies to coal."

(1) Assuming that the neutron flux is due almost entirely to the decay of
U & Th atoms, then the use of measured neutron fluxes from granites
*overestimates* the production of 14C in coal or diamonds from these
neutrons.
(2) However, if the neutron flux measurements were done in deep mines
within ultramafic rocks (i.e. diamond mines) then flux rate is appropriate
for discussing diamonds but *could be 1,000 to 2,000 times too small* for
average coal samples.
(3) This entire argument assumes that Baumgardner's equations for 14C
production rates, based on neutron flux measurements, are correct and that
the equations contain no assumptions based only on a presumed 6,000
year-old earth. I did not evaluate his math for either situation.

Steve
[Disclaimer: Opinions stated in this post are my own and are not to be
attributed to my employer.]
_____________
 Steven M. Smith, Geologist, U.S. Geological Survey
 Box 25046, M.S. 973, DFC, Denver, CO 80225
 Office: (303)236-1192, Fax: (303)236-3200
 Email: smsmith@usgs.gov
 -USGS Nat'l Geochem. Database NURE HSSR Web Site-
  http://pubs.usgs.gov/of/1997/ofr-97-0492/

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Received on Mon Dec 3 19:33:47 2007