From: Steven M Smith (smsmith@usgs.gov)
Date: Mon Oct 06 2003 - 11:33:57 EDT
Allen Roy wrote:
<<Steven M Smith wrote:
Another old critique of an ICR project that preceded the RATE program
(Steve Austin's Grand Canyon Dating Project) can be found at
<www.talkorigins.org/faqs/icr-science.html>. This one addresses the issue
of the young Cenozoic Uinkaret basalts with older "ages" than the
Precambrian Cardenas Basalt mentioned by Allen Roy in a previous post.>>
>This article at talkorigins proves all three of my points.
>1. The acceptance or rejection of isometrically acquired ages for rock
> depends upon factors other than the science/technology of isometric
dating.
I agree with this point ... but so what? Radiometric dating based on
isotopic compositions in rocks is a tool, not a panacea. Radiometric dates
are only one piece of evidence concerning the age of a rock. In addition,
there may be stratigraphic evidence (where in the geologic column the dated
rock is located), correlation with other layers in nearby exposures or
drillholes, superposition (order of layers in the geologic column),
structural evidence (faulting, folding, or other displacement of rock),
evidence from the chemical composition of the rock, inclusions from other
rock units, and evidence from other events that have changed the rock such
as weathering, metamorphism, metasomatism, hydrothermal (hot water)
alteration, or mineralization. When the radiometric date doesn't agree
with the other lines of evidence, then it may be rejected. But that is not
the end of the story, it is just the beginning of trying to figure out why
the test failed. Was the method used inappropriately? Was there
contamination during the collection, processing, or analytical stages?
Were there unrecognized factors in the rock unit that could affect the
isotopic composition? Is the radiometric date telling us something other
than simply the time the rock was emplaced or extruded? etc. etc. etc.
One should not unequivocally accept any test result without corroborating
evidence. As an example, after a recent blood test the doctor told me that
the results indicated that I was a heavy drinker and needed to cut way back
on my alcohol consumption. However, my consumption of alcohol over the
past 40+ years has been very low -- essentially nil. In other words, the
test results didn't agree with the other evidence. Do we immediately
assume that the test is flawed and ignore the results or do we look to see
if there are other factors besides alcohol consumption that are being
indicated? I prefer the latter course of action.
So, in effect, it appears that you are presenting the perfection of
radiometric dating as a strawman argument. Should a radiometric dating
procedure ever give us a result that doesn't agree with the other evidence,
then all radiometric dating is invalid. We can then ignore all radiometric
dates regardless of corroborating or conflicting evidence.
> 2. The dating process of rock units is subjective even when isometric
dates
> are available.
See what I wrote above.
> 3. If any rock unit is believed or known to be young, then any old
isometric
> age is rejected, not as inaccurate, but as irrelevant.
> EVERYONE recognizes that the computed age of the Unikaret basalts based
on
> Rb-Sr cannot be the age of the crystallizing of the lava flows in the
> "Cenozoic" because the "law of superposition" overrides the isotopically
> derived age.
This is incorrect. The radiometric age may be inaccurate but it is not
irrelevant. It is telling us that there are additional factors to be
considered. Your example of the young Unikaret basalts and Austin's 1.3
billion year age is an excellent example of what I am referring to (as well
as evidence that you did not completely read or consider the criticisms of
Austin's work at <www.talkorigins.org/faqs/icr-science.html>). The date
that Austin obtained may be a valid radiometric test result but the method
he used does not give the age of the lava flow sitting at the surface but
more likely the age of the source material in the mantle below the Grand
Canyon. (See the criticism labeled "The wrong meaning is assigned to the
dates.") The tests and the result may be valid but Austin has
misinterpreted the data to mean that the radiometric data is giving the age
of the multiple flows and, since that age is stratigraphically
unreasonable, then all radiometric dating is invalid.
Let me give another non-geological analogy. The Unikaret basalts are like
a fruitcake (except that the fruitcake is probably older! <grin>). Just
like the fruit in the fruitcake, these basalts include pieces of rock from
other sources (known as xenoliths or literally "foreign rocks"). According
to the TalkOrigins FAQ, Austin has deliberately chosen a sampling method
that dates the fruit and then interpreted that as the age of the cake.
Steve
[Disclaimer: Opinions expressed herein are my own and are not to be
ascribed 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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