Claim CD001:
Radiometric dating falsely assumes that the rocks being dated are closed
systems. It inappropriately assumes that no parent or daughter isotopes
were added or removed via other processes through the history of the
sample.
Source:
Morris, Henry M., 1974. Scientific Creationism, Green Forest, AR: Master
Books, p. 139.
Response:
- Absolutely closed systems do not exist even under ideal laboratory
conditions. Nevertheless, many rocks approximate closed systems so
closely that multiple radiometric dating methods produce consistent
results, within 1 percent of each other.
- Some rocks may be open to outside contamination, but not all of them
are. Most ages are determined from multiple mineral and rock samples,
which give a consistent date within 1 and 3 percent. It is extremely
unlikely that contamination would affect all samples by the same
amount.
- Isochron methods can detect contamination and, to some extent, correct
for it. Isochrons are determined from multiple samples, and
contamination would have to affect all of the samples the same way in
order to create an isochron that appeared okay but was wrong (see
CD002).
With uranium-lead dating, closure of the system may be tested with a
concordia diagram. This takes advantage of the fact that there are two
isotopes of uranium (238U and 235U) that decay
to different isotopes of lead (206Pb and 207Pb,
respectively). If the system has remained closed, then a plot of
206Pb / 238U versus 207Pb /
235U will fall on a known line called the concordia. Even
if samples are discordant, reliable dates can often be derived (Faure
1998, 287-290).
- Geochronologists are well aware of the dangers of contamination, and
they take pains to minimize it. For example, they do not use weathered
samples.
Links:
Jessey, David, n.d. Isotope geochemistry.
http://geology.csupomona.edu/drjessey/class/gsc300/isotope1.pdf
Stassen, Chris, 1998. Isochron dating.
http://www.talkorigins.org/faqs/isochron-dating.html
References:
- Faure, Gunter, 1998. Principles and Applications of Geochemistry,
2nd ed. Upper Saddle River, NJ: Prentice Hall.
Further Reading:
Dalrymple, G. Brent, 1991. The Age of the Earth, Stanford University
Press. (Contains an understandable explanation of isochron methods.)
Dickin, Alan P., 1995. Radiogenic Isotope Geology,
Cambridge University Press.
Faure, Gunter, 1986. Principles of Isotope Geology, 2nd Edition,
New York: John Wiley & Sons. (This and Dickin 1995 are standard texts.)
created 2003-5-2, modified 2004-9-2
