Claim CD014:
Isochron dating is unreliable. The method assumes that the samples are cogenetic, that is, that they form at the same time from a reasonably homogeneous common pool. This assumption is invalid. In particular, mixing two sources with different isotopic compositions gives meaningless but apparently valid isochron plots.Source:
Overn, William, n.d. Isochron rock dating is fatally flawed.
http://www.tccsa.tc/articles/isochrons2.html
Response:
- Mixing can usually be detected by plotting the total daughter
isotopes against the ratio of daughter isotopes. These would not
likely fall on a straight line if mixing occurred.
- Isochron plots from mixing can have any slope, even negative slopes.
If mixing were common, we would expect a high percentage of isochron
results to show negative slopes. They do not.
- Other factors can produce false isochrons (Stassen 1998; Zheng 1989).
For example:
- Protracted fractionation. This requires slow cooling (over millions of years) and produces only a small error.
- Inherited ages as from partial melting. The age given by this method is the age of the source material. Furthermore, this factor requires unusual conditions and usually produces scatter in the isochron plot.
- Metamorphosism. This produces apparent ages younger than the age of the source material.
- False isochrons can usually be avoided by choosing appropriate samples. The samples must come from an (apparently) initially homogeneous source and avoiding obvious signs of weathering and metamorphism.
Links:
Stassen, Chris, 1998. Isochron dating. http://www.talkorigins.org/faqs/isochron-dating.htmlReferences:
- Stassen, C., 1998. (see above)
- Zheng, Y.-F., 1989. Influences of the nature of the initial Rb-Sr system on isochron validity. In Chemical Geology (Isotope Geoscience Section) 80, pp. 1-16.
Further Reading:
Dalrymple, G. Brent, 1991. The Age of the Earth. Stanford, CA: Stanford University PressFaure, Gunter, 1986. Principles of Isotope Geology, 2nd ed. New York: John Wiley and Sons.
created 2003-8-4