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Isochron dating gives unreliable results (Talk.Origins)

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Response Article
This article (Isochron dating gives unreliable results (Talk.Origins)) is a response to a rebuttal of a creationist claim published by Talk.Origins Archive under the title Index to Creationist Claims.

Claim CD014:

Isochron dating is unreliable. The method assumes that the samples are cogenetic, i.e., 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.

CreationWiki response: (Talk.Origins quotes in blue)

1. 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.

First of all, how often are dates that agree with what the Evolutionist expects checked for evidence of mixing? There would be more motivation for checking when one does not like the results.

Second, mixing is not the only way of getting false isochrons.

2. 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 don't.

Maybe they do and simply are not published often enough. Furthermore, mixing is not the only way of getting false isochrons.

3. Other factors that can produce false isochrons are
  • 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.

This really does not help Talk Origins' case any, besides they left out two other factors that can produce false isochrons:

  • Open systems. Also known as contamination.
  • Isotopic Fractionation. It is a physical separation of isotopes and it can be caused by heating and cooling, water flow, contact between high and low concentration magma and just normal molecular motion. Evidence for Isotopic Fractionation does show up in isotopic data so it is a factor that needs to be considered.
4. 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.

Yet, an otherwise good isochron will be dismissed as from an open system (contamination) simply because it does not agree with the date determined by the fossils in the material being dated. The fact is that such methods for guarding against false isochrons can and do fail. The ultimate determining factor is the date indicated by fossils.

Reference: New Age Data of Buried Peat Deposits from the Site "Fili Park"

See also

radiometric dating

Isochron dating

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