Saturday, October 11, 2014

Radiometric dating - The assumptions of Radiometric dating

Some of the basic Dating Methods are as follows:
  • Samarium - Neodymium. (Sm-Nd)
  • Rhenium - Osmium (Re-Os)
  • Uranium / Thorium - Lead. ( U/Th-Pb)
  • Ribidium - Strontium (Rb-Sr)
  • Potassium - Argon (K-Ar)
  • Argon - Argon (Ar-Ar)
  • Lutetium - Hafnium (Lu-Hf)
All these methods rely on the changing ratio of  parent or daughter isotopes in a closed system. Now such a closed system does not really exists, but open system affects can't be determined easily, so it is hoped that they about balance out. These methods all have the same basic assumptions.
  1. Constant decay rate.
  2. No gain or loss of parent or daughter isotope.
  3. Known amounts of daughter isotope at start.
Realizing the difficulty of dealing with assumptions #2 and #3 above Isochron Dating was developed in an attempt to solve this problem. According to theory the sample starts out with daughter isotopes ratio with other isotopes of the same element at a constant value, but with the parent isotope is arbitrary. As a result is forms a strait horizontal line on a graph. As parent decays to daughter, the ratios change and the straight line remains but becomes angled. The slope of the line equals the number of half-lives of the parent isotope has passed sense solidification.

A shift  from contamination can take place in all of the data points, but such contamination does not affect all data points equally, so it can cause the data points to shift off the true Isochron completely. Given this when one looks at an Isochron plot how can one really tell where the true Isochron line should be. Sufficient contamination can produce any Isochron pattern regardless of the true Isochron. It is even possible to get a negative slope, this would be equivalent to a negative or future date.

When you look at actual isochron plots such as the ones at above link, there seems to be room for subjectivity. Some are better than others but there is often room for multiple plot lines. Even uniformitarian geologists recognize the existence of false isochron. So how do they distinguish good data from bad? The answer is where the sample fits in the Geologic Column.

The unique key assumption of Isochron dating is that the affect of contamination does on the Isochron can be determined. However the quality of an Isochron is still judged by where the sample fits in the Geologic Column. Also like all forms of radiometric dating it assumes that nuclear decay rates are constant, an assumption which will later be shown false.

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