The author replies to Bartelt.

1. The Point of Contention

The real issue is this: Given the task of solving a problem, is it possible to gather too much data or to perform too many experiments?

My answer to this question is that scientists should collect as much information about the nature of the problem as possible before drawing conclusions. Chemistry is subtle, and chemists can be tricked into believing results that are not true. The best defense against being deceived is to be thoroughly familiar with the scientific literature, to get as much data as possible, and to try to reproduce results a number of times. There are no unnecessary experiments. It is better to perform an irrelevant experiment than not to perform the experiment and then later discover that the results would have been very important.

In stark contrast, Bartelt seems to think that gathering extra data is not important. She has stated that the ⁴⁰K:⁴⁰Ar ratio alone is necessary for accurately assessing a mineral’s age. Bartelt does not see the need to learn more about the unknown chemistry resulting from the decay of the ⁴⁰K nucleus in minerals. She does not comprehend how a knowledge of the stoichiometry of the reaction may aid in the interpretation of the age of the mineral. Therefore, her view is that balancing the potassium–argon reaction in minerals is a red herring and does not need to be done.

I respectfully but firmly disagree. If a geologist determines that a mineral is a closed system suitable for potassium–argon dating, then the potassium–argon reaction in that mineral should balance. Identifying and quantifying the products could be used to corroborate the geologist’s claim that the mineral is indeed a closed system.

2. Is the ⁴⁰K:⁴⁰Ar Ratio the Only Quantity Necessary for Determining a Mineral’s Age by the Potassium–Argon Method?

Bartelt’s assertion that the ⁴⁰K:⁴⁰Ar ratio is the only quantity necessary for determining a mineral’s age is correct, as long as the mineral is a closed system and there was not a significant quantity of ⁴⁰Ar homogeneously distributed in the mineral at time zero. (For a discussion of minerals known to be enriched in ⁴⁰Ar at time zero, see ref 1.)

Consider dating a sanidine mineral (KAlSi₃O₈) by the potassium–argon method. For the sake of argument, we shall adopt the following conditions for this hypothetical experiment: (1) a team of qualified geologists agree that this sample of sanidine is a perfectly closed system and an excellent candidate for potassium–argon dating; (2) there are no phenocrysts or xenoliths trapped in this sample; (3) there is no air contamination of the sample; (4) there are no redox-active impurities; and (5) this sample of sanidine is protected from all outside environmental influences.

A hypothetical balanced reaction is

100 ⁴⁰KAlSi₃O₈→ 11 ⁴⁰Ar + 78 ⁴⁰Ca(Al^II)Si₃O₈ + 11 ⁴⁰CaAl₂Si₆O₁₆

This reaction is balanced with the understanding that ⁴⁰K converts into ⁴⁰Ar 11% of the time and into ⁴⁰Ca 89% of the time. It is mathematically impossible to balance the potassium–argon reaction without forming a reduced product. In the reaction shown above, that reduced product is divalent aluminum in⁴⁰Ca(Al^II)Si₃O₈.

(Of course, it is not really known that divalent aluminum is a product since the reaction shown above is conjecture, and the real potassium–argon reaction in sanidine is unknown. Divalent aluminum is presented as a possible product because naturally occurring samples of paramagnetic quartz doped with aluminum are known, and aluminum-based electron spin resonance signals have been used to date such materials (2).)

The stoichiometric Al^II:⁴⁰Ar ratio is 78:11. If the geochronologist identifies and quantifies the products from the reaction and actually finds an Al^II:⁴⁰Ar ratio reasonably close to 78:11, then the stoichiometry confirms that the ⁴⁰Ar found within the mineral really is radiogenic. In this case, the geologists were right to certify this sample as an excellent candidate for potassium–argon dating.

If the geochronologist finds an Al^II:⁴⁰Ar ratio significantly different from 78:11 however, then there is a problem! There must be an explanation for why the expected 78:11 ratio was not found. Perhaps one or more of the five conditions listed above was not met, despite the approval of the geologists! Or, perhaps the Al^II was oxidized to Al^III by some unknown mechanism. (This oxidation does not really solve the problem however, because that odd electron had to go somewhere and the reaction must still be balanced!) In this case, it is not clear that the mineral age can be accurately determined from the ⁴⁰K:⁴⁰Ar ratio, and using the ⁴⁰K:⁴⁰Ar ratio as the only quantity for determining the mineral’s age may or may not be correct.

So, is the ⁴⁰K:⁴⁰Ar ratio the only quantity necessary for determining a mineral’s age by the potassium–argon method? Provided that the sample was certified by geologists as appropriate for potassium–argon dating, Bartelt’s answer would be affirmative, and she would strongly object to gathering any additional information. My answer, on the other hand, is that I don’t know. There are too many unknowns, and I would like to learn more about the chemistry resulting from the decay of ⁴⁰K in sanidine.

3. Comment on Questioning Well-Established Fields of Science

Radiometric dating is a well-established field of science, but this fact should not be used to intimidate someone from asking questions. Scientific knowledge advances only when well-established practices are questioned, and questioning radiometric dating is the responsibility of all professional scientists everywhere.

When teaching chemistry, I introduce my students to a number of theories, such as Atomic Theory, Quantum Theory, Valence Bond Theory, Molecular Orbital Theory, Crystal Field Theory, and more! These theories are well-established and supported by a great deal of experimental evidence. Nevertheless, I encourage my students to question the experimental support and to think of new experiments that could potentially falsify the theories. This activity is extremely important for producing first rate scientists. The students are well aware that these theories are not “controversial”, and that we question these ideas simply as an intellectual exercise.

In conclusion, Dr. Bartelt’s letter has failed to convince me of any genuine problems. The concepts presented in my article and in this response letter are general chemistry textbook concepts: (1) reactions need to be balanced, and (2) the stoichiometry of the reaction should be considered when trying to determine the reaction lifetime. These ideas are very easy to understand and are commonly taught in general chemistry courses throughout the world.

Literature Cited

1. Kelley, S. Chemical Geology 2002, 188, 1–22.
2. Rink, W. J. Radiation Measurements 1997, 27 (5–6), 975–1025.

See also the replies of two reviewers (1, 2) of Howard's article.