The impact of the article on the polonium halo argument would depend on the half-life of plutonium and the rate of cooling of the granite. If the half-life is short relative to the cooling time, then rapid emplacement (as envisioned by the article) would not affect the argument. However, it already has several problems. The basic argument is that the presence of polonium halos in granites show that they formed rapidly, in less than a couple polonium half-lives. However, as polonium can be generated by radioactive decay within the cooling magma and then incorporated by a crystal, the premise of the agument has problems.
There are also two problems in identifying halos as coming from authigenic polonium. In the case of Gentry's original samples, the radioactive atoms are not necessarily authigenic, as the granite shows signs of hydrothermal alteration. The polonium could have been emplaced after the granite formed. Even if the halos are from authigenic atoms, the identification as polonium halos is tricky. Identifying the causitive radioisotope depends on the size of the halo, which is a spherical hole. If the cross-section does not go precisely through the center of the sphere, or if the halo has been distorted after formation, the measurement will be incorrect.
Finally, it should be noted that the assumptions necessary to identify the halo as polonium and to identify it as authigenic are the same assumptions necessary for radiometric dating. Identification of the halos rests on the assumption that the pattern of radioactive decay has not changed since the formation of the halo. Identification of the polonium as authigenic requires the ability to test rocks for later alteration and reject those that have problems.
Dr. David Campbell
"Old Seashells"
Biology Department
Saint Mary's College of Maryland
18952 E. Fisher Road
St. Mary's City, MD 20686-3001 USA
dcampbell@osprey.smcm.edu, 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'. And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!
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