Radiohalo

This article is about the geophysical phenomenon. For the astronomical phenomenon, see radio halo.

Radiohalos or pleochroic halos are microscopic, spherical shells of discolouration within minerals such as biotite that occur in granite and other igneous rocks. The shells are zones of radiation damage caused by the inclusion of minute radioactive crystals within the host crystal structure. The inclusions are typically zircon, apatite, or titanite which can accommodate uranium or thorium within their crystal structures (Faure 1986). One explanation is that the discolouration is caused by alpha particles emitted by the nuclei; the radius of the concentric shells are proportional to the particle's energy (Henderson & Bateson 1934). The phenomenon of radiohalos has been known to geologists since the early part of the 20th century, but wider interest has been prompted by the claims of creationist Robert V. Gentry that radiohalos in biotite are evidence for a young earth (Gentry 1992).

Production

Uranium-238 follows a sequence of decay through thorium, radium, radon, polonium, and lead. These are the alpha-emitting isotopes in the sequence. (Because of their continuous energy distribution and greater range, beta particles cannot form distinct rings.)

Isotope Half-life Energy in MeV
U-2384.47×109 years4.196
U-2342.455×105 years4.776
Th-23075400 years4.6876
Ra-2261599 years4.784
Rn-2223.823 days5.4897
Po-2183.04 minutes5.181
Po-214163.7 microseconds7.686
Po-210138.4 days5.304
Pb-206stable0

The final characteristics of the radiohalo depends upon the initial isotope, and the size of each ring of a radiohalo is dependent upon the alpha decay energy. A radiohalo formed from U-238 has theoretically eight concentric rings, with five actually distinguishable under a lighted microscope, while a radiohalo formed from polonium has only one, two, or three rings depending on which isotope is the starting material (Weber 2010). In U-238 haloes, U-234, and Ra-226 rings coincide with the Th-230 to form one ring; Rn-222 and Po-210 rings also coincide to form one ring. These rings are indistinguishable from one another under a light microscope (Pal 2004), but Rn-222 and Po-210 rings can be distinguished by other means (Gentry 1974).

Giant radiohaloes caused some excitement when Robert V. Gentry proposed that they resulted from the decay chain of unidentified primordial Superheavy elements (Gentry 1970).

Controversy

Robert V. Gentry studied halos which appeared to have arisen from Po-218 rather than U-238 and concluded that solid rock must have been created with these polonium inclusions, which decayed with a half-life of 3 minutes. They could not have been formed from molten rock which took many millennia to cool (the standard theory) because polonium decays in a few minutes. This is taken by creationists as evidence that the Earth was formed instantaneously (Gentry 1992).

Critics of Gentry, including Thomas A. Baillieul (Baillieul 2005) and John Brawley (Brawley 1992), have pointed out that Po-218 is a decay product of radon, which as a gas can be given off by a grain of uranium in one part of the rock and migrate to another part of the rock to form a uraniumless halo. Apparently a large number of radon atoms are caught or absorbed at a particular point. This has not been proved experimentally, but is supported by the fact that Gentry's "polonium halos" are found along microscopic cracks in rocks that also contain uranium halos (Wakefield 1988).

Gentry's work has been continued and expanded by the creationist Radioactivity and the Age of the Earth (R.A.T.E.) project that was operating between 1997 and 2005 (Wieland 2003). However, Collins (1997), Wakefield (1988) offered rebuttals of the radiohalo evidence for a young Earth.

Citations

    External links

    Favoring a young earth interpretation

    Disputing a young earth interpretation

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