Pleochroic halo

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 sphene which can accommodate uranium or thorium within their crystal structures.(Faure 1986) The most widely accepted 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 was prompted by the claims of creationist Robert V. Gentry that radiohalos in biotite are evidence for a young earth (Gentry 1992). The claims are contested by the mainstream scientific community as an example of creationist pseudoscience (Wakefield 1988).

Production

Uranium-238 follows a sequence of decay through thorium, radium, radon, polonium, and lead. These are the alpha-emitting isotopes in the sequence. (Beta particles do not discolour the rock .)

Isotope	Half-life	Energy in MeV
U238	4.47×109 years	4.196
U234	2.455×105 years	4.776
Th230	75400 years	4.6876
Ra226	1599 years	4.784
Rn222	3.823 days	5.4897
Po218	3.04 minutes	5.181
Po214	163.7 microseconds	7.686
Po210	138.4 days	5.304
Pb206	stable	0

The final characteristics of the radiohalo depend 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. In U-238 haloes, U-234 and Ra-226 rings coincide with the Th-230 to form one ring; Ra-222 and Po-210 rings also coincide to form one ring. These rings are indistinguishable from one another under a lighted microscope (Pal 2004), but Ra-222 and Po-210 rings can be distinguished by other means (Gentry 1974).

A few radiohaloes cannot be explained by any currently known radioactive element. 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

See also: Creation geophysics

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 adsorbed 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) and others have repeatedly and soundly rebutted the radiohalo evidence for a young Earth in peer-reviewed publications.

Citations

• Baillieul, T.A. (2005), "Polonium Haloes" Refuted: A Review of "Radioactive Halos in a Radio-Chronological and Cosmological Perspective" by Robert V. Gentry, TalkOrigins Archive (published 2001–2005){{citation}}: CS1 maint: date format (link)

• Brawley, J. (1992), Evolution's Tiny Violences: The Po-Halo Mystery, TalkOrigins Archive

• Collins, L.G. (1997), "Polonium Halos and Myrmekite in Pegmatite and Granite", Expanding Geospheres, Energy And Mass Transfers From Earth’s Interior, Calgary: Polar Publishing Company, pp. 128–140 {{citation}}: Unknown parameter |editors= ignored (|editor= suggested) (help).

• Durrani, S.A.; Fremlin, J.H. (October 1979), "Polonium Haloes in Mica", Nature, 278: 333–335, doi:10.1038/278333a0.

• Ellenberger, C.L., with reply by Gentry, R.V. 1984. "Polonium Halos Redux," Physics Today. December 1984. pp. 91-92

• Ellenberger, C.L. 1986. "Absolute Dating," unanswered surrebuttal to Gentry, Physics Today. March 1986. pp. 152, 156

• Faure, Gunter (1986), Principles of Isotope Geology, Wiley, pp. 354–355.

• Gentry, R.V. (1970), "Giant Radioactive Halos: Indicators of Unknown Alpha-Radioactivity?" (PDF), Science, 169 (published August 1970): 670–673 {{citation}}: Cite has empty unknown parameter: |1= (help).

• Gentry, R.V. (1975), "Spectacle Haloes", Nature, 258 (published October 1975): 269–270.

• Gentry, R.V. (October 1973), "Radioactive Halos", Annual Review of Nuclear Science, 23: 347–362, doi:10.1146/annurev.ns.23.120173.002023.

• Gentry, R.V. (1974), "Radiohalos in a Radiochronological and Cosmological Perspective" (PDF), Science, 184 (published October 1974): 62–66.

• Gentry, R.V. (1992), Creation's Tiny Mystery, Earth Science Associates (published 2004).

• Henderson, G.H.; Bateson, S. (1934), "A Quantitative Study of Pleochroic Haloes, I", Proceedings of the Royal Society of London, Series A, Containing Papers of a Mathematical and Physical Character, 145 (855): 563–581, doi:10.1098/rspa.1934.0120 {{citation}}: More than one of |number= and |issue= specified (help).

• Henderson, G. H., A quantitative study of pleochroic halos, V, The genesis of halos, Proc. Roy. Soc. , A, 173, 250-264, 1939.

• Henderson, G. H., and F. W. Sparks, A quantitative study of pleochroic halos, IV, New types of halos, Proc. Roy. Soc., A, 173, 238-249, 1939.

• Lide, David R. (Ed.) (2001). CRC Handbook of Chemistry and Physics, 82nd Ed. London: CRC Press. ISBN 0-8493-0482-2.

• Odom, L.A., and Rink, W.J., 1989, "Giant Radiation-Induced Color Halos in Quartz: Solution to a Riddle," Science, v. 246, pp. 107-109.

• Osmon, P., 1986, "Gentry’s pleochroic halos: Creation/Evolution," Newsletter, Feser, Karl D., Editor, v. 6, no. 1, Concord College, Athens, West Virginia

• Pal, Dipak C. (2004), "Concentric rings of radioactive halo in chlorite, Turamdih uranium deposit, Singhbhum Shear Zone, Eastern India: a possible result of 238U chain decay" (PDF), Current Science, 87 n5 (published 10 September 2004): 662–667.

• Schadewald, R., 1987. "Gentry’s tiny mystery, Creation/Evolution" Newsletter, Fezer, Karl D, Editor, v. 4, no. 2 & 3. Concord College. Athens. West Virginia, p 20.

• Schnier, C (August 2002), "Indications for the existence of superheavy elements in radioactive halos", Journal of Radioanalytical and Nuclear Chemistry, 253: 209–216, doi:10.1023/A:1019633305770{{citation}}: CS1 maint: extra punctuation (link).

• Wakefield, J. R. (1988), "The geology of 'Gentry's Tiny Mystery'", Journal of Geological Education, 36: 161–175.

• Wakefield, J. R., 1987-88, "Gentry’s Tiny Mystery - unsupported by geology," Creation/Evolution, v. 22, p. 13-33.

• Wieland, C. (21 August 2003), RATE group reveals exciting breakthroughs!, Answers in Genesis

• Moazed, Cyrus; Richard M. Spector; Richard F. Ward, 1973, Polonium Radiohalos: An Alternate Interpretation, Science, Vol. 180, pp. 1272-1274.

• York, D., 1979, Pleochroic Halos and Geochronology, EOS, v. 60, no. 33, pp. 617-618, Aug. 14, 1979 (publication of the American Geophysical Union).

External links

Favoring a young earth interpretation

• Polonium radiohalos: still "a very tiny mystery", from the Institute for Creation Research
• Fingerprints of Creation

Disputing a young earth interpretation

• Geology of Gentry's "Tiny Mystery", J. Richard Wakefield, Journal of Geological Education, May 1988.
• Examining radiohalides, R. H. Brown, H. G. Coffin, L. J. Gibson, A. A. Roth, and C. L. Webster, Origins 15(1):32-38 (1988).
• Talkorigins FAQ on the criticisms of Gentry's interpretation
• Answers In Creation expose showing that radiohalos do not support a young earth