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RADIOHALOS AND EARTH HISTORY
Pleochroic halos are microscopic, ring-like discolorations observed when thin sections of certain minerals are examined by polarized light. They were first discovered some years before the turn of the present century and were named for their "pleochroism," a term introduced more than a century ago to denote "the property of exhibiting different colors in different directions by transmitted polarized light (Dana)."(1) But they remained an unexplained curiosity of nature for many years.
It was not until 1907 that the mystery of their origin was cleared up. In that year, John Joly, an Irish physicist and geophysicist, and O. Mügge, a German scientist, almost simultaneously came up with the explanation: The discolorations are the result of radioactivity in tiny inclusions (impurities) at the halo centers; prolonged alpha particle emission by atoms undergoing radioactive decay produces a halo-like zone of damage surrounding each inclusion, disrupting the surrounding crystal structure and giving rise to the pleochroism.
More recently, in recognition of the actual cause of the halos, the term "radiohalo" has come into use in place of "pleochroic halo.”(2)
Historically, radiohalos helped to put the science of geochronology on a quantitative footing. More recently, they have thrown the entire discipline into disarray.
The idea of dating rocks radiometrically was first advanced in 1905 by Ernest Rutherford.(3) He reasoned that the helium found in radioactive minerals must come from the decay of radium and other radioactive elements. He wrote: "If the rate of production of helium from known weights of the different radioelements were experimentally known, it should thus be possible to determine the interval required for the production of the amount of helium observed in radioactive minerals, or, in other words, to determine the age of the mineral."
Considering the possibility that helium, a gas, might tend to escape from a source mineral over long periods of time, Rutherford also suggested that lead, should it indeed prove to be an end-product of radioactive decay, as suggested by some then-recent work reported by B. B. Boltwood,(4) might better serve as an indicator of age for radioactive minerals. The role of lead as a final descendant in several chains of radioactive decay was later firmly established.
So it was only a matter of a year or so between Rutherford's first proposal for radiometric dating and the recognition of radiohalo origins by Joly and Mügge. And as early as 1909 Joly further suggested that the progressive nature of halo discoloration might also provide a way to measure the ages of certain minerals. Half a century was to pass before techniques sensitive enough to test this idea were perfected, and then the results were disappointing: Different crystals, even from the same rock, showed different sensitivities to alpha-particle darkening; and it was found that discoloration proceeds only up to a saturation point, beyond which the process tends to reverse itself.(5)
But during this period when radiohalos were still untested as time-keepers, they came into their own in a much more fundamental way.
In some of his earlier papers on pleochroic halos, Joly argued that halos of varying sizes indicated that decay rates for uranium had not been constant throughout geological time. But later he reversed himself and insisted that pleochroic halos actually proved what everyone else had always assumed - that radioactive decay rates have always been constant.(6)
As presented some years later by astronomer Otto Struve, the argument goes like this: ". . . a uranium atom of atomic number 92 and atomic mass 238 (92U238) successively releases eight alpha particles (helium nuclei) to become lead of atomic number 82 and atomic weight 206 (82Pb206 ) [See Figure 2]. Similarly, another isotope of uranium, U235, loses seven alpha particles and decays to Pb207. The disintegration of thorium (90Th232) follows a like pattern, forming Pb208. In all three cases, the end products are lead and the gas helium.
"There is excellent evidence that the rates of radioactive processes measured in the laboratory at the present time are valid also for the remote past. If a radioactive element and its decay products are imbedded in a crystal, each alpha particle emitted during disintegration travels a certain distance that depends only on the rate of that particular decay step. The more rapid this rate, the greater the energy of the alpha particles, and the farther they go before being stopped and producing a color change in the crystal.
RADIOACTIVE DECAY OF URANIUM-238
Nucleus Emitted Half-Life
92U238 alpha (helium nucleus) 4.50 x 109 years
90Th234 beta (electron) 24.1 days
91Pa234 beta 1.18 minutes
92U234 alpha 2.50 x 105 years
90Th230 alpha 8.0 x 104 years
88Ra226 alpha 1620 years
86Em222 (Radon) alpha 3.82 days
84Po218 alpha 3.05 minutes
82Pb2l4 beta 26.8 minutes
83Bi2l4 alpha 19.7 minutes
81Ti210 beta 1.32 minutes
82Pb2l0 beta 22 years
83Pb2l0 beta 5.0 days
84Po210 alpha 138.3 days
82Pb206 stable - - - - -
Emission of an alpha particle decreases the electric charge on the nucleus (subscript) by two and the mass number (superscript) by four. Emission of a beta particle (negative electron) increases the charge by one and leaves the mass number unchanged. This list omits several branch reactions. After Struve, Reference 7.
"Suppose a speck of U238 has remained undisturbed since the formation of a mineral containing it. Then, because the rate of disintegration at each successive alpha-particle emission is different, eight concentric rings of mineral discoloration will be found surrounding the particle of uranium. These rings, known as pleochroic halos, have been found in many rocks of different geological ages, and the diameters of the respective rings are always the same.
"Thus, it can be concluded that the rates of disintegration of uranium and thorium are constant ... “(7)
The first premise of this argument may seem strange when first encountered. Is it true that the energy of an emitted alpha particle depends on the rate of decay? Apparently so. This relationship was discovered experimentally in the early years of radioactivity studies, and for a long time it constituted something of a paradox -- not only for the regularity of the increase in alpha-particle energy with increase in decay rate (or decrease in half-life; see Figure 1), but especially for the fact that none of the observed energies was sufficient to explain how the alpha particles escaped from radioactive nuclei in the first place. It would not be until the late 1920's that quantum theorists would claim to have dispelled this mystery in terms of probabilities.(8)
But does an observed relationship between alpha-particle-emission energies and rates of nuclear decay prove anything about decay rates in the distant past? Not directly, of course. Thus it was that Joly suggested examining ancient minerals, or minerals of presumed antiquity, to see whether or not their radiohalos exhibited dimensions -- indicative of emission energies -- predictable on the basis of modern decay rates.
Early investigators reported that some of the radiohalos indeed showed rings in proper sequences and of the expected sizes.(9) And when a theoretical explanation for the relationship between emission energies and decay rates emerged, it was generally assumed that the case for decay rates never wavering down through geological time was at last ironclad.
But recent work on radiohalos has again cast this premise into doubt. The man responsible for most of this work, Robert V. Gentry, is understandably cautious in discussing its broadest implications, but on one point he stands firm: Radiohalos do not support the idea of constant decay rates.
If this is so, the findings of geochronology, to the extent they are based on radiometric dating, are clearly questionable.
In the early 1960's, Gentry, now of the Chemistry Division of the Oak Ridge National Laboratory, found himself wondering: "Can the earth's age be measured by radioactive dating of its rocks, as most geologists and geochemists believe? For some time I had been intrigued with the thought that certain scientists were using the terms 'age of the universe' and 'age of the earth' quite freely in conjunction with each other. How did they know, I thought. Was the earth actually as old as the universe? Were the earth's elements in fact synthesized in some gigantic primeval nuclear event?"(10)
Noting that radiohalos "provide the only means for studying the radioactive transformation of elements in the earth," Gentry set out to review previous work on the subject, then began his own painstaking study of thousands of halos in rocks from around the world. Almost immediately he found that all was not in order in this long neglected field.
Gentry discovered that, although uranium halos, for example, are readily identifiable by the number and relative rough diameters of their rings, their actual dimensions often vary substantially, even within a single crystal.(11) While explanations might be contrived to account for such variability and thus seemingly preserve the idea of invariant decay rates, the evidence, on its face, denied the idea of a decay "constant" valid for all time.
As Gentry explains in a more recent paper, "experimental uncertainties in measuring U halo radii preclude establishing the constancy of [the decay constant] to within 35 percent, and under certain assumptions U halos provide no information at an in this respect . . ." In a footnote, the author elaborates by pointing out that if alpha-decay energies do not vary for given decay rates, while radiohalo radii do vary (which they do), then there must also be some variation in the decay constant. "In such a case," writes Gentry, "halos furnish no proof that [the decay constant] is constant."(12)
Gentry's demonstration that Joly's conclusion about pleochroic halos proving the constancy of radioactive decay rates was overdrawn was only the beginning. Soon to be announced were findings even more disturbing.
In 1966 Gentry reported "Abnormally Long Alpha-Particle Tracks in Biotite (Mica).”(13) He called attention to earlier investigations disclosing the existence of halo rings much larger than could be attributed to known alpha-emissions in the uranium and thorium decay chains. The largest ring to be expected in these series would have a radius of about 42 microns, due to 8.78-million-electron-volt alpha particles from polonium-212. Yet radii as large as 75 microns had been noted by others, (14) and now Gentry had similar findings to report.
After examining and ruling out suggestions that the "giant halos" might be due to protons from atoms of low atomic number, or to extraordinarily large central inclusions, he pointed out that certain known alpha-emitters - actinium-213, radium-216, radium-217, and francium-215 might have given rise to the oversized halos. But these radionuclides are unrelated to the uranium and thorium decay chains. Gentry added: "This suggests the possibility of other decay series which may have existed in times past ... [Furthermore,] any attempt to trace the progenitors of At2l3, Ra2l6, Ra217, and Fr215 through the transuranium elements reveals a very interesting point: all possible precursors of these nuclides have fairly short half-lives. Since these halos would begin to form upon solidification of the host mineral, it is conceivable that further study of the giant halo parent nuclides (and their precursors) would establish a maximum time period between nucleosynthesis and crustal formation."
This cautiously worded statement barely disguises the bombshell it contains. Nearly every modern cosmological theory assumes that all the heavy, radioactive elements were created "in the beginning" or at least prior to the formation of the Earth -- and were assembled as terrestrial elements as the planet formed. Gentry's studies seemed to indicate that radionuclide-creation in some instances occurred mere minutes before those nuclides became trapped in rocks solidifying to form parts of the Earth's crust. If so, they must have been created on Earth long after the planet itself was formed.
In the years to follow, Gentry was to turn up much more radiohalo evidence pointing to this same conclusion. And most of this additional evidence is much more straightforward, not depending on speculative identifications of halo-inclusion nuclides belonging to unknown decay series.
Early in 1967 Gentry reported his observations of certain intermediate-sized "Y halos" found in biotite of supposedly Pre-Cambrian age from Canada.(15 ) The most likely parent nuclide for these radiohalos, he concluded, was bismuth-211, an isotope with a half-life of only 2.15 minutes.
In another paper that same year, Gentry described halos due to polonium nuclides of atomic weights 210, 212, 214, and 218, all of short half-lives.(16) Although these nuclides are ordinarily members of familiar uranium and thorium decay series, none of the halos reported gave any evidence that the usual polonium precursor elements had ever been present in their central inclusions, or "radiocenters," as Gentry was later to call them.
In a less-technical article published in Medical Opinion and Review,(17) Gentry discussed at some length the problem posed by the "parentless-polonium" halos. Earlier workers, in attributing the halos to polonium and noting the puzzling absence of uranium and thorium from the radiocenters, had speculated that polonium had precipitated from uranium-bearing solutions flowing in microscopic channels through the mineral grains; this neatly sidestepped the unthinkable idea of parentless polonium. But Gentry's examination of many of these halos had turned up no sign whatever, either of cracks or conduits in which solutions might once have flowed, or of the staining characteristic of such flow. More importantly, "the absence of uranium fission tracks [which are always present around uranium radiocenters] after the mineral was etched with hydrofluoric acid demonstrated that there was no uranium."
The studies again had disclosed "overwhelming evidence for the presence of several different types of short-half-life radioactivity in crustal rocks around the world . . . Only elements with extremely long half-lives could have survived the time span from nucleosynthesis to a period several billion years later when the crustal rocks of the earth finally solidified. Yet, the polonium and other variant halos of very short half-life existed side by side with the uranium and thorium halos in the same piece of mica. . ."
By 1970 Gentry could report finding additional halos that seemed to require explanation in terms of "unknown alpha radioactivity which may no longer be present in the earth's crust"(18) or perhaps even "superheavy elements."(19)
New evidence that many polonium halos were "parentless" was reported in 1971.(20) This was the discovery that an overabundance of radiogenic lead Pb-206, which would be predictable for parentless polonium halos, was actually present in polonium radiocenters. When a decay series starts with U-238, a parallel decay series stemming from U-235 also gets under way: the two uranium isotopes are always found together in nature. The U-238 ultimately decays to Pb-206, while the U-235 decays to Pb-207. Thus, inclusions initially containing uranium should later contain the two leads, mixed in proportions that will depend upon how long uranium-decay has been in progress, as well as on the initial ratio of U-238 to U-235 in the inclusion. (Since this initial ratio for all occurrences of the two uraniums is assumed to be a known, fixed ratio,(21) geochronologists have come to put great faith in uranium-lead dating techniques.) Assuming that none of the daughter products of either decay chain is lost from the mineral under study, the ratio of Pb-206 to Pb-207 will vary from about 20 for a very young mineral to about 4 for a mineral more than three billion years old. Gentry points out that "the theoretical maximum possible radiogenic 206 Pb/207 Pb ratio, on the basis of an instantaneous production of Pb from U decay, would be 21.8.”(22) Yet in polonium radiocenters Gentry found that the ratio ranges from about 20 to as high as 60.
Such high values are explainable only if parentless polonium decayed to produce most of the Pb-206.
Thus, in the course of research spanning little more than half a dozen years, Gentry had cast doubt on the theoretical foundations of radiometric dating by demonstrating that the constancy of decay rates cannot be proved by radiohalos, and he had turned up strong evidence that seemed to imply an inconceivable process -- the crystallization of terrestrial rocks within minutes of the creation of radioelements trapped within them.
Such disturbing results could hardly be ignored or left unchallenged. In 1973, three Wayne State University (Michigan) professors sought to deflate Gentry's findings. (23 ) These investigators -- two physicists and a geologist - frankly stated what they saw as the need for their challenge: The existence of polonium halos, unsupported by uranium-decay, would cause "apparently insuperable geological problems . . ." But their effort was not up to the task, and Gentry turned their attack aside with little difficulty.(24)
The problem remains. And again it seems, to judge from the silence that has settled over the subject, that another in a long list of geological "anomalies" is in danger of being swept under the rug.
Can Velikovsky's work throw light on this problem? I believe so. Furthermore, it seems entirely reasonable to suspect that Gentry's studies of radiohalos have unearthed strong, new support for the thesis of Worlds in Collision.
First, consider Velikovsky's conclusion that interplanetary electric discharges in historical times produced a certain amount of element building (nuclear fusion) on Earth. (The possibility that similar discharges to the Moon may be held responsible for concentrations of radioactivity on that body has been discussed elsewhere.(25)) Might we not imagine that new polonium (and uranium, too, for that matter) was created on Earth, and in place, by powerful thunderbolts?
It seems unnecessary to speculate that all parentless polonium must date from recent (historical) electrical events. If fusion was achieved catastrophically in historical times, it undoubtedly was achieved in earlier times as well, perhaps under comparable circumstances or perhaps on vastly larger scales. The point to be considered is that electric discharges of cosmic proportions should be capable of creating new elements; even atmospheric lightning is credited with producing radionuclides,(26) and all artificial element-creation starting with the first fusion reaction ever achieved in the laboratory producing technetium from molybdenum, in 1937 -- has involved harnessing the forces of the electric discharge.(27)
Of course, creating parentless polonium (or any other heavy element) is one thing; getting it into crustal rocks before it decays seems quite another. But here, too, we may be able to draw clues from Velikovsky. The same electrical disturbances that led to interplanetary arcing and sparking may have temporarily prolonged the half-lives of heavy elements that now decay in mere moments.
All matter, as far as we know, is electrically constituted. Atomic nuclei may be said to be "constituted" in spite of electrical forces tending to disrupt them; but electrical forces are there to be reckoned with. If all this is true, what role might environmental electrification play in setting the rules for nuclear stability, radioactive-decay rates, and energies of particle-emissions in decay processes?
"If, according to our present interpretation, the elements are made up of electropositive and electronegative particles, the combinations which these particles will form will depend upon the relative numbers of the two kinds. We suppose that every possible stable grouping of these ultimate (for our purpose) particles will exist. Some will be very stable and some will be upon the verge of instability. The most stable groups will be the most numerous. A relatively small change in the total number of either electrons or protons will cause some of the elements to become unstable . . ." [emphasis added].
These words were written almost 50 years ago by Fernando Sanford, then professor of physics, emeritus, at Stanford University -- a highly respected physicist who happened to stray from doctrinaire paths thanks to a penchant for letting evidence take precedence over preconception. The quoted passage is part of an inquiry as to "what phenomena should be observed on an electrified planet which would not appear on an unelectrified one"; in this passage he is describing conditions that actually prevail on Earth today and help to demonstrate its intense electrification.(28)
We may interpret Sanford's reference to the relative numbers of the two kinds of charged particles as a reference to the state of electrification -- the ambient electrical stress -- of the Earth.
Now what if the Earth's state of electrification were altered, even if only temporarily? It would seem to follow that decay rates for radionuclides might well differ radically from today's norms. Polonium isotopes now exhibiting very little stability might then acquire -- briefly, but long enough -- half-lives in keeping with the evidence of the Earth's crustal rocks.
This is speculation, certainly, but speculation that might be checked experimentally by actually subjecting radionuclides to unaccustomed electrical stress and seeing whether or not they behave differently.
It may also be highly significant that radiohalos are found only in plutonic rocks -- granites, pegmatites, and the like -- and in metamorphosed rocks in close association with plutonic rocks. These rocks are themselves the subject of a centuries-old debate as to their origin.(29) At issue is what used to be called "the granite problem," an intriguing mystery much too complex to discuss here, but whose ultimate solution may also require consideration of physical forces well-known today but seldom, if ever, mentioned in the entire course of training of a professional geologist.(30)
In their continuing searches for "the oldest" materials to be found in the crust of the Earth, the finding of which brings publicity and prestige, geochronologists ardently seek samples of rock of plutonic origin from every continent. Yet so long as the origins of such rocks evade understanding, so long as doubt remains as to the reality of decay "constants," so long as the evidence for element-creation on Earth is ignored, and so long as geophysicists fail to examine the full spectrum of physics, all announcements of “success" in this endeavor must remain questionable. And in the meantime, the explanation of discrepant radiohalos seems to require consideration of Earth history in terms of cosmic quietude interrupted ftom time to time by episodes of Velikovskian violence.
NOTES AND REFERENCES
1. Glossary of Geology and Related Sciences (Washington: American Geological Institute, 1960), p. 225.
2. In the late 1960's R. V. Gentry referred to pleochroic halos as radioactive halos, but by 1971 (Science 173, 727) he had simplified the term to radiohalos.
3. E. Rutherford, Radioactive Transformations (New York: Charles Scribner's Sons, 1906); quoted here from excerpts republished in Source Book in Geology 1900-1950, K. F. Mather, ed., (Cambridge: Harvard University Press, 1967), 115-119.
4. B. Boltwood (as cited by Rutherford), Philosophical Magazine, April 1905; American Journal of Science, October 1905.
5. Cf., H. Faul, Ages of Rocks, Planets, and Stars (New York: McGraw-Hill, 1966), pp. 21-22.
6. J. Joly, Nature 109 (1922), 480.
7. O. Struve, "Finding the Age of the Earth," Sky & Telescope 18 (June 1959), 433-435.
8. G. Gamow wrote in his book, A Star Called the Sun (New York: Bantam Books, 1965, p. 111): "Using wave mechanics, the author was able to explain the 'paradox' of alpha decay of radioactive nuclei, and to give a perfect [sic] theoretical interpretation of the dependence between the energies of the emitted alpha particles and the half-lives of . . . elements . .
9. CL, 0. Struve, op. cit.
10. R. Gentry, "Cosmology and Earth's Invisible Reahn," Medical Opinion & Review (October 1967), 65-79.
11. R. Gentry, American Journal of Physics 33 (1965), 878.
12. R. Gentry, Science 184 (5 April 1974), 62-66.
13. R. Gentry, Applied Physics Letters 8 (1 February 1966), 65-67.
14. (As cited by Gentry): E. Wiman, Bulletin, Geological Institute, University of Uppsala 23 (1930), 1; also H. Hirschi, ORNL-tr-702.
15. R. Gentry, Nature (February 4, 1967), 487-489.
16. R. Gentry, abstract in American Physical Society Bulletin 12 (1967), 32.
17. R. Gentry, Medical Opinion & Review (October 1967), 65-79.
18. R. Gentry, abstract in EOS, Transactions, American Geophysical Union 51 (April 1970), 338.
19. R. Gentry, Science 169 (14 August 1970), 670-673. Quite recently Gentry's name has been prominent in on again-off again reports of finding evidence for extinct superheavy elements; see, e. g., reports by A. L. Robinson, Science 193 (16 July 1976), 219-220 and Science 195 (4 February 1977), 473-474.
20. R. Gentry, Science 173 (20 August 1971), 727-731.
21. The abundances of U-235 and U-238 are such that about 7 of every 1000 uranium atoms found on Earth are U-235; the remaining 993 are U-238. TMs, within very narrow limits, is an observational fact.
22. R. Gentry, op. cit.
23. C. Moazed, R. M. Spector, and R. F. Ward, Science 180 (22 June 1973), 1272-1274.
24. R. Gentry, Science 184 (5 April 1974), 62-66.
25. R. Juergens, Pensee, Vol. 4, No. 3, Summer 1974,4546.
26. It has been suggested that lightning may be held accountable for the creation of carbon14 and introducing an unknown into radiocarbon-dating schemes.
27. AR of man's devices for accelerating ions to fusion energies are basically modeled after the capabilities of natural electric discharges.
28. F. Sanford, Terrestrial Electricity (Palo Alto: Stanford University Press, and London: Oxford University Press, 1931; reproduced in 1967 by microfilm-xerography by University Microfilms, Ann Arbor), Chapter III: "Electrical Charges of the Earth and the Sun."
29. Cf., M. Walton, Science 131 (4 March 1960), 635-645; see also references cited therein. In the near future I plan to prepare a paper suggesting an answer to W. R Bucher's question ("The Crust of the Earth," Scientific American, May 1950): "Where did the granite come from, and where did the sediments [that formerly occupied regions now filled with granite] go?" The question is one that has never yet been answered satisfactorily, and I believe that the explanation for this puzzling situation, as well as for the fact that the Earth's radioactive elements are concentrated in granitic rocks, is to be found in granite origins as a result of electrical breakdown of pre-existing sediments during Earth catastrophes of the kind described by Velikovsky.
30. I refer, of course, to the forces pertaining to electrical science.