Darth> Check my reply to JediHDM's post in the evolution thread... It should answer "it"... sighs...
To fully understand Gentry's hypothesis a basic background in geology, mineralogy, and radiation physics is helpful. The boxes on the next few pages present a brief tutorial in rocks, minerals, and radioactivity. Certain minerals, such as zircon and monazite, which form as common trace constituents in igneous rocks, have crystal structures which can accommodate varying amounts of the naturally occurring radioactive elements, uranium and thorium. When these minerals occur as inclusions in certain other minerals, most notably the mica family, they are often seen to develop discoloration, or "pleochroic" haloes. The haloes are caused by radiation damage to the host mineral's crystalline structure. Figure 1 shows a typical discoloration halo around a radioactive mineral inclusion in the mineral pyroxene. The zone of damage is roughly spherical around a central mineral inclusion or radioactive source. Note that the halo has the highest intensity of discoloration near the source, gradually fading with distance in the host mineral to a "fuzzy" edge.
Radiation damage haloes around mineral inclusions are well known from the geological literature. Discoloration haloes in younger rocks tend to be smaller and less intense than in older rocks, indicating that the zone of crystal damage increases with time. From these observations early attempts were made to use the dimensions of haloes as an age dating technique. This was never fully successful as the size/intensity of an observed damage halo was also a function of the abundance of radionuclides present in the inclusion, and the crystalline structure of the host mineral.
Gentry's thesis has several components. First is his contention that the granitic rocks from which samples reportedly came constitute the "primordial" crust of the Earth. Within these rocks are biotite (an iron-bearing form of mica) and fluorite crystals which bear a relatively uncommon class of tiny, concentric discoloration "haloes" (figure 2). These haloes were considered to be the result of damage to the crystal structure of the host minerals caused by high energy alpha particles. In numerous papers published in scientific journals in the 1970s and 1980s, Gentry built the case that the different alpha decay energies of various naturally occurring radioactive isotopes resulted in distinctly different halo diameters. Thus, Gentry concluded that he could distinguish haloes resulting uniquely from the radioactive decay of various isotopes of the element polonium. Polonium, part of the decay chain of natural uranium and thorium, has a very short half-life - measured in microseconds to days, depending on the specific isotope. Concentric haloes associated with polonium decay - but without any rings corresponding to any other uranium decay series isotopes were taken to be evidence that the host rock had formed almost instantly rather than by the slow cooling of an original magma over millions of years. Gentry extrapolates that all Precambrian granites - his primordial crustal rock - must have formed in less than three minutes, and that polonium haloes are therefore proof of the young Earth creation model according to Genesis.
1) Do the rocks from which Gentry drew his samples represent the "primordial" basement rocks of the originally created Earth?
Gentry is a physicist, not a geologist. He doesn't follow accepted geologic reporting practice and consistently fails to provide the information that a third party would need to collect comparable samples for testing. For his research, Gentry utilized microscope thin sections of rocks from samples sent to him by others from various places around the world. Thus, he is unable to say how his samples fit in with the local or regional geological setting(s). He also does not provide descriptive information about the individual rock samples that make up his studies - i.e., the abundance and distribution of major, accessory, or trace minerals; the texture, crystal size and alteration features of the rocks; and the presence or absence of fractures and discontinuities.
Gentry does not acknowledge that the Precambrian time period represents fully 7/8 of the history of the Earth as determined by decades of intensive field and laboratory investigations by thousands of geologists. Consequently, he does not recognize the wide diversity of geologic terranes that came and went over that enormous time span. His claim that his samples represent "primordial" basement rocks is patently incorrect . In Gentry's model, any rock looking vaguely like a granite and carrying the label Precambrian is considered to be a "primordial" rock. True granites are themselves evidence of significant crustal recycling and elemental differentiation (see for example, Taylor and McLennan, 1996), and cannot be considered primordial. A little detective work by Wakefield (1988) showed that at least one set of rock samples studied by Gentry are not from granites at all, but were taken from a variety of younger Precambrian metamorphic rocks and pegmatite veins in the region around Bancroft, Ontario. Some of these rock units cut or overlie older, sedimentary and even fossil-bearing rocks.
Gentry provides no explanation for how polonium alone finds its way into biotite and fluorite, or why radiation damage haloes in these minerals are common in areas of known uranium enrichment, but rare where uranium abundance is low. Gentry's hypothesis would seem to suggest that there should be a uniform distribution of all polonium isotopes in primordial rocks, or at least no particular spatial association with uranium. Gentry (1974), himself, notes that haloes have not been found in meteorites or lunar samples, rocks known to be very low in uranium abundance. Lorence Collins (1997) has noted these and several other contradictory situations between the polonium halo hypothesis and observed geological relationships in the field.
Polonium haloes in mica are found only in granitic, or granitic-type rocks, and not in mica from adjacent rocks of other compositions
Polonium haloes are found only in rocks which contain myrmekite, a replacement mineral intergrowth - a clear indication that the rock is not "primordial."
2) Are the concentric haloes observed by Gentry actually caused by alpha particle damage to the host crystal structure?
Going back to Gentry's early research (Gentry, 1968, 1971; Gentry, et al., 1973), it is apparent that the association of concentric colored haloes with polonium is actually speculative. Gentry adopts and expands on the work of Joly (1917) that polonium isotopes were the most likely cause of the features observed. Joly did most of his work with discoloration haloes in the first decade of the Twentieth Century, a time when the structure of the atom was just being discovered, and before the crystal structure of minerals had been unraveled. This was also the period when the nature of radioactivity was just being uncovered. Joly made the very speculative assumption that if alpha particles could travel 3-7 centimeters in air, then they would only travel 1/2000 of that distance in biotite mica. From this generalization, and without considering the variability in the density and the crystal structure of the host mica (or even the variable density of air), Joly attempted to correlate the radial size of the concentric ring haloes with the alpha particles of specific isotopes (he was first to suggest polonium). He also tried to develop an age dating technique based on the diameter of the halo features - the larger the halo, the longer the radiation had been affecting the host mineral grain. Henderson (1939) carried Joly's work further, developing a classification scheme for the different patterns of discoloration haloes he observed, and deriving hypotheses for how short-lived polonium could find its way into the host crystal structure.
Here's more explanation... by Tom Baillieul 😉
http://earthfriendarts.tripod.com/evolve/gentry.html
I do have to say, this is the first time I've seen this... I must have gone to a sucky high school too.