Life In The Millennium


URANIUM AND THORIUM RADIOHALOS IN MINERALS



Download 6.3 Mb.
Page31/37
Date08.12.2018
Size6.3 Mb.
1   ...   27   28   29   30   31   32   33   34   ...   37

URANIUM AND THORIUM RADIOHALOS IN MINERALS
A radioactive halo is generally defined as any type of discolored, radiation-damaged region within a mineral and usually results from either alpha or, more rarely, beta emission from a central radioactive inclusion. When the central inclusions, or radiocenters, are small (1 :m), the U and Th daughter alpha emitters produce a series of discolored concentric spheres, which in thin section appear microscopically as concentric rings whose radii correspond to the ranges of the various alpha emitters in the mineral.
Ordinary radiohalos are herein defined as those which initiate with 238U and/or 232Th alpha decay (1), irrespective of whether the actual U or Th halo closely matches the respective idealized alpha decay patterns. In a few instances the match is very good.
Compare, for example, the idealized U halo ring pattern in Fig. 1a with the well developed U halos in biotite (Fig. 1f) and fluorite (Fig. 1h,h'); these halos have ring sizes that agree very well (1,2) with the 4He ion accelerator-induced coloration bands in these minerals (see Table 1). In general a halo ring can be assigned to a definite alpha emitter with confidence only when the halo radiocenter is about 1:m in size.
In other cases, however, such as the halos in fluorite (1,2) shown in Fig. 1(g, i-m), much work was required before these halos could be reliably associated with U alpha decay (2). As explained elsewhere (2), reversal effects accompanying extreme radiation damage caused the appearance of rings that could not be associated with definite alpha emitters of the U decay chain. Thus some halos may exhibit a ring structure different from the idealized U and/or Th alpha decay patterns because of reversal effects. And even though most other halos exhibit blurred ring structures due to the large size of the inclusions, nevertheless the outer dimensions allow them to be classified as U and/or Th types.
Modern analytical techniques such as Scanning Electron Microscope X Ray Fluo-resence (SEMXRF) and Ion Microprobe Mass Spectrometry (IMMA) methods have been utilized to show that U and Th and their respective end-product isotopes of Pb are contained within the U and Th halo radiocenters. As is noted shortly, these modern analytical techniques have proved quite valuable in demonstrating that Po halo radiocenters in minerals contain little or no U or Th, which is in direct contrast to the abundance of these elements detected in the U and/or Th halo radiocenters (2,3).
RADIOACTIVE HALOS AND THE QUESTION OF INVARIANT DECAY RATES
A most important question pertaining to the evolution/creation issue is whether radioactive decay rates have remained invariant during the course of earth history. If they have, geochronologists are justified in interpreting various parent/daughter isotope ratios found in undisturbed rocks in terms of elapsed time. If on the other hand there have been periods in earth history where the decay rate was higher (i. e., during a singularity), then in general the isotope ratios in rocks would not reflect elapsed time except in the specific case where secondary rocks or substances containing only the parent radio-nuclide formed at the end of the most recent singularity. The practical significance of this last statement will be evident in the discussion of the secondary, U halos found in coalified wood specimens from the Colorado Plateau.
Even though most of Joly's (4) measurements of U and Th halos showed their radii were about the sizes expected from the alpha decay energies of the U and Th decay chains, nevertheless he claimed there were slight discrepancies which raised questions about whether the radioactive decay rate had been constant over geological time. His result was not confirmed however by later halo radii measurements (5-10), which agreed to within experimental error with the theoretical sizes. To eliminate any uncertainty about this correspondence I irradiated specimens of various minerals with He ion beams of varying energies to produce different size coloration bands whose widths corresponded to the various alpha energies of the U decay chain. The results of these experiments, presented in Table 1, show there is excellent agreement between the U and Th halo radii and equivalent He ion produced penetration depths (2).
The basis for thinking that standard size U and Th halos imply an invariant decay rate throughout geological time proceeds from the quantum mechanical treatment of alpha decay, which in general shows that the probability for alpha decay for a given nuclide is dependent on the energy with which the alpha particle is emitted from the nucleus. The argument is that if the decay rate had varied in the past, then the U and Th halo rings would be of different size now because the energies of the alpha particles would have been different during the period of change. This argument assumes that a change in the decay rate must necessarily be explainable by quantum mechanics, which is of course an integral part of the uniformitarian framework. Thus, the usual proof of decay rate invariance based on standard size U and Th halos is nothing more than a circular argument which assumes the general uniformitarian principle is correct. In fact, the failure of the uniformitarian principle to explain the evidence for creation presented herein invalidates the basis for the above proof.


POLONIUM, DWARF, AND GIANT HALOS IN MINERALS
Of the three types of unusual halos that appear distinct from those formed by U and/or Th alpha decay, only the Po halos, Fig.1 (b-d, n-r, r’), can presently be identified with known alpha radioactivity (1-3,11-13). Po halos occupy a special niche in my creation model, and these halos will be discussed in more detail subsequently. Several lines of evidence which indicate the enigmatic dwarf halos (see Fig. 2) were produced by some presently unidentified radioactivity have been summarized (1,12,14,15). The rapid etch from HF and the K/Ca inversion are strongly characteristic of highly radiation-damaged regions.
T
he characteristics of the giant halos found in a certain Madagascan mica have also been summarized (1,14,16), and while no definitive evidence as yet exists for a radioactive origin, some halos with opaque inclusions in this same mica exhibit isotopic anomalies which raise questions about the uniformity of U and Th alpha decay. For example, the mass scans and x-ray fluorescence analyses shown in Fig. 3 clearly indicate that, whereas both the monazite and opaque inclusions exhibit 206Pb and 207Pb from U decay, the opaque inclusions exhibit a marked deficiency of 208Pb from 232Th decay (14).


Share with your friends:
1   ...   27   28   29   30   31   32   33   34   ...   37


The database is protected by copyright ©sckool.org 2019
send message

    Main page