Radiohalos in Coalified Wood: New Evidence Relating to
Fig. 1. (a) Coalified wood halos with U radiocenters in transmitted light (× 125) [see (7)]. (b) The same halos in reflected light. The bright central spot in each halo is the radiocenter (× 125)
Another class of more sharply defined halos was discovered possessing smaller inclusions ( 1 to 4 μm in diameter) than the α-active sites. These inclusions exhibit a distinct metallic-like reflectance when viewed with reflected light. Three different varieties of this halo exist: one with a circular cross section, another with an elliptical cross section with variable major and minor axes, and a third most unusual one that is actually a dual halo, being a composite of a circular and an elliptical halo around exactly the same radiocenter (see Fig. 3, a to c).
Although the elliptical halos differ radically from the circular halos in minerals (6), the circular type resembles the 210Po halo in minerals and variations in the radii of circular halos approximate the calculated penetration distances ( 26 to 31 μm) of the 210Po α-particle (energy Eα = 5.3 Mev) in this coalified wood (7). Henderson (8) theorized that Po halos might form in minerals when U-daughter Po isotopes or their β-precursors were preferentially accumulated into small inclusions from some nearby U source. Although this hypothesis was not confirmed for U-poor minerals (9), it did seem a possibility in this U-rich matrix.
The EMXRF analyses (Fig. 2b) showed that the halo inclusions were mainly Pb and Se. This composition fits well into the secondary accumulation hypothesis for both of the U-daughters, 210Po (half-life, t1/2 = 138 days) and its β-precursor 210Pb (t1/2 = 22 years), possess the two characteristics that are vitally essential for the hypothesis: (i) chemical similarity with the elements in the inclusion and (ii) half-lives sufficiently long to permit accumulation prior to decay. This latter requirement is dependent on the radionuclide transport rate. In minerals the diffusion coefficients are so low that there is a negligible probability that 210Po or 210Pb atoms would migrate even 1 μm before decaying, and thus the origin of Po halos in minerals is still being argued (6, 10).
However, in this matrix the situation is quite different. A solution-permeated wood in a gel-like condition would exhibit a much higher transport rate as well as unusual geochemical conditions which might favor the accumulation of 210Po and 210Pb nuclides. Evidence that this accumulation was essentially finished prior to complete coalification comes from the fact that most Po halos are plastically deformed; furthermore, after coalification it is much more difficult to account for such rapid and widespread migration of the radionuclides (that is, within the 210Po half-life). For example, a hundred or more 210Po halos are sometimes evident in a single thin section (2 cm by 2 cm) of coalified wood, and they occurred quite generally in the thin sections examined (11). Of the thousands of Po halos seen in this matrix, only three show any trace of a ring that could possibly be attributed to 214Po α-decay [that is, from the accumulation of the U-daughters 214Pb (t1/2 = 27 minutes), 214Bi (t1/2 = 20 minutes), or 214Po (t1/2 = 164 μsec)], and none has been seen with a ring from 218Po α-decay [that is, from the accumulation of short-lived 218Po (t1/2 = 3 minutes)]. (Possibly these faint outer rings are of chemical rather than radioactive origin.)
Positive identification for the 210Po halos comes from the IMMA analyses. Compared to a 238U halo radiocenter. a 210Po halo inclusion should contain much less 238U (perhaps none at all) and much more of the 210Po decay product 206Pb. The IMMA analyses of Po halo inclusions showed that the 238U content was low, the 238U/206Pb ratios varying from 0.001 to 2.0. [These values were corrected for the different ionization efficiencies (~ 2 : 1) of Pb+ and U+ in this matrix.] This small 238U content implies that only an extremely small amount of Pb could have been generated by in situ U decay. There are certainly three other possible sources for the Pb in these inclusions: (i) common Pb, (ii) Po-derived radiogenic Pb generated by in situ decay of secondarily accumulated 210Pb and 210Po, or (iii) U-derived "old" radiogenic Pb that had accumulated in the hypothesized (12) Precambrian U ore deposit (which is one possible source of the U now in the Colorado Plateau) prior to the time it was carried with the U in solution into the wood. Since the 204Pb count rates, which are unique indicators of common Pb, ranged from undetectable to a few counts per second above background when 206Pb count rates were several thousand counts per second, it was evident that relatively little common Pb was present. Thus only 206Pb/207Pb ratios had to be measured to obtain evidence of 206Pb originating from the decay of 210Po: the results were indeed confirmatory.
Fig. 2. Curve a, EMXRF spectrum of a U-rich radiocenter. Curve b, EMXRF spectrum of the radiocenter of a 210Po halo.
The ratios obtained were as follows: 206Pb/207Pb = 8 ± 0.5, 11.6 ± 0.3, 11.7 ± 0.4, 13.3 ± 0.7, 13.4 ± 1.0, 13.7 ± 0.6, 13.9 ± 0.6, 14.8 ± 0.9, 15.8 ± 1.1, and 16.4 ± 0.5. The variation in this ratio can easily be understood to have resulted from the addition of an increment of 206Pb (generated by in situ 210Po decay) to the isotopic composition of the "old" radiogenic Pb. The lowest Pb ratio, obtained from a very lightly colored 210Po halo, differs slightly from the lowest Pb isotope ratio previously determined on bulk samples of Colorado Plateau U ore specimens (12).
What is the meaning of these Po halos? Clearly, the variations in shape can be attributed to plastic deformation which occurred prior to coalification. Since the model for 210Po formation thus envisions that both 210Po and 210Pb were accumulating simultaneously in the Pb-Se inclusion, a spherical 210Po halo could develop in 0.5 to 1 year from the 210Po atoms initially present and a second similar 210Po halo could develop in 25 to 50 years as the 210Pb atoms more slowly α-decayed to produce another crop of 210Po atoms. If there was no deformation of the matrix between these periods, the two 210Po halos would simply coincide. If, however, the matrix was deformed between the two periods of halo formation then the first halo would have been compressed into an ellipsoid and the second halo would be a normal sphere. The result would be a dual "halo" (Fig. 3c). The widespread occurrence of these dual halos in both Triassic and Jurassic specimens (13) can actually be considered corroborative evidence for a one-time introduction of U into these formations (1, 2), because it is then possible to account for their structure on the basis of a single specifically timed tectonic event. The fact that dual halos occur in only about 1 out of 100 single Po halos is of special significance (14).
In halos with U radiocenters, the low Pb abundance made it generally quite difficult to measure U/Pb ratios with EMXRF (Fig. 2a) techniques. More sensitive IMMA measurements on these U radiocenters revealed 238U/206Pb ratios (15) of approximately 2230; 2520; 8150; 8300; 8750; 18,700; 19,500; 21,000; 21,900; and 27,300 (again corrected for different ionization efficiencies). Typically, the U+ ion signals from which these ratios were derived were greater than 3 × 104 counts per seconds (cps); for example, the 19,500 value was obtained from a halo with a U+ signal of 106 cps (± 5 percent) with background 3 cps. We checked the 238U/235U ratio independently (and found it normal) by excising several radiocenters and analyzing them directly on the filament of a high sensitivity thermal ionization mass spectrometer (16).
Even without attempting to subtract out the 206Pb component of the common and "old" radiogenic Pb (15), these 238U/206Pb ratios raise some questions. For example, if the 238U/206Pb = 27,300 value is indicative of the formation time of the radiocenter, this is more recent by at least a factor of 270 than the minimum (Cretaceous) and more recent by a factor of 760 than the maximum (Triassic) geological age estimated for the introduction of U into the logs (12, 17, 18). To obtain 238U/206Pb ratios that more accurately reflect the amount of Pb from in situ U decay, a search was made for sites with even higher ratios, for such areas possibly contained negligible amounts of extraneous Pb. Two halo radiocenters were found that exhibited 238U+ signals of 4 × 104 and 6.4 × 104 cps, respectively while the 206Pb+ signals were indistinguishable from background ( 3 cps) in both cases (207Pb also absent).
Such extraordinary values admit the possibility that both the initial U infiltration and coalification could possibly have occurred within the past several thousand years. At the same time it may be argued that this view is quite improbable for there exists another explanation that could invalidate the association of the U/Pb ratios with the initial introduction of U. This explanation would admit that, although Po halos constitute evidence that U infiltration and hence U radiocenter formation occurred prior to coalification, some U may have been added or Pb may have been selectively removed, or both, by groundwater circulation after coalification. Hence variable U/Pb ratios would be expected, and the highest ratio would simply reflect the last time when U remobilization or Pb remobilization, or both, occurred. Although this hypothesis has been used to account for U disequilibrium (18, 19) in bulk specimens of U-impregnated Colorado Plateau material, there are some questions about its applicability here.
For example, if Pb was removed from the U sites, it must have been a very selective removal for both the EMXRF and IMMA results show that considerable quantities of Pb still remain in the nearby (within 50 μm of the U sites) Po halo Pb-Se inclusions. If Pb loss was minimal, then to explain the high 238U/206Pb ratios by remobilization requires that significant quantities of U were introduced into the U radiocenters quite recently. In any event, whether the hypothesis is U addition or Pb removal, the crucial point that seems quite difficult to explain under either assumption is the fact that, in general, the halos around U sites are embryonic (20). That is, since it seems clear that the U radiocenters formed during the initial introduction of U and if this were as long ago as the Triassic or Jurassic are generally thought to be, then there should be evident not only fully developed, but overexposed U halos as well (21).
Clearly, it was important to determine whether these phenomena were characteristic only of the U-rich Colorado Plateau coalified wood (2, 3). We therefore initiated studies on coalified wood fragments which are occasionally found in the Chattanooga shale (3, 11, 22). Thus far only embryonic halos have been seen, and the 238U/206Pb ratios are much too high (>103) to correlate with the geological age of the formation (Devonian). The low U content of the Chattanooga shale (1 to 50 parts per million) makes it quite difficult to see how U remobilization could account for these very high isotope ratios. Thus the evidence does not appear to support the remobilization hypothesis as a general explanation of these unusual 238U/206Pb ratios in either the Colorado Plateau or Chattanooga shale specimens.
Fig. 3. (a) Circular 210Po halo (× 250). (b) Compressed 210Po halos (× 250). (c) Circular and compressed 210Po halo (× 250).
If remobilization is not the explanation, then these ratios raise some crucial questions about the validity of present concepts regarding the antiquity of these geological formations and about the time required for coalification. Finally, in addition to again focusing attention on the question of the origin of Po halos in minerals (6, 10), the existence of U-derived single and dual Po halos in different formations suggests that the original source of U may have been a Precambrian ore deposit that was geographically not far removed from the present Colorado Plateau. Thus, in view of America's energy requirements, it might be profitable to search for such an ore deposit by deep drilling into selected areas around and within the Colorado Plateau.
|ROBERT V. GENTRY|
Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37830
|WARNER H. CHRISTIE|
DAVID H. SMITH
J. F. EMERY
S. A. REYNOLDS
|Analytical Chemistry Division,|
Oak Ridge National Laboratory
|S. S. CRISTY|
|Laboratory Development Division,|
Oak Ridge National Laboratory
|P. A. GENTRY|
|Columbia Union College,|
Takoma Park, Maryland 20012