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 (t_1/2 = 27 minutes), ²¹⁴Bi (t_1/2 = 20 minutes), or ²¹⁴Po (t_1/2 = 164 μsec)], and none has been seen with a ring from ²¹⁸Po α-decay [that is, from the accumulation of short-lived ²¹⁸Po (t_1/2 = 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; 8l50; 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).