Abstract. The discovery of embryonic halos around uranium-rich sites that exhibit very high ²³⁸U/²⁰⁶Pb ratios suggests that uranium introduction may have occurred far more recently than previously supposed. The discovery of ²¹⁰Po halos derived from uranium daughters, some elliptical in shape, further suggests that uranium-daughter infiltration occurred prior to coalification when the radionuclide transport rate was relatively high and the matrix still plastically deformable.

Even though the biological fossil record has been extensively documented, the rather abundant fossil record of radiohalos that exists in the coalified wood from the Colorado Plateau has remained virtually undeciphered. Jedwab (1) and Breger (2) have determined some important characteristics of such halos; in fact, earlier (1, 2) as well as present investigations on these samples (3) agree that: (i) the microscopic-size radiocenters responsible for halos (Fig. 1a) in coalified wood are actually secondary sites that preferentially accumulated α-radioactivity during an earlier period of earth history when uranium-bearing solutions infiltrated the logs after they had been uprooted; (ii) although autoradiography shows some α-activity dispersed throughout the matrix (1, 2), most of it is still concentrated in the discrete halo radiocenters; (iii) variations in coloration among radiohalos cannot necessarily be attributed solely to differences in the α-dose because there is evidence that the coalified wood was earlier far more sensitive to α-radiation than at present (1); (iv) halos that appear most intensely colored in unpolarized transmitted light also show evidence of induration; that is, when polished thin sections of coalified wood are viewed with reflected light (Fig. 1b), such high α-dose halos exhibit high reflectivity and pronounced relief; and (v) some areas of coloration are of chemical rather than radioactive origin (1).

In addition to the above verifications, the studies reported here mark the first time that (i) radii measurements have been made to determine the type and stage of development of halos in coalified substances and (ii) the radiocenters of such halos have been analyzed by modern analytical techniques. The discoveries reported herein raise questions relative to when U was introduced into the wood, the duration required for coalification, and the age of the geological formations.

Specifically, it was discovered that the halos (Fig. 1a) surrounding the α-active sites are typically embryonic, that is, they do not generally exhibit the outer ²¹⁴Po ring characteristic of fully developed U halos in minerals (4). Such underdeveloped halos generally imply a low U concentration in the radiocenter. However, electron microprobe x-ray fluorescence (EMXRF) analyses (Fig. 2a) show many such radiocenters contain a large amount of U with the amount of daughter product Pb being generally too small to detect by EMXRF techniques (Fig. 2a). Although we discuss below the application of ion microprobe mass spectrometer (IMMA) techniques (5) to the problem of quantitatively determining the ²³⁸U/²⁰⁶Pb ratios, two important points deserve mention here: (i) if there was only a one-time introduction of U into the wood (2), these radiocenters date from that event unless subsequent mobilization of U occurred, and (ii) if U was introduced prior to coalification (1), then the ²³⁸U/²⁰⁶Pb ratios in these radiocenters also relate to the time of coalification.

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 E_α = 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, t_1/2 = 138 days) and its β-precursor ²¹⁰Pb (t_1/2 = 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).