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References and Notes

1. J. Jedwab, Coal Science (American Chemical Society, Washington, D.C., 1966).
2. I. A. Breger, in Formation of Uranium Ore Deposits, Proceedings of a Symposium, Athens, 6-10 May 1974 (International Atomic Energy Agency, Vienna, 1974), pp. 99-124.
3. I. A. Breger donated Colorado Plateau coalified wood specimens from the following mines: (i) Jurassic—Peanut and Virgin No. 3, Colorado; Corvusite, Utah; and Poison Canyon, New Mexico; (ii) Triassic—Lucky Strike No. 2, Dirty Devil No. 2, Adams, and North Mesa No. 9, all in Utah; and (iii) Eocene—Docamour, Colorado. J. S. Levinthal provided 16 other specimens. However, only those from the Rajah 49 mine [Salt Wash member of the Morrison Formation (Jurassic)] were sufficiently well preserved to exhibit halos. The Chattanooga shale coalified wood (Devonian), which came from near Nashville, Tennessee, was donated by I. A. Breger and V. E. Swanson. Breger's analysis of this coalified wood yielded 0.001 to 16 percent U, 54 to 84 percent C, 3 to 7.5 percent H, 0.3 to 1.8 percent N, 6 to 38 percent O, and 0.6 to 14.5 percent S. Except where stated, all experimental results refer to work on Colorado Plateau coalified wood (Triassic and Jurassic formations). A thin section of a coalified wood specimen (earlier obtained from I. A. Breger) was provided by J. Jedwab and was used along with Breger's other specimens. Although personal communications with Breger and Jedwab proved of great value, this in no way implies that either Jedwab or Breger necessarily concurs with the results presented here.
4. R. V. Gentry. Annu. Rev. Nucl. Sci. 23, 347 (1973). The halo in Fig. 1a would extend another 20 μm if fully developed.
5. C. A. Andersen and J. R. Hinthorne. Science 175, 853 (1972).
6. R. V. Gentry, ibid. 184, 62 (1974).
7. If the appropriate formulas [G. Friedlander, J. W. Kennedy, J. M. Miller, Nuclear and Radiochemistry (Wiley, New York, ed. 2, 1964), pp. 95-98] are used for computing α-ranges in various solids, the ranges of a 5.3-Mev α-particle in coalified wood [see (3)] of density 1.3 and 1.6 g/cm³ would be 31 and 25 μm respectively. Uniform shrinkage of the matrix could also reduce the radius.
8. G. H. Henderson, Proc. R. Soc. London Ser. A 173, 250 (1930).
9. R. V. Gentry, Science 160, 1228 (1968).
10. ______. Nature (London) 252, 564 (1974); ibid. 258, 269 (1975).
11. This occurrence of Po halos refers to the Colorado Plateau coalified wood.
12. L. R. Stieff, T. W. Stern, R. G. Milkey, U.S. Geol. Surv. Circ. 271 (1953).
13. Dual halos have thus far been found in specimens from the North Mesa No. 9 mine in Utah and the Virgin No. 3 and Rajah 49 mines [see (3)].
14. The coloration pattern of the dual halo provides the key to understanding its rarity. If U with its daughters were concurrently flushed out of some Precambrian ore deposit, even with a relatively short transit time from the ore deposit to the wood, equilibrium conditions still require that more than 50 times as much ²¹⁰Pb as ²¹⁰Po be available for accumulation. If the wood exhibited constant sensitivity to α-induced coloration, then the outer circular halo resulting from ²¹⁰Pb accumulation would be expected to be much darker than the elliptical halo resulting from ²¹⁰Po accumulation. The fact that just the opposite is true is in good agreement with the evidence found by Jedwab [(1) and private communication] indicating that during the U infiltration the gel-like wood exhibited much higher sensitivity to a induced coloration as compared to the later stages of coalification. Possibly then, a relatively dark halo could have formed rather quickly from as few as 10⁴ to l0⁵ Po atoms, whereas some 20 to 50 years later the change in the coloration sensitivity of the matrix might require an α-dose 50 to several hundred times higher from the ²¹⁰Pb decay sequence to produce even a light halo. Thus possibly only in rare cases would the Pb-Se inclusions accumulate large enough quantities of ²¹⁰Pb to subsequently generate the outer circular halo.
15. The variation in the ²³⁸U/²⁰⁶Pb ratios may be attributed primarily to the "old" radiogenic Pb component and secondarily to ²²⁶Ra and ²¹⁰Pb, which, in varying amounts, were also incorporated into the U-rich radiocenters. Evidence for this "old" radiogenic Pb was also found in larger, millimeter-size U-rich regions which also contained varying amounts of Na, Al, K, Ca, Ti, V, Fe, Y, Zr, Ba, and the rare earths. Such regions exhibit variable (but not very high) U/Pb ratios and very little common Pb.
16. D. H. Smith, W. H. Christie, H. S. McKown, R. L. Walker, G. R. Hertel, Int. J. Mass Spectrom. Ion Phys. 10, 343 (1972-1973).
17. R. P. Fischer, in Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Geneva, August 1955 (United Nations, New York, 1956), vol. 6, p. 605; Econ. Geol. 65, 778 (1970).
18. S. C. Lind and C. F. Whittemore, U.S. Bur. Mines Tech. Pap. 88 (1915), p. 1; T. W. Stern and L. R. Stieff, U.S. Geol. Surv. Prof. Pap. 320 (1959), p. 151; J. N. Rosholt, in Proceedings of the Second U.N. International Conference on the Peaceful Uses of Atomic Energy, Geneva, September 1958 (United Nations, New York, 1958), vol. 2, p. 231.
19. Nondestructive γ-ray spectrometry was utilized to check on U disequilibrium in gram-size specimens of the Colorado Plateau coalified wood. We found significant differences in the γ-spectra that could reasonably be attributed to U disequilibrium. By removing microportions of U-rich areas and physically smearing the material onto steel planchets for α-counting, we observed one α-spectra that unambiguously indicated U disequilibrium between ²³⁴U and ²³⁰Th, or ²³⁰Th and ²²⁶Ra, or both. Excess α-activity in the ~ 4.7-Mev region was not attributed to excess ²³⁴U because mass spectrometry measurements on a separate specimen showed an equilibrium ²³⁸U/²³⁴U value.
20. Less than 2.5 percent of the halos with U radio-centers have any trace of an outer ring. It is difficult to associate these with sequential α-decay from ²³⁸U because such weak rings do not correlate with the U content. These weak rings may have resulted from diffusion of α-radioactivity out of the radiocenter prior to induration of the halo region by the α-radioactivity. Alternatively, these weak rings may have resulted from the accumulation of small amounts of ²²²Rn, ²¹⁴Pb, or ²²⁶Ra. In fact, the size of the dark halo region around the U-rich sites admits of the possibility that the inner halos may have formed from the accumulation of minute amounts of ²²⁶Ra or ²¹⁰Pb, or both. Their more diffuse radiocenters, however, would prevent the formation of well-defined boundaries as in the case of the Pb-Se inclusions.
21. This would be true even if coalified wood is only 1/10 as sensitive to α-coloration as biotite.
22. I. A. Breger and J. M. Schopf, Geochim. Cosmochim. Acta 7, 387 (1955); V. E. Swanson, U.S. Geol. Surv. Prof. Pap. 300 (1956), p. 451. J. Jedwab informed me of halos in this material.
23. I thank I. A. Breger, J. S. Levinthal, V. E. Swanson, and J. Jedwab for supplying coalified wood specimens. Research sponsored by the Energy Research and Development Administration under contract with Union Carbide Corporation, and by Columbia Union College under NSF research grant DES 74-23451.

15 September 1975, revised 30 June 1976

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