Creation's Tiny Mystery
Appendix: "Differential Lead Retention in Zircons"
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For example, Ringwood (11, 12) has
suggested that highly radiation-damaged minerals that have successfully
retained U, Th, and Pb (13) over a significant fraction of earth
history might also serve to immobilize high-level nuclear waste in synthetic
rock (SYNROC) containers, which could be buried in deep granite holes. Even
though zircons are not envisioned as part of Ringwood's special type of
synthetic rock waste container, our results are relevant since they show that
Pb, which is much more mobile in zircons than U and Th (12, 14), has
been highly retained at depths (960 to 4310 m) which more than span the
proposed burial depths (1000 to 3000 m) for synthetic rock containers in
granite (11). The inclusion of this elevated temperature effect in our
samples means that our results provide data which have heretofore been
unavailable in support of nuclear waste containment in deep granite. In
addition, the contamination-free method we used to analyze the zircons for
radiogenic Pb may prove valuable in searching for other minerals suitable for
synthetic rock waste containment.
Because it has been suggested that temperatures
in the granite formation are rising (15), we do not know precisely how
long the zircons have been exposed to the present temperatures. However, by
using diffusion theory and the measured diffusion coefficient of Pb in zircon
(16), we can estimate future loss of Pb by diffusion in synthetic
rock-encapsulated zircons buried at the proposed depths of 1000 to 3000 m (11)
if we assume a temperature profile similar to that in the drill holes. At a burial
depth of 3000 m (~ 200°C), we calculate that it would take 5 × 1010
years for 1 percent of the Pb to diffuse out of a 50-μm crystal. At 2200 m (~
150°C) it would take 7.4 × 1013 years, and at 1000 m (~ 100°C) it
would take 7.7 × 1017 years for 1 percent loss to occur (16).
Since all these values greatly exceed the 105 to 106
years estimated for waste activity to be reduced to a safe level (11), and
since, as noted earlier, U and Th are bound even more tightly than Pb in
zircons (12, 14), our results appear to lend considerable support to
the synthetic rock concept of nuclear waste containment in deep granite
Robert V. Gentry*|
Thomas J. Sworski
Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37830
|Henry S. McKown|
David H. Smith
R. E. Eby
W. H. Christie
|Analytical Chemistry Division,|
Oak Ridge National Laboratory
References and Notes
A. W. Laughlin and A. Eddy, Los Alamos Sci. Lab. Rep. LA-6930-MS (1977).
A. W. Laughlin provided the core samples used in this work.
R. Laney and A. W. Laughlin, Geophys. Res. Lett. 8, 501 (1981).
D. H. Smith, W. H. Christie, H. S. McKown, R. L. Walker, G. R. Hertel, Int. J. Mass Spectrom. Ion Phys.
10, 343 (1972).
W. H. Christie and A. E. Cameron, Rev. Sci Instrum. 37, 336 (1966).
A. E. Cameron, D. H. Smith, R. L. Walker, Anal. Chem 41, 525 (1969).
D. H. Smith, W. H. Christie. R. E. Eby, Int. J. Mass Spectrum, Ion Phys. 36, 301 (1980).
O. Kopp and H. McSween of the Department of Geological Sciences, University of Tennessee, Knoxville, provided
zircons from Gjerstad, Norway; Oaxaca, Mexico; and Henderson County, North Carolina.
R. E. Zartman, Los Alamos Sci. Lab. Rep. LA-7923-MS (1979).
If the 204
Pb in Zartman's (8
) Pb isotopic abundances in his zircons is attributed to common
lead, the corrected 206
Pb ratio for the zircons from 2900 m is 11.03.
This criterion resulted in the rejection of four single zircon analyses whose average
206Pb/207Pb ratio was 8.8 ± 1.3. These lower ratios
imply that some zircons contain more
initial Pb than others, as noted in some other runs.
A. E. Ringwood, Safe Disposal of High Level Nuclear Reactor Wastes: A New Strategy (Australian
National Univ. Press, Canberra, 1978).
A. E. Ringwood, K. D. Reeve, J. D. Tewhey, in Scientific Basis for Nuclear Waste Management, J. G. Moore,
Ed. (Plenum, New York, 1981), vol. 3, p. 147.
V. M. Oversby and A. E. Ringwood, J. Waste Manage., in press. See also A. E. Ringwood, Lawrence
Livermore Natl. Lab. Rep. UCRL-15347 (1981).
R. T. Pidgeon, J. O'Neill, L. Silver, Science 154,
1538 (1966); Fortschr. Mineral. 50, 118
D. G. Brookins, R. B. Forbes, D. L. Turner, A. W. Laughlin, C. W. Naeser, Los Alamos Sci. Lab. Rep.
In general, if R
is the gas constant, T
is the absolute
temperature, and D
respectively, the diffusion coefficient and activation energy of a certain nuclide in a given diffusing medium,
= D0 e−Q/RT
temperature-independent parameter. In particular, if C0
is the initial concentration of that
nuclide within a sphere of radius a
, then the average nuclide concentration
within that sphere at some later time t
[see L. O. Nicolaysen, Geochim. Cosmochim. Acta 11
We used measured values of D0
2.2 × 10−2
kcal/mole for diffusion of Pb in zircon [see Sh. A. Magomedov, Geokhimiya 2,
263 (1970)] and a computer program to calculate the times when
= 0.99 for T
100°, 150°, and 200°C.
Research sponsored by the U.S. Department of Energy, Division of Basic
Energy Sciences, under contract W-7405-eng-26 with the Union Carbide
* Visiting scientist from Columbia Union
College, Takoma Park, Md. 20112.
3 November 1981; revised 22 January 1982
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