16 April 1982, Volume 216, pp. 296-298
Copyright © 1982 by the American Association for the Advancement of Science
Differential Lead Retention in Zircons:
Implications for Nuclear Waste Containment
Robert V. Gentry, Thomas J. Sworski, Henry S. McKown, David H. Smith, R.
E. Eby, and W. H. Christie
An innovative ultrasensitive technique was
used for lead isotopic analysis of individual zircons extracted from granite
core samples at depths of 960, 2170, 2900, 3930, and 4310 meters. The results
show that lead, a relatively mobile element compared to the nuclear
waste-related actinides uranium and thorium, has been highly retained at
elevated temperatures (105° to 313°C) under conditions relevant to the burial
of synthetic rock waste containers in deep granite holes.
We report here the measurement
of Pb isotope ratios of whole, undissolved zircons, which were loaded
directly onto the rhenium filament of a thermal ionization mass spectrometer.
This innovation eliminates the Pb contamination introduced in standard
chemical dissolution procedures. By using this technique, we were able to
measure contamination-free Pb isotope ratios on single, microscopic (~ 50 to
zircon crystals, which we estimate contained only ~ 0.2 to 0.5 mg of Pb. We
applied this ultralow-level detection method to study the differential
retention of Pb in zircons (ZrSiO4 ) extracted from Precambrian
granite core samples (1) taken from depths of 960, 2170, 2900, 3930,
and 4310 m. These depths correspond to presently recorded temperatures of
105°, 151°, 197°, 277°, and 313°C, respectively (2). We measured about
the same 206Pb/207Pb ratio for zircons from all five
depths, and we found that the total number of Pb counts measured per
individual zircon was, to the limit of our experimental procedures,
independent of depth. Taken together, these results strongly suggest that
there has been little or no differential Pb loss, which can be attributed to
the higher temperatures existing at greater depths. As discussed below, this
evidence for high Pb retention under adverse environmental conditions appears
to have immediate and practical application to the question of long-term
containment of hazardous nuclear wastes.
Samples of granite (2)
from Los Alamos National Laboratory drill holes GT-2 and EE-2 from all five
depths were individually crushed and then passed through different heavy
liquid (methylene iodide) separatory funnels to obtain the high-density
fraction containing the zircons. This procedure was repeated several times
with different samples from each depth. The high-density fraction was then
washed thoroughly with acetone to eliminate the methylene iodide residue
before being placed on a standard 1 by 3 inch glass microscope slide. Under a
polarizing microscope, the zircons were picked out of the high-density
fraction with a fine-tipped needle and then loaded either onto pyrolytic
graphite disks for ion microprobe analysis or onto V-shaped rhenium
filaments, which were mechanically compressed before mass spectrometric
measurements. (Surficial residues on the zircons burned off at temperatures
well below that used to measure Pb from within the zircons.) Some zircons
were analyzed by x-ray fluorescence before mass analysis.
Our efforts to measure lead isotope ratios in
zircons with an Applied Research Laboratory ion microprobe failed because of
molecular ion interferences. We then concentrated on determining relative
abundances of U, Th, and Zr, using mostly an 16O−
primary ion beam. Ion count rates were obtained on the 90Zr+,
232ThO+, and 238UO+ peaks. The
data were then quantified with sensitivity factors obtained from six
different National Bureau of Standards glass standards containing Zr, Th, and
U. Two or three zircons from three depths were analyzed, and usually four
determinations were made from each zircon. Frequently, there were significant
differences in the U and Th concentrations from two different locations on
the same zircon. The results are given in Table 1 as a range of values
obtained from each zircon.
Table 1. Ion microprobe determinations of U and Th
concentration ranges in atomic parts per million on separate zircons from
960, 3930, and 4310 m. Calculations were based on a comparison of 238UO+,
232ThO+, and Zr+ peak sizes and on the
assumption that the zircons were pure ZrSiO4.
The most important results came from the thermal
ionization experiments. The thermal ionization mass spectrometer used in this
work is similar to others described previously (3). It has a single
magnet with 90° deflection and a 30-cm central radius of curvature. It is
equipped with a pulse-counting detection system to allow complete isotopic analyses
to be made on small quantities(<1 ng) of suitable elements ionized from a single filament. The filaments, made of
V-shaped rhenium foil 0.64 cm long and 0.08 cm deep (4),
were baked out at 2000°C before loading the zircons. Ions are formed by
resistive heating of the filament; typical temperatures for this work were
1400° to 1470°C (uncorrected pyrometer readings).
Previous work done to develop a technique for
analyzing small lead samples led to the use of silica gel to enhance ionization
efficiency (5). Because individual zircons are chemically somewhat
similar to silica, we decided to try to analyze lead from individual zircons
loaded directly on the rhenium filament. Such a technique would have several
advantages over traditional methods: contamination would be essentially
eliminated because no chemical separation would be required and, since the
zircons are small (~ 50 μm in diameter), they would provide an approximate
point source of ions, which is known to optimize ion-optical conditions in
the mass spectrometer (6).
Test experiments with zircons from other
localities (7) were uniformly successful; ion signals were observed at
masses (m) 206, 207, and 208 which could definitely be ascribed to Pb
isotopes. To help ensure that we were at the correct ion lens conditions, we
focused on the 138BaO+ peak (the zircons contained some
Ba), which was reasonably intense at 1200°C. Surficial residues left on the
zircons after the acetone wash burned off before the operating temperature of
1450°C, where the lead signal was measured. Great care had to be exercised to
avoid making the temperature too high; very rapid evaporation of the lead
occurred only a little above the operating temperature. Typical count rates
were 100 to 3000 counts per second for 206Pb+. Traces
of thallium (m = 203 and 205) were sometimes observed, but burned out more
rapidly than the lead. Other than thallium, lead gave the only substantive
peaks in the range m = 202 to 210. There was, however, a general background
generated by the sample; chemically unseparated samples such as these zircons
almost always yield such backgrounds. This background has little effect on
the 206, 207, and 208 peaks, but made precise measurement of the 204Pb
signal, which was very small, impossible. For example, in an analysis typical
of these experiments, 1.6 × l05 counts from 206Pb were
collected; the background correction was about 40 counts and, after
correction, 18 counts remained at mass 204. Although these counts are listed
as 204Pb counts in Table 2, more work is needed to determine how
much may be uncompensated background.
Table 2 shows the results of our mass analyses of
filaments loaded with single and multiple zircons from five granite cores.
The range of 206Pb/208Pb values reflects the fact that
this ratio varied from one group of zircons to another, and sometimes varied
during measurements on a single zircon. These variations are not surprising
in view of the ion microprobe analyses, which showed significant U/Th
variations at different points on a single zircon (232Th decays to
208Pb and 238U decays to 206Pb). These
variable 206Pb/208Pb ratios do not furnish any direct
information on differential Pb retention in these zircons. For that purpose,
it is generally accepted that the Radiogenic 206Pb/207Pb
ratios derived from 238U/235U decay are more specific.
We note that Zartman's (8) isotopic measurements of Pb, which was
chemically extracted from zircons taken from the GT-2 core at 2900 m, yield
an adjusted 206Pb/207Pb ratio (9) that
approximates our ratios.
In a conventional chemical extraction of lead
from zircons, the lead measured in the mass analysis is considered to be a
combination of radiogenic lead (from U and Th decay) and nonradiogenic lead
(from common lead contamination and from some initial lead in the zircon).
The radiogenic component is obtained by subtracting out a nonradiogenic
component proportional to the amount of 204Pb. In our experiments,
however, the direct loading procedure virtually eliminated the common lead
contamination, and we circumvented the need to make adjustments for initial
lead in the zircons by accepting only analyses (10) showing a ratio of
204Pb to total Pb of less than 2 × 10−3. Thus the 206Pb/207Pb
ratios shown in Table 2 represent highly radiogenic lead and hence are
potential indicators of Pb retention.
We consider that the most important
observations on the data in Table 2 are: (i) the fact that the 206Pb/207Pb
ratios on single zircons closely approximate the ratio obtained when a group
of similar zircons was loaded simultaneously on a single filament, (ii) the
relative uniformity of the 206Pb/207Pb ratios for
zircons from all depths, and (iii) the fact that the total number of Pb
counts per zircon (the counts in column 4 of Table 2 divided by the product
of columns 2 and 3) shows no systematic decrease with depth, as would be
expected if differential Pb loss had occurred at higher temperatures. Taken
together, items (ii) and (iii) provide strong evidence for high Pb retention
in zircons even for a prolonged period in an environment at an elevated
temperature. These results have possible implications for long-term nuclear
Table 2. Results of thermal ionization
mass measurements for zircons with a 204Pb/total Pb ratio of
less than 2 × 10−3. The background correction was taken from the
208.5 mass position; it was applied to the raw data to obtain the isotopic abundances,
which were used to compute the isotopic ratios. Standard deviations are
listed with the Pb isotopic ratios.
||1.2 × 106
||2 × 10−4
||9.6 ± 0.3
||1.3 × 105
||2.7 × 10−4
||9.9 ± 0.4
||8.9 × 105
||3 × 10−4
||10.0 ± 0.4
||4.1 × 105
||2.8 × 10−4
||11.2 ± 0.3
||6.5 × 105
||2 × 10−4
||11.0 ± 0.4
||8.0 × 104
||5.8 × 10−4
||10.4 ± 0.1
||5.6 × 104
||2.5 × 10−4
||9.7 ± 0.6
||1.6 × 105
||6 × 10−4
||9.8 ± 0.4
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 × l010
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 × l0−2
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