Differential Lead Retention in Zircons:
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 × 105 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 waste disposal.
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.
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 holes.
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
* Visiting scientist from Columbia Union College, Takoma Park, Md. 20112.
3 November 1981; revised 22 January 1982