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Creation's Tiny Mystery
Appendix: "Differential Helium Retention in Zircons"

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Knowledge of the zircon mass and the appropriate compensation factor (to account for differences in initial He loss via near-surface α-emission) enabled us to calculate the theoretical amount of He which could have accumulated assuming negligible diffusion loss. This compensating factor is necessary because the larger (150-250 μm) zircons lost a smaller proportion of the total He generated within the crystal via near-surface α-emission than did the smaller (40-50 μm) zircons. For the smaller zircons we estimate as many as 30-40% of the α-particles (He) emitted within the crystal could have escaped initially whereas for [p. 264] the larger zircons we studied only 5-10% of the total He would have been lost via this mechanism. The ratio of the measured to the theoretical amount of He is shown in the last column of Table 1. The uncertainties in our estimates of the zircon masses and compensation factors probably mean these last values are good only to ±30%.

In spite of these uncertainties, it is quite evident from Table 1 that the zircons from 960 m seem to have retained considerable amounts of He, and perhaps more significantly, differential He loss with increasing depth (and temperature) has occurred rather slowly down to 2900 m (197°C) before a precipitous drop is observed at 239°C (3502 m). In fact, at present we are not certain whether the minute amounts of He recorded from the deepest zircons (3930 and 4310 m) are actually residual He in the zircons or derived from some other source. That is, in the two deepest zircon groups (3930 and 4310 m), we observed only short bursts of He (~l-2 sec) in contrast to the prolonged 20 sec or more evolution of He which was typical of He liberation from zircon groups down to and including 3502 m. In fact, it was this prolonged He liberation profile seen in two 150-250 μm size zircon groups from 3502 m which convinces us that some residual He is still trapped in the zircons down to that depth (239°C).

Now it was recently noted that the high retention of Pb in even the deepest granite cores had favorable implications for nuclear waste containment in deep (1000 to 3000 m) granite holes (Gentry, et al., 1982). The rationale for these implications is straightforward: If zircons, which have been exposed to the same type of elevated temperature environment anticipated in deep granite burial, show no detectable Pb loss either from higher temperatures or from aqueous solution corrosion effects, then nuclear wastes buried in that same granite should, if anything, experience even greater retention because of the comparative immobility of waste-type elements as compared to Pb.

The present results are important in that they provide clear evidence that the dominant factor in slow He loss down to 2900 m is attributable to greater diffusion loss at higher temperatures rather than any corrosion induced losses from the zircons. This is not at all surprising because microscopic examination shows first that zircons from all depths exhibit well-defined prismatic faces without any evidence of external corrosion, and secondly that the delicate internal inclusions within the zircons do not show any evidence of alteration from aqueous intrusion via any microstructural defects. Indeed, the relatively slow liberation of He over several 20 sec intervals observed in zircons from the surface all the way down to 2900 m is strong evidence that these zircons are virtually free of any microfractures which would have permitted a more rapid He escape. In fact, considering the Precambrian age of the granite cores (Zartman, 1979), our results show an almost phenomenal amount of He has been retained at higher temperatures, and the reason for this certainly needs further investigation for it may well turn out to have a critical bearing on the waste storage problem.

Thus the additional evidences reported herein considerably reinforce the view that deep-granite storage should be a very safe corrosion-resistant waste containment procedure. The certainty of these results stands in clear contrast with the uncertainties about how well alternative storage sites (e.g., salt domes) could withstand corrosion and/or dissolution from intruding aqueous solutions.

Acknowledgments. This research was sponsored by the U. S. Department of Energy, Division of Basic Energy Sciences, under contract W-71105-eng-26 with Union Carbide Corporation. We thank A. W. Laughlin of the Los Alamos National Laboratory for providing the core samples.

References

Gentry, R. V., T. J. Sworski, H. S. McKown, David H. Smith, R. E. Eby, and W. H. Cristie, Differential lead retention in zircons: Implications for nuclear waste containment, Science, 216, 296-298, 1982.

Laney, R., and A. W. Laughlin, Natural annealing of pleochroic haloes in biotite samples from deep drill holes, Fenton Hill, New Mexico, Geophys. Res. Lett., 8, 501-504, 1981.

Magomedov, S. A., Migration of radiogenic products in zircon, Geokhimiya, 2, 263-267, 1970.

Zartman, R. E., Uranium, thorium and lead isotopic composition of biotite granodiorite (Sample 9527-2b) from LASL Drill Hole GT-2, Los Alamos Sci. Lab. Rep. LA-7923-MS, 1979.

(Received August 6, 1982; accepted September 3, 1982.)

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