Appendix: "Differential Lead Retention in Zircons"
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16 April 1982, Volume 216, pp. 296-298
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
Copyright © 1982 by the American Association for the Advancement of Science
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.
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