EOS, vol. 61, no. 27, July 1, 1980, page 514.
I welcome York's contribution [York, 1979] to the
exchange of views concerning the possible existence and
potential cosmological implications of polonium halos in
Precambrian granites [Damon, 1979; Gentry, 1979], but I
must take exception to some omissions from his
comments about my results on Po halos.
York seems to regard even the existence of Po halos as
only tentative. But notwithstanding the uncertainties, his
article leans heavily toward the proposition that Po halos
do exist, at least in micas. York's thesis is that Po halos
are most probably explainable within the accepted
framework because the interlocking nature of various
radiometric dating techniques provides powerful evidence
that conventional geochronoiogy is correct. York faults me
for ignoring this internal consistency. Contrary to his
understanding, I do not ignore these data. But neither do I
accept the idea that the presumed agreement between
techniques is really coercive evidence for the correctness
of the uniformitarian assumption which undergirds the
present model. There was no discussion of the 238U 206Pb
ratios [Gentry et al., 1976], which raise significant questions
about the accepted geochronological scheme.
While I can appreciate York's desire to emphasize
internal consistency, it should be evident that irrespective
of how much data has been or yet can be fitted into the
present model, the question of its ultimate reliability hinges
on whether there exist any observations which falsify the
Given the presumption that polonium halos exist, York
considers Henderson's hypothesis [Henderson, 1939]
quite reasonable, i.e., that Po halos in micas formed from
selective accumulation of U-daughter Po atoms that had
slowly migrated away from the source of uranium. Here
York takes me to task for presuming to question
Henderson's hypothesis. However, when he states that my
observations (on mica halos) "do nothing to detract from
Henderson's theory of their mode of origin," he did not
mention either the original report [Gentry, 1968] or the
subsequent review article [Gentry, 1973] in which I
discussed my lengthy but unsuccessful efforts to confirm
Henderson's hypothesis for Po halos in micas by using α-recoil
York's surprise that I would accept Henderson's
hypothesis for Po halos in coalified wood [Gentry et al.,
1976] but reject this explanation for mica because of the
slowness of solid state diffusion suggests first that the
same type of Po halos has been found in both substances
and second that my only objection to accepting
Henderson's hypothesis in mica was the slowness of solid
state diffusion. Here some very important data has been
Mica contains three types of Po halos, but coalified
wood only one. Much evidence suggests the 210Po
halos in coalified wood formed from selective
accumulation of 210Po and 210Pb, which have half-lives
sufficiently long (138 days and 22 years, respectively)
to have migrated to the radiocenters before serious loss
occurred from decay. Likewise, the relatively short half-lives
of 214Pb and 218Po (27 minutes and 3 minutes,
respectively) mean these nuclides generally decayed
away before reaching the accumulation sites, which
explains the absence of 214Po and 218Po halos. Thus the
crucial question is: If Henderson's model results in only
2l0Po halos being formed under ideal conditions of rapid
transport (plus an abundant supply) of U-derived Po
atoms, then how can this model account for all three Po
halo types in mica, where both the U content and the
transport rate are considerably lower? Indeed, the close
proximity in clear mica (i.e., without any conduits) of two
or more types of Po halos presents what may be
incontrovertible evidence against explaining these halos
by Henderson's hypothesis [Feather, 1 978].
Finally, York failed to mention that my hypothesis that
Po halos in Precambrian granites are primordial [Gentry,
1974] could in theory be falsified (and Feather's
objections negated) by the experimental synthesis of a
biotite crystal that contained at least two dissimilar Po
halos in close proximity [Gentry, 1979].
Damon, P. E., "Time: Measured responses," EOS Trans. AGU, 60, 474,
Feather, N., "The unsolved problem of Po-haloes in Precambrian
biotite and other old minerals," Commun. Roy. Soc.
Edinburgh, 11, 147-158, (1978).
Gentry, R. V., "Fossil α-recoil analysis of variant Po halos in biotite,"
Science, 160, 1228-1230, (1968).
Gentry, R. V., "Radioactive halos," Annu. Rev. Nucl. Sci., 23, 347-362,
Gentry, R. V., "Radiohalos in radiochronological and cosmological
perspective," Science, 184, 62-66, (1974).
Gentry, R. V., W. H. Christie, O. H. Smith, J. F. Emery, S. A.
Reynolds, R. Walter, S. S. Cristy, and P. A. Gentry, "Radiohalos
in coalified wood: New evidence relating to the time of U
introduction and coalification," Science, 194, 315-318, (1976).
Gentry, R. V., "Time: Measured responses," EOS Trans. AGU, 60, 474,
Henderson, G. H., "A quantitative study of pleochroic halos," 5, "The
genesis of halos," Proc. Roy. Soc. of London, A173, 260-264, (1939).
York, D., "Polonium halos and geochronology," EOS Trans. AGU,
60, 617-618, (1979).
Robert V. Gentry
Columbia Union College
Takoma Park, Maryland
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