(March 29, 1990)
Ms. Christine Gilbert
Dear Ms. Gilbert:
I am submitting a revised reply to the comments of A. L. Odom and W. J. Rink concerning my work on giant halos and Po halos in micas. As you may observe, this reply focuses on the technical aspects of their comments. As we both know, Science regularly grants the opportunity for researchers to respond when incorrect evaluations are published concerning their results. I do hope that the same opportunity given to others is not denied me in this case.
/s/ Robert V. Gentry
xc: A. L. Odom
Giant Halos and Po Halos in Micas
Based on their studies of three giant halos in quartz, A. L. Odom and W. J. Rink (Reports, 10/6/89, p. 107) propose both Po halos and giant halos in micas are artifacts of charge migration resulting from excess U or Th in the halo centers. Considerable evidence negates this view. First, the hundred or more giant halos I found in a Madagascan mica specimen (1) sometimes overlap the many normal Th halos. Such closeness means the region around giant and Th halos is identical in chemical composition, a fact confirmed by ion-probe analyses (2). Clearly then, giant halos cannot arise from migration effects associated with some trace element enhancement around their centers. Closeness of both halo types also rules out size variations due to differences in age and thermal history. Lastly, neither ion probe nor synchrotron radiation experiments (2,3) show any systematic U/Th differences between giant-halo and Th-halo centers. Giant halos in Madagascan mica are not artifacts of excess U/Th.
Neither are Po halos in micas artifacts of this effect. If that were true, then as Odom and Rink admit, there would have to be excess U in Po-halo centers to induce this effect. But this is disproved by autoradiographic, induced fission-track, microprobe, and scanning electron microscope x-ray fluorescence (SEMXRF) studies (4-6) — all of which showed virtually no U in Po halo centers at present — and by fossil fission-track studies (4), which showed no U was in them in the past. Moreover, what those microprobe and SEMXRF studies did show was that Po-halo centers are highly enriched in Pb-206, which is the expected decay product of the Po isotopes whose alpha energies exactly match the respective ring sizes in the three most abundant types of Po halos (4-5). Thus, Po halos in granites are confirmed by exactly the same techniques used to identify U and Th halos, and the evidence is that they originated with primordial Po (7), not secondary Po from U decay (8).
Robert V. Gentry
Earth Science Associates