HR: 1330h
AN: V32C-1046    [PDF]
TI: Abundant Po Radiohalos in Phanerozoic Granites and Timescale Implications for Their Formation
AU: Snelling, A A
EM: aasnelling@ozemail.com.au
AF: Geo-Research Pty Ltd, P.O. Box 1208, Springwood, Qld 4127 Australia
AU: * Baumgardner, J R
EM: baumgardner@lanl.gov
AF: Los Alamos National Laboratory, MS B216, Los Alamos, NM 87545 United States
AU: Vardiman, L H
EM: lvardiman@icr.edu
AF: Institute for Creation Research, P.O. Box 2667, El Cajon, CA 92021 United States
AB: Radiohalos are significant as a physical, integral, historical record of the decay of radioisotopes in their tiny central mineral inclusions. In thin section the typical dark concentric rings in the host minerals are due to the $\alpha$-emissions, with the ring radii related to the distinctive a energies of the different radioisotopes in the $^{238}$U and $^{232}$Th decay series. $^{238}$U and $^{232}$Th radiohalos typically form around zircon and monazite inclusions, respectively, commonly in biotite, within granitic rocks. Radiohalos are also observed without central mineral inclusions and consisting only of rings from the last three $\alpha$-emitters in the $^{238}$U series: $^{218}$Po, $^{214}$Po and $^{210}$Po. Because rings for all the Po precursors are missing, one infers there may have been migration of a Po precursor, most likely $^{222}$Ra, away from a $^{238}$U source in the genesis of such halos. Early research to understand how Po radiohalos might have formed focused on Precambrian granitic rocks. Thus it was claimed that the Po radiohalos were largely confined to such rocks. Furthermore, their formation was described as "a tiny mystery", because the half-lives for $^{218}$Po of 3.1 minutes, $^{214}$Po of 164 $\mu$s, and $^{210}$Po of 138 days place severe time constraints on the processes for separating the Po precursor from parent $^{238}$U and concentrating it and/or Po prior to halo formation. We report new research which establishes that Po radiohalos are also common in Phanerozoic granites, for example, in the Lachlan Fold Belt of southeastern Australia and the Peninsular Ranges Batholith of southern California. Their abundance is approximately ten $^{210}$Po radiohalos for every $^{214}$Po radiohalo, while $^{218}$Po radiohalos are rare. The frequency of $^{238}$U halos in these rocks is typically comparable to that of the $^{210}$Po halos. Po halos are usually found in the same biotite grains as $^{238}$U halos. The zircon inclusions in the latter often contain $>$100 ppm U and therefore represent a potentially adequate source of precursor $^{222}$Rn and Po for the Po halos. Hydrothermal fluids appear to play a critical role in the formation of these Po halos, both in nuclide transport and in chemical reactions to precipitate Po at localized sites. Because of $\alpha$-track annealing, the halos can form only below 150$\deg$ C. The time window for the required hydrothermal activity in the cooling granite hence would have been extremely short compared with the timescale of $^{238}$U decay. As a consequence the amount of $^{222}$Rn available during this brief cooling window falls far short of the amount required to generate the observed mature halos. We view this seeming paradox as a hint that nuclear decay processes may have been occurring more rapidly during the interval in which these granites were cooling.
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting