ABUNDANT PO RADIOHALOS IN PHANEROZOIC GRANITES AND TIMESCALE IMPLICATIONS FOR THEIR FORMATION Andrew A. Snelling John R. Baumgardner Larry Vardiman Geo-Research Pty Ltd, P.O. Box 1208, Springwood, Queensland, 4127, Australia Los Alamos National Laboratory, 1965 Camino Redondo, Los Alamos, NM 87544, USA Institute for Creation Research, PO Box 2667, El Cajon, CA 92021 USA Rn, Po U S (c) However, at temperatures >150°C the α-tracks are annealed, so no radiohalos form and there is no α-track record of the hydrothermal fluids containing Rn and Po flowing along the cleavage plane. A few S atoms are in lattice defects downflow of the zircon crystal. (f) With further passing of time and more α-decays both the 238U and 210Po radiohalos are fully formed, the granite cools completely and hydrothermal fluid flow ceases. Note that both radiohalos have to form concurrently below 150°C. The rate at which these processes occur must therefore be governed by the 138 day half-life of 210Po. To get 218Po and 214Po radiohalos these processes would have to have occurred even faster. Rn, Po U (d) As the temperatures approach 150°C and 222Rn decays to 218Po, the Po isotopes in the hydrothermal fluids, which have a geochemical affinity for S, precipitate to form PoS as the fluids flow past the S in the lattice defects. The U in the zircon continues to decay and replenish the supply of Rn and Po in the fluids. Po Po Po sheet structure of biotite separated by perfect cleavages included zircon crystal 238U radiohalo 210Po radiohalo (a) Diagrammatic cross-section through a biotite flake showing the sheet structure and perfect cleavage. A tiny zircon crystal has been included between two sheets and its 238U content has generated a 238U radiohalo. A 210Po radiohalo has also developed around a tiny radiocenter between the same two sheets. U (e) Once the temperature drops to below 150°C, the α-tracks produced by continued decay of U in the zircon and of Po in the PoS are no longer annealed and so start discoloring the biotite sheets. More Po isotopes in the flowing hydrothermal fluids replace the Po in the PoS as it decays. There are not enough α-decays of Rn and Po in transit to also discolor the biotite sheets along the fluid flow path. Po Po Po α Rn, Po α α α U Rn, Po α α α α hydrothermal fluid flow along cleavages (b) Enlarged diagrammatic cross-section through a biotite flake that has crystallized from a granite magma to 300°C. The U in an included zircon crystal is emitting α-particles, while hydrothermal fluids released from the cooling magma are flowing along the cleavage plane dissolving decay products Rn and Po from the zircon and carrying them downflow where they also emit α-particles. U Figure 5. Time sequence of diagrams to show the formation of 238U and 210Po radiohalos concurrently. Nuclide 238U 234U 230Th 226Ra 222Rn 218Po 214Po 210Po Eα(Mev) 4.19 4.77 4.68 4.78 5.49 6.00 7.69 5.30 210Po 210Po 214Po 210Po 214Po 218Po (a) 218Po Halo (c) 214Po Halo (d) 210Po Halo (b) 238U Halo Figure 3. Composite schematic drawing of (a) a 218Po halo, (b) a 238U halo, (c) a 214Po halo, and (d) a 210Po halo with radii proportional to the ranges of the α-particles in air. The nuclides responsible for the α-particles and their energies are listed for the different halo rings. Z α β 92 90 88 86 84 82 206Pb ∞ 210 Po 138 d 210 Bi 50 d 210 Pb 22 y 214 Po 2x10-4s 214 Bi 20 m 214 Pb 2.7 m 218 Po 3.1 m 222 Rn 3.8 d 226 Ra 1662 y 230 Th 75 ky 234 U 248 ky 234 Pa 1.2 m 235 Th 24 d 238 U 4.5 By Figure 1. Part of the chart of the nuclides showing the species in the 238U decay series and their half-lives. Note the eight α-decayers, the Po isotopes being the last three. 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 α-emissions, with the ring radii related to the distinctive α energies of the different radioisotopes in the 238U and 232Th decay series (Figure 1). 238U and 232Th radiohalos typically form around zircon and monazite inclusions, respectively, commonly in biotite, within granitic rocks (Figure 2). Radiohalos are also observed without central mineral inclusions and consisting only of rings from the last three α-emitters in the 238U series: 218Po, 214Po and 210Po (Figures 3 and 4). Because rings for all the Po precursors are missing, one infers there may have been migration of a Po precursor, most likely 222Rn, away from a 238U 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 218Po of 3.1 minutes, 214Po of 164 μs, and 210Po of 138 days place severe time constraints on the processes for separating the Po precursor from parent 238U 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 (Table 1). Their abundance is approximately ten 210Po radiohalos for every 214Po radiohalo, while 218Po radiohalos are rare. The frequency of 238U halos in these rocks is typically comparable to that of the 210Po halos. Po halos are usually found in the same biotite grains as 238U halos. The zircon inclusions in the latter often contain >100 ppm U and therefore represent a potentially adequate source of precursor 222Rn 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 (Figure 5). Because of α-track annealing, the halos can form only below 150°C. The time window for the required hydrothermal activity in the cooling granite hence would have been extremely short compared with the timescale of 238U decay. As a consequence the amount of 222Rn 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. Figure 2. 238U radiohalos in biotite flakes from granitic rocks. The diameters of the halos are approximately 60–70 μm. (a) Two 238U radiohalos in the Silurian Cooma Granodiorite, Lachlan Fold Belt, southeastern Australia. The halos are overexposed so that the inner rings are indistinguishable. Within the upper halo the zircon radiocenter is visible, making the diameter of that halo slightly larger. (b) Two 238U radiohalos in the Ngaeri Granite, Japan. The inner rings are more easily distinguished, though the lower halo is faint. Figure 4. Po radiohalos in biotite flakes from granitic rocks. (a) Two 214Po radiohalos in the Triassic Stanthorpe Adamellite, New England Fold Belt, Queensland, Australia. The diameter of their outer rings is approximately 68 μm. The upper halo clearly has no visible inclusion for its radiocenter. (b) A 218Po radiohalo in the Ngaeri Granite, Japan. Its outer ring diameter is approximately 68 μm. (c) A 210Po radiohalo in the Carboniferous Stone Mountain Granite near Atlanta, GA. Its diameter is approximately 39 μm. (d) A series of 218Po radiohalos along a crack in a biotite flake from the Triassic Stanthorpe Adamellite, New England Fold Belt, Queensland, Australia. This confirms that the Po halos are of secondary origin due to hydrothermal fluid transport of the Po isotopes. Rock Unit Location Age Samples (slides) Radiohalos 210Po 214Po 218Po 238U 232Th Indian Hill granites San Diego 90 Ma 4 (180) 279 11 0 45 0 La Posta Pluton County, CA 93 Ma 8 (383) 96 4 0 8 0 Granodiorite of Mono Dome Yosemite, CA 93 Ma 1 (50) 6 0 0 0 0 San Jacinto Pluton Palm Springs, CA Late Cretaceous 9 (450) 96 0 0 9 0 Bass Lake Tonalite Yosemite, CA 114 Ma 1 (50) 84 0 0 0 3 Ward Mountain Trondhjemite Yosemite, CA 115 Ma 1 (50) 63 0 0 0 0 Granodiorite of Arch Rock Yosemite, CA 114–117 Ma 2 (100) 106 0 7 10 0 Tonalite of the Gateway Yosemite, CA 114–117 Ma 2 (100) 1 0 0 0 0 Wheeler Crest Granodiorite Mammoth, CA 200–215 Ma 1 (50) 58 0 12 1 0 Lee Vining Canyon Granite Yosemite, CA 200–215 Ma 1 (50) 108 0 2 13 0 Stanthorpe Adamellite Queensland, Australia 232 Ma 1 (48) 520 4 15 68 19 Stone Mountain Pluton Georgia 291±7 Ma 6 (291) 1109 93 2 88 0 Liberty Hill Pluton Lancaster, SC 320 Ma 3 (150) 180 0 0 0 0 Bathurst Granite New South Wales, Australia 330 Ma 1 (51) 45 0 0 3 0 Harcourt Granite Victoria, Australia 369 Ma 1 (31) 107 130 0 198 0 Strathbogie Granite Victoria, Australia 374 Ma 1 (50) 1366 232 1 1582 10 Shap Granite Lake District, England 393 Ma 1 (55) 452 2 0 52 7 Shannons Flat Granite New South Wales, Australia 417–443 Ma 1 (101) 9 18 0 38 0 Jillamatong Granite New South Wales, Australia 417–443 Ma 1 (31) 120 118 0 137 0 Cootralantra Granite New South Wales, Australia 417–443 Ma 1 (43) 230 75 0 276 2 Cooma Granodiorite New South Wales, Australia 433 Ma 1 (41) 373 44 0 418 37 Encounter Bay Granite South Australia 487–490 Ma 1 (45) 362 8 0 1586 161 Palmer Granite South Australia 490 Ma 1 (51) 1352 17 0 631 3 Table 1. Tabulation of radiohalo numbers counted in slides of mounted biotite flakes from granite samples spanning the Phanerozoic from Cambrian to Cretaceous, from southeastern Australia, California and southeastern USA, and northern England. Each slide contained more than 30 small biotite flakes, so at least 1000 small biotite flakes per sample were surveyed. 2a 4a 4b 4d U 2b 4c