atoms is available for capture in a radiocenter at any given time. But these 218Po atoms must also migrate or diffuse from their source at very low diffusion rates through surrounding mineral grains to be captured by the radiocenters before the 218Po decays (Fremlin 1975; Gentry 1968, 1975). Therefore, there are strict time limits for the formation of the Po radiohalos by primary or secondary processes in granites. Studies of some Po radiohalo centers in biotite (and fluorite) have shown little or no U in conjunction with anomalously high 206Pb/207Pb and/or Pb/U ratios, which would be expected from the decay of Po without the U precursor that normally occurs in U radiohalo centers (Gentry 1974; Gentry et al. 1974). Indeed, many 206Pb/207Pb ratios were greater than 21.8 reflecting a seemingly abnormal mixture of Pb isotopes derived from Po decay independent of the normal U-decay chains (Gentry 1971; Gentry et al. 1973). Thus, based on these data, Gentry advanced the hypothesis that the three different types of Po radiohalos in biotite represent the decay of primordial Po (that is, original Po not derived by 238U-decay), and that the rocks hosting these radiohalos must be primordial rocks produced by fiat creation, given that the half-life of 214Po is only 164 microseconds (Gentry 1979, 1980, 1982, 1983, 1984, 1986, 1988, 1989). He thus perceived that all granites must be Precambrian, and part of the earth’s crust created during the Creation Week. As a consequence of Gentry’s creation hypothesis, the origin of the Po radiohalos has remained controversial and thus apparently unresolved. Of the 22 locations then known where the rocks contained Po radiohalos, Wise (1989) had determined that six of the locations hosted Phanerozoic granitic rocks that intruded fossiliferous (and thus Flood-deposited) sedimentary strata, a large enough proportion to severely question Gentry’s hypothesis of primordial Po in fiat created granitic rocks. Many of these Po radiohalo occurrences are also in proximity to higher than normal U concentrations in nearby rocks and/or minerals, suggesting ideal sources for fluid separation and transport of the Po. Subsequently, Snelling (2000b) reviewed the literature on radiohalos. He thoroughly discussed the many arguments and evidences used in the debate that had ensued over the previous two decades, and concluded that there were insufficient data on the geological occurrence and distribution of the Po radiohalos for the debate to be decisively resolved. He then recognized the spatial association of Po radiohalos to 238U radiohalos meant that the Po which parented the adjacent Po radiohalos may have been derived from the 238U decay products in the radiocenters of the 238U radiohalos. He also observed that many radiohalo-hosting biotite flakes had been hydrothermally altered. Furthermore, many of the host Phanerozoic granites had intruded into fossil-bearing sedimentary layers that therefore were deposited during the Flood. Thus, those and other granites intruded into fossil-bearing, Flood-deposited sedimentary layers had to form during the Flood subsequent to the deposition of those sedimentary layers and could not be creation rocks as postulated by Gentry (1988). Of course, this does not preclude many Precambrian granites having been created during the Creation Week. Furthermore, Snelling (2000b) found that there are now significant reports of 210Po as a detectable species in volcanic gases, in volcanic/hydrothermal fluids associated with subaerial volcanoes implies that the radiocenters which produced these Po radiohalos initially contained only either the respective Po radioisotopes that then parented the subsequent α-decays, or a non-α-emitting parent (Gentry 1971; Gentry et al. 1973). These three Po radiohalo types occur in biotite from granitic rocks (Gentry 1968, 1971, 1973, 1974, 1984, 1986, 1988; Gentry et al. 1973, 1974; Wise 1989; Snelling and Armitage 2003; Snelling 2005a). Joly (1917b, 1924) was probably the first to investigate 210Po radiohalos and was clearly baffled by them. Because Schilling (1926) saw Po radiohalos located only along cleavages and cracks in fluorite from Wölsendorf in Germany, he suggested that they originated from preferential deposition from secondary fluid transport of Po in U-bearing solutions. Henderson (1939) and Henderson and Sparks (1939) invoked a similar but more quantitative hypothesis to explain Po radiohalos along conduits in biotite. However, those Po radiohalos found occurring along much more restricted cleavage planes, similar to those found by Gentry (1973, 1974), have been more difficult to account for. The reason given for these attempts to account for the origin and formation of the Po radiohalos by some secondary process is simple – the half-lifes of the respective Po isotopes are far too short to be reconciled with the Po having been primary, that is, originally in the granitic magmas which are usually claimed to have slowly cooled to form the granitic rocks that now contain the Po-radiohalo-bearing biotite grains. The half-life of 218Po, for example, is 3.1 minutes. However, this is not the only formidable obstacle for any secondary process that transported the Po into the biotite as, or after, the granitic rocks cooled. First, there is the need for the isotopic separation of the Po isotopes, or their α-decay precursors, from their parent 238U having occurred naturally (Gentry et al. 1973). Second, the radiocenters of very dark 218Po radiohalos, for example, may need to have contained as much as 5 x 109 atoms (a concentration of greater than 50%) of 218Po (Gentry 1974), yet the host minerals contain only ppm abundances of 238U, which apparently means only a negligible supply of 218Po daughter Nuclide 238U 234U 230Th 226Ra 222Rn 218Po 214Po 210Po Ea (MeV) 4.19 4.77 4.68 4.78 5.49 6.00 7.69 5.30 (b) 238U Halo (d) 210 Po Halo 210Po 210Po 214Po 210Po 214Po 218Po (c) 214Po Halo (a) 218 Po 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 α-particles in air. The nuclides responsible for the α-particles and their energies are listed for the different halo rings (after Gentry 1973). SNELLING Radiohalos through earth history 2023 ICC 542
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