the radiocenters around them have been unequivocally demonstrated to be products of the α-emitting members of the 238U decay series to stable 206Pb, and occasionally the 232Th decay series to stable 208Pb. It has been demonstrated that the radii of these rings correspond to the energies of specific α-particles in the 238U decay series and their travel ranges (Gentry 1984). Thus, a fully-developed 238U radiohalo should have eight visible concentric rings which correspond to the eight α-decay steps in the 238U decay series. It has also been determined that such a halo requires between 500 million and 1 billion α-decays to be fully-developed and darkened (Fig. 1) (Gentry 1988). The radii of the concentric multiple spheres, or rings in thin sections, correspond to the ranges in the host minerals of the α-particles from the α-emitting radioisotopes in the 238U and 232Th decay series (Gentry 1973, 1974, 1984) (Fig. 2). 235U radiohalos have not been observed. This is readily accounted for by the scarcity of 235U (only 0.7% of the naturally-occurring U), since large concentrations of the parent radionuclides are needed to produce the concentric ring structures of the radiohalos. Ordinary radiohalos can be defined, therefore, as those that are initiated by 238U or 232Th α-decay, irrespective of whether the actual halo sizes closely match the respective idealized patterns (Fig. 2). In many instances the match is very good, the observed sizes agreeing very well with the 4He ion penetration ranges produced in biotite, fluorite, and cordierite (Gentry 1973, 1974). U and Th radiohalos usually are found in igneous rocks, most commonly in granitic rocks and in granitic pegmatites. While U and Th radiohalos have been found in over 40 minerals, their distribution within these minerals is very erratic (Ramdohr 1933, 1957, 1960; Stark 1936). Thus far there have been no hypotheses proposed to explain the cause(s) of this erratic distribution. Radiohalos have even been found in diamonds (Gentry 1998; Armitage and Snelling 2008; Schulze and Nasdala 2017). Biotite is quite clearly the major mineral in which U and Th radiohalos occur (Bower et al. 2016a, b). Wherever found in ubiquitous large (1-5 mm in diameter) biotite flakes the radiohalos are prolific, and are associated with tiny (1-5 μm in diameter) U-bearing zircon grains or Th-bearing monazite grains as the radiocenters. The ease of thin section preparation, and Figure 1. Sunburst effect of alpha-damage trails. The sunburst pattern of α-damage trails produces a spherically colored shell around the halo center. Each arrow represents approximately 5 million α-particles emitted from the center. Halo coloration initially develops after about 100 million α-decays, becomes darker after about 500 million, and very dark after about 1 billion (after Gentry 1988). (a) 238U Halo 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 Nuclide 232Th 228Th 224Ra 220Rn 216Po 212Bi 212Po Ea (MeV) 4.0 5.33 5.42 5.68 6.28 6.77 6.05 8.78 { (b) 232Th Halo Figure 2. Schematic drawing of (a) a 238U halo, and (b) a 232Th 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). the clarity of the radiohalos in these sections, have made biotite an ideal choice for numerous radiohalo investigations, namely, those of Joly (1917a, b, 1923, 1924), Lingen (1926), Iimori and Yoshimura (1926), Kerr-Lawson (1927, 1928), Wiman (1930), Henderson and Bateson (1934), Henderson and Turnbull (1934), Henderson et al. (1934), Henderson and Sparks (1939), Gentry (1968, 1970, 1971), Snelling and Armitage (2003), and Snelling (2005a). U, Th, and other specific halo types in most of these studies have been observed mainly in Precambrian rocks, so much remains to be learned about their occurrence in rocks from the other geological periods of the strata record. However, some studies have shown that they do exist in rocks stretching from the Precambrian to the Tertiary (Holmes 1931; Stark 1936; Wise 1989; Snelling and Armitage 2003; Snelling 2005a). Unfortunately, in some of those published studies the radiohalo types were not specifically identified. Some unusual radiohalo types that appear to be distinct from those formed by 238U and/or 232Th α-decay have been observed (Gentry 1970, 1971, 1973, 1984, 1986; Gentry et al., 1973, 1976a, 1978; Snelling, 2000b). Of these, only the Po (polonium) radiohalos can presently be identified with known α-radioactivity (Gentry 1967, 1968, 1973, 1974; Gentry et al. 1973, 1974). There are three Po isotopes in the 238U-decay chain. In sequence they are 218Po (halflife of 3.1 minutes), 214Po (half-life of 164 microseconds), and 210Po (half-life of 138 days). Po radiohalos contain only three rings, two rings or the one ring produced by these three Po α-emitters respectively (Fig. 3). They are designated by the first (or only) Po α-emitter in the portion of the decay sequence that is represented. The presence in Po radiohalos of only the rings of the three Po α-emitters SNELLING Radiohalos through earth history 2023 ICC 541
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