Degree of natural zircon crystalline structure damage depends on self-radiation dose, i.e. time of structure defects accumulation and concentration of U and Th. The self-radiation dose Dc can be most accurately determined by equation
Dc = (c - c0)/k (1)
where: c is unit-cell parameter of investigated sample crystalline residue; c0 is unit-cell parameter of the zircon without structure radiation defects; k is empirical coefficient, k=3.1.10-21 nm.g/decay.
This equation (1) is true both for samples gem quality zircon from Sri Lanka (Holland and Gottfried, 1955; Fairbairn and Hurley, 1957; Murakami et al., 1991) and for synthetic zircon (Weber, 1991) with isomorphous impurity of hyghly radioactive 238Pu.
On the basis of zircon unit-cell parameters, U-concentration and radiogenic ratios 208Pb/206Pb in the sample the radiation age tc was calculated by means of the equation (Makeev et al., 1981):
Dc/U=2.51.1015{8[exp(l8tc)-1]
+7r[exp(l5tc)-1]+6j[exp(l2tc)-1]}, (2)
where: l8, l5, l2 are 238U, 235U and 232Th decay constants, respectively;
r=235U/238U=1/137.88; j=232Th/238U=0.968(208Pb/206Pb)[exp(l8t)-1]/[exp(l2t)-1];
t is U-Pb zircon age.
Unit-cell parameters were calculated using powder
X-Ray data from the set of 8-12 reflections. Isotopic measurements were performed on a MAT 261 8-collector mass-spectrometer in static mode. All errors are reported at the 2s level.
The c0=0.5979(1) nm was obtained for: a) natural zircon from kimberlites formed at the high P-T-conditions with minimum of isomorphous admixtures; b) some natural archean zircons annealed at 1375-1430ƒC during 2.0-2.5 hrs in air; c) synthetic zircons synthized from stoichiometrical mix of ZrO2 and SiO2
To obtain the zircon crystalline residue without the metamict part which are present practically in all natural zircon samples the treatment of the pulvered samples in 48% HF at 200-220ƒC during 4 hrs was used. The increase of zircon treatment duration in 48% HF at 220ƒC up to 28 hrs does not result in distortion of either radiation or U-Pb ages.
Crystalline residue of the investigated zircons from
10 different geological formations of the Baltic shield
(t=2.5-3.1 Ga) had U concentration 38-113 ppm and unit-cell parameters a=0.6607-0.6618 nm and c=0.5988-0.6010 nm, which correspond to self-radiation doses Dc =(0.29-1.0).1018 decay/g. Zircon crystalline residue radiation ages 1.5-2.0 Ga and they coinside within analitical error with K-Ar ages
(1.5-1.9 Ga) of host rocks. The accuracy of radiation age calculation is 4-10%. Consequently, the radiation ages reflect the period of the Svecofennian activisation of endogenic processes.
Fairbairn, H.W. & Hurley, P.M., Trans. Amer. Geophys. Union. 38, 99-107 (1957).
Holland, H.D. & Gottfried, D., Acta Cryst. 8, 291-300 (1955).
Makeev, A.F., Levchenkov, O.A. & Bubnova, R.S., Intern. Geochemitry 17, 92-97 (1981).
Murakami, T., Chakoumakos, B.C., Ewing, R.C., Lumpkin, G.R. & Weber, W.J., Amer. Mineralogist. 76, 1510-1532 (1991).
Weber, W.J., Radiation Effects and Defects in Solids. 115, 341-349 (1991).