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Comments on the paper ‘Fission-track dating of British Ordovician and Silurian stratotypes’ by R. J. Ross and others

SIR Fission track ages of British Ordovician and Silurian stratotypes were first published in abbreviated form by Ross et al. (1978); now that they have been fully published (Ross et al. 1982) it is possible to assess their significance more carefully. The earlier brief account of their data was hailed by some (e.g. Compston, 1979; McKerrow, Lambert & Chamberlain, 1980) as providing a considerable advance in the numerical calibration of the Phanerozoic time scale. This is an opinion not shared by fission track experts (e.g. Hurford & Green, 1982; Storzer & Wagner, 1982). Though the stratigraphy of the horizons which Ross et al. (1982) attempted to date by the fission track method is unexceptionable it is necessary to take account of Hurford & Green's (1982) warning that adequate safeguards must be taken against the addition of misleading data resulting from the related problems of uncertainties in both Xf (the 238 U fission track decay constant) and neutron dosimetry. Storzer & Wagner (1982), though suggesting that future improvements in neutron dosimetry and age standards may eventually reduce errors in the accuracy of fission track ages below the present realistic figure of +10%, commented that Ross et al. were apparently unaware of the efficiency problems in the external detector technique for zircons, that they did not consider adequately the possibility of fossil-track fading in zircon, and expressed surprise that they quoted fission track ages with errors as small as ±2%. Ross et al. (1982) consistently and seriously underestimated the errors to be attached to their fission track ages. Apart from apparently unassessed errors due to flux perturbation (see Storzer & Wagner, 1982, pp. 213-14), both Storzer & Wagner (1982) and Hurford & Green (1982) stressed the difficulty in the fission track method of accurate measurement of the neutron fluence, even when this is attempted by use of the National Bureau of Standards standard glasses SRM 961-4. The only effective way to avoid this inaccuracy, together with the important remaining uncertainties in the fission decay constant and the fission cross section, is to use an age-standard which is irradiated simultaneously with each batch of samples of unknown age. Apart from the fact that there is as yet no fission track age standard which meets all the necessary criteria of reliability for comparison with other radiometric dating schemes (see Storzer & Wagner, 1982, p. 214), Ross et al. did not follow this procedure. Instead they attempted to relate their fission track ages to the K-Ar age of a standard via neutron fluence measurement made by including in each run their laboratory standard glass, itself calibrated against SRM 962 and SRM 963 using the Cu calibration. Their procedure for relating fission track ages to the K-Ar age of a standard is given by Naeser, Hurford & Gleadow (1977). As was demonstrated by Hurford & Green (1982), this procedure introduces an extra component of at least ±3% into the absolute error to be attached to each fission track age quoted by Ross et al. (1982). As an example, the error for their Ludlovian Sample 76 Sh 25 cannot be as low as the quoted ± 9 Ma; on an absolute basis the compounded error must be at least as large as ± 15 Ma (lcr) for the quoted age of 407 Ma. The lack of a suitable K-Ar standard caused Ross et al. (1982) to write: 'It is possible, therefore, that fission track ages in the early Palaeozoic may differ by a per cent or two from K-Ar'. More serious is the fact that in their statistical computation of errors Ross et al. (1982) used a statistical formula, given by Naeser, Johnson & McGee (1978), which few believe to be applicable to fission track data. Green (1981) criticized the restatement of this approach by Johnson, McGee & Naeser (1979), who in our view failed in their reply (Johnson, McGee & Naeser, 1982) to establish the correctness of their procedure. Briefly, the crux of the matter is that Johnson, McGee & Naeser (1979) proposed a formula which reduced the estimate of the standard deviation of the ratio pjp(, used to compute a fission track age (ps is the spontaneous track density in the mineral to be dated, p( is the thermal neutron induced track density), by introducing an extra negative term depending on an assumed statistical correlation between ps and p(. In their defence of this procedure Johnson, McGee & Naeser (1982) stated correctly that both p, and p( are functions of the uranium concentration in the mineral to be dated and deduce that p, and p( are correlated, and are physically dependent